Automatic Control and User Interface for Central Tire Inflation System

Transcription

1 Automatic Control and User Interface for Central Tire Inflation System Björgvin Rúnar Þórhallsson Thesis of 30 ETCS credits Master of Science in Electrical Engineering 7. May 2015

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3 Automatic Control and User Interface for Central Tire Inflation System Björgvin Rúnar Þórhallsson Thesis of 30 ECTS credits submitted to the School of Science and Engineering at Reykjavík University in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering 7. May 2015 Supervisor: Baldur Þorgilsson, Supervisor Adjunct, Reykjavik University Examiners: Indriði Sævar Ríkharðsson, Examiner Lector, Reykjavik University Guðni Ingimarsson, Examiner Engineer, Össur hf.

4 Copyright Björgvin Rúnar Þórhallsson 7. May 2015

5 Automatic Control and User Interface for Central Tire Inflation System Björgvin Rúnar Þórhallsson Student: 30 ECTS thesis submitted to the School of Science and Engineering at Reykjavík University in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering. 7. May 2015 Björgvin Rúnar Þórhallsson Supervisor: Baldur Þorgilsson Examiners: Indriði Sævar Ríkharðsson Guðni Ingimarsson

6 The undersigned hereby grants permission to the Reykjavík University Library to reproduce single copies of this project report entitled Automatic Control and User Interface for Central Tire Inflation System and to lend or sell such copies for private, scholarly or scientific research purposes only. The author reserves all other publication and other rights in association with the copyright in the project report, and except as herein before provided, neither the project report nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author s prior written permission. Date Björgvin Rúnar Þórhallsson Master of Science

7 Automatic Control and User Interface for Central Tire Inflation System Björgvin Rúnar Þórhallsson 7. May 2015 Abstract The aim of this study was to compile a list of requirements for the automatic control system and user interface part of central tire inflation system (CTIS) specialized for off road snow driving conditions in Iceland. Furthermore to design and build a prototype that fulfils all requirements. CTIS enables the driver of a vehicle to adjust the tire pressure while the vehicle is moving. It can reduce the risk of damaging tires because of over deflated tires, increase average travelling speed and reduce fuel consumption. Manual CTI systems have been used in Iceland for snow driving purposes in modified 4x4 vehicles and are getting more popular. Commercial automatic solutions exists but none fulfils the requirements of Icelandic drivers of snow driving vehicles. Solutions that are aimed to fulfil this market gap are currently being developed in Iceland. Experienced people within the field of modified 4x4 vehicles in Iceland were interviewed to collect opinions about requirements for the system. Experiments were conducted on realistic setups of pneumatic systems to analyse and model the system as steps toward fulfilling requirements. The designed and build prototype fulfils 12 out of 18 requirements defined for the system, more experiments are needed to confirm four and implementation towards two requirements were not finished. The list of requirements coming out of this study is the first one compiled for Icelandic snow driving conditions. It is believed that it will be a cornerstone in the discussion and development for automatic control systems in Iceland in the coming years.

8 Sjálfstýring og notandaviðmót fyrir miðlægan úrhleypibúnað við íslenskar aðstæður Björgvin Rúnar Þórhallsson 4. maí 2015 Útdráttur Markmið þessa verkefnis var annars vegar að komast að þeim kröfum sem notandaviðmót og sjálfstýring fyrir úthleypibúnaði í íslenska fjallajeppa þarf að uppfylla. Jafnframt var markmiðið að hanna og smíða sjálfstýringu og notandaviðmót sem uppfyllir allar kröfur. Úrhleypibúnaður gerir ökumanni kleift að stilla loftþrýsting í dekkjum á meðan á akstri setendur. Slíkur búnaður getur dregið úr hættu á dekkjaskemmdum af völdum aksturs á of úrhleyptu dekki, aukið meðal ferðahraða í snjó og dregið úr eldsneytiseyðslu. Notkun á úrhleypibúnaði í fjallajeppum á Íslandi er að aukast mikið en hingað til hefur þessi búnaður verið nær eingöngu handstýrður. Til eru sjálfstýrilausnir á markaði en engin uppfyllir kröfur íslenskra jeppamanna. Sjálfstýrilausnir sem ætlaðar eru fyrir snjó akstur eru nú þegar í þróun hér á landi. Rætt var við fólk sem tengist íslenskum fjallajeppum og skoðunum þeirra varðandi kröfur fyrir úrhleypibúnað safnað saman. Tilraunir voru framkvæmdar á raunverulegum úrhleypibúnaði þar sem eiginleikar kerfisins voru fundnir sem skref í þá átt að uppfylla kröfur kerfisins. Sú frumgerð sem hönnuð og smíðuð var í verkefninu uppfyllir 12 af þeim 18 skilyrðum sem sett voru fram. Ekki náðist að framkvæma tilraunir sem sýna fram á að fjögur af þessum fimm skilyrðum eru uppfyllt og ekki náðist að klára lausn á tveimur skilyrðum. Kröfulistinn sem settur var saman í verkefninu er sá fyrsti sem gerður hefur verið fyrir íslenskar snjóaðstæður. Höfundur telur víst að listinn muni verða miðpunktur umræðu og þróunar á sjálfstýringum og notandaviðmótum fyrir úrhleypibúnað á næstu árum.

9 v Contents List of Figures List of Tables viii xii 1 Introduction Modifications, for off-road snow driving Off-road snow driving Central tire inflation system Controlling the CTIS Aim and structure of the thesis 9 3 Requirements System s tire pressure total error band Setpoint pressure range Measure tire pressure relative to ambient pressure Adjustable setpoint Number of tire pressure setpoints Setpoint resolution Control constraints Real time requirements Stability Number of channels Isolated channels Indication of leakage Displaying tire pressure measurements Adjustable user interface screen brightness User interface dimensional limitations Simplicity of the user interface

10 vi 3.17 Pneumatic tank pressure sensor Temperature Design of a prototype Pneumatic system The pneumatic configuration Tire pressure sensor Over pressure protection Design options Tank pressure sensor Experimental setup User interface Screen brightness adjustments The user interface case User inputs Variables to display Processing unit and communications Scheduling Software The control model Fulfilling control constraints requirement Isolated or interconnected channels Stability Automatic tire pressure sensor offset calibration Control states Background tasks Electronic design Control unit printed circuit board User interface extension cable User interface circuit Bill of materials Design summary Future work Necessary improvements on the current prototype Future extensions to the system Future research suggestion Experiments 75

11 vii 5.1 Inflation and deflation analysis Analysing tire deflation Analysing tire inflation Pressure settling time and response time Pressure disturbance form driving Worst case latency Overall performance of the system Results Deflation analysis Inflation analysis " MTZ Mickey Thompson tire " Baja Claw Mickey Thompson tire /65R15 tire Maximum pressure change rate Pressure settling time and response time Pressure disturbances form driving Worst case latency Overall performance of the system Comparison of valve configurations Model parameters Overshoots Discussion The aim The requirements The prototype Future research suggestion Impact Conclusion 111 A Schematics 117 B Programming code for the microcontroller 121

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13 ix List of Figures 1.1 The first four wheel drive vehicle imported to Iceland Footprint experiment done by the company Arctic Trucks Deflated Mickey Thompson tires on the Vatnajökull glacier, These tires have 38 diameter and are commonly used under modified vehicles for snow driving in Iceland Cross section of a wheel hub with gear reduction Externally connected tube with a rotational joint at the centre of the wheel The overall system and the scope of this project The over all system is shown and possible location of each part of the system Design flow describing possible design paths of various parts of the system The 2N valve configuration with 2/2 way valves The 2N valve configuration with 3/3 way valves The N+2 valve configuration Total error band of the Trafag ECT2.5A pressure sensor Temperature error band for both MPX4250 and MPX5050 pressure sensors Comparison of design options for valve and pressure sensor configurations An implementation of the N+2 configuration Connection between the rubber tube and the wheels in the pneumatic system experimental setup The user interface and processing unit of the industrial solution LCD screen brightness adjustment circuit The first two prototypes of the user interface Third and current version of the user interface The user interface screen layout Location of the components that needs to interact with the processing unit Control loop latency

14 x 4.17 Block diagram of the tire pressure control system System stability Control state chart Non linear translation of raw ADC values to setpoint pressure The screen layout in front tire offset adjustment mode The layout of the printed circuit board in the control unit Fully build printed circuit board in the control unit The circuit driving the inductors of the pneumatic valves Pneumatic setup in the overall performance test Deflation results for the 38" MT and the 46" MT tires Deflation results for the 195/65R15 tire Logarithmic plot of the tire pressure during deflation of a 38" MT and 46" MT tires Logarithmic plot of the tire pressure for 195/65R15 tire Simplified model of the pneumatic system Inflation processes for the 38" MTZ Mickey Thompson tire Inflation processes for the 46" Baja Claw Mickey Thompson tire Inflation process for a 195/65R15 tire with different supply pressures Response and settling time experimental results Pressure disturbances in a tire during a drive on a bumpy gravel road Pressure disturbances in a tire during a drive on a paved road Delay histogram for the function: background tasks Delay histogram for the function: printing tank pressure Deflation from 5 to 2 psi of four 44" DC tires, connected to the N+2 valve configuration with interconnected channels Deflation from 5 to 2 psi of four 44" DC tires, connected to the N+2 valve configuration with isolated channels Deflation from 5 to 2 psi of one 44" DC tire, connected to the 2N valve configuration Inflation from 2 to 5 psi of four 44" DC tires, connected to the N+2 valve configuration with interconnected channels Inflation from 2 to 5 psi of four 44" DC tires, connected to the N+2 valve configuration with isolated channels Inflation from 2 to 5 psi of four 44" DC tires, connected to the 2N valve configuration A.1 Schematics for the control unit circuit

15 A.2 Schematics for the user interface circuit xi

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17 xiii List of Tables 2.1 Connections between requirements, experiments and implementations Setpoint resolution requirements Stability test Dimensional requirement for the user interface Operating and storage temperature range for the system components depending on location Comparison of three possible tire pressure sensors Considered design options for a the valve and pressure sensor configuration Pneumatic part numbers Specifications for MPX5700 pressure sensor Requirements related to the user interface Pin specification of the components that interact with the processing unit I/O pin requirement for the processing unit Comparison of the best accuracy and the ADC resolution for the three pressure sensors Communication options Channel interconnections or isolation depending on the front axle offset and the state of interconnected/isolated channels Status LED The main components of the control unit printed circuit board Bill of materials for the prototype Fulfilment of requirements The tires and vehicles used for the inflation and deflation experiments Pressure disturbance experiments An overview of the six experiments conducted in the overall performance test

18 xiv 6.1 Deflation curve fitting results for the three tires Comparisons of measured and calculated time constants for deflation Fit results for the inflation process for the 38" MTZ Mickey Thompson tire Fit results for the inflation process for the 46" Baja Claw Mickey Thompson tire Fit results for the inflation process for the 195/65R15 tire Pressure change rates for a 38" tire The response times of the components in the chain of reaction of valve activation Statistical summary for the worst case latency experiments Comparison of three valve configurations Statistical results for the automatic control parameter updating experiment. 105

19 1 Chapter 1 Introduction Figure 1.1: The first four wheel drive vehicle imported to Iceland called FWD. (Source: [2].) The first 4x4 vehicle in Iceland was a truck called FWD, manufactured by the Four Wheel Drive Auto Company, seen in Fig It was imported in 1927 by the Icelandic Road Administration, mainly for snow plowing. It turned out to be very useful at that time for road construction and other work [1]. The next 4x4 vehicles imported to Iceland came with the United States Army in July Before arriving, the army knew that travelling around Iceland could be difficult so they brought a lot of small 4x4 vehicles. Soon after the vehicles were imported, Icelanders noticed how much the vehicles improved mobility in Iceland. During the war, some vehicles were lent to Icelandic people and after the war the army sold most of them to Icelanders instead of transporting them back. These were the first 4x4 vehicles owned by the public in Iceland and they proved to be revolution in transportation [1].

20 2 Automatic control and user interface for CTIS Ever since then, 4x4 vehicles have been important part of transportations in the highlands of Iceland. Today during the summer time a big part of the highlands is accessible to unmodified 4x4 vehicles. However, in the winter time the area is only accessible to 4x4 vehicles that are modified for snow driving because of high snow levels and frequent bad weather conditions. Data shows that snow depth is on average 28 times deeper at Hveravellir than in Reykjavík 1. During the winter it is popular to travel off-road in the highlands and on glaciers. Even though off-road driving in Iceland is in general forbidden it is allowed to drive on frozen soil covered in snow and on glaciers [3]. Lifted and modified 4x4 vehicles increase the mobility in the highlands of Iceland and enable winter travelling in these areas. Modified 4x4 vehicles are mostly privately owned and are used for travelling. Rescue teams, tourist companies and electric maintenance crews also use them to reach the highlands in all seasons Modifications, for off-road snow driving Modifications of 4x4 vehicles for snow driving consists of changes to many parts of the vehicle. Most visible are the bigger tires and added wheel fenders. Suspension is a very important part and is in some cases rebuilt to make it stronger and increase the suspension travel. Lower differential gear ratios and differential locks are added to further increase the mobility and the steering strengthened as well to handle bigger tires. Stronger axles are sometimes added as well. The first attempts to modify and add bigger tires to a 4x4 vehicle in Iceland was around 1970 [4]. Since then, a lot of experience in modification of 4x4 vehicles has been gained over the years in Iceland. This knowledge is now being exported throughout the world, mainly by the company Arctic Trucks in the form of modification solutions and conducting expeditions in the Antarctica Off-road snow driving A key factor in successful off-road snow driving is low ground touching pressure 2, sometimes called contact pressure, between the tiers and the ground. Lower ground touching 1 The weather station at Hveravellir is located in the highlands of Iceland, about 650 m over sea level. The data covers the time range from January 1990 to May Data provider: Icelandic Meteorological Office. Available under "Veðurmælingar á Íslandi" at 2 Pressure is force (or weight) divided by area.

21 Björgvin Rúnar Þórhallsson 3 Figure 1.2: Results from an experiment conducted by the company Arctic Trucks. The figure shows three different tires at two pressure levels on a forceplate. The footprint of the tires increases when deflated from 20psi to 3psi. These tires are good examples of common tires used under 4x4 vehicles in Iceland for snow driving. (Source: [5].) pressure allows the vehicle to float on top of the snow witch improves mobility. There are three ways to decease the contact pressure. First, increase the size of the tires. Bigger tires have larger ground touching surface that decreases the ground pressure. Second, lightening the car. Third, deflate the tires, as seen in Fig 1.3. This makes the tire deform and increase its ground touching surface. Kaczmarek showed that by deflating a 12:00-18 bias ply tire on a ZIL-157 military truck from 50 to 10psi the net footprint more than doubled [6]. That results in better mobility in soft ground conditions such as sand, soil and snow [7][8]. The company Arctic Truck showed that for three different 4x4 vehicle tires the ground touching surface increased dramatically by reducing the tire pressure from 20 to 3 psi [5]. This can be seen in Fig Deflating big 4x4 vehicle s tires is an essential part of being able to travel in challenging snow conditions. A common tire pressure range for typical off-road snow driving condi-

22 4 Automatic control and user interface for CTIS Figure 1.3: Deflated Mickey Thompson tires on the Vatnajökull glacier, These tires have 38 diameter and are commonly used under modified vehicles for snow driving in Iceland. tions 3 is from 2 to 8 psi. In special cases where conditions are extremely difficult and big tires are being used the tire pressure can go as low as 1 psi or even less. However driving on deflated tire generate heat in the rubber. The heat generation depends on the driving speed and the level of deflation. Cooling is very important to prevent damage and is done by the surrounding snow. The conventional procedure of inflating 4 and deflating tires on 4x4 vehicles in the highlands of Iceland is done manually by the driver and individually for each tire. The driver has to stop the vehicle and perform the operation by hand. This is time consuming and a slow process. Normally people try to minimize the number of pressure readjustments during travelling because it is a slow process. That usually means driving for a long time with very low tire pressure that limits the maximum driving speed due to heat generation. The reason for this low tire pressure is that the toughest part of the trail defines the tire pressure. 3 Normal snow driving conditions will in this study be defined as cm deep snow where driving speed is from 10 to 50 km/h. 4 Most 4x4 vehicles that travel Iceland s highlands carry an air pump.

23 Björgvin Rúnar Þórhallsson Central tire inflation system Figure 1.4: The figure shows cross section of a wheel hub like the Hummer H1. The blue path indicate the pressure tube from the valve control unit to the tire. The purple indicate the reduction gears. The lower gear turns a short final axle which the wheel is attached to. This final gear is not explicitly shown with a specific colour in the figure. The smaller upper gear is connected to drive axle from the differential. (Source: [9].) An optimal procedure of deflating and inflating tires in snow driving would be to adjust tire pressure continuously so that the energy spent per distance travelled is minimized. This means that during a short difficult section of the trail the pressure would be decreased down to the point where the vehicle is able to traverse the train and then increase again after the section. Applying this procedure increases the average travelling speed because the level of deflation limits the rolling speed of the tire. It would also reduce the risk of damaging a tire because of driving too fast on very deflated tires. It is very time consuming to follow the optimal procedure of tire pressure adjustments in case of the conventional way of inflating and deflating is used where the driver has to stop the vehicle and get out of it to readjust the pressure. A way to allow the driver to follow the optimum procedure of inflation and deflation is a central tire inflation system (CTIS). Kaczmarek defines CTIS as: "A system incorporated in a wheeled vehicle which permits the vehicle tire pressure to be regulated by the vehicle driver/crew from within the vehicle cab while on the move" [6]. In other words, CTIS enables the driver to adjust the tire pressure in each tire while driving. To achieve this, a pneumatic tube is connected to each

24 6 Automatic control and user interface for CTIS Figure 1.5: Externally connected tubes with a rotation joint at the centre of the wheel. The figure shows a 44x18,5R15 Dick Cepek front tire on a Toyota Hilux. Picture: Björn Ingi Óskarsson tire, either through the wheel hub or externally connected on the outside of the vehicle. Connection through the wheel hub with portal axles is more mechanically complex and is usually implemented in vehicles with gear reduction at the wheel hub as seen in Fig The gears allows for a tube or pressure sealed tunnel through the short final axle. Vehicles equipped with this kind of system are for example Hummer H1 and Benz Unimog. The externally connected tubes are more commonly used than the through-hub method in modified 4x4 vehicle for snow driving in Iceland. The tube for each tire reaches out from the wheel fender down to the centre of the rim where a rotational joint enables the rotation of the wheel. From the joint a short tube is connected to the tire valve. This is shown in Fig Modified 4x4 vehicles for snow driving in Iceland are more commonly equipped with the externally connected tubes than the though-hub method because very few vehicles actually offer the possibility of the through-hub method. The system of externally connected tubes and rotational joint can be installed without much effort in any vehicle. The external tubes themselves are only installed when the CTIS is in use and disconnected during normal highway drive. The disadvantage with the externally connected tubes is that they are vulnerable to damages in rough off-road conditions. They even might have to

25 Björgvin Rúnar Þórhallsson 7 be removed in some conditions to prevent damages to them. The advantage of this method is that in case of tube or rotation joint damage it is easy and inexpensive to replace. The advantage of the connection through the wheel hub in contrast to the externally connected tubs is that the system is always connected and no tubes are vulnerable for damages. However, maintenance is both more complex and expensive Controlling the CTIS The most popular way to control a CTIS in Iceland today is to use manual valves and pressure gauges located within reach of the driver. This requires at least six valves, tubes and a gauges inside the vehicle. The driver has full control over the system but the driver s attention is needed during operation. Commercial automatic CTIS solutions are available. Following is a quick overview of some of the automatic CTIS solutions on the market. The information available about functionalities and specification from the manufactures are often limited or unavailable which makes comparison hard. CTIS in its simplest form is only used to maintain a fixed tire pressure intended for trucks and trailers. Three examples of this kind of systems are the TIREMAAX PRO system from the company Hendrickson [10], T3 system by the company Airgo [11] and Vigia system [12]. An example of more functional systems capable of inflating and deflating the tires are the Spicer system from the company Dana Corporation designed for trailer trucks and military vehicles [13]. It is equipped with user interface reachable to the driver s seat that enables the driver to choose between six predefined pressure levels: loaded/unloaded and three surface conditions. A similar system is the SmartFlow system from the company Meritor, designed for military vehicles [14]. The predefined pressure levels for the SmartFlow system are: Highway, cross country, mud/sand/snow and emergency. Other systems are the CTIS from the company AxleTech International and SYEGON by the company Nexter that include similar functionalities as the systems mentioned above with six and four pressure levels respectively [15][16]. The TireBoss system from the company Tire Pressure Control International is intended for agriculture, military and trailer trucks that allows the driver to select the setpoint pressure for each axle with 1 psi resolution [17]. It features a big user interface module located in the drivers cabin.

26 8 Automatic control and user interface for CTIS None of the above mentioned commercial systems suit the snow driving conditions in Iceland. They lack either pressure range, setpoint 5 resolution, pressure accuracy or have too big user interfaces that are unlikely to fit in the driver s cabin of modified 4x4 vehicle. In the last years some people in Iceland have been experimenting with industrial controllers to automate the control process. The most notable solution was developed by Haraldur Þorkelsson and Tryggvi Valtýr Traustason. It features a Unitronix SM35-J-R20 industrial controller, equipped with 3.5 inch coloured touch screen. The unit is 11x11x6 cm in size and has to be mounted in the interior of the vehicle [18]. The controller regulates the pressure by activating a set of electrically controlled pneumatic valves. The valve setup in this solution is called the N+2 configuration with one pressure sensor. More discussion about this configuration can be found in chapter The type of the pressure sensor used is ECT2.5A manufactured by Trafag [19]. This solution was put on the market for a public sale in October This is currently the only automatic control solution for CTIS specialized for Icelandic conditions. From this point on this solution will be referred to as the industrial solution. More solutions are currently being developed. Sölvi Oddsson is working on a solution where a smartphone is used for control processing and as a user interface. The control unit containing the valves, pressure sensors and communication module is located behind the back seats in the current test vehicle. The smartphone communicates wirelessly with the control unit. The phone is mounted in the interior of the vehicle facing the driver and displaying the user interface. The big advantage with this implementation in contrast to the industrial solution is the small user interface and how simple it is to install it in a vehicle since no wiring is needed except a charging cable to the phone. The valve configuration used in this solution is called the 2N configuration with four MPX4250 pressure sensors, discussed in chapter This solution will be referred to as the smartphone solution. 5 Setpoint is the user defined value indicating the preferred tire pressure. 6 Price for the controller and the pressure sensor is ISK. Website: (26. November 2014)

27 9 Chapter 2 Aim and structure of the thesis The aim of this study can be divided into the two following parts: Compile a list of requirements for the automatic control system and the user interface part of CTIS designed for snow driving conditions in Iceland. Design, build and test a prototype of the automatic control system and the user interface part of CTIS that fulfils all requirements. In Fig. 2.1 and 2.2 an overview of a complete CTIS is shown. The overall system contains the previously mentioned parts along with connections to each tire, pump, pneumatic pressure tank, and a set of valves. This study only deals with the control system and the user interface part of the system. The systems requirements related to the scope of the study are listed in chapter 3. The list of requirements is compiled after interviews with experienced people in the field. Each requirement is described and acceptance criteria stated. The design of a prototype, aimed to fulfil all requirements, is discussed in chapter 4. Chapter 5 contains descriptions of all the experiments related to the design process of the prototype. Results for those experiments are discussed in chapter 6 where corresponding sub chapters have the same name. Cross references are used to point the reader to the experiments and the results as relevant throughout chapter 4. Figure 4.1 shows that design flow paths for various parts of the prototype can be different. Table 2.1 describes the relations between requirements, experiments and implementation in the design of the prototype. It can be a useful guide through this study. The prototype in chapter 4 will be referred to as the Thorhalb solution from this point on.

28 10 Automatic control and user interface for CTIS Figure 2.1: The overall system and the scope of this project. The dashed line shows that the valve system had to be partly taken into the scope of the study. Figure 2.2: The over all system is shown and possible location of each part of the system. (Source: [21], [22].)

29 Björgvin Rúnar Þórhallsson 11 Requirement Requirement Related experiments Implementation Requirement defined in in chapter: confirmed? chapter: in chapters: System s tire pressure total Yes error band Setpoint pressure range Yes Measure tire pressure relative Yes to ambient pressure Adjustable setpoint No Number of tire pressure setpoints Yes Setpoint resolution Yes Control constraints , 5.2, Yes Real time requirements Yes Stability No Number of channels Yes Isolated channels Yes Indication of leakage No Displaying tire pressure measurements Yes Adjustable user interface Yes screen brightness User interface dimensional No limitations Simplicity of the user interface No Pneumatic tank pressure sensor No Temperature , 4.1.5, 4.2, 4.3, 4.6 Yes Table 2.1: The table shows connections between requirements, experiments and implementations.

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31 13 Chapter 3 Requirements In this chapter requirements related to the control system and user interface part of a central inflation system (CTIS) for Icelandic snow driving conditions will be discussed. In the making of the list the author conducted informal interviews with experienced people from within the modified 4x4 vehicle community in Iceland. The following list contains names and titles of some of the people interviewed. Páll Halldór Halldórsson, pioneer in off-road modification of Mercedes Sprinter and driver of a modified 4x4 vehicle for tourists. Sölvi Oddsson, currently developing an automatic control system for CTIS using smartphone for the processing and user interface. Hjörtur Már Gestsson, engineer and current user of manual controlled CTIS in a 4x4 vehicle. Valur Sveinbjörnsson, engineer and current user of manual controlled CTIS in a 4x4 vehicle. Tryggvi Valtýr Traustason, developer and user of the industrial solution, an automatic control system and user interface for CTIS. Ari Arnórsson, driver of a modified 4x4 vehicle for tourists. Hinrik Jóhannsson, engineer working for the company Arctic Trucks. In addition to the interviews, discussions on an Icelandic modified 4x4 vehicle forum was started to gain more opinions from people from within the community 1. Furthermore 1 The website hosting the forum is

32 14 Automatic control and user interface for CTIS all threads containing discussions about CTIS where studied and people s opinions about CTIS collected. The results shows that opinions about requirements for the control and user interface part for CTIS vary quit a lot between people. There are however many common issues that most people talk about. The requirements listed in this chapter are influenced by all the people the author interviewed. It is not expected that all people fully agree with all of the requirements. These requirements are intended to be as general as possible and apply to all user groups of CTIS for snow driving in Iceland. The rest of this chapter contains sub-chapter where each requirement is introduced, explained and acceptance criteria listed. The acceptance criteria for each requirement are put down as an ideal criteria to confirm the requirement. 3.1 System s tire pressure total error band Total error band (TEB) is defined as combination of all possible errors. The difference between accuracy and TEB is that accuracy only covers nonlinearities, hysteresis and non repeatability. With TEB, temperature related errors over some specified temperature range are also taken into account [23]. If the conventional way of deflating manually is examined, it turns out that the pen style pressure gauge often used are not very accurate 2. Sometimes deflation is performed without any gauge. An example of this could be when a vehicle is having trouble passing through a short difficult section of a trail. The driver estimates the deflation time needed for each tire, based on his experience, to make the vehicle pass through the section. Few things need to be considered when requirements for pressure sensor accuracy are decided. First, the importance of minimizing pressure difference between tires. In practice the driver adjust the tire pressure based on how the vehicle is performing at that moment, sometimes irrelative to the current tire pressure. The mobility in snow driving increases with lower tire pressure down to the limit when the tire becomes over deflated 3. Over deflation reduces the mobility in snow because the touch down pressure distribution of 2 No accuracy information was found for any pen style pressure gauge on the Internet. When no information is given about accuracy it usually means that the accuracy is bad, estimated ±2 4 psi for a gauge rang of 0 to 20 psi. 3 Over deflated tire is defined as a tire deflated down below the point of maximum mobility in snow conditions that results in very uneven pressure distribution of the ground touching area.

33 Björgvin Rúnar Þórhallsson 15 the tire becomes uneven. To be able to approach that limit, for all tires without crossing the limit, the relative error between tires has to be minimized. Absolute pressure accuracy requirement is an unknown function of pressure because more accuracy is needed in the lower end of the pressure range. This requirement indicate the system s tire pressure total error band. That means it covers combined errors for the pressure sensor, the analogue to digital converter and the control tolerances. The developers of the two automatic control CTIS solutions currently being developed in Iceland, the industrial and the smartphone solutions, have both come to the conclusion that the accuracies of the pressure sensors that they are using are sufficient for the task. This requirement is based on their experience. The determination of this requirement is not considered to be sufficient. It would require a separate study to find out experimentally what total error band is required for a CTIS in off-road snow driving. Discussed further in chapter Requirement: The total error band (TEB) 4 of the tire pressure for the temperature range from 25 C to 85 C has to be less than or equal to ±2.18 psi 5. Acceptance criteria: The specifications from the manufacturer of the sensor has to fulfil the requirement of TEB over the entire operating temperature range. 3.2 Setpoint pressure range If only off-road conditions are considered the necessary setpoint range is from 0.5 to 10 psi. However if highway tire pressure is included as well, the setpoint pressure range increases to 0.5 to 35 psi. For most individuals using this system for private travelling the smaller range is enough. It is however preferred to be able to use the system to fully inflate the tires. 4 This number may appear to be very high but it is the worst case error that is indeed unlikely to be the case. Typical error band figures are expected to be at least half the TEB. 5 See comparison between pressure sensors in table 4.1.

34 16 Automatic control and user interface for CTIS Requirement: The system has to support setpoint adjustments from 0.5 to 35 psi. Acceptance criteria: Using tire pressure sensor that covers the specified range. The user interface offering the possibility to adjust the tire setpoint pressure within the full specified range. 3.3 Measure tire pressure relative to ambient pressure One important features of an automatic CTIS is the ability to measure tire pressure relative to the ambient pressure. Then travelling in highlands and on glaciers altitude difference of few hundred meters can be experienced during relatively short time. For example when travelling up to Grímsvötn caldera on the Vatnajökull glacier. The altitude when entering the glacier at the west side is about 700m over see while Grimsvötn hut is in about 1700m over sea. This altitude difference results in about 1.6 psi atmospheric pressure difference [24]. When no CTIS is installed and driving towards higher altitude it turns out to be necessary to stop the vehicle occasionally to deflate the tires to maintain the same level of deflation. The opposite is experienced when driving towards lower altitude which can increase the changes of damaging the tires. Then the atmospheric pressure increases, towards lower altitude, the level of deflation increases. That increases the changes of damaging a tire because of driving on too low tire pressure, specially if the driver is not aware of this effect. Requirement: The tire pressure sensor has to measure pressure relative to the ambient pressure. Acceptance criteria: Change the ambient pressure by one psi and observe the system react to the changes.

35 Björgvin Rúnar Þórhallsson Adjustable setpoint An important element of the automated CTIS is for the driver to be able to adjust the setpoint pressure 6. Furthermore the driver should be able to do it while driving in challenging conditions. Requirement: A driver in a vehicle equipped with automatic CTIS has to be able to adjust the tire setpoint while driving in challenging conditions. Acceptance criteria: An experienced person in driving a 4x4 vehicle, has to drive at 40 ± 5 km/h on a twisty gravel road and be able to adjust the setpoint from 2 to 10 psi in 4 s and the other way around in 4 s. The user interface has to be mounted in the interior of the vehicle within reach of the driver. 3.5 Number of tire pressure setpoints The minimum number of setpoint needed is one. In that case one setpoint applies to all the tires. It is sufficient for the general traveller in most cases to have one setpoint for all tires. In case of a vehicle having very different weights on its axles it becomes necessary to allow for different tire pressure setpoint depending on the axle. Both for a four and six wheel vehicle a front and rear pressure settings is sufficient. There are also rare cases of lateral gradient when different tire pressure, depending on which side of the vehicle the tire is, would be beneficial. That is, however, not considered a requirement for a CTIS. Requirement: One adjustable tire pressure setpoint for all four wheels with an optional front axle pressure offset covering the pressure range of ±4 psi with half psi resolution. 6 Setpoint pressure is the target tire pressure that the driver is requesting.

36 18 Automatic control and user interface for CTIS Acceptance criteria: Connect two additional pressure sensors, one to the front tire and one for the rear tire. Change the front axle offset and see the corresponding change on the pressure sensors. 3.6 Setpoint resolution Tire pressure setpoint resolution defines the step size a user is able to adjust the setpoint. More resolution is needed in the pressure range from 0.5 to 5 psi than the rest of the range. Requirement: The resolution requirements are listed in table 3.1. Pressure range Resolution requirement psi 0.5 psi 5 35 psi 1 psi Table 3.1: Setpoint resolution requirements. Acceptance criteria: Connect an additional pressure sensor to a tire which is connected to a CTIS. Change the setpoint by one step in each of the two pressure ranges and see the corresponding change in the additional pressure sensor.

37 Björgvin Rúnar Þórhallsson Control constraints A performance requirement regarding the duration of a certain inflation or deflating operation 7 is not possible to determine for a general control system. The time needed for an operation depends heavily on the properties of the pneumatic system including air tank, pipes, valves and connectors. These properties can vary a lot between implementations in vehicles and therefore the performance of the system. The control system is, however, partly responsible for the time length of an operation. There are two requirements that the control system has to fulfil in order to minimize the operation time. First, eliminating overshoots for both inflation and deflation. Overshoots increase the duration of an operation more than necessary. Second, minimizing inactive time during an operation. Inactive time can be due to measurements being taken in case of a pneumatic configuration that does not allow for pressure readings while inflating/deflating. Requirement: Inflation and deflation overshoots are eliminated. Inactive time cannot exceed more than 10% of the total duration of an operation. Acceptance criteria: The following procedure has to be performed while fulfilling both requirements: 1. Initial tire pressure is set to 5.0 psi. 2. Adjust setpoint to 2.0 psi. 3. Wait until all channels are within 0.05 psi tolerance from setpoint. 4. Adjust setpoint to 5.0 psi. 5. Wait until all channels are within 0.05 psi tolerance from setpoint. 7 An operation is defined as the time interval from the moment then the system finds a channel (tire or interconnected tires) that are not within tolerance form the setpoint pressure, until it is back within tolerance. If the setpoint pressure is changed within an operation a new operation has started.

38 20 Automatic control and user interface for CTIS 3.8 Real time requirements The real time requirements for the control system are defined such that the worst case latency does not result in deflation/inflation overshoot more than the tire pressure setpoint tolerance. Requirement: The following equation has to be fulfilled: max control loop latency [s] pressure tolerance [psi] max pressure change rate [psi/s] (3.1) where maximum control loop latency is measured and depends on the processing unit and the scheduling algorithm of the control system. The term pressure tolerance the tire pressure setpoint tolerance and is determined by the developer of the system. The term max pressure change rate is measured and depends on the tires and properties of the pneumatic system. Acceptance criteria: Equation 3.1 has to be fulfilled. 3.9 Stability The stability of the system is defined as the ability to ignore pressure disturbances coursed by driving while still be able to regulate the tire pressure in case of a leakage or setpoint change. One of main purposes of having a CTIS automated rather than manual is for the driver to concentrate on other things, including driving the vehicle, while the system guarantees that tire pressures are within tolerances. That makes the stability of the system while driving important.

39 Björgvin Rúnar Þórhallsson 21 Requirement: Ones the system has reached setpoint ± tolerance on all tires/channels and the setpoint is not changed, the system has to remain stable even if the vehicle is moving 8. The system is assumed to have no leakage. Acceptance criteria: Stability has to be maintained throughout the experiment in table 3.2. The conditions should be: Gravel road with cm of snow. Each pressure level is tested over specified distance and speed. The vehicle used in this experiment need to be equipped with 38 to 46 inch diameter tires. Part Tire pressure Speed Distance 1 20 psi 40 km/h 400 m 2 10 psi 30 km/h 400 m 3 5 psi 20 km/h 100 m Table 3.2: Stability test Number of channels The system has to support four channels which are sufficient both for four and six wheel vehicle. Requirement: The system has to support four channels. Acceptance criteria: The pneumatic system has to provide connections for four channels. 8 The system is stable when all channels are within tolerance.

40 22 Automatic control and user interface for CTIS 3.11 Isolated channels Isolation of channels is an important feature of CTIS. In case of a vehicle, equipped with CTIS, having different weight resting on the tires it becomes necessary to be able to isolate the channels so that the air does not flow from heavier side to the lighter side. This happens if the vehicle is heavier on one side than the other or the vehicle is driving in a slope. Then air is allowed to flow freely between channels in a slope for long enough time the tires on the heavier side become over deflated which reduces heavily the mobility of the vehicle. Requirement: The system has to provide either permanent isolation between channels or the option of isolate channels selectable through the user interface. Acceptance criteria: Vehicle is equipped with CTIS and tires are inflated to 20 psi. Unplugging a tube in one channel and observe no effects to the tire pressure in the other channels Indication of leakage One advantage of using automated CTIS rather than manual is the ability to maintain a certain tire pressure in case of a leakage in a tire or in the pneumatic system. How much leakage the system can handle depends entirely on the properties of the pneumatic system and are out of the scope of this study to determine. When a leakage occur that the system can handle no specific action is needed other than keep refilling the tire. However it is preferred that the driver is informed about the situation for possible reaction to the leakage. If the driver decide to do nothing about the leakage the indication cannot be implemented in such a way that is annoying to the people in the vehicle over a few hours of travelling.

41 Björgvin Rúnar Þórhallsson 23 Requirement: The user interface has to indicate a leakage in the system. The implementation of the warning sign has to be ignorable in the case when the driver chooses not to respond to the warning. Acceptance criteria: An experienced driver, driving a vehicle with CTIS installed should notice a leak at least two minutes after the leak occur. The level of leakage should be large in enough to require the system to refill a tire at least every minute Displaying tire pressure measurements One of the main purpose of the user interface is to display tire pressure information to the driver. All pressure readings have to be displayed in the psi pressure unit 9. Requirement: The most resent pressure readings for all channels has to be displayed in the psi pressure unit with one decimal point. All values have to be readable from the driver s seat. Acceptance criteria: A person with normal vision has to be able to read the pressure values for all channels on the user interface sitting in the driver s seat of a vehicle equipped with a CTIS Adjustable user interface screen brightness When travelling in 4x4 vehicle in the highlands of Iceland the light conditions can vary a lot. The brightest conditions are probably sunny days on glaciers with clear sky where 9 Pound per square inch.

42 24 Automatic control and user interface for CTIS the snow reflects the sunlight. The other end of the scale are dark nights. For the screen to be useful on the glacier it has to bright. In dark conditions a darker screen is required to not disturb the driver with too much light. Requirement: The brightness of the user interface screen has to be adjustable. Acceptance criteria: An experienced person in modified 4x4 vehicle driving has to confirm that the screen is readable while not too bright for the two following conditions: Driving outside in snow with clear skies and sun, or equivalent. Driving during a night with no surrounding lights except the headlamps of the vehicle User interface dimensional limitations In modified 4x4 vehicles are all kinds of gadgets are added to the interior. Extra gauges, radio transceivers and laptop or a tablet are examples of added equipment. This usually results in rather crowded interior around the dashboard. When automated CTIS is added as well the space available is very limited. A precise and general requirement for the size of the user interface is impossible to make. The size and shape of available space in interiors vary between vehicles but smaller user interface is in general better. Three different ways of implementing the requirement for dimensions of the user interface are listed in table 3.3. It would be preferred to have a requirement that is quantitatively measurable and applies to all vehicles. None of the methods listed in the table fulfils that. The second option was, however, used as a requirement is measurable and applies to all vehicles.

43 Björgvin Rúnar Þórhallsson 25 Method Pros Cons Limitations on the dimensions of the user interface (length, width, hight, volume) Limitations on the time it takes for a mechanic to permanently install the user interface to a vehicle. Limitations on disruption to the interior of the vehicle. Easy to measure. Dimensional requirement different between vehicles. The same requirement applies to all vehicles. Easy to measure. The same requirement applies to all vehicles. There are more thing than installation time that affect the quality of the outcome. Difficult to measure quantitatively. Table 3.3: Three different methods of implementing the dimensional requirement for the user interface. Requirement: It has to be easy to permanently mount the user interface in a modified 4x4 vehicle. Acceptance criteria: A mechanic has to be less than two hours to permanently mount the user interface and cabling in a Toyota Hilux 2014 model Simplicity of the user interface Simplicity and accessibility of the user interface is very important. For most privately owned vehicles equipped with CTIS the user and the buyer are the same person. In that case the level of complexity and user configurable parameters can be higher. There are also companies that buy automatic control systems for CTIS. For those vehicles it has to be assumed that the driver is inexperienced user of CTIS. For that reason it is important that the user interface is as simple as possible.

44 26 Automatic control and user interface for CTIS Requirement: Any person that knows the basic functionality of CTIS should be able to figure out how to use it without reading the manual. Acceptance criteria: A person with basic understanding of the functionality of CTIS has to be able to proceed though the following list in less than 30 seconds: 1. The device is turned off. 2. Instructions to the person: "This is a device to control the tire pressure of a 4x4 vehicle where the driver can adjust the setpoint pressure." 3. A stopwatch is started. 4. Instructions to the person: "Turn on the device and adjust the setpoint to 5 psi". When the task is accomplished go to next step. 5. Instructions to the person: "Adjust the setpoint to 14 psi and turn of the device". When the task is accomplished go to next step. 6. Stop the stopwatch Pneumatic tank pressure sensor Throughput of the air pump is usually the speed limiting factor of inflating tires. That is why the driver s awareness of the pressure in the air tank helps to understand the state of the system. A standalone pressure gauge could be used for this purpose. But like described in chapter 3.15 the available space in the interior of a modified 4x4 vehicle is very limited. It is preferred that the displaying of the tank pressure is included in the user interface to save space. The total error band requirement for the sensor are not intense. The main purpose of the sensor is for the driver to know if the tank is empty or full.

45 Björgvin Rúnar Þórhallsson 27 Requirement: The air tank pressure readings should be displayed on the screen of the user interface. The frequency of measurements and screen update rate has to be 4 Hz. The total error band has to be less than or equal to ±5 psi. Pressure range from 0 to 120 psi. Displayed resolution 0.1 psi. Acceptance criteria: The datasheet for the sensor being used has to indicate that the pressure range and TEB requirements are met. User interface has to display the the measurements four times per second Temperature The location of the systems components can vary between implementations. However, every implementation has an user interface located in the driver s cabin. The temperature of the driver s cabin is expected to be close to room temperature most of the time. The control unit, valves, and pressure sensors can be located anywhere in the vehicle. Most common locations for the control unit and the valves are under the bonnet or in the trunk. It means that temperature can vary from 25 C to 85 C depending on the location, environment temperature and engine temperature. Requirement: Operating temperature Storage temperature Control unit 25 C to 85 C 40 C to 100 C User interface 0 C to 30 C 40 C to 100 C Table 3.4: Operating and storage temperature range for the system components depending on location.

46 28 Automatic control and user interface for CTIS Acceptance criteria: All components have to fulfil the operating and storage temperature requirements listed in table 3.4.

47 29 Chapter 4 Design of a prototype In this chapter the design process behind the prototype of CTIS control system and user interface is described. The design process of this prototype turned out to be more complex than a series of independent decisions. Every design decision affects many aspects of the system which makes parallel design flow of many parts of the system necessary. Fig. 4.1 shows how many design paths are possible for each individual part of the system. Figure 4.1: Design flow describing possible design paths of various parts of the system.

48 30 Automatic control and user interface for CTIS 4.1 Pneumatic system To be able to design a control system it is necessary to define the valve configuration being controlled. In this chapter the options for pneumatic configurations and pressure sensors are discussed and the choice of configuration justified The pneumatic configuration Figure 4.2: Pneumatic schematic for the 2N valve configuration using two 2/2 way valves per channel. Only one channel are drawn to the figure. Other channels are connected like the first one by getting air supply directly from the tank. The schematic does not contain any over pressure safety components. Two pressure sensors are shown, one for the tank and one for channel one. One pressure sensor is needed for each channel. The pneumatic valve system has to able to inflate and deflate each tire/channel 1 individually, as stated in chapter There are two configurations of valves available to fulfil that requirement. The first configuration uses two 2/2 way 2 valves or one 3/3 way 3 valve per channel. The 2/2 way valve configuration is shown in Fig. 4.2 and the 3/3 way version is shown in Fig For N number of channels 4 the number of valves needed is 2N. For the 2N configuration four pressure sensors are needed, one for each channel. The advantage of this configuration is the ability of individual and parallel control of each channels. The disadvantage is the high number of valves and pressure sensors needed. The second configuration has one inflation valve and one deflation valve. Then one valve for each channel. All the vales are 2/2 way. The schematic is shown in Fig Channel is an individual pipe to a tire/tires that is connected to the valve system. 2 2/2 way valve has two ports and two positions. One activator is needed for each valve. 3 3/3 way valve has three ports and three positions. Two activators is needed for each valve. 4 N is the number of individual channels. Usually one channel is needed for each tire. This

49 Björgvin Rúnar Þórhallsson 31 Figure 4.3: Pneumatic schematic for the 2N valve configuration using one 3/3 way valve per channel. Only one channel is drawn here. Other channels are connected like the first one by getting air supply directly from the tank. The schematic does not contain any over pressure safety components. Two pressure sensors are shown, one for the tank and one for channel one. One pressure sensor is needed for each channel. configuration uses N + 2 number of valves for N number of channels, instead of 2N valves for the previously mentioned configuration. When using the N+2 configuration with a four wheel vehicle the number of valves need is reduced by two compared to the 2N configuration. The cost saving for a system with four channels is estimated 25% of the valve system cost 5 with some of the fittings needed. Component s part numbers are shown in table 4.3. The N + 2 configuration is able to control the channels individually but not more than one channel at a time. It is not possible to inflate one channel while for example deflating another. How ever it is possible to interconnect some or all channels and inflate/deflate them all simultaneously. The disadvantage of interconnecting all channels together is that air can flow between them as discussed in chapter Isolation is, however, not needed at all times. There are two ways of connecting a pressure sensor to this configuration. The first one is to connect one sensor to each channel. That requires four sensors but enables pressure readings of all channels at all times. The second option is to connect one pressure sensor directly to the sheared node seen in Fig This reduces the number of sensors needed by three and pressure reading variation between channels are reduced. The setup allows the system to choose the channels which it is operating on that can be anything form one channel to all of them.

50 32 Automatic control and user interface for CTIS Figure 4.4: Pneumatic schematic for the N+2 valve configuration. Only two channels are drawn to the figure. Other channels are connected to the inflation and deflation valves like the first two channels. The schematic does not contain any over pressure safety components. Two pressure sensors are shown, one for the tank and one for the tires Tire pressure sensor The tire pressure sensors currently being used in the two automatic control solutions for CTIS in Iceland are listed in table 4.1. The two sensors in these solutions are quite different. One is intended for industrial environment while the other is intended for microcontroller based products. The industrial sensor is almost nine times more expensive than the other while having larger total error band in three out of four temperature points compared in table 4.1. The sensor ECT2.5 is available both with current and voltage output while MPX4250 is only available with 0 5 V analogue output. No other reasonably priced pressure sensors could be found after extensive search on the Internet that could potentially fulfil the requirements for the pressure sensor. This was confirmed by the developer of the smartphone solution. The total error band range includes all possible errors [23]. It is clear from Fig. 4.5 and 4.6 that the total error band depends heavily on temperature. This raises the question if the effort of implementing a temperature regulated environment for the pressure sensor would be beneficial. That is, however, not considered in this study. 5 N+2 uses six valves while 2N uses eight January Januray Januray 2015.

51 Björgvin Rúnar Þórhallsson 33 Used in: Industrial solution Smartphone solution None Manufacturer Trafag Freescale Freescale Type ECT2.5A MPX4250 MPX5050 Pressure range psi psi psi 25 C ±2.18 psi ±1.14 psi ±0.408 psi 0 C ±1.27 psi ±0.508 psi ±0.181 psi 25 C ±0.363 psi ±0.508 psi ±0.181 psi 85 C ±2.18 psi ±0.508 psi ±0.181 psi Price ISK ISK(16.09 USD 7 ) 2079 ISK(16.09 USD 8 ) Source Samey ehf Digikey.com Digikey.com Table 4.1: Comparison of three possible tire pressure sensors. The first two sensors are currently being used in automatic control solutions for CTIS in Iceland. Typical values can be expected to be half the range of the TEB. (Sources: [19][20][25].) Figure 4.5: Total error band of the Trafag ECT2.5A pressure sensor as a function of temperature. Note that both maximum and typical total error band is shown. (Source: [19]) If more accuracy is needed a second sensor could be added. That could be the third sensor in table 4.1, MPX5050. The Pressure range is only from 0 to 7.25psi. That does, however, cover the lower end of the pressure range which is the range used for snow driving. The problem is that the sensor s burst pressure is 29 psi which is lower than the maximum tire pressure required. It could be solved by adding a small 2/2 way valve to block the sensor when the pressure is more than a certain value. This option was not implemented in this study. When the specifications in table 4.1 are considered the choice of the MPX4250 over ECT2.5A is obvious. It fulfils the requirements of TEB over the specified operating temperature range.

52 34 Automatic control and user interface for CTIS Figure 4.6: Temperature error band for both MPX4250 and MPX5050 pressure sensors. The temperature error factor is multiplied with a given accuracy for the temperature range 0 to 85 C to get the total error band. Total error band values for some chosen values are shown in table 4.1. (Sources: [20], [26].) The pneumatic system configurations N+2 and 2N, discussed in chapter 4.1.1, do not allow for continuous pressure readings while inflating or deflating. That is because the pressure sensors are connected to the path of air stream to and from the tire. The flow in the pipe disturbs the pressure as Bernoulli s principle indicates so that the measured pressure is not the same as in the tire. A solution to this problem could be a wireless pressure transmitters located in the rim, like in tire pressure monitoring systems (TPMS). TPMS are only intended to inform the driver about tire pressure and give warning signs when tire pressure is to low. These systems are standard equipment in many new cars today. There are also a lot of after market systems available. The accuracy requirements for pressure sensors in TPMS, according to the ISO standard [27], are ±10 kp a or ±1.45 psi over the entire temperature range which is from 40 C to 85 C. This accuracy is very close to what the pressure sensor MPX4250 offers so a TPMS that follows the standard could be integrated to a CTIS. In this study the option of integrating wireless pressure transmitters is considered as a potential a future improvement to the system. The problem of not being able to measure the tire pressure while inflating or deflating is solved and discussed in chapter

53 Björgvin Rúnar Þórhallsson Over pressure protection A correct functionality of the control system can not be guaranteed at all times. It has to be assumed that failure will occur. The worst thing that can happen is that the air tank is connected to a tire over long enough time to blow the tire. There are two solutions available to prevent this from happening in case of a control system failure. First, use a pressure regulator to reduce the supply pressure to the valve system that reduces the maximum possible tire pressure. Second, use a relief valve to limit the maximum pressure in the pipes connecting to each tire. These two solutions have different effect on the control system. When a pressure regulator is used, the supply pressure can be seen as constant which makes the predictive inflation time model simple as discussed in chapter However, the disadvantage is that the supply pressure is reduced which reduces the inflation flow rate. That is not necessary a bad thing because the bottleneck in the process of inflation is the throughput of the pump but not the resistance in the pneumatic system. In case of a relief valve being used the supply pressure variations increase a lot compared to the pressure regulator which causes the inflation parameter to fluctuate a lot in the system implemented in chapter For the prototype designed and build in this study a pressure regulator was be used Design options Design Config No. of Comment Pressure Deflation Cost option sensors sensor time [min] [ISK] 1 N+2 1 Interconnected MPX : N+2 1 Isolated MPX : N+2 1 Interconnected ECT2.5A 2 : N+2 1 Isolated ECT2.5A 8 : N+2 4 Interconnected MPX : N+2 4 Isolated MPX : N+2 4 Interconnected ECT2.5A 2 : N+2 4 Isolated ECT2.5A 7 : N 4 - MPX : N 4 - ECT2.5A 1 : Table 4.2: Considered design options for a the valve and pressure sensor configuration. Table 4.2 was constructed to summarize and compare the two valve configurations described above. The table contains ten design options. The N+2 configuration can be operated in two different ways, interconnected or isolated channels. In addition to the two

54 36 Automatic control and user interface for CTIS 1.6 x 105 Estimated cost of implementation [ISK] Better solution Deflation time (5 to 2 psi) [min] Figure 4.7: Comparison of the solutions listed in table 4.2. The points in the table represents design options for valve and pressure sensor configurations. The arrow points to the direction of better solution. Two solutions number one and nine are classified as Parato optimal ways of operating the N+2 configuration it can be implemented with one or four pressure sensors. The 2N configuration is included to the comparison. These five options are all listed twice with two types of pressure sensors each. The deflation times in the table are results from the experiment in chapter 5.5. Results can be found in chapter 6.6. The experiment was designed to replicate the most time critical execution of CTIS in snow driving. That is deflation from about 5 to 2 psi. There are no experimental data available for the design options number six and eight but they where estimated to be 15% faster than design options number two and four 9. The experiment was only conducted with the MPX4250 pressure sensor. The deflation times are estimated to be the same for both pressure sensors as they do not affect the deflation times. 9 When four pressure sensors are being used with the N+2 configuration, operated in isolated-mode, there is no need to wait for pressure measurements before start inflating/deflating a channel.

55 Björgvin Rúnar Þórhallsson 37 Component Part no. Manufacturer Orifice diameter Inflation valve [28] Q3C124.BV0 FLO Control 2.4 mm Tire valves [29] Q2C140.BV0 FLO Control 4 mm Deflation valve [30] U2D180.BB0 FLO Control 8 mm Inductors /666 V 12 FLO Control - Table 4.3: The table shows the part numbers for the valves and inductors used in the prototype. All valves are normally closed. The cost of the configurations in the table include only the cost of the valves, some of the fittings to connect the valves and the pressure sensors 10. The particular valves used in the configurations compare in table 4.2, are listed in table 4.3. To simplify comparison of the design options in table 4.2 the options are plotted in Fig. 4.7 as a function of cost and deflation time. Note that there are four pairs of points in the graph where each pair represents the same hardware solution 11 but operated in a different way. The problem of finding the best solution is set up as a multi objective optimization because there is no objective function available. The aim is to minimize the cost and at the same time to minimize the deflation time. To determine a set of optimal solutions the idea of Pareto dominance is applied to find the Pareto optimal set [31]. Two points in the graph satisfies the conditions of Pareto optimal solutions. These are points number one and nine. Point number one is the N+2 configuration with one MPX4250 pressure sensor operated in interconnected mode. Point number nine is the 2N configuration with four MPX4250 pressure sensors. It has to be noted that point number one in the graph does not fulfil the requirement of isolated channels but can, however, be operated in isolated mode that fulfils the requirement. Design option number one, and two, is chosen to be the configuration in this study because it is less expensive and has fewer valves and pressure sensors. The implementation of the N+2 valve configuration can be seen in figure The following prices are used: N+2 config (6 valves): ISK (Landvélar, September 2014), 2N config: ISK (estimated form the N+2 config), MPX4250 pressure sensor: 2000 ISK ( Trafag ECT2.5A pressure sensor: ISK (Samey ehf, January 2015). 11 The pairs are the following: (1,2) (3,4) (5,6) (7,8).

56 38 Automatic control and user interface for CTIS Figure 4.8: The N+2 configuration that was used in the prototype. The blue path indicates deflation on channel one which is activated by opening the deflation and the tire/channel valves. The red path indicates inflation on channel one which is activated by opening the inflation and the tire/channel valves. The same procedure applies for the other channels. The figure shows a regulator in series with the connection to the air tank. It is used to reduce the pressure to the valve system, see more in chapter Tank pressure sensor An additional pressure sensor is used to measure the pressure in the air tank as required in chapter The only sensor found that was reasonably priced, available and sufficient for the developing of the prototype was the MPX5700, seen in table 4.4. It meets all requirements except it does not cover the pressure range from 100 to 120 psi. Information about the TEB from 25 C to 0 C are not available. The TEB for the sensor in the range from 25 C to 85 C is expected to be close to the TEB requirement of ±5 psi January 2015.

57 Björgvin Rúnar Þórhallsson 39 Manufacturer Freescale Type MPX5700DP Pressure range psi 25 C No info available 0 C ±2.53 psi 25 C ±2.53 psi 85 C ±2.53 psi Price 1832 ISK(13.99 USD) 12 Source Digikey.com Table 4.4: Specifications for MPX5700 pressure sensor. (Source [26].) Experimental setup The experimental setup in this chapter was used in number of experiments in this study. Cross references will be used later to direct the reader back to this chapter. The setup can be considered realistic setup apart from the air source that is usually a small tank and a pump in a typical 4x4 vehicle. The setup contains the following sequence of interconnected parts in this order: Pressurized air source, 7 bar. Can be considered a constant pressure source. Rubber tube, 8 mm inner diameter, with a coupling, unknown length. Pressure regulator of type: Air Comp MR 1/4" R SRU, adjusted to 30 psi except other specified. One inch long 1/4 diameter pipe. The valve system seen in Fig The valves are specified in table 4.3 and assembled in the N+2 configuration described in chapter A male coupling terminal is screwed on tire valve one as seen in Fig A 7.7 m long, 8 mm inner diameter rubber tube, with a female coupling connected to tire valve one in the valve system. The rubber tube connects to the rim valve as seen in Fig The valve core was removed.

58 40 Automatic control and user interface for CTIS Figure 4.9: Connection between the rubber tube and the wheels in the experiments. Note that the inner core of the rim valve was removed in the experiments, as seen in the small figure. 4.2 User interface The user interface of an automatic tire inflation system is very important part of the system. It has to be as simple and easy to operate as possible while fulfilling requirements of functionality. Requirements related to the user interface are listed in table 4.5. Lets begin by examine the two other solutions being developed in Iceland, first mentioned in chapter 1. The first one is the industrial solution using the Unitronics SM35-J-R20 industrial controller seen in Fig The unit is both processing unit and user interface, equipped with a 3.5 colour touch screen. Access to parameter adjustments and manual control are easy to implement in this unit. Permanently mounting this unit in a 4x4 vehicle within reach of the driver is not a straight forward job in most vehicles because of it s size. The unit needs electrical connections to the tire pressure sensor, valves and power supply. The smartphone solution uses, as the name indicates, a smartphone as a user interface. Control processing is also executed on the phone. Like the industrial controller the smartphone interface allows for different menus. The communications to the control unit, which can be located anywhere in the vehicle, is accomplished using Bluetooth. What is needed in the interior of the vehicle is a permanent cellphone mount and access to changing cable. The challenging part of this solution is to write the program for the smartphone. The program has to be supported on wide range of platforms and operating systems to be successful as a general product. The advantages are no wiring to the user interface and distribution of new software updates can be done through the Internet.

59 Björgvin Rúnar Þórhallsson 41 Req. in Description chapter 3.2 Setpoint pressure range 3.4 Adjustable setpoint 3.5 Number of tire pressure setpoints 3.6 Setpoint resolution 3.12 Indication of leakage 3.13 Displaying tire pressure measurements 3.14 Adjustable user interface screen brightness 3.15 User interface dimensional limitations 3.16 Simplicity of the user interface 3.17 Pneumatic tank pressure sensor 3.18 Temperature Table 4.5: Requirements related to the user interface. Figure 4.10: The user interface and processing unit of the industrial solution: Unitronics SM35-J-R20 industrial controller. (Source: [32].) The conclusion coming out of the comparison of the two solutions above are that the industrial computer is to big and the smartphone programming is out of reach for the author of this study. The Thorhalb solution s 13 control unit is based on a microcontroller, controlling the valves and reading the pressure sensor. The available options for an user interface from the control unit s point of view are the following. First, wireless module connecting to a smartphone or a tablet. Before, people used laptops to host navigation programs but tablets are now getting more popular. Second, small standalone colour touch screen like the 2.8" TFT LCD with touchscreen breakout board from the webstore adafruit.com 14. This would require some building of a frame around the screen and possible a printed circuit board with a microcontroller. Third, using 2 by 16 character LCD screen. These 13 Thorhalb solution is the name of the prototype designed in this study. 14 Price: 30 USD ( ).

60 42 Automatic control and user interface for CTIS screens have standardized parallel interface and are manufactured by a lot of companies. They come in prices down to 3.28 USD 15. This option requires that separate components are used for user inputs such as the setpoint. The 2x16 char LCD screen was chosen for the prototype because the components are cheap and connecting them to the microcontroller is simple. However the operating temperature range this kind of displays is a concern. In temperatures close or below 0 C the updated speed capabilities of the screen reduces significantly. So if a vehicle is started in a cold weather the screen will not be working properly until the vehicle has wormed up Screen brightness adjustments The LCD backlight is a LED. Testing showed that the current through the LED has to be adjustable in the range from 2 to 20 ma to fulfil the requirement in chapter It was implemented using the LM7805 voltage regulator in a constant current configuration seen in Fig The resistance between V out and GND on the regulator can be calculated: R high = R low = 5 V 2 ma 5 V 20 ma = 2.5 kω (4.1) = 250 Ω (4.2) The resistor and the potentiometer in the circuit are picked according to the calculations. The potentiometer in the circuit is used to adjust the brightness. It is not included on the front panel of the current prototype. The power dissipation in the resistor in the brightest adjustment is: so a 1/4W resistor is sufficient. P res = (5 V )2 20 ma = 0.1 W (4.3) A light dependent resistor with decreasing resistance with increasing light intensity could be used as well instead of the potentiometer for automatic adjustments. 15 Product: "16 x 2 character LCD display module with blue backlight" at the webstore: ( ).

61 Björgvin Rúnar Þórhallsson 43 Figure 4.11: LCD screen brightness adjustment circuit. A voltage regulator used in a constant current configuration with a potentiometer to adjust the current through the back light diode The user interface case (a) First prototype of user interface. (b) Second prototype of user interface. Figure 4.12: The first two prototypes of the user interface. Fig. (a) is made of paper while (b) is made of aluminium extrusion with aluminium front and back. The design of the case, containing the screen and the additional components, is challenging. The case has to fulfil the requirement of dimensional limitations and simplicity, see table 4.5. The first idea was to include the microcontroller to the user interface case and make it dimensionally similar to a car radio as can be seen in Fig. 4.12a. This prototype was designed around the display with space for the switches in the front and an Arduino Uno microcontroller platform inside. The minimum dimensions of the case for this design turned out to be 114 x 43 x 90 mm (w x h x d). The second prototype, seen in Fig. 4.12b, is an aluminium extrusion with aluminium plates for front and back covers. The outer dimensions of the extrusion are 137 x 47 x 90 mm, which is bigger than the first prototype. The second version has the power switch and a power LED on the right hand sight of the screen. On the other side is a potentiometer, switch and a LED. If the device is assumed

62 44 Automatic control and user interface for CTIS Figure 4.13: Third and current version of the user interface. The dimensions of the case without knob and switches is 137 x 47 x 27 mm (w x h x d). The functionality of the "auto" switch in the figure has been changed since this prototype was build. The new functionality of the switch chooses between isolated or interconnected channels and is discussed in chapter to be located on the right hand sight of the steering wheel, in the interior of the vehicle, the driver will use his right hand to operate the device. In that case he will be blocking the view to the screen with his hand. This can be solved by switch sides for all components around the screen, seen in Fig In the current version the microcontroller was been moved to the control unit so the profile thickness of the user interface was reduced down to 27 mm. That gives a lot of new possibilities of mounting it in an interior of a vehicle. With the thinner design it is not necessary to embed the device in the interior, instead it can be a standalone. After considerable amount of testing and operating the device, further potential improvements have been spotted. The power LED turns out to be useless because the backlight of screen is always on when the device is on. The brightness of the other LED is too much and has to be decreased with a higher series resistance. In chapters and the additional components for the user interface are described. Options for communications between the control unit and user interface are discussed in chapter User inputs The user interface is equipped with a power switch. It drives the primary side of the main relay for the control unit, located at the control unit, and turns the user interface on.

63 Björgvin Rúnar Þórhallsson 45 The pressure setpoint is an important part the system. It is the user input that tells the system what tire pressure is preferred at a given moment. It has to fulfil the requirements specified in chapters 3.2, 3.4, 3.5 and 3.6. There are a few options to implement the hardware part of the setpoint. One option could be two push buttons. One for increment setpoint and one for decrement. The step size would be the resolution defined in the requirements. Another option could be a continuous rotary encoder with a knob. The advantages with this solution is that the translation between the encoder s output and setpoint is very flexible and could even be angular speed dependent. The disadvantage is that when the system is restarted the previous setpoint is lost unless the variable is stored in a non volatile memory. Another possibility is a potentiometer with a knob. It does not loose its position when the device is restarted but it has a limited rotational range. Typical rotational range of potentiometer can be close to 270. The last option, the potentiometer, was chosen for the prototype because it requires one pin on the microcontroller. Requirements in chapters 3.6 and 3.2, indicating setpoint range and resolution, are easy to fulfil and are implemented in software. However, resolution of the analogue to digital converter (ADC) in the microcontroller has to be sufficient. The chosen microcontroller has a 10-bit ADC which provides 1024 steps. The required number of setpoint steps is 39, see table 3.1, so the ADC resolution is sufficient. The requirement in chapter 3.4 indicate that the setpoint has to be adjustable even during a challenging drive. A potentiometer with a physical knob is considered more likely to fulfil that requirement than a touchscreen device. After doing some tests with the prototype a flaw in the functionality of the setpoint potentiometer was discovered. When the position of the potentiometer was on the margin between two steps of the ADC the setpoint value trended to jump constantly between steps. That made it impossible for the system to remain stable. The solutions was to add software hysteresis to the setpoint changes. According to requirement in chapter 3.5 the user has to be able to set the front axle pressure offset with respect to the rear axle. This functionality gets a dedicated switch on the front panel. The current version does not include the switch. By flipping the switch, the preexisting potentiometer is used to adjust the offset in the range from 4 psi to 4 psi with 0.5 psi resolution. The last user input to the system is a dedicated switch where the user can select between interconnected channels or isolated channels. This functionality is described in chapter

64 46 Automatic control and user interface for CTIS Variables to display The user interface has to report variables to the user. The requirements regarding these variables and their specifications are discussed in chapters 3.2, 3.10, 3.13 and The setpoint has the pressure range from 0.5 to 35.0 psi with resolution of 0.1 psi. The maximum number of characters needed to display the setpoint including the decimal point is four. The tire pressure measurement range is from 0 to 35 psi with displayed resolution of 0.1 psi. The maximum number of characters needed to display the tire pressure measurements are four. The newest measurement of each of the four channels have to be displayed simultaneously. The tank pressure measurement range is from 0 to 120 psi, with resolution of 1psi. This requires maximum of three character to display. The resulting display setup is shown in Fig Figure 4.14: The screen layout. All decimal points have fixed locations. The "M" sign indicate what channels is being measured at that time. Instead of "M" an upward or downward arrows are displayed in the same locations when inflation or deflation is being performed respectively. (See also Fig. 4.13). The requirement in chapter 3.12 discusses the need for indication of tire or pneumatic system leakage. The implementation towards fulfilment of this requirement is to add a dedicated green LED to the front panel of the user interface. The LED represents a programming boolean variable called allw ithint ol. This variable is high and the LED is on if all channels are within pressure tolerance from the setpoint. When one or more channels are not within tolerance the variable is low and the LED is off. The LED makes it possible for the driver to quickly check the status of the system by looking at the LED instead of reading the display. Then the system experience no leakage and all channels are within tolerance the LED should just stay on. In case of a leakage the LED is toggling all the time as the system is refilling the tire. This should catch the attention of the driver but

65 Björgvin Rúnar Þórhallsson 47 if he chooses not to respond to the leakage this kind of indication should not be annoying over some period of time. 4.3 Processing unit and communications. In chapters 4.1 and 4.2 components interacting with the processing unit have been defined. A complete list of components interacting with the processing unit is listed in table 4.6 along with the type and number of pins required to interact with them. Function A/D I/O Number of pins Location LCD screen Digital Output 6 User interface Set point POT Analog Input 1 User interface Switches Digital Input 2 User interface Status LED Digital Output 1 User interface Pressure sensors Analog Input 2 Control unit Valve control Digital Output 6 Control unit Table 4.6: Pin specification of the components that interact with the processing unit. The direction of the pins are noted from the processing unit point of view. The processing unit has to offer the pins or communication protocols to communicate with all the components. These pin requirements are summarized in table 4.7. A possible microcontroller (MC), the Atmega32u4 in the Arduino Micro platform has 20 I/O pins and thereof 12 analogue to digital converting (ADC) pins. The platform is small and contains all the extra components needed for the MC to run. Its processing power is likely to be sufficient and the support community around the Arduino platforms is big. It fulfils the requirement of operating temperature in chapter 3.18 and has sufficient ADC resolution as can be seen in table 4.8. The absolute accuracy of the ADC is ±2 LSB. This platform was chosen because it seems to fit the requirements and the author is familiarized with these platforms. The search for other possible platforms or MC is considered a future work for later versions at this point. Type Voltage Number of pins Analog in 5 V 3 Digital in 5 V 2 Digital out 5 V 13 Table 4.7: I/O pin requirement for the processing unit. The direction of the pins are noted from the processing unit point of view.

66 48 Automatic control and user interface for CTIS Purpose Pressure Accuracy at ADC resolution Displayed sensor 0 C to 85 C resolution Tire pressure sensor MPX4250 ±0.508psi psi 0.1 psi Tank pressure sensor MPX5700 ±2.54 psi psi 1 psi Table 4.8: Comparison of the best accuracy and the ADC resolution for the three pressure sensors. In both cases the ADC resolution is at least 25 times smaller than the accuracy. The location of the components in table 4.6 are shown in Fig The problem is where to locate the MC. It could be located in the user interface (UI) but as discussed in chapter 4.2 it is important to minimize the size of the UI. That leaves us with one MC at the control unit. By allocate a MC to the control unit the length of the analogue voltage signals from Figure 4.15: Location of the components that needs to interact with the processing unit. The location of the processing unit could be on either side. the pressure sensors to the MC can be minimized by locating the MC as close as possible to the MC. This is important to minimize the noise picked up by the wire, as discussed in chapter 4.6. The communications between the MC, located at the control unit, and components of the UI have to be solved. The distance can vary form about one to four meters depending on the location of the control unit in the vehicle. Number of options for communicating between the control unit and the components of the user interface are available. Some of these options are listed in table 4.9. In the time scope of this project the only solution considered is the most simple one to implement for a prototype, the parallel solution. Choosing the best communication method is considered a future work for later versions at this point. Some wireless solution is likely to be implemented for the next version of the prototype. It was decided to implement the communications in the most simple way and put more emphasis on testing and development of the control part. One of the wires from the UI is the analogue voltage signal from the setpoint potentiometer. It is expected, then this wire is extended over one to four meters in the environment of 16 Citation needed.

67 Björgvin Rúnar Þórhallsson 49 Communication method CAN bus I 2 C bus RS232 RS485 Bluetooth All wires parallel - no communication protocol used Comment Bus used in vehicles for MC communications. Can be implemented with a MC with CAN bus capabilities. The maximum wire length 16 is a limiting factor and it is sensitive to noise. Solutions available for 5 V microcontrollers like the MAX3323E chip. Solutions available for 5 V microcontrollers like the MAX485 chip. Lot of modules available has the advantage of no wiring between control unit and user interface. Requires connectors with lots of pins. Table 4.9: Some of the options available for communications between the control unit and the user interface. a running vehicle, for the wire to pick up a lot of noise. Two measures are implemented to reduce this effect of the noise. First, a bypass filter located at the ADC oin of the MC, discussed in chapter 4.6. Second, hysteresis in the translation of raw ADC data to the setpoint value in software, discussed in chapter To summarize the design decisions the microcontroller Atmega32u4 in the Arduino Micro platform was be located in the control unit. The communication to the components in the user interface was implemented simply be extending the wires. Other design options will be considered for later versions of the prototype. 4.4 Scheduling The first two scheduling algorithms that were considered were the Round Robin and Earliest deadline first. Round robin algorithm executes tasks cyclically but with maximum time for each task. It prevents a task to hold a resource for a long time. The implementation of this algorithm needs interrupts to take over the control in case of a maximum round time achieved. The earliest deadline first scheduling algorithm assumes independent tasks and arbitrary arrival times of tasks. It arranges processes in a priority queue and executes the task closest to its deadline [33]. Another way of implementing the control system on a microcontroller is so called nonpreemptive loop-based timer polling which does not rely on any interrupts. It is based

68 50 Automatic control and user interface for CTIS Thresholds for all three functions. Backgr. task 1 executed Backgr. task 2 executed TPC task executed Timer polled for TPC task t Timer polled for backgr. task 2 Timer polled for backgr. task 1 Figure 4.16: An example of a latency from the moment a timer reaches a threshold for the tire pressure control (TPC) task and until it is discovered. The red arrow indicates the latency. on polling a timer and comparing the current timer value to a threshold value and when the timer has reached the threshold some action is taken. This method can be used to run functions periodically or with arbitrary time between executions. One advantage of avoiding using interrupts is the potential problem with global variables shared between the interrupt service routine and the main program. The disadvantage of this method is deviation from the actual time specified between executions of a function. This deviation or latency is defined as the time from the moment a time reaches a threshold until the program notices it. An example of this can be seen in Fig The real time requirements for the system are defined in chapter 3.8 as the following equation: max control loop latency(s) pressure tolerance(psi) max pressure change rate(psi/s) (4.4) The pressure tolerances are decided to be 0.05 psi, discussed further in chapter The maximum change rate of 0.17 psi was obtained in the experimental results in chapter An experiment was conducted on the timer polling scheduling method described in chapter 5.4 to find the maximum latency or the maximum control loop latency. The results in chapter 6.5 indicate that maximum latency for the tire pressure control function is 9.6 ms. The equation below show that the system is well within real time requirements: ms 0.05 psi 0.17 psi/s = 294 ms (4.5)

69 Björgvin Rúnar Þórhallsson 51 This confirms that the timer polling scheduling fulfils the real time requirements for the system. This method was chosen because it both fulfils requirements and is simple to implement on the microcontroller. The timer polling method is used to execute three time dependent functions. The main function is the tire pressure control function which is the core functionality of the system, discussed in chapters to The other two functions are dedicated to background tasks, discussed in chapter Software This section discusses the tire pressure control algorithm. The tire pressure control functionality is discussed in chapters to Other background tasks are discussed in chapter The control model The input variables to the tire pressure control system are pressure setpoint, tire pressure measurements and two user configurable binary variables. The output variables are the digital signals controlling the position of the pneumatic valves. Fig shows the block diagram of the tire pressure control system. Figure 4.17: Block diagram of the tire pressure control system Fulfilling control constraints requirement The requirements in chapter 3.7 indicates that pressure overshoots both for inflation and deflation should be eliminated and that inactive time should not exceed 10% of the total

70 52 Automatic control and user interface for CTIS duration of an operation. This chapter contains discussion of implementations in order to fulfil the requirements. As mentioned is chapter the pneumatic configuration does not allow tire pressure readings while inflating/deflating the tires. When tire pressure is measured the air flow in or out of the tire has to be stopped. To minimize the inactive time due to measurements an experiment was conducted to find out the pressure settling time 17 after valve opening. Before every measurement a valve reconfiguration has to be performed which makes it necessary to know the pressure settling time. The experiment is described in chapter 5.2 and the results in chapter 6.3. The results indicate a settling time of 76 ms. The algorithm then has to wait 80 ms after valve reconfiguration before it can start measuring the tire pressure. To be able to calculate the exact duration of inflation/deflation in order to get from current pressure to setpoint pressure a model of the deflation and inflation processes was obtained. The experiments are described in chapter 5.1 and the results are discussed in chapters 6.1 and 6.2. The model obtained for deflation is equation 6.26: p tire (t) = p(0)e t/k (4.6) where p tire (t) is the tire pressure during deflation at time t, p(0) is the tire pressure at t = 0, and k is a time constant. This model can be used to determine the deflation time length by rewriting equation 4.6: ( ) pcurrent t = ln k (4.7) p setp oint where p current = p(0) is the current tire pressure and p setp oint = p tire (t) is the requested tire pressure or setpoint pressure. Given that k is know the time duration for deflation can be calculated. The obtained model for inflation is equation 6.28: p tire (t) = a t + b (4.8) where a is the pressure change rate and b is pressure offset. The equation indicates how pressure in a tire increases with time when inflated. In the experimental setup a pressure regulator was used to reduce the pressure. 17 Settling time is the time length from the moment the control signal is toggled until the sensor output is within ±2% of the final value [34].

71 Björgvin Rúnar Þórhallsson 53 The inflation model can be used to determine the time length by rewriting equation 4.8: t = p tire(t) b a (4.9) Equation 4.9 is then used to find time required to get between the press points p current and p setp oint, that gives t = t setp oint t current = p setp oint(t) b a p current(t) b a t = p setp oint p current a (4.10) (4.11) Given that a is know, the time duration for inflation can be calculated. The parameters k and a have to be determined for each individual system. There are two ways of determine or measure these parameters. One could run a calibration program after installation to a vehicle that does automatic experiments and determines the parameters. The disadvantages with this manually started calibration is that it adds an additional step to the installation process and this process most likely have to be repeated periodically. If these parameters change over time they can not be redetermined unless doing the manually started calibration again. The alternative approach is to tune the parameters continuously and automatically while the system is in general use. For every instance of inflation or deflation attempt the respective parameter is calculated from the results for that attempt. The automatic method was chosen because it is more general solution. The method was implemented so that a new parameter is calculated in the second measurement done state seen in Fig The advantage of this method is that the user does not have to do anything else than use the system for it to calibrate it self. When some characteristics of the system changes with time the system will automatically adapt without the user noticing. To summarize the following equations are used to calculate the parameters for the inflation and deflation models. In case of deflation the following equation is used: k = t ln( pcurrent p setpoint ) (4.12)

72 54 Automatic control and user interface for CTIS where p current is the pressure before and p setpoint is the pressure after the deflation. For inflation the following equation is used a = p setpoint p current t (4.13) The formulation in chapter 6.1 shows that the parameter k is at least dependent on tire volume and tube diameter. The weight of the vehicle was not considered in those models but the parameter is likely to be dependent on the weight of the vehicle as well. That makes continuous calibration necessary because the fuel weight in a vehicle travelling in the highlands of Iceland can easily vary by 150 kg through out one trip. When using the continuous self calibration method the system has to be able to store the parameters then the system is turned off. The chosen microcontroller has 1024 bytes of non-volatile EEPROM memory that can be used to store the parameters. However the manufacturer only guaranties that the memory can be written 100 thousands times [35]. The total live time of a single device can be estimated to be: 30 years 50 usage year 10hours min 60 usage hour = min (4.14) An EEPROM memory address can be written once every minute with out a problem. The parameter is stored in a global variable and once every minute the value is written to an EEPROM address Isolated or interconnected channels As the requirement in chapter 3.11 indicates it is necessary to be able to isolate the channels in some circumstances. That is, however, not needed at all times. The pneumatic configuration N+2, in chapter 4.1.1, allows for both interconnection of channels and isolation of channels. Interconnecting them increases the throughput of the system as seen in chapter 6.6. A dedicated switch on the user interface allows the driver to optimize the system by flipping a switch according to the environment at each time. The microcontroller reads the switch and updates the isolated/interconnected state four times a second. A possible future work would be to integrate an accelerometer to sense the tilt angle in both front-back and left-right directions. When a certain angle is reach the system switches to isolated channels but interconnected otherwise. The actual implementation of isolated / interconnected setting has to take into account the status of the front axle offset. This can be seen in table 4.10.

73 Björgvin Rúnar Þórhallsson 55 The two positions of the switch on the user interface Front axle offset = 0 Front axle offset 0 All channels interconnectenecteconnected All channels intercon- Front channels inter- and rear channels interconnected Individual channels All channels individual All channels individual but with different setpoints Table 4.10: Channel interconnections or isolation depending on the front axle offset and the state of interconnected/isolated channels Stability To fulfil the requirement of stability of the system at all times, in chapter 3.9, a pressure average over some time interval has to be taken when the tire pressure is to be determined. The average, which is a low pass filter, is intended to filter out pressure disturbances from driving. This filter could be implemented electronically or digitally. Digital method was chosen in this case for its flexibility and will be discussed in more details later in this section. Two different conditions have to be considered when the low pass filter is designed. First, a vehicle that is not moving. No significant pressure disturbance are generated in the tires and no need for a low pass filter when the vehicle is still. Second, a vehicle that is driving in an off-road conditions. A low pass filter is necessary for the control system to observe the real tire pressure during an off-road drive. The digital low pass filter is implemented to calculate the average pressure over some period of time. The length of this time period defines the frequency response of the filter. To minimize the inactive time of an operation it is necessary to minimize the length of the time period. The control system, described in chapter 4.5.1, controls six activators each with two stages. To make it possible for the system to remain stable around the setpoint, a tolerance is added to the pressure setpoint. It is, however, important to minimize this tolerance because it has to be added to the pressure sensor TEB to get the overall system TEB. Now these two parameters, tolerance size and averaging interval length, need to be determined. Fig shows how theses parameters effect the stability of the system. The resolution of the tire pressure measurements displayed on the screen of the user interface, see Fig. 4.13, is 0.1 psi. If the pressure sensor uncertainty is ignored an acceptable tolerance could be half the displayed resolution or ±0.05 psi. From now on the toler-

74 56 Automatic control and user interface for CTIS Less system accuracy Stable Very stable Larger tolerance Unstable Stable More system accuracy Faster system Pressure average over longer time Slower system Figure 4.18: The effect of the two parameters, set point tolerance and averaging interval length, on the stability of the system. ance is a constant and the averaging interval length will be determined with respect to the tolerance. Experiments where conducted to observe the properties of pressure disturbances during a drive. Experimental procedure is described in chapter 5.3 and results in chapter 6.4. The results show that for a drive at 25 to 30 km/h on a gravel road an averaging interval of 2.6 s is needed to keep the signal inside the tolerance boundaries. For a stationary vehicle no filter is needed. From the results a system of two filters was implemented. One filter for a stationary vehicle with a very short averaging interval to speed up the control process. The longer averaging interval is used when the vehicle is moving. The procedure of measuring the tire pressure starts with ten measurements over a period of half second. If the difference between highest and lowest pressure among these ten measurements is lower than a certain limit then the average value is used. If on the other hand the difference is higher then the system takes more measurements over longer period of time and uses the average as the results for the pressures reading.

75 Björgvin Rúnar Þórhallsson 57 The exact values of the threshold and averaging interval lengths for both cases have to be determined with further testing with the system installed in a vehicle Automatic tire pressure sensor offset calibration According to the datasheet for the sensor a typical variations are expected to be ±0.5%, which corresponds to ±0.18 psi. It would be beneficial to eliminate this variations. The implementations of a calibration processes is simple to perform and takes a shot time. To perform the calibration in the N + 2 valve configuration, seen in Fig. 4.8, the deflation valve is opened, while keep all other valves closed, which equalizes the pressure on both terminals of the tire pressure sensor. That allows the system to read the sensor and store the offset value. This automated calibration is performed every time the system is started Control states A state chart of the tire pressure control system is shown in Fig The chart shows one control iteration for a channel or interconnected channels. The control system cycles through the channels according to the setting in table It is preferred that some maximum inflation and deflation duration is respected when circling through the channels. This is to eliminate large pressure difference between channels. The requirement in chapter 3.7 indicates that the inactive time of an operation can not exceed more than 10% of the total duration of the operation. The maximum inflation and deflation time can be calculated based on the following: 10% inactive time total time = inactive time inactive time + t max (4.15) The inactive time can be estimated: inactive time = 2 t settlingtime + 2 t measurement = 1160 ms (4.16)

76 58 Automatic control and user interface for CTIS Figure 4.19: A state chart of one tire pressure control iteration. This procedure is executed cyclically on the channels being controlled that can be anything from four single channels to a channel that is formed by interconnecting all four channels together. where t settlingtime = 80 ms and t measurement = 500 ms. Now equation 4.15 can be solved for t max : t max 10.4 s (4.17) The maximum inflation or deflation duration in a single control iteration has to be at least 10.4 s to fulfil the requirement in chapter 3.7. The following unnumbered sections describe what happens in each of the states seen in Fig Wait settling time In this state the system has already configured the valves in a new way and is waiting for the pressure to settle down. Settling time is defined as the delay from the time then the control signal is activated until the sensor output is within 2% of the final value [34]. The length of this waiting period was determined in an experiment described in chapter 5.2 and the results in chapter 6.3. The results indicate that the total delay is about 76ms. The settling time is set to 80 ms.

77 Björgvin Rúnar Þórhallsson 59 In this state the program is waiting for a timer to reach a threshold to proceed to next state. First/Second measurement ongoing In this state the number of measurements already taken are compared to the number of measurements that needs to be taken before the average is calculated. The number of measurements taken can vary. It depends on the pressure disturbances described in chapter Wait between measurements Wait between individual measurements. A fixed time length of 50 ms. The program is waiting for a timer to reach a threshold to proceed to next state. First measurement done At this point the desired number of measurements have been added together. The procedure is the following: 1. The average is calculated. 2. The tire pressure on the screen for the current channel/channels is updated using the new measurement. 3. A decision is made on whether an inflation, deflation or nothing is needed for the channel. The length of the inflation/deflation period is based on the model described in chapter Valves are configured according to the decision in step 3. If the channel is within tolerance from the setpoint the program configures the valves to measure the next channel. 5. Update the boolean variable allw iothint ol indicating that all channels are within tolerance from the setpoint. It is used to wait an extra time between channels when all of them are within tolerance. 6. Proceed to next state.

78 60 Automatic control and user interface for CTIS All channels within tolerance - extra waiting Because of the chosen valve and pressure sensor configuration it is necessary to change position of valve each time a new channel is being measured. Then all channels are within tolerance from the setpoint it is not necessary to cycle through the channels at full speed. To reduce the number of unnecessary valve toggles the system waits an extra time between channels. This wait is eight seconds for the first minute and 25 s after that. Inflation/Deflation ongoing The valves have been configured to either inflate or deflate and the program is waiting for a timer to reach a threshold to proceed to next state. Inflation/Deflation done The valves are configured to measure the current channel. The program proceeds to the next state. Second measurement done At this point desired number of measurements have been added together. The procedure is the following: 1. The average is calculated. 2. The tire pressure on the screen for the current channel is updated using the new measurement. 3. The control parameters are updated. Detailed description in chapter The valves are configured to measure the next channel. 5. Proceed to next state Background tasks All background tasks except one are grouped together in a function called background task and executed four times per second. One of the background tasks could not be implemented with a fixed period between executions so a separated function was used to service that task, called Tank pressure update.

79 Björgvin Rúnar Þórhallsson 61 Setpoint update The potentiometer in Fig is read and the raw data is converted to setpoint pressure according to Fig The non linearity conversion is because considerable more time 35 Translation of raw ADC value to setpoint pressure pressure [psi] raw ADC value Figure 4.20: Non linear translation of raw ADC values to setpoint pressure. The low pressure range is used more than the upper so bigger portion of the potentiometer range is dedicated to the lower range than the upper. The proportion of the potentiometer range dedicated to the lower range is adjustable with a parameter in the program. is spent in the range from 0 to 5 psi than the rest of the range during operation of the system. The requirement for set point resolution in chapter 3.6 are fulfilled. It says that the resolution in the range from 0 to 5 psi has to be 0.5 psi and in the range from 5 to maximum pressure 1 psi. Hysteresis was added to the transitions between setpoint pressure levels in order to stabilize the setpoint as described in chapter Update isolated / interconnected state A dedicated switch on the user interface chooses between isolated and interconnected channels, as discussed in chapter The switch can be seen in Fig labelled as Auto. The auto functionality is part of a early design that has been updated since it was

80 62 Automatic control and user interface for CTIS build. The new functionality of the switch is to choose between interconnected or isolated channels. Front tire offset switch To enter the front tire offset adjustment mode the user has to flip the switch. The screen changes to the layout in figure The user uses the preexisting potentiometer on the user interface to adjust the offset in the range from -4 to 4 psi as stated in chapter 3.5. Then the adjustment is finished the switch is toggled back. Figure 4.21: The screen layout in front tire offset adjustment mode. Update status LED The status LED is updated according to the state of the system. It has two states described in table The purpose of the LED is for the driver to be able to see quickly the status of the system without having to read numbers of the screen as discussed in chapter State LED off LED on Indicating that System is currently regulating some channels that are not within tolerance from the setpoint. All channels within tolerance from setpoint Table 4.11: Status LED. Tank pressure update According to requirement 3.17 the user interface has to display the air tank pressure. The implementation of this can be seen in Fig and An additional functionality was added to the displaying of air tank pressure. The number on the screen blinks with 1.4 s period and 50% duty cycle when the air tank pressure is lower than 30 psi. This is done to indicate to the driver that the supply air tank pressure

81 Björgvin Rúnar Þórhallsson 63 is low. Further more all inflation activity is disabled to prevent interconnection of the air tank and a tire when the supply pressure is low. 4.6 Electronic design This chapter discusses the design of two printed circuit boards for the control unit and the user interface Control unit printed circuit board All the components of the control unit are populated on a single printed circuit board (PCB). Table 4.12 summarize the main components of the PCB. The implementation regarding these components are discussed in details in the following unnumbered section. For simplifications the first prototype of PCB was a single sided board only with through hole components. For later versions double sided board with surface mount components will be used. Fig and 4.23 show the PCB layout and fully assembled PCB. The Component Arduino Micro Pressure sensors Pneumatic valve inductor driver Power input Voltage regulator and noise filtering User interface extension connector Design requirements Removable from sockets. Need to leave space for the USB cable to be plugged in. Removable from sockets. Alignment with respect to the pressure tubes that connect to them. Minimizing the analogue wire length. Need to handle the current and voltage specifications of the inductor, limiting the kickback voltage. Reverse polarity protection. Short circuit protection. Main relay control. Power supply stability for analogue sensors and analogue to digital converters. Has to provide the number of pins required. Non reversible connector. Table 4.12: The main components of the control unit printed circuit board. schematic can be seen in Fig. A.1.

82 64 Automatic control and user interface for CTIS Figure 4.22: The printed circuit board in the control unit. Bottom layer traces are blue and red ones are top layer. The board is only single sided so the top layer traces are jumpers. Power supply The device is be powered by the vehicle s 12 V supply. Estimated maximum current consumed by the device is 6 A. The power switch on the user interface does not handle 6A so a relay is necessary. A 12V supply is fed from the control unit through the extension cable to the user interface. The power switch connects the 12 V supply to a wire leading back to the control unit and powering the coil of the main relay. The 12 V from the power switch also supplies the local 5 V regulator in the user interface. The main relay supplies power to all components in the control unit. Schematics can be seen in Fig. A.1 and A.2. The microcontroller and the pressure sensors runs on 5 V supply so a voltage regulator is required. Since the 5 V components requires in total less than 1 A the LM7805 regulator was chosen [36]. Bypass filter capacitors are placed both on the 12 V supply and the 5 V rail. The capacitors are connected to the rail being filtered and to ground as seen in Fig. A.1. These capacitors serve the purpose of shorting high frequency noise to ground and