ISO11783 a Standardized Tractor Implement Interface

Transcription

1 ISO a ized Tractor Implement Interface Peter Fellmeth, Vector Informatik GmbH The upcoming ISO standard will be the preferred tractor Implement interface in the agricultural industry. Therefore the ISO standard becomes a strategically important interface that has to be supported. The major tractor manufacturers as well as Implement suppliers already support ISO. Serial products will be available in the year So it is time to publish what the ISO group worked out in a period of more than 10 years. At the beginning The need to standardise the communication between the tractor and Implement came up with the experience out of the first proprietary networks used to control various aspects of a tractor. In Germany standardisation activities started in 1988 under control of the VDMA (Agricultural Machinery Association) and results in a national standard DIN9684 named LBS (Agricultural Bus System). These activities eventually forced the creation of an international ISO based standard. In 1991 a working group (WG1) was founded with the goal to develop a international compatible data bus system. The WG1 is part of ISO TC23/SC19 (Technical Committee 23 Tractors and Machinery for Agriculture and Forestry, Subcommittee 19 Agricultural Electronics). Motivation The tractors as well as the Implements have become more and more complex. This is because of the requirements with respect to costs, effectively and usability is constantly growing. Also interoperability especially at the implement level and data interchange with farm management systems are topics that has to be taken into consideration. And last but not least systems designed for Precision Farming and Autonomous Harvesting Devices needs a standardized network to deploy its benefits. What is ISO and what does ISOBUS mean? This question cannot be answered within a single sentence. But ISO is more than an attempt to simply network various electronic control units (ECU). ISO covers all aspects of a networked system like Physical Layer, Power management, Controlling Implements and Tractor, interaction with the operator and communication with the Farm management systems. To ensure compatibility and interoperability an independent conformance test procedure is defined as well as implementation levels. An implementation level is a subset of the official ISO documents that allows machine builders to implement devices in an early stage when the ISO papers are not fully available or simply defines the minimum requirements of an ECU for practical usage. An implement level also clarifies upcoming problems during implementation phase and reports this to the ISO committee. The current implementation level is 2. ISOBUS is a term for an ISO network that is compatible to the current implementation level. Responsible for the definition of the implementation levels are two groups. One is the Implementation Group ISOBUS of the VDMA witch coordinates activity in Europe, and the North American ISOBUS Implementation Task Force (NAIITF) witch coordinates activity in the USA. 08-8

2 ISO Components The ISO standard will comprise about 13 parts. See Table 1 for a complete overview and the current state of the documents. In the following this paper will give an overview of ISO and does not claim to be a complete reference. Figure 4 shows the schematics of an ISO network. Physical Layer (Wiring and Connectors) In general a twisted quad un-shielded cable with four wires is used. CAN_L and CAN_H for the CAN Bus and TBC_PWR and TBC_RTN for the termination bias circuit. The network is designed for hot plug-in and removal of ECUs during operation. Therefore an automatic termination bias circuit is needed. Up to 3 different connectors are specified. Bus extension connector: This connector is provided to extend the implement bus within the tractor as needed. It is also known as the incab connector. Because this plug does not allow to supply the connected ECU with power an additional connector with power supply capabilities will be proposed as an option. This could be helpful for retrofitting when no other power source is available. Breakaway connector: This receptacle shall be mounted at the rear of a tractor to connect the Implements to the tractor. Diagnostic connector: Used to plug diagnostic tools to the tractor. See figure 1 as an example of a physical layer architecture, showing all connector types. For the data transmission a CAN Bus is used with a bit rate of 250kbit/s and a maximum segment of 40m length. A single sample point is used and set to 80% relative to the bit. Up to 30 ECUs can be connected to one segment. The physical CAN definitions are compatible to ISO11898 (ISO Highspeed). This allows to connect directly SAE J1939 or NMEA2000 ECUs. Normally the physical layer works with 12V power supply but it is also designed for 24V operations. Data Link Layer ISO based on 29bit CAN Identifier (CAN 2.0b protocol). The data transmission is compatible and harmonized with SAE J1939. The CAN frame is divided up into two main parts. The CAN Identifier and the data field. The CAN Identifier specifies the data in the data field and the address information for source and destination. The data field contains the signals described as parameters and grouped together as Parameter Groups (PG). Each PG is identified by a unique number (PGN). ISO Title Current Status Part 1 General for mobile Working Draft data communication 2 Physical Layer International 3 Data Link Layer International 4 Network Layer International 5 Network Management International 6 Virtual Terminal International 7 Implement Message Layer International 8 Power Train Messages Working Draft 9 Tractor ECU International 10 Task controller & Working Draft management computer interface 11 Data Dictionary Working Draft 12 Diagnosis Working Draft 13 File Server Working Draft Table 1 - Overview current ISO document status 08-9

3 Since a single CAN frame can only move up to 8 bytes most of the PGs are designed with a length of 8 bytes. For PGs with more than 8 bytes three different transport protocols are defined: 1) BAM (Broadcast Announce Messaging) allows transmission to all ECUs that are interested in this PG. The CAN frames are transmitted with a defined transmission delay between each frame. 2) CMDT (Connection Mode Data Transfer) allows the transmission between two single nodes. Transmitter and receiver can control the message flow. The maximum amount of data bytes that can be transmitted is limited to 1785 bytes. 3) ETP (Extended Transport Protocol) is similar to CMDT but designed for more data. It is possible to move more than 16 Mbytes of data. Such a high amount of data is normally used to transfer an Object Pool to the Virtual Terminal. 4) FPTP (Fast Packet Transport Protocol) is the very same as BAM but optimised for higher transmission rate. There is no delay between each frame and some protocol overhead is removed. This transport protocol is used for NMEA2000 GPS information. Network Layer In practice most ISO systems will have multiple bus segments. To connect these segments in a transparent way a bridge has to be used. It is not possible to use a repeater (echoes the electrical signals from one segment to the other) because of bus timing limitations. The bridge may have at least two CAN controllers each connected to a bus segment and moves the CAN frames between the segments. Part 4 of the ISO standard defines filter capabilities for these interconnection ECUs. Normally the Tractor ECU is an interconnection ECU between the tractor Bus and the Implement Bus. Also an Implement can be connected to the Implement bus via such a Gateway. This could be helpful to save bandwidth if the Implement is not interested in all PGNs provided by the Implement Bus or the Implement uses a lot of proprietary PGNs for internal use only. Each segment shall not be connected in more than one place to another segment. This will prevent building loops and avoids multiple transmission of the same PGN. Tractor Implement Automatic TBC with network power connection VT Bus in-cab connector Automatic with network power connection Implement Bus breakaway connector ECU n Implement Bus Diagnostic Connector Battery Tractor ECU ECU_PWR (25A) PWR (60A) ECU n-1 With automatic Tractor Bus connectors ECU stub connector Optional connectors ECU 1 SAE J1939/ 11 ECU 2 Proprietary connectors connectors * TBC = Terminating Bias Circuit Figure 1 - Example of physical layer architecture 08-10

4 Network Management Each ECU is assigned a unique name (64bits). This name represents the functionality of the ECU and gives also information about the manufacturer and the mounted location (e.g. identifying each nozzle at sprayer #1 of manufacturer X). An important feature of the network design is to allow peer-to-peer communication. Using that 64Bit name as address for transmitter and receiver is not practical while using CAN frames. Therefore every ECU claims a unique 8 bit address during power up or when connected to the network. This could lead to different addresses each time the network is started or the node is connected. Therefore each ECU must provide a Network Management that allows to claim a unique address within a network and get all the address information about its communication partners. Valid addresses are 0 to is the NULL address and is used for example while address claiming when the claiming ECU has no address. Using 255 (global address) as destination address means broadcast communication. In case of multiple ECUs running on a single node each ECU shall have a unique name and address. Working Set An Implement may have multiple ECUs. Most of the ECUs are for controlling the application. Therefore these Implements can be grouped together to a Working Set where one ECU is the Working Set Master and all the others are the Working Set Members. The Working Set Master acts as the interface to the Implement Bus. This means that the Working Set Master is addressed for several services (e.g. handling the communication to a Virtual Terminal) while the Members are still active in performing their tasks. A Working Set Member shall be able to receive PGNs addressed to the Working Set Master. It is possible to group several Working Sets together to one single Working Set. This allows combining multiple instances of single Implements to one big Implement. Two sprayers can be combined to a single one with multiple boom sections. Virtual Terminal The Virtual Terminal (VT) provides a very comfortable user interface. Each Implement can output information to the VT and can also query actions and user inputs as well as application rates. The VT provides each Implement the exclusive Soft Key Soft Key Designator Data Mask Area Navigation keys Input control keys Figure 2 Simulated VT with CANoe rights of usage the VT and it s functions. It is in the responsibility of the VT to manage multiple ECUs. Therefore the VT has to provide a way for the user to select a specific ECU s output. The VT also provides several Physical Soft Keys with corresponding Soft Key Designators and a Data Mask Area. The display of the Data Mask Area as well as the Soft Key Designators are controlled by the Implements. This allows the user to interact with multiple Implements on a single VT, mounted in the tractor s cabin. If available it is also possible that the user can assign auxiliary inputs to Implement functions provided by the VT. As example of a VT figure 2 shows a screenshot of a simulated VT with CANoe. For displaying data an Implement (Working Set Master) transfers an Object Pool to the VT. The Object Pool covers all graphical elements to be displayed. A graphical element for example can be a simple button, textual input/output field, bitmaps or more complex elements like a bar graph. All elements can be moved, modified, hide and shown as needed by the Implement. Also grouping of elements is possible. This allows modifications to the whole group with a single command

5 Because of the size of an Object Pool transferring the Object Pool to the VT on every boot up needs a lot of time and consumes bandwidth, especially if multiple Object Pools are transferred. For that reason an Object Pool can be stored on the VT s non-volatile memory. This will limit boot up delays only to the first time an Implement is connected to a tractor (VT). A retransmission is only necessary if the Object Pool changes or the VT is replaced for some reasons. Tractor ECU The Tractor ECU is the interface between the Implement Bus and the tractor. Most often the Tractor ECU is a gateway between the internal tractor J1939 bus (Power train) and the Implement Bus. In case the tractor does not have an Internal J1939 Bus, the Tractor ECU will generate the necessary PGNs like wheel based speed information out of some internal information. It doesn t matter how the Tractor ECU gets the information. This is an important aspect in case of retrofitting old tractors. A tractor (represented by the Tractor ECU) is divided up into 3 different levels providing information to the Implements and allows remote control of the tractor. A class two tractor includes the functionality of a class one tractor plus class two features. A class three tractor includes the functionality of a class two and one as well as additional class three features. The classes are important for implements to check if the necessary functionality is available. Each implement will be specified for a minimum tractor class. In the case where a tractor does not fulfil all features of a higher class the tractor will be downscaled to the class that is completely matched. A class one tractor provides basic information for power management, speed information, hitch position, PTO, tractor lightning control and preferred language selection. Class one should not be used for new tractors. Class one is typically used for retrofitting. Class two provides more information about driving speed and direction. Also the rear draft of the hitch and auxiliary valve states are provided. Class two allows for complex implements, better control and security strategy. Class three provides a command interface to the implement. The implement can control the rear hitch position, PTO speed and set point as well as auxiliary valves. An additional Class extension of N means support of navigational messages and F support of front mounted implements. The extensions N and F can be combined with the classes described above. Task controller The Task Controller is an ECU running on the Implement Bus. A Task Controller can be configured with data from a farm management system to control one or more tasks. A task contains all necessary data for a field operation. The Task Controller also sends out all commands to a controlled implement during field operation. A Task Controller is typically used for example for precision farming. When the application rate of a sprayer depends on the current position the Task Controller will prepare the sprayer with updated data during field operation. Another important aspect of the Task Controller is the logging capability. A Task Controller will not only accept and run tasks. A Task Controller will also log and report actual field application data back to the farm management system. The amount of logged data can be adapted to XML Jobs Farm Management System Task controller Results XML Figure 3 Task Controller Data exchange 08-12

6 the needs of the user. It is also possible to query information before, after and while the task is running from the operator via the Virtual Terminal. An operator for example can acknowledge its identity or can enter missing data. If necessary an operator can define a complete task on the fly. The interface between the Task Controller and farm management system is currently reviewed by the ISO SC19/ Working Group 1. It is planned to have an open XML based interface. This allows the farm management system to generate tasks without specific knowledge about the Task Controller. Only for the transmission with the Task Controller a manufacturer specific driver may be necessary. See figure 3 for data flow. ECU 1 (Transmission) ECU 2 (Hitch control) Tractor ECU 3 (Valves) ECU 4 (GPS) Tractor Bus ECU 5 Tractor ECU Implement Bus Figure 4 - Typical Topology ECU 11 ECU 10 ECU 9 ECU 8 ECU 6 (VT) Implement 2 Implement 1 ECU 7 (Task Controller) Virtual Terminal Conformance Test In 2002 the German organisation DLG (German Agricultural Society) together with the VDMA worked out a conformance test for the ISO standard. This test will ensure interoperability and stability for ISOBUS systems. Up to now the test covers ISO part 2, 3, 5, 6, 7 and 9. The quality of the test was demonstrated at many Plugfests where manufacturers of implements and tractors hooked together their devices and run against each other and also the conformance test. References For more information about ISO please refer to the standard s documents and visit the following Web pages: Peter Fellmeth Vector Informatik GmbH Ingersheimer Str Stuttgart Phone Fax Peter.Fellmeth@Vector-Informatik.com