Solenoid Control System for Air Flow to Pneumatic Valves

Transcription

1 Solenoid Control System for Air Flow to Pneumatic Valves Project for Chemical Engineering H194 [Summer 2015] Submitted by: Aditya Nandy Advisors: Velencia Witherspoon Jun Xu, Ph.D. Prof. Jeffrey A. Reimer, Ph.D.

2 Nandy 1 Abstract: The purpose of the solenoid control system was to regulate the airflow through a network of pneumatic valves, allowing air to flow (or not flow) through certain areas of the given setup, therefore triggering certain pneumatic valves. A Raspberry Pi 2 Model B (RPi2B) microcomputer was used as a microcontroller to regulate the current flow to the different solenoid valves, allowing each valve to be triggered individually, completely independently of the other valves. Upon the successful actuation of a single solenoid using a python code through the RPi2B, the single circuit was scaled up and replicated ten times to control ten solenoids. Upon the production of the ten-solenoid circuit, each solenoid was successfully actuated individually. SEE NEXT PAGE OF REPORT FOR MORE INFORMATION

3 Nandy 2 Cost Analysis: Predicted (w/ Rabbit Board Module): ITEM COST QTY TOTAL Rabbit SBC BL4S200 (RCM4100 Module) $ $ Rabbit SBC Grounded Case $ $ Normally Closed 1/8" 12V DC Inch Plastic Electric Air Gas Water Solenoid Valve $ $ Male-Male Wires (Assorted Sizes) $ $ Male-Female Wires (for GPIO pins) $ $ 4.99 Solderboard $ $ TOTAL COST $ Predicted (w/ Raspberry Pi 2 Model B): ITEM COST QTY TOTAL Raspberry Pi 2 Model B $ $ Raspberry Pi 2 Model B Grounded Case $ $ 9.95 Normally Closed 1/8" 12V DC Inch Plastic Electric Air Gas Water Solenoid Valve $ $ Male-Male Wires (Assorted Sizes) $ $ Male-Female Wires (for GPIO pins) $ $ 4.99 Solderboard $ $ kω Resistors $ $ A10 Rectifier (Flyback) Diodes (1000V, 6A) $ $ 9.26 TIP120 NPN Bipolar Transistors $ $ 8.24 Copper Heat Sink $ $ 3.95 TOTAL COST $ The predicted cost for the setup using the RPi2B was merely 34% of the cost of the setup using the Rabbit SBC BL4S200 microcomputer. Note that neither the breadboard for prototyping nor the enameled wire were not noted in the cost analysis due to the fact that these items were already present in lab. The breadboard was a temporary setup for the solderboard (permanent setup). Actual (w/ Raspberry Pi 2 Model B): ITEM COST QTY TOTAL Raspberry Pi 2 Model B $ $ Raspberry Pi 2 Model B Grounded Case $ $ 9.95 Normally Closed 1/8" 12V DC Inch Plastic Electric Air Gas Water Solenoid Valve $ $ Male-Male Wires (Assorted Sizes) $ $ Male-Female Wires (for GPIO pins) $ $ 4.99 Solderboard $ $ kω Resistors $ $ A10 Rectifier (Flyback) Diodes (1000V, 6A) $ $ 9.26 TIP120 NPN Bipolar Transistors $ $ 8.24

4 Nandy 3 Copper Heat Sink $ $ 3.95 International Rectifier IRLB3034PBF N-CH MOSFET, (40V, 195A) TO-220AB $ $ 5.33 TOTAL COST $ Note: the actual cost for the setup was $5.33 more than the predicted cost, due to the purchase of a MOSFET (metal oxide semiconductor field-effect transistor), which was not used in the final circuit. Circuit Diagrams Figure 1: Circuit for Actuation of a Single Solenoid RPi 2 Model B Pins 3.3 V 5 V 2, SDA 5 V 3, SCL 4 14, TXD Sol Valve 1 15, RXD Figure 1 is the initially proposed circuit for powering one solenoid valve. The pins on the left side of the figure are the GPIO pins on the RPi2B. The RPi2B is powered independently in the setup. The blue resistor is the 1 kω base resistor, and the power source here is the 15V, 1.00A power supply. The resistor feeds into the transistor, which attaches to ground. The ground for the RPi2B and the ground for the valve must be connected so that the ground is common among the two, so that there is no unwanted potential. Problems / Questions that arose with Figure 1 (answered in figures below): 1. Should I use an N-Channel MOSFET, a P-Channel MOSFET, an NPN Darlington bipolar transistor, or a PNP Darlington bipolar transistor? 2. Why must the flyback diode be placed parallel to the solenoid? As current is induced by the solenoid upon closure, where will it go? What is the purpose of the flyback diode? Transistor Considerations: Figure 2: MOSFET and Bipolar Transistor Schematics d- Drain g- Gate s- Source g d s P-Channel MOSFET g d s N-Channel MOSFET b c e PNP Bipolar Transistor b c e NPN Bipolar Transistor c- Collector b- Base e- Emitter

5 Nandy 4 Figure 2 demonstrates the differences in terminology and diagrams between the MOSFET and the bipolar transistor. Any of these transistors can be replaced in the single solenoid circuit diagram. So which one should be used? MOSFETs are voltage-controlled transistors, whereas bipolar transistors are current-controlled. The MOSFET can be eliminated due to the fact that the RPi2B cannot provide the voltage necessary to close the gate and complete the circuit. Therefore, a bipolar transistor was necessary, and since current was to flow from the collector to the emitter (collector connected to the positive terminal of the battery, while emitter connected to ground), the correct transistor to use was the NPN bipolar transistor. The base of the transistor is where the RPi2B applies a current, thus completing the circuit between the collector and the emitter and allowing the circuit to be completed, thus actuating the solenoid. When the RPi2B stops emitting current (and thus actuating the base of the transistor), the gate opens back up and current can no longer flow, thus returning the solenoid valve to its original state. Diode Considerations: Figure 3: Flyback Diode Schematic Base Current Applied on Transistor Base Current Not Applied on Transistor Sol Valve 1 Sol Valve 1 Figure 3 demonstrates two different situations: when the base current is applied on the transistor (thus completing the switch and allowing current to flow in the direction of the blue arrows), and when the base current is not applied, thus opening the transistor and breaking the circuit which initially allowed the solenoid to actuate. When the circuit is broken, there is residual current in the black wire that can no longer flow to ground. Thus, the only way for the current to flow is back. The current will take the path of least resistance (thus through the diode), and will enter back into the valve rather than jamming into the power source (note that this is the path of least resistance). The current will continue in this circle until it is completely dissipated, thus not damaging any components. As the induced current from the solenoid (as it closes) cannot flow back into the diode (due to the diode direction), the current is dissipated. The dissipation of current stops any circuit components from being damaged, therefore ensuring longevity of the setup. After making the decisions with regards to the flyback diode and bipolar transistor, the full ten-solenoid circuit could be built. With a single power source, the necessary 12 V could be established across as many solenoids as necessary. However, due to Kirchoff s law of junctions, the current that can flow through the solenoids will be split by the number of solenoid valves that are turned on at any given time. Calculations are made later in the report. However, increasing the current while keeping the power source voltage the same will allow more solenoids to turn on simultaneously. However, running excess current through a single solenoid valve may result in damage of the valve. The ten-solenoid circuit is essentially the same as the single solenoid circuit, repeated ten times. Each solenoid corresponds to a different GPIO pin, used for actuation.

6 Nandy 5 Figure 4: Ten Solenoid Control System RPi 2 Model B 3.3 V 5 V 2, SDA 5 V 3, SCL 4 14, TXD Sol Valve 1 15, RXD Sol Valve 2 27 Sol Valve 3 Sol Valve V 24 Sol Valve 5 Sol Valve 6 Sol Valve 7 10 MOSI 9 MOSI 25 Sol Valve 8 11 SCLK 8 CS0 7 CS1 EPROM EPROM 5 Sol Valve Sol Valve MISO MOSI 21 SCLK

7 Nandy 6 Figure 4 demonstrates the necessary circuit to control ten solenoids simultaneously. The current can be followed from the positive terminal of the battery into ground. The setup is essentially ten of the units in Figure 1 combined into one. Theoretically, if the power supply is correctly managed, more than ten solenoids can be controlled with the RPi2B microcontroller. The resistors have 1 kω resistance, the transistors are TIP120 transistors, and the flyback diodes are 6A10 rectifier diodes, rated for 1000V and 6A. When using the RPi2B in this case, it is important to place the copper heat sink on the CPU, due to the amount of power necessary to actuate ten solenoids. Results: As described in the abstract, initial tests were performed on an LED control system, without an external power source. All supplies were present in lab and no extra purchases had to be made. After the solenoid valve shipment arrived, a single solenoid apparatus was built and tested (with the help of an external power source), after which a much more compact, tensolenoid apparatus was formed. All three setups are shown below. Note that the ten-solenoid setup is complicated and needs intricate placement of wires to stop short-circuiting. Figure 5: Individual LED Control

8 Nandy 7 Figure 5 is the proof-of-concept circuit that allows each solenoid to be controlled independently. The rainbow colored wires (white, purple, blue1, teal, brown, yellow1, red, orange, yellow2, and blue2) are male-female wires that lead to the GPIO pins of the RPi2B. The gray wire leads to ground. The resistor is placed between the LED light and ground. In this case, an external power source was unnecessary, because the RPi2B contained enough voltage and power to light up each of the LED lights individually. As current was run through each GPIO pin, it passed through the diode and rushed to ground, resulting in that GPIO pin s LED light being lit accordingly. Figure 6: Single Solenoid Valve Actuation (a) Full Breadboard (b) Zoomed in on circuit

9 Nandy 8 Figure 6 (a) and (b) demonstrates the amount of space necessary for the circuit in order to actuate a single solenoid. A considerable amount of wiring is necessary for the required setup. Figure 7: Ten Solenoid Actuation (a) Full Breadboard (b) Full Setup (c) Pneumatic Valve System Figure 7 (a) and (b) demonstrate the components of the complete solenoid setup, which is to be implemented in coordination with the pneumatic valve system (c). The solenoids are to let in compressed air, which then actuates the pneumatic valves. Simple Circuit Power Calculations:!!.!! 15V, 1.00A power source with 12V, 6.5 W valves: 𝑃 = 𝑉𝐼 𝐼 =! =!!! = 𝐴.!.!!! = Based on the calculations, the given power source could have powered 1.85 solenoids (thus one solenoid). However, the solenoids did not require the full power to actuate (though they may not be completely open), and thus four valves could be actuated at once.!.!!"!

10 Nandy 9 Code: Below is the code that is used to actuate valves individually, as well as the code that is used to actuate numerous (up to four) valves at a single moment. Currently, I am producing a Python graphical user interface (GUI) that can be used to actuate the necessary solenoids for the times inputted by the user. (a) Individual Solenoid Activation (b) Multiple Solenoid Activation As the project continues forth, code is being developed to make the setup more efficient and user-friendly. The first step will be to get a basic GUI up and running (it s close!) After that, a more detailed GUI will be developed with the layout of the ten pneumatic valves. If a more powerful power source is to be used to power more solenoids at any given time, then a code for current control using a mechanical relay may be necessary.

11 Nandy 10 Conclusion: Prior to receiving the shipment of solenoid valves, initial tests were performed on threewire direct action solenoid valves from the previous setup (that were already present in lab) as well as LED lights (also already present in lab). A setup of ten LED lights was built using 100Ω resistors and male-male / male-female wires, and each light was controlled individually and independently of the other nine LEDs, demonstrating a proof-of-concept for actuating ten solenoid valves individually and independently. When the normally-closed, ⅛, water / gas solenoid valves arrived, they were tested for actuation. A 15 Volt, 1.00 Ampere power source was initially used in order to trigger the 12 Volt, 6.5 Watt solenoid valves. Current calculations demonstrated that only one solenoid could be powered with the power source. However, after testing the valves, it was found that the given power source could trigger up to four solenoids at once, though these solenoids may not be actuated completely (the valves produce a clicking sound, which is assumed to be the valve opening, though the degree of opening cannot be determined from merely looking at the valves). Though the solenoid valve may not be completely actuated, there will still be room for air to flow. This is to be tested, once the solenoid control is attached to the pneumatic valve system. In order to solve the problem of lacking current, a 15 Volt, 2.00 Ampere power source was purchased in order to power the valves. In theory, this power source should be able to power up to three solenoids at one time, though in practice, the power source may be able to power more than three valves. However, when using this power source on a single valve, it is important to consider the large amount of power flowing through the solenoid, which could potentially burn out the valve. This is highly unlikely, however, because the solenoid valves were able to handle power from a 19 Volt, 3.42 Ampere power source (a Toshiba laptop charger). After the individual solenoids were checked for actuation using the power source, a circuit was planned and diagrammed to control a single solenoid with the RPi2B. The components of this circuit included a breadboard, a 1 kω resistor, a TIP120 transistor, numerous male-male and male-female wires, a 6A10 rectifier (flyback) diode, and a power source. The resistor was used as a base resistor to stop any large voltages from damaging the RPi2B, the transistor as a switch to control current flow, the diode to stop flyback voltage and current, and the power source to power the solenoid (independently of the RPi2B, since the solenoids are high current / high voltage devices that cannot be powered by the general purpose input / output [GPIO] pins on the RPi2B). Through the use of the correct circuitry, an extremely powerful microcomputer, python, and a Linux-based operating system, I was able to successfully produce a stand-alone tensolenoid control system that can be accessed remotely and controlled, thus making Velencia s job easier, as she will be able to control the pressure setup from Germany. I was able to save the Reimer group $ in funds by using a RPi2B microcomputer, as opposed to the Rabbit Core Module as suggested to Velencia at ENC2015. Future: 1. Attach the solenoid to the pneumatic valves and actuate pneumatic valves successfully. 2. Remotely control the RPi2B from another site, and control the pneumatic valves through the use of a GUI that is able to take text input for time. 3. Solder the solenoid setup onto a permanent solderboard for permanent use. 4. Extend wire lengths so that the solenoid setup can reach far lengths.

12 Nandy 11 Acknowledgements: I would like to acknowledge Velencia Witherspoon for providing me with this interesting project, which allowed me to learn a significant amount about high-current / high-voltage circuitry. I would also like to thank David Zarrin for his help on double-checking the tensolenoid circuit diagram. I would also like to acknowledge Jun Xu for his help on other projects throughout the summer, and Professor Reimer for allowing me to join and help conduct research with the Reimer group.