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ECE 792 Final Project Report Electric Snowmobile Team Members Lindsey Chiron Roger Gauthier Patrick Hingston Parker McDonnell Michael Swanson ECE Faculty Advisor Dr. Gordon Kraft Project Completion Date May 9, 2011 Courses Involved ECE 541,548, 617, 618, 651, 649 Abstract The objective of this project was to enhance the design of a zero-emission, low-noise snowmobile in order to provide the user with a safer and more reliable vehicle. A charging system was designed to improve the practicality of charging the snowmobile batteries. A user interface was implemented through the use of a touch panel and programmable automation controller (CompactRIO). The touch panel displayed battery voltages, speed, instantaneous current consumption, and temperature. In order to obtain this information, sensors were installed in the snowmobile and outputted data to the controller. A gasoline generator was also mounted on the back of the snowmobile, serving as an emergency backup power supply. Introduction The wilderness of New England can be a harsh and dangerous place especially during the winter months. Snowmobiles are used for recreational activity and also have uses in search and rescue. While snowmobiling can be enjoyable, it can also be dangerous at times. Making improvements to modern day snowmobiles can minimize some of the risks and inherent side effects. Becoming stranded in the wilderness is a very real possibility when long trips are taken or insufficient amounts of fuel are brought. Gasoline powered snowmobiles produce a lot of noise pollution which can make search and rescue missions harder to accomplish. By designing an electric snowmobile, noise and fuel pollution concerns can be diminished. Building off of last year’s electric snowmobile we made a number of improvements to help reduce risk and increase usability. A main concern was for a user to be able to view what was going on with the snowmobile while driving it. In order to warn the user of any potential problems with the vehicle, a heads-up display system was added, allowing the user to monitor system status. A programmable automation controller (NI CompactRIO) was used to take data from various sensors and display them on touch panel. The controller used Labview to analyze and present the information to the user. In order to gather the vital diagnostic information, a Hall Effect current sensor and RPM sensor were added, along with a battery voltage monitoring circuit. To further increase the safety of the vehicle, functioning headlights were installed, as well as a redesigned PWM throttle linkage. Another main objective of the project was the charging of the snowmobiles batteries. A charging unit was designed to allow an easier method for the user to charge the batteries. This charging system allows the user to avoid removing each individual battery and charging them all separately. In addition to the charging unit, a gasoline generator was mounted on the back of the vehicle. This safety feature allows the user to charge the batteries even when a wall outlet is not available. The charging system plugs directly into the generator, allowing the generator to substitute for a wall outlet. A 12V auxiliary port was implemented to allow for the user to plug in a GPS system. Design Three major systems were designed, fabricated and tested during the past two semesters. These consisted of a charging circuit, monitoring/GUI system and an emergency generator. The following diagram details the completed systems. figure 1 Monitoring and GUI NI CompactRIO 9074 The heart of the monitoring system consists of the national instruments compactRIO embedded control and acquisition system. The cRIO was used to acquire analog and digital values in addition to exporting analog signals through its hot swappable modules. For our project purposes three NI modules were selected including the NI 9411 digital input module, NI 9263 Analog output module and the NI 9205. Signals which stored there information in terms of frequency or pulse width were applied to the 6 channels of the NI 9411. All standard DC voltages were read in using the 32 channel analog input NI 9205 module. The three modules perform the task of converting analog and digital values, conditioning the signals and writing them to the cRIO’s volatile memory. figure 2 National Instruments Labview Labview served as the programming language used to control the CompactRIO’s functionality. Labview is known as a graphical programming language where wires, blocks and figures are used instead of lines of code. This serves the user the ability to rapidly deploy code and easily transition from flow diagrams to functioning programs. The NI 9205, 9263 and 9411 served as the gateways between physical hardware (sensors, circuits, signals) and our Labview code. All incoming signals were converted to digital values and stored in memory allowing for our code to be updated. The module update timing or scan engine was set to update volatile memory every 10ms allowing for real time monitoring of all vital systems. For this particular project two .Vi files were created similar to .c or .java source code in other programming languages. One .Vi was downloaded to the cRIO to read in values, crunch data and make decisions based on these values. The other .Vi was sent to the touch panel to control the graphical user interface. The two .Vi’s communicated through shared variables with the cRIO publishing values to the network and the touch panel fetching these values from the network Touch Panel The TPC-2206 touch panel runs Windows XP and sports a dual core 1.33 GHz atom processor. The NI cRIO is known as a headless system as it has no way of displaying calculated data. Connecting a touch panel to the cRIO we were able to displayed vital system information including, throttle, MPH, Armature current, RPM and Battery voltages. As mentioned earlier the code running on the touch panel fetched updated variables from the network and displayed the values on dials, graphs and indicators. Through some tweaking of startup programs and the .Vi options we were able load our .Vi at the boot of windows. Network Shared Variable (figure 3) Touch panel Instrument Cluster (figure 4) Sensors and Signals o Ferromagnetic Hall effect Sensor: To measure the revolutions per minute of the motor a ferromagnetic Hall Effect sensor was mounted 2mm from the edge of one of the motors gears. The sensor outputs a voltage equal to Vsource at the lack of a gear tooth and drops to zero when a tooth is present. This function is performed through the use of an open collector output connected to a pull up resistor hooked to Vsource. Therefore a square wave is generated with a frequency proportional to that of the motor RPM. To interpret the incoming signal the high pulse time was measured using the NI 9411. To calculate the revolutions per minute we divided 60 by the number of gear teeth (PPR) times the pulse high time (PHT) 𝑅𝑃𝑀 = (60)/(𝑃𝑃𝑅 × 𝑃𝐻𝑇). This equation was realized in Labview and the output values were sent to the touch panel to display. RPM Sensor Configuration (Figure 5) o Pulse Width Modulation Signal: the snowmobile motor is controlled by a PWM signal which is effectively equivalent to throttle for a gasoline engine. The output of the PWM circuit was wired back to the NI 9411 for high pulse and period measurement. To find the duty cycle from the signal the high pulse width was divided by the period and multiplied by 100. PWM Measurement (Figure 6) o Current Sensor: This Hall Effect sensor is used to measure the amount of current being drawn by the DC motor. The 2AWG wire which carries the motors armature is passed through the sensor closed core structure. As current increases a stronger magnetic field is generated causing the Vout of the sensor to drop by about 5mv per amp. The voltage is passed to the NI 9205 for signal condition and calculation of current in Labview using the simple linear relation between the sensors voltage and the armature current. Current Sensor Configuration (Figure 7) o o NTC Thermistor: A negative temperature coefficient thermistor was used to measure the temperature of the motor driver IBGT transistor. The thermistor was connected in series with a 5KΩ and the voltage was measured over the thermistor. As the temperature increases the resistance of the thermistor decreases along with the voltage across the device. This voltage is read into the NI 9205 analog input module and temperature is calculated in Labview using the B parameter equation. Voltage Monitor Sensor: in order to measure the voltages of the 10 snowmobile batteries an electro isolator voltage step reduction circuit was developed. Originally each battery was to be measured differentially across the terminals. Concerns were raised since the NI 9205 has a max input voltage of 10v while the Open circuit voltage of each lead acid battery is 13.2v. Therefore the voltage will need to be stepped down linearly in order to be within the range of the modules input channels. Additionally noise was another consideration as the DC motor generated large amounts of back EMF and high frequency noise. To perform the task of electrical isolation and voltage reduction optoisolators were used. The following circuit diagram shows the left battery side of the circuit is isolated from the right side by a dielectric medium. With large battery voltages the output voltage a V is relatively small as the transistor is driven on. When the battery voltage drops lower and the transistor effectively off and Vout is pulled to Vsupply. The voltage at the output of the transistor is linearly related to the battery voltage at the input. The transfer function of each optoisolator was found and an equation was derived for the battery voltage as a function of the output. Measuring the voltage at the transistors collector we could solve for the batteries voltages. Opticoupler Circuit and Voltage Output (figure 8) Isolated Grounds A major concern with an electrically noisy device such as a DC motor is the possibility of damaging or inducing error in a sensitive embedded system. To prevent noise coupling to the cRIO’s sensitive modules, measures were taken to retrain accuracy and safety of our devices. The motor and battery bank of the snowmobile is electrically isolated from that of the PWM driver circuit, cRIO embedded computer, touch panel module and various sensors. The battery bank and motor are floating as they are not grounded to the vehicle chassis. The sensors, data acquisitions and displays are grounded to the chassis using metal grounding strips. All signals that flow between the two systems are optically coupled preventing noise from propagating from the motor. Voltage Regulator: Two 12V batteries were added to the front of the snowmobile to power the electronics. These batteries powered a voltage regulator, which outputted a constant 12V and a constant 19V. The CompactRIO and touch panel ran off of the 19V signal, while the sensors ran off of the 12V signal. The sensors’ output was dependent on the input voltage, so if the input voltage varied, the output from the sensor would not be consistent. The voltage regulator provided constant reference voltage, ensuring accuracy in the data. Warning LEDs: The CompactRIO contains an analogue output module. This module was used to power warning LEDs that notified the user of potential hazards. The motor batteries, PWM battery, and IGBT temperature each had their own warning LED. When the voltages of the batteries got too low, the CompactRIO sent out a signal that caused green LEDs to start blinking. When the temperature of the IGBT became too high, the controller powered a red LED. The LEDs are located right next to the touch panel display, making it easy for a user to see when they are lit. These LED are an important safety feature that prevents potentially hazardous situations. Generator The generator provides the user with an emergency backup charging system. It is able to provide 3KW of power for 2.5 hours under full load and will allow the user to charge the batteries enough to return to safety and avoid being stranded in the woods. The charging system plugs directly into the generator as a substitute for the wall outlet as the generator provides the same AC input signal. Charging Circuit/System Two important factors come in to play when designing a battery charging circuit. For the system designed for this particular snowmobile, both voltage regulation and current limitation proved to be staples of the design specifications. o Voltage Regulation: A regulated voltage was necessary since under certain circumstances the batteries would have different voltages in addition to different internal impedances. One of the significant benefits in terms of application for using the optima Yellowtop batteries is their extremely low internal impedance. This was an important trait to strive for since the “series” internal resistance of the batteries would essentially be negligible when driving the motor. It does however cause an issue when charging since the batteries act almost like a short circuit. The regulated voltage was achieved using a fairly simple configuration which employs a high power Darlington transistor with a base voltage biased using a zener diode. The zener diode was selected to hold the base of the transistor at thirty volts which regulates the emitter voltage to be thirty volts minus the characteristic base-to-emitter voltage. This left the approximate regulated output voltage at 27.8 volts. Another important part to account for is the turn-on current for the base of the transistor. The datasheet for the particular transistors used showed that the base required eight milliamps in order to function. This base current was generated by connecting the collector to the base with a 2K ohm power resistor. This allowed for an adequate current to enter the base and operate the transistor properly. With the desired output voltage achieved, the final aspect to account for was the current limiting. o Current Limiter: Not only was it necessary to limit the current for safety purposes and to prevent the circuitry from overheating, it was important to think of the real life application of the charger as well. A standard wall outlet operates off a 30 amp breaker which, if exceeded, trips and shuts off that network of outlets. Ideally, the current for each charging circuit would not exceed 6 amps in order to accommodate the circuit breaker issue in addition to matching the power specifications for the emergency generator. Again, a Darlington NPN transistor is the focal point of the circuitry used to achieve the desired current. A 2K ohm resistor connected to the base and collector provides enough current to turn on the transistor. Unlike the voltage regulator, the current limiter employs standard diodes to connect the emitter to the base. A .3 ohm sense resistor was selected at the output of the emitter which provides a reference voltage for the diodes. As the current through the transistor increases(which also flows through the sense resistor) the voltage drop over the sense resistor also increases. Once the combination of the voltage loss over the sense resistor and the base-to-emitter voltage matches the forward voltage of the two diodes that connect the emitter to the base, the diodes then begin to conduct. With the diodes conducting, there is essentially a short between the base and the emitter which draws current away from the base of the transistor. Since the transistor operates by generating a current gain from the base current, the output current is limited due to the current into the base being reduced by the conducting diodes. See attached schematic diagram: Q1 ZT X605 Q2 ZT X605 R9 .3Ω R8 2k Ω V2 120 V D1 D2 1N 4938 1N 4938 R2 R6 2k Ω 5Ω 95% Ke y =A D4 B ZV 90-C30 D3 1N 4938 V1 24 V Charging Circuit (Figure 9) Conclusion Through the completion of the snowmobile project mistakes were made and lessons were learned. As a team we struggled in the beginning to find a way to work effectively together but by the second semester progress had become very efficient. The biggest problem at the start was finding individual task to assign to everyone so we could work independently without tripping over each other. By working in parallel as much as possible tasks were accomplished exponentially faster. Working independently is not without its faults as many times team members needed components from there project to be compatible with that of another. Through the practice of good communication many of these compatibility issues were reduced or avoided outright. Scheduling was another issue as there were times when parts hadn’t arrived which were necessary for the continuation of the project. These issues were ironed out by transitioning our resources to other tasks which could be completed while these parts were shipping. Staying flexible throughout the project was a critical skill that we learned early on. Often time simple tasks or shipping of parts would take much longer than we ever expected. Staying flexible allowed us to focus our energy elsewhere in order to maximize our working time. Compatibility of components was also a serious issue as many sensors and circuits required conditioning before there functions could be performed. For example the open circuit voltages of the snowmobile batteries were greater than the max input voltage to the analog input module. The biggest issues we ran into in terms of system functionality was getting the touch panel to talk to the CompactRIO. After spending a good amount of time writing code and debugging the problem a simple call the National Instruments solved all of our problems. Critical software which was supposed to ship with the touch panel never made it on the system which was necessary for communication between the CompactRIO and touch panel module. Other issues on the project were heating issues with the charging and voltage regulator circuits. These problems were solved with heat sinks and proper airflow allowing for longer operation of the systems. As our team continued a project built the year previous, lots of debugging needed to be completed to determine the functionality of the included systems. Overall this project has taught us to work well independently or as a team effectively and efficiently. Contributors University of New Hampshire UNH Parents Association UNH ECE Department UNH Energy Club Dr. Gordon Kraft Dr. Barbara Kraft Vincent Pelliccia Kevin White Tom Reis (Substructure) Stephen Doran Luke Vartuli Doug McMillan Lesley Yu (National Instruments) Generator Connections Daniel Mooney Adam Perkins Joyce Perkins Kathy Reynolds Future Project Automatic charging of the batteries using the generator when the voltage levels drop too low Charging with alternative systems such as regenerative breaking or solar panels Emergency communication systems Increase operating frequency of PWM to reduce noise