Electric Vehicle Circuit and Electrical System Senior Lab Project

As part of a multidisciplinary team, electrical engineering students worked with computer and mechanical engineering students to create a small-scale electric vehicle. The major tasks of the team were design and performance prediction; fabrication of the vehicle, control circuits, and computer data acquisition board; system integration and testing; racing the vehicles against other teams; and comparing performance data to predictions. This paper will discuss the electrical engineering students’ design efforts for the project. The project enhanced student learning and provided a practical approach to multidisciplinary teamwork. In addition to the design, build, and test tasks typical of the engineering design process, electrical engineering students learned about Failure Mode and Effects Analysis (FMEA) and how to design circuits to provide for safe outcomes when portions of the system fail. The paper provides an overview of project; describes the learning objectives, as related to the electrical engineering aspects of the project; describes the FMEA process; and includes detailed descriptions of the circuits in the appendix. Introduction Western New England University has a long history of incorporating engineering design into laboratories and courses. 2012 marks the university’s 50 th annual capstone design effort. Additionally, interdisciplinary team efforts are initiated in the freshman year and continue for all four years 1 . This paper describes the electrical engineering aspects of one of the interdisciplinary lab exercises that occurs during the senior year fall semester. In the latest implementation of the class, electrical engineering students were introduced to Failure Mode and Effects Analysis (FMEA) as part of the design process. Project Description During the fall semester at Western New England University, students enrolled in computer, electrical, and mechanical engineering senior lab courses worked together to design and produce a prototype electric vehicle. The vehicles were fairly small, typically 5 to 10 pounds and less than 2 feet long. The power source was a rechargeable 12.8V LiFePO4 battery pack that had a 3000mAh rating. For safety, a battery pack with built in electronics was chosen. The protection included: overcharge, over discharge, a 7A current limit, and short circuit. The vehicle was designed to complete a drag race and figure 8 race. The drag race course is about 100 yards along a road that has around a 10 foot elevation gain. Each of the three disciplines had specific design, build, and testing tasks. During the semester, students from the disciplines worked together to integrate their designs. The mechanical engineers designed, built, and tested: the chassis, drive train, suspension, steering, and breaking systems. The computer engineers designed, built, and tested a data P ge 23469.2 acquisition system that collected: vehicle speed from a beam break sensor and analog voltages representing the motor current and battery voltage. The computer engineering board also provided an enable signal to the electrical engineering student’s circuit board; together the boards enabled or disabled the motor. The electrical engineering students designed, built, and tested a control circuit that acted as a current governor for the motor. The remainder of the paper will focus on the efforts of the electrical engineering students. Electrical Engineering Current Governor Circuit Design Overview During the first week of the project, teams were formed and students were given an overview of the project. After the interdisciplinary group meeting, each set of students met with the professor in their discipline. The electrical engineering students met weekly for two 1.5 hour lab sessions. During these labs, aspects of the design were covered. The current governor consisted of five parts. First, a triangle wave generator and open-loop pulse width modulator (pwm) was covered. Second, a current sensor and an amplifier with a 4 th order low-pass filter was covered. This circuit converted the current into a voltage representation of the average current flowing through the motor. Third, a differential amplifier and a Proportional + Integral controller was covered. Fourth, a potentiometer interface / amplifier circuit was covered. Finally, a FET & Flyback diode circuit was covered. A representative schematic is attached in the appendix. After the students built breadboard prototypes of each circuit, they began to design their circuits for the vehicle. In order to complete the design, build, and test cycle in about 9 weeks, portions of the control circuit were given to the students. The triangle wave generator and open-loop pwm circuit, the FET & Flyback diode circuit, and the diff amp and PI controller circuit designs were given to the students. The 4 th order low-pass filter, the potentiometer interface circuit, and the failure mode and enable circuits were designed by the students. The project had some important specifications. Most notably for safety and current capacity of the FET and Flyback diode, the motor was allowed 0A to 4A average current. The DC gains of the 4 th order low-pass filter and the potentiometer interface needed to be matched in order for the command and feedback circuits to work correctly together. Students also needed to design in some flexibility to adapt to the mistakes in the mechanical system. For example, the servo that turns the potentiometer may not turn the potentiometer enough or may turn it too much (the potentiometer was the input to the command portion of the circuit). In these cases, the potentiometer interface circuit would require a gain adjustment to keep the motor current command signal in the proper range. Course Learning Objectives and FMEA In the course, there were many learning objectives that students mastered to varying degrees. Table 1 lists the objectives and what items were measured to assess how well the students achieved each objective. Many of the objectives are used in the department’s assessment of ABET a-k outcomes 2 . Because the scope of this paper is limited to the electrical engineering aspects of the project, this paper will focus solely on the objectives related to the electrical systems and the FMEA aspects used to teach robust design. The last three learning objectives: teamwork, written and oral communication, and societal impact are not covered in this paper. P ge 23469.3 The first learning objective was an ability to design circuits to solve open-ended problems. For this objective, students were given a motor to test to determine the characteristics of the current flowing through the motor. Figure A9 (in the appendix) shows the circuit used to test the motor’s characteristics. When the motor current was being controlled by the PWM signal, the motor current characteristics were determined mostly by the frequency of the oscillator that controlled the PWM timing. For this project, students used a signal in the 1kHz range. A second set of tests were performed to determine the motor current characteristics when the motor was driven at 100% duty cycle. Here, the motor was connected to a generator that had a variable load. This allowed the students to test the motor with different loads in order to see the how varying the motor torque effects the motor current with regard to amplitude and frequency. After the testing was complete, students needed to design a 4 th order low-pass filter and amplifier that would convert the motor current into a DC voltage representing the average current flowing through the motor. Most students chose a Sallen-Key 4 th order filter with a DC gain of 20V/V. Students were given a specification of 10mVpp ripple for the output of the filter when the motor had an average of 4A flowing through it. After the filter/amplifier was designed and tested, students needed to determine the proper gain for the potentiometer interface circuit. Here students were told work with the mechanical engineers to determine a rotation range for the potentiometers that were driven by a radiocontrolled (RC) servo motor. The teams were supposed to work together to reach agreement on a common design such that all the potentiometers would rotate the same amount. The reason for this constraint was to make all of the electrical engineers’ circuit boards compatible with any of the vehicles. Students were taught that common design allows for interchangeability. If one team’s circuit board failed they could borrow a circuit board from another team and still allow their team to compete in the race. During the race, two cars competed against each other in each heat. During the past iteration of the project, the teams did not cooperate to determine a common rotation range; there were 3 different ranges between all the teams. So, some compatibility was preserved but a few teams did not have another circuit board compatible with their vehicle. For the most part, students performed well in these open-ended design tasks. The average assessment score was 3.5 on a 4 point scale. While the next two learning objectives were largely subsumed by the first learning objective, there are some subtle differences. The ability to collect data and analyze the results was comprised mostly of the filter/amplifier and potentiometer interface design, but also included converting raw data, collected during or before the race, to physical units. Here only about 2/3 of the students performed well. The average assessment score was 2.3. Problems with the mechanical systems delayed the race by a few weeks and students had less time to write their reports. This led to many reports being weaker than reports from previous years – many students presented results with voltages from the A/D and did not convert the A/D voltages back to the correct unit or scale. The ability to test and debug circuits was exercised heavily during the project. Students built their circuits on breadboards and th