Motorsports PCB Design

Transcription

1 Motorsports PCB Design Each year, university students compete internationally to create winning formula style race cars. The University of Washington Formula Motorsports Team designed and assembled four challenging PCBs that were key to vehicle performance. By Kevin A. Luo, Omair Ahmad, James Lindsay, Amrit Puri, and William Voit A student driver trains with the University of Washington Formula Motorsports Team 25 ( ) combustion car in the rain. University of Washington Formula Motorsports Team The University of Washington Formula Motorsports Team is a non profit, academic organization committed to giving the next generation of engineers the skills and work experience needed to excel in their careers. We train student engineers by building two small formula style race cars each year: one with a combustion engine (ccar) and one with an electric drivetrain (ecar) We design, manufacture, and test our cars, then enter them in Formula SAE, an annual international competition hosted by the Society of Automotive Engineers (SAE), our parent organization. At competition, teams begin by performing in static events, presenting each vehicle s design philosophy, cost efficiency, and business plans. Then they perform in dynamic events that test each vehicle s acceleration and lateral grip.

2 Finally, each team competes in autocross, a type of race which emphasizes maneuverability and acceleration over speed, as well as endurance. The endurance race is a grueling 22 km drive through multiple laps of the autocross course, which tests each vehicle s reliability and fuel efficiency. Unlike other teams who sometimes hire professional drivers, we choose to train our own student members to drive during competition. To date, the University of Washington Formula Motorsports Team has built 26 combustion vehicles and 3 electric vehicles. Each year, over 500 teams compete in the Formula SAE competition, and the University of Washington combustion car is currently ranked 31st internationally and 9th in the United States. Mentorship is key to our training and student development. Three student directors lead the entire U of W FSAE team and manage 14 technical and administrative leads. They, in turn, manage individual members. We take pride in our efforts to create well rounded engineers who can explain their projects just as well as they can design, fabricate, and test their systems. Our alumni have gone on to become excellent engineers, often working at companies that have sponsored our competition efforts was the 25th year that the University of Washington competed in Formula SAE (Team 25 pictured above), and 2015 will be the 26th year. The student organization aims to produce world class engineers through a commitment to learning and innovation by designing, building, testing, and tuning two cars for an international competition.

3 This article shares our team s process as we created our vehicles for the 2015 Formula SAE competition. It also showcases four important PCBs designed specifically for the project and describes their objectives, challenges, and processes for testing and revisions: 1. Accumulator Indicator Light (AIL) and Tractive System Active Light (TSAL) Driver board 2. Precharge/Discharge Module board 3. Brake Plausibility Check (BPC) board 4. Insulation Monitoring Device (IMD) Latch board We design our racecars for the weekend autocross racer. They have excellent handling, braking, and acceleration capabilities. However, sheer performance is not the only goal. The cars have to be reliable, easy to maintain, and well engineered. Additionally, we consider their ergonomics and aesthetics Competition Teams: Designing PCBs As we prepared for the June 2015 Formula SAE competition, we gave each student responsibility over a part of a race car. Members focused on one of eight technical aspects of the cars: Aerodynamics, Chassis, Drivetrain, Electronics, Engine, etrain,

4 Manufacturing, and Suspension. Each student also joined one of our organization s administrative teams, so that in addition to engineering training, they would gain experience in public relations, fundraising, resource management, IT management, and business. The Electronics and etrain technical teams designed printed circuit boards. The etrain team managed all high voltage systems used in our electric vehicle (e.g., accumulator cell boards, DC DC converter, etc.), while the Electronics team dealt with all low voltage systems used in both vehicles (e.g., sensors, brake plausibility check, dashboards, etc.). A printed circuit board used in the electric car is reflowed to create solid connections between surface mount components and the solder pads. PCB Characteristics and Design Goals Our main goal was to design each board to be as reliable as possible. During past competitions, the main failure point was usually related to electronics. Our boards had to function reliably in incredibly noisy and thermally dynamic environments courtesy of the fierce driving during competition. We designed and revised our boards for

5 improved heat management and more circuit protection elements, such as TVS diodes and fuses. In addition, if an electronics failure occurred, it was often difficult to diagnose the problematic system. Thus, we tried to make our boards simple, reliable, and modular so we could quickly switch boards if there was a problem. We also prepared fully assembled spare PCBs to bring to the competition to eliminate troubleshooting time. A Formula SAE car weighs 400 to 500 lbs., with horsepower figures ranging from 50 to 90 hp. With a 0 to 60 mph time in the 3 to 4 second range, lateral acceleration of up to 2.0 g and a 60 to 0 braking distance of about 115 ft., a FSAE car will leave most production streetcars in the dust. This Team 25 electric car weighs in at only 370 lbs, one of the lightest in the competition. Our Process: Car Design, Manufacturing, and Assembly & Testing We created our ccar and ecar in three stages lasting 10 weeks each: Design, Manufacturing, and Assembly & Testing. Design (Fall) Every member on the team individually tackled a project necessary for the car to function, such as the dashboard PCB, brake plausibility check, GPS, or telemetry. Students researched all necessary information needed to design and manufacture their assigned parts, learned about the high level process of designing a formula style race car, and studied how each technical team was interconnected. Manufacturing (Winter) Members manufactured and tested their projects with lab equipment, including power supplies and function generators, and oscilloscopes. They revised their parts after

6 testing and collaborated on larger projects with teammates, like designing and assembling the accumulator or assembling our carbon fibre monocoque chassis. Students tasked with creating PCBs revised their board designs at least once in order to increase their longevity and reliability. They also made sure the revisions were neater and more aesthetically pleasing than the original. We used Sierra Circuits s Better DFM (Design for Manufacturability) tool to verify manufacturability. This decreased the time we spent revising boards, because the tool clearly highlighted the parts of the PCB that needed revising. After the boards met our standards and were verified by the DFM tool, Sierra Circuits manufactured the boards and shipped them to our home base in Washington State. Assembly & Testing (Spring) We compiled all the parts into two single chassis to form the complete ecar and ccar automobiles. Next, we tested them so they would perform at peak efficiency during competition. Finally, we trained our drivers extensively so they would be comfortable with the vehicles on race day. Aerodynamics team sands a mold in preparation for aero package manufacturing.

7 Challenges Lack of PCB design and fabrication experience Many new members of the team had little to no prior knowledge in PCB design and fabrication. To get up to speed, students spent the first 10 weeks of the year learning about basic electrical circuit behavior and how to use PCB design tools like Eagle and Altium. Non profit budget UWFSAE is a non profit, so our students fundraised and secured sponsorships from various donors and suppliers. As a result, our budget was extremely tight, so we tried to make our boards as simple as possible. In house assembly Members assemble our PCBs in house to accumulate practical, hands on experience. However, this sometimes resulted in incorrectly or poorly assembled boards, as well as accidentally damaged or lost components. This is why we always purchased several extra boards and components so members didn t have to fear making mistakes.

8 PCB Showcase: Four Boards The following boards were crucial for our cars to function: 1. Accumulator Indicator Light (AIL) and Tractive System Active Light (TSAL) Driver board 2. Precharge/Discharge Module board 3. Brake Plausibility Check (BPC) board 4. Insulation Monitoring Device (IMD) Latch board Below, we ll showcase the objectives of each PCB, describe our design and manufacturing challenges, and explain the testing and revisions we performed to prepare the boards for race day.

9 1.AccumulatorIndicatorLight&TractiveSystemActiveLightDriver AccumulatorIndicatorLight(AIL)board TractiveSystemActiveLight(TSAL)Driverboard

10 Objectives We designed two similar boards: the Tractive System Active Light (TSAL) and Accumulator Indicator Light (AIL). These would serve as indicators when high voltage (defined at 60 volts) was present in the system. The TSAL needed to flash between 2 Hz and 5 Hz to indicate that the tractive system had high voltage. The AIL was a single solid light that illuminated when the battery pack was turned on. The competition rules dictated that the high and low voltage sides of our boards must have at least 3mm of space between them and be galvanically separated. On the TSAL module, we did this with an opto isolator. The rules also mandated that the boards should have no modified programmable components. Both boards had a high voltage PTC fuse and decoupling capacitor next to the input. We tuned a linear regulator to regulate the voltage to the low voltage side of our board. In order to regulate only high voltage currents, we also added a 60 Watt Zener diode before the regulator. From that point in the circuit, the AIL module ran a module to the light, and the TSAL module had an opto isolator to the low voltage side of the board. We supplied 12 volts to the low voltage side of the board from our DC/DC converter and used p fets to supply power to the light when high voltage caused the opto isolator to switch. We had two p fets; one continuously supplied power and put out a constant high voltage, while the other supplied voltage from a 555 timer adjusted to output at ~3 Hz. Challenges The first challenge we faced was to design the boards to be as small as possible, and the second was to regulate any voltage from volts to 24 volts. Testing and Revisions We tested the board by connecting it to two adjustable power supplies. We ensured the board operated as it was intended to as we adjusted voltages. We also monitored the temperature with an IR gun and ran brownout tests on each board. The tests on our first revision showed that both boards got uncomfortably hot, so we made the bottom plane of the boards a heatsink for the linear regulators and populated the pad with vias to dissipate the heat. Earlier in the process, we also switched to using p fets instead of n fets and a NOR gate to shrink the size of the board.

11 2.Pre charge/dischargemodule Pre charge/dischargemoduleboard Objectives ThePre charge/dischargemodulehadresistorsonittomaketherccircuitcharge anddischargethepackcapacitors.weusedresistorsthatwouldchargeand dischargethecapacitorsto90%withinfiveseconds.theboardhadtworelaysthat switchedtogether.onewasanormallyopenpre chargerelay,andtheotherwasa normallycloseddischargerelay. Challenges Attheratethatwepre chargedanddischarged,theresistorsdissipatedasignificant amountofheat. Revisions Tocooltheboard,weconnectedtheheatdissipationpadsdirectlytothegroundplane aroundtheperimeteroftheboardandaddedseveralvias.

12 3.BrakePlausibilityCheck(BPC) BrakePlausibilityCheck(BPC)board Objectives Thebrakeplausibilitydevicewouldtriggertheshutdowncircuitiftheracecardriver brakedhardandpowerwasdeliveredtothemotorformorethan0.5seconds.the triggeringpointforthebrakesensoroccurredwhenitsentavalueof2.5voutofa rangeof0.5vto4.5v(approx.800psiinthebrakeline),andthepowersensorwasat 0.2Vindicatingavalueofaround5kW. Challenges Itwashardtocontrolthedelaytimetobeingwithin0.5seconds.TheuseofanRC chargingcircuitdidprovideadelaybeforetriggeringtherelaycircuit,butitstillneeded someimprovement.also,themovementofthevehiclemayhaveinfluencedthe potentiometersweusedasreferencevoltages. Testing Thereweretwocomparators:onecheckedwhetherthebrakeencodersignalwas smallerthan2.5v,andtheothercheckedwhetherthepowerbeingdeliveredtothe motorwaslessthan0.2vofitstotalvalue.wheneitheroftheseconditionsweretrue, therespectivecomparatoroutputalogiclowsignalandwhenfalse,theoutputran high.whenbothoftheconditionswerefalse,theoutputoftheandgatewashighand thevoltagereferencecircuitdelayed0.5secondsusingrccharging.theoutputthen wentintoanothercomparator,whichboosteditto5vbeforeenteringthedflip flop, whichactedasalatch.thedinputtotheflip flopwaspermanentlyhigh,sowhenthe

13 rising edge of the inverted output of the counter delay came, the flip flop output a logic low to the power transistor that drove the shutdown relay. This opened a relay on the shutdown circuit. An RC circuit was connected to the CLR of the flip flop for resetting. Revisions From the first revision to the final one, one of the most important changes was the total circuit layout. We made the wiring on the board was generally cleaner and neater. We also added a few more pull up resistors to maintain a 5V voltage signal going into the relay shut down circuit. The board may have needed a change to the RC charging circuit and comparator output voltage. By changing values of R and C, the delay time would be changed accordingly. Changing the comparator output voltage decreased the accuracy of the voltage reference, but we compensated by using another comparator before the D flip flop to create a 5V input for triggering the relay circuit.

14 4.InsulationMonitoringDevice(IMD)Latch InsulationMonitoringDevice(IMD)board Objectives TheInsulationMonitoringDevice(IMD)LatchconnectedtheIMDtotheshutdown circuit.itnormallyallowedcurrenttoflowthroughtheshutdowncircuit.whentheimd detectedaproblem,itslatchopenedtheshutdowncircuittoshutdownthevehicle tractivesystem.theboardthenlatchedsothetractivesystemwouldnotreactivate eveniftheimdnolongerdetectedaproblem.thelatchwouldresetwhenthelow voltagepowerwasturnedoff. Challenges Asrequiredbythecompetitionrules,theIMDlatchdidnotincludeanyprogrammable logic.wefacedthechallengeofensuringthatthelogiccomponentsinitialized correctlywhenpowerwasapplied.thefirstrevisiondidnotworkwiththeimd becauseoftheimd sunexpectedstartupbehavior. Testing InitialtestingdidnotinvolvetheIMD.Theboardwaspowered,andtheIMDinputwas connectedto+5v.weverifiedthattheshutdowncircuitwasinitiallyclosed,but openedwhentheinputwasdisconnectedanddidnotclosewhentheinputwas reconnected.latertestingattemptedtodothesamecheckswiththeimd.

15 Revisions Since the first version did not work correctly with the IMD, we revised it to ignore the input from the IMD until the IMD finished its initialization.

16 ThedashboardforthevehicleswasacollaborativeprojectbetweenanAerodynamics TeammemberanElectronicsTeammember. In2014,74teamsrepresenting15differentcountriesparticipatedintheFormula StudentGermanyinternationalcompetition.

17 We look forward to putting our boards to the test as we compete in the Formula SAE race in June We will determine the success of the design and assembly of our PCBs when we assemble our automobiles and test the electrical systems. 3, 2, 1, go! The University of Washington Formula SAE cars run off of fuel, electricity, and the generosity of supporters like you. To contribute to next year s team, visit donate.uwformula.com.