Telemetry helps Formula SAE team close the loop on design

  • 07-Aug-2012 10:17 EDT
BurrussTeam.jpg

The 2012 Virginia Tech Formula SAE Team and car.

The premise behind Formula SAE is that a fictional manufacturing company has contacted a design team to develop a small Formula-style racecar for the nonprofessional weekend autocross racer. Thousands of students at hundreds of colleges and universities participate in Formula SAE teams every year. Each team designs, builds, and tests a prototype, and at the end of the year, the teams gather in Michigan or Omaha to compete on the basis of cost, presentation, design, acceleration, skid pad, autocross, endurance, and economy.

“We run our team continuously,” said Virginia Tech Associate Professor and team faculty adviser Dr. Bob West. “With our philosophy of balancing education and competition, it is very difficult to get our students up to speed in just a year or so. We encourage freshmen and sophomores to get involved. The serious discussion about the design and concepts for our 2013 entry started in the fall of 2011, so we can devote the 2012-2013 academic year to building, testing, and optimization.”

The very first thing West asked the team to do in the fall of 2011 was to break down and analyze the results from the last five years of competition and make a forecast of what the winning times will be in 2013.

From that forecast, the team could begin to establish design goals for reliability, performance, serviceability, and driveability. The team started to play with design assumptions and trade-offs such as, “If we can build a 300-lb car, how much horsepower will we need? If the car weighs 350 lb, what does that do to the power requirements?”

Brice Collamer, Leader for VT’s 2012 Formula SAE team, emphasizes the rules are very open to encourage creativity. “You could have a steel space frame chassis, carbon fiber monocoque, whatever you want, and any engine so long as it is under 610 cc. Most of the rules are for safety and to keep costs within reason. VT’s entry this year had a Yamaha WR450F bored and stroked to 502 cc with a shared gearbox featuring electropneumatic paddle shifters.”

“We design for dynamics and fatigue,” West said. “You have to know the loads. This is not a static vehicle. You have to look at dynamics, time-variant loading: you’re accelerating or decelerating, loading the suspension, shifting gears, hitting bumps, torqueing the half-shafts, applying lateral thrust...you have to understand the loading so you can design the vehicle properly for the dominant inputs. If you design too heavy, you’re giving away performance; if you design too light, you destroy your reliability.”

According to West, actual operational loads are often “hard to get.” VT began using a torque telemetry system from Accumetrics Associates that could help the team compile a dynamic load history. Accumetrics has much experience helping organizations that need to make measurements on rotating equipment. General Motors, for example, used an Accumetrics system in the development of the Cadillac CTSV racecar.

The AT-5000 telemetry system that VT used was designed to be easily applied to rotating shafts with Kevlar straps and to run for long durations on its internal battery. Its transmitter mounts opposite an equally weighted adjustable yoke to maintain balance, and only about 0.7 to 0.9 in (18 to 23 mm) of radial clearance is needed around the shaft. The Kevlar strap has a breaking strength of 3000 psi and can withstand up to 20,000 rpm, depending upon shaft diameter.

Battery life can be as long as 150 hours on a single lithium battery when used with a 1000 ohm full bridge strain gage. The system can accommodate other sensor signals, including voltages and thermocouples and can sample data at 7812 times a second or faster. Receiving antenna options include a rigid brass loop, a flexible loop, and a miniature stub antenna.

By gathering data on the rotor and digitizing it before transmission, the AT-5000 EasyApp telemetry system provides high accuracy and resolution even in the face of noise sources such as ignition spikes. The AT-5000 receiver converts the transmitted torque signal digital data to a simple +/-10-V output. If rpm is also monitored, the torque and rpm data can be combined to show instantaneous delivered horsepower.

“We need to get a dynamic history of what loads are going through the drivetrain, and we can only do that through a torque telemetry system with a transmitter and full torsional bridge on one of the half shafts,” said Collamer. 

“We have learned a lot about actual loads in our drivetrain, including the transitions associated with shifting and the torque profile for different events,” said West. “The Accumetrics telemetry system helps [us] see how the real-world measurements agree—or don’t agree—with our software simulations.”

VT’s 2012 Formula SAE car now becomes a laboratory for the VT students. “The 2012 car will see a continuing extensive test program that involves learning how to use the torque telemetry system more effectively and build signal processing and drivetrain design/optimization algorithms around it,"said West. "The data acquired with the telemetry system will feed into our designs, shifting, drivetrain analysis, traction and launch control programs, and tuning of torque bias ratios in the differential. We’ll use that data to feed forward to the 2013 design, closing the loop between design, analysis, fabrication, and test.”

In 2012, the VT Formula SAE team finished 12th overall in acceleration, 7th on the skid pad, 23rd overall in autocross. The team finished 43rd overall (out of 105), despite being black flagged for a nonworking tail light (disabled by inadvertently toggling a software switch during a tuning of the automatic shifting control) in the endurance event.

During the first 10 laps on the endurance course, the VT car used just 2 oz of fuel. “We look forward to doing better next year, thanks to better data to close the design loop,” said West.

This article was written for SAE Off-Highway Engineering by Jock Elliott, Accumetrics Associates Inc.

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