The Society of Automotive Engineers hosts the annual Formula SAE student design competition. This year, 70 teams will design, build, and race a one man, open-wheeled road racer. Formula SAE is considered the most prestigious university engineering design competition in the world. This year for the first time, a team of undergraduate BYU students will participate in the Formula SAE competition.
Vehicle Data Acquisition System
Testing is a critical part of the design process. By fabricating a prototype vehicle, the team identified which systems to improve prior to construction of the final race vehicle. To validate the design, our tests and experiments rely on instrumentation, on-board data acquisition, and statistical analysis.
We chose an NI CompactRIO system primarily for its configuration flexibility and input channel density (input channel/weight ratio). It is shown mounted to the test vehicle in Figure 1. The CompactRIO provides a solution that was not available with any other device. The varied modules available for the CompactRIO offer flexibility in sensor selection. The networking capabilities of the controller provide a wireless telemetry option needed for remote race vehicle monitoring. Deterministic loop options allow accurate high-speed acquisition and low-power consumption is critical to preserve auxiliary power.
The system weighs less than nine pounds, can be configured with to up to 64 analog input channels and operates on less than 24 watts. Our CompactRIO is configured with two NI cRIO-9211 thermocouple modules, an NI cRIO-9421 digital input module, an NI cRIO-9472 digital output module, and four NI cRIO-9201 analog input modules. We wrote virtual instrument programs in LabVIEW to gather data, chart speeds and accelerations, display temperatures, and record information. Wireless telemetry was established by linking the TCP/IP connection on the CompactRIO to an off-the-shelf wireless router. We used a simple 12 volt to 5 volt DC to DC converter to power the router with the vehicle battery. With the wireless connection, we can interpret the data in real time, which greatly simplifies the data analysis process and allows members of the team to prepare to modify the engine or suspension before the car enters the pits. We have 27 sensors to measure acceleration, wheel speed, air temperature, fuel temperature, tire temperature, pressure, throttle position, brake position, steering position, and suspension displacement.
Accurate Temperature Measurements
Tire temperature significantly impacts traction, and consequently, track times. We have three infra-red temperature sensors mounted to a cantilevered bracket on the front of each tire. As the tires warm up, their ability to grip the track increases until they reach their peak performance, after which an increase in temperature reduces the tires’ ability to grip. With the 24-bit accuracy of the cRIO-9211 thermocouple couple card, we can detect even the slightest increase in temperature. Our testing process consists of three stages:
- Initially we adjust the static camber of suspension and the tire pressure until the tires heat evenly across the tread.
- Coupled with a lateral accelerometer, LabVIEW helps us plot the coefficient of friction as a function of temperature. This information determines the temperature at which the tire grip peaks.
- Finally, we experiment with suspension settings (toe and camber) to closely maintain the peak temperature.
Due to the technical nature of the Formula SAE course, fast acceleration and high cornering speeds are crucial to success at the competition. We used a three-axis accelerometer in our driving tests to quantify cornering ability. Lateral acceleration is affected by static camber, tire pressure, and tire temperature. A factorial experiment helps find the optimal settings for camber and tire pressure. The test is repeated with several drivers.
A Hall Effect sensor is mounted to one of the front uprights to measure wheel speed. We drilled evenly spaced holes into the brake rotor to be used as targets. The sensor does not produce a strong enough voltage signal to switch the state of a digital input module, so we used the cRIO-9201 analog input module to read the analog data from the sensor and derive wheel speed in miles per hour. With 12 holes per revolution and a wheel speed of up to 24 revolutions per second, a minimum sample rate of 576 S/s is necessary. We used a timed loop with a high priority in LabVIEW to ensure that this data is sampled at a sufficient rate.
Minimizing weight transfer in cornering makes our vehicle more predictable and easier to control. Linear potentiometers mounted to the shock absorbers are used to measure how much the suspension travels as the car maneuvers through corners. As adjustments to the suspension are made, we find the settings that minimize travel while maintaining grip.
The traction of the tires determines a limit on lateral and braking acceleration. A skilled driver drives just within this envelope of maximum acceleration. With a three axis accelerometer, we can measure the magnitude of acceleration of the vehicle. A throttle position sensor, brake pressure transducers and a steering angle potentiometer are used to identify trends in successful drivers. Radio communication is used to relay this information the driver. Immediate feedback helps the driver understand the limits of the vehicle, which is beneficial in training for the competition.
At the time of writing the Formula SAE competition is three months away. With more than 200 hours of testing projected, BYU race design engineers continue to depend on National Instruments hardware. The flexibility of the CompactRIO and the short development times of LabVIEW programming continue to benefit the performance of the race vehicle up to the competition. Quality testing equipment and procedures helped BYU achieve its goal of a first place finish.
Lucas B. Graham
Brigham Young University
Tel: (801) 494-9520