Developing a Solar Racecar

Measurement technology from FUTEK Advanced Sensor Technology has been used in the development of a dynamometer used to test the motor of a solar car destined for the gruelling transcontinental World Solar Challenge.

In early 2013, the students that comprise the Stanford Solar Car Project (SSCP) were working on Luminos, the racecar they hoped would qualify for competition in the Challenger Class of the World Solar Challenge.

The World Solar Challenge is the Solar Car Race; and within this international engineering contest, the Challenger Class is the highest echelon of competition, demanding a vehicle with aerodynamics, electronics and mechanics that approach perfection. The 2013 Challenger Class event was slated to be a gruelling transcontinental 'ultra-marathon' spanning over 1850 miles on a coast-to-coast route through the heart of the Australian Outback. In this event, the success of the SSCP would be largely contingent upon the optimisation of a prototype motor the team was designing specifically to power their flagship.

FUTEK's IHH500 digital handheld display alongside its TRS605 sensors

However, characterising the performance of an intricate system like a FUTEK motor is an extremely challenging measurement application. One aspect of this challenge is that the most consequential parameters at play have complex relationships: the rotational frequency (RPM) of the motor functions as the independent variable in the system, while torque (lb-ft) and power (HP) are both nonlinear func
tions of rotational frequency, but independent of each other.

The dynamometer test stand

To emulate the conditions the motor will be subjected to on race day, a test system called a dynamometer (aka dyno) is used. A dyno consists of three basic components: a test stand, an energy absorber, and measurement instrumentation. The test stand securely situates the motor during testing, dampens vibrational modes and allows the crankshaft to rotate indefinitely. The energy absorber applies opposing loads to the crankshaft, approximating what the motor is subjected to when in service; and the measurement instrumentation must capture useful data during the test so it can be analysed.

The SSCP team, however, sought to add an interesting twist to their dyno - to use one of the regenerative brakes from their racecar as the energy absorber. This setup would allow them to simultaneously study the efficiency of their regenerative braking system while testing their motor. To realise this concept, the students teamed up with FUTEK Advanced Sensor Technology.

A measurement system
The application engineers at FUTEK knew the clear choice was a measurement system based around a TRS605 rotary torque sensor paired with an IHH500 Elite digital handheld display. The TRS family of sensors have a single compact package that combines a rotary torque transducer with an angular displacement encoder. Of benefit, these can capture the measurement in situ - in this case, the TRS was securely positioned directly between the crankshaft of the motor and the regenerative braking system. Although this TRS sensor can only measure the torque and angular displacement about the crankshaft, the IHH500 can use these dimensions to derive and display critical parameters such as power (HP) and rotational frequency (RPM) in real time.

Complex measurement applications like this generally require graphical analysis to identify meaningful relationships among the various parameters. To help, the entire FUTEK instrumenta
tion portfolio is compatible with the SENSIT test and ' measurement software platform, which allows measurement data to be logged, graphed, analysed, and reported with simplicity and flexibility.

The software displays graphical data in terms of: directly measured parameters like load, displacement, and time; derived parameters like velocity and power; and even user-defined units of measure through its Math Channel function. By applying a simple scalar or formula to a given input channel, the Math Channel feature can transform torques into torsions, forces into stresses, and displacements into strains. It also has the ability to formulaically relate multiple input channels to generate differential and ratiometric measurements.

Armed with the actionable data provided by an advanced FUTEK measurement system, the SSCP refined their prototype powertrain to exceed its ambitious performance targets.

The result
Luminos and the Stanford Solar Car Team went on to qualify for competition in the Challenger Class, successfully completing the event in well under 40 hours. With an average speed of 47mph, the SSCP placed 4th of the 22 Challenger Class contestants. This was also the first U.S. team to cross the finish line.

February 2014

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