Dynamic Desingn Lab
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Estimating Tire Parameters Using GPS

People

  • Brett Youngberg 
  • Shannon Miller 
  • Alex Millie 
  • Patrick Schweizer 

Description

The tires of a car are what allow a vehicle to drive and handle. However, what actually occurs at the contact patch between the tires and the driving surface is still unknown. Many systems on the car such as ABS brakes and active suspension require some knowledge of what the tires are doing. Currently, the frictional forces, slip, and dynamic radius of the tire are assumed, calculated using complex or over simplified models, or obtained from experimental tire tests done in a restrictive testing environment. Most experimental data requires the absolute velocity or acceleration of the car. Accelerometers have a bias, which when integrated, accumulates error. Some use the throttle sensor to determine acceleration but engines and transmissions have many friction losses and only estimates can be made as to how much energy actually reached the wheels. Comparisons between non-driven and driven wheels of front or rear wheel drive vehicle are currently used to deduce tire slip. Nonetheless, this deduction is made on the assumption that the non-driven wheels do not slip. Regardless of the method used, it is difficult to obtain accurate results good enough to improve on or maximize the effectiveness of the vehicle's dynamic systems. However, with recent development of GPS and the removal of SA (Selective Availability), a vehicles absolute velocity can be obtained with an accuracy of 2 cm/s. From this information it is quite easy to get the velocity and acceleration of a vehicle. In addition to GPS, using a high resolution wheel speed sensor mounted on the hub of a wheel, a comparison can be made between the speed of the wheels and actual speed of the vehicle to determine how much the wheels actually slip. From this information, other tire characteristics such as dynamic radius and tire stiffness can be obtained.

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