Investigations of the State Dependent Riccati Equation Based 4WS/4WD Controller for Electric Buses

Paper #:
  • 2017-01-1579

  • 2017-03-28
The electric buses have gained more popularity aiming to reduce pollutions due to transportation. Equipping the electric buses with four independent driving motors leads to the utilizations of smaller motors, which generally implies lower cost. Additionally, it also opens up the possibility of implementing direct yaw moment control (DYC) for the electric buses. While DYC has been widely used to improve yaw plane stability, its usage has to be limited since the differential torque inevitably increases the loadings on the lateral tire force which may increase the risk of instability. Four wheel steering (4WS), on the other hand, has the capability to mitigate the vehicle understeer or oversteer situations. However, since the yaw rate response is immediately affected by the additional rear wheel steering, it is usually used only in critical situations with limited amount of control so that the yaw rate will not quickly deviate from the desired value based on the roadway curvature. Both approaches have the potential to improve the vehicle yaw plane stability, through different control channels and limitations. The electric buses, due to its larger size and weight, may benefit from integrating both control channels, but the control algorithm need to be carefully designed to successfully integrate them. In this work, the integrated control design for the 4WS/4WD electric buses is investigated, and the control objectives include the yaw rate tracking and side slip angle reduction, which are the basics of the vehicle yaw plane dynamics. Due to the highly nonlinear nature of vehicle dynamics near the critical situations, a state-dependent Riccati equation based optimal control method is employed in order to compromise the different requirements in the vehicle control. The TruckSim vehicle model is modified to represent the electric version of the bus and used as the evaluation platform for the proposed control method.
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