Abstract
According to the progress in an electronic technology, a lot of electronically controlled systems for the car have been developed and put into practical use. Moreover, the onboard network has been standardized to communicate among the electronic control units. To extend the function or improve the performance of one system, the other systems can be utilized through the network. In this paper, the cooperative control of the electronic stability control (ESC) and electric power steering (EPS) is described. The ESC and EPS are communicated through the in-vehicle network and can share mutual information (e.g. sensor and processing signals). The EPS can offer the steer information including the steer torque.
ESC can provide the vehicle behavior information including handling characteristics of the vehicle. It is also possible to estimate a new vehicle state using the provided information. Furthermore, not only the information sharing between the ESC and EPS but also it becomes possible to improve the vehicle stabilization performance of the ESC by cooperating with EPS.
The self-aligning torque characteristic of a pneumatic tire has the peak value around half the slip angle where the lateral force is saturated, and decreases while the lateral force increases as the slip angle increases. The lateral grip margin, which indicates the tire grip level against the road-tire friction, is estimated based on the relation between the wheel slip angle and selfaligning
torque. The wheel slip angle is calculated from the vehicle slip angle in the ESC and the self-aligning torque is obtained from the steer torque and motor drive current of the EPS. The estimated lateral grip margin can be use for the warning system, which informs the driver when the road is slippery and the tire grip level becomes low. Also, the vehicle stability can be improved by integrating ESC with the brake control based on the lateral grip margin. To verify these concepts, experimental results are introduced.
The EPS can control the steer torque in aid of the driver's steering wheel operation by coordinating with the ESC. For instance, it is known that the vehicle deflects toward the high friction side when ABS braking is applied on the -split road where the road friction is laterally different. To reduce this vehicle deflection, the driver should operate the steering wheel toward the low friction side for compensating the yaw moment due to the braking force difference. This steering operation is known as the counter-steer. The steer torque can be controlled to assist the driver's counter-steer operation. The supplementary steer torque is decided based on the vehicle state variables processed in the ESC and applied to the steering wheel to help the driver's counter-steer. The effect of this counter-steer assist is confirmed showing the experimental results.
Keywords - Electronic Stability Control, Electric Power Steering, Coordinated Control, Lateral Grip Margin, -Split Braking