Model Based Control to Decouple Electric Steering Feel and Loop Gain 2017-01-1571
An approach to electric steering control and tuning is developed using vehicle dynamics and quantitative steering objectives. The steering objective chosen is the torque vs. lateral acceleration target for the driver termed the “steering gain”. Two parameters are derived using vehicle dynamics that substantially determine driver feel: the vehicle’s “manual gain” (total steering torque divided by lateral acceleration) and the vehicle’s lateral acceleration gain (lateral acceleration divided by steering angle). Lateral acceleration gain is a well-known quantity in the literature but “manual gain” is a nonstandard point of view for steering control systems. The total gain inside the controller is the loop gain; generally, the higher the loop gain, the better the controller rejects unwanted effects such as friction. For a typical torque-input electric steering topology, it is shown that the relationship between loop gain and steering gain is unique. The practical impact of this result explains the trade-off between steering response and friction rejection that often exists when tuning electric steering systems. The mathematical proof supports the reality of the tuning trade-off. A topology that decouples the loop gain from the steering gain is proposed and implemented. Test results validate the theoretical predictions.