Image Number 4 for United States Patent #4509611.
A torque transducer senses the operator exerted steering torque and provides an output signal which varies in magnitude from a low value representative of a large operator exerted steering torque in one direction through a mean value representative of no operator exerted steering torque to an upper limit value representative of a large operator exerted steering torque in the opposite direction. The controller develops a zero torque reference which has an initial value substantially corresponding to the mean value of the torque transducer output signal, and steering torque assist is developed in relation to the difference between the zero torque reference and the transducer output signal. In operation, the zero torque reference is shifted according to a predetermined time response characteristic in a direction to reduce the difference between it and the transducer output signal. Over a period of time, the shifting of the zero torque difference due to operator exerted steering torque tends to cancel out and the remaining shifting adaptively compensates the zero torque reference for drift and other like sources of error affecting the magnitude of the transducer output signal.