Abstract: The torque gain of an electric motor assisted power steering system is modified as a function of the estimated level of road load friction such that the relationship between the operator exerted steering torque and the resulting rate of steer is substantially insensitive to a reduction in the level of road load. The road load is estimated as a function of the operator exerted steering torque, the torque assist provided by the electric motor, and the resulting steer rate.

Abstract: A driver circuit for a transistor bridge is selectively controllable to produce four state operation of a DC motor. The driver circuit has two control inputs yielding four distinct input combinations, each combination producing a different state of motor operation. The states which may be selectively produced are (1) motor off or coasting, (2) motor on, counterclockwise rotation, (3) motor on, counterclockwise rotation and (4) motor off, dynamic braking.

Abstract: 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.