Patent ID: 7530422

Claim:
A steering system comprising: a position overlay unit for applying one of an assist force to a rack and a motor command to a motor, wherein the position overlay unit includes a variable ratio gain, the steering system further comprising a position sensor for providing a position signal to the variable ratio gain and further comprising a position overlay command provided to output of the variable ratio gain; a torque overlay unit for applying the other of the assist force to a rack or the motor command to a motor; and, one of an electric power mechanism and a hydraulic power mechanism for providing the assist force to the rack, wherein a load torque is sent from a position loop of the position overlay unit to a force loop of the torque overlay unit, and wherein association between the position overlay unit and the torque overlay unit follows the equations: τ s =k Tbar (θ i −θ HW ) θ i =τ s /k Tbar +θ HW Δθ=(τ sc −τ s )/ k Tbar θ ic =θ i +Δθ θ o =θ i +θ m θ mc =θ o −θ ic where τ s is hand wheel torque sensor signal, k Tbar is torsion bar stiffness, θ i is calculated input shaft position, θ HW is hand wheel position sensor signal, Δθ is calculated position correction, τ sc is torque sensor command based on the load torque, θ ic is commanded input shaft position, θ o is output shaft position signal, θ m is motor position, and θ mc is commanded motor position.