H-infinity control and gain scheduling method for electric power assist steering system

A steering system ( 10 ) and method ( 70 ) for controlling the steering of a vehicle having a steering assembly including a steering wheel ( 12 ), a steering column ( 14 ) connected to said steering wheel ( 12 ), and an electric motor ( 20 ) operatively engaged with the steering assembly for supplying torque assist. A steering angle sensor ( 32 ) is employed for sensing an angular position thetac of the steering column ( 14 ). The steering system has first and second H-infinity controllers ( 64 A and 64 B) coupled in a feedback path ( 44 ) for generating first and second feedback signals as a function of the driver torque and first and second characteristics (JC and KC) of the steering system. One of the first and second feedback signals is selected and the selected feedback signal is combined with a feedforward signal to generate a motor control signal (U) as a function of the estimated torque.