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

An electric power assist steering system for a vehicle, said steering system comprising:

• a steering assembly including a steering wheel connected to a steering column;
• an electric motor operatively engaged with said steering assembly for supplying steering torque assist;
• a torque detector determining a torque signal that is substantially indicative of driver torque applied to the steering assembly;
• a steering column sensor for sensing a steering angle of the steering column and generating a signal indicative thereof;
• a first H-infinity controller coupled in a feedback control loop for generating a first feedback control signal as a function of said torque signal and a first characteristic of the steering system;
• a second H-infinity controller coupled in the feedback control loop for generating a second feedback control signal as a function of said torque signal and a second characteristic of the steering system;
• and
• a switch for selecting one of the first and second feedback control signals as a function of the sensed steering angle.

A control system for controlling electric motor torque assist to an electric power assist steering system for a vehicle having a steering assembly including a steering wheel connected to a steering column and an electric motor operatively engaged with the steering assembly for supplying steering torque assist, wherein said control system comprises:

• a torque detector for detecting driver torque and generating a driver torque signal indicative thereof;
• a steering column sensor for sensing angular position of the steering assembly;
• a first H-infinity controller coupled in the feedback control loop and generating a first feedback control signal as a function of the driver torque signal and a first characteristic of the steering system;
• a second H-infinity controller coupled in the feedback control loop and generating a second feedback control signal as a function of the driver torque signal and a second characteristic of the steering system;
• and
• a switch for selecting one of the first and second feedback control signals, wherein a motor control signal is generated as a function of the selected feedback control signal.