Rack disturbance test for determining the frequency response of an electric power steering system

A method of determining a frequency response by a power steering system for either a steer-by-wire or an autonomous vehicle, wherein the power steering system includes a steering assist motor configured to generate an assist torque about an axis of a steering shaft when activated, the method comprising:

• transmitting, by a controller, a commanded steering angle to a rotary actuator connected to the steering shaft;
• in response to receiving the commanded steering angle, actuating the rotary actuator into the commanded steering angle, wherein the rotary actuator maintains position by exerting a steering wheel torque and the steering shaft is connected to the rotary actuator by a compliant torsion bar and inertial wheel;
• transmitting, by the controller, a rack disturbance force control signal to at least one actuator, wherein the at least one actuator is coupled to a rack and is configured to apply a rack disturbance force on the rack in a substantially linear direction;
• in response to receiving the rack disturbance force control signal, generating a first frequency sweep by the at least one actuator, wherein the steering assist motor is deactivated during the first frequency sweep;
• generating a second frequency sweep by the at least one actuator, wherein the steering assist motor is activated and the commanded steering angle is set to zero during the second frequency sweep;
• generating a steering angle frequency sweep based on the commanded steering angle, wherein the steering assist motor is activated and the at least one actuator is disconnected during the steering angle frequency sweep;
• monitoring, by the controller, a rotary encoder coupled to the steering shaft, wherein the rotary encoder measures a steering angle experienced during the first frequency sweep, the second frequency sweep, and the steering angle frequency sweep;
• determining, by the controller, a transfer function based on the steering angle monitored during the first frequency sweep and the second frequency sweep, wherein the transfer function indicates the frequency response of the power steering system;
• and
• determining, by the controller, effects of a feedforward algorithm upon the power steering system based on the steering angle monitored during the steering angle frequency sweep.

A power steering system, comprising:

• a rack defining a first end portion and a second end portion;
• a first actuator disposed on the first end portion of the rack and a second actuator disposed on the second end portion of the rack;
• a steering shaft rotatable about a steering axis;
• a pinion gear connecting the rack to the steering shaft;
• a steering assist motor configured to generate an assist torque about the steering axis of the steering shaft;
• a torque transducer configured to measure a steering output torque of the steering shaft;
• a rotary actuator;
• a compliant torsion bar and inertial wheel configured to connect the rotary actuator to the steering shaft;
• and
• a controller in communication with the first actuator, the second actuator, and the rotary actuator, the controller configured to:
• transmit a commanded steering angle to the rotary actuator, wherein the rotary actuator into the commanded steering angle and maintains position by exerting a steering wheel torque;
• transmit a rack disturbance force control signal to the first actuator and the second actuator;
• instruct the first actuator and the second actuator to generate a first frequency sweep and a second frequency sweep, wherein the steering assist motor is deactivated during the first frequency sweep and activated during the second frequency sweep;
• monitor the torque transducer for the steering output torque of the steering shaft experienced during the first frequency sweep and the second frequency sweep;
• and
• determine a transfer function based on the steering output torque monitored during the first frequency sweep and the second frequency sweep, wherein the transfer function indicates a frequency response of the power steering system.