Electric power steering motor control

An electric power steering motor control apparatus comprising:

• a steering torque sensor providing a torque signal indicative of steering system torque;
• an electric motor for providing power assist to an electric power steering system;
• a controller, coupled to the torque sensor, receiving the torque signal and, responsive thereto, providing a voltage command and a current command to the motor, wherein the controller comprises
• a voltage command generator that generates the voltage command in a voltage mode, wherein the voltage command commands a first torque that is greater than a desired torque assist responsive to low torque signals which are lower than a predetermined value, that is substantially equal to the desired torque assist responsive to high torque signals which are higher than the predetermined value, and that comprises a substantially smooth and continuous function over a range of said low and high torque signals;
• and
• a current command generator that generates the current command in a current mode, wherein the current command commands a second torque that is substantially equal to the desired torque assist responsive to said low torque signals, that is greater than the desire torque assist responsive to said high torque signals, and that comprises a substantially smooth and continuous function over the range of said low and high torque signals,
• wherein the electric motor voltage is no greater than the voltage command and the electric motor current is no greater than the current command and the motor alternately operates in said voltage and current modes.

In an electric power steering system in which a torque assist command is generated for controlling a motor to provide electric power assist torque at a desired torque, a method of motor control according to the steps of:

• determining a voltage mode motor command responsive to the torque assist command, wherein the voltage mode motor command commands a first motor torque that is (a) greater than the desired torque when the desired torque is lower than a predetermined value and (b) substantially equal to the desired torque when the desired torque is higher than the predetermined value;
• determining a current mode motor command responsive to the torque assist command, wherein the current mode motor command commands a second motor torque that is (a) substantially equal to the desired torque when the desired torque is lower than the predetermined value and (b) greater than the desired torque when the desired torque is higher than the predetermined value;
• and
• controlling the motor responsive to the voltage and current mode motor commands, wherein the motor assist torque is equal to a lesser of the first and second motor torques.

In an electric power steering system in which a torque assist command is generated for controlling a motor to provide electric power assist torque at a desired torque, a method of motor control according to the steps of:

• determining a voltage offset responsive to the torque assist command;
• determining a current offset responsive to the torque assist command;
• summing the voltage offset and the torque assist command to determine a voltage torque command commanding a first motor torque, wherein the first motor torque is greater than the desired torque at low desired torques which are lower than a predetermined value and substantially equal to the desired torque at high desired torques which are higher than the predetermined value;
• summing the current offset and the torque assist command to determine a current torque command commanding a second motor torque, wherein the second motor torque is substantially equal to the desired torque at said low desired torques and greater than the desired torque at said high desired torques;
• and
• controlling the motor responsive to the voltage and current torque commands, wherein the motor output torque is equal to the lesser of the first and second motor torques, wherein when the lesser of the first and second torques is the first motor torque, the motor operates in voltage mode and wherein when the lesser of the first and second torques is the second motor torque, the motor operates in a current mode, wherein the motor obtains high stability during high torque events and responsiveness during low torque events.