Electric power steering motor control

• US 5,668,721 A
• Filed: 10/02/1995
• Issued: 09/16/1997
• Est. Priority Date: 10/02/1995
• Status: Expired due to Term

• Current Assignee: GM Global Technology Operations Incorporated (General Motors Company), Steering Solutions IP Holding Corporation (Nexteer Automotive Group Ltd.)
• Original Assignee: General Motors Corporation (General Motors Company)
• Inventors: Chandy, Ashok
• Primary Examiner(s): Chin, Gary
• Application Number:
  US08/537,604
• Time in Patent Office: 715 Days
• Field of Search: 364/424.051, 364/424.052, 318/432, 318/459, 180/412, 180/413, 180/422, 180/443, 180/446
• US Class Current: 180/446
• CPC Class Codes: B62D 5/0466 for returning the steering wheel to neutral position

Abstract

In an electric power steering system in which a torque assist command is generated for controlling a motor that provides 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 low and (b) substantially equal to the desired torque when the desired torque is high; 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 low and (b) greater than the desired torque when the desired torque is high; 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.

Claims

1. An electric power steering motor control method according to the steps of:

determining a desired assist torque function indicating a desired assist torque in response to at least one steering system input parameter; and

then

sensing a steering wheel torque;

responsive to the sensed steering wheel torque, determining a voltage torque command in a voltage mode, wherein the voltage torque command commands a first torque that is greater than the desired assist torque at low desired assist torques which are lower than a predetermined value, substantially equal to the desired assist torque at high desired assist torques which are higher than the predetermined value, and is substantially smooth and continuous over a range of said desired assist torques;

responsive to the sensed steering wheel torque, determining a curtain torque command in a current mode, wherein the current torque command is substantially equal to the desired assist torque at said low desired assist torques, greater than the desired assist torque at said high desired assist torques, and is substantially smooth and continuous over the range of said low and high desired assist torques; and

controlling an electric motor responsive to the voltage and current command torques, wherein the electric motor voltage is no greater than a voltage corresponding to the voltage torque command and the electric motor current is no greater than a current corresponding to the current torque command, wherein the motor provides torque assist to an electric power steering system and alternately operates in said current and voltage modes.

2. An electric power steering motor control method comprising the steps of:

sensing steering system torque of an electric power steering system;

when the sensed torque is above a transfer point which is indicative of operation of the electric power steering system at a torque level where enhanced stability is desired, controlling an electric motor responsive to a command voltage in a voltage mode, wherein the command voltage is determined responsive to the sensed torque;

when the sensed torque is below the transfer point, controlling the electric motor responsive to a command current in a current mode, wherein the command current is determined responsive to the sensed torque, wherein the electric motor provides torque assist to the electric power steering system and alternately operates in said current and voltage modes.

3. An electric power steering motor control according to claim 2, wherein the transfer point varies with operating conditions of the electric motor.

4. 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.

5. An electric power steering motor control apparatus comprising:

a torque sensor providing an output 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 output signal and, responsive thereto, controlling the electric motor in a voltage mode with a voltage command when the output signal is above a transfer value which is indicative of operation of the electric power steering system at a torque level where enhanced stability is desired and controlling the electric motor in a current mode with a current command when the output signal is not above the transfer value.

6. 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.

7. 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.

8. An electric power steering motor control apparatus comprising:

a torque sensor providing an output signal indicative of steering system torque;

an electric motor for providing power assist to an electric power steering system responsive to a motor command;

a controller, coupled to the torque sensor, receiving the output signal and, responsive thereto, determining a motor command that is output to the electric motor, wherein the motor command controls the electric motor in a voltage mode with a voltage command when a gain of the motor command with respect to the output signal is above a transfer value which is indicative of operation of the electric power steering system at a torque level where enhanced stability is desired and wherein the motor command controls the electric motor in a current mode with a current command when the gain of the motor command with respect to the output signal is not above the transfer value.