Method and apparatus for controlling electric power steering system
AssigneeNsk Ltd.
InventorsCaoMinh Ta, ChunHao Jiang, Hideyuki Kobayashi
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Abstract
Disclosed herein is a method for controlling an electric power steering system, wherein a motor is controlled by vector control and driven by a current command value calculated on the basis of a parameter such as a steering torque to provide a steering assist power to a vehicle steering system, and further wherein the current command value is limited on the basis of desired output characteristics and a desired angular speed during a field-weakening control and a non-field-weakening control of the vector control.
Claims12
A method for controlling an electric power steering system wherein a d-axis current command value Id and a q-axis current command value Iq are calculated based on a steering assist current command value Iref calculated from a steering torque, a motor is controlled by an inverter with a vector control based on the d-axis current command value Id and the q-axis current command value Iq and is driven to provide a steering assist power to a vehicle steering system, the method comprising:
- carrying out a field-weakening control on a d-axis of the vector control in accordance with a following equation;
- Id=−|Iref|·sin(cos−1(ωb/ωe))
- where, ωe is an angular speed of the motor and ωb is a base angular speed,
- obtaining a first limit value of the steering assist current command value Iref and a first base limit value based on the angular speed ωe and a rated output characteristic of the motor;
- obtaining a second limit value of the steering assist current command value Iref based on the steering assist current command value Iref and the first limit value;
- obtaining a second base limit value based on the steering assist current command value Iref and the first base limit value;
- calculating the d-axis current command value Id based on the angular speed ωe and the second limit value;
- switching the second base limit value to an output when the angular speed ωe is equal to or smaller than the base angular speed ωb and the second limit value to an output when the angular speed ωe exceeds the base angular speed ωb;
- and
- limiting the steering assist current command value Iref by the switching of the second base limit value and the second limit value.
A control apparatus for an electric power steering system, comprising a steering assist current command value calculator for calculating a steering assist current command value Iref based on a steering torque and a drive controller for controlling a drive of a permanent magnet type brushless DC-motor driven by a vector control of a d-axis current command value Id and a q-axis current command value Iq which are calculated from the steering assist current command value Iref, and providing a steering assist power by means of the permanent magnet type brushless DC-motor to a vehicle steering system, wherein:
- the control apparatus carries out a field-weakening control on a d-axis of the vector control in accordance with a following equation;
- Id=−|Iref|·sin(cos−1(ωb/ωe))
- where, ωe is an angular speed of the permanent magnet type brushless DC-motor and ωb is a base angular speed,
- the control apparatus further comprises a current command value calculator to limit the steering assist current command value Iref based on a rated output characteristic of the permanent magnet type brushless DC-motor, the steering assist current command value Iref and the angular speed ωe of the permanent magnet type brushless DC-motor;
- the current command value calculator comprises a first look-up table for calculating a first limit value based on the angular speed ωe to output the first limit value, a second look-up table for calculating a base limit value based on the angular speed ωe to output the base limit value, a first comparator for comparing the steering assist current command value Iref and the first limit value to output a second limit value, a second comparator for comparing the steering assist current command value Iref and the base limit value to output a base command value, a third look-up table for calculating the d-axis current command value Id based on the angular speed ωe and the second limit value to output the d-axis current command value Id, a switch unit to switch the second limit value and the base command value to output the second limit value or the base command value, and a detecting and switching unit for detecting the angular speed ωe and outputting a limit value of the steering assist current command value Iref;
- and
- the control apparatus outputs the limit value of the steering assist current command value Iref and the d-axis current command value Id during the field-weakening control and non-field-weakening control of the d-axis.
Dependencies are inferred from the claim text.