Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
2002-01-14
2002-07-30
Camby, Richard M. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S042000, C180S446000
Reexamination Certificate
active
06427105
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an electric power steering control system for an automobile and to a control method thereof, in which an assist force when manipulating a steering wheel is generated by means of a motor.
2. Background Art
As a power steering system for an automobile, an electric power steering apparatus has been developed which apparatus detects a vehicle speed and a steering torque acting on a steering shaft, drives a motor by a drive current determined in response to the detected vehicle speed and torque, adds an assist force onto the steering shaft by means of a rotational force of the motor, and then provides a driver a comfortable steering feeling.
Hitherto, in such a type of electric power steering apparatus, as a motor for adding the assist force onto the steering shaft, a DC electric motor with a brush such as a commutator motor has been mainly used. Since the motor is with a brush, however, it is possible that any trouble or abnormal state occurs in the motor itself.
Therefore, recently use of a brushless motor has been proposed instead of the DC electric motor with a brush. However, such a motor and/or a control apparatus come to be complicated and expensive. Thus, to put into practice the control suitable for the power steering apparatus, several inherent problems to be overcome remain in terms of cost or performance.
In the electric power steering control system, the motor for assisting the steering is connected to the steering wheel through a reduction gear. As he driver directly touches the steering wheel, the motor is required to have less fluctuation in torque and less vibration. Moreover, As rotation of the motor follows a steering speed of the steering wheel, the motor is forced to abruptly start or stop depending upon steering conditions.
Considering an arrangement of the control system of the electric power steering control unit using a brushless motor, as motor current of the brushless motor is an alternating current, the current control may get worse due to the steering conditions depending upon the arrangement of the current control system. As a result, an error in torque control and transient vibration are increased and the steering assist performance of the electric power steering control system is lowered in some cases.
For example,
FIG. 4
shows an arrangement of an electric power steering control system which a PM (permanent magnet) brushless motor is used as a steering assist motor and the electric current is controlled with a three-phase alternating current.
Referring to
FIG. 4
, a PM brushless motor
5
a
is PWM driven by a PWM drive section
101
. A control computer
200
computes three-phase voltage command values Vu*, Vv*, Vw* on the basis of a vehicle speed detected by a speed sensor
6
, a steering torque detected by a torque sensor
3
, a rotational position of the brushless motor
5
detected by a position sensor
103
, and three-phase alternating currents Iu, Iv detected by current sensors
102
a,
102
b.
Then the PWM drive
101
is controlled by the mentioned three-phase voltage command values Vu*, Vv*, Vw*.
The control computer
200
includes a d-q axis current command section
200
a
for computing d-axis and q-axis current command values Id*, Iq* for generating a steering assist force in response to a predetermined characteristic on the basis of the steering torque and the vehicle speed, an electrical angle computing section
200
b
for computing an electrical angle signal &thgr; in response to the rotational position of the brushless motor
5
detected by the position sensor
103
, a d-q coordinate inverse transformation section
200
c
for performing d-q coordinate inverse transformation of the d-axis and q-axis current command values Id*, Iq* on the basis of the electrical angle signal &thgr; thereby computing three-phase current command values Iu*, Iv*, and a three-phase voltage command section
200
d
for computing the three-phase voltage command values Vu*, Vv*, Vw* in accordance with deviation values of the three-phase current command values Iu*, Iv* and the three.-phase alternating currents Iu, Iv inputted to the brushless motor
5
.
The PM brushless motor
5
generates a torque by delivering sine-wave currents to the three phases in accordance with the electrical angle as shown in
FIG. 6
, for example. As the motor output torque is in proportion to amplitude of the sine-wave currents, controlling the current amplitude controls the output torque of the PM brushless motor
5
.
The mentioned electrical angle is an angle in which pairs of S and N poles of a rotor in the PM brushless motor are arranged to be 360 degrees. By way of example,
FIG. 5
shows the relation between electrical angle and mechanical angle in case of an eight-pole motor.
In the electric power steering control system as described above, under the condition of holding the steering wheel, generating a torque without rotating the brushless motor
5
performs steering assist. For example, supposing that the electrical angle is positioned at 50 [deg] (on a broken line in FIG.
6
), the electric current flowing in the form of three phases become a direct current as shown in FIG.
7
. Therefore the current command value is coincident with the actual electric current without deterioration in current control characteristic, and the steering assist characteristic is not deteriorated.
Under the condition of rotating the steering wheel, as the brushless motor
5
rotates, it is required that the three-phase current depicts sine waves as described above. Under the condition of relatively gentle steering such as in the case of driving the vehicle at a high speed, the current control system can follow up the steering condition and the steering assist characteristic is not deteriorated.
However, in the case that the brushless motor
5
rotates at a high speed, there is a possibility that, in the control by means of the three-phase alternating currents, a phase shift is generated between the current command value and the actual electric current depending on the current control characteristic.
More specifically, in the electric power steering control system, the rotation speed of the brushless motor
5
follows up the rotation speed of the steering wheel, and therefore when the steering wheel is turned sharply, the motor is abruptly rotated to a speed over the rotation speed performance thereof. As the motor is connected to the steering wheel through the reduction gear (for example, at gear ratio
28
), frequency of the sine-wave current flowing into the motor becomes as follows. That is, gear ratio times (28 times) the steering rotation frequency×pole pair number times (4 times in case of the eight-pole motor)=112 times. Accordingly, frequency of the three-phase alternating current applied to the brushless motor
5
rises abruptly, and therefore in a system in which the electric current is controlled by the three-phase alternating current as shown in
FIG. 4
, the current control cannot follow up such a sharp rise and a phase shift takes place between the current command value and the actual electric current.
In the PM brushless motor, the phase shift means increase in reactive current, and supposing a phase shift with respect to an ideal current phase as &agr;, the output torque is in proportion to COS &agr;. In other words, when any phase shift is generated, the output torque of the steering assist motor is lowered, and as a result, manipulation of the steering wheel becomes heavy.
In the electric power steering control system, it is essential that the steering torque does not increase even when the steering wheel is turned sharply in terms of emergency behavior to avoid accident, and it is important to arrange the electric power steering control system so as not to increase the steering torque even when the steering wheel is turned sharply.
SUMMARY OF THE INVENTION
The present invention was made to solve the above-discussed problems and has an object of providing an electric power ste
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