Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2000-10-18
2002-06-18
Masih, Karen (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S433000, C318S798000, C318S802000, C318S803000
Reexamination Certificate
active
06407524
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a control apparatus for an electrical power steering apparatus for applying steering assistance force by means of a motor to the steering system of an automobile or vehicle, and relates more particularly to a control apparatus for an electrical power steering apparatus whereby torque ripple is reduced and steering feel is improved using a DC motor or a brushless motor.
RELATED ART
Electric power steering systems that use the torque from a motor to assist the steering system of an automobile or other vehicle use a transfer mechanism such as gears or belts to transfer drive power from the motor via a speed reducer to assist turning the steering shaft or rack shaft. Such conventional electrical power steering systems use motor current feedback control to accurately generate the assistance torque (steering assistance torque). Feedback control adjusts the voltage applied to the motor so that the difference between the current control value and the detected motor current becomes smaller, and adjusting the voltage applied to the motor is generally accomplished by adjusting the duty ratio of PWM (Pulse Width Modulation) control.
To describe the general structure of an electrical power steering system with reference to
FIG. 1
, a shaft
2
of a steering wheel
1
is connected to tie rod
6
of the steering wheels by way of speed reducer gear
3
, universal joints
4
a
and
4
b,
and rack and pinion mechanism
5
. A torque sensor
10
for detecting the steering torque of steering wheel
1
is disposed to the shaft
2
, and a motor
20
for assisting the steering power of the steering wheel
1
is linked to the shaft
2
by intervening a clutch
21
and speed reducer gear
3
. Power is supplied by way of ignition key
11
and relay
13
from a battery
14
to a control unit
30
, which controls the power steering system; the control unit
30
calculates steering assistance command value I of the assistance command based on steering torque T detected by the torque sensor
10
and speed V detected by a vehicle speed sensor
12
, and controls the current supplied to the motor
20
based on the calculated steering assistance command value I. The clutch
21
is turned on and off by the control unit
30
, and is on (engaged) during normal operating conditions. When the power steering system is determined by the control unit
30
to have failed, and when the power (voltage Vb) from the battery
14
is turned off by the ignition key
11
and the relay
13
, the clutch
21
is off (disengaged).
The control unit
30
consists primarily of a CPU; the general functions executed by a program in the CPU are as shown in FIG.
2
. For example, phase compensator
31
does not indicate a phase compensator provided as a separate hardware component, but rather indicates a phase compensation function executed by the CPU. To describe the functions and operation of the control unit
30
, steering torque T detected and inputted by the torque sensor
10
is phase-compensated by a phase compensator
31
to improve the stability of the steering system, and the phase-compensated steering torque TA is then inputted to a steering assistance command value calculator
32
. The steering assistance command value calculator
32
determines the steering assistance command value I, which is the control target of the current supplied to the motor
20
, based on the input steering torque TA and speed V. The steering assistance command value I is inputted to a subtracter
30
A and a differential compensator
34
of the feed forward system for improving response speed, the deviation (I−i) of the subtracter
30
A is inputted to a proportional calculator
35
, and the proportional output is inputted to an adder
30
B and an integral operator
36
. Outputs from the differential compensator
34
and the integral operator
36
are also additively inputted to the adder
30
B, and the resulting current control value E, that is, the sum obtained by the adder
30
B, is inputted to a motor drive circuit
37
as the motor drive signal. Motor current i of the motor
20
is detected by a motor current detecting circuit
38
, and the motor current i is inputted to the subtracter
30
A and is fed back.
To describe the structure of the motor drive circuit
37
with reference to
FIG. 3
, the motor drive circuit
37
comprises an FET gate driver
371
for driving the gates of field effect transistors (FET) FET
1
to FET
4
based on the current control value E from the adder
30
B, an H-bridge consisting of FET
1
to FET
4
, and a step-up power supply
372
for driving the high side of FET
1
and FET
2
. FET
1
and FET
2
are switched on and off by a PWM (pulse width modulation) signal of duty ratio D
1
determined based on the current control value E, and the size of current Ir actually flowing to the motor
20
is controlled. FET
3
and FET
4
are driven by a PWM signal of duty ratio D
2
, defined by a specific first degree equation (D
2
=a·D
1
+b, where a and b are constants) in the range where duty ratio D
1
is small, and go on/off according to the direction of the motor
20
rotation, which is determined by the sign of the PWM signal, after duty ratio D
2
reaches 100%. For example, when continuity exists through FET
3
, current flows through FET
1
, the motor
20
, FET
3
, and resistor R
1
, and forward current flows to the motor
20
. When continuity exists through FET
4
, current flows through FET
2
, the motor
20
, FET
4
, and resistor R
2
, and reverse current flows to the motor
20
. Therefore, current control value E from the adder
30
B is also PWM-output. The motor current detecting circuit
38
detects the size of the forward current based on the voltage drop at both ends of resistor R
1
, and detects the size in the reverse direction based on the voltage drop at both ends of resistor R
2
. The motor current i detected by the motor current detecting circuit
38
is inputted to the subtracter
30
A for feedback.
In conjunction with the desire for high output in such conventional electrical power steering systems, there has also been demand for systems achieving a sense of high quality steering, as well as a small size due to layout considerations. To achieve a sense of high quality, it is necessary achieve a feeling of smooth steering. The torque ripple of the motor is generally the factor that determines whether smooth steering is achieved, and measures have been conventionally taken to reduce the torque ripple of the motor as a means of achieving smooth steering. However, conventional technology for reducing motor torque ripple typically reduces the center angle of the magnet in the motor or applies a skew angle to the magnet. In either case, if torque output is maintained, overall motor size increases such that the desire for smaller size cannot be satisfied.
On the other hand, rare earth magnets with high residual magnetic flux density have been used in brushless motors in conjunction with demand for high output and small size in conventional electrical power steering systems. However, smooth steering in response to the operator's manipulation of the steering wheel is needed even with brushless motors, and there is therefore a great need to reduce the motor's torque ripple and cogging torque.
A motor with a small skew angle and magnetization in which the magnetic flux density of the magnetization is close to a trapezoidal wave is needed to reduce torque ripple in a square wave drive brushless motor. However, because this need conflicts with reducing motor cogging torque, it is in reality difficult to reduce torque ripple. Furthermore, as a result of a degraded steering feel due to the effect of torque variation resulting from a shift in the commutation position (due to variation in the relative positions of magnetization and the coil, and the effect of position detection precision, as a result of such factors as armature reaction), and the effect of torque variations due to current variation during commutation, practical use of a high torqu
Chen Hui
Endo Shuji
Masih Karen
NSK Ltd.
Sughrue & Mion, PLLC
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