Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2001-11-08
2003-08-19
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S489000, C180S443000
Reexamination Certificate
active
06608457
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system for controlling electric power steering.
2. Description of Related Art
FIG. 8
shows a conventional electric power steering system, and
FIG. 9
is a block diagram showing the control for the electric power steering system.
As shown in
FIG. 8
, a pinion
2
is placed on one end of an input shaft
1
associated with a steering wheel W. A rack
5
is provided on a rod
4
having both end portions with which wheels
3
R,
3
L are respectively associated. The pinion
2
on the input shaft
1
engages the rack
5
on the rod
4
. An electric motor
6
is associated with a reduction gear
7
having an output shaft on which a pinion is provided. The pinion also engages the rack
5
on the rod
4
. A controller CT is connected to steering-torque detection means
8
for detecting steering torque acting on the input shaft
1
, and vehicle-speed detection means
9
for detecting vehicle-speed.
As shown in
FIG. 9
, the controller CT includes basic assist-command-value determination means
10
for determining a basic assist command-value in response to a steering torque signal from the above sensor
8
, and differentiation command-value determination means
11
for differentiating the steering torque signal.
The controller CT further includes a motor-current control means
12
which receives inputs of the basic assist command value from the basic assist command-value determination means
10
, a differentiation command value from the differentiation command-value determination means
11
, and a vehicle-speed signal from the sensor
9
. The motor-current control means
12
applies motor current to the electric motor
6
in response to the basic assist command value, differentiation command value and vehicle-speed signal.
In other words, the motor-current control means
12
determines the torque current for the electric motor
6
in response to a torque-current control signal consisting of the basic assist command value and the differentiation command value. Therefore, the motor-current control means
12
applies the torque current responsive to the steering torque signal to the electric motor
6
.
Further, the motor-current control means
12
multiplies the torque current by a gain in response to the vehicle-speed signal. For example, the gain is determined to increase when traveling at low speeds. For that, the torque current for the electric motor
6
can be increased when traveling at low speeds to increase the assist force.
In this way, a motor torque current according to the steering torque and vehicle-speed flows into the electric motor
6
, and motor torque according to the current is generated as an assist force.
The aforementioned system for controlling the electric power steering is designed such that the electric motor
6
generates the assist force in response to the traveling speed or steering force. The control of the electric motor
6
for generating the assist force will be explained next on the basis of the electric-motor characteristics.
For this control, the electric motor
6
controls the torque current under a constant strength of a field. Typically, permanent magnets are employed to obtain the constant strength of the field. Due to the constant strength of the field, regarding the characteristics of the electric motor
6
, the relationship between the number of revolutions N of the motor and motor torque T forms a straight line with a uniform gradient as shown in FIG.
10
. As is clear from
FIG. 10
, as the number of motor-revolutions N is increased, the motor torque T decreases, and as the motor torque T is increased, the number of motor-revolutions N reduces.
On the other hand, the steering speed of the wheels
3
R,
3
L is proportional to the number of motor-revolutions N represented by the vertical axis of the graph in FIG.
10
. The assist force of the power steering is proportional to the motor torque represented by the horizontal axis of the graph in FIG.
10
.
In
FIG. 10
, if the motor is needed to have torque Ta for generating a constant assist force, the torque Ta can be provided up to the number of motor-revolutions Na.
However, when the steering speed is further increased, the number of motor-revolutions should exceed the value Na, and therefore the relationship changes from the point a on the graph in the direction of the arrow, resulting in a reduction in the motor torque.
The number of motor-revolutions increases with the increase of the steering speed in this way, whereas the assist force decreases, thus putting more load on the driver in operating the steering wheel, that is presenting the problem of faulty steering response.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a system for controlling electric power steering in which the strength of the field in an electric motor is controlled to control the output of the electric motor.
A system for controlling an electric power steering according to a first aspect of the present invention includes: an electric motor for generating an assist force; steering-torque detection means for detecting steering torque; basic assist command-value determination means for determining a basic assist command value on the basis of the steering torque signal; differentiation command-value determination means for determining a differentiation command value on the basis of the steering torque signal; vehicle-speed detection means for detecting vehicle-speed; field-current determination means for determining field-current for the electric motor in response to a vehicle-speed signal; and motor-current control means for controlling the electric motor, in which the motor-current control means controls the output of the electric motor in response to a field-current control signal from the field-current determination means and a torque-current control signal for the electric motor determined on the basis of the basic assist command value and the differentiation command value.
In a second aspect premising on the first aspect, motor angular-velocity detection means for detecting an angular velocity of the electric motor is further included, and the motor angular-velocity signal is input to the field-current determination means, and the field-current determination means determines field-current in response to the motor angular-velocity signal and the vehicle-speed signal.
A system for controlling an electric power steering according to a third aspect of the present invention includes: an electric motor for generating an assist force; steering-torque detection means for detecting steering torque; basic assist command-value determination means for determining an basic assist command value on the basis of the steering torque signal; differentiation command-value determination means for determining a differentiation command value on the basis of the steering torque signal; field-current determination means for determining field current for the electric motor in response to a torque-current control signal for the electric motor based on the basic assist command value and the differentiation command value; vehicle-speed detection means for detecting vehicle speed; and motor-current control means for controlling the electric motor, in which the motor-current control means controls output of the electric motor in response to a signal resulting from multiplying the torque-current control signal by gain fixed by the vehicle-speed signal, and a field-current control signal from the field-current determination means.
In a fourth aspect premising on the third aspect, motor angular-velocity detection means for detecting an angular velocity of the electric motor is further included, and the motor angular-velocity signal is input to the field-current determination means, and the field-current determination means determines field-current in response to the motor angular-velocity signal and the torque-current control signal.
According to the first aspect, the field-current determination means is provided for determining field cur
Kayaba Industry Co. Ltd.
Ro Bentsu
Steinberg & Raskin, P.C.
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