Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2001-08-07
2004-06-01
Fletcher, Marlon (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S433000
Reexamination Certificate
active
06744232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control unit for an electric power steering apparatus that provides steering assist force by motor to the steering system of an automobile or a vehicle. The invention particularly relates to a control unit for an electric power steering apparatus that generates a current dither signal for removing the influence of static friction in a completely stationary status of the motor.
2. Description of the Related Art
An electric power steering apparatus for applying assist load to the steering apparatus of an automobile or a vehicle using turning effort of a motor applies the driving force of the motor to a steering shaft or a rack axis based on a transmission mechanism like gears or belts via a reduction gear. Such a conventional electric power steering apparatus carries out a feedback control of a motor current for accurately generating an assist torque(a steering assist torque). The feedback control is for adjusting a motor application voltage so as to minimize a difference between a current control value and a motor current detection value. The motor application voltage is generally adjusted based on a duty ratio of a PWM (Pulse Width Modulation) control.
A general structure of an electric power steering apparatus will be explained with reference to
FIG. 1. A
shaft
2
of a steering wheel
1
is connected to a tie rod
6
of running wheels through a reduction gear
3
, universal joints
4
a
and
4
b
, and a pinion rack mechanism
5
. The shaft
2
is provided with a torque sensor
10
for detecting a steering torque of the steering wheel
1
. A motor
20
for assisting the steering force of the steering wheel
1
is connected to the shaft
2
through a clutch
21
and the reduction gear
3
. A control unit
30
for controlling the power steering apparatus is supplied with power from a battery
14
through an ignition key
11
. The control unit
30
calculates a steering assist command value I of an assist command based on a steering torque T detected by the torque sensor
10
and a vehicle speed V detected by a vehicle speed sensor
12
. The control unit
30
then controls a current to be supplied to the motor
20
based on the calculated steering assist command value I. The clutch
21
is ON/OFF-controlled by the control unit
30
, and is kept ON (connected) in an ordinary operation status. When the control unit
30
has decided that the power steering apparatus is in failure, and also when the power source(voltage Vb) of the battery
14
has been turned OFF with the ignition key
11
, the clutch
21
is turned OFF (disconnected).
The control unit
30
is mainly composed of a CPU.
FIG. 2
shows general functions to be executed based on a program inside the CPU. For example, a phase compensator
31
does not show a phase compensator as independent hardware, but shows a phase compensation function to be executed by the CPU.
Functions and operation of the control unit
30
will be explained below. A steering torque T detected by the torque sensor
10
and then input is phase-compensated by the phase compensator
31
for increasing the stability of the steering system. The phase-compensated steering torque TA is inputted to a steering assist command value calculator
32
. A vehicle speed V detected by the vehicle speed sensor
12
is also inputted to the steering assist command value calculator
32
. The steering assist command value calculator
32
calculates a steering assist command value I as a control target value of a current to be supplied to the motor
20
, based on the inputted steering torque TA and the inputted vehicle speed V. The steering assist command value I is inputted to a subtractor
30
A, and is also inputted to a differential compensator
34
of a feedforward system for increasing a response speed. A difference (I−i) calculated by the subtractor
30
A is inputted to a proportional calculator
35
, and is also inputted to an integration calculator
36
for improving the characteristic of a feedback system. Outputs from the differential compensator
34
and the integration calculator
36
are inputted to an adder
30
B, and are added together by the adder
30
B. A result of the addition by the adder
30
B is obtained as a current control value E, and this is inputted to a motor driving circuit
37
as a motor driving signal. A motor current value i of the motor
20
is detected by a motor current detecting circuit
38
, and the motor current value i is inputted to the subtractor
30
A and is fed back.
According to a widely-distributed hydraulic power steering apparatus, the apparatus has a characteristic that the friction of a cylinder section increases in proportion to a cylinder pressure P (a horizontal axis T represents a steering torque), as shown in FIG.
3
. The apparatus has hysteresis because of the frictional characteristic. When a vehicle is cornering, for example, the hysteresis prevents the steering wheel from being suddenly returned by a self-aligning torque. This improves the steering of the driver.
FIG. 4
shows this status. When the steering torque T has suddenly changed by A T, the cylinder pressure P changes by P1 in the absence of hysteresis. However, in the presence of hysteresis, the cylinder pressure P changes by P2(<P1). Therefore, in the presence of hysteresis, it is possible to make smooth the change in the cylinder pressure P in relation to a change in the steering torque T. It has been known that the hysteresis width changes according to a size of friction. In the case of a rubber packing of a hydraulic cylinder, the rubber is compressed along an increase in the cylinder pressure. The hysteresis width increases based on an increase in Coulomb friction. It is important for the steering that the driver feels strong self-aligning torque at a neutral point, and does not feel so strong self-aligning torque when the vehicle is cornering. In this sense, it is ideal that, like in the hydraulic power steering apparatus, the friction(hysteresis) becomes small in an area of a small steering angle &thgr;, and the friction (hysteresis) becomes large in an area of a large steering angle &thgr;.
On the other hand, according to an electric power steering apparatus, the apparatus has constant friction independent of the assist torque T, as shown in FIG.
5
.
FIG. 6
shows a relationship between a motor angular velocity &ohgr; and friction F. Static friction works in an area “A”(−&ohgr;1 to +&ohgr;1) where the motor angular velocity &ohgr; is small. In other angular velocity areas (lower than −&ohgr;1, and higher than +&ohgr;1), the friction F appears as an addition of Coulomb friction B of a constant value (+F2, −F2) and viscous friction C that increases two dimensionally along an increase in the motor angular velocity &ohgr;.
From the relationship between the motor angular velocity &ohgr; and the friction F shown in
FIG. 6
, the friction characteristic becomes discontinuous at a position where the motor angular velocity &ohgr; is near to zero. Therefore, there is an inconvenience that when the steering angle has become completely stationary at certain timing, the driver feels that the steering wheel has been fixed to this position. The electric power steering apparatus carries out various kinds of compensation, including the compensation for the motor inertia for improving the steering performance, and compensation for removing the influence of friction. As the above compensation control is carried out based on the motor angular velocity, it is not possible to remove the influence of a status that the motor is completely stationary. In other words, it is not possible to remove the influence of the static friction of the electric power steering apparatus.
SUMMARY OF THE INVENTION
The present invention has been made in the light of the above situations. It is an object of the present invention to provide a control unit for an electric power steering apparatus capable of obtaining continuous and stable feeling of steeri
Fletcher Marlon
NSK Ltd.
LandOfFree
Control unit for electric power steering apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control unit for electric power steering apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control unit for electric power steering apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3346039