Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2001-09-12
2004-02-17
Morris, Lesley D. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C180S443000, C701S041000
Reexamination Certificate
active
06691818
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 a motor to the steering system of an automobile or a vehicle. The present invention particularly relates to a control unit for an electric power steering apparatus that gives safe and comfortable steering performance by removing the influence of motor inertia and through the provision of continuous steering feeling in a low-cost structure.
2. Description of the Related Art
An electric power steering apparatus that applies auxiliary load to the steering apparatus of an automobile or a vehicle with 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 reduction gears
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 gears
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
mainly comprises 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
determines 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 added together there. 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 this motor current value i is inputted to the subtractor
30
A and is fed back.
An example of a structure of the motor driving circuit
37
will be explained with reference to FIG.
3
. The motor driving circuit
37
comprises an FET (field-effect transistor) gate driving circuit
371
for driving each gate of field-effect transistors FET
1
to FET
4
based on the current control value E from the adder
30
B, an H-bridge circuit composed of the FET
1
to the FET
4
, and a step-up power source
372
for driving a high side of the FET
1
and the FET
2
, respectively. The FET
1
and the FET
2
are ON/OFF controlled by a PWM (Pulse Width Modulation) signal of a duty ratio D1 determined based on the current control value E, thereby to control the size of a current Ir that actually flows to the motor
20
. The FET
3
and the FET
4
are driven by a PWM signal of a duty ratio D2 defined by a predetermined linear functional expression (D2=a·D1+b, where “a” and “b” are constants) in an area where the duty ratio D1 is small. When and after the duty ratio D2 has also reached 100%, the FET
3
and the FET
4
are ON/OFF controlled according to a rotation direction of the motor
20
determined by a sign of the PWM signal.
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.
4
. 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 (SAT). This improves the steering of the driver.
FIG. 5
shows this status. When the steering torque T has suddenly changed by &Dgr;T, the cylinder pressure changes by P1 in the absence of the hysteresis. However, in the presence of the hysteresis, the cylinder pressure changes by P2(<P1). Therefore, in the presence of the 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.
6
. The electric power steering apparatus is characterized in that it has a constant friction characteristic independent of steering force, as the Coulomb friction of the motor mainly rules out. Thus, the hysteresis has a constant width as shown in FIG.
7
. However, the hysteresis width is narrower than the hysteresis width of the hydraulic power steering apparatus during its high-torque time. Therefore, in the electric power steering apparatus, the friction is compensated for in the area of a small steering torq
Chen Hui
Endo Shuji
Morris Lesley D.
NSK Ltd.
Sughrue & Mion, PLLC
Yeagley Daniel
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