Electricity: motive power systems – Motor load – armature current or force control during...
Reexamination Certificate
1999-12-14
2001-07-31
Ip, Paul (Department: 2837)
Electricity: motive power systems
Motor load, armature current or force control during...
C318S132000, C318S254100, C318S258000, C318S269000, C318S273000, C318S430000, C318S432000, C318S434000
Reexamination Certificate
active
06268708
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a control device of an electric power steering system which applies a steering assist force from a motor to the steering gear of an automotive vehicle, and particularly to a control device of an electric power steering system wherein an efficient and economical motor protection measure is realized by a protection function being divided into two stages in correspondence with over-current values.
2. Description of the Related Art
An electric power steering system using a rotational force from a motor to apply an assisting force to the steering gear of an automotive vehicle applies a driving force from the motor to a steering shaft or a rack shaft via a speed-reducer by means of a transmission mechanism such as a gear or a belt. In this kind of electric power steering system in related art, to generate an assist torque (steering assist torque) correctly, feedback control of the motor current is carried out. Feedback control adjusts the voltage impressed on the motor so that a detected motor current value approaches a current control value, and the adjustment of the voltage impressed on the motor is generally carried out by adjustment of a PWM (Pulse Width Modulation) control duty ratio.
The general makeup of an electric power steering system is shown in
FIG. 1. A
shaft
2
of a steering wheel
1
is connected via universal joints
4
a,
4
b
and a rack and pinion mechanism
5
to tie rods
6
of steerable wheels. A torque sensor
10
for detecting a steering torque from the steering wheel
1
is provided on the shaft
2
, and a motor
20
for augmenting the steering force of the steering wheel
1
is connected to the shaft
2
by way of a clutch
21
and speed-reducing gears
3
. Power is supplied from a battery
14
through an ignition switch
11
and a relay
13
to a control unit
30
for controlling the power steering system. The control unit
30
calculates a steering assist command value I of an assist command on the basis of the steering torque T detected by the torque sensor
10
and a vehicle speed V detected by a vehicle speed sensor
12
, and controls a current supplied to the motor
20
on the basis of the computed steering assist command value I. The clutch
21
is ON/OFF-controlled by the control unit
30
, and in a normal operating state is ON (engaged). When it is determined by the control unit
30
that the power steering system has failed, and when the power supply (at voltage Vb) from the battery
14
is switched OFF by the ignition switch
11
or the relay
13
, the clutch
21
is switched OFF (disengaged). The relay
13
is switched OFF at times of emergency.
The control unit
30
consists mainly of a CPU, and general functions executed by a program inside this CPU are shown in FIG.
2
. In
FIG. 2
, for example the phase compensator
31
does not denote a phase compensator consisting of independent hardware, but rather denotes a phase compensation function executed by the CPU. The functions and operation of the control unit
30
will now be described. The steering torque T detected and inputted by the torque sensor
10
is phase-compensated by the phase compensator
31
to raise the stability of the steering gear, and a steering torque TA is inputted to a steering assist command value calculator
32
. The 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, which is a control target value of the current supplied to the motor
20
, on the basis of the inputted steering torque TA and vehicle speed V, and the steering assist command value calculator
32
is provided with a memory
33
. The memory
33
holds steering assist command values I corresponding to steering torques with the vehicle speed V as a parameter, and is used in the computation of the steering assist command value I carried out by the steering assist command value calculator
32
. The steering assist command value I is inputted to a subtractor
30
A and to a differential compensator
34
of a feed-forward line for raising response speed; a difference (I−i) from the subtractor
30
A is inputted to a proportional computing element
35
, and a proportional output thereof is inputted to an adder
30
B and is also inputted to an Integral compensator
36
for improving the characteristics of a feedback line. The outputs of the differential compensator
34
and the integral compensator
36
are also additively inputted to the adder
30
B, and a current control value E, which is the addition result of the adder
30
B, 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 fed back by being inputted to the subtractor
30
A.
An example of the construction of the motor driving circuit
37
is shown in FIG.
3
. This motor driving circuit
37
has a FET gate driving circuit
371
for driving the gates of field effect transistors (FETs) FET
1
through FET
4
by way of a gate circuit
373
on the basis of the current control value E from the adder
30
B, an H-bridge circuit made up of the FETs FET
1
through FET
4
, and a boosted power supply
372
for driving the high sides of the FETs FET
1
and FET
2
. The FET gate driving circuit
371
and the boosted power supply
372
constitute a motor control circuit
37
A and the H-bridge circuit, the gate circuit
373
and current detection resistors R
1
, R
2
constitute a motor driving circuit proper
37
B. PWM signals from the FET gate driving circuit
371
are normally supplied directly through the gate circuit
373
to the FETs FET
1
through FET
4
, but when a cutoff signal CS is inputted all the PWM signals are cut off. The FETs FET
1
and FET
2
are switched ON and OFF by a PWM (Pulse Width Modulated) signal with a duty ratio D1 determined on the basis of the current control value E, and the size of a current Ir actually flowing through the motor
20
is thereby controlled. The FETs FET
3
and FET
4
are driven by a PWM signal with a duty ratio D2 defined by a predetermined linear function (D2=a·D1 + b, where “a” and “b” are constants) when the duty ratio D1 is small, and after the duty ratio D2 also reaches 100% are switched ON and OFF in correspondence with the rotation direction of the motor
20
determined by the sign of the PWM signal.
In this kind of electric power steering system, when while the motor is being driven there is a failure such as shorting or grounding of the motor
20
or the motor driving circuit proper
37
B, an over-current arises in the motor
20
or the motor driving circuit proper
37
B, and if left alone would result in an accident such as a fire. Because of this, an over-current detecting circuit
39
has been provided as shown in
FIG. 4
; any over-current has been detected by means of software or hard logic, and the motor current output has been stopped by the cutoff signal CS being inputted to the gate circuit
373
in the motor driving circuit proper
37
B.
However, when, in steering around an acute turn or the like, the wheel lock end is strongly hit as shown in
FIG. 5
, an over-current arises in the motor
20
and the motor driving circuit proper
37
B due to the sudden change in steering. That is, when the steering is being turned at high speed, the duty ratio of PWM is at nearly 100% to compensate for counter-electromotive force of the motor
20
. However, as a result of the rack hitting the rack end in that instant the rotation of the steering suddenly decreases, and the rotation speed becomes substantially zero. In the CPU sampling time, the duty ratio of PWM is still almost 100% of what it was before. Consequently, a current close to the lock current flows through the motor
20
, and also when current control is realized by means of an analog circuit, due to lag of the response time of the current control,
Endo Shuji
Kawada Hideaki
Ip Paul
NSK Ltd.
Smith Tyrone
Sughrue Mion Zinn Macpeak & Seas, PLLC
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