Electricity: motive power systems – Positional servo systems – With protective or reliability increasing features
Reexamination Certificate
2000-06-29
2002-04-16
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Positional servo systems
With protective or reliability increasing features
C318S432000, C318S459000, C180S443000
Reexamination Certificate
active
06373217
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control apparatus of an electric power steering system for applying a steering assist force by means of a motor to a steering system of a car or a vehicle, and particularly to a control apparatus of an electric power steering system with improved control characteristics and safety having two control parts (CPUs or MCUs: Micro Controller Units), a main and a sub, and carrying out motor control with one and monitoring for abnormality of the drive system with the other.
2. Description of the Prior Art
An electric power steering system for urging with an assisting load a steering system of a car or a vehicle using rotational force of a motor applies as an assisting load to a steering shaft or a rack shaft a driving force of the motor by means of a transmission mechanism such as gears or a belt by way of a speed-changer. This electric power steering system of prior art, to generate an assist torque (steering assist torque) correctly, performs feedback control of a motor current. The feedback control regulates a motor impressed voltage so that the difference between a current command value and a motor current detected value decreases, and regulation of the motor impressed voltage is generally carried out by regulation of a duty ratio of PWM (Pulse Width Modulation) control.
Here, showing in FIG.
1
and explaining a common construction of an electric power steering system, a shaft
2
of a steering wheel
1
is connected to a tie rod
6
of steering vehicle wheels via universal joints
4
a
and
4
b
and a rack and pinion mechanism
5
. A torque sensor
10
for detecting a steering torque in the steering wheel
1
is provided on the shaft
2
, and a motor
20
for assisting a steering force of the steering wheel
1
is connected to the shaft
2
by way of a clutch
21
and reduction gears
3
. To a control unit
30
for controlling the power steering system power is supplied from a battery
14
via an ignition key
11
and a relay
13
, and the control unit
30
performs computation of a steering assist command value I of an assist command on the basis of a 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 has been determined by the control unit
30
that the power steering system has failed, and when due to the ignition key
11
or the relay
13
the power supply (voltage Vb) from the battery
14
is OFF, the clutch
21
is made OFF (disengaged).
The control unit
30
consists mainly of a CPU, and general functions executed by a program inside the CPU are shown in FIG.
2
. For example a phase compensator
31
does not denote a phase compensator consisting of independent hardware, but rather denotes a phase compensation function executed by the CPU. Explaining the functions and operation of the control unit
30
, the steering torque T detected by the torque sensor
10
and inputted is phase-compensated by the phase compensator
31
to raise the stability of the steering system, and a phase-compensated steering torque TA is inputted to a steering assist command value computing element
32
. The vehicle speed V detected by the vehicle speed sensor
12
is also inputted to the steering assist command value computing element
32
. The steering assist command value computing element
32
determines on the basis of the inputted steering torque TA and vehicle speed V a steering assist command value I, which is a control target value of the current supplied to the motor
20
. The steering assist command value I is inputted to a subtractor
30
A and is also inputted to a differential compensator
34
of a feedback system, the differential (I-i) of the subtractor
30
A is inputted to a proportional computing element
35
, and the proportional output thereof is inputted to an adder
30
B and is also inputted to an integral computing element
36
for improving the characteristics of the feedback system. The outputs of the differential compensator
34
and the integral computing element
36
are also additively inputted to the adder
30
B, and a current command value E, which is the addition result in the adder
30
B, is inputted to a motor drive circuit
37
as a motor drive signal. The 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.
As described above, in the past a current command value has been computed on the basis of a torque signal (the steering torque T) and the current detected value i with one CPU, and the motor drive-controlled on the basis of that current command value. In this case, with respect to the determined motor current drive direction, by hard logic or another CPU a steering torque direction signal has been generated on the basis of the torque signal, and the motor has been driven only when this steering torque direction signal and the motor drive direction match. When detecting an abnormality of the motor drive system, the time from detecting to confirming the abnormality has been fixed and not correct always.
In the past the one CPU has been provided with an externally added WDT (Watch Dog Timer), and racing of the CPU has been monitored by clear pulses from the CPU being inputted to the WDT. If no clear pulse is inputted within a predetermined time, a reset signal has been outputted to the CPU from the WDT and the CPU has thereby been restarted. Also, in a system using two CPUs they have each outputted a pulse of a predetermined period to the other and monitored racing of the other CPU by mutually monitoring the period of the pulse.
In an electric power steering system of the kind described above, as the output of the motor increases the inertia of the motor increases, and to secure steering characteristics the need to compensate for the inertia of the motor has emerged. As a result of the control compensating for inertia, the direction according to the torque signal which has been carried out in the past and the direction in which the motor is actually driven have come to not match. Consequently, the problem has arisen that notwithstanding that the control containing the inertia compensation function is operating normally, due to another control part outputting a torque direction signal based on the torque signal, the motor drive has temporarily been stopped. That is, direction interlock based on the direction of the steering torque, which has been carried out in the past, is becoming unsuited to actual systems.
In the past, when detecting abnormality of the motor drive system, the time from detecting to confirming the abnormality has been fixed. However, when the difference between the current command value and the motor current detected value is obtained to detect abnormality, when there Is a large difference it can be considered that there is clearly abnormality, and it is necessary to stop the system without delay. And, when the difference is small, because a mis-detection caused by the influence of noise or the like is conceivable, it is necessary to take time to confirmation.
When racing of the CPU is monitored using an externally added WDT as in the past, when racing arises in the CPU due to the program not operating normally, it is restarted by a reset signal from the WDT. However, in the case of a failure of a kind such that the program causes racing again at a similar place, an outputting state and a stopped state of the motor occur alternately, and have created a dangerous state for the driver.
On the other hand, in a system having two control parts, when one control part performs control computation and the other control part monitors that, to check that the monitoring control part is operating normally it is necessary for the two control parts to m
Endo Shuji
Kawada Hideaki
Sakaguchi Toru
NSK Ltd.
Ro Bentsu
Sughrue & Mion, PLLC
LandOfFree
Control apparatus of electric power steering system 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 apparatus of electric power steering system, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control apparatus of electric power steering system will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2860447