Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
2001-08-14
2002-04-30
Camby, Richard M. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C180S443000
Reexamination Certificate
active
06381528
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 is equipped with two controllers of a main and a sub (CPU (Central Processing Unit) or MCU (Micro Controller Unit)) for controlling a motor of a large inertia using a digital servo, the one controller for controlling the motor, and the other for monitoring an abnormality of a driving system, to have improved controllability and safety.
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 mechanism. 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
and a relay
13
. 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
and the relay
13
, 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. 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
. A proportional output from the proportional calculator
35
is inputted to an adder
30
B, and is also inputted to an integration calculator
36
for improving the characteristic of the feedback system. Outputs from the differential compensator
34
and the integration calculator
36
are also inputted to the 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 command 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.
As explained above, conventionally, one CPU has calculated the current command value based on the torque signal (the steering torque T) and the current detection value i. The CPU then has controlled the driving of the motor based on the current command value. In this case, a signal in the steering torque direction is generated to the determined motor current driving direction, based on the torque signal using hard logic or other CPU. Only when the signal in the steering torque direction coincides with the motor driving direction, the motor is driven. In the case of detecting an abnormality of the motor driving system, the time taken from a detection to a fixing of an abnormality is constant, and this has not always been accurate.
Further, conventionally, one CPU is provided with an external WDT (Watch Dog Timer), and the CPU inputs a clear pulse to the WDT to monitor a runaway of the CPU. When a clear pulse has not been inputted within a predetermined time, a reset signal is outputted to the CPU from the DWT, thereby to restart the CPU. Further, when the system uses two CPUs, the two CPUs mutually output pulses of predetermined periods, and mutually monitor the periods of the pulses, thereby to monitor a runaway of the other CPU.
According to the above electric power steering apparatus, the inertia of the motor becomes large along with an increase in the output of the motor. Thus, in order to secure the steering characteristic, it has become necessary to compensate for the inertia of the motor. Because of the control of compensating for the inertia, the direction of the torque signal has come not to coincide with the actual driving direction of the motor, unlike the conventional practice. As a result, in spite of the fact that the control of the inertia compensation function operates normally, the other controller outputs a torque direction signal based on the torque signal, and there occurs the inconvenience that the motor driving is stopped temporarily. In other words, the direction interlocking based on the direction of the steering torque that has been practiced conventionally has become not adaptable to the actual system.
Further, conventionally, in the case of detecting an abnormality of the motor driving system, the time taken from a detection of an abnormality to a fixing of the abnormality has been constant. However, in the case of detecting an abnormality by obtaining a difference between a current command value and a motor current detection value, it is considered that the existence of a large difference is clearly abnormal. In this case, it is necessary to quickly stop the system. When a small difference has been detected, an erroneous detection due to the influence of noise or the like is considered. In this case, it is necessary to take time for fixing the abnormality.
Further, according to the conventional practice of monitoring a runaway of the CPU by using an external WDT, the CPU generates a runaway when a program does not operate normally, and the CPU is restarted based on a reset signal from the WDT. However, in the case of a failure that the program generates a runway at the same place again, the output status and the stop status of the motor are generated alternately. This has brought about a dangerous status to the driver.
When the system has two contr
Itakura Yusuke
Kawada Hideaki
Sakaguchi Toru
Camby Richard M.
NSK Ltd.
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