Motor vehicles – Steering gear – With electric power assist
Reexamination Certificate
2000-10-17
2002-12-24
Dickson, Paul N. (Department: 3611)
Motor vehicles
Steering gear
With electric power assist
C180S443000
Reexamination Certificate
active
06497303
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an electric power steering system for controlling steering of a vehicle and, more particularly, to a fail-safe.
2. Background Art
In electric power steering system for controlling steering of a vehicle, control is sometimes suspended by its fail-safe function when any trouble occurs. This control suspension secures steering performance of the vehicle by suspending turning operation of steering wheel with a motor and switching the operation to manual operation mode by the driver. As a specific example of the control suspension, an electric current flowing to the motor which outputs an auxiliary torque is interrupted not only by suspending a motor driving circuit but also by opening a relay for supplying a power in order to interrupt the power supply. However, there is a possibility that contact point of this relay is welded and does not open, and therefore it is necessary to inspect the function of the relay. Japanese Patent No. 2715473 proposes a method for detecting the relay welding.
Described below is such a problem of relay welding with reference to FIG.
8
. In the drawing, reference numeral
1
is a torque sensor for detecting a steering torque. Numeral
2
a
is means for calculating controlled variable, which calculates an auxiliary torque on the basis of the torque information and outputs a control signal, and is a microcomputer (hereinafter referred to as CPU). Numeral
13
is an interface circuit
1
which transmits this-torque information to the CPU
2
a
. Numeral
3
is a motor which is arranged in the steering system and generates the auxiliary torque, and is driven by switching elements
4
to
7
. Numeral
8
is a first driving circuit mainly composed of the switching elements
4
to
7
. Numeral
9
a
is a second driving circuit for driving the switching elements
4
to
7
according to a control signal from the CPU
2
a
. Numeral
10
a
is a voltage monitoring circuit which detects voltages at two ends of the motor
3
, and the outputs therefrom are inputted to the CPU
2
a
as E
1
and E
2
. Numeral
20
is a relay which supplies a power to the motor
3
and the switching elements
4
to
7
, and is driven by a switch driving circuit
21
depending on a signal from the CPU
2
a.
Described now is how to detect welding of the relay
20
. First, contact point of the relay
20
is opened in advance by the switch driving circuit
21
. The switching element
4
or
5
is driven, and the CPU
2
a
inputs the result from the voltage monitoring circuit
10
a
. In this case, the voltage monitoring circuit is an inverter circuit comprising a transistor element, and each driving mode thereof is shown in FIG.
9
.
When the switching element
4
is turned on and the element
5
is turned off, both of the signals of E
1
and E
2
are high (hereinafter referred to as H) under normal condition. All other cases indicate an abnormal condition. In the same manner, when the switching element
4
is turned off and the element
5
is turned on, both of the signals of E
1
and E
2
are H under normal condition, and all other cases indicate an abnormal condition.
FIG. 9
clearly shows that the contact point of the relay is welded except under normal condition. However, the driving circuit
9
a
, voltage monitoring circuit
10
a
, and others are under normal condition.
On the other hand, Japanese Patent Publication (unexamined) No. 62-231871 proposes that a relay is interposed also between the driving circuit and the motor (hereinafter referred to as motor relay).
FIG. 10
is a block diagram in which a motor relay
12
is connected in series to the motor
3
. Diodes are inserted in parallel between the switching elements
4
to
7
. Even if the relay
20
(hereinafter referred to as power supply relay) is opened, the motor is rotated when the driver turns the steering wheel, and a regenerative current flows by means of these diodes. Therefore, in case where one of the switching elements is short-circuited and broken, there arises a condition of different load in the turning direction of the steering wheel depending on the turning direction of the steering wheel or the motor, and this obstructs smooth turning of the steering wheel. To cope with this, the motor relay
12
is added.
It may be obvious to combine the foregoing two inventions, however, actual control unit (hereinafter referred to as ECU) is further provided with a fail-safe function, and this brings about a problem in that it is impossible to easily inspect the motor relay. For example, in consideration of any trouble in the CPU
2
a
, means for inhibiting drive of the motor (hereinafter referred to as motor drive inhibiting means) is required other than the CPU. Such motor drive inhibiting means has made it difficult to inspect the relay. In other words, it is certainly possible to inspect the motor relay so long as the motor drive inhibiting means is in a motor driving condition. But when the motor drive inhibiting means is in a motor drive inhibiting condition, the switching elements are not driven even if the motor relay is inspected and it is impossible to judge whether or not the motor relay is in malfunction.
This problem is now described in more detail with reference to FIG.
10
. Drive inhibition circuit
18
g
acts so that the driving circuit
9
a
may become inoperative in a case where the steering wheel is little turned, i.e., when torque is little produced. A switching element for left turning is suspended during a clockwise turning, while a switching element for right is suspended during a counterclockwise turning.
FIG. 11
is a graph showing the relation between the torque and the output of the torque sensor. The switching element
4
or
7
for clockwise turning is suspended when the torque is smaller than Tq
2
, and the switching element
5
or
6
is suspended when the torque is larger than Tq
1
. Less torque is produced in a range from v
2
to v
1
in torque sensor output voltage. It is therefore unnecessary to drive the motor
3
in this range. Accordingly, the drive inhibition circuit
18
g
, which acts so that the driving circuit
9
a
may become inoperative, is added in addition to the CPU
2
a
. More specifically, the drive inhibition circuit
18
g
in
FIG. 10
judges whether or not the torque sensor output voltage is within v
1
and v
2
through comparison using two comparators
18
b
,
18
c.
When the torque sensor output is larger than v
1
, a transistor
18
d
is turned on and the switching element
5
or
6
is turned off. On the other hand, when the torque sensor output is smaller than v
2
, the circuit is connected so that the transistor
18
e
may be turned on and the switching element
4
or
7
may be turned off. When the drive inhibition circuit
18
g
is working, i.e., when the transistor
18
d
or
18
e
is turned on, the switching elements
4
,
5
or
6
,
7
still remain in the off condition even if the CPU
2
a
outputs a control signal. Therefore, it is impossible to inspect the motor relay
12
in this condition.
As described above, the conventional system has a problem in that the drive inhibition circuit obstructs inspection of the relay and the relay cannot be easily inspected.
SUMMARY OF THE INVENTION
The present invention was made to resolve the above-discussed problems and has an object of providing an electric power steering system in which the fail-safe function is improved making it possible to detect a trouble.
An electric power steering system according to the invention, which possesses torque detecting means for detecting a steering torque, controlled variable calculating means for calculating a controlled variable, by which a motor conducts steering of a vehicle, on the basis of the torque information from the torque detecting means, and driving means for driving the motor by switching elements upon receiving the controlled variable, comprises: drive restricting means for restricting drive which restricts at least a part of operation of the driving means on the basis of the torque inf
Kikuta Kazuo
Nishimura Hiroshi
Ohtagaki Shigeki
Takatsuka Yuji
Dickson Paul N.
Lum L.
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