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Claims  |
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I claim:
1. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a vehicle speed of
said motor vehicle, steering torque detecting means for detecting a
steering torque applied to a steering mechanism of said motor vehicle, and
an electric motor operatively coupled to said steering mechanism through
the medium of coupling effort control means, wherein an assist torque is
applied to said steering mechanism by driving said electric motor in
dependence on said vehicle speed,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value which corresponds to the
vehicle speed detected by said vehicle speed detecting means;
load torque arithmetic means for arithmetically determining a load torque
on the basis of at least a steering torque detected by said steering
torque detecting means;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value while decreasing the coupling effort of said coupling
effort control means wherein said coupling effort control means comprises
an electric clutch and wherein when the coupling effort of said coupling
effort control means is decreased, said electric clutch assumes a
semi-coupling state.
2. A control apparatus according to claim 1, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
3. A control apparatus according to claim 1, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
4. A control apparatus according to claim 1, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
5. A control apparatus according to claim 1, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
6. A control apparatus according to claim 1,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
7. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value in dependence on the vehicle
speed detected by said vehicle speed detecting means;
assist torque determining means for determining assist torque applied by
said electric motor;
load torque arithmetic means for arithmetically determining a load torque
on the basis of a steering torque detected by said steering torque
detecting means and on the determined assist torque;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value while decreasing the coupling effort of said coupling
effort control means.
8. A control apparatus for a motor-driven power steering system according
to claim 7, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
9. A control apparatus according to claim 7, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
10. A control apparatus according to claim 7, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
11. A control apparatus according to claim 7, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
12. A control apparatus according to claim 7,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
13. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value and a reference time in
dependence on the vehicle speed detected by said vehicle speed detecting
means;
load torque arithmetic means for arithmetically determining a load torque
on the basis of at least a steering torque detected by said steering
torque detecting means;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operating of said electric
motor when said load torque exceeds said abnormal torque decision
reference value and when in duration in which said load torque exceeds
said abnormal torque decision reference value is longer than said
reference time, while decreasing the coupling effort of said coupling
effort control means.
14. A control apparatus according to claim 13, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
15. A control apparatus according to claim 13, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
16. A control apparatus according to claim 13, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of the voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
17. A control apparatus according to claim 13, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
18. A control apparatus according to claim 13,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
19. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value and a reference time in
dependence on the vehicle speed detected by said vehicle speed detecting
means;
assist torque determining means for determining assist torque applied by
said electric motor;
load torque arithmetic means for arithmetically determining a load torque
on the basis of a steering torque detected by said steering torque
detecting means and on the determined assist torque;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value and when duration in which said load torque exceeds said
abnormal torque decision reference value is longer than said reference
time while decreasing the coupling effort of said coupling effort control
means.
20. A control apparatus according to claim 19, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
21. A control apparatus according to claim 19, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
22. A control apparatus according to claim 19, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of the voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
23. A control apparatus according to claim 19, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
24. A control apparatus according to claim 19,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
25. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value in dependence on the vehicle
speed detected by said vehicle speed detecting means;
load torque arithmetic means for arithmetically determining a load torque
on the basis of at least a steering torque detected by said steering
torque detecting means;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value while controlling the coupling effort of said coupling
effort control means so that the steering torque value detected by said
steering torque detecting means does not exceed a predetermined value at
which a steering wheel of said motor vehicle can be manipulated by a
driver wherein said coupling effort control means comprises an electric
clutch and wherein when the coupling effort of said coupling effort
control means is decreased, said electric clutch assumes a semi-coupling
state.
26. A control apparatus according to claim 25, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
27. A control apparatus for a motor-driven power steering system according
to claim 25, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
28. A control apparatus according to claim 25, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
29. A control apparatus according to claim 25, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
30. A control apparatus for a motor-driven power steering system according
to claim 25,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
31. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value in dependence on the vehicle
speed detected by said vehicle speed detecting means;
assist torque determining means for determining assist torque applied by
said electric motor;
load torque arithmetic means for arithmetically determining a load torque
on the basis of a steering torque detected by said steering torque
detecting means and on the determined assist torque;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value while controlling the coupling effort of said coupling
effort control means so that the steering torque value detected by said
steering torque detecting means does not exceed a predetermined value at
which a steering wheel of said motor vehicle can be manipulated by a
driver.
32. A control apparatus according to claim 31, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
33. A control apparatus according to claim 31, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
34. A control apparatus according to claim 31, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
35. A control apparatus according to claim 31, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
36. A control apparatus for a motor-driven power steering system according
to claim 31,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
37. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value and a reference time in
dependence on the vehicle speed detected by said vehicle speed detecting
means;
load torque arithmetic means for arithmetically determining a load torque
on the basis of at least a steering torque detected by said steering
torque detecting means;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value and when duration in which said load torque exceeds said
abnormal torque decision reference value is longer than said reference
time while controlling the coupling effort of said coupling effort control
means so that the steering torque value detected by said steering torque
detecting means does not exceed a predetermined value at which a steering
wheel of said motor vehicle can be manipulated by a driver.
38. A control apparatus according to claim 37, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
39. A control apparatus according to claim 37, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
40. A control apparatus according to claim 37, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
41. A control apparatus according to claim 37, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
42. A control apparatus for a motor-driven power steering system according
to claim 37,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero.
43. In a motor-driven power steering system for a motor vehicle which
includes vehicle speed detecting means for detecting a speed of said motor
vehicle, steering torque detecting means for detecting a steering torque
applied to a steering mechanism of said motor vehicle, and an electric
motor operatively coupled to said steering mechanism through the medium of
coupling effort control means, wherein an assist torque is applied to said
steering mechanism by driving said electric motor,
an apparatus for controlling said motor-driven power steering system,
comprising:
abnormal torque decision reference value arithmetic means for determining
an abnormal torque decision reference value in dependence on the vehicle
speed detected by said vehicle speed detecting means;
assist torque determining means for determining assist torque applied by
said electric motor;
load torque arithmetic means for arithmetically determining a load torque
on the basis of at least a steering torque detected by said steering
torque detecting means and on the determined assist torque;
torque comparison means for comparing said load torque with said abnormal
torque decision reference value; and
motor fault monitor/control means for stopping operation of said electric
motor when said load torque exceeds said abnormal torque decision
reference value and when duration in which said load torque exceeds said
abnormal torque decision reference value is longer than said decision
reference time while controlling the coupling effort of said coupling
effort control means so that the steering torque value detected by said
steering torque detecting means does not exceed a predetermined value at
which a steering wheel of said motor vehicle can be manipulated by a
driver.
44. A control apparatus for a motor-driven power steering system according
to claim 43, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein when said motor rotation decision means decides that said electric
motor is rotating, operation of said motor fault monitor/control means is
inhibited.
45. A control apparatus according to claim 43, further comprising:
motor voltage detecting means for detecting a voltage applied to said
electric motor;
motor current detecting means for detecting a motor current flowing through
said electric motor; and
motor rotation decision means for estimating rotation number of said
electric motor on the basis of said motor voltage and said motor current
to thereby decide whether or not said electric motor rotates;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
46. A control apparatus according to claim 43, further comprising:
motor regeneration voltage detecting means for detecting a voltage
generated by said electric motor; and
motor rotation decision means for deciding on the basis of said voltage
whether or not said electric motor is rotating;
wherein operation of said motor fault monitor/control means is inhibited
when said motor rotation decision means decides that said electric motor
is rotating.
47. A control apparatus according to claim 43, further comprising:
motor rotation decision means for deciding whether said electric motor is
rotating or not;
wherein for a predetermined time from the time point when said motor
rotation decision means decided that said electric motor was rotating,
operation of said motor fault monitor/control means is inhibited.
48. A control apparatus according to claim 43,
wherein said coupling effort control means is constituted by a disk type
electromagnetic clutch having a clutch gap substantially equal to zero. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a motor-driven power steering
system of a motor vehicle or automobile which is adapted for assisting a
driver in manipulating a steering wheel or handle by making use of an
assist torque generated by a reversible motor which is operatively coupled
to the steering system. In particular, the invention is concerned with a
control apparatus for the motor-driven power steering system which allows
smooth transition of steering operation from a motor-aided mode to a
manual mode when a fault or abnormality occurs in the assist torque
generating motor or other parts. The invention is also concerned with an
apparatus for controlling an electromagnetic clutch through which an
assist torque generated by the motor is transmitted to the steering system
such that a coupling effort of the clutch decreases continuously to a
level allowing the manual operation of the steering wheel upon occurrence
of abnormality in the motor. The combination of the motor-driven power
steering system and the control apparatus may also be referred to as the
motor-driven power steering control system.
2. Description of the Related Art
For better understanding of the present invention, description will first
be directed to a motor-driven power steering control system known
heretofore. FIG. 6 is a schematic diagram showing a general arrangement of
a conventional motor-driven power steering control system described in,
for example, Japanese Unexamined Patent Application Publication No.
255273/1987 (JP-A-62-255273) or Japanese Unexamined Patent Application
Publication No. 215461/1988 (JP-A-63-215461).
Referring to the figure, a steering mechanism is comprised of a steering
wheel 1 to which a steering torque is applied by a driver of the motor
vehicle (not shown), a steering shaft assembly 2 for transmission of the
steering torque, universal joints 4 provided for operatively coupling the
individual shafts of the steering shaft assembly 2, a pinion shaft 5
provided at a bottom end of the steering shaft assembly 2, and a rack 6
disposed to mesh with the pinion shaft 5. A steering torque sensor 3 is
provided in association with the steering shaft assembly 2 for detecting a
steering torque T transmitted there through. Further, a steering speed
sensor 7 is provided for detecting a steering speed A at which the
steering wheel 1 is manipulated by the driver.
For the purpose of controlling the steering mechanism, there is provided a
control apparatus 9 which includes a microcomputer for controlling an
electric motor (or reversible motor) 13 and others in a manner which will
be elucidated later on. A vehicle speed sensor 10 is provided for
inputting a vehicle speed V to the control apparatus 9. A power source for
the control apparatus 9 and the electric motor 13 is constituted by an
onboard battery 11, wherein a key switch 12 is interposed between the
control apparatus 9 and the battery 11. The power supplied to the electric
motor 13 is controlled by the control apparatus 9. To this end, an
electromagnetic clutch 14 is disposed at the output side of the electric
motor 13 and adapted to be controlled by the control apparatus 9. The
electromagnetic clutch 14 has an output shaft coupled operatively to a
worm shaft 15 which constitutes a part of a reduction gear unit. Provided
so as to mesh with the worm shaft 15 is a worm wheel 16 meshing with a
pinion shaft 18 which in turn meshes with the rack 6.
As is apparent from the above description, the electric motor 13 is
operatively coupled to the steering mechanism via the electromagnetic
clutch 14 for generating a steering assist torque which is applied to the
steering mechanism.
FIG. 7 is a functional block diagram showing a configuration of the control
apparatus 9. As can be seen in the figure, the control apparatus 9 is
comprised of a current calculating/driving means 91 for calculating a
motor current command value I.sub.0 on the basis of the steering torque T
and the vehicle speed V to thereby drive the electric motor 13 on the
basis of the motor current command value I.sub.0, a clutch control means
92 for changing over a clutch control signal C for the electromagnetic
clutch 14 from an ON-level to an OFF-level when the vehicle speed V
attained a predetermined reference vehicle speed V.sub.0 (e.g. 50 km/h),
and a motor current detecting means 93 which serves for detecting an
actual motor current I of the motor 13, wherein the signal indicative of
the actual motor current I is supplied to the current calculating/driving
means 91.
With the arrangement of the control apparatus 9 described above, the
control apparatus 9 controls a voltage applied to the electric motor 13 so
that a current corresponding to the motor current command value I.sub.0
flows through the electric motor 13 by monitoring the actual motor current
I as detected and fed back. On the other hand, the clutch control means 92
is adapted for interrupt (i.e., turn off) the clutch control signal C when
occurrence of a fault in a low-speed operation range of the motor vehicle
is determined on the basis of detection signals available from outputs of
the relevant sensors (not shown) which the motor vehicle is equipped with.
Incidentally, the motor current detecting means 93 may be constituted by,
for example, a resistor having one end grounded.
FIG. 8 is a view for illustrating changes in the clutch control signal C
and the motor current command value I.sub.0. As is shown at (a) in FIG. 8,
the clutch control signal C is at the ON-level (indicating the clutch
current on the order of e.g. 1 ampere) when the vehicle speed V is lower
than the predetermined reference vehicle speed V.sub.0 and changed over to
the OFF-level (i.e., the level corresponding to zero clutch current) when
the vehicle speed V is higher than the predetermined reference vehicle
speed V.sub.0 inclusive thereof. Further, as shown at (b) in FIG. 8, the
motor current command value I.sub.0 is so changed as to increase when the
vehicle speed V decreases or as the steering torque T increases so long as
the vehicle speed V is lower than the predetermined reference vehicle
speed V.sub.0 and is interrupted (i.e., set to zero level) when the
vehicle speed V is higher than the predetermined reference vehicle speed
V.sub.0 inclusive thereof. In general, the steering wheel 1 is heavy to
steer (i.e., the steering torque T as required is large) when the motor
vehicle is driven at a low speed, while the steering wheel 1 is easy to
steer (i.e., the steering torque T as demanded is low) when the motor
vehicle is running at a high speed, even th | | |
