 |
Claims  |
|
|
What is claimed is:
1. An electric power steering apparatus including an electric motor for
providing an assist torque to a steering system, a steering torque
detecting unit for detecting a steering torque exerted on said steering
system to generate a steering torque signal, a target current determining
unit for determining, on the basis of at least said steering torque
signal, a target current to be supplied to said electric motor so as to
generate a target current signal, a current detecting unit for detecting a
current flowing through said electric motor to generate a motor current
signal, an offset calculating unit for calculating an offset between said
target current signal and said motor current signal to generate an offset
signal, a motor operation control unit for generating, on the basis of
said offset signal, a drive control signal to drive said electric motor,
and a motor drive unit for driving said electric motor on the basis of
said drive control signal, said electric power steering apparatus
comprising:
a judgment unit which passes a judgment that a condition in which one of
said offset signal and said drive control signal exceeds a preset
threshold has continued for a preset allowable time;
a forced drive control unit for generating a forced drive control signal
tending to forcibly drive said electric motor when said judgment is passed
by said judgment unit; and
a fault judgment unit which judges said current detecting unit to be at
fault if said motor current signal does not vary even though said electric
motor is forcibly driven.
2. An electric power steering apparatus according to claim 1, wherein said
motor operation control unit includes a drive current switching section
for switching said offset signal and said forced drive control signal
generated from said forced drive control unit.
3. An electric power steering apparatus according to claim 1, wherein said
allowable time is set to a response time during which a current supplied
to said electric motor on the basis of said offset signal is fed back to
tend said offset below said threshold.
4. An electric power steering apparatus according to claim 1, wherein said
judgment unit includes
an absolute-value circuit for determining an absolute value of said offset
signal to generate an absolute-value signal,
a threshold circuit for generating a preset threshold signal,
a comparator circuit for generating a comparison output signal if said
offset signal is greater than said threshold, and
a timer circuit for timing a duration of said comparison output signal to
generate a judgment signal as an output signal from said judgment unit
when the timed duration reaches said preset allowable time.
5. An electric power steering apparatus according to claim 1, wherein said
fault judgment unit, when it judges said current detecting unit to be at
fault, generates a current feedback control stop command signal to stop
generation of said drive control signal from said motor operation control
unit.
6. An electric power steering apparatus according to claim 1, wherein said
motor operation control unit includes an adder for adding together said
offset signal and said forced drive control signal from said forced drive
control unit, so as to forcibly vary the current to be supplied to said
electric motor.
7. An electric power steering apparatus according to claim 1, wherein said
fault judgment unit, when it judges said current detecting unit to be at
fault, generates an interrupting command signal, further including an
interrupting device for interrupting supply of power from a battery to
said electric motor on the basis of said interrupting command signal.
8. An electric power steering apparatus according to claim 2, wherein said
motor operation control unit includes a control mode switching section for
switching, on the basis of a control mode switching command signal
generated by said fault judgment unit when said fault judgment unit judges
said current detecting unit to be at fault, the control mode of said motor
operation control unit so as to perform operation control of said electric
motor based on said target current signal.
9. An electric power steering apparatus including an electric motor for
providing an assist torque to a steering system, a steering torque
detecting unit for detecting a steering torque exerted on said steering
system to generate a steering torque signal, a target current determining
unit for determining, on the basis of at least said steering torque
signal, a target current to be supplied to said electric motor so as to
generate a target current signal, a current detecting unit for detecting a
current flowing through said electric motor to generate a motor current
signal, an offset calculating unit for calculating an offset between said
target current signal and said motor current signal to generate an offset
signal, a motor operation control unit for generating, on the basis of
said offset signal, a drive control signal to drive said electric motor,
and a motor drive unit for driving said electric motor on the basis of
said drive control signal, said electric power steering apparatus
comprising:
a judgment unit which passes a judgment that a variation of said motor
current signal per unit time is smaller than a predetermined value when
one of said offset signal and said drive control signal is greater than a
preset threshold;
a forced drive control unit for generating a forced drive control signal
tending to forcibly drive said electric motor when said judgment unit has
passed said judgment; and
a fault judgment unit which judges said current detecting unit to be at
fault if said motor current signal does not vary even though said electric
motor is forcibly driven.
10. An electric power steering apparatus according to claim 9, wherein said
motor operation control unit includes a drive current switching section
for switching said offset signal and said forced drive control signal
generated from said forced drive control unit.
11. An electric power steering apparatus according to claim 9, wherein said
judgment unit includes
a pulse width measuring section for calculating a duty cycle of said drive
control signal to generate the result of calculation as a detected duty
cycle,
a threshold setting section for generating a preset threshold signal,
an operating condition judgment section for making a judgment based on said
detected duty cycle and said threshold signal to determine whether or not
said electric motor is in the operating state and, if said judgment is
indicative of said electric motor being in the operating state, for
generating a motor operating condition detecting signal,
a current variation detecting section for determining a variation of said
motor current per a predetermined unit time while said motor operating
condition detecting signal is being supplied from said operating condition
judgment section, so as to generate an absolute value of the detected
variation as a current variation,
a fault judgment variation setting section for generating a preset fault
judgment variation, and
a current variation fault judgment section for comparing said current
variation with said said fault judgment variation and, if said current
variation is smaller than said fault judgment variation, for generating a
judgment signal.
12. An electric power steering apparatus according to claim 9, wherein said
fault judgment unit, when it judges said current detecting unit to be at
fault, generates a current feedback control stop command signal to stop
generation of said drive control signal from said motor operation control
unit.
13. An electric power steering apparatus according to claim 9, wherein said
judgment unit acts to detect a condition in which a predetermined current
is supplied to said electric motor, monitor a variation of the detected
current which is the output from said current detecting unit, and generate
a judgment signal if said variation is smaller than a preset value.
14. An electric power steering apparatus including an electric motor for
providing an assist torque to a steering system, a steering torque
detecting unit for detecting a steering torque exerted on said steering
system to generate a steering torque signal, a target current determining
unit for determining, on the basis of at least said steering torque
signal, a target current to be supplied to said electric motor so as to
generate a target current signal, a current detecting unit for detecting a
current flowing through said electric motor to generate a motor current
signal, an offset calculating unit for calculating an offset between said
target current signal and said motor current signal to generate an offset
signal, a motor operation control unit for generating, on the basis of
said offset signal, a drive control signal to drive said electric motor,
and a motor drive unit for driving said electric motor on the basis of
said drive control signal, said electric power steering apparatus
comprising:
a first judgment unit which passes a judgment that a condition in which one
of said offset signal and said drive control signal exceeds a first
threshold has continued for a preset allowable time;
a second judgment unit which passes a judgment that a variation of said
motor current signal per unit time is smaller than a predetermined value
when said drive control signal is greater than a second threshold;
a forced drive control unit for generating a forced drive control signal
tending to forcibly drive said electric motor when said judgment of said
first judgment unit and said judgment of said second judgment unit have
passed concurrently; and
a fault judgment unit which judges said current detecting unit to be at
fault if said motor current signal does not vary even though said electric
motor is forcibly driven.
15. An electric power steering apparatus according to claim 14, wherein
said motor operation control unit includes a drive current switching
section for switching said offset signal and said forced drive control
signal generated from said forced drive control unit.
16. An electric power steering apparatus according to claim 14, wherein
said fault judgment unit, when it judges said current detecting unit to be
at fault, generates a current feedback control stop command signal to stop
generation of said drive control signal from said motor operation control
unit. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an electric power steering
apparatus which provides power assist of an electric motor directly to a
steering system so as to reduce necessary steering power to be applied by
the driver. More particularly, it relates to such an electric power
steering apparatus which is capable of detecting an operation failure of a
current detecting unit provided to detect a current flowing through the
electric motor.
2. Description of the Related Art
In general, the electric power steering apparatus includes an electric
motor incorporated in a steering system and a control unit or controller
which controls operation of the electric motor to eventually reduce a
manual steering effort or force required by the driver.
FIG. 17 of the accompanying drawings shows in block diagram the general
construction of a conventional controller 200A. The controller 200A
includes a target current determining unit 210, a PWM (pulse width
modulation) signal generating section or generator 220, and a motor drive
unit 230. The target current determining unit 210 determines a target
assist torque on the basis of a steering torque signal Tp and generates a
target current signal IT required for an electric motor 100 to exert the
target assist torque to a steering system. The PWM signal 220 generates,
on the basis of the target current signal IT, a PWM signal as a drive
control signal 220a which is used for controlling operation of the
electric motor 100.
The motor drive unit 230 has a gate drive circuit 240 and an electric motor
drive circuit 250 including four power FETs (field-effect transistors)
connected together to form a bridge H-network. The gate drive circuit 240
drives, on the basis of the drive control signal (PWM signal) 220a, the
gate of each of the FETs to turn on or conduct the individual FETs. Thus,
on the basis of the steering torque signal Tp corresponding to a steering
torque detected by a steering torque detecting unit 120, electric power
supplied from a battery power source BAT to the electric motor 100 is
altered into a pulse-like shape in an effort to control output power
(steering assist torque) of the electric motor 100.
FIG. 18 shows in block diagram the construction of another conventional
controller 200B. In the controller 200B, a motor current IM actually
supplied to an electric motor 100 is detected so that a feedback control
of the electric motor 100 is performed on the basis of the motor current
IM, so as to improve the control characteristics of the electric motor
100. The controller 200B of this motor current feedback control type
differs from the controller 200A shown in FIG. 17 in that it further
includes a current detecting unit 260 for detecting a current flowing
through the electric motor 100 so as to generate a motor current signal
IM, an offset calculating unit 270 for determining an offset between the
motor current signal IM and a target current signal IT, and a motor
operation control unit 280 for generating, on the basis of the offset
signal 270a generated from the offset calculating unit 270, a drive
control signal 220a to drive the electric motor 100.
The motor operation control unit 280 includes a current feedback (F/B)
control section 290 and a PWM signal generator 220. The current F/B
control section 290 performs control actions, such as proportional,
integral, derivative and so forth. for the offset signal to generate a
drive current signal 290a for controlling a current to be supplied to the
electric motor 100 in such a manner as to tend the offset toward zero. The
PWM signal generator 220 generates, on the basis of the drive current
signal 290a, a PWM signal for the drive control of the electric motor 100
and outputs the generated PWM signal as the above-mentioned drive control
signal 220a.
The controller 200B of the motor current feedback control type shown in
FIG. 18 is able to improve the control accuracy and response because
operation of the electric motor 100 is controlled in such a manner that
the offset between the target motor current IT and the actual motor
current IM tends to zero.
However, when the current detecting unit 260 causes an operation failure
which, for example, tends to fix the detected current at a particular
value (including zero), the controller 200B of this type cannot perform
the normal feedback control operation any more, failing to supply a
desired steering assist torque to the steering system.
FIGS. 19A and 19B are graphs illustrative of an example of the operation
failure in which the output (detected current) IM from the current
detecting unit 260 is fixed at a constant value. FIG. 19A exemplifies the
time-current characteristic of the target current IT and the detected
current IM, and FIG. 19B shows the time-current characteristic of the
drive control signal 220a generated from the motor operation control unit
280 on the basis of the offset 270a between the target current IT and the
detected current IM shown in FIG. 19a. In the case where the output from
the current detecting unit 260 is fixed at a current value IM.alpha., the
motor operation control unit 280 generates, on the basis of an offset
between a target current IT and the detected current IM.alpha., a drive
control signal 220a tending to vary a current to be supplied to the
electric motor 100 in such a manner as to tend the offset closer toward
zero. In this instance, however, since the detected current value is fixed
and does not vary any more, the drive control signal 220a generated from
the motor operation control unit 280 has a larger level in the direction
to cancel out the offset.
Japanese Patent Laid-open Publication No. HEI 8-91239 discloses a
controller for an electric power steering apparatus, which can detect a
failure in a motor current detecting unit. The disclosed controller is
constructed to check the motor current detecting unit for a possible
failure immediately after an ignition key is turned on. To this end, the
controller applies a voltage V across an electric motor for a time T which
is much less than a mechanical time constant Tm of the motor and which is
much greater than an electric time constant Te of the motor (Te<<T<<Tm).
While applying the voltage to the electric motor, the controller
determines whether or not the motor current detecting unit is at fault,
through a comparison made between an estimated motor current is which is
estimated from a motor terminal voltage V and a motor terminal resistance
R, and a motor current i appearing in a condition in which an angular
velocity .omega. of the motor is approximately zero, i.e., the motor is
substantially not rotating.
The controller disclosed in Japanese Patent Laid-open Publication No. HEI
8-91239 is so constructed as to check the motor current detecting unit
(motor current detecting unit) against a failure immediately after the
ignition key is turned on, and so this controller cannot detect any
operation failure in the motor current detecting unit occurring during
movement of the motor vehicle. In order to determine the estimated motor
current is based on the motor terminal voltage V and the motor terminal
resistance R, detection of a battery voltage is inevitable. As a
consequence, the controller further requires a battery voltage detector
and so on. Additionally, the motor terminal resistance R needs to be
registered in advance. Accordingly, when the motor is replaced with a
different type of motor, re-registration of the motor terminal resistance
R is needed.
SUMMARY OF THE INVENTION
With the foregoing drawbacks in view, it is an object of the present
invention to provide an electric power steering apparatus which is capable
of detecting an operation failure of a motor current detecting unit during
movement of a motor vehicle without the need for registration of a battery
terminal voltage and a motor terminal resistance in advance.
According to a first aspect of the present invention, there is provided an
electric power steering apparatus in which an assist torque proportional
to a manual steering torque is supplied from an electric motor to a
steering system by controlling operation of the electric motor on the
basis of a drive control signal generated based on an offset between a
target current set in accordance with the steering torque and a motor
current detected by a current detecting unit, wherein the electric power
steering apparatus includes: a judgment unit which generates a judgment
signal when a condition in which one of an offset signal representing the
offset and the drive control signal exceeds a preset threshold has
continued for a preset allowable time; a forced drive control unit which
generates, on the basis of the judgment signal, a forced drive control
signal tending to forcibly drive the electric motor; and a fault judgment
unit which judges the current detecting unit to be at fault if the motor
current signal does not vary even though the electric motor is forcibly
driven.
With this arrangement of the electric power steering apparatus, when the
judgment unit detects the fact that a condition, in which the offset
signal or the drive control signal is greater than the threshold and,
hence, large power is supplied to the electric motor, has continued for
the allowable time, the forced drive control unit drive the electric motor
forcibly. The fault judgment unit monitors the motor current and, if
forced driving of the electric motor does not vary the motor current, it
judges that the current detecting unit to be at fault. Accordingly, even
when a failure in the current detecting unit occurs during movement of a
motor vehicle in which the power steering apparatus of this invention is
incorporated, such failure in the current detecting unit can reliably be
detected. Additionally, even if the above-mentioned judgment of the
judgment unit has passed as a result of continuous application of a large
manual steering torque, misjudgment does not occur because the fault
judgment against the current detecting unit is performed after the
electric motor is driven forcibly.
The allowable time may be set to a response time during which a current
supplied to the electric motor on the basis of the offset signal is fed
back to tend the offset below the threshold.
The motor operation control unit preferably includes a drive current
switching section for switching the offset signal and the forced drive
control signal generated from the forced drive control unit.
The motor operation control unit may include a control mode switching
section for switching, on the basis of a control mode switching command
signal generated by the fault judgment unit when the fault judgment unit
judges the current detecting unit to be at fault, the control mode of the
motor operation control unit so as to perform operation control of the
electric motor based on the target current signal.
The motor operation control unit may further include an adder for adding
together the offset signal and the forced drive control signal generated
from the forced drive control unit, so as to forcibly vary the current to
be supplied to the electric motor.
Preferably, the judgment unit includes an absolute-value circuit for
determining an absolute value of the offset signal to generate an
absolute-value signal, a threshold circuit for generating a preset
threshold signal, a comparator circuit for generating a comparison output
signal if the offset signal is greater than the threshold, and a timer
circuit for timing a duration of the comparison output signal to generate
a judgment signal as an output signal from the judgment unit when the
timed duration reaches the preset allowable time.
Preferably, the fault judgment unit, when it judges the current detecting
unit to be at fault, generates a current feedback control stop command
signal to stop generation of the drive control signal from the motor
operation control unit.
Additionally, the fault judgment unit, when it judges the current detecting
unit to be at fault, may generate an interrupting command signal, further
including an interrupting device for interrupting supply of power from a
battery to the electric motor on the basis of the interrupting command
signal.
According to a second aspect of the present invention, there is provided an
electric power steering apparatus in which an assist torque proportional
to a manual steering torque is supplied from an electric motor to a
steering system by controlling operation of the electric motor on the
basis of a drive control signal generated based on an offset between a
target current set in accordance with the steering torque and a motor
current detected by a current detecting unit, wherein the electric power
steering apparatus includes: a judgment unit which generates a judgment
signal if a variation of the motor current signal per unit time is smaller
than a predetermined value when one of the offset signal and the drive
control signal is greater than a preset threshold; a forced drive control
unit for generating, on the basis of the judgment signal, a forced drive
control signal tending to forcibly drive the electric motor; and a fault
judgment unit which judges the current detecting unit to be at fault if
the motor current signal does not vary even though the electric motor is
forcibly driven.
With this arrangement, when the judgment unit detects the fact that even
though the motor current is to be varied widely due to the offset signal
or the drive control signal which is greater than the threshold, a
variation of the motor current signal per unit time is small (less than
the predetermined value), the forced drive control unit drive the electric
motor forcibly. The fault judgment unit monitors the motor current and, if
forced driving of the electric motor does not vary the motor current, it
judges that the current detecting unit to be at fault. Accordingly, even
when a failure in the current detecting unit occurs during movement of the
motor vehicle, such failure in the current detecting unit can reliably be
detected. Additionally, even when the variation of the motor current
signal per unit time is restricted to a small value due to a counter
electromotive force generated during high speed rotation of the electric
motor caused by abrupt manual steering operation, misjudgment does not
occur because the fault judgment for the current detecting unit is
performed after the electric motor is driven forcibly.
The judgment unit preferably acts to detect a condition in which a
predetermined current is supplied to the electric motor, monitor a
variation of the detected current which is the output from the current
detecting unit, and generate a judgment signal if the variation is smaller
than a preset value.
Preferably, the judgment unit includes a pulse width measuring section for
calculating a duty cycle of the drive control signal to generate the
result of calculation as a detected duty cycle, a threshold setting
section for generating a preset threshold signal, an operating condition
judgment section for making a judgment based on the detected duty cycle
and the threshold signal to determine whether or not the electric motor is
in the operating state and, if the judgment is indicative of the electric
motor being in the operating state, for generating a motor operating
condition detecting signal, a current variation detecting section for
determining a variation of the motor current per a predetermined unit time
while the motor operating condition detecting signal is being supplied
from the operating condition judgment section, so as to generate an
absolute value of the detected variation as a current variation, a fault
judgment variation setting section for generating a preset fault judgment
variation, and a current variation fault judgment section for comparing
the current variation with the the fault judgment variation and, if the
current variation is smaller than the fault judgment variation, for
generating a judgment signal.
According to a third aspect of the present invention, there is provided an
electric power steering apparatus in which an assist torque proportional
to a manual steering torque is supplied from an electric motor to a
steering system by controlling operation of the electric motor on the
basis of a drive control signal generated based on an offset between a
target current set in accordance with the steering torque and a motor
current detected by a current detecting unit, wherein the electric power
steering apparatus includes: a first judgment unit which generates a first
judgment signal when a condition in which one of an offset signal
representing the offset and the drive control signal exceeds a threshold
has continued for a preset allowable time; a second judgment unit which
generates a second judgment signal if a variation of the motor current
signal per unit time is smaller than a predetermined value when the drive
control signal is greater than a second threshold; a forced drive control
unit which generates a forced drive control signal tending to forcibly
drive the electric motor when the first and second judgment signals are
supplied concurrently to the forced drive control unit; and a fault
judgment unit which judges the current detecting unit to be at fault if
the motor current signal does not vary even though the electric motor is
forcibly driven.
With this arrangement, when the first and second judgment units
concurrently generates the respective judgement signals, the forced drive
control unit forcibly drive the electric motor. The fault judgment unit
judges the current detecting unit to e at fault if the motor current doe
not vary even though forcible driving of the electric motor is performed.
Thus, a failure in the current detecting unit occurs during movement of
the motor vehicle, such failure can reliably be detected. Judgment for a
possible failure in the current detecting unit is performed after the
electric motor is drive forcibly, migs-judgment does not occur even when
abrupt manual steering operation continues.
The above and other objects, features and advantages of the present
invention will become manifest to these versed in the art upon making
reference to the detailed description and the accompanying sheets of
drawings in which preferred structural embodiments incorporating the
principles of the invention are shown by way of illustrative examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatical view showing the general construction of an
electric power steering apparatus to which the present invention pertains;
FIG. 2 is a block diagram showing a control unit or controller, for use
with the electric power steering apparatus of FIG. 1, according to a first
embodiment of the present invention;
FIG. 3 is a block diagram showing a first judgment unit;
FIG. 4 is a graphical representation of an offset output and operation of
the first judgment unit;
FIGS. 5A, 5B and 5C are graphs showing various examples of a forced drive
control signal;
FIG. 6 is a flowchart showing a failure detecting operation performed by a
current detecting unit of the controller shown in FIG. 2;
FIG. 7 is a block diagram showing a first modification of the controller in
the first embodiment shown in FIG. 2;
FIG. 8 is a block diagram showing a second modification of the controller
in the first embodiment shown in FIG. 2;
FIG. 9 is a block diagram showing a controller, for use with the electric
power steering apparatus of FIG. 1, according to a second embodiment of
the present invention;
FIG. 10 is a block diagram showing a second judgment unit;
FIG. 11 is a graphical representation of the operation of the second
judgment unit;
FIG. 12 is a flowchart showing a failure detecting operation performed by a
current detecting unit of the controller shown in FIG. 9;
FIG. 13 is a block diagram showing a modification of the controller in the
second embodiment shown in FIG. 9;
FIG. 14 is a block diagram showing a controller, for use with the electric
power steering apparatus of FIG. 1, according to a third embodiment of the
present invention;
FIG. 15 is a flowchart showing a fault detecting operation performed by a
current detecting unit of the controller shown in FIG. 14;
FIG.16 is a flowchart showing a fault detecting operation performed by the
current detecting unit on the basis of a judgment signal supplied from
either the first judgment unit or the second judgment unit;
FIG. 17 is a block diagram showing a conventional control unit or
controller;
FIG. 18 is a block diagram showing another conventional controller; and
FIGS. 19A and 19B are graphs illustrative of the operation of the
conventional controller taken place when a detection output from the
current detecting unit is fixed at a constant value.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Certain preferred embodiments of the present invention will be described
below in greater detail with reference to the accompanying drawings,
wherein like reference characters designate like or corresponding parts
throughout several views.
FIG. 1 shows an electric power steering apparatus 1 to which the present
invention pertains. The electric power steering apparatus 1 includes a
steering system equipped with an electric motor 10, and a control unit or
controller 20 for controlling power of the electric motor 10 so as to
reduce manual steering effort or power to be applied by the driver.
The steering system includes a steering wheel 2 attached to an end of a
steering shaft 3. The opposite end of the steering shaft 3 is connected to
one end of a connecting shaft 4 via a first universal joint 4a, the other
end of the connecting shaft 4 being connected via a second universal joint
4b to a pinion 6 of a rack-and-pinion mechanism 5. The pinion 6 meshes
with a rack 7 which is a long bar with teeth 7a cut into one side. The
rack-and-pinion mechanism 5 translates a rotary motion of the pinion 6
into an axial reciprocating motion of the rack 7. Opposite ends of the
rack 7 are connected via tie rods 8 to steerable left and right front
wheels 9. When the steering wheel 2 is manually turned or rotated in a
desired direction, the rack-and-pinion mechanism 5 and the tie rods 8
cause the front wheels 9 to pivot in the same direction to thereby change
the direction of movement of a motor vehicle.
In order to reduce the manual steering effort or power required by the
driver, the electric motor 10 is disposed in concentric relation to the
rack 7 and supplies an assist torque (steering assist torque) to the rack
7. A ball screw mechanism 11 is disposed substantially parallel with the
rack 7 so that rotational power of the electric motor 10 is converted by
the ball screw mechanism 11 into an axial thrusting force acting on the
rack 7. The electric motor 10 has a rotor equipped with a drive helical
gear 10a which meshes with a drive helical gear 11b attached to an end of
a screw shaft 11a of the ball screw mechanism 11. A nut 11c of the ball
screw mechanism 11 is connected to the rack 7.
A steering torque detecting unit (steering torque sensor) 12 is disposed in
a steering box (not shown) for detecting a manual steering torque Ts
acting on the pinion 6. The steering torque detecting unit 12 supplies a
steering torque signal Tp corresponding to the detected steering torque
Ts, to the controller 20. The controller 20 performs controlled driving of
the electric motor 10, with the steering torque signal Tp taken as a main
signal in such an effort to control output power (steering assist torque)
of the electric motor 10.
FIG. 2 shows in block diagram the construction of a controller 20 according
to a first embodiment of the present invention. The controller 20 shown in
this figure differs from the conventional controller 200B in that a first
judgment section or unit 31, a forced drive control section or unit 32, a
fault judgment section or unit 33 and an indicator 34 are added. A motor
operation control unit 35 of the controller 20 has a drive current signal
switching section 37 provided between a current feedback (F/B) control
section 29 and a PWM signal generating section or generator 36. An offset
calculating unit 27 produces an offset signal 27a which in turn is input
into the current feedback control section 29 and the first judgment unit
31.
FIG. 3 shows in block diagram an example of the first judgment unit 31. The
first judgment unit 31 includes an absolute-value circuit 41, a first
threshold setting circuit 42, a comparator circuit 43 and a timer circuit
44.
The absolute-value circuit 41 determines an absolute value of the offset
signal 27a and generates an absolute-value signal 41a. For an offset
signal 27a of positive polarity, the absolute-value circuit 41 outputs
this offset signal 27a without modification. For an offset signal 27a of
negative polarity, the absolute-value circuit 41 generates an output
signal which is equal in magnitude to the input offset signal but has an
inverted polarity, i.e., the positive polarity. The absolute-value signal
41a is supplied to a comparison input terminal (+) of the comparator
circuit 42.
The first threshold setting circuit 42 generates a preset first threshold
signal 42a. The first threshold is set with reference to an offset output
value produced when the manual steering operation is done positively. In
other words, the first threshold is set at a value which is greater than
an offset output value produced when the steering wheel is operated to
adjust the steering direction little by little. The first threshold signal
42a is supplied to a reference input terminal (-) of the comparator
circuit 43.
The comparator circuit 43 generates a comparator output signal 43a if the
offset signal 27a is greater than the first threshold signal 42a. The
comparator output signal 43a is supplied to the timer circuit 44.
The timer circuit 44, upon receipt of the comparator output signal 43a,
times a duration of the comparator output signal 43a and produces a first
judgment signal 31a when the timed duration the comparator output signal
43a reaches a preset allowable time. The allowable time is set in
consideration of a response time during which the offset is rendered below
the first threshold as a result of the feedback control effected on the
current supplied to the electric motor 10 on the basis of the offset
signal 27a.
This is because the longer the allowable time is, the greater is the delay
in fault detecting timing of the current detecting unit. Conversely, a
shorter allowable time causes frequent generation of a forced drive signal
even though the current detecting unit is normally operating. The
allowable time is set to a value of the order of 50 to 100 milliseconds,
for example.
FIG. 4 is a graphical representation of the offset output and the operation
of the first judgment unit 31 (FIGS. 2 and 3).
FIG. 4(a) illustrates the change in characteristic of the target current IT
and the change in characteristic of the detected current (motor current IM
as an output from the current detecting unit 26). When the current
detecting unit 26 (FIG. 2) is normally operating, the current supplied to
the electric motor 10 varies with the target current IT. Accordingly, the
current IM detected by the current detecting unit 26 varies with the
target current IT. In the case where the current detecting unit 26 is at
fault, the detected current IM is fixed at a constant value, as indicated
by the phantom line shown in FIG. 4(a).
FIG. 4(b) illustrates the change in characteristic of the offset output.
When the target current IT and the detected current IM vary in the manner
shown in FIG. 4(a), the offset (offset signal 27a) between the target
current an the detected current varies in the manner indicated by the
solid line shown in FIG. 4(b). In the case where the detected current IM
is fixed at a constant value due to fault of the current detecting unit
26, a variation of the target current IT directly forms an off set output,
as indicated by the phantom line shown in FIG. 4(b). In this instance, if
the offset output is negative in polarity, the absolute-value signal 41a,
an output from the absolute-value circuit 41 in the first judgment unit 31
(FIG. 3), is converted into a signal | | |
