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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electric power steering
apparatuses for use in automotive vehicles which provide an electric
steering assist of an electric motor to the vehicle steering system to
reduce a steering effort that is to be manually applied by a vehicle
operator or driver. More particularly, the present invention relates to an
improved electric power steering apparatus which, in case of an anomaly or
abnormal condition in a microprocessor constituting a main power steering
control, turns off or deactivates the motor to prevent the motor from
producing an abnormal steering assist.
2. Description of the Related Art
Various types of electric power steering apparatuses have been proposed and
known today. One of the known types of electric power steering apparatuses
is designed in such a manner that, once manual steering torque is applied
beyond a predetermined dead zone (namely, a zone where no steering assist
is produced by an electric motor, i.e., no current flows through the
electric motor, in spite of a driver's steering operation to the left or
right from the center or neutral position), the apparatus inhibits the
operation or driving of the electric motor in a direction opposite to the
direction of the driver-applied manual steering torque and prevents an
abnormal target motor current from being generated due to an abnormal
condition in a microprocessor constituting a main power steering control
for controlling the overall operation of the motor.
More specifically, in the electric power steering apparatus of the
above-mentioned type, when the microprocessor has got into an abnormal
condition to generate a maximum target motor current in a single direction
(e.g., rightward) irrespective of presence or absence of driver-applied
manual steering torque, the motor is caused to rotate rightward. If the
driver is holding the steering wheel at that time, leftward steering
torque would be detected and the operation of the motor is inhibited once
the detected leftward steering torque exceeds a threshold value of the
dead zone. Then, as the electric motor is deactivated, the detected
steering torque decreases to enter the dead zone, so that the motor is
again turned on or activated and thus the detected steering torque in
again increases. Such occurrences are repeated, which would cause an
undesired "hunting" of steering torque in the neighborhood of the dead
zone, thus creating a possibility of unstable steering characteristics.
Japanese Patent Laid-open Publication No. HEI-8-108856 discloses an
electric power steering apparatus which is designed to provide a solution
to the undesired hunting. Specifically, the disclosed electric power
steering apparatus detects a motor current in an opposite direction to a
direction of steering torque or abnormal motor current and triggers a
timer upon detection of the motor current of the opposite direction to the
steering torque or abnormal motor current. Then, upon lapse of a time
period preset in the timer, a latch circuit is activated and a motor drive
inhibition circuit is activated in response to an output signal from the
latch circuit so that a motor driver circuit is deactivated to stop the
motor current of the opposite direction to the steering torque or abnormal
motor current. The motor drive inhibition circuit remains activated as
long as the latch circuit is activated, to thereby stop the motor current
of the opposite direction to the steering torque or abnormal motor
current. The activated condition of the motor drive inhibition circuit
continues until the latch circuit is deactivated by a non-driven state
detection circuit detecting that the target motor current has reached a
zero (0) level or value.
However, because of the arrangement that the motor drive inhibition circuit
is activated upon lapse of the timer-set time period after detection of
the motor current of the opposite direction to the steering torque or
abnormal motor current, the disclosed electric power steering apparatus
encounters the problem that such a motor current of the opposite direction
to the steering torque or abnormal motor current would undesirably
continue flowing during the timer-set time period.
Further, because the latch circuit is deactivated when the zero target
motor current is detected by the non-driven state detection circuit, the
disclosed electric power steering apparatus has another problem that, even
when the normal operation of the microprocessor is restored, the target
motor current would not decrease to the zero value as long as the steering
wheel is being operated to produce steering torque, so that the latch
circuit, and hence the motor drive inhibition circuit, would remain
activated to thereby keep disabling the motor driver circuit. Further,
when steering torque is being generated during the activation of the latch
circuit, a target motor current is generated with no driving current
actually flowing through the motor, and thus, in a situation where an
offset between the target motor current and the actual detected motor
current is subjected to PID (Proportional, Integral and Differential)
control operations, the duty ratio would reach almost 100% due to the I
(Integral) control operation. Consequently, the motor would be driven at
the 100% duty ratio to create a possibility of an overcurrent flowing
through the motor the moment the latch circuit is deactivated.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an electric
power steering apparatus for an automotive vehicle which can promptly and
reliably inhibit generation of an abnormal motor current due to an anomaly
in a microprocessor constituting a main power steering control and also
can promptly and reliably cancel the inhibition of the abnormal motor
current generation.
Control unit of the electric power steering apparatus, in accordance with
an aspect of the present invention, includes: a zero-value setting section
for, when drive inhibition signals are output by a motor drive inhibition
section, setting a target motor current signal to a zero level or value
irrespective of a value of a steering torque signal output by a steering
torque sensor; and an inhibition cancellation section for cancelling the
drive inhibition signals when an offset between the target motor current
signal and detected motor current signal becomes zero after the target
motor current signal is set to the zero value by the zero-value setting
section. With this arrangement, instructions can be communicated
bidirectionally from the motor drive inhibition section to a main power
steering control (microprocessor) where an anomaly has occurred and from
the main power steering control (microprocessor) to the inhibition
cancellation section, so that the inhibition of the motor drive can be
canceled promptly.
The zero-value setting section in the invention includes: a zero-value
generator section for, on the basis of the drive inhibition signals output
by the motor drive inhibition section, detecting a motor drive inhibition
condition where driving of the electric motor is to be inhibited and
outputting a prestored coefficient signal of value 0 upon detection of the
motor drive inhibition condition; and a multiplier for, when the motor
drive inhibition condition is detected by the zero-value generator
section, multiplies the target motor current signal by the coefficient of
value 0 to thereby compulsorily set the target motor current signal to the
zero value.
In one implementation, the zero-value generator section includes: a first
storage section prestoring a value 1 as a coefficient; a second storage
section prestoring a value 0 as a coefficient; a logic operator section
for performing an exclusive OR operation on the drive inhibition signals
output by the motor drive inhibition section, to detect whether or not the
motor drive inhibition section is signaling the motor drive inhibition
condition; and a selector section for selecting between the coefficient of
the value 1 stored in the first storage section and the coefficient of the
value 0 stored in the second storage section, to supply the coefficient of
the value 1 or the coefficient of the value 0 to the multiplier for
multiplication with the target motor current signal.
The above-mentioned inhibition cancellation section include: a first
inverter for inverting the coefficient signal of value 0 output by the
zero-value setting section; a second inverter for inverting the offset
signal of value 0 output by the offset calculator section; and an AND
circuit for performing an AND operation on the coefficient signal inverted
by the first inverter and the offset signal inverted by the second
inverter. Output from the AND circuit is supplied to the motor drive
inhibition section as the inhibition cancellation signal, so as to cancel
inhibition of the driving of the electric motor.
The motor drive inhibition section includes: a drive-inhibition-condition
determination section for making a comparison between the steering torque
signal output by the steering torque sensor and prestored reference torque
values and outputs the drive inhibition signals when a result of the
comparison indicates that the driving of the electric motor is to be
inhibited; and an output inhibition section for, when the
drive-inhibition-condition determination section determines that the
driving of the electric motor is to be inhibited, inhibiting output of the
motor control signal from the motor drive section to the motor in
accordance with the drive inhibition signals. The motor drive inhibition
section inhibits output of the motor control signal on the basis of at
least the steering torque signal from the steering torque sensor and the
detected motor current signal.
BRIEF DESCRIPTION OF THE DRAWINGS
Certain preferred embodiments of the present invention will be described in
greater detail with reference to the accompanying sheets of drawings, in
which:
FIG. 1 is a block diagram showing a general organization of an electric
power steering apparatus for an automotive vehicle according to a first
embodiment of the present invention;
FIG. 2 is a functional block diagram of a control unit employed in the
electric power steering apparatus of FIG. 1;
FIG. 3 is a block diagram showing in detail an exemplary construction of a
zero-value generator section shown in FIG. 2;
FIG. 4 is a block diagram showing in detail an exemplary construction of a
drive-inhibition-condition determination section shown in FIG. 2;
FIG. 5 is a diagram showing an exemplary circuit structure of an inhibition
cancellation section shown in FIG. 2;
FIG. 6 is a diagram showing an exemplary circuit structure of an output
inhibition section shown in FIG. 2;
FIG. 7 is a diagram showing an exemplary circuit structure of a drive
control section shown in FIG. 2;
FIG. 8 is a diagram showing an exemplary organization of a motor driver
circuit shown in FIG. 2 which is in the form of an FET bridge arrangement;
FIG. 9 is a block diagram showing a general organization of an electric
power steering apparatus for an automotive vehicle according to a second
embodiment of the present invention;
FIG. 10 is a block diagram showing in detail an exemplary construction of a
drive-inhibition-condition determination section shown in FIG. 9;
FIG. 11 is a diagram showing an exemplary control curve of a target motor
current signal versus a steering torque signal with a vehicle velocity
used as a parameter;
FIG. 12 is a diagram illustrating exemplary relationship between an anomaly
in a microprocessor constituting a main power steering control and the
steering torque signal;
FIG. 13 is a diagram illustrating regions where output of a detected motor
current signal is inhibited by the drive-inhibition-condition
determination section of FIG. 4 depending on the value of the steering
torque signal; and
FIG. 14 is a diagram illustrating regions where output of the detected
motor current signal is inhibited by the drive-inhibition-condition
determination section of FIG. 10 depending on the value of the steering
torque signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention as described in detail hereinbelow is constructed to
inhibit a flow of an excessive motor current or motor current of an
opposite direction to the direction of manual steering torque due to an
anomaly in a microprocessor constituting a main power steering control and
also promptly and reliably cancel the motor current flow inhibition by
communicating a zero target motor current instruction and a zero offset
instruction bidirectionally between the main control and a motor drive
inhibition section or motor-drive-inhibition cancellation section, to
thereby improve a steering feel at the time of the motor drive inhibition
and motor-drive-inhibition cancellation.
FIG. 1 is a block diagram showing a general organization of an electric
power steering apparatus for an automotive vehicle according to a first
preferred embodiment of the present invention. Mechanically, the electric
power steering apparatus 1 comprises a steering wheel 2, a steering shaft
3, a hypoid gear 4, a rack-and-pinion steering gear mechanism 5 including
a toothed pinion 5a and a rack shaft 5b, left and right steerable front
wheels 7 connected to the opposite ends of the rack shaft 5b via tie rods
6, and an electric motor 8 for generating and supplying an electric
steering assist to the vehicle steering system.
As electric components, the electric power steering apparatus 1 comprises a
vehicle velocity sensor 11, a steering torque sensor 12, a control unit
13, a motor driver circuit 14 and a motor current detector 15.
The vehicle velocity sensor 11 detects a velocity of the automotive vehicle
and generates a vehicle velocity signal V that is an electric signal
representing the detected vehicle velocity. The steering torque sensor 12
detects steering torque manually applied to the steering wheel 2 and
generates a steering torque signal T that is an electric signal
representing the detected steering torque. The steering torque signal T,
which has intensity and direction (i.e., polarity), is supplied to the
control unit 13 for processing to be described later. In the following
description, the direction or polarity of the steering torque signal T is
assumed to be positive (plus) when the steering torque is in the clockwise
direction and negative (minus) when the detected steering torque are in
the counterclockwise direction.
The control unit 13 of the electric steering apparatus 1, which comprises a
microprocessor, a memory etc., generates a target motor current signal
corresponding at least to the steering torque signal T, as well as an
ultimate motor control signal V.sub.O corresponding to the target motor
current signal. Thus, the control unit 13 controls the motor 8 with a PWM
(Pulse-Width-Modulated) motor voltage V.sub.M by means of the motor driver
circuit 14.
The motor driver circuit 14 includes a bridge circuit composed of a
plurality of (e.g., four) switching elements, such as power FETs (Field
Effect Transistors) or IGBTs (Insulated-Gate Bipolar Transistors), and
generates the PWM motor voltage V.sub.M on the basis of the ultimate motor
control signal V.sub.O so that the motor 8 is driven to rotate in the
forward or reverse direction in response to the motor voltage V.sub.M.
The motor current detector 15 converts an actual or detected motor current
I.sub.M into voltage by means of a resistor, Hall effect device or the
like connected in series with the electric motor 8, and sends a detected
motor current signal I.sub.MF, representing the motor current I.sub.M, to
the control unit 13 for negative feedback to the target motor current
signal I.sub.MS.
As the vehicle driver turns the steering wheel 2 to the left or right, the
manual steering torque applied to the steering shaft 3 is converted, via
the rack-and-pinion steering gear mechanism 5, into an axial linear
movement of the rack shaft 5b, which changes the direction of the
steerable front wheels 7 by way of the tie rods 6. To assist the driver's
manual steering effort, the electric motor 8 is driven in accordance with
the steering torque signal T, and output power or torque thus generated by
the electric motor 8 is increased twofold via the hypoid gear 4 and
applied to the steering shaft 3 as electric steering assist torque to
reduce the driver's manual steering effort.
FIG. 2 is a functional block diagram of the control unit 13 in the electric
power steering apparatus 1 of FIG. 1. The control unit 13 includes a
target motor current setting section 21 for setting a target motor current
signal corresponding at least to the detected steering torque signal T, an
offset calculator section 22 for calculating a difference or offset
between values of the target motor current signal and the detected motor
current signal, and a drive control section 23 for generating the ultimate
motor control signal V.sub.O (e.g., a composite of ON/OFF signal and PWM
signal) based on the difference (negative feedback) between the values of
the target motor current signal and the detected motor current signal
I.sub.MF corresponding to the motor current I.sub.M detected by the motor
current detector 15. The control unit 13 controls operation of the motor
driver circuit 14 so that the difference or offset between the target
motor current signal and the detected motor current signal I.sub.MF
promptly becomes zero (0).
The control unit 13 also includes a motor drive inhibition section 18 that
inhibits output of the motor control signal V.sub.O on the basis of the
steering torque signal T or a combination of the steering torque signal T
and detected motor current signal I.sub.MF. More specifically, this motor
drive inhibition section 18 generates drive inhibition signals S.sub.K1
and S.sub.K2 for inhibiting output of the motor control signal V.sub.O
within a value range to be limited in correspondence with the steering
torque signal T or the motor control signal V.sub.O within a value range
to be limited in correspondence with the combination of the steering
torque signal T and detected motor current signal I.sub.MF.
The control unit 13 further includes a zero-value setting section 17 that,
when the drive inhibition signals S.sub.K1 and S.sub.K2 are output from
the motor drive inhibition section 18, sets the target motor current
signal I.sub.MS to a zero value irrespective of the value of the steering
torque signal T, and an inhibition cancellation section 19 that cancels
the drive inhibition signals S.sub.K1 and S.sub.K2 when the offset between
the values of the target motor current signal and detected motor current
signal becomes zero after the target motor current signal I.sub.MS is set
to the zero value by the zero-value setting section 17.
The above-mentioned target motor current setting section 21, offset
calculator section 22, drive control section 23 and zero-value setting
section 17 together constitute the microprocessor-based main power
steering control in the inventive steering apparatus. The motor drive
inhibition section 18 and inhibition cancellation section 19, on the other
hand, together constitute a subsidiary power steering control that is
based on a digital circuit made up of a microprocessor and other discrete
components.
The target motor current setting section 21 of the control unit 13 includes
a memory, such as a ROM, which has prestored therein data indicative of a
control curve of the target motor current signal I.sub.MS versus the
steering torque signal T with the vehicle velocity V as a parameter, as
shown in FIG. 11, where "V.sub.1 " represents a low vehicle velocity
level, "V.sub.m " a medium vehicle velocity level and "V.sub.h " a high
vehicle velocity level. Upon receipt of the steering torque signal T from
the steering torque sensor 12 and the vehicle velocity signal V from the
vehicle velocity sensor 11, the target motor current setting section 21
reads out one of the prestored data or values of the target motor current
signal I.sub.MS corresponding to the values of the received steering
torque signal T and vehicle velocity signal V and passes the read-out
value to the zero-value setting section 17 as the target motor current
signal I.sub.MS. As clear from FIG. 11, the target motor current signal
I.sub.MS in the preferred embodiment is set to decrease in value relative
to the steering torque T as the vehicle velocity V increases
(V.sub.1.fwdarw.V.sub.m.fwdarw.V.sub.h), so that great electric steering
assist torque is given at low vehicle velocities while stable vehicle
behavior is achieved at high vehicle velocities.
The offset calculator section 22 computes a difference or offset .DELTA.I
between values of a modified target motor current signal I.sub.O supplied
from the zero-value setting section 17 and the detected motor current
signal I.sub.MF from the motor current detector 15 (i.e., .DELTA.I=I.sub.O
-I.sub.MF) and then supplies the drive control section 23 with an electric
signal indicative of the thus-computed offset .DELTA.I. By thus employing
the offset .DELTA.I between the modified target motor current signal
I.sub.O and the detected motor current signal I.sub.MF, the control unit
13 forms a negative feedback loop.
As shown in FIG. 7, the drive control section 23 includes a PID controller
41 and a motor control signal generator section 42. The drive control
section 23 performs P (Proportional) control, I (Integral) control and D
(Differential) control on the offset signal .DELTA.I fed from the offset
calculator section 22, and the motor control signal generator section 42
generates a motor control signal V.sub.D on the basis of the offset signal
.DELTA.I having undergone the above-mentioned PID control. The
thus-generated motor control signal V.sub.D is supplied to the motor drive
inhibition section 18.
The motor drive inhibition section 18 outputs the drive inhibition signals
S.sub.K1 and S.sub.K2 for inhibiting output of the motor control signal
V.sub.D, on the basis of the steering torque signal T supplied from the
torque sensor 12. The motor drive inhibition section 18 includes a
drive-inhibition-condition determination section 25 and an output
inhibition section 26. The drive-inhibition-condition determination
section 25 in the first embodiment includes a memory, such as a ROM, a
comparator section and an output section. Thus, the
drive-inhibition-condition determination section 25 makes a comparison
between the steering torque signal T given from the torque sensor 12 and a
plus reference torque value T.sub.P (or minus reference torque value
-T.sub.P) prestored in memory and supplies the output inhibition section
26 and zero-value setting section 17 with the drive inhibition signals
S.sub.K1 and S.sub.K2 corresponding to the result of the comparison.
When the drive inhibition signals S.sub.K1 and S.sub.K2 are supplied from
the motor drive inhibition section 18, the zero-value setting section 17,
which includes a zero-value generator section 27 and a multiplier 28, sets
the target motor current signal I.sub.MS to a zero value irrespective of
the value of the steering torque signal T. The zero-value generator
section 27, including a logic operator and a memory such as a ROM, detects
a motor drive inhibition condition of the electric motor 8 on the basis of
the drive inhibition signals S.sub.K1 and S.sub.K2 supplied from the motor
drive inhibition section 18 and then supplies a prestored coefficient
K.sub.S of value "0" to the multiplier 28. When the motor drive inhibition
condition of the electric motor 8 is not being detected, the zero-value
generator section 27 supplies a prestored coefficient K.sub.S of value "1"
to the multiplier 28.
When the zero-value generator section 27 is detecting the drive inhibition
condition of the electric motor 8, the multiplier 28 multiplies the target
motor current signal I.sub.MS from the section 21 by the "0" coefficient
K.sub.S from the zero-value generator section 27, to compulsorily set the
modified target motor current signal I.sub.O to the zero value. The
thus-set modified target motor current signal I.sub.O is supplied to the
offset calculator section 22. When the zero-value generator section 27 is
not detecting the drive inhibition condition of the electric motor 8, the
multiplier 28 multiplies the target motor current signal I.sub.MS by the
"1" coefficient K.sub.S given from the zero-value generator section 27, to
supply the offset calculator section 22 with the target motor current
signal I.sub.MS itself as the modified target motor current signal
I.sub.O.
FIG. 3 is a block diagram showing an exemplary detailed construction of the
zero-value generator section 27 shown in FIG. 2. In the illustrated
example of FIG. 3, the zero-value generator section 27 includes a first
storage section 31, a second storage section 32, a logic operator 33 and a
selector section 34. The first storage section 31 comprises a memory, such
as a ROM, having stored therein a coefficient "1" that is passed to the
selector section 34 as a coefficient signal K.sub.1. The second storage
section 32 also comprises a memory, such as a ROM, having stored therein a
coefficient "0" that is passed to the selector section 34 as a coefficient
signal K.sub.0.
The logic operator 33 has an exclusive OR function. More specifically, the
logic operator 33 performs an exclusive OR operation on the drive
inhibition signals S.sub.K1 and S.sub.K2 supplied from the
drive-inhibition-condition determination section 25 of the motor drive
inhibition section 18 as shown in FIG. 2, to thereby detect whether the
motor drive inhibition condition has been identified by the determination
section 25. Detection signal S.sub.K0 output from the logic operator 33 is
passed to the selector section 34. More specifically, when the drive
inhibition signals S.sub.K1 and S.sub.K2 are both at high (H) level, for
example, the logic operator 33 passes, to the selector section 34, a low
(L)-level detection signal S.sub.K0 indicating that the electric motor 8
is to be driven. When one of the drive inhibition signals S.sub.K1 or
S.sub.K2 is at high level and the other of the drive inhibition signals
S.sub.K2 or S.sub.K1 is at low level, the logic operator 33 passes, to the
selector section 34, a high-level detection signal S.sub.K0 indicating
that the operation or driving of the electric motor 8 is to be inhibited.
The selector section 34, on the basis of the detection signal S.sub.K0 from
the logic operator 33, selects either the coefficient signal of value 1
K.sub.1 supplied from the first storage section 31 or the coefficient
signal K.sub.0 of value 0 supplied from the second storage section 32, and
passes the selected coefficient K.sub.1 or K.sub.0 to the multiplier 28 as
the coefficient K.sub.S. More specifically, when the detection signal
S.sub.K0 is at low level, the selector section 34 determines that the
motor 8 is to be driven and thus selects the coefficient K.sub.1 to
thereby output the coefficient K.sub.S of value "1". When the detection
signal S.sub.K0 is at high level, the selector section 34 determines that
the driving of the motor 8 is to be inhibited and thus selects the
coefficient K.sub.0 to thereby output the coefficient K.sub.S of value
"0".
As mentioned, when the condition for driving the motor 8 is detected, the
zero-value setting section 17 of FIG. 2 is caused to output the
coefficient K.sub.S of value 1 to the multiplier 28, which in turn
multiplies the target motor current signal I.sub.MS supplied from the
target motor current setting section 21 by the coefficient "1" K.sub.S, so
that the target motor current signal I.sub.MS unmodified in value is
output from the zero-value setting section 17 as the modified target motor
current I.sub.O.
When, on the other hand, the condition for inhibiting the driving of the
motor 8 (i.e., motor drive inhibition condition) is detected, the
zero-value setting section 17 is caused to output the coefficient K.sub.S
of value 0 to the multiplier 28, which thus multiplies the target motor
current signal I.sub.MS supplied from the target motor current setting
section 21 by the coefficient "0" K.sub.S, so that the target motor
current signal I.sub.MS having been compulsorily set to the zero value is
output from the zero-value setting section 17 as the modified target motor
current I.sub.O.
FIG. 4 is a block diagram showing in detail an exemplary structure of the
drive-inhibition-condition determination section 25 in the motor drive
inhibition section 18. In the illustrated example of FIG. 4, the
drive-inhibition-condition determination section 25 includes a torque
value storage section 36, a comparator section 37 and an inhibition signal
output section 38. The torque value storage section 36 comprises a memory
such as a ROM, where there are provided a first memory section 36a for
storing a positive or plus (+) reference torque value T.sub.P, and a
second memory section 36b for storing a negative or minus (-) reference
torque value -T.sub.P. The plus reference torque value T.sub.P and minus
reference torque value -T.sub.P are both supplied to the comparator
section 37.
The comparator section 37 includes first and second comparators 37a and
37b. The first comparator 37a makes a comparison between the plus
reference torque value T.sub.P from the first memory section 36a and the
steering torque signal T from the steering torque sensor 12. Result of the
comparison by the first comparator 37a is passed to a first flip-flop 38a
of the inhibition signal output section 38, via which it is supplied to
the output inhibition section 26 (FIG. 2) as the drive inhibition signal
S.sub.K2.
When the value of the steering torque signal T is equal to or smaller than
the plus reference torque value T.sub.P (T.ltoreq.T.sub.P), the first
comparator 37a gives a low level signal to a set (S) terminal of the
flip-flop 38a. Then, the flip-flop 38a provides, through one of its
outputs (Q), a high-level drive inhibition signal S.sub.K2 as the
inversion of the low-level input signal to the set (S) terminal. The
high-level drive inhibition signal S.sub.K2 is supplied to the output
inhibition section 26 as mentioned above. When, on the other hand, the
value of the steering torque signal T is greater than the plus reference
torque value T.sub.P (T>T.sub.P), the first comparator 37a gives a high
level signal to the set (S) terminal of the flip-flop 38a, in response to
which the flip-flop 38a is set to provide, through the output (Q), a
low-level drive inhibition signal S.sub.K2 as the inversion of the
high-level input signal to the set (S) terminal. The high-level drive
inhibition signal S.sub.K2 is supplied to the output inhibition section 26
as mentioned above. The high-level drive inhibition signal S.sub.K2
permits the electric motor 8 to rotate in the reverse direction (to give a
leftward steering assist), while the low-level drive inhibition signal
S.sub.K2 inhibits the electric motor 8 from rotating in the reverse
direction (i.e., inhibits the leftward steering assist).
The second comparator 37b makes a comparison between the minus reference
torque value -T.sub.P from the second memory section 36b and the steering
torque signal T from the steering torque sensor 12. Result of the
comparison by the second comparator 37b is passed to a second flip-flop
38b of the inhibition signal output section 38, via which it is supplied
to the output inhibition section 26 (FIG. 2) as the drive inhibition
signal S.sub.K1.
When the value of the steering torque signal T is equal to or smaller than
the minus reference torque value -T.sub.P (T.ltoreq.-T.sub.P), the second
comparator 37b gives a high level signal to a set (S) terminal of the
flip-flop 38b. Then, the second flip-flop 38b provides, through one of its
outputs (Q), a low-level drive inhibition signal S.sub.K1 as the inversion
of the high-level input signal to the set (S) terminal. The low-level
drive inhibition signal S.sub.K1 is supplied to the output inhibition
section 26 of FIG. 2. When, on the other hand, the value of the steering
torque signal T is greater than the minus reference torque value -T.sub.P
(T>-T.sub.P), the second comparator 37b gives a low level signal to the
set (S) terminal of the flip-flop 38b, in response to which the flip-flop
38b is set to provide, through the output (Q), a high-level drive
inhibition signal S.sub.K1 as the inversion of the low-level input signal
to the set (S) terminal. The high-level drive inhibition signal S.sub.K1
is supplied to the output inhibition section 26 similarly to the
above-mentioned. The high-level drive inhibition signal S.sub.K1 permits
the electric motor 8 to rotate in the forward direction (to give a
rightward steering assist), while the low-level drive inhibition signal
S.sub.K1 inhibits the electric motor 8 from rotating in the forward
direction (i.e., inhibits the rightward steering assist).
FIG. 13 is a diagram illustrating regions where output of the detected
motor current signal I.sub.MF is inhibited by the
drive-inhibition-condition determination section depending on the value of
the steering torque signal T. In the example of FIG. 13, output of the
minus (-) or negative detected motor current signal I.sub.MF is inhibited
over the region where the value of the steering torque signal T exceeds
the plus reference torque value T.sub.P (T>T.sub.P) (hereinafter,
"rightward assist inhibition region"). Output of the plus (+) or positive
detected motor current signal I.sub.MF is inhibited over the region where
the value of the steering torque signal T is below the minus reference
torque value -T.sub.P (T<-T.sub.P) (hereinafter, "leftward assist
inhibition region"). Note that the rightward assist inhibition corresponds
to inhibiting the motor 8 from rotating in the reverse dire | | |
