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Claims  |
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What is claimed is:
1. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle, comprising:
a motor controlling section that outputs a rotational direction drive
command current to a motor drive circuit connected to an electric
reversible motor to regulate a rotational direction of the electric motor
in accordance with the rotational direction drive command current;
a drive current detection circuit to detect magnitude and direction of a
rotational direction drive current flowing through the motor; and
an abnormality determining section that detects a state in which the
detected rotational direction drive current indicates a direction opposite
to the direction of the rotational direction drive command current and
determines whether the state is in excess of a predetermined abnormality
determination criterion so as to determine an occurrence of an abnormality
of the power-assisted steering system, the abnormality determining section
comprising a sensitivity reducing section that lowers a sensitivity of the
abnormality determination criterion as an absolute value of the detected
rotational direction drive current becomes smaller.
2. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 1, wherein the predetermined
abnormality determination criterion is a number of times the rotational
direction drive current is reverse to the rotational direction drive
command current and the abnormality determining section determines whether
the number of times the rotational direction drive current is reverse to
the rotational direction drive command current has exceeded a
predetermined number of times so as to determine whether the abnormality
occurs in the power-assisted steering system.
3. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 2, wherein the sensitivity
reducing section lowers the sensitivity in the abnormal determination
criterion by increasing the abnormality determination criterion, as
compared with a case of a larger absolute value of the rotational
directional drive current, when the absolute value of the rotational
direction drive current becomes smaller.
4. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 2, wherein the sensitivity
reducing section lowers the sensitivity in the abnormal determination
criterion by decreasing an incremental count number on the number of times
the rotational directional drive current is reverse to the rotational
direction drive command current, as compared with a case of a larger
absolute value of the rotational direction drive current, when the
absolute value of the rotational direction drive current becomes smaller.
5. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 4, wherein the abnormality
determining section comprises diagnostic circuitry installed in the
microcomputer together with the motor controlling section and the drive
current detection section comprises a voltage difference measuring section
that measures a voltage difference between both ends of a resistor
interposed between the motor and converts the voltage difference
therebetween into a current value I which is caused to flow through the
motor.
6. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 2, wherein the abnormality
determining section determines the occurrence in the abnormality of the
motor-driven power-assisted steering system by determining whether a total
of the number of times the rotational direction drive current is reverse
to the rotational direction drive command current exceeds an abnormal
determination criterion in a predetermined sampling time.
7. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 6, wherein the abnormality
determining section repeats the abnormality determination for a
predetermined time longer than a product of the other predetermined number
for the abnormality determination by the sampling time.
8. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 6, wherein the abnormality
determining section comprises a counter to count the number of times the
rotational direction drive current is reverse to the rotational
directional drive command current for each predetermined sampling time in
an accumulation manner, the counter being reset to zero when the direction
of the rotational direction drive current is the same as the rotational
direction drive command current.
9. A diagnostic apparatus for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 1, wherein the rotational
direction drive torque command value is generated by the motor controlling
section constituted by a microcomputer and is supplied to the motor drive
circuit comprising an H bridge circuit with the electric motor.
10. A diagnostic apparatus for a motor driven power-assisted steering
system of an automotive vehicle as claimed in claim 1 further comprising a
system disconnecting section that disconnects a battery line connecting
the electric motor to a battery to execute a system interruption of the
power-assisted steering system when the abnormality determining section
determines the occurrence of the abnormality in the power-assisted
steering system.
11. A diagnostic apparatus for a motor driven power-assisted steering
system of an automotive vehicle as claimed in claim 1, wherein the
abnormality determining section detects the state in which the detected
rotational direction drive current indicates a direction opposite to the
direction of the rotational direction drive command current by a
comparison of the rotational direction drive current and the rotational
direction drive command current.
12. A diagnostic apparatus for a motor driven power-assisted steering
system of an automotive vehicle as claimed in claim 11, wherein the
detected rotational direction drive current corresponds to a magnitude and
a direction of an actual drive torque.
13. A diagnostic apparatus for a motor driven power-assisted steering
system of an automotive vehicle as claimed in claim 12, wherein the
electric motor is a DC motor.
14. A diagnostic apparatus for motor driven power-assisted steering system
of an automotive vehicle, comprising:
motor controlling means for outputting a rotational direction drive command
current to a motor drive circuit connected to an electric reversible motor
to regulate a rotational drive of the electric motor and a direction
thereof in accordance with the rotational direction drive command current:
drive current detecting means for detecting a magnitude and a direction of
a rotational direction drive current flowing through the motor;
abnormality determining means for detecting a state in which the detected
rotational direction drive current indicates a direction opposite to the
direction of the rotational direction drive command current and for
determining whether the state is in excess of a predetermined abnormality
determination criterion so as to determine an occurrence of an abnormality
in the power-assisted steering system, the abnormality determining means
comprising sensitivity reducing means for lowering a sensitivity of the
abnormality determination criterion as an absolute value of the detected
rotational direction drive current becomes smaller.
15. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle, comprising:
outputting a rotational direction drive command current to a motor drive
circuit connected to an electric reversible motor to regulate a rotational
direction of the electric motor in accordance with the rotational
direction drive command current;
detecting a rotational direction drive current flowing through the motor;
detecting a state in which the detected rotational direction drive current
indicates a direction opposite to the direction of the rotational
direction drive command current;
determining whether the state is in excess of a predetermined abnormality
determination criterion so as to determine an occurrence of an abnormality
of the power-assisted steering system; and
lowering a sensitivity of the abnormality determination criterion as an
absolute value of the detected rotational direction drive current becomes
smaller.
16. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 15, wherein the predetermined
abnormality determination criterion is a number of times the rotational
direction drive current is reverse to the rotational direction drive
command current and, at the abnormality determining step, a determination
is made of whether the number of times the rotational direction drive
current is reverse to the rotational direction drive command current has
exceeded a predetermined number of times so as to determine whether the
abnormality occurs in the power-assisted steering system.
17. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 16, wherein, at the
abnormality determining step, the occurrence in the abnormality of the
motor-driven power-assisted steering system is determined by determining
whether a total of the number of times the rotational direction drive
current is reverse to the rotational direction drive command current
exceeds an abnormal determination criterion in a predetermined sampling
time.
18. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 16, wherein, at the
abnormality determining step, the abnormality determination is repeated
for a predetermined time longer than a product of the other predetermined
number for the abnormality determination by the sampling time.
19. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 16, wherein, at the
abnormality determining step, the sensitivity in the abnormal
determination criterion is lowered by decreasing an incremental count
number on the number of times the rotational direction drive current is
reverse to the rotational direction drive torque command current, as
compared with a case of a larger absolute value of the rotational
direction drive current, when the absolute value of the rotational
direction drive current becomes smaller.
20. A diagnostic method for a motor driven power-assisted steering system
of an automotive vehicle as claimed in claim 16, wherein, at the
abnormality determining step, the sensitivity in the abnormal
determination criterion is lowered by increasing the abnormality
determination criterion, as compared with a case of a larger absolute
value of the rotational direction drive current, when the absolute value
of the rotational direction drive current becomes smaller. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to diagnostic apparatus and method for a
motor driven power-assisted steering system of an automotive vehicle.
b) Description of the Related Art
A Japanese Patent Application First Publication No. Heisei 11-342859
published on Dec. 14, 1999 exemplifies a previously proposed diagnostic
apparatus for a motor driven power-assisted steering system of an
automotive vehicle.
The previously proposed diagnostic apparatus includes a direction
abnormality detection circuit to determine whether there is an abnormality
in a power-assisted steering direction on the basis of a sub torque signal
from a steering torque sensor and a command from a CPU (Central Processing
Unit) of a control unit.
The direction abnormality detection circuit determines the abnormality in
the power-assisted steering direction depending upon whether a
normal/reverse revolution drive command current to command a revolution
drive of the motor and a normal/reverse revolution drive command current
to command a revolution drive direction.
SUMMARY OF THE INVENTION
However, since the actual normal/reverse drive current supplied to the
electric motor includes noises, a current which is reverse to the
normal/reverse revolution drive command current is instantaneously caused
to flow to the electric motor irrespective of the fact that the
power-assisted steering direction is normal in a case where the steering
torque is small and an absolute value of the normal/reverse drive current
is small.
Therefore, even if the operation of the power-assisted steering system is
normal, the diagnostic apparatus erroneously determines that the
power-assisted direction is abnormal and a fail-safe facility is activated
on the basis of this erroneous determination so that the power-assisted
steering is stopped.
Although this problem would be solved to some degree by setting a
sensitivity in an abnormal criterion to a duller direction, this causes a
delay in the abnormality determination due to a reduction in a sensitivity
when the abnormality occurs in the power-assisted steering system in a
case where the steering torque is large and the absolute value of the
normal/reverse drive current is large.
It is, hence, an object of the present invention to provide improved
diagnostic apparatus and method for a motor driven power-assisted steering
system of the vehicle which can reduce an erroneous determination of
abnormality due to an influence of the noises included in the
normal/reverse drive current even if the steering torque is small and the
current value of the normal/reverse drive current supplied to the motor is
small without reduction in the sensitivity in the abnormality
determination when the power-assisted steering operation is required.
The above-described object can be achieved by a diagnostic apparatus for a
motor driven power-assisted steering system of an automotive vehicle,
comprising: a control section that regulates a magnitude of a drive torque
and a direction of the drive torque to be applied to an electric motor in
a form of a normal/reverse drive torque command current; a drive current
detection circuit to detect a normal/reverse drive current to be supplied
to the motor; an abnormality determining section that detects a state in
which the detected normal/reverse drive current indicates an opposite to a
direction of the normal/reverse drive torque command current and
determines whether the state is in excess of a predetermined abnormality
determination criterion so as to determine an occurrence in abnormality of
the power-assisted steering system, the abnormality determining section
comprising a sensitivity reducing section that varies a sensitivity of the
abnormality determination criterion in a decrease direction as an absolute
value of the detected normal/reverse drive current becomes smaller.
The above-described object can also be achieved by providing a diagnostic
method for a motor driven power-assisted steering system of an automotive
vehicle, comprising: regulating a magnitude of a drive torque and a
direction of the drive torque to be applied to an electric motor in a form
of a normal/reverse drive torque command current: detecting a
normal/reverse drive current to be supplied to the motor; detecting a
state in which the detected normal/reverse drive current indicates an
opposite to a direction of the normal/reverse drive torque command
current; determining whether the state is in excess of a predetermined
abnormality determination criterion so as to determine an occurrence in
abnormality of the power-assisted steering system; and varying a
sensitivity of the abnormality determination criterion in a decrease
direction as an absolute value of the detected normal/reverse drive
current becomes smaller.
The other objects and features of this invention will become understood
from the following description with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a whole configuration view of a motor-driven power-assisted
steering system of an automotive vehicle to which a diagnostic apparatus
of a first preferred embodiment according to the present invention is
applicable.
FIG. 2 is a detailed circuit block diagram of the diagnostic apparatus for
the motor-driven power-assisted steering system shown in FIG. 1.
FIG. 3 is a more detailed circuit block diagram of the diagnostic apparatus
for the motor-driven power-assisted steering system shown in FIG. 2.
FIG. 4 is a current value conversion graph used in the diagnostic apparatus
of the first preferred embodiment shown in FIGS. 1 through 3.
FIG. 5 is a timing chart representing the current value I with number of
times sampling is carried out during an abnormality determination as a
time axis.
FIG. 6 is another timing chart representing the current value I with number
of times sampling is carried out during an abnormality determination as a
time axis.
FIG. 7 is an operational flowchart representing a method of modifying a
sensitivity in an abnormality determination criterion of the motor-driven
power-assisted steering system in a second preferred embodiment according
to the present invention.
FIG. 8 is a characteristic graph of an error certainty threshold value
variable characteristic of the diagnostic apparatus with respect to the
current value in a third preferred embodiment according to the present
invention.
FIG. 9 is a characteristic graph of an error certainty threshold value
count value of the diagnostic apparatus with the number of time the
sampling is carried out during the abnormality determination as the time
axis in the third preferred embodiment according to the present invention.
FIG. 10 is a characteristic graph of an error count value of the diagnostic
apparatus with the number of time the sampling is carried out during the
abnormality determination as the time axis in the third preferred
embodiment according to the present invention.
FIG. 11 is a characteristic graph of an error certainty threshold value
variable characteristic of the diagnostic apparatus with respect to the
current value in a fourth preferred embodiment according to the present
invention.
FIG. 12 is a timing chart representing the error certainty threshold value
with the number of times the sampling is carried out during the
abnormality determination as the time axis in the diagnostic apparatus of
the fourth embodiment according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference will hereinafter be made to the drawings in order to facilitate a
better understanding of the present invention.
(First Embodiment)
FIG. 1 shows a whole configuration of a motor driven power-assisted
steering system for an automotive vehicle to which a diagnostic apparatus
in a first preferred embodiment according to the present invention is
applicable.
As shown in FIG. 1, when a steering wheel SW is manually rotated, this
manual rotation is converted through a rack L and pinion P into a straight
line motion by the rack L. This can change (steering wheel operation)
direction of front left and right road wheels TL and TR. In addition,
pinion P is rotatable via speed-reduction gear G via an electric motor M
so that the power assistance for a steering force generated by a
multiplication of the steering wheel can be achieved.
In addition, a drive force exerted by an electric motor (reversible motor)
M is controlled on the basis of a signal from a torque sensor TS detecting
a manual steering force and steering direction thereof and performs a
drive control by a microcomputer A incorporated into a vehicular control
unit (called, ECU). This drive control performs an assistance control of a
steering force in accordance with the manually set steering direction.
In FIG. 1, R denotes a fail-safe purpose relay and B denotes a vehicular
battery.
Next, FIG. 2 shows a system block diagram of the motor driven
power-assisted steering system to which the diagnostic apparatus in the
first preferred embodiment according to the present invention is
applicable.
FIG. 3 shows an electric circuit wiring diagram of the motor-driven
power-assisted steering system shown in FIG. 2.
As shown in FIGS. 2 and 3, the control unit (ECU) includes: a
control/arithmetic operation section (microcomputer) A; a motor drive
circuit C; a power supply circuit D; a motor current detecting circuit E;
a fail-safe relay R for the fail-safe purpose; and a shunt resistance SR.
A diagnostic circuit F is installed.
In FIGS. 2 and 3, I denotes an ignition switch and FL denotes a
failure-indicative lamp.
It is noted that the microcomputer A receives a vehicular velocity
indicative signal S, an engine speed signal r, a torque signal t from a
torque sensor TS, and a self-diagnostic signal f.
In details, the microcomputer A outputs a normal/reverse torque command
current Is to motor drive circuit C in accordance with the torque signal t
(manual directional and steering force) from the torque signal t from the
torque sensor TS.
The motor drive circuit C serves to drivingly control the electric motor M
(reversible motor) in either the normal or reverse direction. As shown in
FIG. 3, the motor drive circuit C is constituted by an H bridge circuit of
drive power supply side switching transistors FET H1 and FET H2 and of
ground side switching transistors FET L1 and FET L2. Each switching
transistor FET H1 through FET L2 is drivingly controlled in an on-and-off
control mode by means of a pre-driver PD on the basis of the
normal/reverse torque command current Is from the microcomputer A so that
the normal/reverse drive control of electric motor M is carried out in an
on-and-off mode.
The fail-safe relay R is intervened in a battery line connecting between
the electric motor M and battery B.
That is to say, this relay R serves to disconnect the battery line during
the fail-safe mode of the motor-drive power-assisted steering system and
an on-and-off control of the relay R is carried out by means of the
microcomputer A.
The shunt resistance SR is a resistor serially inserted in a midway through
a motor line ML in order to detect a current flowing through the drive
motor M.
The motor current detector E detects a voltage difference (.DELTA.Vn=V1-V2)
between both ends of the shunt resistance SR. A current value
(normal/reverse drive current) I supplied to the electric motor M is
determined according to a current value conversion graph shown in FIG. 4
according to the voltage difference (voltage drop value) .DELTA.Vn.
The diagnostic circuitry F detects a state in which the current value I
detected by the motor current detection circuit E is a reverse current to
the normal/reverse torque command current Is and determines an abnormality
of the power-assisted steering system depending on whether the state is in
excess of a predetermined abnormality criterion. The contents of
determination in the diagnostic circuitry F will be described below with
reference to FIGS. 5 and 6. FIG. 5 shows a timing chart representing the
current value I with the number of times a sampling for the current value
I at a time of the abnormality determination is carried out as a time
axis. FIG. 6 shows a timing chart representing an error count value with
the number of times the sampling is carried out as the time axis during
the abnormality determination.
That is to say, in the first preferred embodiment according to the present
invention, the abnormality occurrence determination criterion is the
number of times the current value I reverse to the normal/reverse torque
command current Is is generated. The abnormality determination of the
normal/reverse steering system is carried out by determining whether a
total (error count value) of the number of times the reverse current I has
been generated are counted for each predetermined sampling time t are in
excess of 100 counts (error certainty threshold value).
The above-described diagnostic circuitry F repeats the abnormality
determination for each predetermined period of time which is longer than
an product of the other predetermined number for the abnormality
determination by the sampling time t.
In addition, the above-described diagnostic circuitry F counts in an
accumulation manner for each IO count whenever the reverse current value I
is once generated, as denoted by a broken line of FIG. 5 in a case where
an absolute value of the current value I detected by the motor current
detecting circuit E is in excess of IO. This causes the number of times
the sampling is carried out to be ten times and the error count value to
be equal to or larger than the error certainty threshold value (100).
Consequently, an earlier abnormality determination can be carried out. In
other words, a high sensitivity in the abnormality determination criterion
can be achieved.
As denoted by a solid line in FIG. 5, in a case where the absolute value of
the detected current value I by the motor current detection circuit E is
equal to or below IO, such a modification as an accumulation count for
each one count of the error count for each count of the error count value
is made so that when the number of times the sampling has been carried out
has reached to 100 number of times, the error count value C indicates a
value equal to or larger than the error certainty threshold value (100).
That is to say, the sensitivity in the abnormality determination criterion
can be increased.
Dot-and-dash lines in FIGS. 5 and 6 denote varied states of the current
value I in which the current crosses over I.sub.0.
As described above, in the diagnostic apparatus for the motor-driven
power-assisted steering apparatus in the first embodiment according to the
present invention, when the steering torque is small and the absolute
value of the current value I is small, the error count value is counted in
the accumulation manner for each count so that the sensitivity in
abnormality determination can be reduced. Hence, the erroneous
determination of the abnormality occurrence due to an influence of the
noise included in the current value I can be reduced. When the power
assistance steering is required, i.e., when the steering torque is so
large that the absolute value of the current value I is large, I.sub.0
counts of the error count value is counted in the accumulation manner to
maintain the high sensitivity in the abnormal determination. Hence, the
abnormal state is determined at an earlier time and the fail-safe
structure is activated so that the power-assisted steering operation is
halted.
In addition, as described hereinabove, since the error certainty threshold
value itself is maintained at a predetermined time (100) and the current
value I is varied periodically at a large amplitude, a stable abnormality
determination can be achieved.
Since the number of times the current value I which is reverse to the
normal/reverse torque command current Is has been developed serves as the
abnormality determination criterion, such as erroneous determination as
would be developed when the reverse current value I is determined by the
corresponding threshold value even if the reverse current value I
instantaneously due to the noise influence is caused to flow can be
prevented from occurring so that the abnormal condition only can be
detected without failure.
Since the abnormality occurrence determination is made depending on whether
the total number of times the current value I reverse to the
normal/reverse torque command current is developed and is counted for each
predetermined sampling time t is in excess of the error certainty
threshold value, the determination of whether the abnormality occurs can
be made since the number of times per sampling period t are counted even
if such a continuation state as that in which the reverse current value I
is developed so that the abnormality state can be determined.
Furthermore, even if the abnormality occurrence determination is repeated
whenever a predetermined period of time has passed longer than the product
of the other predetermined number for the abnormality determination by the
sampling time so that the state in which the reverse current value I is
developed is continued, the number of times are counted and the
abnormality occurrence determination is carried out for each constant
period of time.
Consequently, the abnormality state can stably be determined.
(Second Embodiment)
The diagnostic apparatus for the motor driven power-assisted steering
system in a second preferred embodiment according to the present invention
is generally the same as that described in the first embodiment. However,
a part of method for modifying the sensitivity in the abnormality
determination criterion is made different from the first embodiment.
Hence, an operation in the second embodiment will be described with
reference to a flowchart of FIG. 7.
At a step 101, the diagnostic circuitry F determines whether an abnormality
(an abnormal operation or failure) in the power-assisted steering system
has been detected.
If Yes at step 101, the routine goes to a step 102.
If No at step 101, the routine jumps to an end step and this present
routine is ended.
At step 102, the diagnostic apparatus determines whether the motor current
(the current value I supplied to the motor M) is equal to or below
I.sub.0. If Yes at step 102, the routine goes to step 103. At step 103,
the error count value C.sub.0 is counted in the accumulation manner and
the routine goes to a step 109. If No at step 102, the routine goes to a
step 104.
At step 104, the motor current I is determined whether it is equal to or
below than I.sub.1. If Yes at step 104, the routine goes to a step 105 to
accumulate the error count value C.sub.1 and, thereafter, the routine goes
to a step 109. If No at step 104, the routine goes to a step 106.
At step 106, the diagnostic circuitry F determines whether the motor
current I is equal to or below I.sub.n-1.
If Yes at step 106, the routine goes to a step 107 at which the error count
value C.sub.n-1 is accumulated and the routine goes to a step 109. If No
at step 106, the routine goes to a step 108. At step 108, after the error
count value C.sub.n is accumulated, the routine goes to a step 109. At the
step 109, diagnostic circuitry F determines whether the error count value
is equal to or larger than the error certainty threshold value.
If Yes at step 109, after the routine goes to a step 110 to ensure the
abnormality in the power-assisted steering system, this routine is ended.
If No at step 109, this routine is directly ended.
It is noted that a relationship between each current value of motor
I.sub.0, I.sub.1, I.sub.n-1 and error count value C.sub.0, C.sub.1, - - -
, C.sub.n-1 and C.sub.n will be described as follows:
I.sub.0 <I.sub.1 < - - - <I.sub.n-1 ;
C.sub.0 <C.sub.1 < - - - <C.sub.n-1 <C.sub.n
In this way, such an incremental count number C.sub.0, C.sub.1, - - - ,
C.sub.n-1, C.sub.n on the number of times the normal/reverse drive current
which is reverse to the normal/reverse drive command current is developed
as described above is varied in the decrease direction as the motor drive
current I.sub.n-1, - - - , I.sub.1, I.sub.0 becomes reduced.
In details, in the second embodiment, the switching in the sensitivity in
the abnormality determination criterion is carried out at a multiple
stage.
It is noted that the error count value is reset to zero when the
normal/reverse drive current whose direction is the same as the
normal/reverse drive command current is developed.
Hence, the same advantages as those of the first embodiment can be obtained
in the second embodiment and a more accurate determination of abnormality
can be achieved.
(Third Embodiment)
As shown in FIG. 8, the diagnostic circuitry F in the diagnostic apparatus
for the motor driven power-assisted steering system in a third preferred
embodiment sets the error certainty threshold value to 30 in a case where
the absolute value of current value I detected by the motor current
detection circuit E is in excess of I.sub.0. Consequently, the abnormality
determination is made at the earlier stage. That is to say, the
sensitivity in the abnormality determination criterion can be enhanced.
On the contrary, in a case where, as shown in FIG. 8, the absolute value of
the current value I detected by the motor current detection circuitry E is
equal to or lower than I.sub.0, the error certainty threshold value is set
to 100. Thus, a discreet or careful abnormality determination can be
carried out. That is to say, the sensitivity in the abnormality
determination criterion can be reduced.
FIG. 9 shows a timing chart representing the error certainty threshold
value count with the number of times the sampling is carried out during
the abnormality determination.
FIG. 10 shows a timing chart representing the error count value with the
number of times the sampling is carried out during the abnormality
determination.
In FIGS. 9 and 10, a solid line denotes the absolute value of the current
value I which is equal to or below I.sub.0. In this case, the abnormality
determination is made at the first time when the error count value
indicates 100. In FIGS. 9 and 10, a broken line denotes the absolute value
of the current value I which is in excess of I.sub.0. In this case, the
abnormality determination is made at a time at which the error count value
indicates 30. In FIGS. 9 and 10, a dot-and-dash line denotes the absolute
value of the current value I which is in excess of I.sub.0 from a state in
which the absolute value thereof is equal to or smaller than I.sub.0. In
this case, the abnormality determination is carried out at the time at
which the error count value indicates 30.
Hence, the same advantages as described in the first embodiment can be
obtained in the third embodiment. In addition, even if the current value I
is periodically varied at a large amplitude, the abnormality determination
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