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CROSS REFERENCE TO RELATED APPLICATION
The present application is based on and claims priority from Japanese
Patent Application Hei 11-137019 filed May 18, 1999, the contents of which
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DC motor-assisted power steering
apparatus.
2. Description of the Related Art
A power assisting DC motor (hereinafter referred to as P.A. motor) of an
ordinary motor-assisted power steering apparatus is driven by a motor
drive unit which has a H-bridge circuit, a current detecting circuit, and
current control means. The H-bridge circuit has four power transistors to
supply the DC motor with pulse-width-modulated-drive-current. The current
detecting means detects drive current of the DC motor. The current control
means controls the H-bridge circuit in a PID (proportional integral
derivative) control on the basis of a difference between a command current
value and an actual drive current.
The command current value is provided according to a steering torque signal
and a vehicle speed signal. An ordinary current control means controls a
motor drive circuit in the manner of pulse width modulation (hereinafter
referred to as PWM-control) on the basis of the command current value and
the detected actual current value of the P.A. motor. In the current
control means, PI (proportional integral) feedback control is carried out
by the PI control operation means on the basis of the current difference
between the command current value and the actual current value to
determine a duty ratio. In addition, a direction command value is provided
to indicate the direction of the command current value.
However, if the PI feedback control is repeated according to an integration
formula having an integral compensation term, the integral compensation
term may increase to a value that cannot be neglected due to accumulation.
As a result, a large impulsive drive current may flow in the P.A. motor,
thereby causing a large impulsive torque or vibration applied on the
steering wheel.
Therefore, driving feels unnatural due to unpleasant bumps on the steering
wheel.
JP-A 10-203384 discloses a motor-assisted power steering apparatus which
basically carries out a digital feedback control on the basis of a
difference between command current value and actual current value. If an
actual current value is excessively large, it is limited to a
predetermined value, while an internal parameter is changed, so that the
response characteristic of the steering can be improved.
Although the duty ratio is limited when power assisting motor (P.A. motor)
is reversed, a considerable amount of the accumulated portion
corresponding to the integral term still remains. As a result,
abnormal-current detecting means may be erroneously operated, or the DC
motor may suddenly vibrate due to an excessive amount of impulsive current
flows in the P.A. motor when it is reversed. This results in bumps on the
steering wheel during the reversal of steering operation, thereby giving
the driver an unpleasant feeling.
SUMMARY OF THE INVENTION
Therefore, the present invention has an object of providing a P.A. motor
drive unit in which an impulse drive current, shocking motion, or
vibration is restrained even when the P.A. motor is reversed.
Another object of the invention is to provide a motor-assisted power
steering apparatus adopting the P.A. motor drive unit.
According to a feature of the invention, a P.A. motor drive unit for
driving a P.A. motor according to a command current value includes an
H-bridge circuit for supplying PWM-controlled drive current, current
detecting means for providing an actual current value, and current control
means for carrying out a PI feed back control according to difference
between the absolute values of the command current value and the actual
current value. In the above motor drive unit, the current control means is
comprised of PI operation control means for providing a PI control
equation having an integral compensation term, and compensation term
operation means for resetting the integral compensation term to a suitable
value when operation of the P.A. motor is changed from one state to
another.
Even if the integral compensation term increases when the P.A. motor is
reversed or changed to operate in a different state, integral compensation
term is forced to reset to a value close to zero or a negative value.
Accordingly, a large impulsive drive current may not be generated since
the duty ratio of the drive current is controlled to be small. As a
result, the response characteristic of the drive current to the command
current value is improved, and the P.A. motor can operate smoothly. Since
an impulsive large drive current is restrained, an erroneous operation of
a fail-safe means can be prevented when detecting abnormal current.
Moreover, since an impulsive current change is restrained, EMI trouble due
to radiation of noises to the surrounding devices can be restrained.
In the P.A. motor drive unit as defined above the change of operation of
the P.A. motor is judged by a change of the symbol of the command current
value.
In the above defined P.A. motor drive unit, the compensation term is reset
when the symbol of the command current value is changed and the direction
of the command current value becomes different from the rotation direction
of P.A. motor.
If the integral compensation term is reset only at the state of operation
of the P.A. motor when such excessive accumulation may very likely occur,
linearity of the control system will be maintained.
In the P.A. motor drive unit as defined above, the compensation term is
reset when the P.A. motor is started or when it is restarted after an
emergency stop due to an abnormal operation.
According to another feature of the invention, a power steering control
apparatus for a vehicle is comprised of command current operation means
for providing a command current value according to a steering torque
applied to a steering shaft and a vehicle speed, the P.A. motor drive unit
defined above, and a P.A. motor.
If the operation of the P.A. motor is changed, bump is prevented from being
applied to the steering wheel. As a result, the feeling of the steering
vehicle is improved. Because an abrupt change of the impulsive drive
current is restrained, surrounding devices are not negatively affected by
radiation of the noises, including EMI.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and characteristics of the present invention as
well as the functions of related parts of the present invention will
become clear from a study of the following detailed description, the
appended claims and the drawings. In the drawings:
FIG. 1 is a schematic diagram of a motor-assisted power steering system,
which includes a P.A. motor drive unit according to a first embodiment of
the invention;
FIG. 2 is a block diagram of a power steering control apparatus according
to the first embodiment;
FIG. 3 is a block diagram of the power steering apparatus according to the
first embodiment;
FIG. 4 is a flow diagram of a control logic of the P.A. motor drive unit
according to the first embodiment;
FIG. 5 is a graph demonstrating operation of the P.A. motor drive unit
according to the first embodiment;
FIG. 6 is a flow diagram of a control logic of a P.A. motor drive unit
according to a variation 1 of the first embodiment;
FIG. 7 is a block diagram of a power steering apparatus according to a
variation 2 of the first embodiment;
FIG. 8 is a block diagram of a main portion of a power steering apparatus
according to a second embodiment of the invention;
FIG. 9 is a flow diagram showing a control logic of the P.A. motor drive
unit according to the second embodiment;
FIG. 10 is a flow diagram of calculating reduction in the number of motor
revolutions in the second embodiment;
FIG. 11 is a graph demonstrating operation of the P.A. motor drive unit
according to the second embodiment;
FIG. 12 is a graph demonstrating operation of the second embodiment;
FIG. 13 is a flow diagram showing a control logic of the P.A. motor drive
unit according to a variation 2 of the second embodiment;
FIG. 14 is a flow diagram of a self diagnosis logic according to the
variation 2 of the second embodiment; and
FIG. 15 is a flow diagram of a temporary operation logic according to the
variation 2 of the second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
As shown in FIG. 1, a motor-assisted power steering apparatus according to
a first embodiment of the invention includes ECU 1000, which controls P.A.
motor 3 on the basis of a signal from steering torque sensor 1 and a
signal from vehicle speed sensor 2. P.A. motor 3 assists, via a speed
reduction unit, steering torque applied to a steering wheel by a driver to
change the direction of vehicle wheels. As shown in FIG. 2, ECU 1000
includes LPF (low pass filter), a wave-shaping circuit, microcomputer 100
having A-D converter 11 and CPU 111, drive circuit 27, and H-bridge
circuit 30. ECU 1000 controls P.A. motor 3 to increase steering torque in
response to driver's steering of a steering wheel. ECU 1000 changes the
drive or rotation direction of P.A. motor 3 according to the positive or
negative symbol of a command current value, which is mainly related to an
output signal of steering torque sensor 1.
As shown in FIG. 3, the power steering apparatus is functionally comprised
of command current value unit 10, current control unit 20, H-bridge
circuit 30, and current detecting unit 40. Current control unit 20,
H-bridge circuit 30, and current detecting unit 40 form a P.A. motor drive
unit according to the present embodiment of the invention. ECU 1000
includes this P.A. motor drive unit. In other words, ECU 1000 includes
current control unit 20, the P.A. motor drive circuit having H-bridge
circuit 30 and current detecting unit 40, and command current value unit
10.
Command current value unit 10 provides a command current value according to
a steering torque signal Tq that corresponds to a torque applied to a
steering shaft and a vehicle speed signal V that corresponds to a vehicle
having wheels to be steered. Command current value unit 10 includes A-D
converter 11, pulse measuring means 12, phase-compensation means 13
operated by software stored in CPU 111, current map 14, inertia
compensation means 15, and adding means 16.
After noises are removed by low-pass filter LPF, the output signal of
steering torque sensor 1 is converted into a digital signal A-D converter
11 to be inputted to phase compensation means 13 and inertia compensation
means 15 as a steering torque signal Tq.
The torque signal Tq that is inputted into phase compensation means 13 is
compensated to advance in phase by a digital calculation. The digital
calculation corresponds to the following transfer function:
H(s)=(.tau.s+1)/(A.tau.s+1), and is converted to a compensation torque
signal Tp that is inputted into current map 14.
If a vehicle speed signal from vehicle speed sensor 2 interrupts CPU 111
via the wave shaping circuit, pulse operation means 12 calculates a
vehicle speed signal V on the basis of time elapsed from the last vehicle
speed pulse. The vehicle speed signal V is also inputted to current map 14
together with the compensation torque signal Tp. Current map 14 provides a
basic command current value Icb for the steering assisted by an
interpolation or a functional calculation. The basic command current value
Icb becomes larger as the compensation torque signal Tp increases and
becomes smaller as the vehicle speed V increases.
Regarding the steering torque signal Tq inputted to inertia compensation
means 15, a difference thereof corresponding to a derivative value is
multiplied by a suitable gain and outputted from inertia compensation
means 15 as an inertia-compensated command current value Ici.
Subsequently, the basic command current value Icb and the
inertia-compensated command current value Ici are added to provide a
command current value Ic=Icb+Ici.
Current control unit 20 is an electronic unit that controls H bridge
circuit 30 according to the command current value Ic and the detected or
actual drive current Ima (that is an absolute value) supplied to P.A.
motor 3 from current detecting unit 40. In other words, current control
unit 20 is comprised of respective software operation means 21-26 formed
by CPU 111 and drive circuit 27, and is controlled according to the
direction command value Dir and duty ratio Dt provided by the same
software operation means.
The command current value Ic is inputted to absolute value operation means
21 and to direction command means 23 that are included in current control
unit 20. Direction command means 23 sets the direction command value Dir
that indicates the rotation direction of P.A. motor 3 to one of -1, 0, and
1 according to the positive or negative symbol of the command current
value Ic and supplies it to compensation term reset means 24 and to drive
circuit 27. During this process Dir=1 represents the normal direction
command, Dir=0 represents the stop command and Dir=-1 represents the
reversal command.
Absolute value operation means 21 calculates a command current value Ica
that is the absolute value .vertline.Ic.vertline. of the command current
value Ic, which is sent to subtraction means 22. Subtraction means 22
subtracts the above-stated detected drive current Ima (that is an absolute
value) from the command current value Ica to provide a current deviation
Id, which is sent to PI control means 25.
PI control means 25 performs a proportional-integration
feedback-control-calculation according to the current deviation Id
(=Ica-Ima) to calculate an appropriate limited drive voltage Vdg, which is
sent to duty ratio operation means 26. At this stage, compensation term
reset means 24 provides a reset command signal that is supplied to PI
control means 25 when the symbol of the command current value Ic is
changed between positive and negative in response to the direction command
value Dir.
Consequently, PI control means 25 resets the proportional integral term,
which has accumulated by then to zero, to a small positive or small
negative value. Duty ratio operation means 26 calculates a duty ratio Dt
between 0 and 100% according to the appropriately set limit drive voltage
Vdg and sends it to drive circuit 27.
Drive circuit 27 controls H-bridge circuit 30 according to the direction
command value Dir and the duty ratio Dt. In other words, drive circuit 27
performs the pulse width modulation (PWM) according to the direction
command value Dir and the duty ratio Dt and drives P.A. motor 3 by turning
on and off MOS-FETs 31-34.
H-bridge circuit 30 has four MOS-FETs 31-34, which are disposed around P.A.
motor 3 and connected to one another in the shape of the letter H.
H-bridge circuit 30 is applied a certain voltage via relay 5. MOS-FETs
31-34 are turned on or off by the PWM signal so that H-bridge circuit 30
can drive P.A. motor 3 in a designated direction.
Current detecting unit 40 includes shunt resistor 41 connected between an
end of H-bridge circuit 30 and ground, operational amplifier 42 for
detecting the drive current of P.A. motor 3 by a voltage drop across shunt
resistor 41 and A-D converter 43 for converting the output signal of
operational amplifier 42 into a digital signal. The digital current value
Ima is supplied to a subtraction means 22 of current control unit 20 as a
feedback signal.
Thus, the power steering control apparatus includes command current value
unit 10, current control unit 20, H-bridge circuit 30 and current
detecting unit 40. H-bridge circuit 30 supplies pulse-width-modulated
drive current to DC P.A. motor 3 to drive the same. On the other hand,
current detecting unit 40 is an electronic circuit that detects the
absolute value of the drive current of P.A. motor 3 to provide the
detected current value Ima.
Current control unit 20 performs PI feedback control according to the
absolute value Ica of the command current value and the current deviation
Id=Ica-Ima. Subsequently, a duty ratio Dt and the direction command value
Dir are provided to control H-bridge circuit 30. At this stage, current
control unit 20 has compensation-term-reset means 24, which resets the
proportional integral compensation term to a suitable reset value Vrst
when the operation of P.A. motor 3 is changed from one condition to
another in a prescribed manner.
The change of the operation can be detected when the symbol of the command
current value changes between either positive or negative, and when the
reset value Vrst is zero or a value smaller than a positive value or
larger than a negative value.
The power steering control apparatus functions according to the following
control logic, as shown in FIG. 4.
This control logic is started when microcomputer 111 is interrupted every
250 .mu. sec. The direction command value Dir and the duty ratio Dt are
provided according to the command current value Ic and the detected
current value Ima.
At steps S1-S5, a direction command operation is carried out, and the
direction command value Dir is provided according to the command current
value Ic. If Ic>0, then Dir=1; if Ic=0, then Dir=0; and if Ic<0,
then Dir=-1.
At steps S6-S10, the limited drive voltage Vdg is calculated from the
command current value Ic and the detected current value Ima. In other
words, the absolute value Ica=.vertline.Ic.vertline. is calculated by
absolute value operation means 21 at step S6, and the detected current
value Ima is read at step S7. At the next step S8, the current deviation
Id=Ica-Ima is calculated by subtraction means 22. At step S9, PI feedback
operation by the current deviation Id is performed according to the
limited drive voltage Vdg, which is guarded in a discrete-time by control
means 25 so that the drive voltage Vd can be provided from the following
formula.
[F1]Vd(n)=Vdg(n-1)+N Id(n)-M Id(n-1)
Here, N and M are proportional integration coefficients for the PI feedback
control performed during discrete-time; (n) represents a present value;
and (n-1) represents a last value.
At step S10, the limited drive voltage Vdg is provided so that the range of
the drive voltage Vd(n) is limited by PI control means 25 within Vlim. In
other words, if Vd(n) is smaller than -Vlim or larger than Vlim, Vdg(n) is
limited to .+-.Vlim and, otherwise, Vdg (n) is set equal to Vd(n).
Then, at steps S11 and S12, when the operation of P.A. motor 3 is changed
from one condition to another, the compensation term Vdg(n) is reset to a
predetermined voltage Vrst. That is, a judgement is made by compensation
term reset means 24 at judging step S11, and step S12 follows only if
Dir(n) Dir(n-1) and Dir(n) 0. Otherwise, step S12 is skipped. At step S12,
the limited drive voltage Vdg is reset by PI control means 25 to a
predetermined voltage Vrst (e.g. zero) only if the symbol of the command
current value Ic is changed from one to the other, or from the neutral
(i.e. Ic=0) to the negative or the positive.
At steps S13-S15, the duty ratio Dt is set, and the direction command value
Dir and the duty ratio Dt is outputted. At step S13, duty ratio operation
means 26 multiplies the limited drive voltage Vdg and a positive
coefficient Kdt together to provide a provisional duty ratio Dta.
Subsequently, at step S14, the duty ratio Dt is set within 0-100% on the
basis of the provisional duty ratio Dta. That is: if Dta<0%, then
Dt=0%; if Dta>100%, then Dt=100%; and otherwise, Dt=Dta. At the last
step, the direction command value Dir and the duty ratio Dt are sent from
direction command means 23 and duty ratio operation means 26 to drive
circuit 27.
When the direction command value Dir and the duty ratio Dt are inputted to
drive circuit 27, drive circuit 27 controls MOS-FETs 31-34 according to
both the input signals. If the direction command value Dir is 1, drive
circuit 27 turns off two MOS-FETs 32 and 33 and PWM-controls MOS-FETs 31
and 34, which turn on and off at the duty ratio Dt, so as to rotate P.A.
motor 3 in the normal direction. If, on the other hand, the direction
command value is -1, drive circuit 27 turns off two MOS-FETs 31 and 34 and
PWM-controls MOS-FETs 32 and 33, which turn on and off at the duty ratio
Dt, so as to rotate P.A. motor 3 in the reverse direction. If the
direction command value Dir is 0, drive circuit 27 turns off all MOS-FETs
31-34 to cut current supplied to P.A. motor 3.
The above described formula F1 is formed as follows. In the PI feedback
control operation, the current deviation Id is transferred via an internal
value of a transfer function G(s)=Kp+Ki/s to form a drive voltage
Vd(s)=G(s) Id(s). Here, Kp is a proportional gain, Ki is an integral gain,
and s is the Laplace operator.
With operation cycle t, transfer function G(s) is Z-transformed by the
following bilinear transformation equation: s=(2/t) (z-1)/(z+1) to provide
a transfer function G(z) at the discrete-time.
[F2]G(z)={(Kp+Ki t/2)-(Kp+Ki t/2) z.sup.-1 }/(1-z.sup.-1)
The drive voltage Vd can be expressed by the formula F1 if N=Kp+Ki t/2, and
M=Kp-Ki t/2, because the current deviation Id is transferred via the
transfer function G(z).
When the operation of P.A. motor 3 is changed, the limited drive voltage
Vdg is reset to the reset voltage Vrst (e.g. zero). Even if the integral
compensation term increases when P.A. motor 3 is reversed, the limited
drive voltage Vdg is reset to zero.
As shown in FIG. 5, if the symbol of the command current value Ic is
changed from the positive to the negative, the limited drive voltage Vdg
is reset to zero as soon as the direction command value Dir is changed
from 0 to -1. Subsequently, the duty ratio Dt is reset to zero to start
PWM-control P.A. motor 3 at the duty ratio Dt=0 so that the detected
current value Im gradually increases in the reversal direction. Since the
feedback integral term is zero, a large impulsive drive current Im will
not flow in P.A. motor 3.
A fail-safe function that detects an abnormality of the drive current Im
can be provided to reduce erroneous operation, thereby improving safety
and the feeling of the steering.
Variation 1 of First Embodiment
A variation of the first embodiment is described with reference to a flow
diagram shown in FIG. 6.
This variation has different control logic for a digital operation
performed by microcomputer 111 of current control unit 20. Therefore, it
can be changed by easily replacing the software from the original control
logic of the first embodiment.
Accordingly, just after the direction command operation performed at steps
S1-S5, the absolute value Ica of the command current value is reset to a
reset current value Irst when the command current value Ic is inverted.
The reset current value Irst is set to a certain large negative value
compared to the maximum drive current. For example, the reset current
value could be -20 A.
If the direction command value Dir is not zero and different from the last,
Ica=Irst is set at step S12'. Otherwise, Ica=.vertline.Ic.vertline. is set
at step S6. In other words, the command current value Ica is forcibly
reset to a certain negative value so that the output of PI feedback
control means 25 can be zero when the steering wheel is reversed.
Since the reset current value Irst is set to a certain large negative value
by comparing it with the maximum drive current, the current deviation
Id=Ica-Ima becomes a large negative value just after the reset at the next
step S8. Accordingly, the drive voltage Vd defined by the formula F1
becomes small at step S9. Since the newly calculated drive voltage Vd
renews the next limited drive current Vdg, the same result as the result
when the limited drive voltage Vdg is reset can be expected as in the
first embodiment.
Variation 2 of the First Embodiment
As shown in FIG. 7, an analog circuit replaces a part of current control
unit 20 of the power steering control apparatus. PI control means 25 of
the first embodiment is replaced by analog PI control means 25', and duty
ratio operation means 26 is replaced by analog duty ratio circuit 26'.
The absolute value of a detected current value Ima is supplied to PI
control operation circuit 25' from current detecting unit 40. PI control
operation circuit 25' is comprised of reset switch SW, input resistors R1,
R1', feedback resistor R2, feedback capacitor C, and operational amplifier
251. Reset switch SW normally opens. It closes only in moments when the
reset signal is supplied by compensation term reset means 24. Duty ratio
set circuit 26' is comprised of input resistor 3, comparator 261, and
saw-tooth-wave circuit 262.
If reset switch SW of PI control operation circuit 25' opens then the PI
feedback control is carried out. In the PI control analog system a ratio
R2/R1 between feedback resistor R2 and input resistor R1 corresponds to a
proportional gain, and a reciprocal number 1/(R1 C) of the product of
feedback capacity and input resistor R1 corresponds to the integral gain.
In duty ratio set circuit 26', the output voltage of operational amplifier
251 and the output current of saw-tooth-wave circuit 262 are compared by
comparator 261 to provide the PWM signal that corresponds to the duty
ratio. Subsequently, the direction command value Dir and the PWM signal
are supplied to drive circuit 27' so that the drive circuit 27' controls
the H-bridge circuit 30 to drive the P.A. motor 3.
If reset switch SW of PI control operation circuit 25' is closed, for
example, when the symbol of the command current value Ic changes from one
to the other, feedback resistor R2 and feedback capacitor C are
short-circuited momentarily so that the electrical charge of the
capacitor, which corresponds to the integral value, is discharged to zero.
This prevents the impulsive drive current. Thereafter, if the direction
command value Dir is constant, reset signal SW remains open so that the
duty ratio Dt is gradually increased by the PI operation. As a result,
P.A. motor 3 can be smoothly driven by the drive current that is highly
responsive to the command current value Ic.
Second Embodiment
A motor-assisted power steering control apparatus according to a second
embodiment of the invention is described with reference to FIGS. 8-12.
Judgment of compensation term reset means 24' is made according to the
rotation direction of P.A. motor 3, in addition to direction command value
Dir.
The power steering apparatus according to the second embodiment includes
operational amplifier 28' as terminal voltage detecting means, A-D
converter 28, and revolution speed operation means 29 in addition to the
parts and elements of the first embodiment. As shown in FIG. 9, the
control logic of compensation term reset operation means 24' additionally
has judging step S11'. The operation of P.A. motor 3 is determined as
being changed not only if the symbol of the command current value Ic
changes between positive and negative, but also if the rotation direction
Dir of the command current value Ic is different from the actual rotation
direction of P.A. motor 3.
Operational amplifier 28' detects the voltage across a pair of terminals of
P.A. motor 3 and sends the output voltage Vm to revolution speed operation
means 29 of microcomputer 111 via A-D converter 28. Microcomputer ill
calculates revolution speed .theta.m' of P.A. motor 3 at revolution speed
operation means 29 and sends it to compensation term reset means 24'.
Revolution speed .theta.m' is calculated by the following formula F3 on
the basis of the terminal voltage Vm and the detected current value Ima.
[F3].theta.m'=Kv (Vm-R Ima Dir), where Kv is a constant that corresponds to
the reciprocal of an induced voltage constant, R is a circuit resistance
of P.A. motor 3 and wire harnesses thereof. Revolution speed operation
means 29 includes a digital low-pass filter (not shown), which removes
noises from the signal of revolution speed .theta.m' of P.A. motor 3
stepwise at steps S21-S24, as shown in FIG. 10.
The compensation term reset means 24' determines whether the compensation
term is reset or not by judging the revolution speed .theta.m' and
rotation direction command value Dir at judging step S11'. In other words,
as soon as the direction command signal Dir changes to the direction
opposite the actual rotation direction of P.A. motor 3 (which can be
detected by the positive or negative symbol of the motor revolution number
.theta.m'), the limited drive voltage Vdg is reset to the reset voltage
Vrst (=0).
The control logic of this embodiment is the same except the judging step
S11'. As shown by arrows in FIG. 11, if the symbol of the command current
value Ic is inverted to change the direction command value Dir, the
limited drive voltage Vdg is reset to the reset voltage Vrst (zero). As a
result, even if the symbol of the command current value Ic is inverted, an
impulsive large current caused by the counter electromotive force due to
reversal of P.A. motor 3 can be prevented. As shown in circle A, if the
direction command value Dir rises again after it drops to zero without
further dropping to the negative the limited drive voltage Vdg is not
reset to zero. Because P.A. motor 3 rotates in the same direction due to
the inertia thereof, the symbol of the motor revolution speed .theta.m' is
not inverted so that the symbol of the direction command value Dir is not
changed. In other words, if the direction command value Dir returns from
zero without inverting the symbol thereof, the limited drive voltage Vdg
is not reset, and a high degree of limited drive voltage Vdg is maintained
to balance the counter electromotive force of P.A. motor 3. As a result,
if the current command value Ic becomes zero for a while, the drive
current Im follows at a high response characteristic, thereby preventing
the operation of P.A. motor 3 from delaying.
A test result of a vehicle equipped with the motor-assisted power steering
control apparatus according to this embodiment is shown in FIG. 12. It was
found from the data that the limited drive voltage Vdg is not reset if the
direction command signal Dir returns from zero. Therefore, no impulsive
current is generated in the detected current value Ima. As a result,
revolution speed .theta.m' of the motor can be controlled smoothly.
This power steering control apparatus provides not only the same effect as
the first embodiment, but also, the effect that a driver does not feel a
"draggingg" of the steering wheel even if the command current value Ic
returns after it becomes zero.
Variation 1 of Second Embodiment
As a variation 1 of this embodiment, the above described reset value Vrst
is changed to balance the counter electromotive force of the P.A. motor 3.
If the symbol of the command current value Ic is inverted, the limited
drive voltage Vdg is reset to the reset voltage Vrst as in the second
embodiment. However, the reset value Vrst changes as the counter
electromotive force Ve of P.A. motor 3 changes in this variation. The
counter electromotive force Ve is expressed in the following formula, as
shown in FIG. 10 at step S22.
[F4]Ve=Vm-R Ima Dir, where R is a resistance of P.A. motor 3 and the wire
harnesses thereof.
The reset voltage value Vrst, which corresponds to the counter
electromotive force Ve, is expressed in the following formula.
[F5]Vrst=Ve Dir For example, if the direction command value Dir changes
from 1 to -1 while the symbol of revolution speed .theta.m' of the motor
is positive, the symbol of the counter electromotive force Ve is positive.
Since Dir is -1, the reset voltage value Vrst becomes negative, that is
-Ve. Accordingly, the duty ratio Dt is set to 0% so that the impulsive
current can be eliminated. On the other hand, if the direction command
value Dir changes from -1 to 1 while the symbol of revolution speed
.theta.m' of the motor is negative, the symbol of the counter
electromotive force Ve is negative. However, the reset voltage value Vrst
has negative symbol, and the duty ratio Dt is also set to 0%.
Occasionally, the direction command signal Dir may be changed to 1 while
revolution speed .theta.m' of the motor is positive, or it may be changed
to -1 while revolution speed .theta.m' of the motor is positive. Since
revolution speed .theta.m' of the motor and the direction command value
Dir is the same in symbol, the limited drive voltage Vdg is not reset.
Thus, the limited drive voltage Vdg is continuously maintained to balance
the counter electromotive force of P.A. motor 3 so that an excellent
response characteristic of the drive current Im to the command current
value Ic can be provided.
Thus, if the counter electromotive force Ve is induced in P.A. motor 3, the
limited drive voltage Vdg is reset to a voltage that can balance the
counter electromotive force. Consequently, the duty ratio Dt is set to 0%,
and the driving torque of P.A. motor 3 disappears, so that the operation
of P.A. motor 3 can be changed smoothly.
Variation 2 of Second Embodiment
The limited drive voltage Vdg is reset either when P.A. motor 3 is turned
on by the ignition switch or operated after it was stopped by the fail
safe means due to an abnormal operation.
The power steering control apparatus has substantially the same structure
as the second embodiment except a modified judgment logic of compensation
term reset means 24'. As shown in FIG. 13, the main control logic includes
a judgment step S11' in addition to the control logic of the second
embodiment. In the judgment step S11', the limited drive voltage Vdg is
reset to an enable condition when P.A. motor 3 is recovered from a disable
condition after an abnormal condition was detected when the system of the
motor-assisted power steering apparatus is checked.
Therefore, an excessive impulsive drive current Im can be more surely
prevented. Since the driving torque of P.A. motor 3 is gradually
recovered, a driver can be prevented from over-rotating the steering wheel
due to his careless operation. The steering can also be carried out
comfortably even when the motor control is recovered from an abnormal
condition.
As shown in FIG. 14, a self-diagnosis logic is installed in CPU 111 for the
system check. When the ignition switch is turned on, the system is
initialized at step S31 before other systems. That is, a memory of
microcomputer 100 and interruption steps including the interruption of the
above-stated control logic are initialized, and P.A. motor 3 is
initialized to the disable condition.
Thereafter, a routine of system check steps S32-S37 is carried out at a
prescribed cycle. At step S32, whether the steering torque signal Tq is
normal or not and whether the detected current value Ima is normal or not
are judged together with other judgments. Thereafter, whether there is any
abnormal condition or not is judged at step S33 according to the above
results. If no abnormal condition is found at step S33, a judge flag is
raised to permit P.A. motor 3 to operate at step S34 (Enable). Each time
the system check routine is repeated, the limited current value Igd that
is the limited value of the drive current Ima is gradually increased until
the limited current value Igd reaches a preset normal value at step S35.
On the other hand, if abnormal condition is judged at step S33, the
limited current value Idg is immediately set to 0 to prevent a runaway of
P.A. motor 3 at step S36, and the judge flag is lowered to zero to stop
P.A. motor 3 at step S37 (Disable).
In the control logic shown in FIG. 13, whether the judging flag is changed
from the disable condition to the enable condition is examined at step
S11'. If the judging flag is changed from the disable condition to the
enable condition, the limited drive voltage Vdg is reset to the reset
voltage Vrst so that the duty ratio Dt becomes 0%. Therefore, P.A. motor 3
gradually operates so that the driving torque of P.A. motor 3 gradually
increases. As a result, abrupt change, which would otherwise frighten a
driver can be prevented effectively.
The operation of command current value unit 10 is controlled according to
the flow diagram shown in FIG. 15.
In the foregoing description of the present invention, the invention has
been disclosed with reference to specific embodiments thereof. It will,
however, be evident that various modifications and changes may be made to
the specific embodiments of the present invention without departing from
the broader spirit and scope of the invention as set forth in the appended
claims. Accordingly, the description of the present invention in this
document is to be regarded in an illustrative, rather than restrictive,
sense.
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