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BACKGROUND OF THE INVENTION
This invention relates to an electric power steering apparatus in which
part of a steering force is provided by an electric motor.
A known apparatus of this general type is disclosed in Japanese Laid-Open
Patent Application No. 227560/1984 and also in FIG. 20 of the drawings.
In this apparatus, a steering rotational shaft 102 is axially displaced by
a plurality of steel ball bearings 103 into and out of a ball nut 101
positioned in a steering gear box 100 so as to slide horizontally as
viewed in FIG. 20. When the shaft 102 is rotated, the steel ball bearings
103 move along a spiral groove formed on the outer surface of the shaft
102 so as to thereby move ball nut 101 axially.
A rack 101a is formed on and extends axially along the outer periphery of
ball nut 101 and meshes with a pinion 104a of a sector shaft 104.
Accordingly, when ball nut 101 moves axially, as described above, the
sector shaft 104 is rotated to thereby move the wheels via a steering
member, not shown, connected to sector shaft 104.
The steering rotational shaft 102 is positioned coaxially with a steering
shaft 105 integral with a steering wheel, not shown. Both shaft members
102 and 105 are substantially hollow and fixedly connected to each other
via a torsion bar 106 inserted therethrough at opposite ends of the
torsion bar. The end of shaft 102 which is connected to torsion bar 106 is
connected by a reduction device 107 to a drive shaft of an electric motor
108. A torque sensor 109 is provided at the end of steering shaft 105
opposite to the steering wheel. The output signal S1 sensed by torque
sensor 109 represents the direction and torque of rotation (hereinafter
referred to as torque) of the steering wheel is input to a controller 112
along with the output signal S2 from a vehicle speed sensor 114. An output
drive signal S3 from controller 112 controls the direction and torque of
rotation of electric motor 108.
As described, the electric power steering apparatus of this known type is
intended to provide an arrangement in which the electric motor shares the
steering force input in accordance with the direction and torque applied
to the steering shaft 105 as sensed by torque sensor 109.
In this known steering apparatus, the direction and torque of the steering
shaft are sensed by a single torque sensor to drive the electric motor, so
that if trouble occurs in the torque sensing unit, a signal will possibly
be produced irrespective of the driver's intention, thereby driving the
electric motor and automatically steering the vehicle and possibly leading
to an accident. In addition, the torque sensor is operated by use of a
relative deviation between the input shaft and the output shaft.
Accordingly, if an abnormality occurs in the operating mechanism, a smooth
feeling of steering will not be obtained. In a worst case scenario, if the
operating mechanism is seized or becomes inoperative during the course of
the steering operation, a signal will continue to be produced regardless
of the driver's intention and an electric current will continue to flow to
the electric motor causing an automatic input to the steering thereby
leading possibly to an accident.
It is a technical subject of this invention to prevent a signal of the
foregoing type from being generated when there is abnormality in the
torque sensor. This would avoid driving the electric motor and reduce the
risk of accidents.
SUMMARY OF THE INVENTION
In order to provide a solution to the above described problem, this
invention provides an electric power steering apparatus which includes:
an axial converting mechanism for converting the relative rotation between
an input shaft and an output shaft to an axial deviation;
a slider element as part of the conversion mechanism and deviatable
axially;
a first sensing unit for sensing the axial deviation of the slider and
having a lever with torque sensing means, the lever being fixed at one end
to a housing and supported at the other end by the slider;
a second sensing unit for sensing the direction of rotation, or the
direction and torque of rotation, of a steering wheel; and
a controller for comparing the output signals from the first and second
sensing units and controlling the direction and torque of rotation of an
electric motor which contributes to the steering force, whereby the
comparison value between the signals from the first and second sensing
units is used to stop the motor when the comparison value is outside a
predetermined range.
When the first and second sensing units are operating normally, the
comparison value between the output signals from the first and second
sensing units compared by the controller falls within a range of
predetermined conditions or values. Therefore, the electric motor is
driven so that the output shaft of the motor is rotated in the same
direction as the input shaft thereof in accordance with signals from both
the sensing units to thereby contribute to the steering force.
If an abnormality occurs in either the first sensing unit or in the second
sensing unit for some reason, for example, if the slider constituting the
axial conversion mechanism is seized or frozen in position, the first
sensing unit will continue to produce a signal corresponding to the
position where the slider is seized irrespective of the driver's intention
while the second sensing unit produces a signal in accordance with the
direction of rotation, or the direction and torque of the steering wheel
so that the comparison value between the signals output from both the
sensing units compared by the controller falls outside the range of the
predetermined condition or value. When this is determined, an electric
current flowing through the electric motor stops, so that the motor also
stops. Thus the signal is prevented from continuing to be produced
irrespective of the driver's intention, so that the motor is not driven
and manual steering is permitted.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of one embodiment of an electric power
steering apparatus according to this invention;
FIG. 2 is a block diagram of a controller of the apparatus shown in FIG. 1;
FIG. 3 illustrates the relationship between input and output of a relay
logic RL of the controller shown in FIG. 2;
FIG. 4 illustrates the relationship between input and output of a main
logic ML of the controller shown in FIG. 2;
FIG. 5 is an electric circuit diagram of a determination circuit B19 of the
controller shown in FIG. 2;
FIG. 6a is a circuit diagram of a strain sensing circuit of each of a first
and a second sensing unit used in the embodiment of FIG. 1;
FIG. 6b is a characteristic diagram of the sensed output voltage from the
strain sensing circuit of the first sensing unit used in the embodiment of
FIG. 1;
FIG. 6c is a characteristic diagram of the sensed output voltage from the
strain sensing circuit of the second sensing unit used in the first
embodiment of FIG. 1;
FIG. 7 is a cross-sectional view of another embodiment of the second
sensing unit in this invention;
FIG. 8 is an electric circuit diagram of a determination circuit B19 used
with the second sensing unit shown in FIG. 7;
FIG. 9 is a characteristic diagram showing the sensing characteristic of
the second sensing unit shown in FIG. 7;
FIG. 10 is a cross-sectional view of another embodiment of the second
sensing unit in this invention;
FIG. 11 is a plan view of a slit in FIG. 10;
FIG. 12 is an arrangement plan of the slits, on enlarged scale, of a slit
as shown in FIG. 10;
FIG. 13 is an arrangement plan of photointerrupters in FIG. 10;
FIG. 14 is an arrangement plan of the slits in FIG. 12 when left torque is
applied to the steering wheel;
FIG. 15 is a arrangement plan of the slits in FIG. 12 when right torque is
applied to the steering wheel;
FIG. 16 illustrates the relationship between input and output of the second
sensing unit shown in FIG. 10;
FIG. 17 is an arrangement plan of photointerrupters showing a modification
of the second sensing unit shown in FIG. 10;
FIG. 18 is an arrangement plan of slits, on enlarged scale, in the
modification shown in FIG. 17;
FIG. 19 is an electric circuit diagram of a determination circuit B19 used
with second sensing unit shown in FIG. 10; and
FIG. 20 is a cross-sectional view of the prior art apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of this invention will now be described more specifically
with reference to FIGS. 1 and 2.
The electric power steering apparatus shown in FIG. 1 is different from the
conventional ball nut type electric power steering apparatus shown in FIG.
20 in that it is a rack and pinion type electric power steering apparatus.
It will be obvious, however, that this invention may be applicable to
either type of apparatus.
In FIG. 1, reference numeral 1 denotes an input shaft connected at one end
to steering wheel 32 via a steering shaft 31 and is integral with a pinion
shaft 6 via a torsion bar 13. Namely, input shaft 1 and pinion shaft 6 are
coupled via pins 14a and 14b, respectively, to the torsion bar 13.
Input shaft 1 is supported by a bearing 3 which is press fitted into an
upper housing 2 on the steering wheel side of the input shaft and at the
other or opposite end by a bearing 4 which is press fitted into a
cylindrical end of the pinion shaft 6. Pinion shaft 6 is supported by a
pair of angular ball bearings 8, 9 which are mounted within a lower
housing 7 so as to prevent axial play.
A pinion gear is formed at the end of pinion shaft 6 and meshes with a rack
5 to steer the wheels (not shown). Input shaft 1 has a gear 6a positioned
at the end opposite the steering wheel. The gear 6a meshes with a gear 12
via intermediate gears 11a, 11b to thereby form a reduction device.
The gear 12, intermediate gears 11a, 11b and gear 6a of pinion shaft 6 are
all formed as helical gears.
A spiral groove 1a is formed on the central portion of input shaft 1 and
cooperates with an opening 17a provided in the inner peripheral surface of
a cylindrical slider 17. The slider has a flange and is loosely fitted
over the input shaft 1 and cooperates with the shaft 1 to hold a ball
element 15 between the shaft and the slider.
An annular groove 17c is provided circumferentially on the outer peripheral
surface of the slider 17. An axial elongated groove 17b is provided on an
end surface of slider 17 facing the pinion shaft 6 and gear 6a. A pin 19
is press fitted into the end surface of pinion shaft 6 and gear portion 6a
is engaged within groove 17b so as to permit the slider 17 to move axially
along input shaft 1 and to rotate in accordance with rotation of pinion
shaft 6. Reference numeral 16 denotes a annular member fitted over the
cylindrical portion of slider 17. Reference numeral 18 denotes an elastic
retainer ring for preventing the annular member from slipping off the
slider.
Upper housing 2 has a casing 23 within which a leaf spring lever 20 having
a ball element 21 at one end, is fixed to the casing 23 at the other end
by a bolt 24. Ball element 21 is fitted into or otherwise received in
annular groove 17c of slider 17. Fixed or secured to lever 20 are strain
gauges 22 (which include an electric element whose resistance varies in
accordance with a strain). Strain gauges 22 and lever 20 constitute a
first sensing unit in which four strain gauges 22 (only one of which is
shown in FIG. 1) are connected in a bridge to form a strain sensor
circuit, as shown in FIG. 6a. The sensor circuit has a sensing
characteristic, as shown in FIG. 6b, and is connected via a cable to
controller 25 so that the electric signal from the sensor circuit is
transmitted to the controller.
Strain gauges 33, which include an electric element whose resistance varies
in accordance with a strain, are fixed to steering shaft 31 which couples
and is connected between input shaft 1 and steering wheel 32. The strain
gauges 33 constitute a second sensing unit and have a sensing
characteristic such as that shown in FIG. 6c. In the same manner as strain
gauges 22 constituting the first sensing unit, four strain gauges are
connected in a bridge to form a strain sensor circuit, as shown in FIG.
6a. The sensor circuit is connected by a cable to controller 25 so that
the electric signal from the sensor circuit is transmitted to the
controller.
Controller 25 will now be described in more detail with reference to FIG. 2
in which controller 25 includes circuit B1 which supplies a voltage Vcc
from a battery 35 to respective circuit blocks; a circuit B2 which
gradually increases its output when the battery voltage falls below a
predetermined value; and a circuit B4 which includes a PID (proportion,
integration and differentiation) compensation circuit which smooths or
damps a fluctuation in the electric signal from the first sensing unit
which may arise due to a delay in the operation of the mechanical system
and amplifies the input DC signal linearly. A comparator B3 includes an
abnormality sensing unit output circuit which changes its output to a low
level when the output from circuit B4 is below a predetermined value and
above a second predetermined value which is higher than the first
predetermined value. A comparator B5 has a sensing unit output direction
determining circuit which produces a high level output when the output of
B4 exceed a reference voltage V1 and produces a low level output when the
output of B4 falls below the reference voltage V1, the comparator B5 has a
hysteresis characteristic for preventing chattering. A linear amplifier B7
includes an absolute value circuit having a output signal representative
of the magnitude of the difference between the output from circuit B4 and
the reference voltage V1. B8 indicates a sawtooth waveform generator and
B9 is an adder which adds a reference voltage V2 to the B output signal of
the waveform generator B2. A comparator B10 has a hysteresis
characteristic and sets the start point where the DC electric motor DM is
energized (dead zone) and produces a high-level output when the output of
B7, which is proportional to the magnitude of the input signal, exceeds a
predetermined value. A comparator B11 compares the output of B9, i.e., the
sawtooth waveform with reference voltage and the output of B7
(proportional to the magnitude of the input signal) and includes a pulse
width modulation (PWM) circuit which turns on and off the electric current
flowing through the DC electric motor DM at predetermined short periods
and changes the on-off duty cycle to variably control a quantity of
driving the DC motor DM. Also included is a motor current sensing circuit
which includes a pair of differential amplifiers B12, B13 which
differentially amplify the voltages across the corresponding shunt
resistors; an adder B14 for the amplifiers B12 and B13 which has an output
proportional to the magnitude of the motor current. An adder B15 adds the
outputs of B2 and B14 and provides an output signal representative of the
sensed current modified by the voltage drop across the battery.
Integration circuit B16 includes an average current sensing circuit which
smooths or damps the sensed electric current signal and including an
average electric current sensing circuit which produces a high-level
output when the sensed current signal is below a predetermined value and
produces a gradually decreasing output when the sensed current signal
exceeds the predetermined value. The integration circuit clamps the output
with a diode before the comparator B11 compares the absolute value of the
output of circuit B16 with a sawtooth waveform. Circuit B17 is an abnormal
current sensing circuit which produces a low level signal when the sensed
current signal exceeds the predetermined level and which provides an
output to a relay logic RL, to be described later, to control a main
relay. Circuit B18 is a temporary current control circuit which changes
its output to a low level so as to switch off a PWM signal when the sensed
current signal exceeds the preset value. The relay logic RL changes its
output to a low level to switch off the main relay as shown by the
relationship between inputs a, b and the output shown in FIG. 3 when the
sensed output from the first or second sensing unit is abnormal or when an
abnormal current occurs. A main logic circuit ML is provided for control
of its outputs to output transistors Q1, Q2, Q3, Q4 via a base driver BD,
as shown in the relationship between inputs c, d, e, f, g from the
respective blocks and its output shown in FIG. 4, to control the rotation
and direction of rotation of DC electric motor DM. A determination circuit
B19 for determining an anti-assist mode (autosteer) when the sensed output
signals from the first and second sensing units are opposite and thereby
change the output to a low level. The graph in each block shows the
electric input-output characteristic of the block in which the axis of
abscissas shows an input level while the axis of ordinates shows an output
level. The set values for average current sensing circuit B16, abnormal
current sensing circuit B17 and temporary current limiting circuit B18
have the relationship B16<B18<B17. The relay logic RL has a filter to
neglect a low level signal of less than tens of milliseconds.
The anti-assist mode (autosteer) determination circuit B19 has a structure
shown in FIG. 5. The output signal from PID compensation circuit B4,
namely, the electric signal output from the first sensor and the electric
signal output from the second sensor are compared with reference voltage
V1 at comparators OP1 and OP2, respectively. Each comparator output has a
high level signal when the corresponding electric signal is higher than V1
and a low level signal when the electric signal is lower than V1. The
respective output signals from the comparators provide an input to an
exclusive OR gate EX-OR1, the output signal from which is input together
with the output signal from the comparator B10 to an AND gate AND1. The
output signal from AND1 is inverted by a NOT gate NOT1 and is an input
signal to an input terminal of the relay logic RL. By such structure, when
the output signal from PID compensation circuit B4 or the electric signal
from the first sensing unit and the electric signal from the second
sensing unit are at different levels, namely, opposite, the determination
circuit B19 determines that the anti-assist mode (autosteer) operation
occurs to change the output signal to a low level which is applied to the
relay logic RL to switch off the main relay MR to allow an electric
current to the electric motor DM to flow through the main relay and stop
the motor.
The operation of this embodiment having the above structure will be
described with reference to FIGS. 1 and 2.
When the steering shaft 32 is rotated, it rotates the input shaft 1 via
steering shaft 31 to twist torsion bar 13 relative to pinion shaft 6 which
is coupled via the rack to the wheels, so that the pinion shaft 6 and
input shaft 1 rotate relative to each other.
The relative rotation of the input shaft 1 causes the helical groove la to
rotate by the difference in rotation between input shaft 1 and pinion
shaft 6 to thereby apply an axial thrust to slider 17 via ball elements 21
to shift slider 17 axially in accordance with the rotation of the input
shaft.
The axial shifting of slider 17 causes lever 20 holding ball element 21,
engaged in the annular groove 17c in slider 17, to flex. This causes
strain gauges 22, having the sensing characteristic shown in FIG. 6, to
generate from the first sensing unit an electric signal corresponding to a
quantity of relative rotation between input shaft 1 and pinion shaft 6.
The electric signal from the strain gauges 22 or the first sensing unit is
transmitted as an input to the controller 25.
Strain gauges 33, constituting the second sensing unit, are fixedly
connected to the steering shaft 31 and has the same sensing characteristic
as strain gauge 22 constituting the first sensing unit as shown in FIG. 6,
generate an electric signal in accordance with the direction and torque of
rotation of steering shaft 32 due to torsion of the steering shaft as
determined by rotation of the steering wheel 32. The electric output
signal from strain gauges 33 or the second sensing unit is transmitted as
an input to the controller 25.
The electric signal transmitted from strain gauges 22 (first sensing unit)
and input to controller 25 is delivered via PID compensation circuit B4 to
determination circuit B19 shown in FIG. 5. Similarly, the electric signal
from strain gauges 33 (second sensing unit) is also transmitted to the
determination circuit 19. When the first and second sensing units are
operating in accordance with normal conditions or within a range of
predetermined conditions, the electric output signals from both strain
gauges 22 and 33 (first and second sensing units) are at the same level,
so that as described above, the output signal from determination circuit
B19 becomes high and is transmitted to the input terminal of relay logic
RL to thereby turn on main relay MR. This results in an electric current
to flow therethrough to electric motor DM. As a result, as shown in the
relationship between inputs c, d, e, f, g (from the corresponding blocks)
and output of the main logic ML in FIG. 4, the output signal from the main
logic is applied via base driver BD to output transistors Q1, Q2, Q3, Q4
to turn the transistors on and off, so that the direction and quantity of
an electric current applied to motor DM are controlled.
Motor DM is decelerated via drive shaft gear 12, intermediate gears 11b,
11a and gear 6a of pinion shaft 6 to rotate pinion shaft 6. This causes
rack 5 to move to thereby alter the direction of the wheels and increasing
the steering force.
At this time, pinion shaft 6, rotated by motor DM, is rotated in the
direction in which the relative rotation between input shaft 1 and pinion
shaft 6 becomes null or zero. As a result, slider 17 is returned to the
position where the torsion of lever 20 becomes null by pin 19 projecting
from end surface of pinion shaft 6.
The operation of the apparatus, when the sensing unit is abnormal, as an
object of this invention will be described. When the first or second
sensing unit become abnormal, i.e., transmits an abnormal signal, for
example, the slider 17 constituting part of an axial change mechanism is
seized, strain gauges 22 (first sensing unit) generate a signal depending
on the position where the slider 17 is seized irrespective of the driver's
intention and movement of the steering wheel. The electric signal so
generated is transmitted as an input to the determination circuit B19. On
the other hand, strain gauges 33 (second sensing unit) generate an
electric signal depending on the direction of rotation and torque of the
steering wheel 32, as discussed above, and the signal is input to
determination circuit B19 so that the electric signals from strain gauges
22 and 33 (first and second sensing units) may be at different levels in
which case the determination circuit B19 applies a low electric signal to
the input terminal of relay logic RL to thereby turn off main relay MR.
This causes motor DM to stop and hence prevents autosteering, i.e.,
permitting unassisted manual steering.
Although not shown in FIG. 1, a clutch may be interposed between motor DM
and gear 12 to interrupt one from the other in order to improve a
sensation of manual steering when motor DM is at rest.
FIG. 7 is a determination unit 50 showing another embodiment of the second
sensing unit which includes strain gauges 33 of the first embodiment
according to this invention, described above. The steering shaft 31 of
FIG. 1 is provided as a double structure which includes an outer shaft 31a
coupled to input shaft 1 and an inner shaft 31b coupled to steering wheel
32. A movable contact 51 is inserted into a first groove 52 in outer shaft
31a and is fixed to inner shaft 31b. Contacts 50a, 50b are fixed in the
groove 52 in outer shaft 31a with the movable contact 51 being loosely
received between fixed contacts 50a and 50b. The fixed contacts are
connected by a cable to the controller 25. Outer shaft 31a has a second
groove 53 at a symmetrical position with reference to first groove 52 and
receives therein a protrusion 54 of inner shaft 31b. Provided under
tension in the second groove 53 are springs 55, 56 which normally maintain
movable contact 51 loosely between fixed contacts 50a and 50b to thereby
maintain the determination unit 50 in an off condition.
The sensing characteristic of this determination unit 50 is shown in FIG.
9. In this case, the determination circuit B19 in the block diagram of
controller 25 shown in FIG. 2 has a different structure, shown in FIG. 8,
which operates in a similar manner to that of the determination circuit
shown in FIG. 5 and used in the previous embodiment. However, this
arrangement provides an example wherein the contact 50a may be removed so
as to leave only contact 50b to turn on the determination unit when the
steering wheel is turned left.
The operation of the determination unit 50 (second sensing unit) will now
be described with reference to FIG. 7.
If the steering wheel 32 is, for example, turned to the left, movable
contact 51 contacts the fixed contact 50b to turn on determination circuit
50, so as to produce a high level electric signal which is delivered to
controller 25 and hence to the input terminal of determination circuit B19
shown in FIG. 8. When the steering wheel is turned to the right, the
determination circuit 50 is turned off to deliver a low level signal. The
operation of the determination circuit B19 shown in FIG. 8 is similar to
that of the previous embodiment, further description on subsequent
operations will be omitted.
FIG. 10 shows a determination unit 60 using a photointerrupter as another
embodiment of the second sensing unit in this invention.
The determination unit 60 uses first and second shafts 61 and 62 which are
connected to the steering wheel and the input shaft, respectively, and
which in turn are connected to a torsion bar 63, instead of the steering
shaft 31 as in the previous embodiment.
First shaft 61 has one end fitted into and fixed to a first sleeve 64 while
a second shaft 62 has one end fitted into and fixed to a second sleeve 65
with slits 66, 67 having the shape as shown in FIG. 11 being fixed to the
adjacent end faces of both sleeves 64 and 65, respectively. It is to be
noted that both slits 66 and 67 are rotatable relative to each other and
take a relative position shown on large scale in FIG. 12 when the driver
applies no steering force to the steering wheel.
A casing 68 is fixed to, for example, the chassis so as to surround shafts
61, 62 and torsion bar 63. Four photointerrupters 69a, 69b, 69c and 69d
are disposed on the inner peripheral surface of casing 68, as shown in
FIG. 13, so as to receive slits 66 and 67 at their grooves. It is to be
noted that these photointerrupters are disposed so as to shift by the
predetermined amount P/4 from the center of vanes of the respective slits
66 and 67, as shown in FIG. 13.
The torsion bar 63 which connects both shafts 61 and 62 to amplify a
torsion due to a torque applied has a stopper mechanism which mechanically
couples shafts 61 and 62 when the relative torsion angle between both
shafts 61 and 62 becomes .+-.P/4.
According to the determination unit 60, when right and left torques are
applied to the steering wheel, both slits 66 and 67 take the relative
positions shown in FIG. 14 (left torque) and in FIG. 15 (right torque),
respectively, so that the output signals from photointerrupters 69a, 69b,
69c and 69d goes to a high level when the optical axes of the
photointerrupters are shut off or interrupted by slits 66 and 67 while the
output signals from the photointerrupters are low level otherwise and the
sensing characteristic of the photointerrupters is shown in FIG. 16. Thus,
the determination circuit B19 of controller 25 shown in FIG. 2 which
receives the output signals from photointerrupters 69a, 69b, 69c and 69d
outputs or transmits a low output signal to be applied to the input
terminal of relay logic RL when the output level from determination unit
60 (second sensing unit) and the output signal level from PID compensation
circuit B4 or the electric signal from the first sensing unit are
different, so as to turn off the main relay MR and stop the motor DM. The
operation of the determination circuit B19 will be understood from FIGS.
19 and 2 and further description thereon will be omitted.
While in this embodiment four photointerrupters which constitute the
determination unit 60 are used, five photointerrupters may be disposed
within casing 68, as shown in FIG. 17 in which both slits 66 and 67 are
disposed at a relative position shown in FIG. 18 and the mechanical
coupling between shafts 61 and 62 is set to be .+-.P/5.
Thus this modification is capable of determining right and left torques
using the output signals from photointerrupters 69a, 69b, 69c, 69d and 69e
and also can be used as a steering angle sensing unit already used and
implemented as a sensing unit for correction of lateral G forces, etc., in
the power steering.
While the determination unit using contact switches and the determination
unit using photointerrupters as other embodiments of the second sensing
unit have been described, a potentiometer and a photosensitive unit may be
used to sense a deviation of slider 17 or lever 20 or otherwise the
direction of an electric current flowing through motor DM when the motor
is rotated forwardly and backwardly.
As described above, according to this invention, the arrangement is such
that electric signals from the first and second sensing units are
compared. When the levels of the electric signals are outside different
predetermined ranges of values, the main relay through which an electric
current flows to the electric motor is turned off to thereby stop the
motor and hence permit manual steering. Therefore, for example, when an
indicated or detected abnormality occurs in the first or second sensing
unit, signals which would otherwise drive the electric motor and leading
to autosteering (anti-assist) are prevented from being produced. Thus, an
electric power steering apparatus excellent in safety is provided.
In order to further solve the problems relating to the conventional
apparatus, it could be conceived to construct the first sensing unit in a
multi-system. However, the first sensing unit would require a high level
of accuracy to control electric power steering and a complicated
large-sized structure which would render the entire apparatus expensive.
In contrast, according to this invention, the first sensing unit is
irrespective of the second sensing unit, so that the individual first and
second sensing units do not require such high accuracy. The apparatus is
simple in structure, inexpensive, and small-sized. In addition, when the
apparatus operates erroneously due to noise, etc., the mechanism
constituted by this invention rapidly turns off the electric system, so
that the steering wheel will not be out of control.
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