Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
2000-07-06
2002-04-30
Nguyen, Tan (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C180S446000
Reexamination Certificate
active
06381525
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an electric power steering apparatus in which a power of an electric motor works directly on a steering system as a steering assist power to thereby reducing the steering power of a driver, and more particularly to an electric power steering apparatus for automatically compensating a torque reference value of a torque sensor.
A conventional electric power steering apparatus is known in which a target current is set on the basis of a steering torque detected by a steering torque sensor, an electric motor is driven with an electric motor voltage in correspondence with this target current, the electric motor current that flows through the electric motor is detected, a negative feedback (NFB) is applied to the electric motor, the electric motor voltage is controlled on the basis of a deviation signal of the target current and the electric motor current so that the electric motor is driven with the electric motor current that is equal to the target current, and the steering assist power works on the steering system.
FIG. 6
is a structural diagram showing this kind of conventional electric power steering apparatus.
FIG. 6
, the electric power steering apparatus is provided with a steering detecting means (steering torque sensor)
1
, a target current setting means
2
for setting a target current Im using the characteristic relationship between the steering torque T and the target current Im shown in
FIG. 7
on the basis of the steering torque T from the steering torque detecting means
1
, an electric motor current detecting means
4
for detecting the current that flows through an electric motor
3
, a deviation determining means
5
for calculating a deviation signal &Dgr;I between the target current Im and the electric motor current I detected by the electric motor current detecting means
4
, and a drive controlling means for converting the deviation signal &Dgr;I into the voltage and for PWM controlling an electric motor drive means
7
to generate an electric motor controlling voltage V
0
for quickly converging the deviation signal &Dgr;I to zero.
The electric motor drive means
7
is composed of, for example, a bridge circuit using four switching elements such as power FETs (Field Effect Transistors) and each of paired power FETs ( every two FETs constitute one pair) is controlled and driven with an electric motor controlling voltage V
0
composed of a PWM control signal and a switch, ON/OFF signal fed from the drive controlling means
6
so that the voltage value and the polarity of the electric motor voltage V to be fed to the electric motor
3
. Incidentally, the polarity of the electric motor voltage V is determined by controlling each of the two pairs of power FETs at the electric motor voltage V
0
outputted from the drive controlling means
6
.
Thus, in the conventional electric power steering apparatus, the feedback loop of the electric motor detecting current I is provided to thereby control the deviation signal &Dgr;I between the target current Im and the electric motor detecting current I. Therefore, in either case where the steering direction is in the clockwise direction or in the counterclockwise direction, the deviation signal &Dgr; is converged into zero so that the electric motor current I quickly becomes the target current Im.
Also, the steering torque sensor of the conventional electric power steering apparatus is known which is composed of a potentiometer and a differential amplifier. The steering torque is detected in terms of a corresponding potential change of the potentiometer. This change is converted into an electric signal and is detected as the steering torque signal.
FIG. 8
shows a specific circuit structure of the steering torque detecting means
1
shown in FIG.
6
. The steering torque detecting means
1
is composed of a potentiometer
1
a
connected between a power source terminal Er and the ground, a differential amplifier
1
b
whose non-inverted input terminal is connected to a slide terminal of this potentiometer
1
a
, a resistor
1
c
connected between an output terminal of this differential amplifier
1
b
and the inverted input terminal, and partial voltage resistors
1
d
and
1
e
connected between a power source terminal Eo and the ground. The connection point between the partial voltage resistors
1
d
and
1
e
is connected to the inverted input terminal of the differential amplifier
1
b.
The steering torque generated by operating the steering wheel (not shown) is detected in terms of the potential change of the potentiometer
1
a
and the output corresponding to this potential change is generated. The output of the potentiometer
1
a
is fed to the non-inverted input terminal of the differential amplifier
1
b
and is compared with a voltage (2.5V) that is half the power source voltage (for example, 5V) from the power source terminal Eo which has been applied to the inverted input terminal. The difference therebetween is amplified and outputted as the steering torque signal Ta.
FIG. 9
shows characteristics between the steering torque T and the steering torque signal Ta.
In
FIG. 9
, according to the characteristics the steering torque signal actually shows (indicated by the solid line), the steering torque signal Ta detected by the steering torque detecting means
1
and corresponding to the steering torque T=0 is 2.5V and is increased in a linear fashion from 2.5V to 5V as the steering torque T is increased in a plus (+) direction (for example, in the clockwise direction), whereas the steering torque signal Ta is decreased in a linear fashion from 2.5 V to 0 V as the steering torque T is increased in a minus (−) direction (for example, in the counterclockwise direction).
Accordingly, the steering torque detecting means
1
is so constructed that the torque reference value is set to 2.5 V and the magnitude and the direction of the steering torque is detected as the steering torque signal Ta of from 0 V to 5 V.
Also,
FIG. 9
shows an example of the characteristics, indicated by the one-dot-and-dash line, in which the steering torque signal Ta of the torque reference value detected by the steering torque detecting means
1
and corresponding to the steering torque T=0 is offset in a Lower direction from the voltage of 2.5 V. This means that the steering torque T is offset by &Dgr;T in the minus direction (−) (for example, in the counterclockwise direction). When the steering torque T is increased by &Dgr;T in the plus (+) direction (for example, in the clockwise direction), the steering torque signal Ta is detected at 2.5 V.
Incidentally, in the adjustment of the torque reference value of the steering torque detecting means
1
, in assembling the electric power steering apparatus, for example, the resistance (variable resistances) of one of the partial resistors
1
d
and
1
e
connected to the inverted input terminal of the differential amplifier
1
b
is adjusted to set the steering torque signal Ta to 2.5 V as the initial setting.
By the way, the above-described conventional power steering apparatus suffers from the following problems.
Namely, the adjustment of the torque reference value of the steering torque detecting means is performed only in assembling work, and thereafter the centering adjustment is not performed. Accordingly, the torque reference value would be changed due to the voltage offset: of the potentiometer that constitutes the steering torque detecting means and the aging change of the differential amplifier, or due to the shift of the adjustable resistance (variable resistance) caused by vibrations or the like, resulting in imbalance in detection of the steering torque. Thus, there is a fear that a driver would feel unsmoothness in steering operation.
Also, depending on the setting of the target value Im and the steering torque T as shown in
FIG. 7
, there is a fear that the electric motor current might continue to flow while the steering operation is not effected due to the offset
Hori Yasuaki
Ikemoto Katsuya
Nguyen Tan
Pipala Edward
Sughrue & Mion, PLLC
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