Electricity: motive power systems – Battery-fed motor systems
Reexamination Certificate
2002-09-17
2004-04-27
Masih, Karen (Department: 2837)
Electricity: motive power systems
Battery-fed motor systems
C318S594000, C180S446000
Reexamination Certificate
active
06727669
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a motor-driven power steering apparatus for imparting assist force to a steering system of an automobile or other vehicle by use of a motor, and more particularly to a motor-driven power steering apparatus having a step-up circuit capable of regulating current supplied from an on-vehicle battery to the motor.
2. Description of the Related Art
Conventionally, there has been used a motor-driven power steering apparatus which assists operation of a steering wheel through utilization of rotational force of a motor. In such a motor-driven power steering apparatus, when a driver rotates the steering wheel to steer the vehicle, the motor imparts to a steering mechanism a steering assist force corresponding to a steering torque that the driver applies to the steering wheel.
Incidentally, the above-described motor-driven power steering apparatus is a system that requires large current in order to produce large torque.
Conventionally, since voltage output from an on-vehicle battery (DC 12 V) is applied directly to a motor-driven power steering apparatus, a motor designed to operate at DC 12V is used therein, with the result that large current is supplied to the motor. Therefore, the size of the motor unavoidably increases, and the capacity (thickness) of wiring lines to be used unavoidably increases.
In order to solve the above problem, there has been proposed a motor-driven power steering apparatus capable of regulating current supplied from an on-vehicle battery to a motor (Japanese Patent Application Laid-Open No. 8-127350).
In the motor-driven power steering apparatus, as shown in
FIG. 51
, a step-up circuit
300
and a step-up circuit controller
301
are provided in a circuit for supplying current to the motor. The step-up circuit
300
is provided between a point P
1
to which battery voltage V
PIG
(DC 12V) is applied from an on-vehicle battery and a point P
2
from which voltage is applied to the motor. The step-up circuit
300
includes capacitors C
1
and C
2
, a coil L, a diode D, and a switching transistor Q
1
.
The step-up circuit controller
301
outputs a duty-ratio drive signal to the transistor Q
1
of the step-up circuit
300
in order to control the transistor Q
1
according to the duty-ratio drive signal (duty control). By virtue of this duty control, the transistor Q
1
performs switching operation as shown in FIG.
52
. As a result, accumulation of energy in the coil L and discharge of energy from the coil L are repeated, and high voltage generated by virtue of discharge appears on the cathode side of the diode D. Note that, in
FIG. 52
, T&agr; represents an ON period; T represents a pulse cycle; and &agr; represents a duty ratio (on duty). These definitions will be used throughout the present specification. When the transistor Q
1
is turned on, current flows through the coil L, and when the transistor Q
1
is turned off, the current flowing through the coil L is cut off.
When the current flowing through the coil L is cut off, high voltage is generated on the cathode side of the diode D so as to prevent magnetic flux from changing in response to cut off of the current. Through iteration of this operation, high voltage is generated repeatedly on the cathode side of the diode D. The high voltage is smoothed by means of the capacitor C
2
and is fed to the point P
2
as output voltage V
BPIG
.
The voltage obtained through step-up effected by the step-up circuit
300
relates to the duty ratio of the duty-ratio drive signal output from the step-up circuit controller
301
. When the duty ratio increases, the output voltage V
BPIG
increases; and when the duty ratio decreases, the output voltage V
BPIG
decreases.
However, since the conventional step-up circuit
300
uses the diode D as described above, a problem arises when the motor enters a regeneration state. That is, even when the motor enters a regeneration state, current cannot flow from the voltage application point P
2
toward the battery B, due to presence of the diode D, so that the output voltage V
BPIG
increases. Due to this increased voltage, the step-up circuit
300
may be broken. In the example shown in
FIG. 51
, the capacitor C
2
and the diode D, which constitute the step-up circuit
300
, may be broken.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a motor-driven power steering apparatus in which a step-up circuit is not broken even when a motor enters a regeneration state.
In order to achieve the above object, the present invention provides a motor-driven power steering apparatus for a vehicle, comprising: a steering mechanism for steering the vehicle in accordance with operation of a steering wheel; a motor mechanically coupled to the steering mechanism and generating assisting force to be imparted to the steering mechanism; control signal generation means for generating a motor control signal on the basis of at least steering torque applied to the steering wheel; motor drive means for driving the motor in accordance with the motor control signal; a step-up circuit provided in a current supply circuit extending from a battery to the motor drive means, the step-up circuit including a step-up coil whose one end is connected to the battery for reception of battery voltage, a first switching element connected between the other end of the step-up coil and the ground, a second switching element connected between the other end of the step-up coil and an output terminal, and a capacitor connected between the output terminal and the ground and smoothing output voltage which is produced by means of the step-up coil and appears at the output terminal; and step-up circuit control means for detecting the output voltage and controlling the first and second switching elements on the basis of difference between a target output voltage and the detected output voltage in such a manner that during power generation periods, at least the first switching element is turned on and off in order to increase the output voltage supplied to the motor through step-up operation, and during regeneration periods, at least the second switching element is turned on and off.
In the motor-driven power steering apparatus having the above-described configuration, the step-up circuit does not break even when the motor enters a regeneration state.
Preferably, the first and second switching elements are each constituted by a field effect transistor. In this case, the reliability and efficiency of the motor-driven power steering apparatus can be improved.
Preferably, the step-up circuit control means turns the first and second switching elements on and off alternately on the basis of the difference between the target output voltage and the detected output voltage in such a manner that during power generation periods, the output voltage supplied to the motor is increased through step-up operation, and during regeneration periods, regenerative current output from the motor flows to the battery.
In this case, the output voltage supplied to the motor can be increased through step-up operation during power generation periods, and regenerative current output from the motor can flow to the battery during regeneration periods.
Preferably, the motor-driven power steering apparatus further comprises steering state judgment means for determining, on the basis of the difference between the target output voltage and the detected output voltage, whether the motor is in a power generation state or in a regeneration state, wherein the step-up circuit control means turns at least one of the first and second switching elements on and off on the basis of the state of the motor determined by the steering state judgment means.
This configuration prevents the output voltage from increasing during regeneration periods.
The step-up circuit control means may turn only the first switching element on and off when the steering state judgment means determines that the motor is in a power generation state, and turn the first
Ito Kazuhiko
Sakai Atsuo
Suzuki Hiroshi
Masih Karen
Toyoda Koki Kabushiki Kaisha
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