Electricity: motive power systems – Motor-reversing
Reexamination Certificate
2002-07-25
2004-03-30
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Motor-reversing
C318S375000, C318S376000, C361S023000, C361S033000, C361S091100
Reexamination Certificate
active
06713979
ABSTRACT:
The present application is based on Japanese Patent Applications No. 2001-226200 and 2001-393317, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an actuator having an inductance, and, more particularly, to a drive circuit for an actuator having an inductance, such as a brushless motor, a DC motor, a voice coil motor, or an electromagnetic actuator.
2. Related Art
Upon exposure to an external wind while remaining stationary, a fan motor using, for instance, a DC motor as a fan motor rotates in reverse, thereby producing an induced electromotive voltage. When a brisk wind arises, the fan motor rotates in reverse at high speed, whereby the resultant induced electromotive voltage becomes higher. When the voltage of a power circuit becomes anomalously higher, circuit components may become defective. For this reason, there have been proposed techniques for controlling a drive circuit, by preventing an excessive increase in the voltage of the power circuit, which would otherwise be caused by the induced electromotive voltage. The technique described in Japanese Patent Laid-open No. 2001-37276A is one such technique.
FIG. 8
shows a circuit described in the foregoing publication.
As shown in
FIG. 8
, an H bridge circuit
4
constituted by connecting transistors in the shape of the letter H comprises two pairs of output transistors; that is, a pair of power-side output transistors and a pair of ground-side output transistors. A DC motor M is connected to a node between the two output transistor pairs. A drive current is supplied to the motor M by use of the two output transistor pairs, thereby activating the motor M. A junction P of the pair of power-side output transistors is connected to a power-circuit
2
. A junction of the pair of ground-side output transistors is connected to the ground. The power circuit
2
is connected to a source voltage detection circuit
3
composed by connecting in series a plurality of Zener diodes D
5
, D
6
, and D
7
. The source voltage detection circuit
3
monitors a supply voltage. If the supply voltage has exceeded a predetermined level as a result of reverse rotation of the motor M, the Zener diodes D
5
, D
6
, and D
7
constituting the supply voltage detection circuit
3
are brought into conduction. As a result, a current flows into a capacitor C
2
, thereby boosting a voltage across the capacitor C
2
.
A PWM circuit
6
increases the duty factor of the DC motor in the direction of forward rotation by way of an error amplification circuit
5
while an increase in the voltage across the capacitor C
2
is taken as a trigger. A logic circuit
7
controls activation or deactivation of the respective output transistors of the H bridge circuit
4
, thereby applying a brake to reverse rotation of the DC motor. In this way, a generated voltage of the motor M is suppressed, thereby maintaining the supply voltage at a predetermined level or less.
Japanese Patent Laid-Open No. 09-182474A describes a technique for reducing a power loss, which would arise in a drive circuit of a brushless motor when a brake is applied to reverse rotation of the motor. In the drive circuit of the brushless motor of the related-art technique, a drive coil is constituted of three phases. Three sensors are provided for the respective-phases of the drive coil, thereby switching energization; The respective sensors detect a magnetic pole of an unillustrated rotor magnet. Outputs from the respective sensors are input to a matrix circuit after having been amplified by a hole amplifier. The matrix circuit outputs a timing signal to be used for switching energization of the drive coil of three phases. The signal is input to a motor drive current output section by way of a pre-driver.
In the drive circuit, when the direction of rotation of the motor is switched during the course of rotation by switching an F/R signal, an electrical reverse brake is applied to the motor until the direction of rotation of the motor is reversed. When a reverse brake is applied to the motor, a resistor Rf detects a drive current, and a comparison circuit compares the drive current with a predetermined current limit level. When a drive current in the direction of reverse rotation has exceeded the predetermined current limit level, a switching circuit is activated, thereby deactivating the power (source) output transistor or the ground (sink) output transistor of the drive current output section through control operation. After lapse of a predetermined period of time, the output transistor is again activated through control operation. When the drive current has exceeded the current limit level, the output transistor is again deactivated through control operation. In this way, individual switching elements of the drive current output section are controlled so as to avoid becoming unsaturated, by subjecting the output transistor to PWM control when a brake is applied to the motor.
In the DC motor drive circuit, a control function, such as the PWM circuit
6
, is inactive when the power circuit
2
remains deactivated. Hence, an induced electromotive voltage of the motor M cannot be suppressed. If power is shut off abruptly during the course of energization of the motor M, the drive coil-produces a counter electromotive voltage for maintaining a motor current. However, according to the related-art technique, when power is shut off, suppressing the counter electromotive voltage becomes impossible.
Specifically, according to the related-art technique, a circuit cannot be protected from an overvoltage produced by the motor unless power is active.
In the drive circuit of the brushless motor, the following problems are encountered in performing PWM control in a reverse brake mode, which has been performed under the related-art technique.
Since the drive current increases at the time of application of a brake to reverse rotation as compared with normal times, a current limit level is achieved within a shorter period of time, and the output transistor remains subjected to deactivation control for a longer period of time. If the power-side output transistor or ground-side output transistor, which is subjected to activation control, is switched to a deactivated state during the period of deactivation control operation for reasons of energization switching operation, a regenerative path for the drive current is momentarily disconnected; whereby the drive current flows in reverse to a motor power line for seeking the regenerative path.
If the motor power source has no sink (absorbing) capacity, the drive current will lose a place to flow. A sharp increase momentarily arises in the motor drive voltage, thereby exceeding the withstand voltage of the drive circuit or supply circuit. A common measure against exceeding withstand voltage is interposing an electrolytic capacitor or Zener diode between the motor supply source and the ground, thereby imparting a sink capacity to the power source. However, such a measure involves a cost hike.
Use of an electrolytic capacitor entails a problem of insufficient suppression of an increase in source voltage, for the following reasons. Specifically, at the time of a reverse rotation braking operation, the period of time during which PWN activation control is to be performed is extremely short. Hence, supply of an external current is substantially not required. Conversely, since reverse flow of the regenerative current arises frequently, a charge current much larger than a discharge current flows through the electrolytic capacitor. For this reason, the supply voltage gradually increases each time the regenerative current flows.
SUMMARY OF THE INVENTION
The invention has been conceived to solve the drawback of the related-art technique and aims at providing an actuator drive circuit having an inductance which activates the actuator by causing a drive current to flow through the inductance, wherein, even when a power supply remains inactive, a circuit can be protected against an overvoltage.
Naito Hayato
Okada Noriaki
Saito Kazuhiko
Colon Santana Eduardo
Fletcher Marlon T.
Kabushiki Kaisha Sankyo Seiki Seisakusho
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