Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2001-07-18
2002-11-19
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000, C318S700000, C318S701000
Reexamination Certificate
active
06483266
ABSTRACT:
1. Technical Field of the Invention
The present invention relates to an apparatus for driving a sensorless motor such as a brushless DC motor or a stepper motor, and more particularly, to an apparatus for driving a sensorless motor by controlling commutation by means of a position detector disposed on an object driven by the sensorless motor.
2. Description of the Related Art
In the prior art of motors using a permanent magnet on a rotor, such as a brushless DC motor or an HB-type stepper motor, it is known to detect the position of the rotor by detecting a counter electromotive force induced in a non-exciting phase (phase in which no current is passed) of stator coils.
That is, the counter electromotive voltage induced in a non-exciting coil is monitored, and the position of the rotor is detected by detecting a zero-crossing point at which the counter electromotive voltage crosses a neutral voltage point. In this technique, commutation is performed, for example, at a point at which the phase is shifted by 30° with respect to the zero-crossing point.
However, in this technique, when the rotor is at rest, no counter electromotive voltage is induced in the stator coil and thus sensorless driving is impossible. Therefore, when the motor whose rotor is at rest is started, the rotor is forcedly driven by performing forced commutation, and the operation is switched into a sensorless driving mode when the rotation speed has become high enough to induce a counter electromotive voltage greater than a predetermined value in the stator coil.
It is also known to provide a Hall device on a motor whereby the position of the rotor is detected thereby controlling the motor in a sensorless control method.
However, in the sensorless control on the basis of the counter electromotive voltage, commutation cannot be controlled in a low-speed range as described above, and thus this technique is unsuitable when the motor is frequently started and stopped. On the other hand, when a Hall device is used to control commutation, although control is possible in a low-speed range, the control accuracy is limited by factors such as a limited accuracy in the width of magnetic poles of the rotor and an installation position error of the Hall device, and thus the accuracy of controlling commutation is poorer than that obtained by the sensorless control on the basis of the counter electromotive voltage in a high-speed range in which the counter electromotive voltage can be detected. Thus, there is a need for a sensorless control method which allows a motor to be controlled precisely even in a low-speed range.
SUMMARY OF THE INVENTION
In view of the above problems in the conventional techniques, it is an object of the present invention to provide an apparatus for driving a sensorless motor, capable of controlling commutation in a precise and highly reliable fashion even in a low-speed range.
To achieve the above object, there is provided an apparatus for driving a sensorless motor, comprising: a position detector for outputting a pulse signal in response to movement of an object driven by a sensorless motor; commutation control means which counts the number of pulses output from the position detector and controls the commutation of the sensorless motor depending upon the counted value; and commutation reference point setting means for setting a commutation reference point employed as a reference point in the counting of the pulses; wherein, when the sensorless motor is started for the first time, the commutation reference point setting means performs phase excitation twice, switching the excited phase such that a pulled-in position resulting from the first-time excitation and that resulting from second-time excitation become different in electrical angle from each other by a magnitude not equal to either 180° or an integral multiple of 180°, and the commutation reference point setting means performs the setting of the commutation reference point when a rotor of the motor has stopped after the second-time excitation.
When the object driven by the sensorless motor moves, a pulse signal is output from the position detector in response to the movement of the object, and commutation is controlled on the basis of the counted number of pulses of the pulse signal. The commutation reference point, which is used as a reference point in counting the number of pulse signals, is set by the commutation reference point setting means on the basis of a position (pulled-in position) at which the rotor stops when a stator coil of the sensorless motor is excited for the first time.
In the case where excitation is performed only once, there is a possibility that, depending upon the position at which the rotor is at rest just before the sensorless motor is started for the first time, the rotor does not move in response to the excitation. However, if excitation is performed twice such that pulled-in positions become different in electrical angle from each other by a magnitude not equal to either 180° or an integral multiple of 180°, it is assured that the rotor moves. If the commutation reference point is set at the position at which the rotor stops after being pulled therein, the resultant commutation reference point becomes coincident with the position at which commutation should be performed during the rotation of the rotor.
Therefore, if the number of pulses is counted starting from the commutation reference point, and if commutation is performed each time the counted value becomes equal to an integral multiple of the predetermined number of pulses per commutation interval, commutation is correctly performed whenever the rotor comes to a position at which commutation should be performed.
The invention also provides a sensorless motor driving apparatus wherein the commutation reference point setting means sets the commutation reference point for each rotation direction of the sensorless motor, and wherein the commutation control means controls the commutation depending upon a present rotation direction in accordance with the number of pulses as counted starting from a commutation reference point set for the present rotation direction.
The invention also provides a sensorless motor driving apparatus wherein the commutation reference point setting means detects an offset value indicating the number of pulses corresponding to a difference in position between the commutation reference points set for the respective rotation directions of the sensorless motor, and wherein the commutation control means counts the pulses with respect to one of commutation reference points and corrects the counted value of pulses on the basis of the offset value each time the rotation direction is switched.
Commutation reference points are set for the respective rotation directions of the sensorless motor. In the case where the rotational motion of the sensorless motor is converted into the linear motion of a belt via a pulley thereby moving an object disposed on the belt, the absolute position of the rotor corresponding to a certain absolute position of the object becomes different depending upon the rotation direction, because the amount of expansion of the belt varies depending upon the rotation direction. When there is such a difference, if commutation is controlled on the basis of the counted value of pulses with reference to one commutation reference point set for one rotation direction, commutation timings determined on the basis of the counted value of pulses in the opposite direction become different from correct timings.
To avoid the above problem, commutation reference points are set for the respective rotation directions of the sensorless motor, and commutation is controlled, depending upon the present rotation direction, in accordance with the number of pulses as counted starting from the commutation reference point set for the present rotation direction, thereby preventing the commutation timings from shifting from the correct timings.
Herein, an offset value, which indicates the number of pulses corresponding to a difference in position betwe
Ikegami Akihiko
Miyazaki Shinichi
Fletcher Marlon T.
Harness & Dickey & Pierce P.L.C.
Seiko Epson Corporation
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