Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2002-01-08
2003-05-27
Masih, Karen (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000
Reexamination Certificate
active
06570351
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a sensorless motor driving apparatus for driving a sensorless motor such as a brushless DC motor or a stepper motor by switching an excited phase in accordance with the position of a rotor, and more particularly, to a sensorless motor driving apparatus which controls commutation of the sensorless motor using a position detector for detecting the position of an object driven by the sensorless motor.
DESCRIPTION OF THE RELATED ART
In the prior art of motors using a permanent magnet on the rotor, such as brushless DC motors or HB-type stepper motors, it is known to detect the position of the rotor by detecting a counter electromotive voltage induced in a non-exciting phase (phase in which no current flows) of the stator coil. 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.
Besides, the above sensorless control method, it is also known to provide a Hall device on a motor whereby the position of the rotor is detected thereby controlling the motor.
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 a movement of an object driven by a sensorless motor; commutation control means which counts a number of pulses of the pulse signal output from the position detector and controls the commutation of the sensorless motor depending upon the count value; commutation reference point setting means for setting a commutation reference point employed as a reference point in the counting of the pulses; wherein, each time the sensorless motor is started for the first time after the apparatus has been turned on, the commutation reference point setting means performs a first and a second phase excitation such that a first pulled-in position resulting from the first phase excitation and a second pulled-in position resulting from the secondphase 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 further performs third-phase excitation such that a third pulled-in position resulting from the third-phase excitation becomes different in electrical angle from the second pulled-in position by a magnitude not equal to either 180° or an integral multiple of 180°, and, thereafter, the commutation reference point setting means performs the setting of the commutation reference point when a rotor of the sensorless motor has stopped after the third-phase 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 the 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 pulses of the pulse signal, 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.
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 the 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°, the rotor surely rotates in the forward or reverse direction and is pulled into the second-pulled-in position. If further excitation is performed such that the pulled-in position resulting from the third phase excitation becomes different in electrical angle from that of the pulled-in position resulting from the second-phase excitation by a magnitude not equal to either 180° or an integral multiple of 180°, the rotor rotates in a desired rotational direction and is pulled into the third-pulled-in position.
If the commutation reference point is set at the position at which the rotor is at rest after being pulled-in by the third excitation, then the resultant commutation reference point becomes coincident with the position at which commutation should be performed during rotation in the specified rotational direction.
Therefore, if this commutation reference point is employed as a counting reference point, and if the pulses are counted with respect to this reference point, and commutation is performed each time the count value becomes equal to an integral multiple of a predetermined number of pulses per commutation interval, commutation is correctly performed whenever the rotor comes to a position at which commutation should be performed.
In the case where an object is driven by the sensorless motor via a mechanism including, for example, a gear transmission mechanism, transmission belt, and pulley, there is a possibility that the object driven by the sensorless motor does not move and thus no pulse signal is output from the position detector, although the rotor actually rotates. This can occur if the gear transmission mechanism has backlash or if the driving force transmission belt expands. As a result, a shift occurs between the actual amount of rotation of the rotor from the rest position and the amount of rotation calculated from the number of pulses which are output from the position detector in response to the rotation.
In the present invention, the above problem is avoided by rotating the rotor in a desired rotational direction by performing excitation three times and then setting the commutation reference point at the pulled-in position where the rotor has stopped after the third excitation. Thus, if, thereafter, the sensorless motor is started in the same rotational direction as that in which the rotor was rotated to the position where the commutation reference point was set, the commutation reference point provides the correct counting reference point with no influe
Ikegami Akihiko
Miyazaki Shinichi
Harness & Dickey & Pierce P.L.C.
Masih Karen
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