Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2001-10-11
2003-06-24
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000, C318S700000, C318S721000
Reexamination Certificate
active
06583593
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for position-sensorless motor control in which a rotor position of a motor is detected without a position sensor, whereby the driving of the motor is controlled, in particular, when the rotor is at a stop or slowly revolves.
A brushless motor does not have a mechanical commutation element such as a brush, but instead has an electric circuit for carrying out the commutation electrically. The electric circuit controls the currents flowing through stator windings, in synchronization with the rotor revolution.
A brushless motor comprises a permanent magnet and thus at least two magnetic poles. A rotor position is defined by an angle around the center axis of a rotor between the direction (d-axis direction) of the center axis of a magnetic pole of the rotor and a reference direction (&agr;-axis direction) fixed to a stator.
Electric commutation needs detection of rotor position. A prior art motor control apparatus for brushless motor has obtained the information on rotor position using a position sensor such as Hall devices, a resolver, a magnetic encoder, and an optical encoder. However, the position sensor has caused a higher cost and a larger size in the prior art brushless motor.
In a position-sensorless motor control apparatus (prior art example, hereafter) disclosed in Japanese Laid-Open Patent Publication No. Hei 10-323099, a rotor position is detected without an above-described position sensor. Thus, the cost and the size of a brushless motor are reduced.
The prior art example detects a rotor position without a position sensor and controls the driving of motor, especially when a rotor is at a stop and slowly revolves, according to the following steps:
(1) Estimating the rotor position, thereby estimating the d-axis direction and the q-axis direction (&ggr;-axis direction and &dgr;-axis direction, respectively) of the rotor based on the estimated rotor position. Here, the q-axis direction is defined as the direction advancing by 90° in terms of electric angle from the d-axis direction in the direction of the rotor revolution.
(2) Superimposing a predetermined current signal or voltage signal for rotor position estimation (rotor-position estimation current/voltage signal, hereafter) on the &ggr;-axis direction component of a target current vector or a target voltage vector of the stator windings. Here, a target current vector of the stator windings is a vector representing target currents in the control over the currents flowing through the stator windings. A target voltage vector of the stator windings is a vector representing target voltages in the control over the voltages applied across the stator windings. In the invention, a target current vector with superimposed wave means a vector representing the target currents on which are superimposed the rotor-position estimation current signal. A target voltage vector with superimposed wave means a vector representing the target voltages that are superimposed the rotor-position estimation voltage signal on.
(3) Converting the target current vector with superimposed wave into the corresponding target voltage vector of the stator windings. A motor driver supplies electric power based on either the target voltage vector of the stator windings or the target voltage vector with superimposed wave, to the stator windings. In particular, in a pulse width modulation (PWM) control over the currents of the stator windings, the motor driver modulates the target voltages represented by either the target voltage vector of the stator windings or the target voltage vector with superimposed wave, through the PWM, and then applies the modulated target voltages across the stator windings. Electric power corresponding to the rotor-position estimation current/voltage signal is applied to the stator windings through the power supplied to the stator windings by the motor drive. Here, the rotor-position estimation current/voltage signal is, for example, an AC signal having a constant period equal to a multiple of the PWM carrier period and a constant amplitude. Each of the rotor-position estimation current/voltage signals is generally referred to as a rotor-position estimation signal hereafter, when distinction between both of the signals is unnecessary. In the above-described PWM control, a constant AC power corresponding to the rotor-position estimation signal is applied to the stator windings. Then, a current response to the AC power is generated in the stator windings.
(4) Detecting the above-described current response in the &dgr;-axis direction at a predetermined phase. For example, a sampling of the current response is carried out at each peak of the rotor-position estimation signal, that is, at each half period of the rotor-position estimation signal.
(5) Correcting the estimated rotor position so that the detected current response approaches zero in the &dgr;-axis direction.
These steps (1) through (5) are repeated during the driving control of the motor.
The amount of deviation of the &dgr;-axis direction from the d-axis direction is designated as &Dgr; &thgr;, equal to the amount of deviation of the &dgr;-axis direction from the q-axis direction, and hereafter referred to as a position estimation error. The amplitude of the current response in the &dgr;-axis direction is substantially proportional to sin (2&Dgr; &thgr;). Accordingly, the estimated rotor position and the actual rotor position coincide with each other within a predetermined error, when the current response converge to zero within a predetermined error in the &dgr;-axis direction.
The prior art position-sensorless motor control uses the rotor-position estimation signal with a constant frequency. In particular, the constant frequency falls within the audio-frequency band from a few tens Hz to a few hundreds Hz. As a result, when the stator teeth and the like vibrate in synchronization with the rotor-position estimation signal, an undesired sound is generated. The undesired sound is large especially near the frequency of the rotor-position estimation signal.
The prior art position-sensorless motor control uses the rotor-position estimation signal with a constant amplitude. Accordingly, the current response to the rotor-position estimation signal has a substantially constant amplitude. On the other hand, the larger the amplitudes of the currents flowing through the stator windings becomes, the larger an electric noise, which is hereafter referred to as a noise, in the &dgr;-axis direction is generally generated, and hence, the greater the ratio (S/N ratio) of the current response's amplitude to the noise is reduced. When the S/N ratio is small, the distinction between the current response and the noise is difficult, and hence, the position estimation error is large. Furthermore, in the prior art position-sensorless motor control, the sampling of the current response is carried out only at each half period of the rotor-position estimation signal, and thus, the number of the samples is small. Accordingly, the position estimation error may be very large in the prior art, when the S/N ratios of the respective samples of the current response are small.
In order to suppress the position estimation error, the S/N ratios of the current response have to increase. To do this, the amplitude of the rotor-position estimation signal may be increased, or the noise may be reduced. However, an increasing of the amplitude of the rotor-position estimation signal is difficult, since the above-described undesired sound is large when the amplitude of the rotor-position estimation signal is large. On the other hand, in order to reduce the noise in the current response, the current response may be sufficiently attenuated through a low pass filter (LPF). Alternatively, a gain may be reduced in a process of calculating the correction of the estimated position from the position estimation error. However, these approaches may delay the estimation of the rotor position, thereby slowing down the response in the drivin
Iijima Tomokuni
Maruyama Yukinori
Narazaki Kazushige
Oyama Ichiro
Tazawa Toru
Akin Gump Strauss Hauer & Feld L.L.P.
Duda Rina I.
Matsushita Electric - Industrial Co., Ltd.
Nappi Robert E.
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