Field of the Invention
The present invention relates to a motor control apparatus for controlling a motor.
Description of the Related Art
A permanent magnet motor (hereinafter referred to simply as a “motor” as well) is excellent for achieving a compact size and higher efficiency, and is used as a drive unit in various fields, including the field of information devices such as printers and copiers. When vector control is used as motor control, torque generated in the motor can be controlled more precisely, and it is possible to realize noise reduction, vibration reduction, and higher efficiency in the motor. Rotational position information regarding a rotor of the motor is needed in vector control. For this reason, in general, a position sensor such as a Hall element or an encoder is used, which leads to increases in cost and size.
In view of this, a sensor-less method has been proposed in which the rotational position of the rotor is estimated based on current flowing in the motor without using a position sensor. One example of a sensor-less method is a counter-electromotive voltage estimation method, in which the rotational position of the rotor is estimated by estimating counter-electromotive voltages generated in the windings of the motor accompanying the rotation of the rotor. The counter-electromotive voltages generated in the windings of the motor are determined according to the magnetization distribution of the permanent magnets in the rotor. The magnetization distribution of the permanent magnets is often a trapezoidal wave magnetization in order to achieve higher efficiency in the motor. Harmonic components, which are integral multiples of the rotation frequency of the motor, are included in the counter-electromotive voltage generated in the winding of the motor with the trapezoidal wave magnetization. Also, even in the case of magnetizing with a sine waveform, an ideal sine wave is not achieved due to errors in manufacturing, and harmonic components are included in the counter-electromotive voltage. With the counter-electromotive voltage estimation method, the rotational position of the rotor is estimated under the assumption that the counter-electromotive voltage normally changes in a sine waveform, and therefore error occurs due to the harmonic components.
For this reason, Japanese Patent No. 5170505 discloses a configuration in which the harmonic components are removed using a low-pass filter, whereby the error in the estimated position is reduced. Also, Japanese Patent No. 4154149 discloses a configuration in which a table of data on the counter-electromotive voltage of a motor is prepared, and a voltage is applied to the motor with consideration given to the harmonic components of the counter-electromotive voltage, whereby the harmonic components are removed. Furthermore, Japanese Patent No. 4631672 discloses a configuration in which harmonic components are extracted using a high-pass filter and a correction amount of the estimated position is obtained based on the extracted harmonic components, whereby the error is reduced.
However, with the configuration disclosed in Japanese Patent No. 5170505, phase lag caused by the low-pass filter occurs, and lag occurs in the estimation of the rotational position and rotational velocity of the rotor. Due to this lag, the responsiveness and stability of the control loop decrease. With the configuration disclosed in Japanese Patent No. 4154149, phase lag does not occur, but in order to obtain the counter-electromotive voltage data, measurement using a dedicated apparatus needs to be performed in advance, which reduces productivity. With the configuration disclosed in Japanese Patent No. 4631672, the extracted harmonic components include harmonic components resulting from counter-electromotive voltage distortion and harmonic components that occur due to velocity variation and load variation, and a distinction cannot be made between these, as a result of which error is included in the correction amount of the calculated rotational position. It is also difficult to eliminate the influence of phase shift caused by the high-pass filter, and error caused by phase shift also occurs.