1. Field of the Invention
The present invention relates to a method of estimating a rotational position of a motor, and a control apparatus of a motor.
2. Description of the Related Art
Sensorless vector control, which eliminates use of a sensor for rotational position detection to reduce the number of wires in a motor, the size of the motor, and a production cost of the motor, has often been used. A high-frequency voltage injection method is known as a sensorless vector control technique. In the high-frequency voltage injection method, a voltage having a frequency higher than a frequency of a drive voltage for a motor is applied to the motor, and a rotational position (i.e., a phase) of the motor is estimated based on a response current.
For example, a paper by Shinnaka, S., “A new speed-varying ellipse voltage injection method for sensorless drive of permanent—magnet synchronous motors with pole saliency—New PLL method using high-frequency current component multiplied signal” (IEEE Transactions on Industry Applications, 44(3), 2008, pp. 777-788), is known. In a method described in this paper, a high-frequency voltage is generated in a γδ rotating reference frame. This voltage is transformed to a αβ stationary reference frame through Park transformation, and the resulting voltage is applied to a motor through space vector PWM and an inverter. Three-phase high-frequency currents flowing in the motor are extracted using band-pass filters, and are transformed to the γδ rotating reference frame. A γ-axis current and a δ-axis current are multiplied together, and the resulting signal is subjected to low-pass filtering to generate a signal of a single channel. This signal is used to estimate the rotational position of the motor through a PLL.
Further, a paper by Corley, M. J. and Lorenz, R. D., “Rotor position and velocity estimation for a salient-pole permanent magnet synchronous machine at standstill and high speeds” (IEEE Transactions on Industry Applications, 34(4), 1998, pp. 784-789), is known. In a method described in this paper, using response currents in a stationary reference frame and the sine and cosine of an estimated rotational position (i.e., angle), a signal representing a d-axis component of a high-frequency current is obtained. This signal is passed through a band-pass filter or the like to obtain an error signal representing a difference between the estimated rotational position and an actual rotational position. This error signal is inputted to an observer. The observer minimizes the error to track the rotational position.
In each of the above methods, a process requiring a high computational load, such as, for example, transformation between stationary and rotating reference frames, needs to be performed in an arithmetic portion. An increase in the computational load on the arithmetic portion may affect precision with which the rotational position of a rotating portion is sensed. Accordingly, there is a demand for a novel technique to precisely sense the rotational position of the rotating portion with a low computational load.