Servo motor maintenance

Alignment between detection phase of servo motor rotor feedback and rotor pole phase

For the problem of how to align the encoder phase of servo motor with the phase zero of rotor pole, Jia Hongwei summarizes his experience and experience on this problem for your reference or a comprehensive understanding.

Why should the encoder phase of permanent magnet AC servo motor be aligned with the rotor pole phase

Its only purpose is to achieve the goal of vector control, decouple the d-axis excitation component and q-axis output component, and make the electromagnetic field generated by the stator winding of permanent magnet AC servo motor always orthogonal to the rotor permanent magnetic field, so as to obtain the best output effect, i.e. "DC like characteristics". This control method is also called field oriented control (FOC), The external manifestation of achieving the FOC control goal is that the "phase current" waveform of the permanent magnet AC servo motor is always consistent with the "opposite potential" waveform. Therefore, it can be inferred that the FOC control goal can be achieved as long as the "phase current" waveform of the permanent magnet AC servo motor is always consistent with the "opposite potential" waveform, Make the primary electromagnetic field of the permanent magnet AC servo motor orthogonal to the permanent magnetic field of the magnetic pole, that is, the difference between the waveforms is 90 degrees. How to find a way to make the "phase current" waveform of the permanent magnet AC servo motor consistent with the "opposite potential" waveform? As can be seen from Fig. 1, as long as the electrical angle phase of the sinusoidal back EMF waveform can be detected at any time, then the sinusoidal phase current waveform consistent with the back EMF waveform can be generated relatively easily according to this phase. Therefore, the phase alignment can be transformed into the alignment relationship between the encoder phase and the back EMF waveform phase. In practice, European and American manufacturers are used to aligning the phase of encoder and rotor magnetic pole by passing DC current less than rated current to the motor winding to orient the motor rotor. When the DC current less than the rated current is applied to the winding of the motor, under the condition of no external force, the primary electromagnetic field interacts with the permanent magnetic field of the magnetic pole, which will attract each other and locate at the balance position of the phase with a mutual difference of 0 degrees. Compared with Fig. 3 and Fig. 2 above, it can be seen that although the position of the U-phase winding (red) is at the peak center (specific angle) of the electromagnetic field waveform, under the control of FOC, The U-phase center is aligned with the q-axis of the permanent magnet, while in the no-load orientation, the U-phase center is aligned with the d-axis, which realizes the a-axis or α Alignment relationship between axis and axis D. at this time, the phase is aligned to the electrical angle of 0 degrees, and the direction of rotor orientation current applied in the motor winding is U-phase in and VW out. Because V-phase and w-phase are in parallel, the current flowing through V-phase and w-phase may be unbalanced, thus affecting the accuracy of rotor orientation. The practical application method of rotor directional current is U-in and v-out, that is, U-phase and V-phase are connected in series, which can obtain U-phase and V-phase currents with exactly the same amplitude, which is conducive to the accuracy of orientation. At this time, the position of U-phase winding (red) is 30 degrees different from d-axis, i.e. axis A or D α The shaft is aligned to an electrical angle position with a difference of (negative) 30 degrees from D. the mainstream servo motor position feedback components include incremental encoder, absolute encoder, sine cosine encoder, resolver, etc. Phase alignment of incremental encoder in this discussion, the output signal of incremental encoder is square wave signal, which can be divided into incremental encoder with commutation signal and ordinary incremental encoder. Ordinary incremental encoder has two-phase orthogonal square wave pulse output signals a and B and zero position signal Z; The incremental encoder with commutation signal not only has ABZ output signal, but also has electronic commutation signal UVW with mutual difference of 120 degrees. The number of cycles per revolution of UVW is consistent with the magnetic pole logarithm of motor rotor. The phase of UVW electronic commutation signal and rotor pole phase of incremental encoder with commutation signal