Incremental servo motor encoder zeroing method
(Phase alignment method of incremental encoder)
In this discussion, the output signal of the incremental encoder is a square wave signal, which can be further divided into incremental encoders with commutation signals and ordinary incremental encoders. Ordinary incremental encoders have two-phase orthogonal square wave pulse output signals A and B, and a zero-position signal Z. Incremental encoders with commutation signals, in addition to the ABZ output signals, also have electronic commutation signals UVW that are 120 degrees apart. The number of cycles per revolution of UVW is consistent with the number of pole pairs of the motor rotor. The alignment method between the phase of the UVW electronic commutation signal of the incremental encoder with commutation signals and the rotor pole phase, or electrical angle phase, is as follows:
1. Apply a DC current less than the rated current to the UV winding of the motor using a DC power supply, with U as the input and V as the output, to orient the motor shaft to a balanced position;
2. Use an oscilloscope to observe the U-phase and Z-phase signals of the encoder;
3. Adjust the relative position of the encoder shaft and the motor shaft;
4. While adjusting, observe the U-phase signal transition edge and the Z signal of the encoder until the Z signal stabilizes at a high level (assuming the normal state of the Z signal is low level), and lock the relative position relationship between the encoder and the motor.
5. Twist the motor shaft back and forth. If the Z signal remains stable at a high level each time the motor shaft freely returns to its balanced position, then the alignment is effective.
After disconnecting the DC power supply, the following verification was performed:
1. Use an oscilloscope to observe the U-phase signal of the encoder and the back EMF waveform of the UV line of the motor;
2. When the motor shaft is rotated, the rising edge of the encoder's U-phase signal coincides with the zero-crossing point of the motor's UV-line back EMF waveform from low to high. The encoder's Z signal also appears at this zero-crossing point.
The verification method described above can also be used as an alignment method.
It is important to note that at this point, the phase zero point of the incremental encoder's U-phase signal is aligned with the phase zero point of the motor's UV line back EMF. Since the motor's U-phase back EMF differs from the UV line back EMF by 30 degrees, this alignment brings the phase zero point of the incremental encoder's U-phase signal to the -30-degree phase point of the motor's U-phase back EMF. Since the phase of the motor's electrical angle is consistent with the phase of the U-phase back EMF waveform, the phase zero point of the incremental encoder's U-phase signal is aligned with the -30-degree point of the motor's electrical angle phase.
Some servo manufacturers are accustomed to directly aligning the zero point of the encoder's U-phase signal with the zero point of the motor's electrical angle. To achieve this, one can:
1. Connect three resistors of equal value in a star configuration, and then connect the three star-connected resistors to the UVW three-phase winding leads of the motor respectively;
2. By observing the midpoint between the motor's U-phase input and the star resistor using an oscilloscope, the U-phase back EMF waveform of the motor can be approximately obtained;
3. Adjust the relative position of the encoder shaft and the motor shaft, or the relative position of the encoder housing and the motor housing, according to the ease of operation;
4. While adjusting, observe the rising edge of the encoder's U-phase signal and the zero-crossing point of the motor's U-phase back EMF waveform from low to high. Finally, make the rising edge and the zero-crossing point coincide, lock the relative position relationship between the encoder and the motor, and complete the alignment.
Since ordinary incremental encoders do not have UVW phase information, and the Z signal can only reflect a point within one revolution, they do not have direct phase alignment potential, and therefore are not the topic of this discussion.
