How a rotary transformer handles "zero"

Why is the encoder phase of a permanent magnet AC servo motor aligned with the rotor magnetic pole phase?

Its sole purpose is to achieve the goal of vector control, decoupling the d-axis excitation component and the q-axis output component, ensuring that the electromagnetic field generated by the stator winding of the permanent magnet AC servo motor is always orthogonal to the rotor permanent magnetic field, thereby achieving the best output effect, namely "classical DC characteristics". This control method is also known as 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 always remains consistent with the "opposite potential" waveform, as shown in the following figure:

Therefore, by reasoning backward, we can conclude that as long as we find a way to ensure that the "phase current" waveform of the permanent magnet AC servo motor always remains consistent with the "opposite potential" waveform, the FOC control objective can be achieved. This will 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 two waveforms will differ by 90 degrees in electrical angle, as shown in the following figure:

How can we find a way to ensure that the "phase current" waveform of the permanent magnet AC servo motor always remains consistent with the "opposite potential" waveform? As shown in Figure 1, as long as the sine-type reverse potential waveform's electrical angle phase can be detected at any time, then it will be relatively easy to generate a sine-type phase current waveform consistent with the reverse potential waveform based on the electrical angle phase.

It is necessary to explicitly state that the so-called electrical angle of the permanent magnet AC servo motor is the sine phase of the opposite potential waveform of phase a (U phase), so phase alignment can be transformed into the alignment relationship between the encoder phase and the opposite potential waveform phase; on the other hand, the electrical angle is also the angle between the d-axis (direct axis) of the rotor coordinate system and the a-axis (U-axis) or α-axis of the stator coordinate system, which is helpful for graphical analysis.

In practical operation, manufacturers in Europe and America are accustomed to using the method of applying a DC current less than the rated current to the motor's windings to align the encoder and the rotor magnetic poles' phases. When the motor's windings are supplied with a DC current less than the rated current, in the absence of external forces, the primary electromagnetic field interacts with the permanent magnetic field of the magnetic pole, attracting and positioning to the balanced position with an inter-phase phase difference of 0 degrees, as shown in the following figure:

By comparing Figure 3 with Figure 2, it can be seen that although the position of the a-phase (U-phase) winding (in red) is at the peak center of the electromagnetic field waveform (at a specific angle), under FOC control, the center of the a-phase (U-phase) aligns with the q-axis of the permanent magnet; while in the no-load directional mode, the center of the a-phase (U-phase) aligns with the d-axis. That is to say, compared to the primary (stator) winding, the d-axis of the secondary (rotor) magnet coordinate system shifts 90 degrees in the electric angle during no-load directional operation, and coincides with the original position of the q-axis under FOC control. Thus, the alignment relationship between the a-axis (U-axis) or the α-axis and the d-axis during rotor no-load directional operation is achieved.

At this point, when the phase alignment is at 0 degrees electric angle, the direction of the rotor directional current applied to the motor winding is from the bc phase (VW phase) into and from the a phase (U phase). Since the b phase (V phase) and the c phase (W phase) are in parallel, the current flowing through the b phase (V phase) and the c phase (W phase) may be unbalanced, thereby affecting the accuracy of rotor directional operation.

The practical method for applying rotor directional current is to apply it into the b phase (V phase) and out of the a phase (U phase), that is, to connect the a phase (U phase) in series with the b phase (V phase). This can obtain a current with the same amplitude for the a phase (U phase) and the b phase (V phase), which is beneficial to the accuracy of directional operation. At this time, the position of the a-phase (U-phase) winding (in red) is 30 degrees in electric angle away from the d-axis, that is, the a-axis (U-axis) or the α-axis is aligned to the position with a difference of (negative) 30 degrees in electric angle from the d-axis, as shown in the figure:

The corresponding opposite potential waveforms of the windings and line counter-potential, as well as the relationship of electrical angles for the above two rotor orientation methods are shown in the following figure. The brown line represents the alignment of the a-axis (U-axis) or the α-axis with the d-axis, that is, directly aligned to the 0 degree electrical angle point; the purple line represents the alignment of the a-axis (U-axis) or the α-axis to an electrical angle position that is (negative) 30 degrees away from the d-axis, that is, aligned to the -30 degree electrical angle point:

The vector relationships of the above two rotor orientation methods in the dq rotor coordinate system and the abc (uvw) or ɑβ stator coordinate systems are shown in Figure 6:

The phase alignment method of incremental encoders

In this discussion, the output signal of the incremental encoder is a square wave signal, which can be divided into incremental encoders with synchronization signals and ordinary incremental encoders. Ordinary incremental encoders have two-phase orthogonal square wave pulse output signals A and B, as well as a zero position signal Z; incremental encoders with synchronization signals also have electronic synchronization signals UVW, which are mutually 120 degrees apart, and each of their per-rotation periods is consistent with the number of magnetic poles of the motor rotor. The phase alignment method between the UVW electronic synchronization signals of the incremental encoders with synchronization signals and the phase of the rotor magnetic poles or the electrical angle phase is as follows:

1. Use a DC power supply to apply a DC current less than the rated current to the UV winding of the motor, V in, U out, and orient the motor shaft to a balanced position;

2. Use an oscilloscope to observe the U signal and Z signal of the encoder;

3. Adjust the relative position of the encoder shaft and the motor shaft;

4. While adjusting, observe the rising edge of the U signal of the encoder and the Z signal, until the Z signal stabilizes at a high level (here, it is assumed that the normal state of the Z signal is low), lock the relative position relationship between the encoder and the motor;

5. Twist the motor shaft back and forth, release it, and if the Z signal can stabilize at a high level each time the motor shaft freely returns to the balanced position, the alignment is effective.

After removing the DC power supply, the following verification is carried out:

1. Use an oscilloscope to observe the U signal of the encoder and the reverse electromotive force waveform of the motor UV line;

2. Rotate the motor shaft counterclockwise, and the rising edge of the U signal of the encoder and the reverse electromotive force waveform of the motor UV line will coincide at the zero-crossing point from low to high, and the Z signal of the encoder also appears at this zero-crossing point.

The above verification method can also be used as an alignment method.

It should be noted that at this time, the phase zero point of the U signal of the incremental encoder is aligned with the phase zero point of the motor's electrical angle. Due to the opposite U electromotive force of the motor and the 30-degree difference with the UV line electromotive force, after such alignment, the phase zero point of the U signal of the incremental encoder is aligned with the -30-degree phase point of the motor's opposite U electromotive force, and the electrical angle phase of the motor is consistent with the phase of the U opposite electromotive force waveform. Therefore, at this time, the phase zero point of the U signal of the incremental encoder is aligned with the -30-degree point of the motor's electrical angle.

Some servo enterprises are accustomed to directly aligning the zero point of the U signal of the encoder with the zero point of the motor's electrical angle. To achieve this purpose, the following can be done:

1. Use a DC power supply to apply a DC current less than the rated current to the UVW winding of the motor, VW in, U out, and orient the motor shaft to a balanced position;

2. Use an oscilloscope to observe the U signal and Z signal of the encoder;

3. Adjust the relative position of the encoder shaft and the motor shaft;

4. While adjusting, observe the rising edge of the U signal of the encoder and the Z signal, until the Z signal stabilizes at a high level (here, it is assumed that the normal state of the Z signal is low), lock the relative position relationship between the encoder and the motor;

5. Twist the motor shaft back and forth, release it, and if the Z signal can stabilize at a high level each time the motor shaft freely returns to the balanced position, the alignment is effective.

The verification method is as follows:

1. Connect three resistors of equal resistance in a star configuration, and then connect the three resistors of the star configuration to the three-phase winding leads of the motor's UVW;

2. Observe the U phase input of the motor and the midpoint of the star resistors with an oscilloscope, approximately obtaining the U opposite electromotive force waveform of the motor;

3. Rotate the motor shaft counterclockwise, and it can be seen that the rising edge of the U signal of the encoder and the U opposite electromotive force waveform of the motor will coincide at the zero-crossing point from low to high.

Since ordinary incremental encoders do not have UVW phase information, and the Z signal can only represent a single point within one rotation, it does not have the potential for direct phase alignment, and thus is not a topic of discussion in this context.

The phase alignment method of the absolute encoder

The phase alignment of the absolute encoder is not significantly different for single-loop and multi-loop cases. In essence, it is aligning the detection phase of the encoder with the phase of the motor electrical angle within one loop. Early absolute encoders would provide the highest bit level of the single-loop phase through individual pins. By using the 0 and 1 flips of this level, the phase alignment of the encoder and the motor could be achieved. The method is asfollows:

1. Supply a DC power source to the UV winding of the motor with a current less than the rated current. V in, U out. Align the motor shaft to a balanced position.

2. Use an oscilloscope to observe the highest count bit level signal of the absolute encoder.

3. Adjust the relative position of the encoder shaft and the motor shaft.

4. While adjusting, observe the transition edge of the highest count bit signal. Until the transition edge accurately appears at the balanced position of the motor shaft, lock the relative position relationship between the encoder and the motor.

5. Twist the motor shaft back and forth. After releasing, if the motor shaft can accurately reproduce the transition edge each time it freely returns to the balanced position, the alignment is effective.

This type of absolute encoder has now been widely replaced by serial protocols such as EnDAT, BiSS, Hyperface, and new absolute encoders with Japanese-specific serial protocols. Thus, the highest bit signal no longer exists. At this point, the method for aligning the encoder and motor phases has changed. One very practical method is to use the EEPROM inside the encoder to store the phase measured after the encoder is randomly installed on the motor shaft. The specific method is as follows:

1. Randomly install the encoder on the motor, i.e., fix the encoder shaft and motor shaft, as well as the encoder housing and motor housing.

2. Supply a DC power source to the UV winding of the motor with a current less than the rated current. V in, U out. Align the motor shaft to a balanced position.

3. Use a servo driver to read the single-loop position value of the absolute encoder and store it in the EEPROM inside the encoder to record the initial phase of the motor electrical angle.

4. The alignment process is completed.

Since the motor shaft is now aligned in the -30 degree direction of the electrical angle, the position detection value stored in the internal EEPROM of the encoder corresponds to the -30 degree phase of the motor electrical angle. Thereafter, the driver will calculate the difference between the single-loop position detection data at any time and this stored value, and perform necessary conversions based on the number of motor poles. Add -30 degrees, and you can obtain the motor electrical angle phase at that moment.

This alignment method requires the support and cooperation of the encoder and servo driver to be realized. The fundamental reason why the phase of Japanese servo encoders is not convenient for end users to directly adjust lies in their unwillingness to provide this alignment method's function interface and operation method to users. One of the major advantages of this alignment method is that only a rotor orientation current with a determined phase sequence and direction needs to be provided to the motor winding, without the need to adjust the angle relationship between the encoder and the motor shaft. Therefore, the encoder can be directly installed on the motor at any initial angle, and there is no need for a precise or even simple adjustment process. The operation is simple and the process is good in terms of craftsmanship.

If the absolute encoder has neither a usable EEPROM nor a detectable highest count bit pin, the alignment method will be relatively complex. If the driver supports the reading and display of single-loop absolute position information, then the following can be considered:

1. Supply a DC power source to the UV winding of the motor with a current less than the rated current. V in, U out. Align the motor shaft to a balanced position.

2. Use a servo driver to read and display the single-loop position value of the absolute encoder.

3. Adjust the relative position of the encoder shaft and the motor shaft.

4. After the above adjustments, the displayed absolute position value of each circle is made to be as close as possible to the absolute position point corresponding to the 30-degree electrical angle of the motor calculated based on the number of poles of the motor. This ensures the relative position relationship between the encoder and the motor is locked.

5. Twist the motor shaft back and forth. After releasing, if the above calculated position point can accurately be reproduced every time the motor shaft freely returns to the balanced position, then the alignment is effective.

If the user cannot obtain the absolute value information, then they can only rely on the original factory's special tooling. While detecting the absolute position detection value, they can also detect the electrical angle phase of the motor. Using the tooling, they can adjust the relative angular position relationship between the encoder and the motor, align the encoder phase with the motor electrical angle phase, and then lock it. In this way, the user will have even less ability to solve the problem of aligning the encoder phase by themselves.

Personally, I recommend using the method of storing the initial installation position in the EEPROM. It is simple, practical, adaptable, and easy to be opened to users, so that users can install the encoder by themselves and complete the phase setting of the motor electrical angle.

The phase alignment method of the sine-cosine encoder

The ordinary sine-cosine encoder consists of a pair of orthogonal sin and cos 1Vp-p signals, equivalent to the incremental signals AB of the square wave signal encoder, repeating numerous signal cycles per revolution, such as 2048, etc.; and a narrow-band symmetrical triangular Index signal, equivalent to the Z signal of the incremental encoder, typically appearing once per revolution; this sine-cosine encoder is essentially an incremental encoder as well. Another sine-cosine encoder, in addition to the above-mentioned orthogonal sin and cos signals, also possesses a pair of mutually orthogonal 1Vp-p sine-type C and D signals that appear only once per revolution. If the C signal is sin and the D signal is cos, when the encoder shaft rotates counterclockwise, the Index signal of the Z signal generally aligns with the rising zero point of the C signal from low to high. Through the high-rate subdivision technology of sin and cos signals, the sine-cosine encoder can achieve a nominal detection resolution finer than the original signal period, such as a 2048-line sine-cosine encoder after 2048 subdivisions can reach a nominal detection resolution of over 4 million lines per revolution. Currently, many European and American servo manufacturers offer such high-resolution servo systems, while domestic manufacturers are still relatively rare; moreover, the C and D signals of the sine-cosine encoder with C and D signals after subdivision can provide higher absolute position information per revolution, such as 2048 absolute positions per revolution. Therefore, the sine-cosine encoder with C and D signals can be regarded as a kind of analog single-rotation absolute encoder.

The initial electrical angle phase alignment method for the servo motor using this encoder is as follows:

1. Supply a DC power supply to the UV windings of the motor with a current less than the rated current, V in, U out, and orient the motor shaft to a balanced position;

2. Use an oscilloscope to observe the waveforms of the C signal and Index signal of the sine-cosine encoder;

3. Adjust the relative position of the encoder shaft and the motor shaft;

4. While adjusting, observe the waveforms of the C signal and Index signal until the zero crossing point of the C signal or the effective level of the Index signal accurately appears at the directional balanced position of the motor shaft, lock the relative position relationship between the encoder and the motor;

5. Twist the motor shaft back and forth, release it, and if the zero crossing point of the C signal or the effective level of the Index signal can accurately reproduce each time the motor shaft freely returns to the balanced position, then alignment is effective.

After removing the DC power supply, verify as follows:

1. Use an oscilloscope to observe the C signal of the encoder and the UV line reverse electromotive force waveform of the motor;

2. Rotate the motor shaft counterclockwise, and the zero crossing point of the C signal or the jump of the Index signal along from low to high coincides with the zero crossing point of the UV line reverse electromotive force waveform from low to high.

This verification method can also be used as an alignment method.

At this time, the zero crossing point of the C signal aligns with the -30-degree point of the motor electrical angle.

If you want to directly align with the 0-degree point of the motor electrical angle, you can consider:

1. Supply a DC power supply to the UVW windings of the motor with a current less than the rated current, VW in, U out, orient the motor shaft to a balanced position;

2. Use an oscilloscope to observe the waveforms of the C signal and Index signal of the encoder;

3. Adjust the relative position of the encoder shaft and the motor shaft;

4. While adjusting, observe the waveforms of the C signal and Index signal until the zero crossing point of the C signal or the effective level of the Index signal accurately appears at the directional balanced position of the motor shaft, lock the relative position relationship between the encoder and the motor;

5. Twist the motor shaft back and forth, release it, and if the zero crossing point of the C signal or the effective level of the Index signal can remain stable at the high level each time the motor shaft freely returns to the balanced position, then alignment is effective.

The verification method is as follows:

1. Connect three resistors of equal resistance in a star configuration, and then connect the three resistors of the star configuration to the three-phase winding leads of the motor's U, V, and W phases respectively;

2. Use an oscilloscope to observe the input of the motor's U phase and the midpoint of the star-shaped resistors, which can approximately obtain the U-phase reverse potential waveform of the motor;

3. Rotate the encoder shaft counterclockwise, and observe the zero-crossing points of the C signal or the Index signal of the encoder from low to high should coincide with the zero-crossing points of the U-phase reverse potential waveform of the motor from low to high.

The above verification method can also be used as an alignment method.

Since ordinary sine and cosine encoders do not have phase information within one circle, and the Index signal can only reflect one point within one circle, it does not have the potential for direct phase alignment, therefore, it is not discussed here.

If the servo driver that can be connected to the sine and cosine encoder can provide users with the single-circle absolute position information obtained from C and D, then consider:

1. Use a DC power supply to apply a current less than the rated current to the UV winding of the motor's U phase, V input, U output, and orient the motor shaft to a balanced position;

2. Use the servo driver to read and display the single-circle absolute position information obtained from the C and D signals;

3. Adjust the relative position of the rotary sensor shaft and the motor shaft;

4. After the above adjustments, make the displayed absolute position value closely approach the absolute position point corresponding to the motor's -30-degree electrical angle calculated based on the number of poles, and lock the relative position relationship between the encoder and the motor;

5. When twisting the motor shaft back and forth, if the above-mentioned calculated absolute position point can be accurately reproduced each time the motor shaft freely returns to the balanced position, the alignment is effective.

After this, the same alignment verification effect as before can be obtained by removing the DC power supply:

1. Use an oscilloscope to observe the C signal of the sine and cosine encoder and the reverse electromotive force waveform of the motor's UV line;

2. Rotate the motor shaft to verify that the zero-crossing points of the C signal of the encoder from low to high coincide with the zero-crossing points of the reverse electromotive force waveform of the motor's UV line from low to high.

If using the non-volatile memory such as EEPROM inside the driver, the phase of the sine and cosine encoder that was randomly installed on the motor shaft can be stored, the specific method is as follows:

1. Randomly install the sine and cosine on the motor, that is, fix the encoder rotation shaft with the motor shaft, and the encoder housing with the motor housing;

2. Use a DC power supply to apply a current less than the rated current to the UV winding of the motor's U phase, V input, U output, and orient the motor shaft to a balanced position;

3. Use the servo driver to read the single-circle absolute position value parsed from the C and D signals and store it in the non-volatile memory such as EEPROM inside the driver to record the initial installation phase of the motor's electrical angle;

4. After the alignment process is completed.

Since the motor shaft has been oriented in the -30-degree direction of the electrical angle at this time, the position detection value stored in the non-volatile memory such as EEPROM inside the driver corresponding to the -30-degree phase of the motor's electrical angle. Thereafter, the driver will calculate the single-circle absolute position value parsed from the encoder at any time and subtract this stored value, and perform necessary conversion based on the number of motor poles, and add -30 degrees, to obtain the motor's electrical angle phase at that time.

This alignment method requires the support and cooperation of the servo driver in domestic and operation to be realized. And since the non-volatile memory such as EEPROM inside the driver is located in the servo driver, if the motor, sine and cosine encoder, or driver needs to be replaced, a new initial installation phase alignment operation and re-binding of the motor and driver's supporting relationship need to be performed.

The phase alignment method of the rotary transformer

The rotary transformer, abbreviated as ROT, is composed of high-performance silicon steel laminations and enameled wires with a specially designed electromagnetic structure. Compared to the encoder using photoelectric technology, it has the ability to withstand high temperatures, vibrations, impacts, oil contamination, and even corrosion in harsh working environments. Therefore, it is widely adopted in applications with harsh conditions such as weapon systems. A pair of poles (single-speed) of ROT can be regarded as a single-loop absolute feedback system and is the most widely used. Thus, in this discussion, only single-speed ROT will be considered. Multi-speed ROT is paired with servo motors, and personally, I think the number of pole pairs should be approximately a factor of the motor's pole pairs to facilitate the correspondence of motor degrees and pole pair decomposition.

The signal leads of the rotary transformer usually consist of 6 wires, divided into 3 groups, corresponding to one excitation coil and two orthogonal sensing coils. The excitation coil receives the input sinusoidal excitation signal, and the sensing coils sense the detection signals with SIN and COS envelopes based on the relative angular position relationship between the rotor and stator. The SIN and COS output signals of the rotary transformer are the modulation results of the excitation sinusoidal signal according to the angle between the rotor and stator. If the excitation signal is sinωt and the electrical angle between the rotor is θ, then the SIN signal is sinωt × sinθ, and the COS signal is sinωt × cosθ. By using the necessary detection circuits based on the SIN, COS signals and the original excitation signal, high-resolution position detection results can be obtained. Currently, the detection resolution of commercial rotary transformer systems can reach 2^12 per revolution, that is, 4096. While in scientific research and aerospace systems, it can even reach above 2^20. However, the volume and cost are also very considerable.

Here, it is assumed that when the rotator with rotation variation rotates counterclockwise, the electrical angle phase of the rotation variation increases; and when the rotator with rotation variation rotates clockwise, the electrical angle phase of the rotation variation decreases.

The alignment method for the electrical angle and phase of the commercial rotary transformer and the servo motor is as follows:

1. Supply the UV winding of the motor with a direct current that is less than the rated current, with V input and U output.

2. Then use an oscilloscope to observe the signal lead output of the rotator's SIN coil.

3. According to the ease of operation, adjust the relative position of the rotator rotor on the motor shaft and the motor shaft, or the relative position of the rotator stator and the motor housing.

4. While adjusting, observe the envelope of the rotator SIN signal until the amplitude of the signal envelope is completely zero, and lock the rotator.

(4') While adjusting, observe the Lissajous figure with the SIN signal of the rotator as the abscissa and the excitation signal as the ordinate until the Lissajous figure becomes a vertical line coinciding with the ordinate axis, and twist the vertical line towards the 1st and 3rd quadrants in the counterclockwise direction, and towards the 2nd and 4th quadrants in the clockwise direction. Lock the rotator.

5. Twist the motor shaft back and forth. After releasing, if the amplitude of the signal envelope can accurately reproduce the zero point each time the motor shaft freely returns to the equilibrium position, or the Lissajous figure can coincide with the ordinate axis as a vertical line, then alignment is effective.

Remove the direct current power supply and perform alignment verification:

1. Use an oscilloscope to observe the SIN signal of the rotary transformer and the reverse electromotive force waveform of the motor's UV line;

2. Rotate the motor shaft to verify that the zero-crossing point of the envelope of the SIN signal of the rotary transformer coincides with the zero-crossing points of the reverse electromotive force waveform of the motor, which increase from low to high.

This verification method can also be used as an alignment method.

At this time, the zero-crossing point of the envelope of the SIN signal coincides with the -30-degree point of the motor's electrical angle phase.

If you want to align directly with the 0-degree point of the motor's electrical angle, you can consider:

1. Supply the UVW windings of the motor with a direct current that is less than the rated current. Connect VW in, U out. Align the motor shaft to a balanced position.

2. Use an oscilloscope to observe the SIN signal of the resolver.

3. Adjust the relative position of the resolver shaft and the motor shaft.

4. While adjusting, observe the envelope waveform of the SIN signal. Keep adjusting until the amplitude of the signal envelope is completely zero, and lock the resolver.

(4') While adjusting, observe the Lissajous figure with the SIN signal as the abscissa and the excitation signal as the ordinate. Keep adjusting until the Lissajous figure becomes a vertical line coinciding with the ordinate axis, and twist the vertical line towards the 1st and 3rd quadrants in the counterclockwise direction, and towards the 2nd and 4th quadrants in the clockwise direction. Lock the resolver.

5. Twist the motor shaft back and forth. After releasing, if the amplitude of the signal envelope can accurately reproduce the zero point each time the motor shaft freely returns to the balanced position, or if the Lissajous figure can coincide with the ordinate axis as a vertical line, then alignment is effective.

The verification method is as follows:

1. Connect three resistors of equal resistance in a star configuration, and then connect the three resistors of the star connection to the three-phase winding leads of the motor's U, V, and W phases respectively.

2. By observing the U-phase input of the motor and the midpoint of the star-connected resistors with a scope, the approximate U-phase reverse voltage waveform of the motor can be obtained.

3. By observing the zero-crossing points of the envelope of the SIN signal from the rotary transformer and the zero-crossing points of the U-phase reverse voltage waveform from low to high, these two zero-crossing points should coincide.

The above verification method can also be used as an alignment method.

It should be noted that in the above operations, it is necessary to effectively distinguish the positive half-cycle and the negative half-cycle of the SIN envelope signal of the rotation change. Since the SIN signal is the modulation result of the excitation signal based on the angle between the stator of the rotation change, the SIN envelope signal corresponding to the positive half-cycle of sinθ has the same phase as the original excitation signal, while the SIN envelope signal corresponding to the negative half-cycle of sinθ has the opposite phase to the original excitation signal. Based on this, it is possible to distinguish and determine the positive and negative half-cycles of the SIN envelope signal waveform of the rotation change output, and during alignment, the zero point of the SIN envelope signal corresponding to the transition point from the negative half-cycle to the positive half-cycle of sinθ should be taken. If the polarity is reversed or not accurately judged, the electrical angle after alignment may be misaligned by 180 degrees, which may cause the speed outer loop to enter positive feedback.

If the servo driver that can interface with the rotary transformer can provide users with the absolute position information related to the motor's electrical angle obtained from the rotary transformer signal, then it can be considered:

1. Supply a DC power supply with a current less than the rated current to the UV winding of the motor, V in, U out, and orient the motor shaft to a balanced position;

2. Use the servo drive to read and display the absolute position information related to the motor electrical angle from the rotation change signal;

3. Adjust the relative position of the rotation change axis and the motor shaft, or the relative position of the rotation change housing and the motor housing according to the convenience of operation;

4. After the above adjustments, make the displayed absolute position value closely approach the absolute position point corresponding to the motor-30 degree electrical angle calculated based on the number of poles of the motor, and lock the relative position relationship between the rotation change rotor and the motor shaft;

5. When twisting the motor shaft back and forth and releasing, if the above calculated absolute position point can accurately reproduce each time the motor shaft freely returns to the equilibrium position, then the alignment is effective.

After this, the same alignment verification effect as before can be obtained by removing the DC power supply:

1. Use an oscilloscope to observe the SIN signal of the rotation change and the U-phase reverse electromotive force waveform of the motor;

2. Rotate the motor shaft to verify that the zero points of the envelope of the rotation change SIN signal coincide with the zero points of the U-phase reverse electromotive force waveform from low to high.

If using the non-volatile memory such as EEPROM inside the driver, the phase measured after the random installation of the rotation change on the motor shaft can be stored, the specific method is as follows:

1. Randomly install the rotation change on the motor, that is, fix the rotation change shaft and the motor shaft, as well as the rotation change housing and the motor housing.

2. Supply the UV winding of the motor with a direct current that is less than the rated current. V in, U out. Align the motor shaft to a balanced position.

3. Use the servo driver to read the absolute position values related to the electrical angle derived from the resolver, and store them in the non-volatile memory such as the EEPROM inside the driver, which records the initial installation phase of the motor's electrical angle.

4. The alignment process is completed.

Since the motor shaft has now been aligned in the -30 degree direction of the electrical angle phase, the position detection values stored in the non-volatile memory such as the EEPROM inside the driver, which correspond to the initial installation phase of the motor's electrical angle, will correspond to the -30 degree phase of the motor's electrical angle. Thereafter, the driver will subtract the absolute position value related to the electrical angle derived from the resolver at any given time from this stored value, and perform necessary conversions based on the number of motor poles, and add -30 degrees to obtain the motor's electrical angle phase at that moment.

This alignment method can only be achieved with the support and cooperation of the servo driver in both domestic and operational aspects. Moreover, since the non-volatile memory such as the EEPROM that records the initial phase of the motor's electrical angle is located inside the servo driver, once the alignment is completed, the motor and the driver are actually bound together. If it is necessary to replace the motor, resolver, or driver, a new initial installation phase alignment operation must be performed, and the matching relationship between the motor and the driver must be re-bonded.

Attention

1. In the above discussion, the term "aligning to the -30-degree phase of the motor electrical angle" is based on the premise that the UV reverse electromotive force waveform lags behind the U phase by 30 degrees.

2. In the above discussion, it is assumed that VU are connected in parallel and the reverse electromotive force waveform of UV is referred to. Some servo systems may adopt UW connected in parallel and refer to the reverse electromotive force waveform of UW.

3. If one wants to directly align to the 0-degree phase of the motor electrical angle, U phase can be connected to the negative terminal of the low-voltage DC source, and V phase and W phase can be connected in parallel to the positive terminal of the DC source. At this time, the orientation angle of the motor shaft will be offset by 30 degrees compared to the method of connecting U and V in series with the UV phase. After aligning according to the corresponding alignment method given in the text, it is theoretically aligned to the 0-degree phase of the motor electrical angle, without the -30-degree offset. This approach may seem beneficial, but considering the inconsistency of the parameters of the motor windings, when V and W are connected in parallel, the currents flowing through the V and W windings may not be consistent, thereby affecting the accuracy of the orientation angle of the motor shaft. However, when VU are connected in parallel, the U and V windings have a simple series relationship, so the currents flowing through the U and V windings must be consistent, and the accuracy of the orientation angle of the motor shaft will not be affected by the current direction of the windings.

4. It cannot be ruled out that the servo manufacturers may intentionally align the initial phase misalignment. Especially in a feedback system that can provide absolute position data, the initial phase misalignment alignment can be easily compensated for by the data's offset quantity. In this way, it might serve as some form of protection for their product line. However, in this case, users will have even less idea about where the initial phase of the servo motor feedback element should be aligned. Users naturally do not want to encounter such suppliers.

Summary of the basic methods for phase alignment in electrical angles

1. Waveform observation method

Applicable to incremental encoders with changeover signals, sine-cosine encoders, and rotary transformers.

1) By directly observing the zero-crossing points of the UV line reverse electromotive force waveform with a scope and the rising edge of the U signal/Z signal or the zero-crossing point of the Sin signal or the zero-crossing point of the Sin envelope signal of the sensor, the phase alignment relationship can be determined. This method can align the above signals' edges or zero-crossing points of the sensor to the -30-degree electrical angle phase.

2) By forming a star with three equal-value resistors within a resistance range appropriate for the application and connecting them to the UVW power lines of the permanent magnet servo motor, observe the virtual U reverse electromotive force waveform between the U phase power line and the center point of the star-shaped equal-value resistors with a scope. Observe the phase alignment relationship between this waveform and the rising edge of the U signal/Z signal or the zero-crossing point of the Sin signal or the zero-crossing point of the Sin envelope signal of the sensor. This method can align the above signals' edges or zero-crossing points to the electrical angle phase 0 point.

2. Rotor orientation method

It is applicable to waveforms alignment of incremental encoders with change signals, sine-cosine encodings, rotary transformers, or for alignment of absolute encoders and sine-cosine encodings, rotary transformers, etc. based on providing single-turn absolute position numerical information.

1) Connect the V phase to the positive terminal of the low-voltage DC source, the U phase to the negative terminal of the DC source, and the motor shaft is oriented.

Then, while adjusting the relative position relationship between the sensor and the motor, observe the sensor signal with an oscilloscope until the rising edge of the U signal or the zero point of the Z signal, or the zero point of the Sin signal, or the zero point of the Sin envelope signal is accurately reproduced. In this way, the above signal edges or zero points of the sensor can be aligned to the -30 degree electrical angle phase;

You can also adjust the relative position relationship between the sensor and the motor while trying to observe the numerical information of the single-turn absolute position until the data zero position is accurately reproduced. In this way, the above signal edges or zero points of the sensor can also be aligned to the electrical angle phase 0 point;

If the numerical value of the single-turn absolute position corresponding to the -30 degree electrical angle is estimated in advance, you can adjust the relative position relationship between the sensor and the motor until this value is accurately reproduced. Then, the single-turn absolute position zero point can be directly aligned to the electrical angle phase 0 point (this method may be more accurate than the last method summarized in 2) below);

Of course, you can also simply install the encoder randomly without adjusting the relative position relationship between the sensor and the motor, and simply take the read single-turn absolute position information as the initial installation offset value. Through subsequent operations, the logical alignment of the single-turn absolute position information and the electrical angle phase zero point can be achieved. This method requires the lowest manual operation.

2) Connect the U phase to the negative terminal of the low-voltage DC source, connect the V phase and W phase in parallel and then to the positive terminal of the DC source, and the motor shaft is oriented.

Then, while adjusting the relative position relationship between the sensor and the motor, observe the sensor signal with an oscilloscope until the rising edge of the U signal or the zero point of the Z signal, or the zero point of the Sin signal, or the zero point of the Sin envelope signal is accurately reproduced. In this way, the above signal edges or zero points of the sensor can be aligned to the electrical angle phase 0 point;

You can also adjust the relative position relationship between the sensor and the motor while trying to observe the numerical information of the single-turn absolute position until the data zero position is accurately reproduced. In this way, the above signal edges or zero points of the sensor can also be aligned to the electrical angle phase 0 point.