Author Topic: Does the brushless servo motor need to measure the phase voltage? What is the pu  (Read 755 times)

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Offline Benjam

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My friend and I discussed about the voltage measurement of the servo drive. The point we are arguing about is whether it is necessary to measure the three-phase voltage? What is the purpose of measurement?
 

Offline David Hess

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The phase between the stator and rotor has to be measured in one way or another for proper commutation.
 
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Offline Benta

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Well, you're generating the voltages. If you trust your drive, why measure them? And if you don't trust them, something is wrong in your concept.
 
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Offline fordem

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The phase between the stator and rotor has to be measured in one way or another for proper commutation.

I'm inclined to ask why?  My train of thought follows Benta's - the brushless motors I've seen have all been relatively small, and whatever feed back was used, was what I would call "positional" - the controller would drive the motor until whatever was being moved reached the desired location - if it stalled or jammed it would be apparent from the lack of a position change.
 

Offline Doctorandus_P

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BLDC PMSM and Stepper motors are all pretty much the same.

Most stepper motors are 2 phase, but 3-phase (ans also 5-phase) stepper motors also exist.

If you take a BLDC motor, and push a DC current to one or two of it's coils, then the motor will "lock" in a fixed position, just like a stepper motor. Current (power) consumption without rotation (= no output power) equals to very low efficiency.

All these motors have the highest efficiency if there is a 90 degree phase difference between the mechanical and the electric position of the magnetic fields, and it is the job of the motor controller to try to maintain this 90 degree phase difference, and to do that it needs some form of feedback.

This feedback is commonly done with Hall sensors (for "BLDC" motors) or with an encoder ( for "PMSM", "Servo" and "Steppermotors with feedback".

It can also be done "sensorless", by measuring the back EMF of the motor coil that does not have current. A limitation of this method is that there is no back EMF (or too little) when the motor is rotating too slow, but it can work nicely for some applications such as for example motors of quad copters.
This technique is called "FOC" (Field Oriented Control) and there is a lot of literature written about this.

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Even Brushed DC motors are very similar. The main difference with brushed DC motors is that this phase shift is maintained by the mechanical position of the brushes.
« Last Edit: December 03, 2021, 12:55:59 pm by Doctorandus_P »
 
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Offline dmills

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Just a note that FOC and back emf rotor angle sensing are sort of orthogonal concepts.
You can do FOC with an encoder, and you can do trapezoidal commutation with back emf sensing.

The key thing about FOC is that you can control the magnetising current separately from the torque, (Magnetising is in phase with the rotor position, torque producing is 90 degrees off), so that you can do interesting things like field weakening for high speed operation while still getting full torque at low speed.
 
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Offline Siwastaja

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Voltage of the output stage (motor windings) may be sensed to provide so called "sensorless" rotor angle estimation. If this scheme is not in use, then voltage measurement isn't needed either. A servo motor, i.e., one with positional feedback, obviously won't be using rotor angle estimation, so doesn't need phase voltage measurement.

Voltage of the input DC bus is important to measure, for cutting regeneration in case the energy can't go anywhere upstream and DC link bus voltage is rising towards dangerous levels. Other reasons might be if some control loop benefits from feedforward from the input voltage.

Doctorandus_P is othewise correct but the name for sensorless commutation is not "FOC", FOC is something completely different. Rotor angle (estimated or directly measured) is an input parameter for FOC, though.

The key of understanding of FOC is not that it provides optimal 90 degree phase shift. This can be provided in simple, totally trivial ways. The reason to use FOC is that it makes the PI control loops (input: current measurement; output: PWM duty cycle), "rotate" in sync with the rotor, making the delay from the PI loop insignificant. Without FOC, you would have three simple PI loops to control phase currents, but because the setpoints would change sinusoidally and rapidly, there would be lag in actual current. All FOC means, is we use a single PI loop and rotate the inputs (phase currents) in sync with the rotor angle, then rotate the outputs (PWM setpoints) back. Then, the current setpoint is a straight line, only changed when you want to change torque generated. The sinusoidal output currents are simply generated by the sin() and cos() functions when calculating standard 2D vector rotation in the FOC implementation.

At least for me, trying to understand what FOC is was originally a bit hard because no one explained the real function / reason properly, it was either advertisements by people who have no idea what they are talking about, or obfuscated mathematical explanation about the implementation of the formulae, which I also wasn't interested about before I know why. Besides, in reality, the math is very simple, such as standard 2D vector rotation. In the context of FOC, this standard formula, familiar from computer graphics, has been just given a separate name for obfuscation.
« Last Edit: December 04, 2021, 05:40:58 pm by Siwastaja »
 
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Offline Benjam

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The phase between the stator and rotor has to be measured in one way or another for proper commutation.

Isn't that what the coder is supposed to do? Do you need to measure the voltage to do that?
 

Offline Benjam

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BLDC PMSM and Stepper motors are all pretty much the same.

Most stepper motors are 2 phase, but 3-phase (ans also 5-phase) stepper motors also exist.

If you take a BLDC motor, and push a DC current to one or two of it's coils, then the motor will "lock" in a fixed position, just like a stepper motor. Current (power) consumption without rotation (= no output power) equals to very low efficiency.

All these motors have the highest efficiency if there is a 90 degree phase difference between the mechanical and the electric position of the magnetic fields, and it is the job of the motor controller to try to maintain this 90 degree phase difference, and to do that it needs some form of feedback.

This feedback is commonly done with Hall sensors (for "BLDC" motors) or with an encoder ( for "PMSM", "Servo" and "Steppermotors with feedback".

It can also be done "sensorless", by measuring the back EMF of the motor coil that does not have current. A limitation of this method is that there is no back EMF (or too little) when the motor is rotating too slow, but it can work nicely for some applications such as for example motors of quad copters.
This technique is called "FOC" (Field Oriented Control) and there is a lot of literature written about this.

-------------
Even Brushed DC motors are very similar. The main difference with brushed DC motors is that this phase shift is maintained by the mechanical position of the brushes.

In other words, if the encoder is used, there is no need to measure the voltage. I want to make a drive to drive PMSM, using FOC
 

Offline Benjam

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Voltage of the output stage (motor windings) may be sensed to provide so called "sensorless" rotor angle estimation. If this scheme is not in use, then voltage measurement isn't needed either. A servo motor, i.e., one with positional feedback, obviously won't be using rotor angle estimation, so doesn't need phase voltage measurement.

Voltage of the input DC bus is important to measure, for cutting regeneration in case the energy can't go anywhere upstream and DC link bus voltage is rising towards dangerous levels. Other reasons might be if some control loop benefits from feedforward from the input voltage.

Doctorandus_P is othewise correct but the name for sensorless commutation is not "FOC", FOC is something completely different. Rotor angle (estimated or directly measured) is an input parameter for FOC, though.

The key of understanding of FOC is not that it provides optimal 90 degree phase shift. This can be provided in simple, totally trivial ways. The reason to use FOC is that it makes the PI control loops (input: current measurement; output: PWM duty cycle), "rotate" in sync with the rotor, making the delay from the PI loop insignificant. Without FOC, you would have three simple PI loops to control phase currents, but because the setpoints would change sinusoidally and rapidly, there would be lag in actual current. All FOC means, is we use a single PI loop and rotate the inputs (phase currents) in sync with the rotor angle, then rotate the outputs (PWM setpoints) back. Then, the current setpoint is a straight line, only changed when you want to change torque generated. The sinusoidal output currents are simply generated by the sin() and cos() functions when calculating standard 2D vector rotation in the FOC implementation.

At least for me, trying to understand what FOC is was originally a bit hard because no one explained the real function / reason properly, it was either advertisements by people who have no idea what they are talking about, or obfuscated mathematical explanation about the implementation of the formulae, which I also wasn't interested about before I know why. Besides, in reality, the math is very simple, such as standard 2D vector rotation. In the context of FOC, this standard formula, familiar from computer graphics, has been just given a separate name for obfuscation.


Thank you, very professional answer.
 

Offline Siwastaja

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The idea is simple to understand, you need to know in which position the rotor is, so that you can apply correctly aligned magnetic field (simplified: turn the right windings on!) that the magnetic field pulls/pushes the rotor forward.

Yes, the options for rotor position sensing are mechanical or optical encoders, or hall sensors that sense the permanent magnets of the motor. Sensorless is relevant in low-cost devices that do not need to run at low speeds at all. For example, drone motors are typically sensorless, but hoverboard motors have sensors because they need to run reliably at low speeds.

Minimum accuracy for acceptable drive is something like +/-30 electrical degrees, which is achieved by the classical configuration of three hall sensors installed in such way you get 6 distinctive steps per electrical rotation. But a servo which is accurately controlled and ran at low speeds, benefits from higher accuracy encoder than that.

If you want to just run the motor at optimum efficiency (as in traction devices), you want the generated magnetic field to be 90 degrees "forward" of the rotor position. OTOH, if you keep it at 0 degrees forward, you can hold the motor stalled, consuming current (of the resistive losses) in the process, resulting in 0% efficiency as no physical work is done. In a servo drive, efficiency and accuracy of control can be both have by gearing down the motor so that even if machine runs very slowly, the motor can run at some clearly non-zero speed, allowing feedback loops (FOC iq, id currents, and current setpoints based on measured speed) to run properly, maintaining high efficiency.
« Last Edit: December 04, 2021, 06:23:56 pm by Siwastaja »
 

Offline Benjam

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The idea is simple to understand, you need to know in which position the rotor is, so that you can apply correctly aligned magnetic field (simplified: turn the right windings on!) that the magnetic field pulls/pushes the rotor forward.

Yes, the options for rotor position sensing are mechanical or optical encoders, or hall sensors that sense the permanent magnets of the motor. Sensorless is relevant in low-cost devices that do not need to run at low speeds at all. For example, drone motors are typically sensorless, but hoverboard motors have sensors because they need to run reliably at low speeds.

Minimum accuracy for acceptable drive is something like +/-30 electrical degrees, which is achieved by the classical configuration of three hall sensors installed in such way you get 6 distinctive steps per electrical rotation. But a servo which is accurately controlled and ran at low speeds, benefits from higher accuracy encoder than that.

If you want to just run the motor at optimum efficiency (as in traction devices), you want the generated magnetic field to be 90 degrees "forward" of the rotor position. OTOH, if you keep it at 0 degrees forward, you can hold the motor stalled, consuming current (of the resistive losses) in the process, resulting in 0% efficiency as no physical work is done. In a servo drive, efficiency and accuracy of control can be both have by gearing down the motor so that even if machine runs very slowly, the motor can run at some clearly non-zero speed, allowing feedback loops (FOC iq, id currents, and current setpoints based on measured speed) to run properly, maintaining high efficiency.


Wow...You are very professional. Thank you for your kind reply. ;D
 


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