PI control of a PMDC motor at low speeds

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Discussion Overview

The discussion revolves around the challenges of implementing PI control for a permanent magnet DC (PMDC) motor at low speeds, specifically focusing on the fluctuations observed when the reference speed is reduced to 5 rad/s. Participants explore potential causes of these fluctuations and suggest various approaches to mitigate them.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports successful PI control at high speeds but experiences fluctuations at low speeds, questioning whether increasing PWM frequency could help.
  • Another participant inquires about the motor's specifications, specifically the number of poles and commutator blocks.
  • A participant suggests that the fluctuations at 5 rad/s correspond to a 15 Hz rumble that may not be controllable with an analog PI controller.
  • There is a suggestion that mechanical solutions, such as adding a flywheel, could potentially address the issue, although this is described as a desperate measure.
  • One participant requests clarification on how the 15 Hz value was calculated and expresses interest in the flywheel option, while also inquiring about alternative diodes for the H-bridge.
  • A participant hypothesizes that the noise from the commutator may be contributing to the rumbling effect and suggests testing the motor in open loop to diagnose the issue.
  • Another participant advises ensuring that the integrator gain is sufficient and that the pole is set correctly in the control system.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of increasing PWM frequency at low speeds, with some suggesting it may not help. There is no consensus on the root cause of the fluctuations, as multiple hypotheses are presented, including mechanical characteristics of the motor and control system settings.

Contextual Notes

Participants mention various assumptions regarding the motor's characteristics and control system parameters, but these remain unresolved. The discussion includes speculation about mechanical solutions and control strategies without definitive conclusions.

Who May Find This Useful

This discussion may be useful for individuals working on motor control systems, particularly those dealing with low-speed operations in PMDC motors, as well as those interested in the implications of PWM frequency and mechanical solutions in control applications.

awaiting
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Hi everybody,

I have implemented PI control of a DC motor and it works properly at high speeds (50rad/s) but if I lower the reference speed to 5 rad/s, there are fluctuations in its motion. Initially, I thought it could be due to the limitation of my current sensor (LTS 6 np) but I added some load, so that current is at a higher value (around 30mA) but the fluctuations are still present. I tried to increase the frequency of the PWM but due to limitations in my board, could not test beyond 20KHz.

Will increasing the PWM frequency have any positive effect at low frequencies?

Your comments and suggestions are welcome.

Thanks,
 

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How many poles, commutator blocks per revolution?
 
It has 30 commutator blocks and 2 poles.
 
awaiting said:
Will increasing the PWM frequency have any positive effect at low frequencies?
No, I don't think so
 
awaiting said:
It has 30 commutator blocks and 2 poles.
At 5 rad/s, you've got a 15 Hz rumble that's just not going to be controlled with an analog PI.

Added comment on edit: You could iron it out mechanically with some monstrous fly wheel, but that's getting pretty desperate.
 
Thanks for the reply!

Could you please explain how did you calculate the 15Hz value. I would also like to try the flywheel option. I am using 1N5822 diode across both the sides of the h-bridge, do you know of any other diode that could be useful.
 
Thirty commutator contacts may be 15 windings on the rotor. As the motor rotates and switches from one to the next, you get a bit of a "jump" that's probably just too rough to iron out with the controller. I guarantee nothing. I'm making a "remote" guess at what might be giving you problems at low speeds.
 
Sounds like at low speeds the noise of the commutator is causing the rumbling effect. Why don't you try to run the motor open loop i.e. since the speed depends on the voltage applied, just start off by applying a very small voltage using a DC source and listen to the rumble and then increase the voltage for higher speeds. If you hear the rumble then it's the characteristic of the motor.

If not, I suspect you don't have enough gain on the integrator. Make sure that the pole is at s= 0 and not higher like s= 1kHz. Just a guess as I can't see any control schematic.
 

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