PI control of a PMDC motor at low speeds

In summary, the motor appears to have some noise issues at low speeds that may be due to the commutator. You may try running the motor open loop and increasing the voltage to see if that helps.
  • #1
awaiting
24
0
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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  • #2
How many poles, commutator blocks per revolution?
 
  • #3
It has 30 commutator blocks and 2 poles.
 
  • #4
awaiting said:
Will increasing the PWM frequency have any positive effect at low frequencies?
No, I don't think so
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
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.
 

Related to PI control of a PMDC motor at low speeds

1. What is PI control and how does it work?

PI control, also known as proportional-integral control, is a type of feedback control system used to regulate the speed of a PMDC motor at low speeds. It works by continuously measuring the difference between the desired speed and the actual speed of the motor. The proportional component adjusts the motor's speed in proportion to this difference, while the integral component takes into account the accumulated error over time and makes adjustments to minimize it. This allows for precise and stable control of the motor's speed.

2. Why is PI control important for PMDC motors at low speeds?

PMDC motors are known for their high starting torque and accurate speed control. However, at low speeds, these motors can experience fluctuations and instability. PI control helps to mitigate these issues by continuously adjusting the motor's speed and ensuring it stays at the desired level, even at low speeds. This is especially important in applications where precise speed control is crucial, such as robotics or automated systems.

3. How is the PI controller tuned for optimal performance?

The PI controller is typically tuned by adjusting the proportional and integral gain values. The proportional gain controls the response to changes in speed, while the integral gain accounts for the accumulated error over time. The optimal values for these gains can vary depending on the specific motor and application, and may require experimentation and fine-tuning to achieve the best performance.

4. Can PI control be used for PMDC motors at high speeds?

Yes, PI control can also be used for PMDC motors at high speeds. However, at higher speeds, other factors such as motor inertia and friction may also need to be taken into account for optimal control. In some cases, a more advanced control method, such as PID control, may be necessary for better performance at high speeds.

5. Are there any limitations to using PI control for PMDC motors?

While PI control is effective for regulating the speed of PMDC motors at low speeds, it may not be suitable for all applications. For example, in applications where the load on the motor can vary significantly, a different control method, such as adaptive control, may be more suitable. Additionally, PI control may not be effective for controlling the motor's position or direction of rotation, which may require a different type of control system.

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