Hello everyone, I am working on a project where I need to control the RPM of a motor accurately over a large range (0 to 6000 RPM). I have tried to use a BLDC to do this, but it didn't work for lower RPMs. Stepper motors also seems to be not an option at higher RPMs. The operation has to be smooth at lower and higher RPMs. Also I need fine control over the RPM values. Currently I am thinking of using a brushed dc motor and some hall sensors for this. I request anyone who has experience in this area to provide some inputs on this.
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What is the application, we may be able to help you if we have more information.
What power rating will the motor be?
What is the available power supply?
Thanks... Tom... 
I am planning to use it as a spin coater. A sample (usually a thin square sheet) will be fixed on top of the motor. A drop of some polymer will be placed on the sample. As the motor rotates, the polymer will form a thin film covering the sample with thickness determined by the RPM. That's why I need precise control over the RPM.
The load the motor has to move is only few grams.
I haven't been able to finalize the motor and power supplies required. Any help on this would be appreciated. I am ok with having to use bulky motors and power supplies if needed.
nithint60:
Hello everyone, I am working on a project where I need to control the RPM of a motor accurately over a large range (0 to 6000 RPM). I have tried to use a BLDC to do this, but it didn't work for lower RPMs.
You have not given us any details of how you tried to control the speed or exactly what the problem was so I can't see how people can suggest improvements.
I built a system to control the speed of a small DC motor using a spot of white paint on a black disk on the motor shaft that was detected by a QRE1113 reflective optical sensor and a PID (actually only PI) control system. It could hold the motor at a constant speed even though the load varied.
Compared to your system, however, I was not concerned about really low speeds (down to zero) and your system probably does not have a varying load. My project just used a single pulse per revolution and you might need more for very low speed control. On the other hand, if your load does not vary one pulse per revolution may be sufficient.
Putting a substantial flywheel on the motor will greatly help with smooth speed control.
The code in this link for detecting the motor speed is derived from my motor control project.
...R
I would have thought that a sensored brushless motor with a proper controller would do the job unless you need to be very accurate at very low speeds. The far more common sensorless BLDC motor won't work because it needs to be moving at a reasonable speed for the controller to kick in properly but that's not really a problem for a motor with built-in hall sensors.
Steve
The problem I faced with BLDC was exactly what slipstick mentioned. At lower RPMs the induced voltage was very low for the controller to work.
I will try out these suggestions and will let you know.
Thanks for the advice.
Can I clarify this - are we talking about a three phase BLDC motor as described here
Does the motor have to ramp between RPMs during its run? Or is it set to an RPM then used?
-jim lee
IF you are going to control the motor RPM, you have to know the motor RPM at all times. Are you able to di=o this now?
Paul
What I tried initially was a sensorless 3 phase brushless motor which didn't work at low RPMs. Now I planning to try a 3 phase brushless motor with hall sensors.
I have to ramp the RPM while running. In the operation, the motor has to operate at different RPMs for different times with a smooth transition between them.
Currently I am not able to measure the RPM. Following the previous answers, I am planning to measure the RPM. Also, I think there are some brushless sensored controllers which output the RPM.
I think you are worrying about nothing, nithint60 .
For spin coating AFAIK you need smooth (not necessarily SO accurate) control of speed, at high rpm. The coating has considerable energy so smoothness is important.
AT low RPM the coating has little energy and only a sudden change in motion will have an effect. A flywheel is your friend; and - most importantly -prevention of vibration that would introduce ripples in the coating.
Thank you for the advice. I will keep these in mind while designing the system.
johnerrington:
A flywheel is your friend; and - most importantly -prevention of vibration that would introduce ripples in the coating.
I agree, put some mass in your turntable, look at vinyl record platters, direct drive types are quite heavy.
You will need a speed feedback from the drive shaft if you are looking for precision and stable readings and operation.
Tom...