DC motor speed control

Thread Starter

NornIron

Joined Nov 19, 2013
12
Hi everyone,
I am trying to make a piezoelectric bimorph exciter, basically a DC motor that will control a piston with the element attached to the top. my problem is that these elements only operate at a specific frequency 27 Hz, 2 Hz either side and it stops operating as a generator so it needs to be extremely accurate!
I have decided to use a DC motor control system, PWM controlled by a potentiometer, that will have a feedback loop from a tacho fed back into an op-amp, the op-amp doesn't need a fast response.
Questions,
1) is PWM necessary if i use DC supply?
2) Are there any other methods in which to control a dc motor at such a specific frequency?
3) can i use the exact same motor as the controlled motor as the tacho and feed the generated voltage back into the op-amp?

Cheers for the feedback
 

Alec_t

Joined Sep 17, 2013
11,389
A stepper motor controlled by a micro could run at an accurately known speed and would probably be simpler overall.
 

Thread Starter

NornIron

Joined Nov 19, 2013
12
Thanks Alec,
sorry for the late reply, was away for a few days!
would a stepper motor be able to rotate at such speeds of 27Hz, 1620 RPM?
I was trying to avoid using an aurdrino board as i'm not great at programing.
 

MaxHeadRoom

Joined Jul 18, 2013
20,874
Very close to 30Hz which is 1/2 N.A. supply, unless that is Wrexham UK then it would be 50Hz/2.
In any event, a crystal may be the solution as a reference source?
Max.
 

Thread Starter

NornIron

Joined Nov 19, 2013
12
Thanks Max,
although this piezoelectric bimorph has a optimum frequency of 27 Hz others are around 100Hz, it will be necessary to change the speed in order to find the frequency of other bimorph elements, crystals have only one frequency!
 

Alec_t

Joined Sep 17, 2013
11,389
would a stepper motor be able to rotate at such speeds of 27Hz, 1620 RPM?
That would be pushing it, and certainly wouldn't cope with 100Hz. A step-up gearbox would increase speed, but adds to the complexity.
can i use the exact same motor as the controlled motor as the tacho
Yes. Attach a shaft-angle encoder to the motor, or use motor commutation current/voltage pulses for timing.
 
Last edited:

Thread Starter

NornIron

Joined Nov 19, 2013
12
Thanks again for the replies everone!
Thanks Max,
I admit i was slightly confused, a crystal oscillator sounds perfect! whats the advantage of using the PWM rather that just reducing the DC voltage through a pot? would it be a heat problem?

Thanks Alec,
I'll certainly look into the Shaft angle encoder, could be a bit on the costly side!

Hi Ron,
i was thinking of using a 12,000 RPM 12v 2.5A motor with a constant load, an example can be seen here
 
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