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

 

A stepper motor controlled by a micro could run at an accurately known speed and would probably be simpler overall.

 

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.

 

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.

 

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!

 

I did mean a crystal oscillator to control motor speed, if there was any confusion?
Max.

 

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.

 

How big is the motor? Does the load vary or is it constant?

 

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

 

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

You could brew your own: painted disc on shaft, plus opto-sensor.