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I'm trying to determine the change in frequency with voltage of a coin vibration motor (https://solarbotics.com/product/vpm2/) using an oscilloscope. So far I have tried two different set ups.

Set up 1:

The coin vibration motor is being powered using a mosfet and an arduino to vary the voltage using PWM. I hooked up two probes to the oscilloscope, one on each of the motor's leads, and subtracted the two signals. The only signal that was picked up was the PWM.

Set up 2:

The coin vibration motor is hooked up to a power supply through a 1 ohm resistor. I used one probe from the oscilloscope to detect the voltage drop across the resistor. This set-up was one I found reading through previous replies to a similar question here. It didn't work, as the oscilloscope only showed what appeared to be noise signals.

I've read through other options such as using an accelerometer. Would something else besides an oscilloscope be a better option? Or is there a way to read the vibration using an oscilloscope and I just haven't got the right set up?

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  • \$\begingroup\$ I think it will be impractical to measure the rpms the way your doing it. I would glue the vibrator onto a pieze-electric disk and measure that with the scope. You will have to attach the edge of the vibrator to the flat of the disk, so the vibration is in the correct direction. \$\endgroup\$ – Drew Nov 21 '20 at 2:09
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    \$\begingroup\$ What makes you believe that measuring the supply voltage or current to the motor will give you any useful indication of its speed? \$\endgroup\$ – brhans Nov 21 '20 at 5:05
  • \$\begingroup\$ use a procedure that is used to tune string instruments ... connect a speaker to a frequency generator ... use your ears to tune the frequency generator to the the same "pitch" as the vibration motor, by listening for "beats" when the two frequencies are almost identical \$\endgroup\$ – jsotola Nov 21 '20 at 18:32
  • \$\begingroup\$ @brhans I'm not sure if 'speed' is the right word to describe it as it's not visibly rotating. From what I know of the motor, the frequency it vibrates at changes with its voltage, but the documentation given doesn't give specifics to that relationship. One thing I don't know for sure is if the frequency does change with voltage, but the amplitude certainly does. If anything, the frequency change might just be too small for me to detect listening to it audibly. \$\endgroup\$ – GeneralSpud Nov 21 '20 at 19:15
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The vibration rate is probably in the low audio range. So here are some ideas:

  1. Connect a microphone to your scope, put it near the motor, and measure the frequency of the waveform.
  2. Use a smartphone to record the buzzing sound, then analyze it with an audio editing app.
  3. Record the sound using a laptop, again analyzing with an audio editing program.
  4. Get an adjustable strobe light, adjust it to “freeze” the vibration and read the frequency from the strobe rep rate.
  5. An accelerometer (analog) and your scope.
  6. An accelerometer (digital) and a microcontroller.
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