How do you find rpm using hall sensors?

[UCF]

Hi,

I am an intern at a local engineering company.

I was asked to use hall sensors in a motor to find the rpm. So far, I have hooked up the hall sensors to the interior of the motor. I have also found the theoretical rpm using a formula based on the voltage put into the motor. On labview, I have a dial to control the speed of the motor using a DAQ. Also, a wavegraph is used to sense the peaks of voltage from the hall sensors. How do I used the frequency of the peaks in the graph to find the instantaneous rpm. Thanks!

[Henrik_Volkers]

Well if you can pickup some magnetic field from the rotor  (and not the stator!!) you can use the extract single tone vi or

make a FFT.  You will see spikes at frequencies n times the rotation speed (in 1/s not RPM)

  If you can post a vi with save data (make current values default) and a coarse guess of the correct RPM value, it will help us to help you 😉

[UCF]

Well, I am not very experienced with LABVIEW, I don't really understand your response. I will post a picture of the block diagram, it may help.

THANKS!

[Cody C]

Good Morning,

You could just perform an FFT of your periodic signal coming in from the motor.  Go to the document at this link and it has some example code that explains how to implement an FFT in LabVIEW.  I hope this helps!

-Cody C