Error finding frequency from tach signal

[Ronster]

I am using the 'Extract Single Tone Information' vi to find the speed of a motor.  The input is from a Monarch Laser Tach AC output.  The tach readout is very steady and the AC output is 5 volts peak.  There are some negative peaks, but these are at the motor speed.  My frequency output, which should read 1800 changes back and forth from 1800 to a frequency near 5400, but not exactly 5400 (5321, etc.).  Is there any signal conditioning that I am missing before feeding the signal to the 'Extract' vi, for example, window it and then use a frequency domain peak search?

Thanks in advance,

Ron

[MattBradley]

I would suggest putting the signal through a low pass filter at 4608 Hz first.  (1800 * 2.56 = 4608, the reason I use 2.56 rather than the Nyquist value of 2 is that sampling at 2 times the max frequency preserves frequency information but not amplitude information).  This can all be done within LabVIEW, and I would probably recommend a Butterworth filter.

[Ronster]

Matt,

Thanks for the feedback.  I don't care about magnitude, since it is just for speed.  The motor speeds can be anywhere from 400 to 3600 rpm, so I will filter from 4000 and up.

I will let you know how it works.

Ron

[MattBradley]

Ron,

That could still be a problem.  If your maximum signal is at 3600, you really need to sample at least 9216 samples/sec.  If you don't, you could be sampling right at the zero crossing points and end up with no signal at all.  So how fast are you sampling?

[Ronster · Accepted solution]

Matt,

Just to let you know that I put a bandpass filter in my vi.  I took the estimated speed input from the user and bracketed the bandpass filter to be from 25% below to 25% above the target and the tach speed input is working very well!

Ron

[Ronster]

Wanted to make sure this was marked as a fix!