Phase calculation with tacho

[James9999z]

Hi there,

Can somebody provide a guidance on how to calculate the phase between a vibration measurement channel and the tacho?

I guess people won't use any time domain calculation method to do such calculation. Time domain signals may have noise, and we want to calculate the phase not only at fundemental frequencies, but also the harmonics.

Should we use cross spectrum method? How the tacho signal, which is essentially a chain of pulses, be represented?

Thanks for the attention.

James

http://www.go-ci.com

8ch dynamic signal analyzer/recorder
102.4 kHz sampling, 130dB dynamic range

 

[GregLocock]

Try FFTing both channels and looking at the transfer function, pick the phase off at each peak in the response signal's power spectrum.

It could be used to drive an ODS animation, for example.

Another alternative would be to use phase locked triggering.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

I'm not quite sure what you are trying to do here. It sounds like you need a sensible reference signal that's used as one of your crosspower inputs. An accelerometer on the engine block or cylinder head will be better than a tach siganal.

- Steve

 

[James9999z]

Greg, Do you mean that we can FFT the tacho time signal? It seems a bit odd to me. Tacho signal will be a series of pulses. The shape of pulses can vary a lot.

Steve, I am trying to measure the phase of an analog channel in reference to the tacho input. It is used for rotor balancing. Estimating the RPM value is not sufficient. We need to know the relative phase info.

James

http://www.go-ci.com

8ch dynamic signal analyzer/recorder
102.4 kHz sampling, 130dB dynamic range

 

[GregLocock]

Yes that is what I mean. In many ways it is similar to using a reference accelerometer.

It will work 100% for first order two plane balancing, I've done it tens, or maybe hundreds, of times.

For first order you'll need to use a 1 pulse per rev tacho, obviously. If you can't get one of the crank use a proximity probe on a cam and that'll give you half pulse per rev.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[James9999z]

Hi Greg, Thanks for the tip. I will give it a try in the nex a few days.

http://www.go-ci.com

8ch dynamic signal analyzer/recorder
102.4 kHz sampling, 130dB dynamic range