Need some help....

If I am doing an ODS using accelerometers (typical) but want to incorporate prox probes on the shaft so that I can tell how the shaft is moving relative to the structure.... how do I determine the position (phase) for the shaft using the probes?

Cant seem to get my head around how to do this.

Thanks,

KB

Original Post

At least for GE/BN convention, the thing to remember is that the phase lag angle represents the angle by which the response of the rotor lags behind the physical reference.  Let's take an example where the signal from a single probe mounted at top dead center displays an amplitude "X" at 135°.  Pictorially, that represents what I've shown in the attached graphic.  To physically locate the equivalent spot, the KØ notch is lined up with the measurement probe, and 135° is measured opposite the direction of rotation.

The response is often termed the "high spot" and is coincident with the heavy spot at very low speed.  But as speed increases the lag angle increases, 90° lag being the point of resonance.

lag_2

 

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If you are looking for non-synchronous frequencies, this is the same as for the accels.  Use the phase of the transfer function in the same manner (same reference).  

If the signals are not coherent at a frequency (as with the accels) don't look for meaning.

Accels lead displacement by 180 degrees - take this into consideration.  The amplitude from the transfer function has to be scaled as well (by frequency squared, in rad/sec).  Also, consider the units, accels might be in g's,  displacement in micrometres, etc.

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