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Difficulty Measuring Low-Frequency, Large-Amplitude Displacement Signals

 

QUESTION

I am having difficulty measuring low-frequency, large-amplitude displacement signals. What could be causing this and is there a solution?

ANSWER

The vb instrument is specified to measure displacement signals up to a maximum of 10 mm 0-peak (400 mils). This level is sufficient to cover practically all industrial vibration.

If your machine is actually vibrating more than 10 mm in displacement, you might want to look for the nearest emergency exit rather than try to measure that machine! However, in a small number of cases, an erroneous distortion of the displacement reading can occur on machines vibrating below 10 mm. This is because the displacement value of the vb instrument is derived by integration (mathematical conversion) from the velocity signal. This signal integration takes place inside the vb instrument.

Note: If you are using a displacement sensor with a vbOnline device or a vb3000 instrument running firmware version 4.30 or higher this problem will not occur as no signal integration takes place when measuring displacement directly.

The upper spec on velocity is 100 mm/s 0-peak (4 in/s). If the velocity of vibration has exceeded the spec limits of the instrument, then the resulting displacement value is subsequently distorted. In these cases of particularly low-frequency, large amplitude signals it is possible to exceed the velocity spec limit.

For example an object vibrating at a frequency of at 15 Hz (900 RPM) and with a displacement amplitude of 5 mm (0.2') the corresponding velocity signal is 470 mm/s (19 in/s). In such a case, the actual displacement level is within spec limits, but electronic distortion is caused by the velocity signal exceeding its spec limits by almost 5 times.

SOLUTION

Fortunately there is an easy work-around to avoid this distortion of the displacement value due to exceeding the specs on the velocity signal. This is to use a low sensitivity accelerometer, for example 10 mV/g. This makes all the signal voltages 10 times smaller inside the vb instrument, so the velocity integrator circuit is much less prone to saturate, thereby reducing the occurrence of distorted displacement readings.

When using a 10 mV/g accelerometer for vibration measurements (waveforms and spectra), the vb instrument's sensor sensitivity should be set to 10 mV/g. Readings will be scaled correctly by the instrument. However, even using such an accelerometer does not change the fact that the velocity peak is exceeding the vb instrument's specified maximum. So for these readings the velocity signal is still not valid. However, both acceleration and displacement measurements are valid. Therefore, if the true velocity value is needed, then record the vibration using either the acceleration or displacement setting in the instrument, and the conversion to velocity can be done easily within the Ascent software.

When using a 10 mV/g accelerometer for Balancing, the vb instrument's sensor sensitivity should be left on the standard 100 mV/g. The readings will be un-distorted, but will be 10x smaller than the actual value. This technique should only be used for the first balance run of extremely imbalanced rotors.