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Accuracy of Low Frequency (slow speed) Response of vb Instrument

 

QUESTION

What is the accuracy of the low frequency (slow speed) response of the vb instrument?

ANSWER

There are three limiting factors to the low frequency response of your system:
1. The response of the sensor(s) being used. See further comments near the end of this article.
2. The effect of the AC decoupling capacitors in the instrument (does not apply when using DC coupling).
3. The integration process from acceleration to velocity (& displacement), which must remove low frequency components to avoid output drift.

AC Coupled Inputs

We specify the vbSeries portable instrument and vbOnline system frequency response down to 1 Hz (60 cpm). You can take readings below this frequency but the instrument's sensitivity drops off rapidly because this region includes the corner frequencies of both the AC decoupling filter and the integrators.

Note: The tables are an indication of the nominal response, not a calibration.


DC Coupled Inputs - to measure very low frequencies

For the best low frequency response you must bypass the AC decoupling filter and use DC coupling. We provide this capability in the vbOnline system and in selected vbSeries portables. To measure ICP® accelerometers using DC coupling:
• Set the sensor's Voltage Range and Coupling to DC (0 V to 20 V).
• Keep the Drive Current enabled.

When measuring acceleration this gives you a FLAT (100%) response down to 0 Hz DC. The trade-off is that there is more low-frequency noise, so more ski-slope in the spectra.

For measurements of velocity or displacement the response is not 100%, there is still significant low frequency attenuation. This is because the integration process from acceleration must remove the DC offset and low frequencies, to avoid output drift. The response is better than that indicated in the AC table above, typically halfway between the values given and 100%.

However for vbOnline firmware versions from v5.81 we have introduced a special low-frequency integration mode. This yields the velocity and displacement responses given in the table below.

Low-frequency integration mode is enabled and used by setting:

Sensor Coupling = DC

Fmax = 100 Hz (6000 CPM)

Fmin = 0

Sensor settling time = 20 ... 30 s

Sensor Sensitivity = 500 mV/g or higher

Notes

The best low frequency response is obtained with Fmax=100Hz. However all Fmaxes up to 500Hz do use this mode, with responses scaled to maintain the -3dB point at 0.2% of Fmax.

High sensitivity sensors should be used because low-frequency vibration (under 1Hz) has extremely low acceleration levels - system noise may dominate the signal.

DC Coupled Inputs (in low-frequency integration mode)


Sensor Bias Checks when using DC coupling

All Commtest instruments now support sensor bias checks for AC sensors (i.e. ICP accelerometers). However that automatic checking is not available for DC coupled sensors. Instead you can take Average Value measurements from the sensor and set Average Value alarms to check that the acceleration offset is as expected for the typical 12V bias:

100 mV/g expect 120 g offset Suggest alarms if outside 80 .. 160 g

500 mV/g expect 24 g offset Suggest alarms if outside 16 .. 32 g

For owners of older vbClassic instruments

vbClassic (vb1000, vb2000, vb3000) instruments prior to serial number 11500 (running firmware versions 3.xx and earlier) used an earlier version of analog input circuit. For these instruments the best low frequency accuracy is obtained by taking velocity or displacement readings, which gives a response similar to the AC coupled table above. We do not recommend the use of acceleration on these instruments below 10 Hz.

Accelerometer low frequency response

Accelerometers are typically specified as having a ± 3 dB response between, say, 0.5 Hz and 15 kHz. Their response usually drops off towards -3 dB at the lower frequency limit so you will need to take this into account if you want to calculate the most accurate system response. Ideally consult the frequency response curve published by the sensor manufacturer; however, we provide the table below as an indication of the likely response based purely on the Low Frequency Limit (LFL, e.g. 0.5 Hz or 30 cpm).



 

Related Information

If you are analyzing a bearing for faults then bearing tones will be at higher frequencies than the running speed, so will be less affected by the low frequency response.

Demodulation is particularly useful for detecting bearing faults on slow speed machines. The signal is derived from high frequency impacts, and the Demod/decimation process does not attenuate low frequencies, so Demod has a flat (100%) response.

On slow speed machines, you can use Demod time waveform measurements to look at the time differences between impact events in the time record and compare with fault frequencies. This is only available on the vbX instruments, running v3.0 firmware or later.

For more detailed information you can purchase the technical paper 'Low Speed Machines' from Technical Associates of Charlotte, P.C.

Balancing

It is possible to balance very slow speed machines using the vb instruments (e.g. 30 RPM) even though the reading may have a reduced response. This is because all balance runs are made at the same speed, and the balancing algorithm analyzes the effect of fitting the trial weight. A low sensitivity at the running speed will affect all of the Initial, Trial and Balance runs to the same extent, so is effectively self compensating.