Sometimes spurious spikes from fast thermal shifts, lightning strikes, and shocks can overload the sensor
and cause a momentary shift in the bias voltage. The shift in bias can trigger alarms and protection system
shutdown devices. To prevent triggering alarms and shutdown, a longer delay can usually be programmed or
hardwired into the monitoring system. The delay prevents the system from taking action until the sensor has
settled.
High frequency, high amplitude vibration signals can also overload the sensor and in severe cases cause bias
shift and erratic time waveform. However, overload problems are usually detected by observing truncated
waveforms and large ski-slope spectrums.
Truncated time waveform: sensor overload
Truncated (flattened) time waveforms indicate that the signal is clipping. Clipping causes the amplifier to
saturate and become overloaded. Some common mechanical causes of an overload in the sensor are severe
pump cavitation, steam release, impacts from loose or reciprocating parts and even gearmesh. One way to
reduce clipping is to use a higher power supply voltage and ensure that the bias voltage is centered between
supply voltage and ground voltage. However the bias voltage and power supply are rarely adjustable. For ex-
ample, if you are using an 18 volt power supply and a 12 volt bias, clipping will occur sooner than if you used a
24 volt power supply.
Long cables in excess of 200 feet can also reduce the amplitude swing at high frequencies and may be a
problem in some applications. The easiest solution is to use a lower sensitivity sensor. A sensor with 10 mV/g
sensitivity will have a hundred times larger amplitude range than a similar 1 V/g sensor.
Ski-slope spectrum
Sensor overload may also produce a ski-slope spectrum. If the amplifier saturates, intermodulation distortion
occurs. This causes low frequency noise, also referred to as washover distortion. Figures 6a shows a normal
spectrum. Figure 6b shows what can happen when the signal becomes overloaded due to excessive vibration.
Figure 6a: Normal operation
Figure 6b: Overload operation
Sometimes the ski-slope signals can be caused by
the circuitry used to integrate acceleration signals to
velocity or displacement. Figure 7 shows integration
noise due to analog integration of an acceleration
signal.
Figure 7: Ski-slope noise caused by analog integration
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