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Troubleshooting accelerometer installations
Accelerometer based monitoring systems can be tested to verify proper installation and operation. Testing
ensures data integrity and can identify most commonly occurring problems.
The troubleshooting techniques presented are very simple and can be performed using most monitoring
systems and data collectors or simple test equipment. Many installation and sensor problems can be
detected by measuring the bias voltage of the sensor. The bias voltage will indicate faulty cable routes and
failed sensors. Many online systems are capable of trending the sensor bias voltage. Other problems can be
detected by analyzing the time waveform and FFT spectrum.
Accelerometer operation and response
Most accelerometer faults can be diagnosed by measuring the bias voltage of the sensor amplifier. If the bias
voltage is within correct limits the sensor is most likely operating properly. Most cabling faults can also be
isolated by measuring the bias. After the bias is checked, the time waveform and FFT spectrum will verify fault
diagnosis or proper operation.
AC coupling and the DC bias voltage
The sensor output is an AC signal proportional to the vibration applied. This AC signal is superimposed on a
DC bias voltage, also referred to as Bias Output Voltage (BOV) or sometimes rest voltage. The DC component
of the signal is generated by the 2 mA constant current diode in the power supply. This DC voltage needs to be
blocked by a coupling capacitor in the measurement equipment, leaving the AC output signal. Most vibration
data collectors, monitors, and sensor power units contain an internal blocking capacitor for AC coupling. If not
included, a blocking capacitor must be field installed.
What is bias voltage?
The majority of accelerometers, PiezoVelocity Transducers (PVT®), and pressure sensors have a biased
output. Biased outputs are characteristic of two-wire sensors used to measure dynamic AC signals. Vibration
and pressure are examples of dynamic signals that vary with time. The external power supply provides a DC
voltage to the accelerometer. This power supply voltage is normally 18 to 30 volts DC. The accelerometer
amplifier circuit design establishes this voltage (or "biases" the voltage) to a preset level. This BOV is normally
12 VDC, although it may vary depending on the manufacturer and sensor design. The accelerometer's
specification sheet provides further information on the BOV. The BOV is determined by the amplifier design
and is not adjustable.
The BOV remains the same regardless of the input power to the accelerometer, as long as the input power is
within the specified range. For example, if the BOV is 12 VDC and the input power is specified as 18 to 30 V,
then the BOV will be 12 VDC if the input power is 18 VDC. If the input power is increased to 30 VDC, the BOV
will remain at 12 VDC. The BOV is set by the interaction of the amplifier circuit in the accelerometer and the
constant current from the stand-alone power supply or the analyzer or data collector.
8435 Progress Drive
Tel: +1 (301) 330 8811
Frederick, MD 21701
info@wilcoxon.com
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Wilcoxon Sensing Technologies
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Table of Contents
Troubleshooting
Related Manuals for Amphenol Wilcoxon TN14
Summary of Contents for Amphenol Wilcoxon TN14
- Page 1 Troubleshooting accelerometer installations Accelerometer based monitoring systems can be tested to verify proper installation and operation. Testing ensures data integrity and can identify most commonly occurring problems. The troubleshooting techniques presented are very simple and can be performed using most monitoring systems and data collectors or simple test equipment.
- Page 2 Figure 1 is a diagram representing the performance of the circuit. The line represented by “Instrument Power” and BOV is one conductor that has two functions. So even though the power supply is pro- viding a higher input voltage, the BOV is the mea- sured output voltage level on the cable connecting the accelerometer to the data collector or analyzer.
- Page 3 The BOV should be measured periodically to check sensor operation. The best measurement device is a voltmeter, however, most portable data collectors can measure the BOV if the sensor is powered from a differ- ent source (other than the data collector). When using the data collector as a voltmeter the DC voltage input setting is used.
- Page 4 is very rare to have a short inside the sensor. The most common fault location is in junction box terminations. Check to make sure that a frayed shield is not shorting across the signal leads. Many times a crushed cable can produce a short.
- Page 5 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.
- Page 6 Mounting resonance spectrum Mounting resonance can give false indication of high frequency machinery faults such as gear mesh and bear- ing problems. The problem is more likely to occur when using probe tips and magnets, and care must be exer- cised to prevent the measured mounting resonance peaks from being falsely identified as machine produced peaks.
- Page 7 Troubleshooting chart Below is a troubleshooting chart for sensors with a 12 volt bias. For sensors with other bias voltages, the same concepts apply — only the stable bias range will be different. Spectrum Time waveform Fault condition Action • Test/turn on power No power or cable/ No signal No signal...
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