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
Frederick, MD 21701
Tel: +1 (301) 330 8811
Wilcoxon Sensing Technologies
