Sensors are at the core of any vibration monitoring program.
The industrial accelerometer is the primary vibration sensor needed for rotating equipment utilizing rolling element bearings. The vibrations generated by the machine will be transmitted through the casing of the machine to the base of the accelerometer where the Piezo ceramic reacts with the mass in a shearing mode to produce a charge output. The charge is converted to a voltage and amplified internally in the accelerometer for measurement by the vibration analyzer, data collector, or online monitoring system.
Sensitivity Tolerance
Every accelerometer will be calibrated to determine the sensitivity, and the actual measured output value must fall within the sensitivity tolerance.
Typical tolerances are ±5%, ±10%, ±15%, and ±20%, where 5% would be considered a premium tolerance and 20% would be considered a marginal tolerance. Typical user values are 10% and 15% for general purpose vibration measurements. If you were to choose a 100 mV/g accelerometer with a ±10% sensitivity tolerance, the calibrated output of the sensor could range from 90 mV/g (-10%) to 110 mV/g (+10%). However, if you were to choose a 500 mV/g accelerometer with a ±10% sensitivity tolerance, the calibrated output of the sensor could range from 450 mV/g (-10%) to 550 mV/g (+10%).
Frequency Response
Given the established sensitivity of the accelerometer, the allowable output at the calibration frequency will be identified as the sensitivity tolerance, but all other amplitudes will be bound by the frequency response limits. These limits are typically established at ±5%, ±10%, and ±3% dB above and below the measured sensitivity value, for those frequencies greater than or less than the calibration frequency. ±3% dB will be the largest frequency range and is normally considered to be the usable range or working range of the accelerometer.
Dynamic Range
The maximum and minimum amplitude that the accelerometer can measure is indicated by the dynamic range.
Typically the dynamic range is limited to ±5 VACPeak, but some 100 mV/g accelerometers are available with a dynamic range extended to ±8 VACPeak. If the normal bias voltage is 12 VDC, but can range from 10-14 VDC, the power supply must have greater potential than the bias voltage ± the voltage of the vibration signal.
The following examples illustrate this concept:
Given an 18 VDC power supply, 12 VDC bias voltage, and maximum vibration amplitude (Dynamic Range) of ±5 VAC, the combined voltage output would vary between 7 to 17 volts. This is within the limit of the power supply.
Given an 18 VDC power supply, 14 VDC bias voltage, and maximum vibration amplitude (Dynamic Range) of ±5 VAC, the combined voltage output would vary between 9 to 19 volts. This exceeds the upper limit of the power supply by 1 volt and will cause clipping of the vibration signal. In this case, a 20 VDC power supply would be required.
