Piezo-electric crystals are man-made or naturally occurring crystals that produce a charge output when they are compressed, flexed or subjected to shear forces. The word piezo is Greek for squeeze. In a piezo-electric accelerometer a mass is attached to a piezo-electric crystal which is in turn mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibration the mass mounted on the crystal wants to stay still in space due to inertia and so compresses and stretches the piezo-electric crystal. This force causes a charge to be generated proportional to the external acceleration. The charge output is either converted to a low impedance voltage output by the use of integral electronics (example: in an IEPE accelerometer) or made available as a charge output (Pico-coulombs /g) in a charge output piezo-electric accelerometer.
Charge Mode System Advantages --
1. Since there are no electronic components contained within the charge mode sensor housing, the upper temperature limit of charge mode sensors is much higher than the limit imposed by the internal electronics of IEPE sensors.
2. Charge amplifier gain is independent of input capacitance, therefore system sensitivity is unaffected by changes in input cable length or type, an important point when interchanging cables.
IEPE System Advantages --
1. Low output impedance makes the sensitivity of the IEPE sensor independent of cable length. Basic system sensitivity does not change when cables are replaced or changed.
2. The low output impedance allows the use of inexpensive coaxial cable (or twin-lead ribbon cable) to connect the sensor to your signal conditioner.
3. Sensitivity and discharge time constant are fixed at time of assembly, setting full scale range and low frequency response. This makes IEPE sensors ideal for dedicated applications such as modal analysis and machine health monitoring.
4. Sealed rugged construction, with high impedance connections contained within the sensor housing, makes IEPE sensors ideal for field use in dirty or moist environments.
5. With proper considerations, very long cables (up to thousands of feet
long) can be driven by IEPE sensors.
6. IEPE power units are relatively simple and much lower in cost to the cost of laboratory charge amplifiers.
7. The tiny IC amplifier chip built into IEPE sensors is very rugged, able to withstand shocks over 100,000 g’s. This makes IEPE accelerometers excellent choices for measurement of very high shocks.
