Question
Can you explain the differences between IEPE accelerometers and the 4-20 mA vibration transmitter sensors now available?
Answer
Though similar and often confused as identical, significant technology differences exist between these two sensors. They are also generally intended for use within different applications.
IEPE (Integral Electronics Piezoelectric) accelerometers are powered by a constant current, typically 2 mA to 20 mA. This is supplied by either a signal conditioner or certain specialized data acquisition systems. The output signal from the accelerometer is a voltage signal that is superimposed onto the same line on which constant current is supplied. Coaxial cable is typically used between the accelerometer and signal conditioner or data acquisition system (although most industrial grade accelerometers use shielded, twisted pair cable).
In contrast, a 4-20 mA vibration transmitter outputs its signal with a varying current, ranging from 4 mA to 20 mA over a wire loop. No voltage signal is involved. 4 mA represents 0 value (e.g., 0 g) and 20 mA represents full scale (i.e., the maximum value at which a transducer can measure). The first 4 mA is used by the transducer itself to power its own internal electronics. Vibration transmitters (not to be confused with wireless systems, by the way) are similar to many "loop-powered" transducers that have been available for decades for measuring pressure, temperature and flow within industrial process industries. Loop-powered transducers are often connected to a plant distributed control system (DCS) or programmable logic controller (PLC) and are used to monitor a parameter (e.g., pressure, temperature, flow) so that an operator can properly manage the process (e.g., chemical or pharmaceutical manufacturing, food and beverage processing, etc.).
Most significantly, though, a vibration transmitter does not output a dynamic signal, and thus no spectral information can be extracted from its output. A transmitter can only report "overall" values of vibration to which it is being exposed. (It should be noted that some vibration transmitters do offer a "dynamic signal" output. But this is available only on a separate connector pin, and not on the 4-20 mA loop.) As an example, consider the case of a vibration transmitter mounted to a vibration shaker. If this shaker was to be set to a sinusoidal waveform at 1000 Hz and amplitude of 5 gpk, what would the output signal from the vibration transmitter look like? It would be a current value of constant amplitude, the value of which would depend upon how the sensor was scaled when it was manufactured (somewhere above 4 mA and at or below 20 mA). From this output signal alone, the user would have no idea that vibration is at 1000 Hz, or even that the vibration waveform is sinusoidal. The user would only know that vibration amplitude is 5 gpk.
In contrast, an IEPE accelerometer mounted to this same shaker would output a dynamic voltage signal, with an amplitude value depending upon how the sensor was scaled at the time of manufacture. From this signal alone, the user could determine not only vibration amplitude, but that it is at 1000 Hz and a sinusoidal waveform.
It is for this reason that IEPE accelerometers are most appropriate for testing-related applications, such as those in which detailed information about structural behavioral response is required. Vibration transmitters, on the other hand, are often found within condition monitoring applications where detailed information is often not needed, but overall condition is.