Many accelerometers used for measuring vibration include a built-in preamplifier to transform the high impedance charge output of the piezoelectric sensor into a low impedance voltage signal, typically 100-300 ohms, that can be transmitted over longer distances. To power this preamplifier, many VTI Instruments products provide an interface that complies with the Integrated Electronics Piezo Electric, or IEPE standard. Using this standard interface ensures that you can connect sensors with an IEPE interface to a VTI instrument and get good results.
The physical layer of an IEPE interface is a single-conductor, shielded cable. This conductor carries both sensor power as well as the sensor signal. This simple connection is one reason for its popularity.
An IEPE interface supplies a constant current between 2 and 20 mA to the sensor. (Do not confuse the IEPE interface with the 4-20 mA current loop interface. Sensors with a current loop interface output a 4-20 mA signal whose value will vary depending on the input.) A common value for the constant current sent to an IEPE sensor is 4 mA. This current will cause the IEPE sensor to output a positive bias voltage, typically between 8 and 14 V when the sensor is not being excited. The signal voltage will then add to this bias voltage.
The higher the current supplied to an IEPE sensor, the longer the possible cable length. Cables of several hundred meters in length can be used without a loss of signal quality. The supply, or compliance, voltage of the constant current source should be 24 to 30 V which is about two times the bias voltage. This ensures maximum amplitudes in positive and negative direction.
IEPE sensors have become the preferred sensors for many applications. There are several reasons for this:
- Low-impedance output is less susceptible to external noise, especially when transmitting over long cables in harsh environments.
- Signal conditioners and cables are inexpensive.
- Self-test. Instruments can detect when sensors and cables are open or shorted.
- Sensors can withstand harsh conditions, such as dirt and humidity, because they are sealed.
Of course, there are some disadvantages. One disadvantage is the required constant current excitation could limit their ability to be used in battery-powered systems. Another is that the operating temperature is limited to what the electronics can bear, generally less than 120 °C.
An example of an instrument that has an IEPE interface is the EMX-4350 Smart 4-Channel Digitizer. It offers exceptional excitation flexibility. You can program the IEPE excitation anywhere from 2 mA to 20 mA, thereby maximizing transducer bandwidth and dynamic range by delivering the exact excitation level independent of external cabling. And, if maximum transducer life is important in your application, you can set excitation levels low when using shorter transducer cables.
For more information on the IEPE interface or the EMX-4350 Smart 4-Channel Digitizer, contact an authorized sales representative by visiting programmablepower.com/contact. AMETEK Programmable Power also can be contacted directly toll free at 800-733-5427 or 858-450-0085 or at firstname.lastname@example.org.