Accelerometer pros and cons

This guide outlines the differences between the types of Accelerometer available.

Voltage Output IEPE Accelerometers 

This type of accelerometer features an integrated circuit within the Accelerometer converting the charge from the piezoelectric crystal before it leaves the sensor, this voltage can then be transmitted across a standard coaxial cable, as it is more stable and inherently immune to noise and interference from the cable. 

These kinds of accelerometers have a maximum operating temperature of 125ºC, with some high-temperature variants of these accelerometers, which can withstand temperatures of up to 185ºC. There is also a water-cooled accelerometer that can operate on surface temperatures in excess of 900ºC. IEPE Accelerometers can be connected to the measuring equipment, or a computer with minimal interfacing/instrumentation, and does not require the charge amplifier. It is important that the Accelerometer is powered by a stable, constant current supply to reduce noise on the signal generated by the Accelerometer. The disadvantage of low impedance accelerometers is that the sensor has an internally fixed range and time constant, potentially limiting their uses in some applications.

Pros        

  • Operation at higher temperatures, up to 185°C for test and measurement accelerometers. 
  • Direct connectivity to the majority of readout instruments, or vibration analysers.
  • More reliable.
  • The signal can be driven down a longer cable without increased noise or loss of resolution.
  • Can be operated from low cost constant current sources.
  • Integrated electronics can provide a higher output from a smaller mass
  • Use a wide range of cable types  Less sensitive to dirt
  • Signal power often built into DAQ systems

Cons

  • Requires more expertise to operate the system.
  • Fixed sensitivity.
  • Range and time constant are fixed within the sensor.
  • More expensive per unit Lower operating temperature than a charge accelerometer
  • Less robust

Charge Output Accelerometers

In this type of accelerometer, a charge will be emitted proportional to the force applied to the piezoelectric accelerometer. This charge will be transmitted directly down the cable and will require a special low noise cable, with a charge amplifier on the receiving end.

Charge accelerometers have high impedance and are particularly suited to high-temperature applications (more than 185°C) where IEPE Accelerometers could not be used.   It is important to note that due to the high impedance nature of these sensors, they require a low noise cable to be used when connecting the sensor to the measurement equipment. This is due to the cable used having an effect on the signal; movement on the cable can distort this signal. It is also important to note that cable length will have a significant effect on the signal from the sensor, and it is advisable to have a short cable between the accelerometer and the charge amplifier.

Low noise charge amplifiers or amplifiers with integrated low pass filters can be used, meaning that the signal from the charge amplifier is useable without any further filtering.

The main advantage of a charge system over an IEPE system is the ability of the accelerometer to operate at higher temperatures. Charge systems are generally more versatile due to the time constant, gain, and normalisation all being controlled by the charge amplifier. 

Pros

  • Operation at higher temperatures, up to 250°C for test and measurement accelerometers, and 400°C for industrial accelerometers.
  • More flexibility in adjusting the time constant, gain, and normalisation via the charge amplifier.
  • Extended low-frequency response.
  • Wider Dynamic Range
  • Robust
  • Lower cost per unit

Cons

  • Noise from the cable affects results and reduces resolution (must use Low Noise Cable).
  • More costly with the requirement for additional instrumentation, and special low noise cables.
  • The high impedance nature of the signal makes it more vulnerable to noise. Ie, Sensitive to cable flex
  • Requires more expertise to operate the system.
  • Sensitive to dirt on connectors