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Accelerometers 101

There is more than one type of accelerometer!

Endevco designs and manufactures a variety of accelerometers for measurements of vibration, shock and inertial motion. To meet various testing requirements, several types of accelerometer technologies are available each differing in terms of their recommended usage, performance specifications, power requirements and signal conditioning characteristics. The following is a general overview of accelerometer technology types and their application considerations.

Piezoelectric (PE) accelerometers

All mechanical accelerometer designs are based on a simple spring-mass principle in which strain is generated in relation to amplitude and frequency of the dynamic motion. In a PiezoElectric (PE) accelerometer, this strain is applied directly to the PE element, which develops an electrical charge proportional to mechanical motion. Different material and configurations of PE accelerometer elements are used to support specific applications.

Advantages of PE sensors

  • The dynamic range of PE accelerometers can be greater than 130dB. When used with a proper charge converter/amplifier, it offers unmatched performance in terms of total measurement range and reliability. Transduction efficiency may also be optimized with PE crystals to allow for the miniaturization of accelerometer within a given physical envelope.
  • Single-ended compression type is optimum for low level measurements because of the high sensitivity that can be achieved by stacking multiple PE crystals and connecting them in parallel.
  • A shear mode design allows for the construction of miniature, lightweight sensors suitable for monitoring of small components and test articles. A key advantage of the shear design is the isolation of the sensing element from the base, which provides excellent protection from base strain and temperature transients.
  • Due to the broad temperature range of most piezoelectric materials, PE accelerometers are often specified for extreme high or low temperature applications, from cryogenic conditions in rocket engines to the extreme heat environment of a turbine generator.
  • PE accelerometers are available in a wide range of shapes and sizes, from micro-miniature in electronics component testing to larger sizes used in seismic or engine applications.

Piezoresistive (PR) accelerometers

Strain gauge accelerometer designs based on a Wheatstone bridge arrangement consist of a rugged monolithic assembly with solid-state MEMS resistors that change in resistance in proportion to applied mechanical strain.

Advantages of PR sensors

  • PR accelerometers feature DC response which makes it useful for measuring long duration pulses. Its acceleration outputs can therefore be integrated to yield velocity and displacement accurately.
  • Monolithic MEMS sensors exhibit high sensitivity with an excellent signal-to-noise ratio and outstanding stability.
  • The simplicity and miniature size of MEMS PR element lend itself to broad bandwidth (>100,000Hz) and wide dynamic range (>100,000g) designs.

Integrated electronics piezoelectric (IEPE) accelerometers

The acronym IEPE refers to a type of piezoelectric (PE) accelerometer with internal electronics (IE) that allows it to convert charge to a low-impedance voltage output. Its temperature response is somewhat limited due to its onboard electronics. This type of accelerometer is primarily specified for applications in which environmental conditions permit its use, including HALT/HASS/ESS testing, industrial vibration monitoring and general purpose vibration and shock testing.

Since both PE and IEPE types are AC-coupled designs, the acceleration output of these accelerometers cannot be used to yield correct velocity and displacement information by numerical integration.

Advantages of IEPE sensors

  • Less operator attention, training and installation expertise required.
  • Uses standard coaxial cable.
  • Drives long cables without noise increase or loss of resolution.
  • Operates directly into many data collectors with built-in constant current input.

Variable capacitance (VC) accelerometers

Feature a silicon MEMS capacitive sensing element that is anodically bonded to a lid and base to form a parallel plate, differential capacitor pair. The accelerometers feature DC response, gas damping for good frequency coverage and rugged construction. Integral electronics with DC excitation provide a high-level, low-impedance output signal that is stable.

Advantages of VC sensors

  • Designed for low-g measurement, yet can also withstand very high g shocks.
  • Suitable for trajectory monitoring, modal analysis,flutter testing, vehicle dynamics measurements for automotive suspensions and ride quality.