Question

Upon reviewing the Endevco Catalog, I find that there are four accelerometer families listed. Why are there four technologies used and which one is the best?

Answer

For starters, no one technology is really better than the other. The "best" accelerometer technology is dependent on the application. This brief article will review the various technologies showing their relative strengths and weaknesses.

Piezoelectric (Charge Mode): This is by far the most used technology for shock and vibration measurements since it is a good fit for many applications. A piezoelectric device offers excellent performance from a few Hz to about 10 kHz (higher in some cases). There are charge mode accelerometers that offer extreme temperature operation from -300°F (184°C) up to 1200°F (650°C). The shock range of these units is also very wide. They are also ideal when the maximum range is unknown as the accelerometer can be paired with a charge amplifier where the gain can be adjusted based on the actual output requirements. Acceleration measurements are limited to dynamic acceleration as they do not respond to constant acceleration.

IEPE (Isotron): This family of accelerometers is also piezoelectric but has a built-in electronic charge to voltage converter. They have characteristics similar to piezoelectric accelerometers, but feature a low impedance voltage output thus no charge amplifier is required. Due to the internal electronics, the high temperature is limited to 257°F (125°C). Endevco has developed technology that allows for continuous operation up to 350°F (175°C). IEPE accelerometers exhibit the same frequency and sensitivity characteristics as piezoelectric devices and also only measure dynamic acceleration. IEPE accelerometers are used for shock and vibration measurements with specialized devices for low noise seismic applications.

IEPE technology offers operator convenience and allows the use of ordinary coaxial cable. While this technology eliminates the need for a charge amplifier, a constant current source is required. Many data acquisition systems and FFT analyzers have built-in current sources making no additional conditioning necessary. Stand-alone power sources and conditioners are also available. The user should be aware of the IEPE limitations described in the above section.

Piezoresistive: Piezoresistive accelerometers are usually a low sensitivity device which makes them desirable for shock measurements. By using micromachine technology, they can be manufactured with a very high resonance frequency allowing for measuring frequencies >100 kHz. They are also "DC" responding allowing for observing long duration shock events. Very small lightweight versions are also available.

Piezoresistive units are especially useful for high level shock measurements and automotive crash testing applications.

Variable Capacitance (VC): VC accelerometers are among the newer accelerometer technologies available. Their strength is the measurement of very low frequencies and motion with little low frequency phase shift. These devices can measure continuous acceleration with high sensitivity making them useable for measuring tilt angle and also acceleration produced during a rocket launch. VC accelerometers exhibit very stable zero offsets and amplitude stability. They have high sensitivity making them desirable for motion and vibration, but have a limited high frequency response to a few kHz.

Applications include ride studies, low frequency modal analysis, aircraft flutter applications. They are desirable for measuring whole body motion following a shock event.

It should be noted that all the above accelerometers, with the exception of the piezoelectric accelerometer, require excitation power.