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What are the advantages and disadvantages of capacitive and piezoresistive accelerometers?

Question

I want to measure linear acceleration (~10g, DC to 20Hz) on a rail vehicle. From looking at your range of accelerometers, either a capacitive accelerometer (7290A) or piezoresistive accelerometer (2262A) would suit. What are the advantages and disadvantages of capacitive and piezoresistive accelerometers (e.g., shock survival, accuracy, drift, overrange recovery time)?

Answer

For your application, the variable-capacitance (VC) accelerometer has it all over the piezoresistive (PR). VC accelerometers are generally available with much higher sensitivities than PR accelerometers. The VC accelerometer best suited for your test, the 7290A-10, has a nominal sensitivity of 200 mV/g. The most closely competitive PR unit, the 7265A-HS, only has a sensitivity of 25 mV/g. Because your test will likely expose a chosen accelerometer to fractions of a "g" at some point, you'll want to go with a higher-sensitivity unit in order to achieve a more desirable signal-to-noise ratio.

VC and PR accelerometers do share two similarities: 1) they both require simple signal conditioning (DC power supply and differential output); and 2) they both can measure accelerations down to 0 Hz (i.e., gravity). The main difference between the two, as stated above, is sensitivity. VC accelerometers are generally much more sensitive than PR units.

Endevco VC accelerometers also generally have much higher shock limits. To continue the example above, the 7290A-10 (VC) has a shock limit of 5,000 g's, and the 7265A-HS (PR) has a shock limit of 2,000 g's. The next higher g-range variable-capacitance model, the 7290A-30, has a shock limit of 10,000 g's. It should be noted that high shock survivability is a characteristic unique to Endevco variable-capacitance accelerometers. There are other VC units that offer equivalent sensitivities, but they are only capable of surviving mechanical shocks 30%-60% the level of g's that Endevco units can.

In the end, perhaps it's best to think of VC and PR units in terms of which is most appropriate for a given application. VC accelerometers are ideally suited to measure low-frequency, low-level accelerations (of an automobile or wind-induced sway on a skyscraper). Endevco VC accelerometers, in addition, are particularly well suited to measuring low-level accelerations before or after being subjected to a violent shock event (measuring low-level vibrations on a space launch vehicle after surviving the launch event or acceleration of a missile on a railcar before it impacts a truck in a transportation safety test). PR accelerometers are ideal for medium-level, lower-frequency shock events like automotive crash testing (200 - 2,000 g's), where a DC response is necessary. We do also offer a PR accelerometer (7270A) to measure high-level shock events. It can measure shocks as high as 200,000 g's and has a mounted resonant frequency in excess of 1 Mhz.