A Selection Guide for Variable Capacitance and Piezoresistive Accelerometers

With the advent of variable capacitance ("VC") transducers and continued availability of piezoresistive ("PR"") devices, engineers now have a wider choice of accelerometers for their measurement needs. VC devices seem to garner comments such as, "They're the same as PRs - why change?" Or, "They're different and hard to understand - so I'm going to stick with what I know." A close look at both of these technologies is in order.

Differences in Technology

VC devices are quite different internally from PRs. PR devices use strain gage technology to measure acceleration, while VC devices use the change in capacitance derived from a moving plate to measure acceleration. PR devices output voltage from the strain gages directly, but VC devices rely on sophisticated electronics to read the change in capacitance and output a voltage.

VC transducers look, from the electrical perspective, much like a PR device. They require an excitation voltage and output a voltage proportional to the applied acceleration. However, there are several important differences. VC devices operate largely independent of differences in excitation voltage and typically operate from an excitation of 9 to 30VDC with no output change. On the other hand, PR device output is ratiometric to the excitation voltage.

VC devices cannot be easily calibrated using shunt cal methods, but their sensitivity is often easily tested by simply using earth's gravity as a "1 g reference.

VC devices have higher output than PR devices. For example, the full scale output of Endevco Microtron® accelerometers is ±2.0 V peak resulting is a high sensitivity, whereas Endevco PR devices are about ±200mVPK (at 10.00VDC excitation).

A PR transducer can be temperature compensated by adding resistors (in parallel or in series) that have an opposite temperature coefficient to the bridge sensing gages. A VC device can be temperature compensated electronically. The Endevco 7290D is temperature compensated by observing the effect of temperature and compensating for it digitally. Compensation of VC transducers digitally is orders of magnitude easier than the multiple-equations with multiple-unknowns that PR devices require.

Differences in Application

Basic application parameters often drive which type of device to select. Using Endevco as an example again, the Microtron® VC accelerometers are limited to 150g or less (one measures ±2g, full scale). But it is also important to keep in mind that Endevco Microtron® devices have 20,000g survivability. This extreme overrange capability is the result of the unique design of the Endevco Microtron accelerometer.

VC devices, with their high sensitivity, are well suited for testing automotive suspension systems, measuring aircraft flutter, safety testing of amusement park rides, and control of specialized railroad passenger cars.

PR devices are excellent shock accelerometers, with the capability of accurately measuring shock events greater than 60,000g. As a result of a micromachined manufacturing process, they rarely exhibit zero shift, often found with piezoelectric accelerometers. Their low sensitivity makes them desirable for high level shock events since they will provide accurate results without overdriving the associated data acquisition equipment.

Since PR devices are often DC coupled, they are capable of capturing long duration shock events. They are the first choice for many transportation safety tests including automotive crash tests, airline seat safety testing, and testing of head protection devices.

In Summary

Advantages for variable capacitance:

• Excitation voltage independence (9.5-18VDC typical)
• Overscale protection (20,000g)
• Line driving (50-500Ω vs. 500-1,000Ω for PR)
• High sensitivity
• High accuracy over temperature
• High output (+/- 2 Volts full scale

Advantages for piezoresistive:

• Noise is resistor noise only (VCs have electronics, so more noise)
• Relatively well understood by lab technicians
• Available in higher ranges
• Small size
• Less expensive
• Low weight
• FS not absolute (VC FS electronically fixed; PR will work, not necessarily linearly, up to breakage point)
• Ease of shunt cal (unnecessary with modern amps, but still in use)

Variable capacitance and piezoresistive are complementary accelerometer technologies. When measuring low level, low frequency events, variable capacitance accelerometers are the accelerometer of choice. For measuring shock or automotive crash applications, piezoresistive devices are best due to their lower sensitivity and wide frequency response.


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