CHOOSING THE RIGHT TYPE OF ACCELEROMETER
As with most engineering activities, choosing the right tool may have serious implications for the measurement results. The information below may help the readers make the proper accelerometer selection. Let’s start with the basic classifications and their technologies.
BASIC ACCELEROMETER TYPES
There are two classes of accelerometers in general:
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AC-RESPONSE
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DC-RESPONSE
In an AC-response accelerometer, as the name implies, the output is ac coupled. An AC coupled device cannot be used to measure static acceleration such as gravity and constant centrifugal acceleration, for example. It is only suitable for measuring dynamic events. A DC-response accelerometer, on the other hand, is DC coupled and can respond down to zero Hertz. It therefore can be used to measure static, as well as dynamic acceleration. Measuring static acceleration is not the only reason a DC-response accelerometer should be selected, however.
SUMMARY
Each accelerometer sensing technology has its advantages and compromises. Before making a selection, it’s important to understand the basic differences between the various types and the test requirements. First and foremost, choose only DC-response accelerometers to measure static or very low frequency (150°C) dynamic measurement applications, charge mode piezoelectric is an obvious choice; or in most cases, the only choice. With a charge mode device, a low-noise coaxial cable should be used due to its high impedance output, and a remote charge amplifier (or an inline charge converter) to condition its charge output. Voltage mode piezoelectric is the most popular type of accelerometer for dynamic measurements. It offers small size, broad bandwidth, and a built-in charge converter which allows direct interface with many modern signal analyzers and data acquisition systems (those that offer integrated IEPE/ICP power source). Voltage mode piezoelectric is typically limited to <125°C applications, but it is no longer necessary to use a low-noise coaxial cable due to its low impedance output. Capacitive design features are critically damped to overdamped response which lends itself to low-frequency measurements. The low-cost, SMD class of devices is suited for high-volume automotive and consumer applications where ultimate accuracy is not a priority. The more expensive instrumentation grade silicon MEMS capacitive accelerometers have good bias stability and very low noise. Capacitive accelerometers have low impedance output and ±2V to ±5V full-scale output. Most designs require a regulated dc voltage for power. Piezoresistive accelerometers are versatile in terms of their frequency and dynamic range capabilities. Being a DC-response device, it can handle static acceleration and produce accurate velocity and displacement data. Its broad bandwidth also covers most dynamic measurement needs. Piezoresistive designs offer various degrees of damping (from ζ =0.1 to 0.8) response which makes it suitable for use in a variety of test conditions, including shock testing. Plain piezoresistive accelerometers (without electronics) are small and lightweight, with a ±100 to ±200mV full-scale output. The amplified models (with built-in ASIC) feature low output impedance (<100Ω) and ±2V to ±5Vfull scale output.