Choose the Right Accelerometer

Charge/Piezoelectric Accelerometers

Piezoelectric accelerometers use a spring-mass system to generate a force equivalent to the amplitude and frequency of vibration. The force is applied to a piezoelectric element, which produces a charge on its terminals that is proportional to the vibratory motion. The unique design of Brüel & Kjær's piezoelectric accelerometers allows for high seismic resonance and ruggedness in the same package, making this type of accelerometer suitable for general-purpose use. Their exceptional high-frequency characteristics are also ideal for high-frequency vibration measurement applications, such as gear noise analysis and monitoring of turbine or high-speed rotating machinery. Piezoelectric materials are self-generating, therefore, do not require an external power source. They are capable of operating at extreme temperatures, but are constrained by low output sensitivity (inherent in any spring-mass transducer design). Because most high-frequency accelerometers   are undamped, high-frequency harmonics from the structure can cause ringing of the accelerometer and overload conditions in downstream electronics. The resonance frequency of the accelerometer should, therefore, be sufficiently high that it stays above the high-frequency signals that are present in the structure.

IEPE Accelerometers

IEPE accelerometers are piezoelectric accelerometers with integral preamplifiers, which give output signals in the form of voltage modulation on the power supply line. Brüel & Kjær's IEPE accelerometers are specifically aimed at measuring vibration on small structures (for example, miniatures). Their high output sensitivity, high signal-to-noise ratio and wide bandwidth make them suitable for both general purpose and high-frequency vibration measurements. These low-cost and lightweight accelerometers are high performance instruments that have higher output sensitivity than standard piezoelectric accelerometers (without integral amplifiers). They are hermetically sealed to protect against environmental contamination, have low susceptibility to radio frequency electromagnetic radiation and low impedance output due to the external constant current power source. Low-impedance output allows you to use inexpensive coaxial cables. IEPE accelerometers are undamped, high-frequency accelerometers, which require measures to be taken to avoid ringing of the accelerometer and overload conditions.

Piezoresistive Accelerometers

Piezoresistive accelerometer strain gauge elements change electrical resistance in proportion to applied mechanical stress. The accelerometers monolithic sensor includes integral mechanical stops and offers outstanding ruggedness, while still maintaining an excellent signal-to-noise ratio. This type of accelerometer is ideal for measuring motion, low-frequency vibration and shock, and is designed for crash testing, flutter testing, rough road testing, biodynamics measurements and similar applications that require minimal mass loading and broad frequency response. They can also be used for shock testing of lightweight systems or structures and meet SAEJ 211 specifications for anthromorphic dummy instrumentation. With a frequency response extending down to DC, or steady-state acceleration, the accelerometers are ideal for  measuring long duration transients as well as short duration shocks. For many of these applications, the sensitivity is high enough that preamplification of the output is unnecessary. Piezoresistive accelerometers have minimum damping, thereby producing no phase shift at low frequency. There are, however, inherent problems with measurements at low frequency, and steps must be taken to overcome these disadvantages.

Variable Capacitance Accelerometers

In variable capacitance accelerometers, a unique variable capacitance microsensor forms a parallel plate capacitive device. The result is a sensor that provides response to DC acceleration inputs, stable damping characteristics that maximise frequency response, and ruggedness to withstand extremely high shock and acceleration loads. These low-g accelerometers are perfect for measuring motion and low-frequency vibration and are intended for applications such as trajectory monitoring, aircraft/vehicle structural evaluation, flutter testing, automotive suspension and brake testing. Gas damping and internal over-range stops enable the microsensors in the accelerometers to withstand the shocks and acceleration loads inherent in typical high-g applications. In the case of high-g testing, physical damage to the sensor often occurs; therefore, we suggest that you overestimate the maximum shock level when selecting the range of a shock accelerometer. A general rule-of-thumb: the closer the accelerometers are to the source (explosive or impact), the higher the input g level. Solder terminals and ribbon wires are also recommended because of their light weight, but extra care is required in installing and handling these delicate connections.