The Principles of Acceleration, Shock, and Vibration Sensors

May 1, 2005 By: Craig Aszkler Sensors

Whenever a structure moves, it experiences acceleration. Measuring this acceleration promotes an understanding of the dynamic characteristics that govern the object's behavior. The data can be used to modify response, enhance ruggedness, improve durability, and/or reduce noise and vibration.

Accelerometers are sensing transducers that provide an output proportional to acceleration, vibration, and shock. The piezoelectric accelerometer is the most popular class of these devices; other types are based on piezoresistive, capacitive, and servo technologies.

Piezoelectric Accelerometers

These self-generating devices (see Figure 1) are characterized by an extended flat-frequency response range, a large linear amplitude range, and excellent durability. Piezoelectric materials, whether natural quartz or manmade ceramic, have the ability to output an electrical signal proportional to applied stress. In an accelerometer, the piezoelectric elements act as a spring with a stiffness k, and connect the base of the accelerometer to the seismic masses. The sensor operates on Newton's second law of motion: F = ma. An input at the base of the accelerometer creates a force, F, on the piezoelectric material proportional to the applied acceleration, a, and size of the seismic mass, m. The frequency response (see Figure 2) is determined by the sensor's resonant frequency, ω, which can generally be modeled as a simple single-degree-of-freedom system. It can be estimated by:

Figure 1. A 3-crystal, shear mode, piezoelectric accelerometer is commonly referred to as a trishear design. This configuration minimizes the effects of thermal transients and base strain.

There are two categories of piezoelectric accelerometers, defined by their mode of operation. Internally amplified, or integral electronic piezoelectric (IEPE) types, have built-in microelectronic signal conditioning. Charge-output devices incorporate only the self-generating piezoelectric sensing element and have a high-impedance charge output signal.

Figure 2. The typical frequency response of a piezoelectric accelerometer is shown as a logarithmic plot. The usable frequency range generally falls along the horizontally "flat" area of the response curve.

The signal-conditioning electronics in IEPE sensors (see Figure 3) convert the high-impedance charge signal generated by the sensing element into a usable low-impedance voltage signal that can be readily transmitted, over ordinary 2-wire or coaxial cables, to any voltage readout or recording device. The circuitry can also include gain, filtering, and self-test features. Simplicity of use, high accuracy, broad frequency range, and low cost suit IEPE accelerometers for use in most vibration or shock applications. Their upper temperature limit is typically 250°F (121°C), but specialty units can operate up to 350°F (175°C).

Figure 3. This schematic diagram of a typical IEPE accelerometer measurement system shows the piezoelectric element, built-in signal conditioning circuitry, interconnecting cables, and power supply.