Sensing Geometries for Piezoelectric Accelerometers

The Piezoelectric Effect is an inherent property of quartz and an induced property of certain manufactured ceramic crystals. Piezoelectric accelerometers are constructed with such crystals as their sensing element. As the crystal undergoes stress due to applied force, negative and positive ions will accumulate onto the opposed surfaces of the crystal in an amount that is directly proportional to the applied force. For an accelerometer, a seismic mass is coupled to the crystal. When under the influence of acceleration, the mass will cause a force to act upon the crystal, thus generating a proportional electrical output. This cause and effect relationship is defined by Newton's Law of Motion F=ma.

There are three predominant methods for inducing stress on a piezoelectric crystal in order to generate an electrical output - compression, shear, and flexural. For an accelerometer, each technique offers certain performance attributes, which can make one design more appropriate for certain applications over others.

The Compression design (or compression mode) offers the advantage of few parts and high stiffness leading to a high frequency range. This design, however, tends to be more susceptible to base strain and thermal transient effects since the crystal is in intimate contact with the base of the housing. Any strain or expansion/contraction influences to the base are easily transmitted to the crystal, which can then respond with an output that is not due to acceleration and is therefore error. As a result, compression designs are not recommended for use on metal panels, which may bend, or in thermally unstable environments.

The Shear design (or shear mode) offers the best overall performance for an accelerometer. Planar shear designs (using crystal plates) and annular shear designs (using a ring shaped crystal) are prevalent.

With each style, the crystal is clamped between a center post and outer mass. The more mass that is attached, the more shear force is applied to the crystal for a given acceleration. The accelerometer structure is rigid, affording a high frequency range and since the crystal is not in intimate contact with the base, strain and thermal transient effects are minimized.

Flexural designs offer the ability to generate exceptionally high output signals since the crystal is subjected to high stress levels. Such designs utilize crystal plates that are rectangular or disc shaped. The bending of the crystal can occur as the result of the crystal's own mass in opposition to acceleration, or to enhance bending, additional weight may be clamped or bonded to the crystal. Flexural mode accelerometers are less stiff when compared to compression or shear designs, providing them with a limited frequency range. Also, since the crystal is subjected to high stress levels, they are more easily damaged than other types if exposed to excessive shock or vibration.