Vibration and shock testing typically require piezoelectric accelerometers. This is because these devices are ideal for measuring high-frequency acceleration signals generated by pyrotechnic shocks, equipment and machinery vibrations, impulse or impact forces, pneumatic or hydraulic perturbations, and so on.
Piezoelectric accelerometers rely on the piezoelectric effect. Generally speaking, when subject to mechanical stress, most piezoelectric materials produce electricity. A similar effect also happens conversely, as applying an electric field to a piezoelectric material can deform it mechanically to a small extent. Details of this phenomenon are quite interesting.
When no mechanical stress is present, the location of the negative and positive charges are such as to balance each other, making the molecules electrically neutral.
The application of a mechanical force deforms the structure and displaces the balance of the positive and negative charges. This leads the molecules to create many small dipoles in the material. The result is the appearance of some fixed charges on the surface of the piezoelectric material. The amount of electrical charges present is proportional to the force applied.
Piezoelectric substances belong to a class of dielectric materials. Being insulating in nature, they are very poor conductors of electricity. However, depositing two metal electrodes on the opposite surfaces of a piezoelectric material makes it possible to produce electricity from the electric field that the piezoelectric effect produces.
However, the electric current that the piezoelectric effect produces from a static force can last only a short period. Such a current flow continues only until free electrons cancel the electric field from the piezoelectric effect.
Removing the external force causes the material to return to its original shape. However, this process now causes a piezoelectric effect in the reverse direction, causing a current flow in the opposite direction.
Most piezoelectric accelerometers constitute a piezoelectric element that mechanically connects a known quantity of mass (proof mass) to the accelerometer body. As the mechanism accelerates due to external forces, the proof mass tends to lag behind due to its inertia. This deforms the piezoelectric element, thereby producing a charge output. The input acceleration produces a proportional amount of charge.
Piezoelectric accelerometers vary in their mechanical designs. Fundamentally, there are three designs, working in the compression mode, shear mode, and flexural mode. The sensor performance depends on the mechanical configuration. It impacts the sensitivity, bandwidth, temperature response of the sensor, and the susceptibility of the sensor to the base strain.
Just as in a MEMS accelerometer, Newton’s second law of motion is also the basis of the piezoelectric accelerometer. This allows modeling the piezoelectric element and the proof mass as a mass-damper-spring arrangement. A second-order differential equation of motion best describes the mass displacement. The mechanical system has a resonance behavior that specifies the upper-frequency limit of the accelerometer.
The amplifier following the sensor defines the lower frequency limit of the piezoelectric accelerometer. Such accelerometers are not capable of true DC response, and hence incapable of performing true static measurements. With a proper design, a piezoelectric accelerometer can respond to frequencies lower than 1 Hz, but cannot produce an output at 0 Hz or true DC.
