Piezoelectric accelerometer

Piezoelectric accelerometers use the principle of mass displacement detection based on the electrical signal generated by a piezoelectric crystal (quartz or ceramic) when subjected to mechanical stress. This effect is exploited by placing a known mass in contact with the crystal, also known as the seismic or test mass, which constitutes both the sensor and the elastic element, so that it exerts a force.

Like other transducers, piezoelectric accelerometers convert one form of energy into another and provide an electrical signal in response to the state, property or quantity. Acceleration acts on a seismic mass held by a spring or suspended from a cantilever, converting a physical force into an electrical signal.

In the presence of acceleration, the mass (which has a certain inertia) compresses the crystal with a force directly proportional to the acceleration, which generates an electrical signal directly proportional to the compression force experienced by the sensor. Considering that the elastic element is a crystal, the characteristics of these devices are peculiar:

  • they have a relatively low sensitivity;
  • they can detect very high accelerations without being damaged (even 1000 g);
  • they cannot detect constant accelerations over time.

A particularly important consideration lies in the fact that the crystals generally used in the construction of the elastic element have a very high value of the elastic constant, as well as high stability and repeatability, which has a profound influence on the differential equation that governs the vibratory phenomenon involving the instrument system.

The last characteristic is worth mentioning: as already mentioned, the crystal generates an electrical signal proportional to the compression, but if the compression remains on the crystal, the generated signal tends to disappear after a short time. As a result of this phenomenon, called leakage, these accelerometers are not able to detect a quasi-static acceleration; in fact, after a few seconds from the acceleration, the first signal “freezes” and then dissipates, and in the output there will be no signal. This is due to the high resistance of the accelerometer or, possibly, to an incorrect setting of the lower cut-off frequency on the preamplifier.

These accelerometers are used in applications where dynamic accelerations, such as those generated by vibrations and mechanical shocks, need to be detected. There are two types of piezoelectric accelerometers: high impedance and low impedance. High impedance accelerometers have a charge output that is converted to voltage using a charge amplifier or external impedance converter. Low-impedance units use the same piezoelectric sensing element as high-impedance units, and incorporate a miniaturized onboard charge-to-voltage converter and an external power supply coupler to power the electronics and decouple the subsequent DC bias from the output signal.

The main advantages of piezoelectric accelerometers are:

  • Extremely wide dynamic range
  • Low output noise
  • Wide frequency range
  • No moving parts (suitable for shock and vibration measurements)
  • Compact, non-contact design
  • Excellent linearity over its dynamic range
  • Acceleration signal can be integrated to provide velocity and displacement
  • High sensitivity
  • Self-powered (no external power required)

Key applications for piezoelectric accelerometers include:

  • Engine testing – combustion and dynamic stress
  • Ballistics – Combustion, explosion and detonation
  • Industrial/Factory – Machining systems, metal cutting, and machine health monitoring
  • Original Equipment Manufacturers – Transportation systems, rockets, engines, flexible structures and shock/vibration testers
  • Engineering – Dynamic response testing, shock and vibration isolation, automotive chassis
  • Structural testing, structural analysis, reactors, control systems, and materials evaluation
  • Aerospace – Ejection systems, rocketry, landing gear hydraulics, shock tube instrumentation, wind tunnel, and modal testing.
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