[Encyclios]

Piezoelectric accelerometer

The piezoelectric accelerometer uses, as a principle for detecting mass displacement, the electrical signal generated by a piezoelectric crystal (quartz or ceramic crystals) when subjected to mechanical stress. This effect is exploited by placing a known mass in contact with the crystal, also known as the seismic mass 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 condition, property, or quantity. Acceleration acts upon a seismic mass that is restrained by a spring or suspended on a cantilever beam and converts a physical force into an electrical signal.

In the presence of acceleration, the mass (which has certain inertia) compresses the crystal with force directly proportional to the acceleration, which will generate an electrical signal directly proportional to the compression force to which the sensor is subjected. 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 phenomenon vibratory which involves the instrument system.

The last characteristic is to be remarked: as mentioned, the crystal generates an electrical signal proportional to the compression, but if the compression on the crystal remains, the generated signal tends to dissipate after a short period. As a result of this phenomenon, called leakage, these accelerometers are unable to detect a quasistatic acceleration; in fact, after a few seconds from the acceleration, the first signal “freezes” and then dissipates, and in output, there will be no signal. This is due to the high resistance of the accelerometer or, possibly, also to an incorrect setting of the lower limit frequency on the preamplifier.

These accelerometers are used in applications where dynamic accelerations such as those generated in vibrations and mechanical shocks must be detected. There are two types of piezoelectric accelerometers: high and low impedance. High impedance accelerometers have a charge output that is converted into a 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 built-in charge-to-voltage converter and an external power supply coupler to energize the electronics and decouple the subsequent DC bias voltage from the output signal.

The key benefits of piezoelectric accelerometers are:

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

Major applications of piezoelectric accelerometers include:

• Engine testing – Combustion and dynamic stressing
• Ballistics – Combustion, explosion, and detonation
• Industrial/factory – Machining systems, metal cutting, and machine health monitoring
• Original equipment manufacturer – Transportation systems, rockets, machine tools,
• engines, flexible structures, and shock/vibration testers
• Engineering – Dynamic response testing, shock and vibration isolation, auto 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.