Accelerometers: Piezoelectric, Servo, and Vibrating element technologies

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Published: 23rd November 2010
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An accelerometer is a device that measures accelerating force experienced relative to freefall. These forces may be static, like the constant force of gravity, or they could be dynamic caused by moving or vibrating. There are many types of accelerometer technologies developed and reported in the literature, among them are piezoelectric, servo or force balance, vibrating quartz, and other technologies.

Piezoelectric accelerometers
Piezoelectric sensors use the piezoelectric effects of the sensing element to produce a charge output. The piezoelectric crystals, which usually are made from quartz or ceramic, produce an electric charge when they become deformed under acceleration. Piezoelectric transducers do not require an external power source for operation so they are self-generating, and that is why they cannot be used to measure steady-state accelerations or force. The strong points of piezoelectric sensors are that they are usually extremely rugged, and, although the sensitivity of piezoelectric accelerometers is relatively low compared with other types of accelerometers, they have the highest range (up to 100,000 g's) and frequency response. Today, piezoelectric accelerometers are widely used for shock and vibration measurements in laboratories, at production facilities.

Servo accelerometers
Servo accelerometers contain a mass whose position is controlled by a servo feedback mechanism. The feedback signal is proportional to acceleration. In the early force balance sensors piezoelectric or magnetic "dithering" mechanisms were used to reduce stiction effects. Nowadays, quartz flexure mechanism with high-resolution null detect systems is utilized in order to eliminate the bearing. This technology is used in Servo quartz accelerometers of Bricett Company. The crystalline quartz which is used by Bricett engineers as a pivot has superior mechanical characteristics providing nearly zero hysteresis performance due to the fact that the mass does not deflect significantly.
Servo Accelerometer scheme

Servo accelerometers have the best temperature stability, high accuracy, and high-level output at a relatively high cost. As they are closed-loop designed, servo accelerometers ensure significant performance advantages such as reduced cross-axis sensitivity, good scale-factor stability, wide dynamic range, and linearity. That is why servo accelerometers are used for the most part in inertial navigation and guidance systems such as INS - Inertial Navigation Systems, IMU - Inertial Measurement Units, AHRS - Attitude Heading Reference Systems, IRU - Inertial Reference Units, IGS - Inertial Guidance Systems, etc.

Vibrating element accelerometers
The main operating principle of a vibrating element accelerometer is that a seismic mass supported by vibrating prestressed wires causes the tension of the wires in a permanent magnetic field. The signal that the wires produce when their frequency of vibration is changed is correlated with the acceleration. Although the precision of such a device is high, it is quite sensitive to temperature variations and is relatively expensive. Vibrating element accelerometers are often used in industrial applications, for acceleration measurement, vibration and shock analysis.

Thus, different accelerometer sensor types have their specific features which may be advantages for certain applications and disadvantages for others. Nowadays, there is wide variety of accelerometers produced and sold on the world market so every company or individual who needs this device can choose the right accelerometer sensor for any specific purpose use.

More information about quartz servo accelerometers, their features and technical characterictics is available here.

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