Accelerometers: Resistive, Capacitive, and Servo technologies
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 resistive, capacitive, servo or force balance, and other technologies.
Resistive accelerometers
The main operating principle of resistive accelerometers is that voltage output of resistor bridge is changed with respect to acceleration. Piezoresistive accelerometers use the electrical properties of piezoresistive material as the primary transducer component. Piezoresistive accelerometers may be fabricated from metal strain gauges, piezoresistive silicon, or as a micro-machined or MEMS. Strain gauge accelerometers, often called "piezoresistive" accelerometers, use strain gauges acting as arms of a Wheatstone bridge to convert mechanical strain to a DC output voltage. Piezoresistive accelerometers are widely used in automobile industry. They are applied in securing safety performance of a vehicle, including anti-lock braking system, automotive safety air-bags and traction control system.
Capacitive accelerometers
Capacitive accelerometers sense a change in electrical capacitance proportionally to applied acceleration. When the proof mass tends to move under acceleration force the voltage across the capacitor is changed corresponding to the applied acceleration. Capacitive accelerometers usually are more sensitive than piezoresistive. Among disadvantages are the cost and size concerned with the necessity of the onboard conditioning. Capacitive accelerometers typically utilize a silicon micro-machined sensing element, and so they achieve low frequency range, high stability and linearity. Capacitive accelerometers are generally used for vibration measurements, R&D and test and measurement, vehicle dynamics, vibration monitoring systems and condition monitoring applications.
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 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. They are applicable also for structural health monitoring, orientation systems for oil drilling industry, gyrocompassing systems for submarines, ROV, AUV.
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.
