Accelerometers- Resistive_ Capacitive_ and Servo technologies

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Accelerometers- Resistive_ Capacitive_ and Servo technologies Powered By Docstoc
					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 鈥?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.