Accelerometers can be single or three axis devices; a typical single axis device is packaged in a 25 x 25 mm casing weighing 45 grams. This contains a pendulum (proof-mass) that senses acceleration. Relative displacement between the pendulum and casing is sensed by a high gain capacitance pick-off and a pair of coils.
A closed loop servomechanism feedback signal (proportional to acceleration) is then amplified and demodulated. This feedback signal (analogue current or digital pulses) is applied to the coils to restrain the pendulum at the null position.
The feedback required to maintain the null position is proportional to the sensed acceleration; this becomes the accelerometer’s output signal. Because of the high gain of the servomechanism electronics used, pendulum displacements are limited to microradians. An integral temperature sensor provides thermal compensation. The IRU contains three devices, measuring acceleration in the longitudinal, lateral and normal axes of the aircraft.
The primary sensors used in inertial navigation systems are accelerometers and gyros to determine the aircraft’s movement. These sensors provide outputs that are processed to provide basic navigation data.
The inertial navigation system needs to establish a local attitude reference and direction of true north for navigation purposes. During this process, the aircraft should not be moved.
MEMS based sensors
Developments in micro-electromechanical systems (MEMS) technology has led to silicon accelerometers that are more reliable and can be manufactured onto an integrated circuit. MEMS is the integration of mechanical elements, sensors and electronics on a common silicon substrate through micro-fabrication technology.