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