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Open Loop Sensors

RG-103 An open loop sensor directly responds to an input stimulus. Its physical structure is directly affected by the input (e.g. a pressure change affecting the level of deformation in a diaphragm). A greater input produces a greater physical change, and thus, greater output signal. A design compromise is always made between the measurement sensitivity and measurement range. For higher sensitivity, one has to sacrifice the range; and for wider range one has to sacrifice the sensitivity. In general, open loop sensors operate linearly over a very narrow range; and have a nonlinear response over the full operational range. These devices require look-up table calibration, or some other means of compensation.


Closed Loop Sensor

ClosedLoop1 The closed loop sensor response does not depend upon the physical change of the sensor directly, but upon an indirect measurement. As the navigational grade accelerometer has to have resolution on the order of a few micro-Gs, it is designed with an extremely compliant suspension (a frictionless hinge will be ideal). The micro-G level change in acceleration tends to produce a measurable displacement in the proof mass, which is detected by a pick-off, that in turn generates a rebalance force to bring the proof mass back to its null (zero G) position. The higher input requires a higher rebalance force through a controller. Thus, the signal that produces the rebalance force to maintain the proof mass at its null position becomes the measure of acceleration.   Similar differential pick-off, forcing, torquing or phase nulling approaches are employed in all closed loop sensors.

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