Feedback Control
Feedback system that applies an equal and opposite force to a proof mass to maintain its null position defines the operation. Closed loop force rebalance prevents the mechanical element from moving through its full range of travel. Approach is standard in high-end accelerometers.
Operational Range
Maintaining the mass at the center of the sensor gap eliminates the non-linearities associated with large displacements. The electrical force required to hold the mass is directly proportional to the external acceleration. Accuracy is maintained across much higher loads than an open-loop design.
Linearity Benefit
Precision is improved because the system does not rely on the spring constant of the flexures, which can change with temperature. Instead, the accuracy depends on the stability of the feedback electronics and the reference voltage. Engineers prefer this method for inertial navigation where small errors accumulate over time.
Bandwidth Constraint
Dynamic response of the sensor is determined by the speed of the control loop rather than the natural frequency of the mechanical structure. A faster loop allows the device to track rapid changes in motion, but it also requires more power and complex circuitry. Designers must optimize the gain and phase margins to prevent oscillations that would corrupt the output.
Stability is verified by injecting a step input and observing the settling time of the mass. Testing ensures that the sensor remains responsive under all operating conditions. High-frequency signals are filtered to prevent aliasing in the digital readout.
