Design and Demonstration of an Electromagnetic Force-Rebalanced Sub-10-Hz Interferometric MEMS Accelerometer

Grating-assisted optical interferometry is advantageous for high integration and electromagnetic immunity and thus powerful for developing microscale accelerometers with high sensitivity. However, the optical intensity of interferometric signal is normally sinusoidal with respect to the phase difference, resulting in imperfect linearity and limited dynamic range. To combat this and enhance the utility of interferometry in seismic applications, a digital closed-loop detection system with electromagnetic force feedback is first equipped for interferometric micro-electromechanical system (MEMS) accelerometers with a resonance below 10 Hz. We validate the control scheme upon system-level simulations and then perform experimental evaluations on an 8.35-Hz high-sensitivity accelerometer prototype. Test results demonstrate that the feedback control improves the dynamic range by 23 times, from 30 to $697~\mu \text{g}$ ; meanwhile, an excellent $R^{2}$ of 0.99995 is achieved in the scale factor, indicating a great linearity in output. More importantly, the 0.2-Hz terrestrial microseism was successfully detected by the closed-loop device, further demonstrating a noise floor of 10 ng/ $\sqrt {\text {Hz}}$ from 0.2 to 7 Hz under calibration of a standard seismometer. This control system not only enables nonlinear correction of our accelerometers but also offers a low-noise solution for inherent narrow dynamic range in most low-frequency interferometric transducers.