A Universal High-Sensitivity Area-Variation Capacitive Displacement Transducer (CDT) Based on Fringe Effect

Capacitive displacement transducers (CDTs) have been widely used in many physical sensors, attributing to high-resolution, simple electricity and easy manufacturing process. Gap-variation CDTs generally have higher displacement resolution due to small electrode gaps but suffer from the pull-in effect, the nonlinear effect and squeeze-film damping; whereas area-variation CDTs have intrinsically good linearity and much smaller slide-film damping. However, the parallel-plate-based area-variation CDTs have the electrode width much larger than the electrode gap with negligible fringe effect; therefore, the sensitivity is limited by periodic electrode numbers. In this paper, we introduce a novel fringe-effect dominated area-variation CDT with a much higher sensitivity within a certain electrode-deployable area. Both theoretical and numerical analysis are applied to investigate the working principle of the CDT design. The proposed fringe-effect-based CDT benefits from a much larger capacitance-to-displacement sensitivity than the traditional periodic array parallel-plate-based CDT, due to the more displacement-sensitive fringe field and more deployable electrode periods. A set of experiments are designed, and the proposed area-variation CDTs are evaluated. Experimental results suggested that the proposed CDT design, which had equal electrode width, separation and gap, could universally be applied to sensors with different featured dimensions either in macroscale or microscale. Angular misalignments with both out-of-plane tilts and in-plane rotations, which affect the output offset and sensitivity, should be minimized or alleviated. The proposed fringe-effect-based CDT are successfully applied to a single-axis in-plane sensing micro-electromechanical systems (MEMS) accelerometer, showing a noise floor as low as 0.25 ng/Hz
$^{\mathrm {1/2}}$
@1 Hz. The corresponding displacement noise of the proposed CDT is 0.1 pm/Hz
$^{\mathrm {1/2}}$
.