Tunable Negative-Stiffness Nonlinear Microspring Design for High Performance MEMS Inertial Sensors

As microfabrication technologies have matured, the development of high-performance microsensors increasingly relies on nonlinear mechanisms to enhance sensitivity or noise performance of these devices beyond their linear limits. In particular, nonlinear spring mechanisms and negative springs are often employed in high-performance inertial sensor designs for applications such as gravimetry. The nonlinear springs proposed to data primarily rely on the snap-through behavior of pre-shaped long curved beams, requiring significant preloading and initial displacement. Herein, we present a new nonlinear microspring design which exhibits a nonlinear force-displacement response and allows for a wide, tunable range of negative stiffness. Through theoretical modeling and finite element analysis, we demonstrate that the negative stiffness and operational range can be controlled simply by adjusting the length of a single beam. A capacitive accelerometer integrating the nonlinear mechanism with linear folded beams and a proof mass was designed and characterized as proof of principle. Experimental results show strong agreement with theoretical and simulated predictions, leading to a reduction in total stiffness from 40 N/m to 2.5 N/m over the 10μm displacement range. The device requires 0.28mN of preload and 10μm of initial displacement to exhibit negative stiffness behavior. Additionally, frequency response measurements indicate a decrease in resonant frequency from 128Hz to 16Hz under preloading. These results demonstrate the effectiveness of the proposed mechanism in enhancing the performance of MEMS inertial sensors, while allowing for a reduction in both the required preload and the chip area.