Design and FEM Simulation of dual mass resonant MEMS Gyroscope

With the rapid development of MEMS technology, silicon micromachined gyroscopes have attracted much attention. Based on the working principle, the MEMS gyroscopes are divided into two main categories including resonant and non-resonant gyroscopes. In resonant MEMS gyroscopes, the device is operated at resonance and both the drive and sense mode resonant frequency values are generally matched which leads to high mechanical sensitivity. A major challenge faced by the MEMS designer is the fluctuation in the performance parameters of resonant MEMS gyroscopes as they are easily affected by any variation in ambient conditions and fabrication imperfections, as these imperfections can cause a shift in resonance frequency which in turn causes a mismatch between the drive and sense mode frequencies. To minimize these effects, complex structures have to be introduced in the design or additional feedback circuitry is required to reduce the mismatch between drive and sense mode frequencies. Resonant MEMS gyroscopes usually operate in a closed-loop in order to maintain the performance characteristics over a wide range of environmental conditions. There are multiple errors which have to be mitigated in order to obtain optimal performance and for this, some design choices have to be made which can help reduce the errors and avoid overly complex feedback electronics. A major error source is the mismatch in the frequencies of drive and sense modes, this can cause the performance of the gyroscope to reduce dramatically as the even a slight mismatch can reduce the amplitude response of a mode matched gyroscope significantly. To mitigate this error frequency tuning is required. This method is based on a phenomenon called electrostatic spring softening which is used to change the stiffness of the structure electronically, hence changing the frequency. This allows the designer to reduce the mismatch and to get the optimal performance of the gyroscope. In this master’s thesis project, a new design of resonant MEMS gyroscope consisting of two separate masses for the drive and sense mode is presented which allows minimizing the cross-axis sensitivity and common-mode error by decoupling the drive and sense mode displacements using a unique configuration mechanical springs. In particular, the new design of resonant mode-matched electrostatic z-axis MEMS gyroscope considers the foundry constraints of relatively low cost and commercially available Silicon-on-Insulator (SOI) based SOIMUMPs process. For the compensation of the frequency mismatch between the drive and sense mode frequency due to micro-fabrication process tolerances and device operating temperature variations, comb-drive based electrostatic tuning is implemented in the design. A FEM analysis of the presented MEMS gyroscope is carried out in ANSYS APDL. Particular attention is taken on the dynamical behaviour of the structure, which is strongly influenced by the electromechanical coupling. In addition, FEM simulation results are compared to the simplified lumped parameter model solutions. The closed-loop mode-matching controller that provides a DC tuning potential is designed regarding the phase relationship between the drive and sense signals. Finally, the effects of the closed-loop control systems on the dynamical behaviour of the presented MEMS gyroscope structure are simulated in Matlab/SIMULINK environment.