Modelling and Control of IRST MEMS microphone

The need for high quality microphones is rising in many application fields, ranging from consumer electronics to distributed environmental control. In this paper, a new MEMS microphone, is presented and its lumped parameter model derived. In the device developed at IRST-ITC, a square planar silicon membrane is suspended over a rigid, gold-plated backplate by means of four flexural springs. This assembly constitutes a capacitor with a mobile armature, which capacitance varies with the acoustic pressure. By measuring such variations of the capacitance, the information on the acoustic pressure is then obtained. However, the measured capacitance variation is not proportional to the acoustic pressure, especially in presence of large signals, and this results in a distortion, affecting the measured quantities. In this paper, we propose the use of a force re-balancing loop, counteracting the acoustic pressure, which maintains the membrane at its equilibrium position during operation. With this solution, the re-balancing command, generated by a properly designed controller, is proportional to the acoustic pressure. Moreover, since the motion of the membrane is negligible, the non-linear effects are almost cancelled, with major benefits on the distortion, when large signals are applied to the microphone. The effectiveness of the solution proposed is demonstrated by simulation results and experiments.