סמינר מחלקתי Erez Benjamin

School of Mechanical Engineering Seminar
Wednesday, March 23, 2016 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Bistable Force\Acceleration Sensor Based on Critical Voltage Monitoring

Erez Benjamin

M.Sc. Student of Prof. Krylov

Micro-electromechanical systems (MEMS) are embedded devices or systems composed of micro machined structures. Micro accelerometers are among the most promising and intensively researched applications of the MEMS technology. In this work we report on fabrication and experimental demonstration of force\acceleration sensing in a fully compliant contactless bistable structure, electrostatically actuated by a single parallel-plate transducer. The operational principle of the sensor is based on the monitoring of the stability boundaries of the device. Previous work found the stability properties of the device to be sensitive to fabrication tolerances. In the present work, careful mapping of the design parameters was carried out, which reduced the sensitivity to fabrication tolerances and assured the bistablility in the device.

The acceleration is measured by tracking the critical voltage corresponding to the instability point, where the sensitivity is enhanced. The proof mass is suspended using initially curved flexible beams pulled by nonlinear electrostatic forces directed along the beams. In contrast to conventional electrostatic actuators, where the so-called pull-in instability is followed by the collapse of the device to the electrode, in our device steep increase in the suspension's stiffness and appearance of an additional stable configuration of the device after the pull-in prevents the contact and eliminates irreversible damage of the structure. The model of the device built using the Rayleigh-Ritz method, incorporates structural geometric nonlinearity of the suspensions and the intrinsic electrostatic nonlinearity. To account for the influence of three-dimensional fringing fields, the electrostatic force was calculated numerically using the IntelliSuite software package. Devices of several configurations were fabricated from a silicon on insulator (SOI) substrate using deep reactive ion etching (DRIE) process.  The devices were operated in ambient air conditions, the motion was video recorded and the voltage–displacement dependence was built using image processing. First, the bistability of the device was demonstrated experimentally at zero acceleration.  Experimental data were in a good agreement with the model predictions. Next, the influence of an external force\acceleration was emulated with an additional parallel plate transducer. The sensitivity of 0.173 V/g shown in the experiments was in a reasonable agreement with the value of 0.149 V/g predicted by the model.