Dynamics of hybrid MEMS sensors and switches for mass and acceleration detection

This dissertation presents an investigation into dynamic instabilities and bifurcations in MEMS devices actuated electrostatically and their use to achieve new methods and functionalities for mass and acceleration sensing and detection. These instabilities are induced into a MEMS microstructure either by a combined loading of shock and DC voltage or by exciting it with nonlinear forcing composed of a DC voltage and an AC harmonic load.In the case of shock and DC loading, early dynamic pull-in instability was noticed and utilized to design a new mechanical switch triggered by shock or acceleration forces. The principle of operation for this switch was verified by simulation and experiment using a capacitive device. The switch can be tuned to be activated at various shock and acceleration thresholds by adjusting the DC voltage bias. Our results indicate that designing these new switches to respond quasi-statically to mechanical shock makes them robust against variations in shock shape and duration. More importantly, quasi-static operation makes the switches insensitive to variations in damping conditions. This can be promising to lower the cost of packaging for these switches since they can operate in atmospheric pressure with no hermetic sealing or costly package required.In the case of AC and DC, the frequency of the AC load is tuned to be near the fundamental natural frequency of the structure (primary resonance) or its multiples (subharmonic resonance). For each excitation method, local bifurcations, such as saddle-node and pitchfork, and global bifurcations, such as the escape phenomenon may occur. Four different MEMS microstructures were used as case studies to reveal experimentally these interesting nonlinear behaviors. One of the tested devices (a capacitive accelerometer) was used in a through theoretical and experimental investigation to explore the utilization of these bifurcations to design novel mass sensors and switches of improved characteristics. One explored concept of a device is a switch triggered by mass threshold (STMT). This device has the potential of serving as a smart switch that combines the functions of two devices: a sensitive gas/mass sensor and an electro-mechanical switch. We address the STMT ability to reject noises and disturbances using a delayed feedback controller. The controller is shown to be promising to enhance the switch stability against shock load and disturbances.A second type of explored devices is a Mass Sensor of Amplified Response (MSAR). The basic principle of this device is based on the jump phenomena encountered in pitchfork bifurcations during mass detection. This leads to an amplified response of the excited structure making the sensor more sensitive and its signal easier to be measured.