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1. A MEMS Threshold Acceleration Switch Powered by a Triboelectric Generator

   https://doi.org/10.1115/DETC2018-85543  

   Pallay, Mark ; Ibrahim, Alwathiqbellah I. ; Towfighian, Shahrzad ( October 2018 , Proceedings of the ASME 2018 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   In this paper a novel electrostatic MEMS combined shock sensor and normally-closed switch is presented. The switch uses combined attractive and repulsive forcing to toggle a cantilever beam to and from the pulled-in position. The attractive force is generated through a parallel plate electrode configuration and induces pull-in. The repulsive force is generated through electrostatic levitation from a third electrode and serves to pull the beam out of its pulled-in position. A triboelectric transducer converts impact energy to electrical energy to provide voltage for the third electrode, which temporarily opens the switch if enough impact energy is supplied. Triboelectricity addresses the high voltage requirement for electrostatic levitation. The multi-electrode sensor also addresses the low current output from the generator because it acts as an open circuit between the parallel plate and levitation electrodes. A theoretical model of the switch is derived to analyze stability and the dynamic response of the cantilever. Threshold voltages to pull-in and release the beam through repulsive forcing is calculated. Output voltage plots from a prototype generator under a single impact are applied to the sensor-switch model to demonstrate the working principle of the sensor-switch is feasible. 

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2. Fabrication and Experimental Characterization of a MEMS Microphone Using Electrostatic Levitation

   https://doi.org/10.1109/SENSORS43011.2019.8956879  

   Ozdogan, Mehmet ; Towfighian, Shahrzad ; Miles, Ronald N. ( October 2019 , 2019 IEEE SENSORS)

   Fabrication and acoustic performance of a microelectromechanical systems (MEMS) microphone are presented. The microphone utilizes an unusual electrostatic sensing scheme that causes the sensing electrode to move away, or levitate from the biasing electrode as the bias voltage is applied. This approach differs from existing electrostatic sensors and completely avoids the usual collapse, or pull-in instability. In this study, our goal is to fabricate a MEMS microphone whose sensitivity could be improved simply by increasing the bias voltage, without suffering from pull-in instability. The microphone is tested in our anechoic chamber and a read-out circuit is used to obtain electrical signals in response to sound pressure at various bias voltages. Experimental results show that the sensitivity increases approximately linearly with bias voltage for bias voltages from 40 volts to 100 volts. The ability to design electrostatic sensors without concerns about pull-in failure can enable a wide-range of promising sensor designs. 

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3. A MEMS Microphone Using Repulsive Force Sensors

   https://doi.org/10.1115/DETC2016-60171  

   Ozdogan, Mehmet ; Towfighian, Shahrzad ( August 2016 , ASME International Design Engineering Technical Conferences)

   We present a MEMS microphone that converts the mechanical motion of a diaphragm, generated by acoustic waves, to an electrical output voltage by capacitive fingers. The sensitivity of a microphone is one of the most important properties of its design. The sensitivity is proportional to the applied bias voltage. However, it is limited by the pull-in voltage, which causes the parallel plates to collapse and prevents the device from functioning properly. The presented MEMS microphone is biased by repulsive force instead of attractive force to avoid pull-in instability. A unit module of the repulsive force sensor consists of a grounded moving finger directly above a grounded fixed finger placed between two horizontally seperated voltage fixed fingers. The moving finger experiences an asymmetric electrostatic field that generates repulsive force that pushes it away from the substrate. Because of the repulsive nature of the force, the applied voltage can be increased for better sensitivity without the risk of pull-in failure. To date, the repulsive force has been used to engage a MEMS actuator such as a micro-mirror, but we now apply it for a capacitive sensor. Using the repulsive force can revolutionize capacitive sensors in many applications because they will achieve better sensitivity. Our simulations show that the repulsive force allows us to improve the sensitivity by increasing the bias voltage. The applied voltage and the back volume of a standard microphone have stiffening effects that significantly reduce its sensitivity. We find that proper design of the back volume and capacitive fingers yield promising results without pull-in instability. 

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4. A Combined MEMS Threshold Pressure Sensor and Switch

   https://doi.org/10.1109/SENSORS43011.2019.8956554  

   Pallay, Mark ; Towfighian, Shahrzad ( October 2019 , 2019 IEEE SENSORS)

   In this study, a combined threshold pressure sensor and switch is introduced. The sensor detects when the pressure drops below a threshold value and automatically triggers a switch without the need for any computational overhead to read the pressure or trigger the switch. This system exploits the significant fluid interaction of a MEMS beam undergoing a large oscillation from electrostatic levitation to detect changes in ambient pressure. The levitation electrode configuration is combined with a parallel-plate system by adding an extra voltage to an electrode that is traditionally grounded, giving the system the ability to simultaneously act as a switch by toggling to and from the pulled-in position. It is experimentally demonstrated that the pressure sensing/switching mechanism is feasible and the threshold pressure to trigger the switch can be controlled by adjusting the voltage applied to the switch electrode. 

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5. Threshold Pressure Sensing Using Parametric Resonance in Electrostatic MEMS

   https://doi.org/10.1109/SENSORS43011.2019.8956679  

   Hasan, Md Nahid ; Pallay, Mark ; Towfighian, Shahrzad ( October 2019 , 2019 IEEE SENSORS)

   This study illustrates the concept of threshold pressure sensing using the parametric resonance of an electrostatic levitation mechanism. The electrostatic levitation allows the oscillations in the opposite direction of the substrate, thereby not limited to small gaps. The pressure sensor detects the pressure drop below a threshold value by triggering the parametric resonance with significant peak to peak dynamic amplitude changes (~ 25 𝝁𝒎). This detection relies on the fact that the instability region expands when the pressure drop forces the amplitude jump up to the higher oscillation branch. This significant change in the resonator amplitude can be related to a large capacitance variation indicating the threshold pressure. A mathematical model of the resonator is presented to show the working principle of the sensor through frequency response. Our experimental results show that the threshold pressure the sensor detects, can be adjusted by the AC voltage it receives. 

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