Nonlinear-based MEMS sensors and active switches for gas and acceleration applications Mechanical Engineering

In this talk, we demonstrate the realization of smart sensors and actuators through the exploitation of principles of nonlinear dynamics at the micro scale. Specifically, we demonstrate combining sensing and actuation into a single device through what is called smart switches triggered by the detection of a desirable physical quantity. The concept aims to reduce the complexity of systems that rely on controllers and complex algorithms to realize on-demand trigger actions. In the first part of the talk, we discuss the category of switches triggered by the detection of gas. Toward this, electrostatically microbeams resonators are fabricated, then coated with highly absorbent polymers (MOFs), and afterward are exposed to gases. Such devices can be useful for instant alarming of toxic gases. In the second part, we demonstrate switches triggered by shock and acceleration. The concept is demonstrated on a millimeter-scale capacitive sensor. The sensor is tested using acceleration generated from shakers. Such devices can be used for the deployment of airbags in automobiles.


Part I: Programmable Switch
• The softening and hardening nonlinear behaviors of the microbeams are exploited to demonstrate the ideas. • For gas sensing, an amplitude-based tracking algorithm is developed to quantify the captured quantity of gas. • Then, a MEMS switch triggered by gas using the nonlinear response of the microbeam is demonstrated. • The proposed switch is promising for delivering binary sensing information, and also can be used directly to activate useful functionalities, such as alarming. • Lower electrode, composed of gold/chrome • Amorphous silicon sacrificial layer • Structural layer made of polyimide (PI) and metal on top.

MOF Coating
• Metal-Organic Framework MOFs are very promising porous materials for gas sensing applications • The clamped-clamped microbeam was coated with a MOF thin film using an inkjet printer using a nozzle with 20 µm of diameter. • We used Cu3(btc)2.xH2O MOF (btc is 1, 3, 5benzenetricarboxylate), also known as HKUST-1. •The linearly-fitted curve can be used to relate the amplitude change to the frequency shift, and hence, the amount of absorbed mass.
• Time history of the beam displacement upon gas exposure indicating the sudden jump down, and hence, the switching event.
•The vertical line indicates a fixed operating frequency of the resonator during vapor exposure.
•Clearly, the amplitude of the resonator increases with the absorption of vapor. Jump-up Switch Triggered by Gas

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•The frequency shift due to thermal fluctuations around the resonator can be related to the phase variation at a given frequency.
•A precision impedance analyzer connected to a PC with National Instruments data acquisition has been used to characterize the microbeam electrically. The phase evolution as a function of time at a fixed frequency has been shown. The phase noise has been calculated to be , which leads to a frequency shift .
where f res,0V = 86.8 kHz is the natural resonant frequency at V DC = 0V and  • The figure shows the gradual increase of the midpoint displacement of the microbeam during the ethanol vapor exposure. This increase in displacement is due to the shift of the pull-in band toward the operating frequency.

Switch Triggered by Mass
• The slope is | ⁄ | = 3.95 × 10 −3 ⁄ . • Based on the slope and the starting operating frequency, one can determine the frequency shift, and hence the absorbed mass, from the measured amplitude. • The calculated frequency shift to reach pull in is = 89.4 corresponding to an added mass threshold of Δm = 536 pg.
• The added mass threshold can be controlled by shifting the operating frequency to lower or higher values. For example, to decrease the mass threshold value, the operating frequency can be moved closer to the pull-in band up to the limit the noise permits ( = 60 ).
• We demonstrated the advantage of using the nonlinear response of an electrostatically actuated resonator for gas sensing. • The clamped-clamped microbeam is coated with HKUST-1 MOF, which is a very sensitive chemical layer. • We demonstrated that frequency shift can be tracked in nonlinear regime using the linearly fitted upper branch in hardening behavior • Two ideas of switches triggered by mass detection were demonstrated based on the jumps a resonator experiences in a hardening or a softening behavior. • Switch triggered by mass detection based on dynamic pull-in, which eliminates the need for controllers, was demonstrated.
Summary and Conclusions Part I Part II: Switch Trigger by Acceleration A new concept of switches (triggers) that are actuated at or beyond a specific level of mechanical shock or acceleration. The principle of operation of the switches is based on dynamic pull-in instability induced by the combined interaction between electrostatic and mechanical shock forces. These switches can be tuned to be activated at various shock and acceleration thresholds by adjusting the DC voltage bias.

Cantilever beams
The Device Under Investigation

Low-g Electrostatically Actuated Resonant Switch
• A new concept is presented of an electrostatically actuated resonant switch (EARS) for earthquake detection and low-g seismic applications.
• The resonator is designed to operate close to instability bands of frequency-response curves, where it is forced to collapse dynamically (pull-in) if operated within these bands. • By careful tuning, the resonator can be made to enter the pull-in instability zone upon the detection of the earthquake signal, thereby snapping down as an electric switch. Such a switching action can be functionalized for alarming purposes or can be used to activate a network of sensors for seismic activity recording.    • We presented experimental and theoretical investigation to the characteristics and performance of a new class of tunable thresholdacceleration switches actuated at or beyond a specific level of mechanical shock or acceleration. • The exploitation of the dynamic pull-in instability of electrostatically actuated resonators to realize a sensor and an actuator at the same time has been demonstrated. • The new concept of an electrostatically actuated resonant switch (EARS) for earthquake detection was validated numerically and experimentally, where measurements showed the EARS ability to switch due to a small amount of acceleration. • The EARS concept could be useful in many applications to do switching action or to activate different devices.