Microresonator-Based Sensors with Feedback-Enabled Nonlinearities
Security concerns in modern transportation and commerce systems have resulted in an urgent need for improvements in explosives detection technologies. The priorities for development include minimizing inconvenience to security agencies and civilians, and maximizing sensitivity while having reliable detection. Trace vapor detection is an important thrust among the various classes of detection technologies due to the potential for relative convenience and throughput, and because it would directly indicate the presence of dangerous materials. Vibration-based sensing using microelectromechanical systems (MEMS) or millimeter-scale devices has shown promise in sensitive mass detection across numerous application spaces. To date, many vibration-based sensing modalities have relied upon monitoring small shifts in the natural frequency of a system to detect structural changes which are attributable to the chemicals that are being detected. Often, this approach carries significant expense, due to the presence of electronics, such as precision phase locked loops or lock-in amplifiers, when high sensitivities are required. Bifurcation-based sensing modalities, in contrast, can produce large, easy to detect changes in response amplitude with high sensitivity to structural change. However, design of such devices for specific applications can be expensive, and fabrication yields of devices that exhibit desired nonlinearity with safe drive amplitudes can be poor. In this work, design and implementation of a tunable, Duffing-like electronic resonator was realized via nonlinear feedback electronics, which used a quartz crystal tuning fork as the device platform. The system in this manifestation used collocated sensing and actuation, along with readily available electronic components, to realize the desired behavior, creating a nonlinear resonator from a linear one. Another challenge for the design for resonant trace vapor sensors is the fact that the sensitivity of microresonators to added mass is dependent on the location of placement of the mass. A novel method was developed that leverages inkjet technology in order to deposit small masses across a device and measure the corresponding change in resonant frequency. This allows for the mapping of the spatial mass sensitivity of microresonators. The method was demonstrated on a 16 MHz quartz resonator, a candidate for sensitivity improvement for the sensing platform. The design of a feedback system to create the desired saddle-node bifurcation in the 16 MHz resonators resulted in additional concerns. High performance circuitry was required and time delay in the feedback loop became important. In order to minimize time delay, an alternative circuit design was created using a pair of diodes in order to approximate a piecewise-linear response, and successfully produced the desired bifurcation behavior. Approximate analytical models were developed to predict the response of the idealized piecewise-linear system as well as more realistic versions, incorporating the non-ideal diode response with a polynomial fit, and also incorporating the effect of small feedback time-delays. Finally, a sensing-focused version of the circuit was developed, and a proof-of-concept of a bifurcation-based trinitrotoluene vapor sensing system is presented with encouraging results.
Chiu, Purdue University.
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