Research
Energy Harvesting: Due to the recent advances in the fields of fabrication, computing, and electronics, small size and low power-consumption sensors and actuators have become a reality. New devices that operate with minimal power requirements invaded the consumer market and are now an essential part of our daily life. To cope with the rapid growth in low-power consumption technologies, researchers have established a new and flourishing area in vibrations known as vibration-based energy harvesting. This science deals with scavenging of otherwise waste mechanical energy and transforming it into electrical energy to self-power and maintain a device operation. Energy harvesting may prove very useful in powering remote sensor networks, bio-implants, and security devices.

Our research interest lies in the modeling analysis and optimization of energy harvesting devices. This includes formulating a deep understanding of the effect of design parameters on the optimal power, utilizing different active materials and comparing their effectiveness, and addressing the effect of the nonlinearities on the output power.

Primary and Autoparametric Resonances in Nonlinear Delay Differential Equations:

Despite the significant body of research that deals with the stability and stabilization of delay systems, most of the previous efforts were directed towards characterizing the stability of the free response by proposing various methodologies to predict and estimate the location of the eigenvalues relative to the imaginary axis. Little attention has been paid to understanding the effect of time delays on the response of nonlinear externally-excited systems. In particular, the nonlinear response of a delayed system to primary-resonance excitations has yet to be addressed comprehensively. Such studies were not necessary in the past due to the limited number of applications in which time delays and external excitations coexist in the operation of a dynamic system. Currently, and due to the emergence of micro and nanodevices as the next generation sensors and actuators, this type of analysis is becoming more imperative. For instance, to realize large dynamic responses, microdevices are usually excited at one of their resonant frequencies. Further, to enhance their dynamic characteristics, feedback control algorithms are being implemented to close the loop and provide real-time information about the states. However, due to their high natural frequencies, the presence of the infinitesimal measurement delays in the control loop can be of the order of magnitude of the response period, thereby channeling energy into or out of the system at incorrect time intervals and producing instabilities that render traditional controllers' performance ineffective.

Nonlinear Dynamics and Control of Suspended Objects: 

Cranes play a very important role in transportation and construction. As a result, there is an increasing demand on faster, bigger, and more efficient cranes to guarantee fast turn-around time, while meeting safety requirements. Current figures indicate that more than 95% of cargo tonnage that moves in and out of the United States relies on ocean transportation. Over seven million loaded marine containers enter U.S. ports each year. Current growth predictions indicate that container cargo will quadruple in the next twenty years. To cope with this rapid growth, our work focuses on developing new control strategies that guarantee faster transfer maneuvers while minimizing the transient and steady-state oscillations of the load. The control strategies include new input-shaping methodologies, delayed-feedback algorithms, and hybrid control techniques.

Nanomechanical Cantilever Sensors:

 When Atomic Force Microscopy (AFM) introduced microcantilevers as a tool for characterizing surface structure and stresses in solids, scientists soon discovered that molecular adsorption on one side of these cantilever generates enough surface stress energy that could culminate in measurable deformations. With cost-effective means for fabrication in place, microcantilevers were readily available for further experimental studies transducing chemical and biological processes into micromechanical motion.  Soon, they were realized to be the ideal choice for detecting the most infinitesimal mechanical responses generated by molecular interactions. As specifically illustrated in the figure, chemical reactions occurring on one side of the sensor result in surface stress changes that cause the cantilever to deflect and shift its resonance-frequency.  These chemically-induced mechanical forces can be estimated by measuring the cantilever deflection (static mode) and/or its resonance-frequency shift (dynamic mode). Our research is mainly concerned with modeling, dynamic characterization, sensitivity enhancement, and self-sensing methodologies for these cantilevers. As of today, our main concerns and research direction lie in the realization of sensors that are extremely sensitive, self-contained, and could work as a stand alone unit.                                                                                                                                                                                                   

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