System Identification, Design, and Implementation of Amplitude Feedback Control on a Nonlinear Parametric MEM Resonator for Trace Nerve Agent Sensing

In this paper, we develop and experimentally verify a novel amplitude feedback control scheme on a silicon microbeam for real-time sensing of parts-per-trillion (ppt) nerve agent concentrations. Utilizing the nonlinear dynamics resulting from parametric excitation of the microbeam for sensing at atmospheric pressure has demonstrated superior performance compared with conventional linear forced sensing. The microbeam is coated with a molecularly imprinted polymer that selectively adsorbs dimethyl methylphosphonate (DMMP) a precursor to G-series nerve agents including sarin. When the molecules attach to the microbeam and increase its mass, the amplitude response described by the nonlinear dynamics shifts in frequency. In tracking this frequency shift, information about the mass load, and, therefore, the nerve agent concentration, can be extracted. Previous sensing schemes have successfully applied this concept while relying on experimentally designed controllers. This paper features a model-based approach starting with the nonlinear dynamics and system parameter identification. After linearization, a control designed with the Glover-McFarlane H
_∞
loop-shaping procedure robustly stabilizes the system. The control scheme is implemented on a LabView field-programmable gate array. Water vapor and trace DMMP real-time experiments at atmospheric pressure demonstrate the control's functionality. A limit of detection of 13.3 ppt DMMP molecules in nitrogen is established.