Capacitive Sensing for 2-D Electrostatic MEMS Scanner in a Clinical Endomicroscope

A flexible fiber-coupled confocal laser endomicroscope has been developed using an electrostatic micro-electro-mechanical system (MEMS) scanner located in at distal optics to collect in vivo images in human subjects. Long transmission lines are required that deliver drive and sense signals with limited bandwidth. Phase shifts have been observed between orthogonal <inline-formula> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula>- and <inline-formula> <tex-math notation="LaTeX">${Y}$ </tex-math></inline-formula>-scanner axes from environmental perturbations, which impede image reconstruction. Image-processing algorithms used for correction depend on image content and quality, while scanner calibration in the clinic can be limited by potential patient exposure to lasers. We demonstrate a capacitive sensing method to track the motion of the electrostatically driven 2-D MEMS scanner and to extract phase information needed for image reconstruction. This circuit uses an amplitude modulation (AM) envelope detection method on shared drive and sensing electrodes of the scanner. Circuit parameters were optimized for performance given high scan frequencies, transmission line effects, and substantial parasitic coupling of drive signal to circuit output. Extraction of phase information further leverages nonlinear dynamics of the MEMS scanner. The sensing circuit was verified by comparing with data from a position sensing detector (PSD) measurement. The phase estimation showed an accuracy of 2.18° and 0.79° in <inline-formula> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula>- and <inline-formula> <tex-math notation="LaTeX">${Y}$ </tex-math></inline-formula>-axes for motion sensing, respectively. The results indicate that the sensing circuit can be implemented with feedback control for precalibration of the scanner in clinical MEMS-based imaging systems.