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Phase detection to measure rotational direction of resonant MEMS mirror driven by parametric excitation



This paper introduces a new method of detecting the rotational direction of an electrostatic, resonant MEMS mirror with in-plane comb (angular vertical comb) fingers driven by parametric excitation, in which its rotational direction strongly depends on its initial condition due to the nature of its actuation method. In AR applications, this ambiguity on its rotational direction could cause a projected image to be flipped in its scanning direction. To avoid this ambiguity, its rotational direction has to be determined before image projection. To do that, its motion-induced current is measured with a transimpedance amplifier (TIA), and the phase of the motion-induced current at its resonance, f0, is determined through quadrature demodulation, while the resonant MEMS mirror is driven at twice its torsional, resonant frequency, 2f0, through parametric excitation. We also validate this concept of phase measurement through a series of experiments with the Laser Doppler Vibrometer (LDV) that optically measures its rotational direction and is used as a reference, as well as the Lock-In Amplifier (LIA) that electrically measures the phase of its motion-induced current at its resonance, f0, with respect to the optical reference signal from LDV.

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