This paper presents a new, low-voltage, and small-form-factor analog front-end (AFE) circuit that measures the torsional angle of an electrostatically-actuated, quasi-static MEMS mirror, while it operates at a frame rate of 50 Hz to 60 Hz or higher, following a given angle profile, such as a sawtooth or triangle profile. The quasi-static MEMS mirror is actuated with two high voltage (HV) differential signals. To enforce this quasi-static MEMS mirror to track a given angle profile or trajectory with minimum error, the MEMS mirror is driven by feedback control that requires a sensor capable of measuring its mechanical or optical angle in real time with high enough bandwidth and sensitivity. Hence, we design and implement the angle sensing circuit (ASC) that meets the low-power, low-cost, and small-form-factor requirements to reduce power consumption, size, and weight for AR applications. This ASC consists of a variable gain amplifier (VGA) and an envelope detector that operates at 3.3 V and draws about 1 mA during operation.
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 resonance, 2f0, through parametric excitation. We also validate this concept of phase measurement through a series of experiments with a Laser Doppler Vibrometer (LDV) that optically measures its rotational direction and is used as a reference, as well as a 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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