A novel closed-loop architecture for accurate micromirror trajectory control in linear scanning MEMS-based projectors

In the field of raster scanning projectors, linear micromirrors used for image scan along the vertical axis are driven by a sawtooth waveform, whose frequency is related to image refresh rate (typically 60-120 Hz). Such driving profile with a fast retrace (10% of the period) excites the fundamental resonant mode, stimulating unwanted ringing of the tilt angle that worsens image quality, requiring a compensation. Open-loop solutions based on pre-distortion of the driving profile require accurate calibration of the device and do not offer enough versatility among different micromirrors. Embedded position sensors enable implementation of closed-loop techniques: this work presents an innovative linear control strategy for mirrors with piezoresistive position sensing, which allows to achieve accurate tracking of the control signal while suppressing resonance. The concept is based on a control approach: the goal is to damp the mirror quality factor, while achieving accurate tracking, within few tens of m degrees, and compensate for mechanical non-linearities by nulling the error between angle and control signal. An analog implementation is studied on a theoretical basis, to determine the fundamental limit in terms of tracking accuracy and noise. Then, a more versatile design is presented, where the controllers are implemented digitally to cover a wide range of mirror parameters. Analytical/behavioral simulations show the capability to achieve accuracy within 20 m degrees. Experimental testing on an analog implementation of the resonance damping loop proves the validity of the approach.