The System-In-Package here presented manages the movement of a couple of piezoelectric mirrors used for Raster Scan Projection applications. Its partitioning consists in two dice stacked one above the other. The first die works in the High Voltage domain and is responsible of the driving of the mirrors; it includes a single coil dual output boost converter that produces high voltage supplies (up to 45V). The linear driver that works up to 42V is composed by a low voltage DAC and a rail-to-rail push-pull output amplifier. The resonant driver, implementing a charge recovery architecture, translates a low voltage pulsing signal into a synchronous high voltage trapezoidal signal that excites the resonant mirror. The second die works in the low voltage domain to sense and control the mirrors movement. The PZR Wheatstone bridge, one for each mirror, is biased with a programmable voltage. Its differential output is applied to a read-out chain composed by a programmable Analog Front-End and a Continuous-Time A/D Converter with 16-bits resolution. A fully integrated digital block processes the signals produced by the two sensing chains that are applied to their relative control loop algorithm. An additional front-end path is present to measure the PZR Bridge working temperature for sensitivity versus temperature compensation. Innovative lock-in and opening angle algorithms allows to control the movement of the resonant mirror against temperature, pressure and aging variations. Similarly, an innovative multi feedback PID algorithm controls the linear mirror movement mainly suppressing the fundamental resonant component as well as other spurious modes.
KEYWORDS: Micromirrors, Projection systems, Mirrors, Position sensors, Raster graphics, Control systems design, Analog electronics, Nulling interferometry, Image quality, Device simulation
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◦ , 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 . Experimental testing on an analog implementation of the resonance damping loop proves the validity of the approach.
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