The adoption of MEMS mirrors has been rapidly growing in recent years, involving different types of applications. One of the most challenging applications for Laser Beam Scanning (LBS) displays are Augmented Reality (AR) glasses, since they require high resolution images, small volume occupation and low power consumption. To comply with these requirements, MEMS scanners are moving to piezoelectric actuation, a technology that enables high actuation forces and high efficiency in a small die area. In this work, two compact MEMS mirrors with piezoelectric actuation are presented: a 27.5kHz resonant mirror (MMR40100) with 1.1mm diameter and 56deg FOV and a quasi-static mirror (MML40100) working at 60Hz with 2.45x1.44mm2 reflective area and 32deg FOV. The working voltage is <40V for both mirrors, keeping the power consumption low (<20mW). To enable the mirror control, diffused piezoresistive sensors in Wheatstone bridge configuration are integrated in both mirrors. In this paper it will be described the working principle of the MEMS designs, the manufacturing process, the FEM simulations and the experimental findings obtained on fabricated samples. Once coupled together, the two mirrors enable a 720p display module with 65deg diagonal FOV and a volume occupation of the entire display module of <0.7cc making this solution one of the most promising for the next-generation AR glasses.
MEMS mirrors are at the core of miniaturized projection systems based on Laser Beam Scanning (LBS) which are involved in a wide range of applications in the field of Augmented/Mixed Reality and LiDAR. Most of these applications require at least one axis to be scanned with constant velocity by a quasi-static micromirror, which needs a good driving voltage to angle linearity. In practical implementations, electro-mechanical response of quasi-static micromirrors is strongly affected by MEMS nonlinearities which can be pure mechanical, like geometric nonlinearity, or can arise from the actuation principle, like electrostatic softening in comb-finger actuators or the hysteresis in piezoelectric actuators. As a result, the open loop response of the opening angle can significantly deviate from the ideal linear one affecting the final system performance. Piezoelectric micromirrors are becoming even more common in the current LBS-based product scenario. A proper and accurate modeling technique for handling the mirror scanning trajectory affected by piezoelectric hysteresis is needed for setting up the most appropriate control strategy. In this work, a modeling approach based on Bouc-Wen model describing the hysteretic behavior of piezoelectric MEMS mirrors is presented. Furthermore, a mono-axial quasi-static PZT actuated MEMS mirror has been investigated through a characterization phase focusing on the relation between the input driving voltage and the output scanning angle. A final validation stage with a comparison between collected data and model results is reported.
MEMS mirrors are used in projection systems based on Laser Beam Scanning LBS and hence they are involved in applications as Augmented Reality Headset and LiDAR. Characterization of the electro-mechanical behavior of MEMS structures plays an important role in the following control strategy definition and system integration. A mono-axial resonant PZT actuated MEMS mirror is addressed and the characteristics of the mechanical structure is investigated by measuring the frequency response of the functional and the main spurious vibrational modes. In addition, a dedicated analysis on the nonlinear behavior is performed Calibration stations with camera sensors are used for optical and electrical analyses while vibrometric and stroboscopic measurement techniques are employed for detecting micrometric movements of the structure. A piezoresistive sensor with Wheatstone bridge configuration is built on board of the MEMS mirror for tracking mirror position during actuation. The combined effects of the functional and spurious modes on the sensor frequency response are evaluated and compared with the MEMS mirror model. In this work, a full characterization on the electro-mechanical parameters of a resonant mono-axial PZT actuated MEMS mirror is presented and a detailed comparison between collected data and model prediction is reported.
Piezoelectric thin films have gained attention as key materials for the actuation of micro devices since they provide high drive forces compared to others actuation mechanisms. One of the most challenging application field is the design of quasistatic MEMS scanners, in which is difficult to have a high torsional angle and large reflective area while maintaining a good device robustness. In this work a monoaxial, quasi-static MEMS mirror with PZT actuation will be presented, surpassing the limitations found in the state-of-the-art solutions. The mirror has a large reflective area in the visible spectrum (4x3mm with aluminum coating) and the half mechanical opening angle is of 9deg (corresponding to a field of view of 36deg) with a relatively low actuation voltage (40V). To enable the mirror control, a diffused piezoresistive sensor in a Wheatstone bridge configuration is integrated in the technology platform. The innovative mirror design guarantees high resonant frequencies of the torsional mode (>500Hz) and higher spurious modes (>2.5kHz), which allow a quasistatic actuation for the typical display refresh rates (up to 120Hz). The high frequencies of the translational spurious modes guarantee a high robustness against shocks and vibrations, which makes this mirror suitable for consumer or automotive products. In this paper it will be described the working principle of the patented MEMS design, the manufacturing process, the FEM simulations and the experimental findings obtained on fabricated samples.
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