Biaxial MEMS Research Articles

Simple modulation of Lissajous MEMS laser beam scanning with reconfigurable structured light patterns for 3D imaging

Structured light 3D imaging systems commonly employ panel-based projectors or 1-axis MEMS mirrors with beam expander lens to project multi-frame barcodes or dot clouds, addressing challenges posed by objects with multi-scale feature sizes. However, these methods often result in large system volumes due to the required projection multi-lens modules, high hardware costs, or limited light pattern generation capabilities that hindering measurement precision enhancement. This paper introduces an innovative approach to reconfigurable spatial light pattern projection using a single bi-axial MEMS mirror with Lissajous scanning. In contrast to the pixel-by-pixel pre-defined image patterns encoding of conventional 2D laser beam scanning, the proposed method simply aligns the MEMS bi-axial resonance frequencies with laser pulse modulation, enabling the projection of diverse structured light patterns such as stripes, lines, dot matrices, and random dot clouds, which can adapt to different 3D imaging algorithms demands. It eliminates the need for multi-frame encoding and streamlines data caching, simplifies digital logic hardware. A prototype 3D imaging system was developed to demonstrate the mathematical model for laser modulation and the technical feasibility based on the proposed principle. Beyond its lens-free essence, the system supports focal-free optics and a compact projection form factor, which accommodates to a broad range of projection distances and field-of-views based on object’s location. 3D depth map of polynomial surface and blocks objects are extracted through single-frame pattern projection with a relative high accuracy. The presented modulation theory for diverse structured light pattern generation opens avenues for versatile and compact 3D imaging applications of LiDAR and robotic 3D vision.

Temperature effect on the modal frequencies of a steel railway bridge

This paper presents the dynamic identification of the Ostiglia-Revere railway bridge, a steel truss girder bridge located in Northern Italy. The bridge is 6.6 m wide, about 940 m long and composed of 12 spans. The accelerations of a bridge span caused by both ambient excitation and train passages have been continuously measured from August to November. The monitoring system consists of 4 temperature sensors and 4 biaxial MEMS accelerometers, acquiring accelerations with a sampling frequency of 80 Hz. Modal properties are estimated adopting two different identification approaches, namely the Enhanced Frequency Domain Decomposition and the Stochastic Subspace Identification. Particular attention is paid to two crucial issues for vibration-based structural health monitoring: the recognition of the same structural mode from results identified during different time windows (i.e., mode clustering), and the temperature effect on estimated modal properties. As the latter is concerned, two regressive models, namely a linear regression model and an ARX model, are fitted to the frequency-temperature data, strongly reducing the eventuality of false vibration-based damage detections in the future.

Thermal Behavior of Biaxial Piezoelectric MEMS-Scanners

This paper presents the thermal behavior of non-resonant (quasi-static) piezoelectric biaxial MEMS scanners with Bragg reflectors. These scanners were developed for LIDAR (LIght Detection And Ranging) applications using a pulsed 1550 nm laser with an average power of 2 W. At this power, a standard metal (gold) reflector can overheat and be damaged. The Bragg reflector developed here has up to 24 times lower absorption than gold, which limits heating of the mirror. However, the use of such a reflector involves a technological process completely different from that used for gold and induces, for example, different final stresses on the mirror. In view of the high requirements for optical power, the behavior of this reflector in the event of an increase in temperature needs to be studied and compared with the results of previous studies using gold reflectors. This paper shows that the Bragg reflector remains functional as the temperature rises and undergoes no detrimental deformation even when heated to 200 °C. In addition, the 2D-projection model revealed a 5% variation in optical angle at temperatures up to 150 °C and stability of 2D scanning during one hour of continuous use at 150 °C. The results of this study demonstrate that a biaxial piezoelectric MEMS scanner equipped with Bragg reflector technology can reach a maximum temperature of 150 °C, which is of the same order of magnitude as can be reached by scanners with gold reflectors.

Biaxial film bulk acoustic resonator magnetic sensor based on the Fe80Ga20 anisotropic ΔE effect

With the increasing application of personal navigation systems in consumer electronics, the demand for multi-axis magnetic sensors based on MEMS is growing. We report a biaxial MEMS DC magnetic sensor consisting of an Mo/AlN/Fe80Ga20 film bulk acoustic resonator, with anisotropy ΔE effect-based sensing principle. Different from the previously reported 1D magnetic sensor based on the ΔE effect, the anisotropic ΔE effect was used to realize in-plane and out-of-plane 2D magnetic field responses on a discrete sensor, and the sensor had two readout methods: resonant frequency f and return loss S11. The magnetic sensor realized the resonant frequency f shifted by 1.03 MHz and 0.2 MHz in the 567 Oe in-plane magnetic field and 720 Oe out-of-plane magnetic field, respectively, and the S11 changes by −30.2 dB and −0.92 dB. As the applied magnetic field increases, the −3 dB bandwidth quality factor Q 3dB of the S11 curve gradually increases, and its maximum values in the in-plane and out-of-plane magnetic fields are 77 143 and 1828, respectively, which reduces the detection limit of the magnetic sensor. The resonant magnetic sensor has stable high linear temperature and frequency drift characteristics, and its temperature frequency coefficient is −48.7 ppm °C−1.

Underwater 3D Scanner Model Using a Biaxial MEMS Mirror

Underwater 3D laser scanners are an essential type of sensor used by unmanned underwater vehicles (UUVs) for operations such as navigation, inspection and object recognition and manipulation. These sensors need to be able to provide highly accurate 3D data at fast refresh rates in order to accomplish these tasks. Usually, these scanners rely on a rotating mirror actuated by a galvanometer. However, the light planes steered by this type of mirrors are typically deformed into cones due to refraction. In order to produce accurate results, this distortion needs to be taken into account, which increases the computational cost of the 3D reconstruction. A novel approach consisting in using a biaxial MEMS mirror is proposed in this paper. The second rotational degree of freedom of the mirror can be used to project optimally curved light shapes, so that the refraction process transforms them into planes. Being able to model the light surfaces as planes rather than cones can significantly reduce the computation time of the 3D reconstruction. In order to do so, an exhaustive model of the complete light trajectories is presented. To the best of the authors’ knowledge, this paper constitutes the first attempt to model and counteract the distortion in the scanning pattern introduced by a biaxial mirror and a double refraction process in the context of underwater robotics.

Effects of Natural Aging in Biaxial MEMS Accelerometers

Effects of a fully natural aging of MEMS accelerometers are evaluated with regard to changes in their performance. Two models of commercial dual-axis accelerometers (two pieces of ADXL 202E and 203 by Analog Devices Inc.) with analog outputs were tested over a period of about 10 and 4 years, respectively. A custom computer controlled test rig was used for performing relevant experimental studies, employing the gravitational acceleration as the reference source. A methodology of determining the proposed indicators of aging phenomena is presented and discussed. Changes of the offset voltage and the scale factor were observed and a way of evaluating the overall error due to combined influence of these two parameters is proposed. It was found out that the changes of the output voltage generated by the tested accelerometers were considerable, resulting in respective maximal errors of about 52 mg (2.6%) (ADXL 202E) or 20 mg (1%) (ADXL 203). Simple ways of reducing the effects of aging are proposed.

Design and Configuration of Folded Platonic Strapdowns of Biaxial MEMS Accelerometers

Abstract The authors report on the design, configure, and test of isotropic accelerometer strapdowns for high-precision inertial measurement unit (IMU) and folded MEMS configuration. The biaxial low-g MEMS accelerometers are based on the Platonic solids. A Platonic strapdown is integrated into an embedded system for acceleration-array signal acquisition targeting rigid-body pose-and-twist estimation. The electromechanical properties for dynamic sensitivity are tested on a haptic manipulator, which shows that the position estimation matches reasonably well the encoder readouts. The Platonic strapdown is promising in folded MEMS IMU with chip-level miniaturization and high estimation precision.

Design of a biaxial high frequency-ratio low-g MEMS accelerometer

The optimum design of a biaxial MEMS accelerometer for low-g applications, along with its fabrication and testing, is reported in this paper. The monolithic structure was optimally designed based on a fully symmetric architecture with a high frequency ratio between the insensitive and the sensitive axes. The sensing substructure was designed, in turn, with a configurable comb-structure for simultaneous biaxial capacitive sensing. This accelerometer was fabricated with high precision and tested under 1-g acceleration, both statically and dynamically. The corresponding static sensitivity is 1.55 μm/g, while the capacitance sensitivity is 113.5 fF/g.

A MEMS Actuator System for an Integrated 3-D Optical Coherent Tomography Scanner

In this paper, we present an electrothermal biaxial MEMS actuator system, which provides $x$ - and $y$ -direction scanning for a fully integrated 3-D optical coherence tomography (OCT) scanner. An angular scanning range of 8° (corresponding to a 7-mm linear scanning range in both directions) is achieved, with an average power consumption of 150 mW. The resonant frequency is 668 and 297 Hz for x- and y-directions, respectively. With a footprint of only $2.5\times2.5$ mm2, this system is part of a device which also integrates an optical waveguide and a collimated lens on the same chip, thus making the fully integrated, self-aligned, and miniaturized 3-D OCT scanners feasible. [2017-0268]

An electrostatically driven 2D micro-scanning mirror with capacitive sensing for projection display

Bi-axial or two-dimensional (2D) MEMS micro-scanning mirrors (or micro-scanners) are considered the key component for laser scanning projectors. Many studies have shown the mechanical characterization of fabricated devices driven by various mechanisms. This work presents an electrostatically driven bi-axial micro-scanner with capacitive position sensing for Lissajous scanning projection. With the added sensing capability, a PLL (phase-locked loop)-based oscillator loop is developed to sustain mechanical resonance and to provide mirror position information, which are equally important for practical applications. The micro-scanner and the required circuits are implemented using bulk micromachining SOI (silicon on insulator) and 0.35-μm CMOS (complementary metal oxide semiconductor) technologies, respectively. The measured resonant frequencies of the bi-axial micro-scanner for the slow and fast-axis scans are 1.4 and 21.9kHz, and the associated optical scan angles are 22.5° and 40°, respectively, under pulse modulation of 48 and 115Vpp. The fabricated micro-scanner is adopted in a laser beam scanning projection system to achieve WVGA (852×480) display resolution.

High-Q MEMS Resonators for Laser Beam Scanning Displays

This paper reports on design, fabrication and characterization of high-Q MEMS resonators to be used in optical applications like laser displays and LIDAR range sensors. Stacked vertical comb drives for electrostatic actuation of single-axis scanners and biaxial MEMS mirrors were realized in a dual layer polysilicon SOI process. High Q-factors up to 145,000 have been achieved applying wafer level vacuum packaging technology including deposition of titanium thin film getters. The effective reduction of gas damping allows the MEMS actuator to achieve large amplitudes at high oscillation frequencies while driving voltage and power consumption can be minimized. Exemplarily shown is a micro scanner that achieves a total optical scan angle of 86 degrees at a resonant frequency of 30.8 kHz, which fulfills the requirements for HD720 resolution. Furthermore, results of a new wafer based glass-forming technology for fabrication of three dimensionally shaped glass lids with tilted optical windows are presented.

Characteristics of a novel biaxial capacitive MEMS accelerometer

A novel MEMS accelerometer with grid strip capacitors is developed. The mechanical and electrical noise can be reduced greatly for the novel structure design. ANSOFT-Maxwell software was used to analyze the fringing electric field of the grid strip structure and its effects on the designed accelerometer. The effects of the width, thickness and overlapping width of the grid strip on the sensing capacitance are analyzed by using the ANSOFT-Maxwell software. The results show that the parameters have little effect on the characteristics of the presented accelerometer. The designed accelerometer was fabricated based on deep RIE and silicon–glass bonding processes. The preliminary tested sensitivities are 0.53 pF/g and 0.49 pF/g in the x and y axis directions, respectively. A resonator with grid strip structure was also fabricated whose tested quality factor is 514 in air, which proves that the grid strip structure can reduce mechanical noise.

NiFe Plated Biaxial MEMS Scanner for 2-D Imaging

A two-axis microelectromechanical systems micromirror actuator is developed for retinal scanning display and imaging applications. The device operation makes use of magnetostatic torque produced by the combination of a flux generating custom-made high-frequency electrocoil and the NiFe layer deposited on the movable part. Modeling of the actuation in the magnetic domain, as well as the experimental characterization of the mechanical part is described. The device is capable of full optical scan angles of 88deg (at 100-mA root-mean-square coil current) and 1.8deg for slow and fast-scan directions, respectively. In combination with a mirror size of 1.5 mm, resulting thetas <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">opt</sub> middotD products are 132degmiddotmm and 2.7degmiddotmm for slow and fast axis, respectively. Atmospheric operation of the device is enabled due to high mechanical quality factors of the order of 3000