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2-Axis MEMS Scanner for a Laser Range Finder I. Aoyagi, K. Shimaoka, S.Kato MEMS Device Lab. Power Electronics Research Div. TOYOTA Central R&D labs., Inc. Aichi, Japan W. Makishi, Y. Kawai, S. Tanaka, T. Ono, M. Esashi, K. Hane Tohoku University Miyagi, Japan Abstract— A moving-magnet-type 2-axis MEMS scanner with a rotation angle detector has been developed. The fabricated MEMS scanner demonstrated a raster scan and 2-axis rotation angles detection with one Hall sensor. Keywords-component; MEMS, scanner, Laser range finder I. INTRODUCTION In recent years, sensors that detect forward obstacles have been developed for automotive safety systems, such as adaptive cruise control and pre-crash safety. The sensors required high spatial resolution and miniaturization. We have been developing the laser range finder (LRF) using a 2-axis MEMS scanner, which has the possibility of meeting these requirements. Figure 1 shows the schematic diagram of the LRF, which is composed of a laser diode (LD), a collimation lens, the MEMS scanner, a focus lens, and a photo diode (PD). A laser beam from the LD is scanned by the MEMS scanner and the distance to obstacles is measured by a time-of flight method. The MEMS scanner for the LRF needs four important points. The first point is to have a large mirror in millimeter size. To realize a long measurement range, a high power LD is needed. As the high power LD has a wide divergence angle, a collimated beam becomes wide. Therefore, the mirror has to be large. The second point is a large rotation angle for measuring a wide region. The third point is to realize a raster scan to make uniform scan of the laser beam. Therefore, at least one axis of the MEMS scanner is better to be driven in non-resonant mode. The fourth point is accuracy of satisfying specifications for an automotive sensor. To satisfying the fourth point, detecting the rotation angle of the MEMS mirror and the feedback control is necessary. In this paper, we present results of the fabricated MEMS scanner with a rotation angle detector. Figure 1. Schematic diagram of the laser range finder II. PRINCIPLE AND DESIGN As a driving principle, we selected an electromagnetic actuation, which is suitable for a large mirror, a wide rotation angle, and the non-resonant drive. The electromagnetic actuation has two different types. One is a moving coil type in which driving coils are patterned on a moving part [1]. The other is a moving magnet type in which permanent magnets are mounted on a moving part [2]. We selected the moving magnet type because this type doesn’t need to form an electric wire on a moving part and have no risks of being broken. Figure 2(a) shows a structure of a moving-magnet-type 2D-MEMS scanner, which consists of a MEMS mirror with a movable mirror, a movable frame, and two pairs of electromagnets (Emags). One pair is a mirror-drive Emag and the other pair is a frame-drive Emag. The movable mirror is 5mm x 5mm in size. The movable mirror and frame is suspended by torsion beams and rotates about the Y and X axes, respectively. The movable mirror has two mirror-drive magnets, and the movable frame has two frame-drive magnets. These four permanent magnets are magnetized in the Z direction and were glued to penetrating holes. By applying electric current to the Emags, a magnetic field H is generated. The magnetic field H works on the permanent magnet with the magnetization M and the volume V. As a result, the torque T represented in equation (1) is generated. T =V |M ×H| (1) When the mirror-drive Emag provides the magnetic field in the X direction, the movable mirror rotates about the Y axis. When the frame-drive Emag provides the magnetic field in the Y direction, the movable frame rotates about the X axis. In this case, the movable mirror was driven in a resonant mode. The movable frame was driven in a non-resonant mode. The full mechanical rotation angle of 10º about the X and Y axis are required, respectively. Next, we explain the principle of rotation angle detection. From the point of view of reliability, it is better to form no electric wires on torsion beams to detect the rotation angle. We propose using a Hall sensor to detect rotation angle of the MEMS mirror. Figures 2(b) and (c) show the detection principle. The Hall sensor is placed under the MEMS mirror, which detects the magnetic field in the Z direction. The magnetic field of the permanent magnet (B Z ) decreases when the MEMS mirror rotates clockwise about the Y axis. On the other hand, the B z increases when the mirror rotates counterclockwise about the Y axis. The rotation angle of the MEMS mirror is detected by the B z variation. The key point is 978-1-4577-0336-2/11/$26.00 c 2011 IEEE 39

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Page 1: [IEEE Nanophotonics - Istanbul, Turkey (2011.08.8-2011.08.11)] 16th International Conference on Optical MEMS and Nanophotonics - 2-axis MEMS scanner for a laser range finder

2-Axis MEMS Scanner for a Laser Range Finder

I. Aoyagi, K. Shimaoka, S.Kato MEMS Device Lab. Power Electronics Research Div.

TOYOTA Central R&D labs., Inc. Aichi, Japan

W. Makishi, Y. Kawai, S. Tanaka, T. Ono, M. Esashi, K. Hane

Tohoku University

Miyagi, Japan

Abstract— A moving-magnet-type 2-axis MEMS scanner with a rotation angle detector has been developed. The fabricated MEMS scanner demonstrated a raster scan and 2-axis rotation angles detection with one Hall sensor.

Keywords-component; MEMS, scanner, Laser range finder

I. INTRODUCTION In recent years, sensors that detect forward obstacles have

been developed for automotive safety systems, such as adaptive cruise control and pre-crash safety. The sensors required high spatial resolution and miniaturization. We have been developing the laser range finder (LRF) using a 2-axis MEMS scanner, which has the possibility of meeting these requirements. Figure 1 shows the schematic diagram of the LRF, which is composed of a laser diode (LD), a collimation lens, the MEMS scanner, a focus lens, and a photo diode (PD). A laser beam from the LD is scanned by the MEMS scanner and the distance to obstacles is measured by a time-of flight method. The MEMS scanner for the LRF needs four important points. The first point is to have a large mirror in millimeter size. To realize a long measurement range, a high power LD is needed. As the high power LD has a wide divergence angle, a collimated beam becomes wide. Therefore, the mirror has to be large. The second point is a large rotation angle for measuring a wide region. The third point is to realize a raster scan to make uniform scan of the laser beam. Therefore, at least one axis of the MEMS scanner is better to be driven in non-resonant mode. The fourth point is accuracy of satisfying specifications for an automotive sensor. To satisfying the fourth point, detecting the rotation angle of the MEMS mirror and the feedback control is necessary.

In this paper, we present results of the fabricated MEMS scanner with a rotation angle detector.

Figure 1. Schematic diagram of the laser range finder

II. PRINCIPLE AND DESIGN As a driving principle, we selected an electromagnetic

actuation, which is suitable for a large mirror, a wide rotation angle, and the non-resonant drive. The electromagnetic actuation has two different types. One is a moving coil type in which driving coils are patterned on a moving part [1]. The other is a moving magnet type in which permanent magnets are mounted on a moving part [2]. We selected the moving magnet type because this type doesn’t need to form an electric wire on a moving part and have no risks of being broken. Figure 2(a) shows a structure of a moving-magnet-type 2D-MEMS scanner, which consists of a MEMS mirror with a movable mirror, a movable frame, and two pairs of electromagnets (Emags). One pair is a mirror-drive Emag and the other pair is a frame-drive Emag. The movable mirror is 5mm x 5mm in size. The movable mirror and frame is suspended by torsion beams and rotates about the Y and X axes, respectively. The movable mirror has two mirror-drive magnets, and the movable frame has two frame-drive magnets. These four permanent magnets are magnetized in the Z direction and were glued to penetrating holes. By applying electric current to the Emags, a magnetic field H is generated. The magnetic field H works on the permanent magnet with the magnetization M and the volume V. As a result, the torque T represented in equation (1) is generated.

T =V |M ×H| (1)

When the mirror-drive Emag provides the magnetic field in the X direction, the movable mirror rotates about the Y axis. When the frame-drive Emag provides the magnetic field in the Y direction, the movable frame rotates about the X axis. In this case, the movable mirror was driven in a resonant mode. The movable frame was driven in a non-resonant mode. The full mechanical rotation angle of 10º about the X and Y axis are required, respectively.

Next, we explain the principle of rotation angle detection. From the point of view of reliability, it is better to form no electric wires on torsion beams to detect the rotation angle. We propose using a Hall sensor to detect rotation angle of the MEMS mirror. Figures 2(b) and (c) show the detection principle. The Hall sensor is placed under the MEMS mirror, which detects the magnetic field in the Z direction. The magnetic field of the permanent magnet (BZ) decreases when the MEMS mirror rotates clockwise about the Y axis. On the other hand, the Bz increases when the mirror rotates counterclockwise about the Y axis. The rotation angle of the MEMS mirror is detected by the Bz variation. The key point is

978-1-4577-0336-2/11/$26.00 c©2011 IEEE 39

Page 2: [IEEE Nanophotonics - Istanbul, Turkey (2011.08.8-2011.08.11)] 16th International Conference on Optical MEMS and Nanophotonics - 2-axis MEMS scanner for a laser range finder

that our method detects rotation angles in two directions with one Hall sensor. This is because the location of the Hall sensor has offsets from the rotating center of the MEMS mirror in the X and Y directions. At this position, the BZ changes in both case when the MEMS mirror rotates about the X axis and about Y axis. In addition, the rotation frequencies about the X and Y axis are different. Therefore, detecting signals have two different frequencies. By separating two signals, and measuring the amplitudes of the separated signals, rotation angles in two directions are detected with the one Hall sensor.

Figure 2. Structure of a 2-axis MEMS scanner

III. FABRICATION The MEMS mirror was fabricated by a double-side-

polished (100)-oriented silicon-on-insulator (SOI) wafer. The thicknesses of device, buried oxide, and handle layers were 100μm, 1μm, and 200μm, respectively. Firstly, to form the movable mirror and frame, the device layer was etched by deep reactive ion etching (DRIE). Secondly, the handle layer was etched by DRIE. Thirdly, the buried oxide layer was removed by hydrofluoric acid (HF). Fourthly, a chromium adhesion layer and a gold reflective layer evaporated through a metal mask. Finally, permanent magnets were mounted. After the process, the MEMS mirror was assembled to the two pairs of Emags and the Hall sensor.

IV. RESULTS Figures 3(a) and (b) show the fabricated MEMS mirror and

MEMS scanner, respectively. The rotation angles of the MEMS mirror were measured by using a laser and screen. The MEMS mirror was driven at 30Hz (non-resonant mode) about the X axis, and driven at 1122Hz (resonant mode) about the Y axis. Figure 3(c) shows the tracks of a scanned laser beam on the screen. By applying an ac of 242mAp-p at 30Hz to the frame-drive Emag, and an ac of 18mAp-p at 1122Hz to the mirror-drive Emag, a full mechanical rotation angle of 10º about the X axis and 24º about the Y axis was obtained. Based on Figure 3(c), it was confirmed that the fabricated MEMS

scanner realized the raster scan and achieved a full mechanical rotation angle of 10º in both directions. The output voltages of the rotation angle detector were measured by an oscilloscope. When the MEMS mirror was rotated about the X axis with 8º at 30Hz and about the Y axis with 10º at 1122Hz, the output voltage showed a superimposed signal of 30Hz and 1122Hz as shown in Figure 3(d). These two signals are clear sinusoidal in shape and easy to be separated by frequency. The sensitivity of an X-axis rotation angle of 90mV/º and a Y-axis rotation angle of 21mV/º was obtained, respectively.

Figure 3. Measurement results

V. CONCLUSION A 2-axis MEMS scanner with one rotation angle detector

for LRF was proposed. A fundamental principle of driving the MEMS mirror was moving-magnet-type. The MEMS scanner actuated a raster scan of a laser beam with a full mechanical rotation angle of 10º in both directions. One Hall sensor detected two rotation angles of the MEMS mirror. The sensitivities of an X-axis rotation angle of 90mV/º and a Y-axis rotation angle of 21mV/º was obtained. The proposed MEMS scanner will be useful for the automotive LRF in the future.

ACKNOWLEDGMENT A part of this work was supported by a special coordination

funds for promoting science and technology and was carried out in the R&D center of excellence for integrated micro-systems. The authors greatly thank the colleagues of Tohoku Univ. for their contribution to this work.

REFERENCES

[1] H. Miyajima, “Development of a MEMS electromagnetic optical scanner for a commercial laser scanninf microscope,” J. of Microlithography microfabrication and microsystems. 3 (2). pp.348-357.

[2] T. Iseki, et al., “Two-Dimensional Deflecting Mirror Using Electromagnetic Actuation,” OPTICAL REVIEW. 13 (4). pp. 189-194.

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