JP7206950B2 - Optical scanning device and control method for optical scanning device - Google Patents

Description

The present invention relates to an optical scanning device and a method of controlling the optical scanning device, and more particularly to control of MEMS (Micro Electro Mechanical Systems) mirrors.

Optical scanning devices with MEMS mirrors are currently being developed. The MEMS mirror reflects light emitted from outside the optical scanning isometry. Light can be reflected in a desired direction by adjusting the angle of the MEMS mirror.

Patent Literature 1 describes an example of a MEMS mirror. A MEMS mirror has a movable plate and a torsion bar. The movable plate is supported by torsion bars. A piezoresistor is integrally formed with the torsion bar. The piezoresistors generate a voltage according to the twist of the torsion bar, which makes it possible to detect the angle of the movable plate. Furthermore, the piezoresistors generate heat due to the bias current, which allows adjustment of the resonance frequency of the vibrator composed of the movable plate and the torsion bar.

Patent Literature 2 describes an example of a MEMS mirror. The MEMS mirror has a support, a scanning body, a first sensor and a second sensor. The scanning body is supported by a support. The first sensor detects the time the scanning body passes through the first position in the vibration of the scanning body, and the second sensor detects the time the scanning body passes the second position in the vibration of the scanning body. With this MEMS mirror, the amplitude of vibration of the scanning body can be calculated from the difference between the time when the scanning body passes through the first position and the time when the scanning body passes through the second position.

Patent Document 3 describes a MEMS element different from the MEMS mirror. US Pat. No. 5,300,004 describes a MEMS element for changing the gap between a movable substrate and a fixed substrate. A MEMS element has an electrostatic actuator. The electrostatic actuator changes the gap between the movable substrate and the fixed substrate by electrostatic attraction between the movable electrode formed on the movable substrate and the fixed electrode formed on the fixed substrate.

JP 2009-229517 A JP 2016-161857 A JP 2015-225153 A

The inventors have investigated methods for controlling the amplitude of vibration in MEMS mirrors.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to control the amplitude of vibration in a MEMS mirror by a novel method.

An optical scanning device according to the present invention includes a movable electrode, a fixed electrode, a DC voltage application section, a rectangular wave voltage application section, and a control section. The fixed electrode is arranged to face the movable electrode. The DC voltage applying section applies a DC voltage to one of the movable electrode and the fixed electrode. The rectangular wave voltage applying section applies a rectangular wave voltage to the other of the movable electrode and the fixed electrode. The control section controls the duty ratio of the rectangular wave voltage applied from the rectangular wave voltage applying section.

According to the control method of the optical scanning device according to the present invention, a DC voltage is applied from the DC voltage application unit to one of the movable electrode and the fixed electrode, and a rectangular wave voltage is applied to the other of the movable electrode and the fixed electrode. A rectangular wave voltage is applied from the section, and the duty ratio of the rectangular wave voltage applied from the rectangular wave voltage applying section is controlled by the control section.

According to the present invention, the amplitude of vibration in MEMS mirrors can be controlled in a novel way.

1 shows an optical scanning device according to an embodiment. (a) is a graph showing an example of the relationship between the DC voltage difference ΔE between the movable electrode and the fixed electrode and the amplitude of the movable electrode, and (b) is a square wave voltage V applied by a square wave voltage applying section. 4 is a graph showing an example of the relationship between the _AC fundamental wave peak voltage _VN and the amplitude of the movable electrode; (a) is the duty when the DC voltage V _B applied by the DC voltage application unit is 20 V and the peak voltage V _p of the rectangular wave voltage V _AC applied by the rectangular wave voltage application unit is 20 V. FIG. _4B is a graph showing the relationship between the ratio D, the DC voltage difference ΔE, and the fundamental wave peak voltage _VN , and FIG. 4 is a graph showing the relationship among duty ratio D, _DC voltage difference ΔE, and fundamental wave peak voltage _VN when peak voltage _Vp of rectangular wave voltage VAC applied by a unit is 20V. Figure 2 shows a first variant of Figure 1; Figure 2 shows a second variant of Figure 1;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, in all the drawings, the same constituent elements are denoted by the same reference numerals, and the description thereof will be omitted as appropriate.

FIG. 1 shows an optical scanning device 20 according to an embodiment.

An overview of the optical scanning device 20 will be described with reference to FIG. The optical scanning device 20 includes a movable electrode 120 , a fixed electrode 140 , a DC voltage application section 210 , a rectangular wave voltage application section 220 and a control section 240 . The fixed electrode 140 is arranged to face the movable electrode 120 . The DC voltage applying unit 210 applies a DC voltage to one of the movable electrode 120 and the fixed electrode 140 . In the example shown in FIG. 1 , the DC voltage applying section 210 applies DC voltage to the movable electrode 120 . The rectangular wave voltage applying section 220 applies a rectangular wave voltage to the other of the movable electrode 120 and the fixed electrode 140 . In the example shown in FIG. 1 , the rectangular wave voltage applying section 220 applies a rectangular wave voltage to the fixed electrode 140 . The control section 240 controls the duty ratio of the rectangular wave voltage applied from the rectangular wave voltage applying section 220 .

According to this embodiment, the amplitude of vibration in the movable electrode 120 can be controlled by the duty ratio of the rectangular wave voltage. As will be described later in detail, the present inventors have newly discovered the relationship between the duty ratio of the rectangular wave and the amplitude of vibration in the movable electrode 120 . In this embodiment, the control section 240 can control the duty ratio of the rectangular wave voltage, that is, the amplitude of vibration in the movable electrode 120 .

In the example shown in FIG. 1, the optical scanning device 20 further includes detection electrodes 150 and charge amplifiers 230 . The detection electrode 150 is arranged to face the movable electrode 120 . The charge amplifier 230 converts the capacitance between the movable electrode 120 and the detection electrode 150 into voltage. Control unit 240 controls the duty ratio according to the output voltage from charge amplifier 230 .

According to this embodiment, the amplitude of vibration in the movable electrode 120 can be controlled while keeping the gain of the charge amplifier 230 constant. As will be described later in detail, the present inventor has newly discovered that the duty ratio of the rectangular wave voltage applied from the rectangular wave voltage applying section 220 can be adjusted without varying the gain of the charge amplifier 230. . In this embodiment, the control section 240 can control the duty ratio without varying the gain of the charge amplifier 230 .

Details of the optical scanning device 20 will be described with reference to FIG.

The optical scanning device 20 includes an optical scanning main body 10 , a DC voltage application section 210 , a rectangular wave voltage application section 220 , a control section 240 and a charge amplifier 230 .

The optical scanning main body 10 includes a frame 110 , movable electrodes 120 , shaft members 130 , fixed electrodes 140 and detection electrodes 150 .

The movable electrode 120 has a reflective surface on which a reflective layer (eg, a metal layer, specifically, an aluminum layer, for example) is formed. Light irradiated from the outside of the optical scanning device 20 is reflected by the reflecting surface of the movable electrode 120 . Light can be reflected in a desired direction by adjusting the angle of the movable electrode 120 .

The movable electrode 120 is attached to the frame 110 by two shaft members 130 . One of the two shaft members 130 is connected to one of two opposing sides of the movable electrode 120, and the other of the two shaft members 130 is connected to the opposing side of the movable electrode 120. connected to the other of the two sides that The shaft member 130 functions as a rotating shaft for the movable electrode 120 .

The movable electrode 120 has one side between the two sides to which the shaft member 130 is connected and on which a comb-tooth electrode is formed, and another side between the two sides to which the shaft member 130 is connected and on which the comb-tooth electrode is formed. are doing.

The fixed electrode 140 has a comb-teeth electrode. The fixed electrode 140 faces the movable electrode 120 such that a gap is formed between the comb-teeth electrode of the fixed electrode 140 and the comb-teeth electrode of the movable electrode 120 . In the example shown in FIG. 1, one fixed electrode 140 faces one comb tooth electrode of the movable electrode 120, and another fixed electrode 140 faces another comb tooth electrode of the movable electrode 120. ing. The movable electrode 120 vibrates due to the electrostatic force generated by the voltage between the movable electrode 120 and the fixed electrode 140 .

The detection electrodes 150 have comb electrodes. The detection electrode 150 faces the movable electrode 120 such that a gap is formed between the comb-shaped electrode of the detection electrode 150 and the comb-shaped electrode of the movable electrode 120 . In the example shown in FIG. 1 , two detection electrodes 150 arranged with one fixed electrode 140 interposed therebetween are opposed to one comb tooth electrode of the movable electrode 120 , and the other one arranged with the other fixed electrode 140 interposed therebetween. Two detection electrodes 150 face another comb-teeth electrode of the movable electrode 120 . A capacitance between the movable electrode 120 and the detection electrode 150 varies according to the vibration of the movable electrode 120 . Therefore, by detecting the capacitance between the movable electrode 120 and the detection electrode 150, the angle of the movable electrode 120 can be detected. The layout of the detection electrodes 150 is not limited to the example shown in FIG. For example, one or more detection electrodes 150 may be arranged on only one of the two comb electrodes of the movable electrode 120 .

In the example shown in FIG. 1, the optical scanning body 10 can be formed by selectively etching a conductive material, for example a silicon substrate.

DC voltage application unit 210 is electrically connected to frame 110 . A DC voltage from the DC voltage applying section 210 is applied to the movable electrode 120 via the frame 110 and the shaft member 130 . The DC voltage application unit 210 can be, for example, a DC voltage source.

The rectangular wave voltage application section 220 is electrically connected to the fixed electrode 140 . A rectangular wave voltage from the rectangular wave voltage applying section 220 is applied to the fixed electrode 140 . The rectangular wave voltage applying section 220 can be, for example, a function generator.

Charge amplifier 230 is electrically connected to detection electrode 150 . The charge amplifier 230 can function as a capacitance voltage converter that converts the capacitance between the movable electrode 120 and the detection electrode 150 into voltage.

Charge amplifier 230 includes operational amplifier 232 , resistor 234 and capacitor 236 . A non-inverting input terminal of the charge amplifier 230 is grounded. An inverting input terminal of the charge amplifier 230 is electrically connected to the detection electrode 150 . A resistor 234 and a capacitor 236 are connected in parallel between the output terminal and the inverting input terminal of op amp 232 .

Control section 240 controls the duty ratio of the rectangular wave voltage applied from rectangular wave voltage applying section 220 according to the output voltage from charge amplifier 230 . The control unit 240 can determine from the output voltage from the charge amplifier 230 whether the movable electrode 120 is vibrating with a desired amplitude. When the amplitude of the movable electrode 120 deviates from the desired amplitude due to various factors (for example, disturbance), the controller 240 can control the amplitude of vibration in the movable electrode 120 by controlling the duty ratio. The controller 240 can be, for example, a computer (eg, personal computer or microcomputer).

Ｖ_Ｂ：直流電圧印加部２１０によって印加される直流電圧
Ｒ_ｆ：抵抗器２３４の抵抗
Ｃ_ｆ：キャパシタ２３６の静電容量
ｊ：虚数単位
チャージアンプ２３０の利得Ｇは、以下の式によって表される。

角周波数ωが十分に大きく、ωＣ_ｆＲ_ｆ＞＞１であるとき、利得Ｇは、以下の式によって表され、一定の値となる。

When the capacitance CM between the movable electrode 120 and the detection electrode 150 oscillates at an angular frequency ω, the output voltage _Vch from the charge amplifier ₂₃₀ is expressed by the following equation.

V _B : DC voltage applied by DC voltage applying section 210 R _f : Resistance of resistor 234 C _f : Capacitance of capacitor 236 j: Imaginary unit Gain G of charge amplifier 230 is expressed by the following equation. .

When the angular frequency ω is sufficiently large and ωC _f R _f >>1, the gain G is expressed by the following equation and has a constant value.

As is clear from equations (1) to (3), the capacitance _{CM between the movable electrode 120 and the detection electrode 150 can be calculated from the output voltage Vch from} the charge amplifier 230. That is, the angle of the movable electrode 120 can be calculated from the output voltage _Vch from the charge amplifier 230. FIG.

According to this embodiment, the amplitude of vibration in the movable electrode 120 can be controlled while the gain G of the charge amplifier 230 is kept constant. The reason is as follows.

Ｄ：矩形波電圧Ｖ_ＡＣのデューティ比
Ｖ_ｐ：矩形波電圧Ｖ_ＡＣのピーク電圧 The rectangular wave voltage _VAC applied by the rectangular wave voltage applying section 220 is represented by the following equation.

D: Duty ratio of rectangular wave voltage _VAC V _p : Peak voltage of rectangular wave voltage _VAC

The rectangular wave voltage V _AC consists of a DC voltage component (the first term on the lower right side of Equation (4)), a fundamental wave component of the AC voltage (the second term on the lower right side of Equation (4)), and a high frequency component of the AC voltage. It contains components (the third term on the right side of the lower row of equation (4) and subsequent terms). The high frequency component hardly affects the vibration of the movable electrode 120 . A DC voltage component V _M and a fundamental wave peak voltage V _N of the rectangular wave voltage V _AC are expressed by the following equations.

2A is a graph showing an example of the relationship between the DC voltage difference ΔE between the movable electrode 120 and the fixed electrode 140 and the amplitude of the movable electrode 120, and FIG. 4 is a graph showing an example of the relationship between the fundamental wave peak voltage _VN of the rectangular wave voltage _VAC applied by V and the amplitude of the movable electrode 120. FIG.

In FIG. 2(a), the DC voltage difference ΔE is determined by the DC voltage component V _M of the DC voltage _VB applied by the DC voltage applying unit 210 and the rectangular wave voltage V _AC applied by the rectangular wave voltage applying unit 220. be done.

As shown in FIG. 2A, the amplitude of the movable electrode 120 increases as the DC voltage difference ΔE increases. By controlling the duty ratio D, the controller 240 can control the DC voltage difference ΔE (DC voltage component V _M ) from the equation (5).

As shown in FIG. 2B, the amplitude of the movable electrode 120 increases as the fundamental wave peak voltage _VN increases. By controlling the duty ratio D, the control unit 240 can control the fundamental wave peak voltage _VN from the equation (6).

In FIG. 3A, the DC voltage VB applied by the _DC voltage applying section 210 is _20V , and the peak voltage _Vp of the rectangular wave voltage VAC applied by the rectangular wave voltage applying section 220 is 20V. FIG. 3B is a graph showing the relationship between the duty ratio D, the DC voltage difference ΔE, and the fundamental wave peak voltage _VN in the case where the DC voltage VB applied by the DC voltage applying unit 210 is _−20V . and the peak voltage _Vp of the rectangular wave voltage V _AC applied by the rectangular wave voltage applying section 220 is 20 _V. is.

As shown in FIG. 3A, when the DC voltage VB is 20 V, that is, a positive voltage, the DC voltage difference ΔE decreases as the duty ratio _D increases, and the fundamental wave peak voltage _VN It takes a maximum value at a duty ratio of 0.5, and decreases as the duty ratio departs from 0.5.

3A, when the DC voltage VB applied by the DC voltage applying unit 210 is a positive voltage, the control unit 240 may control the duty ratio _D within a range of 0.5 or more and less than 1. . In the range where the duty ratio D is 0.5 or more and less than 1, both the DC voltage difference ΔE and the fundamental wave peak voltage _VN decrease as the duty ratio D increases. That is, in this range, both the DC voltage difference ΔE and the fundamental wave peak voltage _VN change so that the amplitude of the movable electrode 120 decreases as the duty ratio D increases (FIGS. 2A and 2B). 2(b)). Therefore, it becomes easy to control the amplitude of the movable electrode 120 .

As shown in FIG. 3( _b ), when the DC voltage VB is −20 V, that is, a negative voltage, the DC voltage difference ΔE increases as the duty ratio D increases, and the fundamental wave peak voltage _VN increases as the duty ratio D increases. It takes a maximum value at a duty ratio of 0.5, and decreases as the duty ratio departs from 0.5.

From FIG. 3B, when the DC voltage VB applied by the DC voltage applying unit 210 is a negative voltage, the control unit 240 may control the duty ratio _D to a range of more than 0 and 0.5 or less. . When the duty ratio D is greater than 0 and equal to or less than 0.5, both the DC voltage difference ΔE and the fundamental wave peak voltage _VN decrease as the duty ratio D decreases. That is, in this range, both the DC voltage difference ΔE and the fundamental wave peak voltage _VN change so that the amplitude of the movable electrode 120 decreases as the duty ratio D decreases (FIGS. 2A and 2B). 2(b)). Therefore, it becomes easy to control the amplitude of the movable electrode 120 .

According to this embodiment, the control unit 240 controls the amplitude of vibration in the movable electrode 120 by controlling the duty ratio D (see formula (3)) that does not affect the gain G of the charge amplifier 230. can be done. For example, when the DC voltage _VB applied by the DC voltage applying section 210 is varied in order to control the amplitude of vibration in the movable electrode 120, the gain G is also varied (see equation (3)). In contrast, in this embodiment, the amplitude of vibration in the movable electrode 120 can be controlled without varying the DC voltage _VB .

Furthermore, according to the present embodiment, the amplitude of vibration in the movable electrode 120 can be controlled by simple control of the duty ratio D. FIG. For example, in order to control the amplitude of vibration in the movable electrode 120, a sine wave voltage applying section is used instead of the rectangular wave voltage applying section 220, and the sine wave voltage applied from the sine wave voltage applying section is varied. circuit is required. In contrast, in this embodiment, the amplitude of vibration in the movable electrode 120 can be controlled without using a complicated circuit.

FIG. 4 shows a first variant of FIG. The example shown in FIG. 4 is similar to the example shown in FIG. 1, except for the following points.

The optical scanning device 20 may not include the detection electrodes 150 and the charge amplifier 230 shown in FIG. Control unit 240 applies a square wave voltage in response to detecting a characteristic (e.g., the direction of light reflected by movable electrode 120) that is different from the capacitance between movable electrode 120 and detection electrode 150 (FIG. 1). The duty ratio of the square wave voltage applied by section 220 may be controlled. Also in this case, the amplitude of the vibration of the movable electrode 120 can be controlled by the duty ratio of the rectangular wave voltage, similarly to the example shown in FIG.

FIG. 5 shows a second variant of FIG. The example shown in FIG. 5 is similar to the example shown in FIG. 4, except for the following points.

The DC voltage applying section 210 may apply a DC voltage to the fixed electrode 140 , and the rectangular wave voltage applying section 220 may apply a rectangular wave voltage to the movable electrode 120 . Also in this case, the amplitude of the vibration of the movable electrode 120 can be controlled by the duty ratio of the rectangular wave voltage, similarly to the example shown in FIG.

Although the embodiments of the present invention have been described above with reference to the drawings, these are examples of the present invention, and various configurations other than those described above can also be adopted.

10 Optical scanning main body 20 Optical scanning device 110 Frame body 120 Movable electrode 130 Shaft member 140 Fixed electrode 150 Detection electrode 210 DC voltage application unit 220 Rectangular wave voltage application unit 230 Charge amplifier 232 Operational amplifier 234 Resistor 236 Capacitor 240 Control unit

Claims (3)

a movable electrode;
a fixed electrode arranged to face the movable electrode;
a DC voltage applying unit for applying a DC voltage to one of the movable electrode and the fixed electrode;
a rectangular wave voltage applying unit for applying a rectangular wave voltage to the other of the movable electrode and the fixed electrode;
When the DC voltage is a positive voltage, the duty ratio of the rectangular wave voltage is controlled in the range of 0.5 or more and less than 1, and when the DC voltage is a negative voltage, the duty ratio of the rectangular wave voltage is set to more than 0. a control unit that controls the amplitude of the vibration in the movable electrode by controlling the amplitude in the range of 0.5 or less and using the controlled duty ratio ;
an optical scanning device comprising: The optical scanning device according to claim 1, wherein
a detection electrode arranged to face the movable electrode;
a charge amplifier for converting the capacitance between the movable electrode and the detection electrode into a voltage;
further comprising
The DC voltage application unit applies the DC voltage to the movable electrode,
The optical scanning device, wherein the rectangular wave voltage applying section applies the rectangular wave voltage to the fixed electrode. A DC voltage applying section applies a DC voltage to one of a movable electrode and a fixed electrode arranged to face the movable electrode, and a rectangular wave voltage applying section applies a DC voltage to the other of the movable electrode and the fixed electrode. applying a square wave voltage from
When the DC voltage is a positive voltage, the control unit controls the duty ratio of the rectangular wave voltage to a range of 0.5 or more and less than 1, and when the DC voltage is a negative voltage, the duty ratio of the rectangular wave voltage. is controlled by the control unit to a range of more than 0 and 0.5 or less, and the control unit controls the amplitude of vibration in the movable electrode according to the controlled duty ratio ;
A method of controlling an optical scanning device, comprising:

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