OPTICAL SCANNING DEVICE AND CONTROL METHOD THEREOF

An optical scanning device causes a mirror portion to perform a spiral rotation operation with a first driving signal applied to a first actuator and a second driving signal applied to a second actuator as cyclic voltage signals. In a case where a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around a first axis, closest to a frequency of the cyclic voltage signal are respectively set as fr1 and Q1, a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around a second axis, closest to the frequency of the cyclic voltage signal are respectively set as fr2 and Q2, and the frequency of the cyclic voltage signal is fd, a relationship of Q1≠Q2, Fr2<fr1, and fr2×(1−1/(1.2×Q2))≤fd≤fr1×(1+1/(6×Q1)) is satisfied.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2022-002608 filed on Jan. 11, 2022. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND

1. Technical Field

The technique of the present disclosure relates to an optical scanning device and a control method thereof.

2. Description of the Related Art

In a field of light detection and ranging (LiDAR), an omnidirectional type that can obtain a 360° field of view is drawing attention. Some omnidirectional LiDAR devices are configured by combining a micro electro mechanical systems (MEMS) mirror and an omnidirectional lens. The LiDAR device using the MEMS mirror is lightweight and can be reduced in cost.

In the omnidirectional LiDAR device, the MEMS mirror needs to scan all over a donut-shaped incident surface of the omnidirectional lens with a light beam. In order to scan the above range more efficiently, the MEMS mirror desirably performs a spiral scan such that a radius vector of the light beam changes linearly over time. For this purpose, a spiral rotation operation is required in which a swing angle amplitude (hereinafter referred to as swing amplitude) of a mirror portion changes at a constant change speed. Further, in a case where such a LiDAR device is mounted on a moving body and the like, scanning of a wider range at a high frame rate is important. For this purpose, a change speed of the swing amplitude of the mirror portion is required to be increased.

JP2008-170500A describes a technique related to the spiral rotation operation of the MEMS mirror. JP2008-170500A discloses an optical scanning device comprising a swing plate, a first swing unit that causes, to the swing plate, a first swing around a first axis parallel to a plane including the swing plate, and a second swing unit that causes, to the swing plate, a second swing around a second axis that is parallel to the plane including the swing plate and perpendicular to the first axis at a frequency identical to that of the first swing and at a phase different from that of the first swing by approximately 90°. Further, JP2008-170500A discloses that a scanning position of light reflected by the swing plate is moved to draw a swirl (that is, the spiral rotation operation is performed) with increase or decrease in amplitudes of both the first swing and the second swing with time.

SUMMARY

In JP2008-170500A, with the MEMS mirror having a structure with high symmetry, the MEMS mirror performs the spiral rotation operation. Specifically, in order to form the MEMS mirror having the structure with high symmetry, characteristics (rigidity, mass, attenuation, and the like) of the first axis and the second axis are completely matched and then a driving frequency is substantially matched with a resonance frequency. This is drive control on the premise that the resonance frequency and a resonance Q value are completely matched between the first axis and the second axis. With the use of the resonance phenomenon, it is possible to cause the MEMS mirror to perform the spiral rotation operation with low power consumption.

However, in reality, the resonance frequency often does not match between the first axis and the second axis due to a process error, temperature dependence, changes in characteristics over time, and the like of the MEMS mirror. In particular, in a case where the resonance frequency changes over time, the driving frequency deviates from the resonance frequency. In this case, in order to maintain the spiral rotation operation, it is necessary to significantly increase a driving voltage. As a result, the power consumption required for driving increases. Further, in order to maintain the spiral rotation operation in a wide temperature range, it is necessary to widen a dynamic range of the power consumption of a drive circuit. This causes a problem that the overall power consumption is increased.

An object of the technique of the present disclosure is to provide an optical scanning device and a control method thereof capable of reducing power consumption required for driving and reducing a change in power consumption with a change over time.

In order to achieve the above object, an optical scanning device of the present disclosure is an optical scanning device comprising a mirror device that has a mirror portion, which is swingable around a first axis and a second axis orthogonal to each other, having a reflecting surface reflecting incident light, a first actuator causing the mirror portion to swing around the first axis by applying a rotational torque around the first axis to the mirror portion, and a second actuator causing the mirror portion to swing around the second axis by applying a rotational torque around the second axis to the mirror portion, and a processor that provides a first driving signal to the first actuator and provides a second driving signal to the second actuator. The processor causes the mirror portion to perform a spiral rotation operation with the first driving signal and the second driving signal as cyclic voltage signals. In a case where a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around the first axis, closest to a frequency of the cyclic voltage signal are respectively set as fr1and Q1, a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around the second axis, closest to the frequency of the cyclic voltage signal are respectively set as fr2and Q2, and the frequency of the cyclic voltage signal is fd, a relationship of Q1≠Q2, Fr2<fr1, and fr2×(1−1/(1.2×Q2))≤fd≤fr1×(1+1/(6×Q1)) is satisfied.

It is preferable that the cyclic voltage signal is a signal whose amplitude and phase change over time.

It is preferable that the spiral rotation operation is an operation in which a swing amplitude around the first axis and a swing amplitude around the second axis of the mirror portion change over time in a range from a first value to a second value, respectively.

It is preferable that the second value is larger than the first value, the resonance frequency and the resonance Q value in a case where the swing amplitude around the first axis is the second value are fr1and Q1, and the resonance frequency and the resonance Q value in a case where the swing amplitude around the second axis is the second value are fr2and Q2.

A control method of an optical scanning device of the present disclosure is a control method of an optical scanning device that includes a mirror device that has a mirror portion, which is swingable around a first axis and a second axis orthogonal to each other, having a reflecting surface reflecting incident light, a first actuator causing the mirror portion to swing around the first axis by applying a rotational torque around the first axis to the mirror portion, and a second actuator causing the mirror portion to swing around the second axis by applying a rotational torque around the second axis to the mirror portion. The control method comprises causing the mirror portion to perform a spiral rotation operation with a first driving signal applied to the first actuator and a second driving signal applied to the second actuator as cyclic voltage signals. In a case where a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around the first axis, closest to a frequency of the cyclic voltage signal are respectively set as fr1and Q1, a resonance frequency and a resonance Q value of a resonance mode, among resonance modes accompanied by mirror tilt swing around the second axis, closest to the frequency of the cyclic voltage signal are respectively set as fr2and Q2, and the frequency of the cyclic voltage signal is fd, a relationship of Q1≠Q2, Fr2<fr1, and fr2×(1−1/(1.2×Q2))≤fd≤fr1×(1+1/(6×Q1)) is satisfied.

According to the technique of the present disclosure, it is possible to provide an optical scanning device and a control method thereof capable of reducing the power consumption required for driving and reducing the change in power consumption with the change over time.

DETAILED DESCRIPTION

An example of an embodiment according to the technique of the present disclosure will be described with reference to the accompanying drawings.

FIG.1schematically shows an optical scanning device10according to an embodiment. The optical scanning device10has a MEMS mirror2, a light source3, and a driving controller4. In the optical scanning device10, under control of the driving controller4, a light beam L emitted from the light source3is reflected by the MEMS mirror2to perform light scanning on a surface to be scanned5. The surface to be scanned5is, for example, a screen. The MEMS mirror2is an example of a “mirror device” according to the technique of the present disclosure.

In a case where the optical scanning device10is applied to a LiDAR device, the MEMS mirror2is configured in combination with an omnidirectional lens. In this case, the MEMS mirror2scans a donut-shaped incident surface of the omnidirectional lens with the light beam L.

The MEMS mirror2is a piezoelectric two-axis driving type micromirror device that can swing a mirror portion20(refer toFIG.3) around a first axis a1and a second axis a2orthogonal to the first axis a1. Hereinafter, a direction parallel to the first axis a1is an X direction, a direction parallel to the second axis azis a Y direction, and a direction orthogonal to the first axis a1and the second axis a2is a Z direction. Further, the swing of the mirror portion20is also referred to as a mirror tilt swing.

In the present embodiment, an example in which the first axis a1and the second axis a2are orthogonal (that is, intersect perpendicularly) is shown, but the first axis a1and the second axis a2may intersect at an angle other than 90°. In the present disclosure, orthogonal means intersecting within a certain angle range including a margin of error centered at 90°.

The light source3is, for example, a laser apparatus that emits a laser beam as the light beam L. The light source3preferably emits the light beam L perpendicularly to a reflecting surface20A (refer toFIG.3) provided in the mirror portion20in a state where the mirror portion20of the MEMS mirror2is stationary.

The driving controller4outputs driving signals to the light source3and the MEMS mirror2based on optical scanning information. The light source3generates the light beam L based on the input driving signal and emits the generated light beam to the MEMS mirror2. The MEMS mirror2swings the mirror portion20around the first axis a1and the second axis a2based on the input driving signal.

As will be described in detail below, the driving controller4causes the mirror portion20to perform a spiral rotation operation including a period in which a swing amplitude around the first axis a1and a swing amplitude around the second axis a2change linearly (that is, spiral rotation operation in which a radius vector changes linearly). With the spiral rotation operation of the mirror portion20, the reflected light beam L is scanned to draw a spiral orbit (that is, a spiral curve) on the surface to be scanned5.

FIG.2shows an example of a hardware configuration of the driving controller4. The driving controller4has a central processing unit (CPU)40, a read only memory (ROM)41, a random access memory (RAM)42, a light source driving unit43, and a mirror driving unit44. The CPU40is a calculation unit that reads out a program and data from a storage device such as the ROM41into the RAM42and executes processing to realize the entire function of the driving controller4. The CPU40is an example of a “processor” according to the technique of the present disclosure.

The ROM41is a non-volatile storage device and stores the program for the CPU40to execute the processing and the data such as the above-mentioned optical scanning information. The RAM42is a non-volatile storage device that temporarily holds the program and the data.

The light source driving unit43is an electric circuit that outputs the driving signal to the light source3under the control of the CPU40. In the light source driving unit43, the driving signal is a driving voltage for controlling an emission timing and emission intensity of the light source3.

The mirror driving unit44is an electric circuit that outputs the driving signal to the MEMS mirror2under the control of the CPU40. In the mirror driving unit44, the driving signal is a driving voltage for controlling a timing, cycle, and deflection angle of swinging the mirror portion20of the mirror driving unit44. As will be described in detail below, the driving signal includes a first driving signal and a second driving signal.

For example, in the mirror driving unit44, the driving signal is created as a digital signal and output via a digital analog converter (DAC) and an amplification amplifier. The driving signal may be output as a stepped waveform based on the number of resolution bits of a digital signal source. Further, the driving signal can be created from a pulse signal, a bandpass filter, and the like.

The CPU40controls the light source driving unit43and the mirror driving unit44based on the optical scanning information. The optical scanning information represents how to scan the surface to be scanned5with the light beam L. In the present embodiment, the optical scanning information represents that the light beam L is scanned to draw the spiral orbit on the surface to be scanned5. For example, in a case where the optical scanning device10is applied to the LiDAR device, the optical scanning information includes a timing of emitting the light beam L for distance measurement, an emission range of the light beam, and the like.

Next, an example of a configuration of the MEMS mirror2will be described with reference toFIG.3.FIG.3is a schematic diagram of the MEMS mirror2.

The MEMS mirror2has the mirror portion20, a first actuator21, a second actuator22, a support frame23, a first support portion24, a second support portion25, a connection portion26, and a fixing portion27. The MEMS mirror2is formed, for example, by etching a silicon-on-insulator (SOI) substrate.

The mirror portion20has the reflecting surface20A reflecting incident light. The reflecting surface20A is formed of, for example, a metal thin film such as gold (Au) or aluminum (Al) provided on one surface of the mirror portion20. The reflecting surface20A is, for example, circular.

The support frame23is disposed so as to surround the mirror portion20. The second actuator22is disposed so as to surround the mirror portion20and the support frame23. The first actuator21is disposed so as to surround the mirror portion20, the support frame23, and the second actuator22.

The first support portion24connects the mirror portion20and the support frame23on the first axis a1and supports the mirror portion20swingably around the first axis a1. The first axis a1is in a plane including the reflecting surface20A in a case where the mirror portion20is stationary. For example, the first support portion24is a torsion bar extending along the first axis a1.

The second support portion25connects the support frame23and the second actuator22on the second axis a2and supports the mirror portion20and the support frame23swingably around the second axis a2. The second axis a2is orthogonal to the first axis a1in the plane including the reflecting surface20A in a case where the mirror portion20is stationary.

The connection portion26connects the first actuator21and the second actuator22on the first axis a1. Further, the connection portion26connects the first actuator21and the fixing portion27on the first axis a1.

The fixing portion27has a rectangular outer shape and surrounds the first actuator21. Lengths of the fixing portion27in the X direction and the Y direction are each, for example, about 1 mm to 10 mm. A thickness of the fixing portion27in the Z direction is, for example, about 5 μm to 0.2 mm.

The first actuator21and the second actuator22are piezoelectric actuators each provided with a piezoelectric element. The first actuator21applies a rotational torque around the first axis a1to the mirror portion20. The second actuator22applies a rotational torque around the second axis a2to the mirror portion20. Accordingly, the mirror portion20swings around the first axis a1and around the second axis a2.

The first actuator21is an annular thin plate member that surrounds the mirror portion20, the support frame23, and the second actuator22in an XY plane. The first actuator21is configured of a pair of a first movable portion21A and a second movable portion21B. The first movable portion21A and the second movable portion21B are each semi-annular. The first movable portion21A and the second movable portion21B have a shape that is axisymmetric with respect to the first axis a1and are connected on the first axis a1.

The support frame23is an annular thin plate member that surrounds the mirror portion20in the XY plane.

The second actuator22is an annular thin plate member that surrounds the mirror portion20and the support frame23in the XY plane. The second actuator22is configured of a pair of a first movable portion22A and a second movable portion22B. The first movable portion22A and the second movable portion22B are each semi-annular. The first movable portion22A and the second movable portion22B have a shape that is axisymmetric with respect to the second axis a2and are connected on the second axis a2.

In the first actuator21, the first movable portion21A and the second movable portion21B are each provided with piezoelectric elements. In the second actuator22, the first movable portion22A and the second movable portion22B are each provided with piezoelectric elements.

In the present example, the first actuator21and the second actuator22are each configured as separate annular structures, but the present disclosure is not limited thereto. The first actuator21and the second actuator22may be configured to coexist in one structure. For example, piezoelectric bodies are disposed into one annular structure in a divided manner. The first driving signal and the second driving signal are provided to two piezoelectric parts separated by the division in this manner, and thus the mirror swings around the first axis a1and around the second axis a2can be realized.

FIGS.4A and4Bdescribe deflection angles in a case where the mirror portion20swings.FIG.4Ashows a deflection angle (hereinafter referred to as a first deflection angle) θ1around the first axis a1of the mirror portion20.FIG.4Bshows a deflection angle (hereinafter referred to as a second deflection angle) θ2around the second axis a2of the mirror portion20.

As shown inFIG.4A, the first deflection angle θ1is an angle at which a normal line N of the reflecting surface20A of the mirror portion20is inclined in a YZ plane. The first deflection angle θ1takes a positive value in a case where the normal line N of the reflecting surface20A is inclined in a +Y direction, and the first deflection angle θ1takes a negative value in a case where the normal line N thereof is inclined in a −Y direction.

The first deflection angle θ1 is controlled by a driving signal (hereinafter referred to as a first driving signal) provided to the first actuator21by the driving controller4. The first driving signal is, for example, a sinusoidal alternating voltage. The first driving signal includes a driving voltage waveform V1A(t) applied to the first movable portion21A and a driving voltage waveform V1B(t) applied to the second movable portion21B. The driving voltage waveform V1A(t) and the driving voltage waveform V1B(t) are out of phase with each other (that is, phase difference is 180°).

As shown inFIG.4B, the second deflection angle θ2is an angle at which the normal line N of the reflecting surface20A of the mirror portion20is inclined in an XZ plane. The second deflection angle θ2takes a positive value in a case where the normal line N of the reflecting surface20A is inclined in a +X direction, and the second deflection angle θ2takes a negative value in a case where the normal line N thereof is inclined in a −X direction.

The second deflection angle θ2is controlled by a driving signal (hereinafter referred to as a second driving signal) provided to the second actuator22by the driving controller4. The second driving signal is, for example, a sinusoidal alternating voltage. The second driving signal includes a driving voltage waveform V2A(t) applied to the first movable portion22A and a driving voltage waveform V2B(t) applied to the second movable portion22B. The driving voltage waveform V2A(t) and the driving voltage waveform V2B(t) are out of phase with each other (that is, phase difference is 180°).

FIGS.5A and5Bshow examples of the driving signals provided to the first actuator21and the second actuator22.FIG.5Ashows the driving voltage waveforms V1A(t) and V1B(t) included in the first driving signal.FIG.5Bshows the driving voltage waveforms V2A(t) and V2B(t) included in the second driving signal.

The driving voltage waveforms V1A(t) and V1B(t) are each represented by the following equations (1A) and (1B).

Here, t is a time. fdis a driving frequency. A1(t) is amplitude and changes over time t. γ1(t) is phase and changes over time t. The phase difference between the driving voltage waveform V1A(t) and the driving voltage waveform V1B(t) is π (that is, 180°).

That is, the first driving signal is a cyclic voltage signal whose amplitude and phase change over time. The driving voltage waveforms V1A(t) and V1B(t) are each applied to the first movable portion21A and the second movable portion21B to swing the mirror portion20around the first axis a1in a cycle Td(=1/fd).

The driving voltage waveforms V2A(t) and V2B(t) are each represented by the following equations (2A) and (2B).

Here, t is a time. fdis a driving frequency. A2(t) is amplitude and changes over time t. γ2(t) is phase and changes over time t. The phase difference between the driving voltage waveform V2A(t) and the driving voltage waveform V2B(t) is π (that is, 180°). Further, φ is the phase difference between the driving voltage waveforms V1A(t) and V1B(t) and the driving voltage waveforms V2A(t) and V2B(t). In the present embodiment, φ=π/2 (that is, 90°) in order to cause the mirror portion20to perform a circular spiral scan operation. A value of φ may be set to a value other than π/2. In a case where φ is the value other than π/2, the mirror portion20performs an elliptical spiral scan operation.

That is, the second driving signal is the cyclic voltage signal whose amplitude and phase change over time. The driving voltage waveforms V2A(t) and V2B(t) are each applied to the first movable portion22A and the second movable portion22B to swing the mirror portion20around the second axis a2in the cycle Td(=1/fd).

The amplitude A1(t) and phase γ1(t) of the first driving signal are each represented by polynomials indicated by the following equations (3) and (4). The amplitude A2(t) and phase γ2(t) of the second driving signal are each represented by polynomials indicated by the following equations (5) and (6). In the present embodiment, the polynomial is a secondary function, but may be a cubic or more function. A degree of the polynomial is determined by a required accuracy of the spiral scan operation and calculation power of the processor. mkpand nkpare coefficients. Here, k is 0, 1, or 2. p is a or b. In the present embodiment, the phase γ2(t) is represented by a polynomial including the phase difference φ.

The coefficients mkpand nkpare determined such that the swing amplitude around the first axis a1and the swing amplitude around the second axis a2of the mirror portion20change linearly over time (that is, the radius vector of the spiral orbit changes at a constant change speed). The swing amplitude around the first axis a1corresponds to a maximum value and a minimum value of the first deflection angle θ1. The swing amplitude around the second axis a2corresponds to a maximum value and a minimum value of the second deflection angle θ2.

For example, the coefficients mkpand nkpare determined by a method in which the driving controller4actually inputs the first driving signal and the second driving signal to the MEMS mirror2and adjustment is performed while checking the first deflection angle θ1and the second deflection angle θ2of the mirror portion20with a sensor or the like.

As the sensor to detect the deflection angle, there is a method of detecting, with an optical sensor, reflected light of the light beam L, which is emitted from the light source3installed outside the MEMS mirror2and reflected by the mirror portion20, a method of incorporating, on the MEMS mirror2, a strain sensor or the like that generates a voltage according to a stress, and the like.

As described above, the applicant suggests in JP2021-102628 that the coefficients mkpand nkprelated to the changes in the amplitude and the phase over time are appropriately determined with the first driving signal and the second driving signal as the cyclic voltage signals whose amplitudes and phases change over time, respectively.

The amplitudes A1(t) and A2(t) and the phases γ1(t) and γ2(t) are cyclic functions with a modulation cycle Tmas a unit. In a case where the optical scanning device10is applied to the LiDAR device that acquires a distance image, the modulation cycle Tmcorresponds to a frame rate of the distance image. In a case where the LiDAR device is mounted on a moving body such as a drone, the modulation cycle Tmis desirably as small as possible. In this case, for example, the frame rate is required to be at least 10 Hz or higher, preferably 20 Hz or higher. That is, the modulation cycle Tmis required to be at least 0.1 seconds or less, preferably 0.05 seconds or less.

A line spacing of the spiral orbit corresponds to a resolution of the distance image. In order to increase the frame rate and narrow the line spacing, scanning at equal spacings without unevenness is most efficient and preferable. In the present embodiment, the spiral rotation operation in which the radius vector changes linearly is realized in order to make line spacings of the spiral orbit equal.

In the present embodiment, the radius vector of the spiral orbit is expanded and contracted in one modulation cycle Tm. That is, one modulation cycle Tmincludes an expansion period TE and a contraction period TS. The expansion period TE is a period in which the swing amplitude around the first axis a1and the swing amplitude around the second axis a2increase linearly. The contraction period TS is a period in which the swing amplitude around the first axis a1and the swing amplitude around the second axis a2decrease linearly.

The MEMS mirror2has a resonance mode frequency (hereinafter referred to as first resonance frequency fr1) accompanied by the swing around the first axis a1of the mirror portion20and a resonance mode frequency (hereinafter referred to as second resonance frequency fa) accompanied by the swing around the second axis a2of the mirror portion20.FIG.6shows a relationship between the first resonance frequency fr1and the second resonance frequency fa. InFIG.6, α1indicates the swing amplitude around the first axis a1, and a2indicates the swing amplitude around the second axis a2. The first resonance frequency fr1is a driving frequency fdat which the swing amplitude α1is maximized in a case where the driving frequency fdis swept in a state where the mirror portion20is caused to swing around the first axis a1. The second resonance frequency fr2is a driving frequency fdat which the swing amplitude α2is maximized in a case where the driving frequency fdis swept in a state where the mirror portion20is caused to swing around the second axis a2.

Ideally, as shown in (A) ofFIG.6, the MEMS mirror2is designed such that the first resonance frequency fr1substantially match the second resonance frequency fr2and the driving frequency fdis set to a value that substantially matches the first resonance frequency fr1and the second resonance frequency fr2.

However, in reality, as shown in (B) ofFIG.6, the first resonance frequency fr1often does not match the second resonance frequency fr2due to a process error, temperature dependence, changes in characteristics over time, and the like of the MEMS mirror2. In a case where the first resonance frequency fr1does not match the second resonance frequency fr2in this manner, in order to cause the mirror portion20to perform the spiral rotation operation, it is necessary to set the driving frequency fdto a frequency between the first resonance frequency fr1and the second resonance frequency fr2and to significantly increase the driving frequency fd. As a result, power consumption required for driving increases. Further, in order to maintain the spiral rotation operation in a wide temperature range, it is necessary to widen a dynamic range of the power consumption of a drive circuit. This causes a problem that the overall power consumption is increased.

Thus, as shown inFIG.7, the applicant has found that it is possible to reduce the power consumption required for driving and reduce the change in power consumption with the change over time by differentiating a resonance Q value around the first axis a1of the MEMS mirror2(hereinafter referred to as first resonance Q value Q1) from a resonance Q value around the second axis a2thereof (hereinafter referred to as second resonance Q value Q2) and setting the driving frequency fdin a frequency range R.

The second resonance frequency fr2is smaller than the first resonance frequency fr1, and the frequency range R is a range from fr2−Δfr2to fr1+Δfr1. Further, Δfr1=fr1/(6×Q1) and Δfr2=fr2/(1.2×Q2).

In other words, the applicant has found that it is possible to reduce the power consumption and to reduce the change in power consumption with the change over time in a case where Q1≠Q2and fr2<fr1and the driving frequency fdsatisfies the following equation (7). That is, in the frequency range R, the change in power consumption according to the driving frequency fdis small and the robustness is high.

The first resonance frequency fr1and the first resonance Q value Q1are a resonance frequency and a resonance Q value of a basic resonance mode closest to the driving frequency fdamong a plurality of resonance modes accompanied by the mirror tilt swing around the first axis a1. Further, the second resonance frequency fr2and the second resonance Q value Q2are a resonance frequency and a resonance Q value of a basic resonance mode closest to the driving frequency fdamong a plurality of resonance modes accompanied by the mirror tilt swing around the second axis a2.

The above equation (7) indicates that the preferable frequency range R includes a region between the first resonance frequency fr1and the second resonance frequency fr2(hereinafter referred to as central region) and an outer peripheral region thereof. Further, the above equation (7) indicates that the outer peripheral region is narrow on a high frequency side and wide on a low frequency side. This represents that the robustness on the high frequency side is narrow and the robustness on the low frequency side is wide in the outer peripheral region.

In the central region, there is a relatively broad region with a low power consumption. It is considered that this is because the mirror portion20vibrates at the same time in the two resonance modes and thus an interaction such as inertial force is generated and the energy efficiency is improved as compared with a case where each axis is independently driven.

Further, the reason why the robustness spreads to the low frequency side in the outer peripheral region is considered that the resonance frequency is effectively shifted to the low frequency side by the interaction between the two axes of the first axis a1and the second axis a2. More specifically, the reason why the effective resonance frequency is reduced is considered that a rotating body mass (inertial moment) around the other axis increases as the mirror portion20rotates around one of the first axis a1and the second axis a2. Accordingly, the robustness spreads to the low frequency side.

Further, with the setting of the resonance Q value to be different between the first axis a1and the second axis a2, the response of the axis having a smaller resonance Q value becomes slow. For example, in a case where the first resonance Q value Q1is smaller than the second resonance Q value Q2, the response of the first axis a1becomes slow. Accordingly, there is also an effect that the low power consumption region having the wider range is formed and the change in power consumption with the change over time becomes slow.

The resonance Q value is determined by the balance between kinetic energy and energy dissipation. Therefore, a target resonance Q value can be realized by designing a structure of the MEMS mirror2such that these factors are appropriate values. For example, a thick frame structure (also referred to as rib) is provided on a back surface of the mirror portion20or the support frame23to increase the inertial moment and thus to increase the kinetic energy. Accordingly, the first resonance Q value Q1and the second resonance Q value Q2can be increased individually. Further, for example, a comb-tooth structure is provided on an outer periphery of the mirror portion20or the support frame23to increase the air resistance and thus to increase the energy attenuation. Accordingly, the first resonance Q value Q1and the second resonance Q value Q2can be reduced individually.

Hereinafter, Examples will be described. The following Examples 1 and 2 show results of measuring the power consumption in a state where the MEMS mirror2is driven and the mirror portion20is caused to perform the spiral rotation operation.

FIG.8shows driving conditions used in Examples 1 and 2. In Examples 1 and 2, the first driving signal and the second driving signal based on the following driving conditions are provided to the MEMS mirror2to cause the mirror portion20to perform the spiral rotation operation. In a state where the mirror portion20performs the spiral rotation operation, the light beam L is emitted from the light source3to the mirror portion20. The light beam L reflected by the mirror portion20is incident on a position sensor diode (PSD) element, and a voltage signal output from the PSD element is converted into an incident position of the light beam L to evaluate the spiral orbit.

The resonance frequency of the MEMS mirror2is measured by the following method. A sinusoidal voltage signal is input only to the first actuator21to cause the mirror portion20to swing around the first axis a1, and a frequency at which the swing amplitude α1is maximized in a case where a frequency of the sinusoidal wave (that is, driving frequency fd) is changed is set as the first resonance frequency fr1. Similarly, a sinusoidal voltage signal is input only to the second actuator22to cause the mirror portion20to swing around the second axis a2, and a frequency at which the swing amplitude α2is maximized in a case where a frequency of the sinusoidal wave (that is, driving frequency fd) is changed is set as the second resonance frequency fr2.

Further, the spiral rotation operation of the mirror portion20is an operation in which the swing amplitude α1around the first axis a1and the swing amplitude α2around the second axis a2are each changed over time in a range from a first value to a second value (for example, from 5° up to 10°). Here, the second value is larger than the first value. In the present disclosure, the resonance frequency and the resonance Q value in a case where the swing amplitude α1around the first axis a1is the second value are defined as the first resonance frequency fr1and the first resonance Q value Q1. Further, the resonance frequency and the resonance Q value in a case where the swing amplitude α2around the second axis a2is the second value are defined as the second resonance frequency fr2and the second resonance Q value Q2.

FIG.9describes the definition of the first resonance Q value Q1. The first resonance Q value Q1is defined by the following equation (8).

Here, f1and f2are the driving frequencies fdin which the swing amplitude α2is 2−1/2times a maximum value, and there is a relationship of f2>f1. The second resonance Q value Q2is also defined in the same manner.

As shown inFIG.8, in Example 1, the modulation cycle Tmis set to 0.0531 seconds. In the modulation cycle Tm, the expansion period TE is set as a period of 0 seconds or more and less than 0.0431 seconds, and the contraction period TS is set as a period of 0.0431 seconds or more and 0.0531 seconds or less. Further, the driving frequency fdis set to 1439.26 Hz.

In Example 1, the coefficients mkpand nkpare adjusted and determined such that the radius vector of the spiral orbit changes linearly with the amplitude A1(t) and the phase γ1(t) of the first driving signal each as secondary functions and with the amplitude A2(t) and the phase γ2(t) of the second driving signal each as secondary functions.

FIG.10shows changes in the driving voltage waveforms V1A(t) and V2A(t) over time in one modulation cycle Tm. The driving voltage waveform V1A(t) shown in (A) ofFIG.10is obtained by applying the coefficients mkpand nkpshown inFIG.8to equation (1A), equation (3), and equation (4). The driving voltage waveform V2A(t) shown in (B) ofFIG.10is obtained by applying the coefficients mkpand nkpshown inFIG.8to equation (1B), equation (5), and equation (6). Since the driving voltage waveforms V1B(t) and V2B(t) are each inverted versions of the driving voltage waveforms V1A(t) and V2A(t), illustrations thereof are omitted.

The first driving signal consisting of the driving voltage waveforms V1A(t) and V1B(t) shown in (A) ofFIG.10and the second driving signal consisting of the driving voltage waveforms V2A(t) and V2B(t) shown in (B) ofFIG.10are provided to the MEMS mirror2. With the above, the first deflection angle θ1and second deflection angle θ2of the mirror portion20that performs the spiral rotation operation are measured. In reality, in order to prevent a polarization reversal of the first actuator21and the second actuator22, which are the piezoelectric actuators, a negative bias of −15 V is added to each of the driving voltage waveforms.

FIGS.11A and11Bshow measurement results of the spiral orbits in one modulation cycle Tm.FIG.11Ashows the spiral orbit in the expansion period TE.FIG.11Bshows the spiral orbit in the contraction period TS. As shown inFIGS.11A and11B, according to Example 1, the spiral rotation operation in which the radius vector expands and contracts linearly in a range of 4.2° to 8.2° is realized.

Next, the amplitudes A1(t) and A2(t) and the phases γ1(t) and γ2(t) are adjusted such that the driving frequency fdis changed to maintain the spiral orbit in the same angular range as described above. Then, a current value during driving is measured with a current probe, and a product of the current value and the driving voltage is subjected to time integration to calculate an average value of the power consumption in one modulation cycle Tm.

FIG.12shows a maximum value of the amplitude A1(t), a maximum value of the amplitude A2(t), the power consumption, and a change amount of the power consumption with respect to the driving frequency fd. The change amount of the power consumption represents a change amount of the power consumption in a case where the driving frequency fdchanges by 1 Hz.

FIG.13is a graph showing dependence of the power consumption and the change amount of the power consumption on the driving frequency fd.FIG.14shows measured values of the first resonance frequency fr1, the second resonance frequency fr2, the first resonance Q value Q1, and the second resonance Q value Q2. From these measured values, “fr1+Δfr1” and “fr2−Δfr2” defining the frequency range R described above are respectively calculated to be 1449.71 Hz and 1434.33 Hz.

As shown inFIG.13, it can be seen that with the setting of the driving frequency fdto the frequency range R described above, it is possible to reduce the power consumption required for driving and reduce the change in power consumption with the change over time.

Next, Example 2 will be described. As shown inFIG.8, in Example 2, the modulation cycle Tmis set to 0.0529 seconds. In the modulation cycle Tm, the expansion period TE is set as a period of 0 seconds or more and less than 0.0429 seconds, and the contraction period TS is set as a period of 0.0429 seconds or more and 0.0529 seconds or less. Further, the driving frequency fdis set to 1445.00 Hz.

In Example 2, as in Example 1, the coefficients mkpand nkpare adjusted and determined such that the radius vector of the spiral orbit changes linearly with the amplitude A1(t) and the phase γ1(t) of the first driving signal each as secondary functions and with the amplitude A2(t) and the phase γ2(t) of the second driving signal each as secondary functions.

The first driving signal consisting of the driving voltage waveforms V1A(t) and V1B(t) shown in (A) ofFIG.15and the second driving signal consisting of the driving voltage waveforms V2A(t) and V2B(t) shown in (B) ofFIG.15are provided to the MEMS mirror2. With the above, the first deflection angle θ1and second deflection angle θ2of the mirror portion20that performs the spiral rotation operation are measured. In reality, in order to prevent a polarization reversal of the first actuator21and the second actuator22, which are the piezoelectric actuators, a negative bias of −15 V is added to each of the driving voltage waveforms.

FIGS.16A and16Bshow measurement results of the spiral orbits in one modulation cycle Tm.FIG.16Ashows the spiral orbit in the expansion period TE.FIG.16Bshows the spiral orbit in the contraction period TS. As shown inFIGS.16A and16B, according to Example 2, the spiral rotation operation in which the radius vector expands and contracts linearly in a range of 3.9° to 8.1° is realized.

Next, the amplitudes A1(t) and A2(t) and the phases γ1(t) and γ2(t) are adjusted such that the driving frequency fdis changed to maintain the spiral orbit in the same angular range as described above. Then, a current value during driving is measured with a current probe, and a product of the current value and the driving voltage is subjected to time integration to calculate an average value of the power consumption in one modulation cycle Tm.

FIG.17shows a maximum value of the amplitude A1(t), a maximum value of the amplitude A2(t), the power consumption, and a change amount of the power consumption with respect to the driving frequency fd. The change amount of the power consumption represents a change amount of the power consumption in a case where the driving frequency fdchanges by 1 Hz.

FIG.18is a graph showing dependence of the power consumption and the change amount of the power consumption on the driving frequency fd.FIG.19shows measured values of the first resonance frequency fr1, the second resonance frequency fa, the first resonance Q value Q1, and the second resonance Q value Q2. From these measured values, “fr1+Δfr1” and “fr2−Δfr2” defining the frequency range R described above are respectively calculated to be 1460.62 Hz and 1441.02 Hz.

As shown inFIG.18, it can be seen that with the setting of the driving frequency fdto the frequency range R described above, it is possible to reduce the power consumption required for driving and reduce the change in power consumption with the change over time.

The first axis a1and the second axis azin the above embodiment are interchangeable. That is, in the above embodiment, the axis along the first support portion24is the first axis a1, and the axis along the second support portion25is the second axis a2. However, the axis along the first support portion24may be the second axis a2, and the axis along the second support portion25may be the first axis a1.

The configuration of the MEMS mirror2shown in the above embodiment can be changed as appropriate. For example, in the above embodiment, the first actuator21and the second actuator22have the annular shape, but one or both of the first actuator21and the second actuator22may have a meander structure. A support member having a configuration other than the torsion bar may be used as the first support portion24and the second support portion25.

The hardware configuration of the driving controller4can be modified in various ways. The processing unit of the driving controller4may be configured of one processor or a combination of two or more processors of the same type or different types (for example, a combination of a plurality of field programmable gate arrays (FPGAs), and/or a combination of a CPU and an FPGA).

All documents, patent applications, and technical standards described in the present specification are incorporated by reference in the present specification to the same extent as in a case where the incorporation of each individual document, patent application, and technical standard by reference is specifically and individually described.