Optical scanning device and control method thereof

Abstract

To provide an optical scanner which reduces variations of a line interval of spiral orbits and a control method thereof.SOLUTION: When a resonant frequency in an order higher by one from a frequency of a basic resonance mode is frH, such a condition that a ratio of a first voltage level to a second voltage level being the maximum value of a voltage level in an entire frequency range of a frequency component of a periodic voltage signal is equal to or less than -55 dBV is satisfied. Here, regarding an axis of a first axis and a second axis where a resonance mode in a lower order exists, the maximum value of a voltage level in a frequency range of (1±1 / 20)×frL and a frequency range of (1±1 / 20)×frH of the frequency component of the periodic voltage signal is the first voltage level, and regarding an axis where the resonance mode in the lower order does not exist, the maximum value of the voltage level in the frequency range of (1±1 / 20)×frH of the frequency component of the periodic voltage signal is the first voltage level.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The technology of the present disclosure relates to an optical scanning device and a control method thereof. [Background technology]

[0002] In the field of LiDAR (Light Detection and Ranging), omnidirectional types that can provide a 360-degree field of view are attracting attention. Some omnidirectional LiDAR devices are configured by combining a MEMS (Micro Electro Mechanical Systems) mirror and an omnidirectional lens. LiDAR devices that use MEMS mirrors are lightweight and can be made low-cost.

[0003] In an omnidirectional LiDAR device, the MEMS mirror must thoroughly scan the doughnut-shaped incident surface of the omnidirectional lens with a light beam. To scan the above range more efficiently, it is desirable for the MEMS mirror to perform a spiral scan so that the radius of the light beam changes linearly with time. To achieve this, the MEMS mirror must perform a spiral rotation operation in which the oscillation angle amplitude (hereinafter referred to as oscillation amplitude) of the mirror changes at a constant speed. Furthermore, when such a LiDAR device is mounted on a moving object or the like, it is important to scan a wider range at a high frame rate. To achieve this, it is necessary to increase the rate of change of the oscillation amplitude of the mirror.

[0004] Patent Document 1 describes a technology related to the spiral rotation of a MEMS mirror. Patent Document 1 discloses an optical scanning device including an oscillating plate, a first oscillating means for causing the oscillating plate to undergo a first oscillation about a first axis parallel to a plane containing the oscillating plate, and a second oscillating means for causing the oscillating plate to undergo a second oscillation about a second axis parallel to the plane containing the oscillating plate and perpendicular to the first axis, at the same frequency as the first oscillation but with a phase shift of approximately 90°. Patent Document 1 also discloses that the amplitudes of both the first oscillation and the second oscillation are increased or decreased over time, thereby moving the scanning position of light reflected by the oscillating plate in a spiral pattern (i.e., performing a spiral rotation).

[0005] In the technique described in Patent Document 1, the amplitude of the sinusoidal drive signal is varied over time to increase or decrease the amplitude of the first oscillation and the second oscillation over time. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-170500 Summary of the Invention [Problem to be solved by the invention]

[0007] When scanning by spiral rotation, the line spacing of the spiral orbit corresponds to the resolution of the range image. To achieve a high frame rate and narrow line spacing, it is most efficient and preferable to scan at equal intervals without varying the density. Therefore, in Japanese Patent Application No. 2021-102628, the applicant proposed that, in order to achieve equal line spacing on the spiral orbit, the drive signal be a periodic voltage signal whose amplitude and phase change over time, thereby realizing a spiral rotation operation in which the radius vector changes linearly.

[0008] However, the applicant has found that even if the drive signal is a periodic voltage signal whose amplitude and phase change over time, there is a problem in that the line spacing (i.e., resolution) varies in a specific region of the spiral trajectory, and further improvement is required. In particular, when the MEMS mirror is driven using a resonant mode, in which the actuator and mirror oscillate in an anti-phase relationship, which has high drive efficiency, the line spacing variation becomes significant.

[0009] The technology of the present disclosure aims to provide an optical scanning device and a control method thereof that can reduce variations in the line spacing of a spiral orbit. [Means for solving the problem]

[0010] In order to achieve the above object, an optical scanning device of the present disclosure is an optical scanning device comprising: a mirror device having a reflective surface that reflects incident light and that is oscillating around a first axis and a second axis that are orthogonal to each other; a first actuator that applies a rotational torque about the first axis to the mirror unit to cause the mirror unit to oscillate around the first axis; and a second actuator that applies a rotational torque about the second axis to the mirror unit to cause the mirror unit to oscillate around the second axis; and a processor that applies a first drive signal to the first actuator and a second drive signal to the second actuator, wherein the processor causes the mirror unit to perform a spiral rotation operation by using the first drive signal and the second drive signal as periodic voltage signals, and wherein at least one of a plurality of resonance modes involving mirror tilt oscillation around the first axis and a plurality of resonance modes involving mirror tilt oscillation around the second axis has a resonance mode of one order lower than a fundamental resonance mode that is closest to the frequency of the periodic voltage signal, and the resonance frequency of one order lower than the frequency of the fundamental resonance mode is set to f for each axis. rL , the next higher resonance frequency from the fundamental resonance mode frequency is f rHIn this case, the ratio of the first voltage level to the second voltage level, which is the maximum value of the voltage level in the entire frequency range, among the frequency components of the periodic voltage signal, is -55 dBV or less. Here, for the axis on which a low-order resonance mode exists, of the first axis and the second axis, the ratio of the frequency components of the periodic voltage signal to the second voltage level, which is the maximum value of the voltage level in the entire frequency range, is -55 dBV or less. rL Frequency range and (1±1 / 20)×f rH For an axis where the maximum value of the voltage level in the frequency range is the first voltage level and no low-order resonance mode exists, the frequency components of the periodic voltage signal are (1±1 / 20)×f rH The maximum voltage level in this frequency range is the first voltage level.

[0011] In the fundamental resonance mode of an axis on which a lower-order resonance mode exists, it is preferable that the actuator that drives the mirror section around the axis on which the lower-order resonance mode exists, either the first actuator or the second actuator, and the mirror section oscillate in an anti-phase relationship with each other.

[0012] Preferably, the processor applies frequency filtering to the first drive signal and the second drive signal so that the ratio of the first voltage level to the second voltage level is −55 dBV or less.

[0013] The frequency filtering is preferably digital or analog filtering.

[0014] The periodic voltage signal is preferably a signal whose amplitude and phase are time-varying.

[0015] The spiral rotation operation preferably includes a period in which the oscillation amplitude of the mirror section around the first axis and the oscillation amplitude of the mirror section around the second axis each change linearly.

[0016] A control method for an optical scanning device according to the present disclosure is a control method for an optical scanning device including a mirror device having a mirror unit having a reflective surface that reflects incident light and that is oscillating around a first axis and a second axis that are orthogonal to each other, a first actuator that applies a rotational torque about the first axis to the mirror unit to cause the mirror unit to oscillate around the first axis, and a second actuator that applies a rotational torque about the second axis to the mirror unit to cause the mirror unit to oscillate around the second axis, wherein a first drive signal given to the first actuator and a second drive signal given to the second actuator are periodic voltage signals, causing the mirror unit to perform a spiral rotation operation, and at least one of a plurality of resonance modes involving mirror tilt oscillation about the first axis and a plurality of resonance modes involving mirror tilt oscillation about the second axis has a resonance mode one order lower than a fundamental resonance mode that is closest to the frequency of the periodic voltage signal, and a resonance frequency one order lower than the frequency of the fundamental resonance mode is set to f for each axis. rL , the next higher resonance frequency from the fundamental resonance mode frequency is f rH In this case, the ratio of the first voltage level to the second voltage level, which is the maximum value of the voltage level in the entire frequency range, among the frequency components of the periodic voltage signal, is -55 dBV or less. Here, for the axis on which a low-order resonance mode exists, of the first axis and the second axis, the ratio of the frequency components of the periodic voltage signal to the second voltage level, which is the maximum value of the voltage level in the entire frequency range, is -55 dBV or less. rL Frequency range and (1±1 / 20)×f rH For an axis where the maximum value of the voltage level in the frequency range is the first voltage level and no low-order resonance mode exists, the frequency components of the periodic voltage signal are (1±1 / 20)×f rH The maximum voltage level in this frequency range is the first voltage level.

[0017] According to the technology of the present disclosure, it is possible to provide an optical scanning device and a control method thereof that can reduce variations in the line spacing of a spiral orbit. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of an optical scanning device. [Figure 2] FIG. 2 is a block diagram showing an example of a hardware configuration of a drive control unit. [Figure 3] FIG. 1 is a schematic diagram of a micromirror device. [Figure 4] 4A and 4B are diagrams for explaining the deflection angle when the mirror section swings, in which (A) shows a first deflection angle and (B) shows a second deflection angle. [Figure 5] 1A and 1B are diagrams showing examples of drive signals applied to a first actuator and a second actuator, where (A) shows a first drive signal and (B) shows a second drive signal. [Figure 6] FIG. 4 is a diagram schematically showing frequency components included in a first drive signal. [Figure 7] FIG. 10 is a diagram showing measurement results of the resonance frequencies of the fundamental resonance mode, lower-order resonance modes, and higher-order resonance modes. [Figure 8] FIG. 10 is a diagram showing the results of a simulation calculation of the shape of a fundamental resonance mode among the resonance modes accompanying tilt oscillation of a mirror around a first axis. [Figure 9] FIG. 10 is a diagram showing the results of calculations performed by simulation on the shape of the resonance mode one order lower than the fundamental resonance mode. [Figure 10] FIG. 10 is a diagram showing the results of calculations performed by simulation of the shape of a resonance mode one order higher than the fundamental resonance mode. [Figure 11] 10A and 10B are diagrams illustrating calculation results of frequencies at the high frequency end and the low frequency end that define the first frequency range and the second frequency range. [Figure 12] FIG. 1 shows seven conditions used in the experiment and the experimental results for each condition. [Figure 13] 10 is a graph showing the relationship between the voltage level ratio and the variation in the line spacing. [Figure 14] 1A and 1B are diagrams showing changes over time in drive voltage waveforms within one modulation period, where (A) shows the waveform of a first drive signal and (B) shows the waveform of a second drive signal. [Figure 15] FIG. 4 is a diagram showing frequency components of a driving voltage waveform. [Figure 16] 10A and 10B are diagrams showing measurement results of a first deflection angle and a second deflection angle within one modulation cycle. [Figure 17] FIG. 10 is a diagram showing the measurement results of a spiral trajectory in an expansion period within one modulation cycle. [Figure 18] 10 is a graph showing the measurement results of the line interval around the first axis. [Figure 19] 1A and 1B are diagrams showing changes over time in drive voltage waveforms within one modulation period, where (A) shows the waveform of a first drive signal and (B) shows the waveform of a second drive signal. [Figure 20] FIG. 4 is a diagram showing frequency components of a driving voltage waveform. [Figure 21] 10A and 10B are diagrams showing measurement results of a first deflection angle and a second deflection angle within one modulation cycle. [Figure 22] FIG. 10 is a diagram showing the measurement results of a spiral trajectory in an expansion period within one modulation cycle. [Figure 23] 10 is a graph showing the measurement results of the line interval around the first axis. [Figure 24] FIG. 10 is a diagram showing the measurement results of a spiral trajectory in an expansion period within one modulation cycle. [Figure 25] 10 is a graph showing the measurement results of the line interval around the first axis. DETAILED DESCRIPTION OF THE INVENTION

[0019] An example of an embodiment of the technology of the present disclosure will be described with reference to the accompanying drawings.

[0020] FIG. 1 schematically illustrates an optical scanning device 10 according to one embodiment. The optical scanning device 10 includes a MEMS mirror 2, a light source 3, and a drive controller 4. The optical scanning device 10 optically scans a surface 5 to be scanned by reflecting a light beam L emitted from the light source 3 by the MEMS mirror 2 under the control of the drive controller 4. The surface 5 to be scanned is, for example, a screen. The MEMS mirror 2 is an example of a "mirror device" according to the technology of the present disclosure.

[0021] When the optical scanning device 10 is applied to a LiDAR device, the MEMS mirror 2 is configured in combination with an omnidirectional lens. In this case, the MEMS mirror 2 scans the donut-shaped incident surface of the omnidirectional lens with the light beam L.

[0022] The MEMS mirror 2 is a piezoelectric two-axis drive micromirror device that can oscillate a mirror section 20 (see FIG. 3) around a first axis a1 and a second axis a2 that is perpendicular to the first axis a1. Hereinafter, the direction parallel to the first axis a1 will be referred to as the X direction, the direction parallel to the second axis a2 as the Y direction, and the direction perpendicular to the first axis a1 and the second axis a2 as the Z direction. The oscillation of the mirror section 20 will also be referred to as mirror tilt oscillation.

[0023] In this embodiment, an example is shown in which the first axis a1 and the second axis a2 are orthogonal (i.e., intersect perpendicularly), but the first axis a1 and the second axis a2 may intersect at an angle other than 90°. In this disclosure, orthogonal means intersecting within a certain angle range, including a tolerance, centered on 90°.

[0024] The light source 3 is a laser device that emits, for example, laser light as the light beam L. It is preferable that the light source 3 irradiates the light beam L perpendicularly to a reflecting surface 20A (see FIG. 3) of the mirror portion 20 of the MEMS mirror 2 when the mirror portion 20 is stationary.

[0025] The drive control unit 4 outputs drive signals to the light source 3 and the MEMS mirror 2 based on the optical scanning information. The light source 3 generates a light beam L based on the input drive signal and irradiates the MEMS mirror 2 with the light beam L. The MEMS mirror 2 oscillates the mirror portion 20 around the first axis a1 and the second axis a2 based on the input drive signal.

[0026] As will be described in detail later, the drive control unit 4 causes the mirror unit 20 to perform a spiral rotation operation (i.e., a spiral rotation operation in which the radius of curvature changes linearly) including a period in which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly. When the mirror unit 20 performs the spiral rotation operation, the reflected light beam L scans the scanned surface 5 so as to trace a spiral trajectory (i.e., a spiral curve).

[0027] 2 shows an example of the hardware configuration of the drive control unit 4. The drive control unit 4 has a CPU (Central Processing Unit) 40, a ROM (Read Only Memory) 41, a RAM (Random Access Memory) 42, a light source drive unit 43, and a mirror drive unit 44.

[0028] The CPU 40 is a computing device that reads programs and data from a storage device such as the ROM 41 into the RAM 42 and executes the processes, thereby realizing the overall functions of the drive control unit 4. The CPU 40 is an example of a "processor" according to the technology of the present disclosure.

[0029] The ROM 41 is a non-volatile storage device that stores programs for executing processes by the CPU 40 and data such as the optical scanning information described above. The RAM 42 is a non-volatile storage device that temporarily stores programs and data.

[0030] The light source driving unit 43 is an electric circuit that outputs a driving signal to the light source 3 under the control of the CPU 40. In the light source driving unit 43, the driving signal is a driving voltage for controlling the irradiation timing and irradiation intensity of the light source 3.

[0031] The mirror driver 44 is an electric circuit that outputs a drive signal to the MEMS mirror 2 under the control of the CPU 40. In the mirror driver 44, the drive signal is a drive voltage for controlling the timing, period, and deflection angle of oscillating the mirror section 20 of the mirror driver 44. As will be described in detail later, the drive signal includes a first drive signal and a second drive signal.

[0032] The mirror driver 44 includes a drive signal generator 45 and a frequency filter processor 46. The drive signal generator 45 generates and outputs a drive signal. For example, the drive signal generator 45 generates the drive signal as a digital signal and outputs it via a DAC (Digital Analog Converter) and an amplifier. The drive signal may be output as a step waveform based on the resolution bit rate of the digital signal source. Alternatively, the drive signal may be generated from a pulse signal and a band-pass filter, etc.

[0033] The frequency filter processing unit 46 performs frequency filtering, which will be described later, on the drive signal output from the drive signal generation unit 45. For example, the frequency filter processing unit 46 is a band-pass filter or a notch filter. The mirror drive unit 44 outputs the drive signal, which is generated by the drive signal generation unit 45 and has been frequency filtered by the frequency filter processing unit 46, to the MEMS mirror 2. If the drive signal is a digital signal, the frequency filter processing unit 46 is a digital filter circuit that performs digital filtering. If the drive signal is a digital signal, the frequency filter processing unit 46 is an analog filter circuit that performs analog filtering.

[0034] The CPU 40 controls the light source driver 43 and the mirror driver 44 based on the optical scanning information. The optical scanning information is information that indicates how the light beam L is to be scanned on the scanned surface 5. In this embodiment, the information indicates that the light beam L is to be scanned so as to trace a spiral trajectory on the scanned surface 5. Note that, for example, when the optical scanning device 10 is applied to a LiDAR device, the optical scanning information includes the timing of irradiating the light beam L for distance measurement, the irradiation range, etc.

[0035] Next, an example of the configuration of the MEMS mirror 2 will be described with reference to Fig. 3. Fig. 3 is a schematic diagram of the MEMS mirror 2.

[0036] The MEMS mirror 2 has a mirror section 20, a first actuator 21, a second actuator 22, a support frame 23, a first support section 24, a second support section 25, a connection section 26, and a fixing section 27. The MEMS mirror 2 is formed by, for example, etching an SOI (Silicon On Insulator) substrate.

[0037] The mirror section 20 has a reflecting surface 20A that reflects incident light. The reflecting surface 20A is formed of a thin metal film such as gold (Au) or aluminum (Al) provided on one surface of the mirror section 20. The reflecting surface 20A is, for example, circular.

[0038] The support frame 23 is arranged to surround the mirror section 20. The second actuator 22 is arranged to surround the mirror section 20 and the support frame 23. The first actuator 21 is arranged to surround the mirror section 20, the support frame 23, and the second actuator 22.

[0039] The first support portion 24 connects the mirror portion 20 and the support frame 23 on the first axis a1, and supports the mirror portion 20 so that it can swing around the first axis a1. The first axis a1 is in a plane that includes the reflecting surface 20A when the mirror portion 20 is stationary. For example, the first support portion 24 is a torsion bar that extends along the first axis a1.

[0040] The second support section 25 connects the support frame 23 and the second actuator 22 on the second axis a2, and supports the mirror section 20 and the support frame 23 so that they can swing about the second axis a2. The second axis a2 is perpendicular to the first axis a1 in a plane including the reflecting surface 20A when the mirror section 20 is stationary.

[0041] The connecting portion 26 connects the first actuator 21 and the second actuator 22 on the first axis a1. The connecting portion 26 also connects the first actuator 21 and the fixed portion 27 on the first axis a1.

[0042] The fixed portion 27 has a rectangular outer shape and surrounds the first actuator 21. The lengths of the fixed portion 27 in the X and Y directions are each, for example, about 1 mm to 10 mm. The thickness of the fixed portion 27 in the Z direction is, for example, about 5 μm to 0.2 mm.

[0043] The first actuator 21 and the second actuator 22 are piezoelectric actuators each equipped with a piezoelectric element. The first actuator 21 applies a rotational torque about a first axis a1 to the mirror section 20. The second actuator 22 applies a rotational torque about a second axis a2 to the mirror section 20. This causes the mirror section 20 to oscillate about the first axis a1 and the second axis a2.

[0044] The first actuator 21 is an annular thin plate member that surrounds the mirror unit 20, the support frame 23, and the second actuator 22 in the XY plane. The first actuator 21 is composed of a pair of a first movable unit 21A and a second movable unit 21B. The first movable unit 21A and the second movable unit 21B are each semi-annular. The first movable unit 21A and the second movable unit 21B are shaped to be line-symmetric with respect to the first axis a1, and are connected on the first axis a1.

[0045] The support frame 23 is an annular thin plate member that surrounds the mirror section 20 in the XY plane.

[0046] The second actuator 22 is an annular thin plate member that surrounds the mirror unit 20 and the support frame 23 in the XY plane. The second actuator 22 is composed of a pair of a first movable unit 22A and a second movable unit 22B. The first movable unit 22A and the second movable unit 22B are each semi-annular. The first movable unit 22A and the second movable unit 22B are shaped to be line-symmetric with respect to the second axis a2 and are connected on the second axis a2.

[0047] In the first actuator 21, a piezoelectric element is provided on each of the first movable portion 21A and the second movable portion 21B. In the second actuator 22, a piezoelectric element is provided on each of the first movable portion 22A and the second movable portion 22B.

[0048] In this example, the first actuator 21 and the second actuator 22 are configured as separate annular structures, but this is not limiting and they may be configured to coexist within a single structure. For example, a piezoelectric body may be divided and placed within a single annular structure. By applying a first drive signal and a second drive signal to the two separate piezoelectric body portions in this manner, it is possible to realize tilting and oscillation of the mirror around the first axis a1 and the second axis a2.

[0049] Figure 4 explains the deflection angle when the mirror section 20 swings. Figure 4(A) shows the deflection angle θ1 around the first axis a1 of the mirror section 20 (hereinafter referred to as the first deflection angle). Figure 4(B) shows the deflection angle θ2 around the second axis a2 of the mirror section 20 (hereinafter referred to as the second deflection angle).

[0050] 4(A), the angle at which the normal N to the reflecting surface 20A of the mirror section 20 is tilted in the YZ plane is referred to as the first deflection angle θ1. When the normal N to the reflecting surface 20A is tilted in the +Y direction, the first deflection angle θ1 takes a positive value, and when it is tilted in the -Y direction, the first deflection angle θ1 takes a negative value.

[0051] The first deflection angle θ1 is controlled by a drive signal (hereinafter referred to as the first drive signal) that the drive control unit 4 provides to the first actuator 21. The first drive signal is, for example, a sinusoidal AC voltage. The first drive signal is a drive voltage waveform V 1A (t) and the driving voltage waveform V applied to the second movable portion 21B 1B (t) and the driving voltage waveform V 1A (t) and the drive voltage waveform V 1B (t) are in opposite phase to each other (i.e., a phase difference of 180°).

[0052] 4(B), the angle at which the normal N to the reflecting surface 20A of the mirror section 20 is tilted in the XZ plane is referred to as the second deflection angle θ2. When the normal N to the reflecting surface 20A is tilted in the +X direction, the second deflection angle θ2 takes a positive value, and when it is tilted in the -X direction, the second deflection angle θ2 takes a negative value.

[0053] The second deflection angle θ2 is controlled by a drive signal (hereinafter referred to as a second drive signal) that the drive control unit 4 provides to the second actuator 22. The second drive signal is, for example, a sinusoidal AC voltage. The second drive signal is a drive voltage waveform V applied to the first movable portion 22A. 2A (t) and the driving voltage waveform V applied to the second movable portion 22B 2B (t) and the driving voltage waveform V 2A (t) and the drive voltage waveform V 2B (t) are in opposite phase to each other (i.e., a phase difference of 180°).

[0054] 5A and 5B show examples of drive signals applied to the first actuator 21 and the second actuator 22. FIG. 5A shows a drive voltage waveform V 1A (t) and V 1B FIG. 5B shows the drive voltage waveform V included in the second drive signal. 2A (t) and V 2B (t) is shown.

[0055] Drive voltage waveform V 1A (t) and V 1B (t) are expressed by the following formulas (1A) and (1B), respectively.

number

number

[0056] where t is time. d is the driving frequency. A1(t) is the amplitude and changes with time t. γ1(t) is the phase and changes with time t. The driving voltage waveform V1A (t) and the drive voltage waveform V 1B The phase difference with (t) is π (i.e., 180°).

[0057] That is, the first drive signal is a periodic voltage signal whose amplitude and phase change over time. 1A (t) and V 1B (t) is applied to the first movable portion 21A and the second movable portion 21B, so that the mirror portion 20 rotates around the first axis a1 with a period T d (=1 / f d ) and oscillate.

[0058] Drive voltage waveform V 2A (t) and V 2B (t) are expressed by the following formulas (2A) and (2B), respectively.

number

number

[0059] where t is time. d is the driving frequency. A2(t) is the amplitude and changes with time t. γ2(t) is the phase and changes with time t. The driving voltage waveform V 2A (t) and the drive voltage waveform V 2B The phase difference with (t) is π (i.e., 180°).

[0060] That is, the second drive signal is a periodic voltage signal whose amplitude and phase change over time. 2A (t) and V 2B (t) is applied to the first movable portion 22A and the second movable portion 22B, respectively, so that the mirror portion 20 rotates around the second axis a2 with a period T d (=1 / f d ) and oscillate.

[0061] Also, φ is the driving voltage waveform V 1A (t) and V 1B(t) and the driving voltage waveform V 2A (t) and V 2B In this embodiment, in order to make the mirror section 20 perform a circular spiral scan, φ is set to π / 2 (i.e., 90°). As a result, the period of one revolution of the circular orbit is T d (=1 / f d ) is realized. The value of φ may be set to a value other than π / 2. When φ is a value other than π / 2, the mirror section 20 performs an elliptical spiral scan operation.

[0062] The amplitude A1(t) and phase γ1(t) of the first drive signal are expressed by the polynomials shown in the following formulas (3) and (4), respectively. The amplitude A2(t) and phase γ2(t) of the second drive signal are expressed by the polynomials shown in the following formulas (5) and (6), respectively. In this embodiment, the polynomials are quadratic functions, but they may also be cubic or higher order functions. The degree of the polynomial is determined depending on the required operational accuracy of the spiral scan and the processing power of the processor. m kp and n kp is a coefficient. Here, k is 0, 1, or 2. p is a or b. In this embodiment, the phase γ2(t) is expressed by a polynomial including the phase difference φ.

[0063]

number

number

number

number

[0064] Coefficient m kp and n kpare determined so that the oscillation amplitude of the mirror section 20 around the first axis a1 and the oscillation amplitude of the mirror section 20 around the second axis a2 change linearly with time (i.e., the radius of the spiral orbit changes at a constant speed). The oscillation amplitude around the first axis a1 corresponds to the maximum and minimum values of the first deflection angle θ1. The oscillation amplitude around the second axis a2 corresponds to the maximum and minimum values of the second deflection angle θ2.

[0065] For example, the coefficient m kp and n kp is determined by a method in which the drive control unit 4 actually inputs the first drive signal and the second drive signal to the MEMS mirror 2, and adjusts the first deflection angle θ1 and the second deflection angle θ2 of the mirror unit 20 while checking them with a sensor or the like.

[0066] As a sensor for detecting the deflection angle, there are a method in which an optical sensor detects the reflected light of a light beam L emitted from a light source 3 installed outside the MEMS mirror 2 and reflected by the mirror part 20, and a method in which a strain sensor or the like that generates a voltage according to stress is incorporated on the MEMS mirror 2.

[0067] As described above, the first drive signal and the second drive signal are periodic voltage signals whose amplitude and phase change over time, and the coefficient m kp and n kp The applicant has proposed in Patent Application No. 2021-102628 that the above be appropriately determined.

[0068] The amplitudes A1(t), A2(t) and phases γ1(t), γ2(t) are determined by the modulation period T m When the optical scanning device 10 is applied to a LiDAR device that acquires a range image, the modulation period T m corresponds to the frame rate of the range image. When the LiDAR device is mounted on a mobile object such as a drone, the modulation period T m It is desirable that is as small as possible. In this case, for example, the frame rate is required to be at least 10 Hz or more, preferably 20 Hz or more. That is, the modulation period T mis required to be at least 0.1 seconds or less, preferably 0.05 seconds or less.

[0069] Furthermore, the line spacing of the spiral trajectory corresponds to the resolution of the distance image. To increase the frame rate and narrow the line spacing, it is most efficient and preferable to scan at equal intervals without varying the density. The line spacing refers to the spacing in the radial direction of the spiral trajectory. In this embodiment, in order to make the line spacing of the spiral trajectory equal, a spiral rotation operation is realized in which the radius changes linearly.

[0070] In this embodiment, one modulation period T m The radius of the spiral orbit expands and contracts within one modulation period T m includes an expansion period TE and a contraction period TS. The expansion period TE is a period during which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 increase linearly. The contraction period TS is a period during which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 decrease linearly.

[0071] The MEMS mirror 2 has a frequency of a resonance mode (hereinafter referred to as a first resonance frequency f r1 ) and the frequency of a resonance mode accompanying the mirror tilt oscillation around the second axis a2 of the mirror section 20 (hereinafter referred to as the second resonance frequency f r2 ) exists. Here, the first resonance frequency f r1 is the driving frequency f among the multiple resonance modes accompanying the mirror tilt oscillation around the first axis a1. d The second resonance frequency f r2 is the driving frequency f among the multiple resonance modes accompanying the mirror tilt oscillation around the second axis a2. d is the resonant frequency of the fundamental resonant mode closest to

[0072] 1st resonance frequency f r1 is the driving frequency f when the mirror part 20 is oscillated around the first axis a1. d The driving frequency f at which the oscillation amplitude becomes maximum whend The second resonance frequency f r2 is the driving frequency f when the mirror part 20 is oscillated around the second axis a2. d The driving frequency f at which the oscillation amplitude becomes maximum when d is.

[0073] The MEMS mirror 2 has a first resonant frequency f r1 and the second resonant frequency f r2 and the driving frequency f d is the first resonant frequency f r1 and the second resonant frequency f r2 However, in reality, the first resonance frequency f r1 and the second resonant frequency f r2 and may not match.

[0074] The applicant has found that even if the drive signal is a periodic voltage signal whose amplitude and phase change over time, there is a problem in that the line spacing (i.e., resolution) varies in a specific region of the spiral trajectory, and further improvement is required. One possible way to reduce the line spacing variation of the spiral trajectory is to lengthen the period of the spiral rotation, but lengthening the period of the spiral rotation reduces the frame rate of the distance image.

[0075] The applicant has found that the variation in the line spacing of the spiral orbit is caused by excitation of unnecessary resonance modes lower and higher than the fundamental resonance mode during driving. Furthermore, the applicant has found that the unnecessary resonance modes accompanying the tilt oscillation of the mirror on either the first axis a1 or the second axis a2 cause the variation in the line spacing. Therefore, the resonance frequencies f of the unnecessary resonance modes of the lower and higher orders on each axis are r ±(f rBy suppressing the frequency components included in the frequency range of (f / Q) / 2, excitation of unnecessary resonance modes is suppressed, and variations in line spacing are reduced. Q represents the resonance Q value. Since the resonance Q value of the resonance mode accompanying the mirror tilt oscillation of the MEMS mirror 2 is generally 10 or more, in this embodiment, the resonance frequencies f of the unnecessary low-order and high-order resonance modes are r ±f centered on r By suppressing frequency components within the / 20 frequency range, robustness is improved even against variations in the Q value of unwanted modes due to dimensional changes, manufacturing variations, and the like.

[0076] By suppressing the frequency components in the above frequency range from the first drive signal and the second drive signal using the frequency filter processing unit 46, it is possible to reduce the variation in the line spacing of the spiral trajectory without lowering the frame rate. Specifically, first, among the multiple resonance modes accompanying the mirror tilt oscillation around the first axis a1, the frequency of the periodic voltage signal (i.e., the drive frequency f d ) and a second frequency range B2 including a resonance frequency one order higher than the frequency of the fundamental resonance mode, the variation in the line spacing caused by unwanted vibrations around the first axis a1 is reduced by suppressing the frequency components in the first frequency range B1 including a resonance frequency one order lower than the frequency of the fundamental resonance mode closest to the frequency of the fundamental resonance mode, and the second frequency range B2 including a resonance frequency one order higher than the frequency of the fundamental resonance mode. Secondly, among the multiple resonance modes accompanying the mirror tilt oscillation around the second axis a2, the frequency of the periodic voltage signal (i.e., the drive frequency f d By suppressing frequency components in a first frequency range B1 including the resonance frequency of the fundamental resonance mode closest to the frequency of the fundamental resonance mode that is one order lower than the frequency of the fundamental resonance mode, and a second frequency range B2 including the resonance frequency of the fundamental resonance mode that is one order higher than the frequency of the fundamental resonance mode, variation in the line spacing caused by unwanted vibrations around the second axis a2 is reduced. Note that if there is no lower-order resonance mode (i.e., if the first frequency range B1 does not exist), it is sufficient to suppress only the frequency components in the second frequency range B2.

[0077] FIG. 6 shows a schematic diagram of the frequency components included in the first drive signal. r1L is the first resonant frequency f r1f r1H is the first resonant frequency f r1 The first frequency range B1 represents the next higher resonant frequency from f B1L ≦B1≦f B1H where f B1L =(1-1 / 20)×f r1L and f B1H =(1+1 / 20)×f r1L That is, the first frequency range B1 is (1±1 / 20)×f r1L The second frequency range B2 is a frequency range of f B2L ≦B2≦f B2H The range is defined by f B2L =(1-1 / 20)×f r1H and f B2H =(1+1 / 20)×f r1H That is, the second frequency range B2 is (1±1 / 20)×f r1H is the frequency range.

[0078] 6, V1 is the maximum voltage level (hereinafter referred to as the first voltage level) of the frequency components of the first drive signal in the first frequency range B1 and the second frequency range B2. V2 is the maximum voltage level (hereinafter referred to as the second voltage level) of the frequency components of the first drive signal in all frequency ranges. The frequency filter processing unit 46 performs filtering on the first drive signal so that the ratio R of the first voltage level VL1 to the second voltage level VL2 (hereinafter referred to as the voltage level ratio R) is −55 dBV or less.

[0079] The same applies to the second drive signal. The frequency filter processing unit 46 performs filtering on the second drive signal so that the voltage level ratio R is −55 dBV or less.

[0080] For the first axis a1 or the second axis a2, whichever axis has a resonance mode that is one order lower than the fundamental resonance mode, the maximum voltage level of the frequency components of the periodic voltage signal in the first frequency range B1 and the second frequency range B2 is the first voltage level VL1. On the other hand, for the axis that does not have a resonance mode that is one order lower than the fundamental resonance mode, the maximum voltage level of the frequency components of the periodic voltage signal in the second frequency range B2 is the first voltage level VL1.

[0081] The first frequency range B1 and the second frequency range B2 are defined by the first resonance frequency f r1 and the second resonant frequency f r2 The range may be determined based on either one of the above. Furthermore, the frequency filter processing unit 46 may be designed to set the voltage level ratio R to -55 dBV or less for either the first drive signal or the second drive signal. For example, the frequency filter processing unit 46 may be designed to set the voltage level ratio R to -55 dBV or less for the first drive signal, and may also perform the same filtering process on the second drive signal as on the first drive signal.

[0082] [Experimental Results] The following shows the results of an experiment on the dependency of filtering on variations in the line spacing of a spiral trajectory.

[0083] First, the resonance frequency of the MEMS mirror 2 used in the experiment was measured by the following method. A sinusoidal voltage signal was input to only the first actuator 21 to cause the mirror part 20 to oscillate around the first axis a1, and the frequency of the sinusoidal wave (i.e., the drive frequency f d ) is changed, the frequency at which the oscillation amplitude becomes maximum is called the first resonance frequency f r1 Similarly, a sinusoidal voltage signal is input to only the second actuator 22 to cause the mirror section 20 to oscillate around the second axis a2, and the frequency of the sinusoidal wave (i.e., the drive frequency f d ) is changed, the frequency at which the oscillation amplitude becomes maximum is called the second resonance frequency f r2 It was decided.

[0084] Furthermore, the spiral rotation of the mirror section 20 is an operation in which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 each change over time within a range from a first value to a second value (for example, from 5° to 10°). Here, the second value is greater than the first value. In the present disclosure, the resonance frequency when the oscillation amplitude around the first axis a1 is the second value is referred to as the first resonance frequency f r1 The resonance frequency when the oscillation amplitude around the second axis a2 is a second value is defined as a second resonance frequency f r2 It is defined as:

[0085] Next, the first resonant frequency f r1 For the lower and higher resonance frequencies f r1L ,f r1H and the second resonant frequency f r2 For the lower and higher resonance frequencies f r2L ,f r2H These resonance frequencies were measured using a vibration analyzer (Polytec MSA-500) that utilizes a laser Doppler measurement device. By inputting a chirp voltage waveform or a noise voltage waveform to each of the first actuator 21 and the second actuator 22 and measuring multiple points on the surface of the MEMS mirror 2 using laser Doppler vibration analysis, it is possible to visualize the vibration shape in the out-of-plane direction.

[0086] Also, the first resonance frequency f r1 and the second resonance frequency f r2 The resonant frequency f r1L ,f r1H ,f r2L ,f r2H It is also possible to measure the first resonance frequency f r1 and the second resonance frequency f r2 By searching the frequency range lower than each of the low-order resonance frequencies f r1L ,f r2L The first resonance frequency f r1 and the second resonance frequency f r2 By searching a wide frequency range higher than each of ther1H ,f r2H can be measured.

[0087] Figure 7 shows the measurement results of the resonance frequencies of the fundamental resonance mode, lower-order and higher-order resonance modes. r1L ,f r1H ,f r2L ,f r2H Measured value of the first resonance frequency f r1 and the second resonance frequency f r2 The measurement values were measured using the same measurement method as above.

[0088] 1st resonance frequency f r1 The second resonance frequency f was 1448.2 Hz. r2 The resonance frequency f r1L ,f r1H The resonance frequencies f r2H was 10400.0Hz.

[0089] Next, various resonance modes will be described in detail. Fig. 8 shows the results of a simulation calculation of the shape of a fundamental resonance mode among the resonance modes involving mirror tilt oscillation around the first axis a1. In this fundamental resonance mode, the first actuator 21 and the mirror section 20 oscillate around the first axis a1 in an anti-phase relationship. In this way, when the first actuator 21 and the mirror section 20 oscillate in anti-phase relationship, leakage of vibration energy to the fixed section 27 is suppressed. This allows the MEMS mirror 2 to be driven efficiently and with low power consumption.

[0090] Fig. 9 shows the results of a simulation calculation of the shape of the resonance mode one order lower than the fundamental resonance mode. In this lower resonance mode, the first actuator 21 and the mirror section 20 oscillate around the first axis a1 in phase with each other. When the first actuator 21 and the mirror section 20 oscillate in phase with each other in this way, vibration energy leaks to the fixed section 27, resulting in relatively poor drive efficiency. Fig. 10 shows the results of a simulation calculation of the shape of the resonance mode one order higher than the fundamental resonance mode.

[0091] Next, in order to examine the difference in drive efficiency between the fundamental resonance mode and the lower-order resonance mode, the first actuator 21 was driven at a resonance frequency f r1 The driving frequency f d The fundamental resonance mode shown in FIG. 8 was excited by inputting a sinusoidal signal having the following formula: and one-dimensional scanning was performed to examine the relationship between the first deflection angle θ1 of the mirror section 20 and the drive voltage (amplitude of the drive signal). In this case, when the drive voltage was set to 4 Vpp, θ1=5°. In addition, when the first actuator was set to a resonance frequency f r1L The driving frequency f d By inputting a sinusoidal signal of the following formula, the low-order resonance mode shown in Fig. 9 was excited. In this case, when the drive voltage was set to 4 Vpp, θ1=2.2°.

[0092] In this way, by using a resonance mode in which the first actuator 21 and the mirror section 20 oscillate in an antiphase relationship, the MEMS mirror 2 can be driven with a low drive voltage even when the mirror section 20 is caused to perform a spiral scan operation, thereby achieving low power consumption and a wide scan angle. However, such a resonance mode is generally not the lowest-order resonance mode among the many resonance modes around the first axis a1. Therefore, at least another resonance mode in which the first actuator 21 and the mirror section 20 oscillate in an in-phase relationship is always present on the low-frequency side. In the present disclosure, it has been found that when the mirror section 20 is caused to perform a spiral scan operation, not only resonance modes on the high-frequency side relative to the fundamental resonance mode but also resonance modes on the low-frequency side as shown in FIG. 9 have a significant impact on the variation in line spacing.

[0093] According to the technology of the present disclosure, by removing both low-frequency and high-frequency components from the drive signal, it is possible to achieve spiral scan operation with equal line spacing while using a highly efficient drive mode.

[0094] In the fundamental resonance mode of an axis on which a resonance mode one order lower than the fundamental resonance mode exists, it is preferable that one of the first actuator 21 and the second actuator 22, which drives the mirror section 20 around the axis on which the lower-order resonance mode exists, oscillates in an anti-phase relationship with the mirror section 20. In the example shown in Fig. 7, it is preferable that the first actuator 21, which drives the mirror section 20 around the first axis a1 on which at least a resonance mode one order lower than the fundamental resonance mode exists, oscillates in an anti-phase relationship with the mirror section 20.

[0095] Next, the first frequency range B1 and the second frequency range B2 were calculated for the resonance mode involving the tilt oscillation of the mirror around the first axis a1, in which the resonance modes exist on the lower and higher order sides of the fundamental resonance mode. B1L ,f B1H , and a frequency f that defines the second frequency range B2 B2L ,f B2HThe calculation results are shown below.

[0096] In this experiment, the mirror section 20 was caused to perform a spiral rotation by applying a first drive signal and a second drive signal, which are periodic voltage signals whose amplitude and phase change over time, to the MEMS mirror 2. Then, while the mirror section 20 was performing the spiral rotation, a light beam L was irradiated onto the mirror section 20 from the light source 3. The light beam L reflected by the mirror section 20 was made incident on a PSD (Position Sensor Diode) element, and the voltage signal output from the PSD element was converted into the incident position of the light beam L, thereby measuring the variation in the line spacing of the spiral trajectory.

[0097] In this experiment, the content of the filtering process by the frequency filtering unit 46 was changed to change the voltage level ratio R, and the dependency of the variation in the line spacing of the spiral trajectory on the voltage level ratio R was evaluated. d was set to 1456Hz.

[0098] 12 shows the seven conditions used in this experiment and the experimental results for each condition. Condition 1 means that no filtering is performed on the first drive signal and the second drive signal. Conditions 2 to 5 mean that the frequency filter processing unit 46 is a Butterworth band-pass filter, and the order and cut-off frequency f of the band-pass filter are cL ,f cH Either one of the two is different. cL is the cutoff frequency at the low frequency end of the passband. cH is the cutoff frequency at the high frequency end of the pass band. Conditions 6 and 7 are that the frequency filter processing unit 46 is a Butterworth low-pass filter, and the cutoff frequency f cH Under conditions 6 and 7, the frequency filter processing unit 46 sets the cutoff frequency f cH The following frequency bands are passbands:

[0099] The mirror section 20 was caused to perform a spiral rotation operation under each of conditions 1 to 7, and the maximum voltage level in the entire frequency range, the maximum voltage level in the first frequency range B1, the maximum voltage level in the second frequency range B2, the voltage level ratio R, and the line spacing variation of the frequency components of the first drive signal were evaluated. The maximum voltage level in the entire frequency range corresponds to the second voltage level VL2 described above. The larger of the maximum voltage level in the first frequency range B1 and the maximum voltage level in the second frequency range B2 corresponds to the first voltage level VL1 described above. The line spacing variation is represented by the tilt angle of the mirror section 20.

[0100] Fig. 13 is a graph showing the relationship between the voltage level ratio R shown in Fig. 12 and the variation in line spacing. Fig. 13 shows that the lower the voltage level ratio R, the smaller the variation in line spacing (i.e., the better the resolution). In particular, when the voltage level ratio R is -55 dBV or less, the variation in line spacing is 0.05° or less. A variation in line spacing of 0.05° corresponds to about half the line spacing of an ideal spiral orbit, and sufficient resolution can be obtained.

[0101] [Details of experimental results under condition 1] Next, the details of the experimental results under the above condition 1 will be described. m The driving voltage waveform V 1A (t) and V 2A (t) shows the time change of the driving voltage waveform V 1A (t). Figure 14(B) shows the driving voltage waveform V 2A (t) shows the driving voltage waveform V 1B (t) and V 2B (t) are the drive voltage waveforms V 1A (t) and V 2A Since this is the inverse of (t), it is not shown in the figure.

[0102] The driving voltage waveform V shown in Figure 14(A) 1A (t) and V 1B(t) and the driving voltage waveform V shown in FIG. 2A (t) and V 2B A second drive signal consisting of (t) and (t) was applied to the MEMS mirror 2. As a result, the first deflection angle θ1 and the second deflection angle θ2 of the mirror part 20 performing spiral rotation were measured. In practice, a negative bias of -15 V was added to each of the above drive voltage waveforms in order to prevent polarization reversal of the first actuator 21 and the second actuator 22, which are piezoelectric actuators.

[0103] In the experiment under condition 1, the frequency filter processing unit 46 did not perform filtering.

[0104] FIG. 15 shows the drive voltage waveform V applied to the first movable portion 21A of the first actuator 21. 1A Fig. 15 shows the frequency components of the driving voltage waveform V 1A 15 plots the frequency components obtained by Fourier transforming (t). It can be seen from Fig. 15 that when no filtering is performed, voltage components with a certain voltage level or higher exist in the first frequency range B1 and the second frequency range B2 shown in Fig. 11.

[0105] Although the frequency components included in the second drive signal are not shown, the high-order resonance frequency f r2H The voltage level ratio R in the second frequency range B2 (range of 9880 Hz to 10920 Hz) including the reference frequency (see FIG. 7) was −65 dBV, which was a sufficiently small value.

[0106] Fig. 16 shows the measurement results of the first deflection angle θ1 and the second deflection angle θ2 within one modulation period Tm. In Fig. 16, the solid line indicates the change over time of the first deflection angle θ1, and the dashed line indicates the change over time of the second deflection angle θ2. As shown in Fig. 16, in the expansion period TE and the contraction period TS, the maximum and minimum values of the first deflection angle θ1 and the maximum and minimum values of the second deflection angle θ2 each change linearly. That is, in one modulation period T mincludes a period in which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0107] According to FIG. 16, it can be seen that a spiral rotation operation is realized in which the oscillation amplitude linearly expands in the range of 5° to 10° in 0.43 seconds and linearly contracts in 0.01 seconds.

[0108] Figure 17 shows the time required for one modulation period T m 17 shows the measurement results of the spiral trajectory in the expansion period TE within the period. Based on the measurement results of the spiral trajectory shown in FIG. 17, the variation in line spacing around the first axis a1 was measured. The variation in line spacing around the first axis a1 means the variation in the spacing of the spiral trajectory crossing the straight line α shown in FIG. 17. Note that the value of the variation in line spacing in FIG. 12 is the maximum absolute value when the absolute value of the difference in line spacing between adjacent line numbers is calculated for all line numbers.

[0109] Fig. 18 shows the measurement results of the line spacing around the first axis a1. Fig. 18 shows the line spacing measured on the straight line α, plotted against the line number. The line number is a number that identifies the line of the spiral trajectory that crosses the straight line α. Specifically, the line numbers are assigned to the multiple lines that cross the straight line α in order of the smallest first deflection angle θ1.

[0110] 18, it can be seen that the line spacing varies greatly under condition 1, where no filtering is performed. Under condition 1, the maximum value of the line spacing variation was 0.342°.

[0111] The line spacing around the second axis a2 (the line spacing along the straight line β shown in FIG. 17) varied little, with the maximum variation being 0.031°. In other words, the variation in line spacing around the second axis a2 was less than 1 / 10 of the variation in line spacing around the first axis a1. [Details of experimental results under condition 4] Next, the details of the experiment results under the above condition 1 will be described. m The driving voltage waveform V 1A (t) and V 2A (t) shows the time change of the driving voltage waveform V 1A (t). Figure 19(B) shows the driving voltage waveform V 2A (t) shows the driving voltage waveform V 1B (t) and V 2B (t) are the drive voltage waveforms V 1A (t) and V 2A Since this is the inverse of (t), it is not shown in the figure.

[0112] The driving voltage waveform V shown in Figure 19(A) 1A (t) and V 1B (t) and the driving voltage waveform V shown in FIG. 2A (t) and V 2B A second drive signal consisting of (t) and (t) was applied to the MEMS mirror 2. As a result, the first deflection angle θ1 and the second deflection angle θ2 of the mirror part 20 performing spiral rotation were measured. In practice, a negative bias of -15 V was added to each of the above drive voltage waveforms in order to prevent polarization reversal of the first actuator 21 and the second actuator 22, which are piezoelectric actuators.

[0113] In the experiment under condition 4, the frequency filter processing unit 46 was an eighth-order Butterworth band-pass filter (see FIG. 12).

[0114] FIG. 20 shows the driving voltage waveform V applied to the first movable part 21A of the first actuator 21. 1A Fig. 20 shows the frequency components of the driving voltage waveform V 1A 20 is a plot of the frequency components obtained by Fourier transforming (t). It can be seen from Fig. 20 that the voltage components in the first frequency range B1 and the second frequency range B2 are significantly reduced by performing filtering.

[0115] Figure 21 shows the time required for one modulation period T m21 shows the measurement results of the first deflection angle θ1 and the second deflection angle θ2 within one modulation period T. In FIG. 21, the solid line indicates the change over time of the first deflection angle θ1, and the dashed line indicates the change over time of the second deflection angle θ2. As shown in FIG. 21, in the expansion period TE and the contraction period TS, the maximum and minimum values of the first deflection angle θ1 and the maximum and minimum values of the second deflection angle θ2 each change linearly. That is, in one modulation period T m includes a period in which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0116] According to FIG. 21, it can be seen that a spiral rotation operation is realized in which the oscillation amplitude linearly expands in the range of 5° to 10° in 0.43 seconds and linearly contracts in 0.01 seconds.

[0117] Figure 22 shows the time required for one modulation period T m 22 shows the measurement results of the spiral trajectory in the expansion period TE within the spiral trajectory. Based on the measurement results of the spiral trajectory shown in FIG. 22, the variation in the line spacing around the first axis a1 was measured. The variation in the line spacing around the first axis a1 means the variation in the spacing of the spiral trajectory that crosses the straight line α shown in FIG.

[0118] Fig. 23 shows the measurement results of the line spacing around the first axis a1. Fig. 23 shows the line spacing measured on the straight line α, plotted against the line number. The line number is a number that identifies the line of the spiral trajectory that crosses the straight line α. Specifically, the line numbers are assigned to the multiple lines that cross the straight line α in order of the smallest first deflection angle θ1.

[0119] 23, it can be seen that under condition 4, the variation in the line spacing is reduced compared to condition 1. Under condition 4, the maximum value of the variation in the line spacing was 0.0183°.

[0120] The line spacing around the second axis a2 (the line spacing on the straight line β shown in FIG. 22) had little variation, with the maximum value of the variation in line spacing being 0.021°.

[0121] [Details of experimental results under condition 6] Next, we will explain the details of the experimental results under the above condition 6. In the experiment under condition 6, the frequency filter processing unit 46 is an eighth-order Butterworth low-pass filter, and of the first frequency range B1 and the second frequency range B2, only the frequency components in the second frequency range B2 are suppressed.

[0122] Figure 24 shows the time required for one modulation period T m 24 shows the measurement results of the spiral trajectory in the expansion period TE within the spiral trajectory. Based on the measurement results of the spiral trajectory shown in FIG. 24, the variation in the line spacing around the first axis a1 was measured. The variation in the line spacing around the first axis a1 means the variation in the spacing of the spiral trajectory that crosses the straight line α shown in FIG.

[0123] Fig. 25 shows the measurement results of the line spacing around the first axis a1. Fig. 25 shows the line spacing measured on the straight line α, plotted against the line number. The line numbers are numbers that identify the lines of the spiral trajectory that cross the straight line α. Specifically, the line numbers are assigned to the multiple lines that cross the straight line α in order of the smallest first deflection angle θ1.

[0124] 25, under condition 6, the variation in line spacing is reduced compared to condition 1, but the variation in line spacing is large in areas with small line numbers, with the maximum value being 0.15°. This is due to the excitation of unwanted resonance modes on the lower frequency side than the fundamental resonance mode among the resonance modes accompanying the tilt oscillation of the mirror around the first axis a1.

[0125] In the above embodiment, the mirror driver 44 is provided with the frequency filter processor 46, but the frequency filter processor 46 may not be provided. That is, the frequency filter processor 46 may not be provided, and the drive signal generated by the drive signal generator 45 may satisfy the relationship R≦−55 dBV.

[0126] The configuration of the MEMS mirror 2 shown in the above embodiment can be modified as appropriate. For example, in the above embodiment, the first actuator 21 and the second actuator 22 are annular, but one or both of the first actuator 21 and the second actuator 22 can also have a meander structure. Furthermore, the first support portion 24 and the second support portion 25 can also be support members having a configuration other than a torsion bar.

[0127] Furthermore, various modifications are possible to the hardware configuration of the drive control unit 4. The processing unit of the drive control unit 4 may be configured with one processor, or may be configured with a combination of two or more processors of the same or different types (for example, a combination of multiple FPGAs (Field Programmable Gate Arrays) and / or a combination of a CPU and an FPGA).

[0128] All publications, patent applications, and technical standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, or technical standard was specifically and individually indicated to be incorporated by reference. [Explanation of symbols]

[0129] 2. MEMS mirror 3 light source 4 Drive control unit 5. Scanned surface 10 Optical scanning device 20 Mirror section 20A reflective surface 21 First actuator 21A 1st moving part 21B 2nd moving part 22 Second actuator 22A 1st moving part 22B 2nd moving part 23 Support Frame 24 1st support part 25 Second support part 26 Connection 27 Fixed part 43 Light source driver 44 Mirror drive unit B1 First frequency range B2 Second frequency range L Light beam N normal TE Expansion Period TS contraction period T m Modulation Period a1 First axis a2 2nd axis α,β straight line

Claims

1. a mirror device including: a mirror portion having a reflecting surface that reflects incident light and that is oscillating around a first axis and a second axis that are orthogonal to each other; a first actuator that applies a rotational torque about the first axis to the mirror portion to cause the mirror portion to oscillate around the first axis; and a second actuator that applies a rotational torque about the second axis to the mirror portion to cause the mirror portion to oscillate around the second axis; a processor for providing a first drive signal to the first actuator and a second drive signal to the second actuator; An optical scanning device comprising: the processor causes the mirror section to perform a spiral rotation operation by setting the first drive signal and the second drive signal to periodic voltage signals; at least one of the plurality of resonance modes involving the mirror tilt oscillation around the first axis and the plurality of resonance modes involving the mirror tilt oscillation around the second axis includes a resonance mode of one order lower than a fundamental resonance mode that is closest to the frequency of the periodic voltage signal; For each axis, the resonance frequency one order lower than the frequency of the fundamental resonance mode is set as f ｒＬ , the resonance frequency one order higher than the frequency of the fundamental resonance mode is f ｒＨ and when the ratio of the first voltage level to the second voltage level, which is the maximum voltage level in the entire frequency range of the frequency components of the periodic voltage signal, is -55 dBV or less. Here, for the axis on which the low-order resonance mode exists out of the first axis and the second axis, (1±1 / 20)×f ｒＬ and (1±1 / 20)×f ｒＨ The maximum value of the voltage level in the frequency range of is the first voltage level, and for an axis where the low-order resonance mode does not exist, the frequency component of the periodic voltage signal is (1±1 / 20)×f ｒＨ the maximum voltage level in the frequency range is the first voltage level; Optical scanning device.

2. In the fundamental resonance mode of the axis on which the lower-order resonance mode exists, one of the first actuator and the second actuator, which drives the mirror unit around the axis on which the lower-order resonance mode exists, oscillates in an anti-phase relationship with the mirror unit.

2. The optical scanning device according to claim 1.

3. the processor performs frequency filtering on the first drive signal and the second drive signal so that the ratio of the first voltage level to the second voltage level is −55 dBV or less; 3. The optical scanning device according to claim 1.

4. The frequency filtering is digital filtering or analog filtering.

4. The optical scanning device according to claim 3.

5. The periodic voltage signal is a signal whose amplitude and phase vary over time. The optical scanning device according to any one of claims 1 to 4.

6. the spiral rotation operation includes a period during which the oscillation amplitude of the mirror part about the first axis and the oscillation amplitude of the mirror part about the second axis each change linearly. The optical scanning device according to any one of claims 1 to 5.

7. A control method for an optical scanning device equipped with a mirror device including: a mirror unit having a reflective surface that reflects incident light and that is oscillating around a first axis and a second axis that are orthogonal to each other; a first actuator that applies a rotational torque about the first axis to the mirror unit to cause the mirror unit to oscillate around the first axis; and a second actuator that applies a rotational torque about the second axis to the mirror unit to cause the mirror unit to oscillate around the second axis, a first drive signal to be applied to the first actuator and a second drive signal to be applied to the second actuator are periodic voltage signals, thereby causing the mirror portion to perform a spiral rotation operation; at least one of the plurality of resonance modes involving the mirror tilt oscillation around the first axis and the plurality of resonance modes involving the mirror tilt oscillation around the second axis includes a resonance mode of one order lower than a fundamental resonance mode that is closest to the frequency of the periodic voltage signal; For each axis, the resonance frequency one order lower than the frequency of the fundamental resonance mode is set as f ｒＬ , the resonance frequency one order higher than the frequency of the fundamental resonance mode is f ｒＨ and when the ratio of the first voltage level to the second voltage level, which is the maximum voltage level in the entire frequency range of the frequency components of the periodic voltage signal, is -55 dBV or less. Here, for the axis on which the low-order resonance mode exists out of the first axis and the second axis, (1±1 / 20)×f ｒＬ and (1±1 / 20)×f ｒＨ The maximum value of the voltage level in the frequency range of is the first voltage level, and for an axis where the low-order resonance mode does not exist, the frequency component of the periodic voltage signal is (1±1 / 20)×f ｒＨ the maximum voltage level in the frequency range is the first voltage level; A method for controlling an optical scanning device.

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