Optical scanning device and control method thereof

Abstract

To provide an optical scanner and a control method for the same that can achieve a spiral rotation action in which a speed of change in oscillation amplitude is high and a radius vector linearly changes even when a resonance frequency and drive frequency do not match.SOLUTION: An optical scanner comprises: a mirror unit that has a reflection surface reflecting incidence light, and can oscillate around mutually orthogonal first axis and second axis; a mirror device that has a first actuator giving the mirror unit a rotation torque around the first axis to thereby cause the mirror unit to oscillate around the first axis, and a second actuator giving the mirror unit a rotation torque around the second axis to thereby cause the mirror unit to oscillate around the second axis; and a processor that gives the first actuator a first drive signal, and gives the second actuator a second drive signal. The processor is configured to determine the first drive signal and the second drive signal respectively as a periodical voltage signal in which amplitude and phase change timewise, and cause the mirror unit to perform a spiral rotation action including a period during which oscillation amplitude around the first axis and oscillation amplitude around the second axis linearly change.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The technology disclosed herein 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 obtain a 360° field of view are attracting attention. Some omnidirectional LiDAR devices are composed of a combination of a MEMS (Micro Electro Mechanical Systems) mirror and an omnidirectional lens. LiDAR devices that use MEMS mirrors are lightweight and can be made at low cost.

[0003] In an omnidirectional LiDAR device, the MEMS mirror needs to thoroughly scan the doughnut-shaped incident surface of the omnidirectional lens with a light beam. In order to scan the above-mentioned range more efficiently, it is desirable for the MEMS mirror to perform a spiral scan so that the moving radius of the light beam changes linearly with respect to time, and for this purpose, it is required to perform a spiral rotation operation in which the oscillation angle amplitude (hereinafter referred to as the oscillation amplitude) of the mirror part changes at a constant speed. Furthermore, when such a LiDAR device is mounted on a moving body or the like and used, it is important to scan a wider range at a high frame rate. For this purpose, it is required to increase the change speed of the oscillation amplitude of the mirror part. Specifically, in consideration of the practicality of the LiDAR device, the oscillation amplitude needs to change at least at a speed of 0.5 rad / s. This corresponds to, for example, a reciprocating motion in which the change amount of the oscillation amplitude is 3° and the time required for the change is 0.1 s, and the minimum required scan range can be achieved at a frame rate of 10 Hz.

[0004] Patent Document 1 describes a technique 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 perform a first oscillation about a first axis parallel to a plane including the oscillating plate, and a second oscillating means for causing the oscillating plate to perform a second oscillation about a second axis parallel to the plane including the oscillating plate and perpendicular to the first axis, at the same frequency as the first oscillation and with a phase difference 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 the light reflected by the oscillating plate in a vortex shape (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 in order 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] JP 2008-170500 A Summary of the Invention [Problem to be solved by the invention]

[0007] When the resonant frequencies and drive frequencies around the first and second axes are all the same, or when the rate of change of the oscillation amplitude is slow, it is possible to perform a spiral rotation operation in which the radius changes linearly by changing only the amplitude of the drive signal, as described in Patent Document 1.

[0008] However, because the resonance frequencies around the two axes vary and change due to processing variations and temperature changes in the MEMS mirror, it is difficult to always match the drive frequency and the resonance frequency. The same is true when nonlinearity exists in the mechanical characteristics of the MEMS mirror. Nonlinearity is a phenomenon in which the resonance frequency changes due to the effect of changing the rigidity or moment of inertia (hard spring effect or soft spring effect) depending on the magnitude of the mirror's deflection angle, for example.

[0009] The applicant has found that when the resonant frequency and the drive frequency do not match perfectly, if one tries to change the oscillation amplitude at high speed, the response phase of the MEMS mirror cannot be kept constant using a drive method that changes only the amplitude of the drive signal as in the conventional technology, and it is difficult to achieve a spiral rotation operation in which the radius changes linearly. This is particularly noticeable during high-speed modulation where the oscillation amplitude changes at a rate of 0.5 rad / s or more.

[0010] The technology disclosed herein aims to provide an optical scanning device and a control method thereof that are capable of achieving spiral rotation operation in which the oscillation amplitude changes at a fast rate and the radius changes linearly, even when the resonant frequency and the drive frequency do not match. [Means for solving the problem]

[0011] In order to achieve the above-mentioned object, the optical scanning device disclosed herein is an optical scanning device comprising: a mirror device having a mirror section having a reflective surface that reflects incident light and capable of oscillating around a first axis and a second axis that are perpendicular to each other; a first actuator that causes the mirror section to oscillate around the first axis by applying a rotational torque about the first axis to the mirror section; and a second actuator that causes the mirror section to oscillate around the second axis by applying a rotational torque about the second axis to the mirror section; and a processor that provides a first drive signal to the first actuator and a second drive signal to the second actuator, wherein the processor makes the first drive signal and the second drive signal periodic voltage signals whose amplitude and phase change over time, respectively, and causes the mirror section to perform a spiral rotation operation that includes a period during which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis change linearly.

[0012] In order to utilize the resonance phenomenon and achieve large oscillation with low power consumption, it is preferable that the frequency of the periodic voltage signal is a frequency close to the resonance frequency around the first axis and the resonance frequency around the second axis.

[0013] It is preferable that the resonance frequency around the first axis and the resonance frequency around the second axis are different, since this makes it possible to prevent unintended coupling operation of both axes.

[0014] It is preferable that the frequency of the periodic voltage signal is different from at least one of the resonant frequency around the first axis and the resonant frequency around the second axis, thereby making it possible to reduce characteristic variations and changes over time due to abrupt changes in characteristics near the resonance point.

[0015] During the period, it is preferable that the absolute value of the rate of change of the oscillation amplitude around the first axis and the absolute value of the rate of change of the oscillation amplitude around the second axis are each 0.5 rad / s or more. This makes it possible to achieve both a wide field of view and a high frame rate when the optical scanning device is applied to a LiDAR device.

[0016] In the period, it is preferable that the absolute value of the rate of change of the oscillation amplitude around the first axis and the absolute value of the rate of change of the oscillation amplitude around the second axis are each 1.0 rad / s or more.

[0017] The period preferably includes an expansion period during which the oscillation amplitude about the first axis and the oscillation amplitude about the second axis increase linearly, and a contraction period during which the oscillation amplitude about the first axis and the oscillation amplitude about the second axis decrease linearly.

[0018] The control method for an optical scanning device disclosed herein is a control method for an optical scanning device equipped with a mirror device having a mirror section having a reflective surface that reflects incident light and that is oscillating around a first axis and a second axis that are perpendicular to each other, a first actuator that causes the mirror section to oscillate around the first axis by applying a rotational torque about the first axis to the mirror section, and a second actuator that causes the mirror section to oscillate around the second axis by applying a rotational torque about the second axis to the mirror section, wherein a first drive signal given to the first actuator and a second drive signal given to the second actuator are periodic voltage signals whose amplitude and phase change over time, and the mirror section is caused to perform a spiral rotation operation that includes a period during which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis change linearly.

[0019] According to the technology disclosed herein, it is possible to provide an optical scanning device and a control method thereof that can achieve spiral rotation operation in which the oscillation amplitude changes at a fast rate and the radius changes linearly, even when the resonant frequency and the drive frequency do not match. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a schematic diagram of an optical scanning device. [Diagram 2] 2 is a block diagram showing an example of a hardware configuration of a drive control unit. FIG. [Diagram 3] FIG. 1 is a perspective view of the appearance of a micromirror device. [Figure 4] 5A and 5B are diagrams for explaining the deflection angle when the mirror portion swings, in which (A) shows a first deflection angle and (B) shows a second deflection angle. [Diagram 5] 5A and 5B are diagrams showing examples of drive signals applied to a first actuator and a second actuator, in which (A) shows a first drive signal and (B) shows a second drive signal. [Figure 6] FIG. 4 is a diagram showing the driving conditions used in Examples 1 and 2. [Figure 7] 5A and 5B are diagrams showing changes over time in the amplitude and phase of a first drive signal within one modulation period, where (A) shows changes over time in the amplitude of the first drive signal, and (B) shows changes over time in the phase of the first drive signal. [Figure 8] 5A and 5B are diagrams showing changes over time in the amplitude and phase of a second drive signal within one modulation period, where (A) shows changes over time in the amplitude of the second drive signal, and (B) shows changes over time in the phase of the second drive signal. [Figure 9] FIG. 2 is a diagram 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 10] 11A and 11B are diagrams illustrating measurement results of a first deflection angle and a second deflection angle within one modulation period. [Figure 11] FIG. 1 shows a spiral trajectory within one modulation period, where (A) shows the spiral trajectory during the diastolic period, and (B) shows the spiral trajectory during the systolic period. [Figure 12] 5A and 5B are diagrams showing changes over time in the amplitude and phase of a first drive signal within one modulation period, where (A) shows changes over time in the amplitude of the first drive signal, and (B) shows changes over time in the phase of the first drive signal. [Figure 13] 5A and 5B are diagrams showing changes over time in the amplitude and phase of a second drive signal within one modulation period, where (A) shows changes over time in the amplitude of the second drive signal, and (B) shows changes over time in the phase of the second drive signal. [Figure 14] FIG. 2 is a diagram 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] 11A and 11B are diagrams illustrating measurement results of a first deflection angle and a second deflection angle within one modulation period. [Figure 16]FIG. 1 shows a spiral trajectory within one modulation period, where (A) shows the spiral trajectory during the diastolic period, and (B) shows the spiral trajectory during the systolic period. [Figure 17] FIG. 13 is a diagram showing driving conditions used in a comparative example. [Figure 18] 1A and 1B are diagrams showing changes in amplitude over time within one modulation period, in which (A) shows changes in amplitude of a first drive signal and (B) shows changes in amplitude of a second drive signal over time. [Figure 19] FIG. 2 is a diagram showing changes over time in drive voltage waveforms within one modulation period, where (A) shows the drive voltage waveform of a first drive signal, and (B) shows the drive voltage waveform of a second drive signal. [Figure 20] 11A and 11B are diagrams illustrating measurement results of a first deflection angle and a second deflection angle within one modulation period. [Figure 21] FIG. 1 shows a spiral trajectory within one modulation period, where (A) shows the spiral trajectory during the diastolic period, and (B) shows the spiral trajectory during the systolic period. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] An example of an embodiment of the technology of the present disclosure will be described with reference to the accompanying drawings.

[0022] 1 is a schematic diagram of an optical scanning device 10 according to an 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.

[0023] 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.

[0024] The MEMS mirror 2 is a piezoelectric two-axis drive micromirror device that can oscillate the mirror section 20 (see FIG. 3) around a first axis a1 and a second axis a2 perpendicular to the first axis a1. Hereinafter, the direction parallel to the first axis a1 is 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.

[0025] 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 around 90°.

[0026] The light source 3 is, for example, a laser device that emits 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.

[0027] 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.

[0028] Although the details will be described 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 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 draw a spiral trajectory (i.e., a spiral curve).

[0029] 2 shows an example of a hardware configuration of the drive control unit 4. The drive control unit 4 has a central processing unit (CPU) 40, a read only memory (ROM) 41, a random access memory (RAM) 42, a light source drive unit 43, and a mirror drive unit 44. The CPU 40 is a calculation device that realizes the overall function of the drive control unit 4 by reading programs and data from a storage device such as the ROM 41 into the RAM 42 and executing processing. The CPU 40 is an example of a "processor" according to the technology of the present disclosure.

[0030] The ROM 41 is a non-volatile storage device that stores programs for the CPU 40 to execute processes, and data such as the optical scanning information described above. The RAM 42 is a non-volatile storage device that temporarily holds programs and data.

[0031] 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.

[0032] 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 swinging the mirror section 20 of the mirror driver 44. The drive signal includes a first drive signal and a second drive signal, which will be described in detail later.

[0033] For example, in the mirror driver 44, the drive signal is generated as a digital signal and output via a DAC (Digital Analog Converter) and an amplifier. The drive signal may be output as a step-like waveform based on the resolution bit number of the digital signal source. The drive signal can also be generated from a pulse signal and a band-pass filter, etc.

[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 indicating 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 draw a spiral trajectory on the scanned surface 5. 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, and the like.

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

[0036] The MEMS mirror 2 has a mirror portion 20, a first actuator 21, a second actuator 22, a support frame 23, a first support portion 24, a second support portion 25, and a fixing portion 26. The MEMS mirror 2 is formed, for example, by 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 first actuator 21 is disposed so as to surround the mirror section 20. The support frame 23 is disposed so as to surround the mirror section 20 and the first actuator 21. The second actuator 22 is disposed so as to surround the mirror section 20, the first actuator 21, and the support frame 23.

[0039] The first support 24 connects the mirror section 20 and the first actuator 21 on the first axis a1, and supports the mirror section 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 section 20 is stationary. For example, the first support 24 is a torsion bar that extends along the first axis a1. The first support 24 is also connected to the support frame 23 on the first axis a1.

[0040] The second support section 25 connects the first actuator 21 and the second actuator 22 on the second axis a2, and supports the mirror section 20 and the first actuator 21 so as to be swingable around 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. In addition, the second support section 25 is connected to the support frame 23 and the fixed section 26 on the second axis a2.

[0041] The fixed portion 26 is connected to the second actuator 22 by the second support portion 25. The fixed portion 26 has a rectangular outer shape and surrounds the second actuator 22. The lengths of the fixed portion 26 in the X and Y directions are each, for example, about 1 mm to 10 mm. The thickness of the fixed portion 26 in the Z direction is, for example, about 5 μm to 0.2 mm.

[0042] 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 and the first actuator 21. This causes the mirror section 20 to oscillate about the first axis a1 and the second axis a2.

[0043] The first actuator 21 is an annular thin plate member surrounding the mirror section 20 in the XY plane. The first actuator 21 is composed of a pair of a first movable section 21A and a second movable section 21B. The first movable section 21A and the second movable section 21B are each semi-annular. The first movable section 21A and the second movable section 21B are shaped to be line-symmetric with respect to the first axis a1, and are connected on the first axis a1.

[0044] The support frame 23 is an annular thin plate member that surrounds the mirror section 20 and the first actuator 21 in the XY plane.

[0045] The second actuator 22 is an annular thin plate member that surrounds the mirror section 20, the first actuator 21, and the support frame 23 in the XY plane. The second actuator 22 is composed of a pair of a first movable section 22A and a second movable section 22B. The first movable section 22A and the second movable section 22B are each semi-annular. The first movable section 22A and the second movable section 22B are shaped to be line-symmetric with respect to the second axis a2, and are connected on the second axis a2.

[0046] In the first actuator 21, the first movable part 21A and the second movable part 21B are provided with piezoelectric elements 27A and 27B, respectively. In the second actuator 22, the first movable part 22A and the second movable part 22B are provided with piezoelectric elements 28A and 28B, respectively.

[0047] In this example, the first actuator 21 and the second actuator 22 are configured as separate annular structures, but the present invention is not limited to this and they may be configured to coexist within one structure. For example, a piezoelectric body is divided and arranged within one annular structure. By applying a first drive signal and a second drive signal to the two individual piezoelectric body parts thus divided, respectively, it is possible to realize the mirror oscillation around the first axis a1 and the second axis a2.

[0048] Fig. 4 explains the deflection angle when the mirror section 20 swings. Fig. 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). Fig. 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).

[0049] 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 called 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 has a positive value, and when it is tilted in the -Y direction, the first deflection angle θ1 has a negative value.

[0050] The first deflection angle θ1 is controlled by a drive signal (hereinafter, referred to as a 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 driving voltage waveform V 1B (t) are in opposite phase to each other (i.e., the phase difference is 180°).

[0051] 4(B), the angle at which the normal N of 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 of the reflecting surface 20A is tilted in the +X direction, the second deflection angle θ2 has a positive value, and when it is tilted in the -X direction, the second deflection angle θ2 has a negative value.

[0052] 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 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 driving voltage waveform V 2B (t) are in opposite phase to each other (i.e., the phase difference is 180°).

[0053] 5 shows an example of a drive signal applied to the first actuator 21 and the second actuator 22. FIG. 5(A) shows a drive voltage waveform V 1A (t) and V 1B FIG. 5B shows the drive voltage waveform V 2A (t) and V 2B (t) is shown.

[0054] Drive voltage waveform V 1A (t) and V 1B(t) is expressed by the following formula (1A) and formula (1B), respectively.

number

number

[0055] 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 V 1A (t) and the driving voltage waveform V 1B The phase difference with respect to (t) is π (i.e., 180°).

[0056] That is, the first drive signal is a periodic voltage signal whose amplitude and phase change over time. 1A (t) and V 1B When the current (t) is applied to the first movable portion 21A and the second movable portion 21B, the mirror portion 20 rotates around the first axis a1 with a period T d (=1 / f d ) and oscillates.

[0057] Drive voltage waveform V 2A (t) and V 2B (t) is expressed by the following formula (2A) and formula (2B), respectively.

number

number

[0058] 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 driving voltage waveform V 2BThe phase difference with (t) is π (i.e. 180°). 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 (t) is a phase difference between the phases of the mirror section 20 and the phases of the mirror section 20. In this embodiment, φ=π / 2 (i.e., 90°) is set so that the mirror section 20 performs a circular spiral scan operation. 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.

[0059] That is, the second drive signal is a periodic voltage signal whose amplitude and phase change over time. 2A (t) and V 2B When the current (t) is applied to the first movable portion 22A and the second movable portion 22B, the mirror portion 20 rotates around the second axis a2 with a period T d (=1 / f d ) and oscillates.

[0060] 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 polynomial is a cubic function, but it may be a quartic or higher order function. The degree of the polynomial is determined according to the required operational accuracy of the spiral scan and the computing power of the processor. m kp and n kp is a coefficient. Here, k is 0, 1, 2, or 3. p is a or b. In this embodiment, the phase γ2(t) is expressed by a polynomial including the phase difference φ.

[0061]

number

number

number

number

[0062] Coefficient m kp and n kp are determined so that the oscillation amplitude of the mirror section 20 around the first axis a1 and the oscillation amplitude 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.

[0063] For example, the coefficient m kp and n kp is determined by a method in which the drive control unit 4 actually inputs a first drive signal and a second drive signal to the MEMS mirror 2, and adjusts the first deflection angle θ1 and the second deflection angle θ2 of the mirror portion 20 while checking them with a sensor or the like.

[0064] As a sensor for detecting the deflection angle, there is a method in which a light beam L emitted from a light source 3 installed outside the MEMS mirror 2 and reflected by the mirror portion 20 is detected by an optical sensor, and a method in which a strain sensor that generates a voltage according to stress is incorporated on the MEMS mirror 2.

[0065] The MEMS mirror 2 has a frequency of a resonance mode accompanied by oscillation of the mirror portion 20 about a first axis a1 (hereinafter referred to as the first resonance frequency) and a frequency of a resonance mode accompanied by oscillation of the mirror portion 20 about a second axis a2 (hereinafter referred to as the second resonance frequency), and the MEMS mirror 2 is designed so that these frequencies approximately match. However, in reality, due to processing variations, temperature changes, etc., either or both of the first and second resonance frequencies may deviate from the design values, and the two may not match. Drive frequency f dis a frequency close to the first and second resonant frequencies, and preferably coincides with the first and second resonant frequencies. However, when the first and second resonant frequencies do not coincide, the driving frequency f d is different from at least one of the first and second resonant frequencies, the response phase of the MEMS mirror 2 cannot be kept constant by the driving method of changing only the amplitude of the driving signal as in the conventional technology. In other words, it is difficult to realize a spiral rotation operation in which the radius vector changes linearly. In particular, when the rate of change of the oscillation amplitude of the mirror part 20 is large, the phase shift that occurs during modulation becomes noticeable.

[0066] Furthermore, if there is nonlinearity in the mechanical characteristics of the MEMS mirror 2, the resonant frequency itself changes during driving, and the response phase of the mirror to the driving force changes, so that a spiral rotation operation in which the radius changes linearly cannot be realized with a driving method that changes only the amplitude of the driving signal as in the conventional technology. The above-mentioned nonlinearity is caused, for example, by the effect of changing the rigidity or moment of inertia according to the magnitude of the deflection angle of the mirror section 20 (hard spring effect or soft spring effect).

[0067] To address these problems, the first drive signal and the second drive signal are periodic voltage signals whose amplitude and phase change over time, and a coefficient m kp and n kp By appropriately determining, it is possible to realize a spiral rotation operation in which the radius changes linearly.

[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 moving object such as a drone, the modulation period T mIt is desirable that the modulation period T is as small as possible. In this case, for example, the frame rate is required to be at least 10 Hz or more, and preferably 20 Hz or more. That is, m is required to be at least 0.1 seconds or less, and preferably 0.05 seconds or less.

[0069] Moreover, the line spacing of the spiral orbit 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 sparse or dense scanning. In this embodiment, in order to achieve the line spacing of the spiral orbit, a spiral rotation operation in which the radius changes linearly is realized.

[0070] In this embodiment, one modulation period T m In other words, the radius of the spiral orbit is expanded and contracted 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] Considering the practicality of the LiDAR device, it is preferable that the absolute value of the rate of change of the oscillation amplitude around the first axis a1 and the absolute value of the rate of change of the oscillation amplitude around the second axis a2 during the expansion period TE and the contraction period TS are each 0.5 rad / s or more, and more preferably 1.0 rad / s or more.

[0072] Examples will be described below. The following Examples 1 and 2 show experimental results of the spiral rotation operation of the mirror portion 20 when the MEMS mirror 2 is driven under various driving conditions.

[0073] 6 shows the driving conditions used in Examples 1 and 2. In Examples 1 and 2, a first driving signal and a second driving signal based on the following driving conditions were applied to the MEMS mirror 2 to cause the mirror section 20 to perform a spiral rotation operation. Then, while the mirror section 20 was performing the spiral rotation operation, 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 evaluating the spiral trajectory.

[0074] [Example 1] As shown in FIG. 6, in the first embodiment, the modulation period T m The modulation period T m In the above, a period of 0 seconds or more and less than 0.0347 seconds was defined as an diastolic period TE, and a period of 0.0347 seconds or more and 0.0695 seconds or less was defined as a systolic period TS.

[0075] The resonance frequency of the MEMS mirror 2 was measured by the following method. A sine wave voltage signal was input only to the first actuator 21 to cause the mirror section 20 to oscillate around the first axis a1, and the frequency of the sine wave was changed to determine the frequency at which the oscillation amplitude was maximum as the first resonance frequency. Similarly, a sine wave voltage signal was input only to the second actuator 22 to cause the mirror section 20 to oscillate around the second axis a2, and the frequency of the sine wave was changed to determine the frequency at which the oscillation amplitude was maximum as the second resonance frequency. When the oscillation amplitude was 5°, the first resonance frequency was 1238 Hz and the second resonance frequency was 1240 Hz. Driving frequency f d That is, the driving frequency f d is different from the second resonance frequency, and the difference between them is 2 Hz. The resonance frequency changes depending on the oscillation amplitude, temperature, etc. In the target oscillation amplitude range of the mirror section 20, the resonance frequency is set to the driving frequency f d In the case where the region includes regions that do not match, the method disclosed in the present application is effective.

[0076] In the first embodiment, the amplitude A1(t) and phase γ1(t) of the first drive signal are each a cubic function, and the amplitude A2(t) and phase γ2(t) of the second drive signal are each a cubic function, and a coefficient m is set so that the radius of the spiral orbit changes linearly. kp and n kp was adjusted and determined.

[0077] Figure 7 shows the time course of one modulation period T m 7A and 7B show the time changes in the amplitude A1(t) and phase γ1(t) of the first drive signal in the driving signal 100. FIG. 7A shows the time changes in the amplitude A1(t) of the first drive signal. FIG. 7B shows the time changes in the phase γ1(t) of the first drive signal. The amplitude A1(t) shown in FIG. 7A is a function of a coefficient m ka The phase γ1(t) shown in FIG. 7B is obtained by applying the coefficient n ka is obtained by applying to the above equation (4).

[0078] Figure 8 shows the time course of one modulation period T m 8A shows the time change of the amplitude A2(t) and phase γ2(t) of the second drive signal in FIG. 8. FIG. 8B shows the time change of the phase γ2(t) of the second drive signal. The amplitude A2(t) shown in FIG. 8A is a function of a coefficient m kb The phase γ2(t) shown in FIG. 8B is obtained by applying the coefficient n kb is obtained by applying to the above equation (6).

[0079] Figure 9 shows the time course of one modulation period T m The driving voltage waveform V 1A (t) and V 2A The time change of the driving voltage waveform V 1A The driving voltage waveform V(t) shown in FIG. 9(B) is obtained by applying the amplitude A1(t) and phase γ1(t) shown in FIG. 7 to the above equation (1A). 2AThe driving voltage waveform V(t) is obtained by applying the amplitude A2(t) and phase γ2(t) shown in FIG. 8 to the above equation (2A). 1B (t) and V 2B (t) is the driving voltage waveform V 1A (t) and V 2A Since this is the inversion of (t), it is not shown in the figure.

[0080] The driving voltage waveform V shown in FIG. 1A (t) and V 1B (t) and the driving voltage waveform V 2A (t) and V 2B A second drive signal consisting of (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 a spiral rotation operation were measured. Note that, in practice, a negative bias of -20V 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.

[0081] Figure 10 shows the time course of one modulation period T m 10 shows the measurement results of the first deflection angle θ1 and the second deflection angle θ2 within one modulation period T. In FIG. 10, 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. 10, 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 during which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0082] Figure 11 shows the time course of one modulation period T m 11(A) shows a spiral trajectory in an expansion period TE. FIG. 11(B) shows a spiral trajectory in a contraction period TS. As shown in FIG. 11, according to the first embodiment, a spiral rotation operation in which the radius of curvature is linearly expanded and contracted in a range of 0° to 9° at a frequency of 14 Hz is realized.

[0083] The change speed of the oscillation amplitude (also called the modulation speed) calculated from the results shown in Fig. 11 was 4.5 rad / s during the diastolic period TE and -4.5 rad / s during the systolic period TS. That is, the absolute value of the change speed of the oscillation amplitude around the first axis a1 and the absolute value of the change speed of the oscillation amplitude around the second axis a2 were both 1.0 rad / s or more.

[0084] [Example 2] Next, a second embodiment will be described. As shown in FIG. 6, in the second embodiment, the modulation period T m The modulation period T m In this example, a period of 0 seconds or more and less than 0.0443 seconds was defined as an expansion period TE, and a period of 0.0443 seconds or more and 0.055 seconds or less was defined as a contraction period TS. In Example 2, a MEMS mirror 2 having dimensions and a shape different from those in Example 1 was used. The measurement results of the resonance frequency at an oscillation amplitude of 5° were that the first resonance frequency was 1408 Hz and the second resonance frequency was 1401 Hz. The driving frequency f d The second resonance frequency was 1401 Hz. The second resonance frequency exhibited nonlinearity that changed with the angle, and was 1405 Hz when the oscillation amplitude was 10°. In other words, the second resonance frequency and the drive frequency have some parts where they match and some parts where they differ within the range of the oscillation amplitude of 5° to 10°.

[0085] In the second embodiment, the amplitude A1(t) and phase γ1(t) of the first drive signal are each a quadratic function, and the amplitude A2(t) and phase γ2(t) of the second drive signal are each a quadratic function, and a coefficient m is set so that the radius of the spiral orbit changes linearly. kp and n kp In order to make the amplitude and phase two functions, the third-order coefficient m 3p and n 3p was set to 0.

[0086] Figures 12 and 13 correspond to Figures 7 and 8 shown in Example 1, and show the time changes in the amplitude and phase of the first drive signal and the time changes in the amplitude and phase of the second drive signal. Figure 14 corresponds to Figure 9 shown in Example 1, and shows the time changes in the drive voltage waveform.

[0087] Other driving conditions and evaluation methods in Example 2 were the same as those in Example 1. In Example 2, in order to prevent polarization inversion of the first actuator 21 and the second actuator 22, which are piezoelectric actuators, a negative bias of −17 V was added to each of the driving voltage waveforms.

[0088] 15 corresponds to FIG. 10 shown in Example 1, and shows the measurement results of the first deflection angle θ1 and the second deflection angle θ2 in Example 2. The solid line shows the change over time of the first deflection angle θ1, and the dashed line shows the change over time of the second deflection angle θ2. As shown in FIG. 15, 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 during which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0089] Figure 16 shows the time course of one modulation period T m 16A shows a spiral trajectory in an expansion period TE. FIG. 16B shows a spiral trajectory in a contraction period TS. As shown in FIG. 16, according to the second embodiment, a spiral rotation operation in which the radius of curvature is linearly expanded and contracted at a frequency of 18.6 Hz in a range of 5° to 10° was realized.

[0090] The rate of change of the oscillation amplitude calculated from the results shown in Fig. 16 was 2.0 rad / s during the diastolic period TE and -8.2 rad / s during the systolic period TS. That is, the absolute value of the rate of change of the oscillation amplitude around the first axis a1 and the absolute value of the rate of change of the oscillation amplitude around the second axis a2 were both 1.0 rad / s or more.

[0091] [Comparative Example] Next, a comparative example will be described. The comparative example corresponds to the conventional technology, and is an example in which the amplitudes of the first drive signal and the second drive signal are changed linearly and the phase is set to a fixed value.

[0092] FIG. 17 shows the driving conditions used in the comparative example. In the comparative example, the modulation period T m The modulation period T m In the driving frequency f, the period from 0 to less than 0.05 seconds was defined as the expansion period TE, and the period from 0.05 to 0.1 seconds was defined as the contraction period TS. In the comparative example, a MEMS mirror 2 having the same structure as that of the first embodiment was used, and the first resonance frequency was 1238 Hz and the second resonance frequency was 1240 Hz. d was set to 1238Hz.

[0093] In the comparative example, in order to make the amplitude A1(t) of the first drive signal and the amplitude A2(t) of the second drive signal a linear function, a third-order coefficient m 3p , and the second order coefficient m 2p were set to 0. In addition, in order to set the phase γ1(t) of the first drive signal and the phase γ2(t) of the second drive signal to fixed values, the third-order coefficient n 3p , quadratic coefficient n 2p , and the first-order coefficient n 1p and 0 were set to 0. Furthermore, for the phase γ1(t) of the first drive signal, the zeroth-order coefficient n 0a is set to 0, and the zeroth-order coefficient n 0b was set to 40. Note that the coefficient n 0b The value of is determined so that the mirror section 20 performs a circular scan with a constant radius (i.e., a circular scan with a constant radius), i.e., so that the oscillation phase difference between the first axis a1 and the second axis a2 is π / 2.

[0094] Figure 18 shows the time course of one modulation period T m18A shows the change over time of the amplitude A1(t) of the first drive signal. FIG. 18B shows the change over time of the amplitude A2(t) of the second drive signal. As shown in FIG. 18, the amplitudes of the first drive signal and the second drive signal increase and decrease linearly.

[0095] Figure 19 shows the modulation period T m The driving voltage waveform V 1A (t) and V 2A (t) shows the time change of the drive voltage waveform V 1A (t) shows the driving voltage waveform V 2A (t) shows the driving voltage waveform V 1B (t) and V 2B (t) is the driving voltage waveform V 1A (t) and V 2A Since this is the inversion of (t), it is not shown in the figure.

[0096] Other driving conditions and evaluation methods in the comparative example were the same as those in Example 1. In the comparative example, in order to prevent polarization inversion of the first actuator 21 and the second actuator 22, which are piezoelectric actuators, a negative bias of −20 V was added to each of the driving voltage waveforms.

[0097] 20 corresponds to FIG. 10 shown in Example 1, and shows the measurement results of the first deflection angle θ1 and the second deflection angle θ2 in the comparative example. The solid line shows the change over time of the first deflection angle θ1, and the dashed line shows the change over time of the second deflection angle θ2. As shown in FIG. 20, 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 nonlinearly. That is, in one modulation period T m does not include a period in which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0098] Figure 21 shows the time course of one modulation period T m21(A) shows the trajectory in an expansion period TE. FIG. 21(B) shows the trajectory in a contraction period TS. As shown in FIG. 21, in the comparative example, the phase difference between the oscillation of the mirror section 20 about the first axis a1 and the oscillation of the mirror section 20 about the second axis a2 is 1 modulation period T m In other words, in the comparative example, it was not possible to realize a spiral rotation operation in which the radius of curvature changes linearly.

[0099] 21, the change speed of the oscillation amplitude was 0.5 rad / s during the diastolic period TE and -0.5 rad / s during the systolic period TS, which was slow. In the comparative example, the maximum value of the first oscillation angle θ1 was 2.76° and the maximum value of the second oscillation angle θ2 was 4.2°, which were small, and it was not possible to obtain a large oscillation amplitude.

[0100] As described above, it has been confirmed that by making the first drive signal and the second drive signal periodic voltage signals whose amplitude and phase change over time, it is possible to cause the mirror section 20 to perform a spiral rotation operation that includes a period during which the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 change linearly.

[0101] [Variations] Modifications of the above embodiment will now be described.

[0102] In the above embodiment, the first drive signal and the second drive signal are expressed by the above formulas (1A), (1B), (2A), and (2B), and the coefficient m kp and n kp However, the waveform may be determined using an equation of motion that represents the dynamic response of the mirror section 20.

[0103] The dynamic response of the mirror section 20 is expressed by the equation of motion of the following equation (7). F is the rotational torque. I is the moment of inertia. θ(t) is the deflection angle of the mirror section 20. ω r is the resonant frequency. Q is the resonant quality factor.

[0104]

number

[0105] The spiral rotation motion in which the radius changes linearly is expressed by the following equations (8A) and (8B).

[0106]

number

number

[0107] By substituting the above equations (8A) and (8B) into the above equation (7), the following equations (9) to (11) are obtained, where β is a proportionality constant.

[0108]

number

number

number

[0109] The above expressions (9) to (11) are equivalent to a sine wave whose amplitude and phase change over time. Based on the results of driving tests performed on the first axis a1 and the second axis a2 of the mirror section 20, Q, ω r By deriving values ​​such as these and substituting the derived values ​​into the above equations (9) to (11), it is possible to create waveforms representing the first drive signal and the second drive signal.

[0110] Therefore, instead of equations (1A) and (1B) in the above embodiment, it is possible to use the following equations (12A) and (12B), and instead of equations (2A) and (2B), it is possible to use the following equations (13A) and (13B).

[0111]

number

number

number

number

[0112] In addition, the mirror section 20 may be driven using the value derived based on the results of the drive test as described above as the initial value, and the coefficients may be fine-tuned to approach the target response while monitoring the actual response of the mirror section 20 with an angle sensor or the like.

[0113] Equations (12A) and (12B) are equivalent to equations (1A) and (1B), and equations (8A) and (8B) are equivalent to equations (2A) and (2B).

[0114] 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 meandering structure. In addition, it is also possible to use support members having a configuration other than a torsion bar as the first support portion 24 and the second support portion 25.

[0115] In the above embodiment, one modulation period T m In the above example, the oscillation amplitude around the first axis a1 and the oscillation amplitude around the second axis a2 linearly expand and contract in all regions within the range, but this is not limited to the above. For example, in order to wait for the timing of irradiating the LiDAR measurement light, a portion where the oscillation amplitude is constant may be provided, or a portion where the oscillation amplitude temporarily expands or contracts in a curved manner may be included. In other words, it is sufficient to include a portion where the oscillation amplitude changes linearly.

[0116] 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).

[0117] All publications, patent applications, and standards mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent application, and standard was specifically and individually indicated to be incorporated by reference. [Explanation of symbols]

[0118] 2. MEMS mirror 3 light source 4 Drive control section 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 Fixed part 27A, 27B, 28A, 28B Piezoelectric element 43 Light source driver 44 Mirror drive unit L Light beam N Normal TE Extension Period TS Contraction Period T m Modulation Period

Claims

1. a mirror device including a mirror section having a reflecting surface that reflects incident light and capable of swinging about a first axis and a second axis that are perpendicular to each other, a first actuator that applies a rotational torque about the first axis to the mirror section to swing the mirror section about the first axis, and a second actuator that applies a rotational torque about the second axis to the mirror section to swing the mirror section about 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, providing, as the first drive signal and the second drive signal, periodic voltage signals whose amplitude and phase change over time, which are determined so as to cause the mirror portion to perform a spiral rotation operation in which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis change linearly at least during a period in which the incident light is incident on the mirror portion, to the first actuator and the second actuator; Optical scanning device.

2. a resonant frequency around the first axis is different from a resonant frequency around the second axis; 2. The optical scanning device according to claim 1.

3. The frequency of the periodic voltage signal is different from at least one of a resonant frequency about the first axis and a resonant frequency about the second axis.

3. The optical scanning device according to claim 2.

4. During the period, the absolute value of the rate of change of the oscillation amplitude around the first axis and the absolute value of the rate of change of the oscillation amplitude around the second axis are each 0.5 rad / s or more. The optical scanning device according to claim 1 .

5. During the period, the absolute value of the rate of change of the oscillation amplitude around the first axis and the absolute value of the rate of change of the oscillation amplitude around the second axis are each 1.0 rad / s or more. The optical scanning device according to claim 1 .

6. The period includes an expansion period during which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis increase linearly, and a contraction period during which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis decrease linearly. The optical scanning device according to claim 1 .

7. A method for controlling an optical scanning device equipped with a mirror device including a mirror section having a reflective surface that reflects incident light and that is oscillating around a first axis and a second axis that are perpendicular to each other, a first actuator that applies a rotational torque about the first axis to the mirror section to cause the mirror section to oscillate around the first axis, and a second actuator that applies a rotational torque about the second axis to the mirror section to cause the mirror section to oscillate around the second axis, comprising: 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 whose amplitude and phase change over time, a time change in the amplitude and phase of the periodic voltage signal is determined so as to cause the mirror portion to perform a spiral rotation operation in which the oscillation amplitude around the first axis and the oscillation amplitude around the second axis change linearly at least during a period in which the incident light is incident on the mirror portion. A method for controlling an optical scanning device.

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