Vibration sensor

Abstract

A vibration sensor (1) comprises: a light projection unit (10) having a laser light source (20) and a light projection optical system (30); a light reception unit (40) having a line sensor (50) and a light reception optical system (60); and an image processing device (70) that calculates an amount of movement between frames of a speckle pattern generated in reflected and scattered light of the laser light, and estimates vibration of an object (10) on the basis of the amount of movement. A first optical axis of the light projection unit (10) and a second optical axis of the light reception unit (40) intersect each other on the object, an X direction in which the line sensor (50) extends is the same as the direction in which a linear spot (91) extends, the light reception optical system (60) is axially asymmetric with respect to the X direction and a Y direction, an image (93) of the object (90) formed by the light reception optical system (60) is not focused on a light reception surface of the line sensor (50) in the X direction, and the image (93) of the object (90) formed by the light reception optical system (60) is focused on the light reception surface of the line sensor (50) in the Y direction.

Description

Vibration Sensor

[0001] The present disclosure relates to a vibration sensor that irradiates an object with laser light and measures vibrations of the object using a speckle pattern that occurs in the reflected and scattered light.

[0002] When a coherent laser beam is irradiated onto the rough surface of an object, a speckle pattern, which is a pattern of bright and dark spots, appears in the reflected and scattered light. If the object vibrates at this time, the observed speckle pattern will displace in accordance with the vibration of the object. Patent Document 1 discloses a technology for measuring the vibration of an object by irradiating the rough surface of the object with laser beam and capturing the displacement of the generated speckle pattern with a two-dimensional camera.

[0003] Furthermore, Non-Patent Document 1 proposes an apparatus for measuring acoustic vibrations of an object by imaging a speckle pattern generated by irradiation with laser light using a line scan camera.

[0004] U.S. Patent No. 8,638,991

[0005] S. Bianchi et al., “Long-range detection of acoustic vibrations by speckle tracking,” Applied Optics, Vol. 58, No. 28, pp. 7805-7809, (2019).

[0006] However, the above-mentioned conventional technology has a problem in that it is necessary to use a laser beam with a high power as the laser beam to be irradiated.

[0007] An object of the present disclosure is to provide a vibration sensor that can use a laser beam with a low power as the laser beam to be irradiated onto an object.

[0008] The vibration sensor of the present disclosure includes a light-projecting unit having a laser light source and a light-projecting optical system that projects the laser light emitted from the laser light source onto an object as a line spot, a light-receiving unit having a line sensor and a light-receiving optical system that focuses the reflected and scattered light of the laser light irradiated onto the object onto the line sensor, and an image processing device that calculates the amount of movement between frames of a speckle pattern generated in the reflected and scattered light received by the line sensor and estimates the vibration of the object based on the amount of movement, and The second optical axis, which is the optical axis of the light receiving unit, intersects with the object, the first direction in which the line sensor extends is the same as the direction in which the linear spot extends, the light receiving optical system is an optical system that is axially asymmetric with respect to the first direction and a second direction perpendicular to the first direction, and in the first direction, the image of the object formed by the light receiving optical system is not focused on the light receiving surface of the line sensor, and in the second direction, the image of the object formed by the light receiving optical system is focused on the light receiving surface of the line sensor.

[0009] According to the vibration sensor of the present disclosure, a laser beam with a low power can be used as the laser beam to be irradiated onto the object.

[0010] 4A is a schematic perspective view showing the configuration and main light rays of a vibration sensor according to an embodiment, and FIG. 4B is a front view showing a linear spot formed on an object by laser light. FIG. 4A is a schematic side view showing the configuration and main light rays of a light projecting unit of a vibration sensor according to an embodiment, and FIG. 4B is a schematic plan view showing the configuration and main light rays of a light projecting unit of a vibration sensor according to an embodiment. FIG. 4A is a schematic side view showing the configuration and main light rays of a light receiving unit of a vibration sensor according to an embodiment, and FIG. 4B is a schematic plan view showing the configuration and main light rays of a light receiving unit of a vibration sensor according to an embodiment. FIG. 4A and FIG. 4B are schematic plan views showing vibration of an object. FIG. 4A and FIG. 4B are diagrams showing the movement of a speckle pattern generated on an observation surface 92 due to vibration of the object shown in FIG. 4A and FIG. 4B are diagrams showing the movement of a speckle pattern generated on a line sensor 50 due to vibration of the object shown in FIG. 4A and FIG. 4B. 1A and 1B are diagrams showing an example of a one-dimensional speckle pattern waveform that moves due to vibration. (A) and (B) are diagrams showing an example of a process for calculating the vibration amount of a speckle pattern from waveform matching. (A) and (B) are diagrams showing an example of a process for calculating the vibration amount of a speckle pattern from waveform matching. (A) to (C) are diagrams showing differences in speckle pattern waveforms due to beam width. (A) to (C) are diagrams showing overlapping between the light receiving surface of a line sensor and an image of a speckle pattern (image of a line-shaped spot). (A) to (D) are diagrams showing an example of the hardware configuration of an image processing device for a vibration sensor according to an embodiment. (A) to (D) are schematic plan views showing image shifts when the distance changes in the vibration sensor according to an embodiment. 1A shows the image shift when the stretching direction is parallel to the X direction and L=L0, (B) shows the image shift when the stretching direction is parallel to the X direction and L and L0 are different, (C) shows the image shift when the stretching direction is inclined with respect to the X direction and L=L0, and (D) shows the image shift when the stretching direction is inclined with respect to the X direction and L and L0 are different.

[0011] Hereinafter, vibration sensors according to embodiments will be described with reference to the drawings. The following embodiments are merely examples, and the embodiments can be modified as appropriate.

[0012] Configuration of the Embodiment Fig. 1A is a schematic perspective view showing the configuration and main light rays of a vibration sensor 1 according to an embodiment. Fig. 1B is a front view showing a linear spot 91 formed by laser light irradiated onto an object 90 that is the target of vibration sensing. The vibration sensor 1 detects vibration of the object 90 by irradiating the object 90 (rough surface) with laser light and observing the reflected and scattered light. The vibration sensor 1 includes a light-projecting unit 10 that irradiates the object 90 with laser light, a light-receiving unit 40 that receives the reflected and scattered light generated by the object 90 irradiated with the laser light, and an image processing device 70 that serves as an information processing device that detects vibration based on a detection signal output from the light-receiving unit 40. Typically, the light-projecting unit 10 and the light-receiving unit 40 are housed within a light-projecting and light-receiving head 2 (e.g., within a single housing).

[0013] The figure shows the coordinate axes of an X'YZ' Cartesian coordinate system as a first coordinate system used to explain the light-projecting unit 10. The figure also shows the coordinate axes of an XYZ Cartesian coordinate system as a second coordinate system used to explain the light-receiving unit 40, the target object 90, and the observation surface 92. The central ray of the laser light emitted from the light-projecting unit 10 is parallel to the Z' axis, and the central ray of the light received by the light-receiving unit 40 is parallel to the Z axis. The Y axis is a coordinate axis normal to a plane including the Z' axis and the Z axis. The X' axis is a coordinate axis perpendicular to both the Y axis and the Z' axis. The X axis is a coordinate axis perpendicular to both the Y axis and the Z axis.

[0014] The light-projecting unit 10 has a laser light source 20 and a light-projecting optical system 30 that projects laser light emitted from the laser light source 20 onto the object 90 as a line spot 91. The light-receiving unit 40 has a line sensor 50 and a light-receiving optical system 60 that focuses the reflected and scattered light of the laser light irradiated onto the object 90 onto the line sensor 50 (i.e., onto the light-receiving surface of the line sensor 50). The image processing device 70 calculates the amount of movement between frames of a speckle pattern generated in the reflected and scattered light received by the line sensor 50, and estimates (i.e., calculates) the vibration of the object 90 based on this amount of movement.

[0015] The light-projecting unit 10 and the light-receiving unit 40 are arranged so that a first optical axis 11, which is the optical axis of the light-projecting unit 10, and a second optical axis 41, which is the optical axis of the light-receiving unit 40, intersect on the target object 90. The light-projecting unit 10 and the light-receiving unit 40 are arranged so that a first direction (X direction) in which the line sensor 50 extends (i.e., the light-receiving surface of the line sensor 50 extends) is the same as the direction in which the linear spot 91 extends. Here, the optical axis of the light-projecting unit 10 may be the axis of the laser light emitted from the light-projecting unit 10. Furthermore, the optical axis of the light-receiving unit 40 may be the axis of reflected and scattered light that can be received by the line sensor 50.

[0016] The light receiving optical system 60 is asymmetric with respect to the X direction, which is a first direction, and the Y direction, which is a second direction perpendicular to the X direction (i.e., axially asymmetric with respect to the X and Y axes). In the X direction, which is the longitudinal direction of the line sensor 50 (i.e., the longitudinal direction of the light receiving surface of the line sensor 50), the image of the object 90 (i.e., the image 93 of the linear spot 91) formed by the light receiving optical system 60 is not focused on the light receiving surface of the line sensor 50. In the Y direction, which is the lateral direction of the line sensor 50 (i.e., the lateral direction of the light receiving surface of the line sensor 50), the image of the object 90 (i.e., the image 93 of the linear spot 91) formed by the light receiving optical system 60 is focused on the light receiving surface of the line sensor 50.

[0017] In this embodiment, the light projecting unit 10 and the light receiving unit 40 are arranged side by side in the X direction (or X' direction). In addition, it is desirable that the light projecting optical system 30 includes an adjustment mechanism 31a that changes the size of the linear spot 91 in the X direction.

[0018] In this embodiment, the projection optical system 30 includes an X-direction beam width conversion element 34, which is a set of two cylindrical lenses 32, 33 that form the linear spot 91. The projection optical system 30 includes the concave cylindrical lens 32 as an asymmetric optical system. The X-direction beam width conversion element 34 is an optical system (for example, a lens or a group of lenses) that can change the width (length Bx) in the X direction of the laser light emitted from the laser light source 20.

[0019] The light projecting unit 10 also has a collimating lens 31. An adjustment mechanism 31 a that changes the size of the linear spot 91 in the X direction is a mechanism for adjusting (for example, by manual operation) the distance Lc between the light emitting point of the laser light source 20 and the collimating lens 31.

[0020] 2A is a schematic side view (i.e., a view of the light-projecting unit 10 viewed in the X' direction) showing the configuration and main light rays of the light-projecting unit 10 of the vibration sensor 1 according to the embodiment. FIG. 2B is a schematic plan view (i.e., a view of the light-projecting unit 10 viewed in the -Y direction) showing the configuration and main light rays of the light-projecting unit 10 of the vibration sensor 1.

[0021] The first optical axis 11 of the light-projecting unit 10 and the second optical axis 41 of the light-receiving unit 40 intersect on the object 90. It is desirable to provide a mechanism for adjusting the direction of the first optical axis 11 of the light-projecting unit 10 or the direction of the second optical axis 41 of the light-receiving unit 40 so that the first optical axis 11 of the light-projecting unit 10 and the second optical axis 41 of the light-receiving unit 40 intersect even if the distance from the vibration sensor 1 to the object 90 changes.

[0022] 2A and 2B show a beam emitted from a single point of the laser light source 20. In FIGS. 2A and 2B, the laser light source 20 is, for example, a semiconductor laser. The beam (laser light) emitted from the semiconductor laser has a divergence. The beam is converted into a roughly parallel beam by a collimating lens 31. The parallel beam is then converted into a beam diverging in the X' direction by an X-direction beam width conversion element 34, which serves as a beam width conversion unit installed downstream of the collimating lens 31, and is irradiated onto a distant object 90 as a linear spot 91 extending in the X direction. Here, the collimating lens 31 is finely adjusted forward and backward in the optical axis direction using an adjustment mechanism 31a, thereby adjusting the Y-direction width of the linear spot 91 on the object 90 to be minimized. In other words, the beam is focused in the Y direction on the object 90, which is located a finite distance from the laser light source 20. Here, it is assumed that the object 90 is located at a distance ranging from several tens of centimeters to approximately 10 meters from the vibration sensor 1 .

[0023] 2A and 2B, an example of the configuration of the X-direction beam width conversion element 34 includes a cylindrical lens 32 which is a concave lens and a cylindrical lens 33 which is a convex lens. The cylindrical lenses 32 and 33 have a curvature in the X' direction, and the focal length of each lens is f ３２ , f ３３ For example, f ３２ =-25 [mm], f ３３ When the light emitted from the collimator lens 31 is completely collimated, the distance between the cylindrical lens 32, which is a concave lens, and the cylindrical lens 33, which is a convex lens, is f = 50 [mm]. ３２ +f ３３ = 25 [mm], the light emitted from the cylindrical lens 33 moves in the X direction as ３２ | / f ３３ = 2 times expanded collimated beam. Focal length f ３３ and f ３２By changing the value of , the width of the light beam can be converted into a collimated beam of any width. In addition, by finely adjusting the distance between the cylindrical lenses 32 and 33, the beam width on the object 90 can be changed.

[0024] 2A and 2B, the light is focused at a finite position in the Y direction, so the light beam emitted from the collimator lens 31 is not a collimated light beam but a weakly convergent light beam, and the lens spacing at which the collimated light beam is emitted is slightly different from the value described above. In this way, the X-direction beam width conversion element 34 can make the beam width in the X' direction (i.e., length Bx) on the object 90 different from the length in the Y direction (i.e., make it axially asymmetric). In this way, the laser light irradiated on the object 90 is focused in the Y direction to form a linear spot 91 having a length Bx in the X direction.

[0025] FIG. 3A is a schematic side view (i.e., a view of the light receiving unit 40 in the X direction) showing the configuration and main light rays of the light receiving unit 40 of the vibration sensor 1 according to the embodiment. FIG. 3B is a schematic plan view (i.e., a view of the light receiving unit 40 in the -Y direction) showing the configuration and main light rays of the light receiving unit 40 of the vibration sensor 1. In this embodiment, the light receiving unit 40 includes a line sensor 50, an imaging lens 61, and a focusing cylindrical lens 62. The imaging lens 61 is an object-side telecentric optical system or a hyper-tric optical system. FIGS. 3A and 3B show an example in which the imaging lens 61 is an axially symmetric telecentric optical system. An object-side telecentric optical system is an optical system in which the chief rays from each image height directed from the lens to the object are parallel to the optical axis. A hyper-tric optical system is a special optical system in which the chief rays from each image height directed from the lens to the object converge toward the optical axis.

[0026] The surface of the object 90 is rough, and the reflected and scattered light from the linear spot 91 generates a speckle pattern throughout the entire space. In Figures 3A and 3B, the points at both ends of the light-receiving surface of the line sensor 50 in the X direction are designated P1 and P3, and the central point is designated P2. Figures 3A and 3B depict light rays that are reflected and scattered on the object 90 and reach points P1, P2, and P3. As shown in Figure 3B, in the X direction, the imaging lens 61 of the light-receiving unit 40 focuses the speckle pattern on an observation surface 92. That is, in the X direction, the speckle pattern that appears on the observation surface 92 is projected onto the light-receiving surface of the line sensor 50.

[0027] By inserting a focusing cylindrical lens 62 that has a concave curvature only in the Y direction, the focal position on the object side in the Y direction can be moved from the observation surface 92 to the target object 90 located farther away. The focal length of the focusing cylindrical lens 62 is f62, and the distance between the focusing cylindrical lens 62 and the observation surface 92 is L62. For example, let f62 = -50 mm. In this case, if L62 = |f62| = 50 mm, the focus will be at infinity in the Y direction. If L62 is made slightly larger than 50 mm, the focus can be adjusted to a point closer than infinity.

[0028] The focus in the Y direction is adjusted on the object 90 by using a position adjustment mechanism 64 that adjusts the position of the condensing cylindrical lens 62 in the Z direction according to the distance to the object 90. In other words, the line sensor 50 and the object 90 are at conjugate positions in the Y direction. Therefore, light reflected and scattered from the object 90 is efficiently propagated in the Y direction to the line sensor 50.

[0029] As described above, by irradiating the object 90 with a linear spot 91 that is narrow in the Y direction and focusing on the object 90 in the Y direction by the axially asymmetric light-receiving optical system 60, an image 93 of the linear spot 91 that is narrow in the Y direction is irradiated onto the line sensor 50. This allows the reflected and scattered light from the object 90 to be efficiently transmitted to the line sensor 50, making it possible to function as a vibration sensor even with a weak power laser light. If the spot irradiated onto the object 90 is circular and wide in the Y direction, only a wide beam can be focused onto the line sensor 50 in the Y direction, resulting in a large amount of light leaking outside the line sensor 50. Furthermore, if the light-receiving optical system 60 is not focused on the object 90 in the Y direction, a blurred image in the Y direction will be projected onto the line sensor 50, again resulting in a large amount of light leaking outside the line sensor 50.

[0030] On the other hand, as will be described in detail later, in order to observe speckles in the X direction, it is necessary to irradiate a linear spot 91 that is wide in the X direction, and the focal position in the X direction of the light-receiving optical system 60 must also be significantly deviated from the object 90. For this reason, in this embodiment, in the optical system for observing a speckle pattern in the X direction, a linear spot 91 that is narrow in the Y direction is irradiated, and the axially asymmetric light-receiving optical system 60 is used to focus the light on the object 90 in the Y direction, thereby achieving highly efficient light propagation.

[0031] However, if the focal position of the light receiving optical system 60 on the object 90 in the Y direction is shifted from the position of the linear spot 91, no light will be transmitted to the line sensor 50, and therefore it is necessary to finely adjust the line sensor 50 or the focusing cylindrical lens 62 in the Y direction so that the linear spot 91 is imaged on the line sensor 50 in the Y direction. The adjustment method will be described later.

[0032] Observation of Speckle Patterns Figures 4A and 4B are schematic plan views (i.e., views of the light receiving unit 40 viewed in the -Y direction) showing the configuration of the light receiving unit 40 of the vibration sensor 1 according to the embodiment, the main light rays, and the displacement (e.g., vibration) of the object 90. Figures 4A and 4B show the displacement due to vibration of the object 90. Figures 5A and 5B are diagrams showing the movement on the observation surface 92 of the speckle pattern generated by the vibration of the object 90 shown in Figures 4A and 4B. Figures 5C and 5D are diagrams showing the movement on the light receiving surface of the line sensor 50 of the speckle pattern generated by the vibration of the object 90 shown in Figures 4A and 4B.

[0033] As shown in Figures 4A and 4B, the speckle pattern observed on the observation surface 92 is a random granular pattern, as shown in Figure 5A, for example. A speckle pattern such as that shown in Figure 5A can be observed with the naked eye by placing a screen on the observation surface 92 to darken the surroundings, or it can be observed as an electronic image by placing a two-dimensional image sensor directly on the observation surface 92. Assume that the surface of the object 90 at the point irradiated with the linear spot 91 is tilted by a small tilt angle α in the X direction due to vibration. Then, the speckle pattern on the observation surface 92 is slightly displaced in the X direction, as shown in Figure 5B. This shift amount δX can be calculated using the distance L from the object 90 and the tilt angle α using the following equation (1): δX = L × α (1)

[0034] In Figures 4A and 4B, the imaging lens 61 is a telecentric lens, so the observation plane 92 is located slightly in front of the light-receiving optical system 60. Since the object 90 is located farther away, the observation plane 92 exists between the light-receiving optical system 60 and the object 90. However, the speckle observation plane 92 may be located behind the object 90 (in the +Z direction) or behind the light-receiving optical system 60 (in the -Z direction). The speckle observation plane 92 is a plane that determines the distance L for calculating the speckle pattern shift amount δX based on Equation (1) in accordance with the tilt of the object 90. Therefore, the actual measurement can be performed even if a virtual image of the observation plane 92 is projected onto the line sensor 50. However, having the observation plane 92 located between the light-receiving optical system 60 and the object 90 has the advantage of making it easier to confirm the location of the focus during device assembly and adjustment. By placing a chart having a pattern in the X direction on the observation surface 92, an image of the chart appears on the line sensor 50, making it possible to confirm whether the observation surface 92 is in the correct position.

[0035] When light propagates from the observation surface 92 to the line sensor 50, a pattern compressed in the Y direction is observed on the light-receiving surface of the line sensor 50, as shown in FIGS. 5C and 5D. FIGS. 5C and 5D are images corresponding to FIGS. 5A and 5B, respectively. As shown in FIGS. 3A and 3B, the focus is on the object 90 in the Y direction, so a narrow width in the Y direction of the linear spot 91 on the object 90 is projected onto the surface of the line sensor 50. Since the linear spot 91 on the object 90 has a slight width in the Y direction, the beam irradiated onto the surface of the line sensor 50 also has a slight width in the Y direction. There is also a weak fluctuation in brightness in the Y direction due to the speckle pattern. In the X direction, the speckle pattern on the observation surface 92 is transferred to the surface of the line sensor 50.

[0036] The line sensor 50 is disposed in a direction extending in the X direction of the beam patterns in Figures 5C and 5D. A luminance signal detected by the line sensor 50 is sent to an image processing device 70.

[0037] Fig. 6 is a diagram showing an example of a one-dimensional speckle pattern waveform that moves due to vibration. Fig. 6 shows wave(a) and wave(b), which are output waveforms from the line sensor 50, corresponding to Figs. 5(A) and (B). As shown in Fig. 6, wave(a) shifts in the X direction by a shift amount δX, as shown in wave(b), due to the tilt angle α (angle change) of the object 90. When used as a vibration sensor, the amount of movement in the X direction of the signal waveform acquired by the line sensor 50 is calculated for each frame, and the amount of movement is integrated to obtain a vibration waveform corresponding to the angle change due to vibration.

[0038] The frame rate of the line sensor 50 is, for example, 3 kHz, but is not limited to this.

[0039] <<Algorithm for Extracting Displacement Amount from Successive Speckle Pattern Images>> To calculate the amount of movement of a signal waveform, for example, waveform matching processing is used. As shown in FIG. 6 , ideally, a waveform is obtained that is shifted by a shift amount δX in the X direction while maintaining its waveform shape. Several methods are known for calculating the shift amount δX from these two waveforms. For example, the sum of absolute values ​​of differences for each pixel (SAD) or the sum of squared differences (SSD) of differences for each pixel can be calculated as the similarity, and the shift amount δX can be calculated from the calculated similarity. The pixel pitch is, for example, a value within a range of approximately 5 μm to 2 μm, but is not limited thereto. Here, a case where similarity is calculated using SAD will be described as an example.

[0040] If the luminance value of wave(a) at pixel number i=1, 2, 3, . . . is fa(i), and the luminance value of wave(b) is fb(i), the similarity R ＳＡＤ (x) is calculated using the following formula (2): ＳＡＤ (x) is pixel number i ｍｉｎ From i ｍａｘ The similarity between the luminance value fa(i) in the pixel range up to and including the luminance value fb(i+x) at a position obtained by shifting the luminance value fb(i) by the unknown quantity x is calculated. ＳＡＤThe smaller the value of (x), the more similar the brightness value fb(i+x) and the brightness value fa(i) are. ＳＡＤ (x) is calculated, and the similarity R is calculated as shown in the following formula (3). ＳＡＤ The x that minimizes (x) is defined as the shift amount δX.

[0041]

[0042] 7A and 7B, and 8A and 8B are diagrams showing an example of a process for calculating the vibration amount of a speckle pattern from waveform matching. In FIG. 7A, the horizontal axis represents x and the vertical axis represents R. ＳＡＤ 7(a) is a graph showing the shift amount δX. The x that gives the minimum value (most similar) of this graph is the shift amount δX, and in the case of FIG. 7(a), δX = 5 [pixels]. By this method, the shift amount δX, which is the amount of movement between consecutive image frames, can be calculated. This is performed between all consecutive frames. An example of calculating the shift amount δX for the first 50 frames is shown in FIG. 7(b). The shift amount δX is the amount of movement of the image between adjacent frames, and to calculate the amount of movement from the start of measurement, it is necessary to take the sum of the shift amounts δX for the frames up to that point. When the image frames are in order, n = 0, 1, 2, ..., the shift amount from the (n-1)th frame to the nth frame is δX. ｎ Then, the movement amount X(n) from the initial position in the nth frame is obtained by integrating the movement amount between frames, as shown in the following equation (4).

[0043]

[0044] FIG. 8A shows an example of the amount of movement X(n) from the initial position in the nth frame. In the case of vibration, data should be distributed above and below the line where the vertical axis is zero. However, actual data contains a low-frequency offset, as shown here. Possible reasons for this offset include the accumulation of errors when calculating the amount of movement of the speckle pattern image, or the gradual movement of the irradiation position of the linear spot 91 on the object 90. To subtract this offset, for example, a process of removing low-frequency components using a high-pass filter or the like can be performed. FIG. 8B shows the data in FIG. 8A after passing it through a high-pass filter, and this is the vibration signal we are interested in.

[0045] It should be noted that the brightness distribution (speckle pattern waveform) of the actually observed speckle pattern gradually changes as the tilt angle α of the object 90 increases. ＳＡＤ The inter-frame shift amount δX obtained by calculating the minimum value of (x) will contain a large error if the tilt angle α is large.

[0046] For example, if the object 90 is ｏｂｊ = 1 [kHz], amplitude angle ±α １ If the amplitude angle is α １ However, when the shift amount δX, which is the amount of movement of the speckle pattern calculated by Equation (1), is large enough to be observed (for example, δX = several pixels), the sampling frequency of the sensor is ν ｓｅｎｓｏｒ = 2 kHz, the shape of the speckle pattern waveform does not change significantly, so the shift amount δX can be obtained correctly and the vibration of the sine wave can be reproduced.

[0047] On the other hand, the amplitude angle α １ is large, and ν ｏｂｊ If the speckle pattern waveform changes significantly within a vibration period of ν = 1 [kHz], ｓｅｎｓｏｒ= 2 [kHz], matching of speckle pattern waveforms between frames is not achieved well, and the error in the shift amount δX becomes large. In such a case, by acquiring speckle pattern waveforms at a higher frame rate, it is possible to suppress deformation of the waveform between frames. For example, when ν ｓｅｎｓｏｒ If the shift amount δX is set to 10 [kHz], the shift amount δX is calculated before the deformation of the speckle pattern waveform becomes large, so that the error in the shift amount δX can be suppressed and a sine wave of 1 [kHz] can be accurately reproduced. The above processing is performed by the image processing device 70 shown in FIG.

[0048] <<Length Bx of Linear Spot 91 in the X-Direction and Size of Speckle Pattern Design>> It is known that the spatial size of the speckle pattern design depends greatly on the diameter W of the laser beam on the object 90, and the larger the diameter W, the finer the pattern. It is known that the typical size P of the speckle pattern on the observation surface 92 of the speckle pattern is approximately equal to (Lλ / W). This is expressed by the following equation (5) and is explained in, for example, Non-Patent Document 1: P ∼ Lλ / W (5) where λ is the wavelength of the laser light, and L is the distance from the observation surface 92 to the object 90.

[0049] A similar discussion can be made about the linear spot 91 in the X direction, which is the direction in which the speckle pattern is observed. That is, a typical size P of the speckle pattern in the X direction on the observation surface 92 in FIG. 1 is expressed by the following equation (6): P~Lλ / Bx (6)

[0050] 9A to 9C are diagrams illustrating differences in speckle pattern waveforms depending on the beam width. Here, Bx is the length of the linear spot 91 in the X direction. Using the relationship in equation (6), the fineness of the speckle pattern waveform measured by the line sensor 50 can be adjusted by adjusting the magnitude of the length Bx. For example, the one-dimensional waveforms of the speckle pattern observed by the line sensor 50 when the length Bx is 1 mm, 2 mm, or 3 mm are shown in FIGS. 9A, 9B, and 9C. When Bx = 1 mm, as shown in FIG. 9A, the speckle pattern waveform is gentle, and therefore waveform matching accuracy is poor for a minute speckle pattern waveform shift amount δX. When Bx = 3 mm (FIG. 9C), the speckle pattern waveform has many peaks and valleys, and waveform matching accuracy is high for a minute speckle pattern waveform shift amount δX. On the other hand, when the displacement of the speckle pattern waveform is large, if the speckle pattern is too fine, the pattern waveform is likely to be deformed, and matching errors are likely to occur. Therefore, it is necessary to adjust the fineness of the speckle pattern waveform appropriately depending on the magnitude of the vibration of the object 90. For this purpose, the X-direction beam width conversion element 34 is provided with a mechanism that can adjust the length Bx of the linear spot 91.

[0051] Furthermore, a typical size P of a speckle pattern is expressed by equation (6), but experiments have shown that size P varies greatly depending on the roughness of the surface condition of object 90. Therefore, it is desirable to adjust length Bx of linear spot 91 while checking the size of the speckle pattern depending on the surface condition of object 90.

[0052] <<Adjustment Method>> FIGS. 10A to 10C are diagrams showing the overlap of the light-receiving surface of the line sensor 50 and the speckle pattern. The light-receiving surface of the line sensor 50 and the surface of the object 90 are conjugate in the Y direction. Therefore, an image 93 of the linear spot 91 on the object 90 on the light-receiving surface of the line sensor 50 has a narrow width in the Y direction. In other words, the reflected and scattered light is focused on the light-receiving surface of the line sensor 50 in the Y direction. FIGS. 10A to 10C show an example of the image 93 of the linear spot 91 projected onto the light-receiving surface of the line sensor 50. As described above, the image 93 of the linear spot 91 has a narrow width in the Y direction and is elongated in the X direction. The image 93 of the linear spot 91 has a brightness distribution in the X direction according to the length Bx of the linear spot 91 in the X direction. FIG. 10A shows a state in which the light receiving surface of the line sensor 50 and the image 93 of the linear spot 91 overlap, and in this state, the vibration sensor 1 of this embodiment can operate normally.

[0053] However, as shown in Figure 10 (B), if the Y-direction position of the image 93 of the linear spot 91 is deviated from the optical axis position of the light receiving unit 40, the light receiving surface of the line sensor 50 and the image 93 of the linear spot 91 do not overlap, and the speckle pattern cannot be observed with the line sensor 50.

[0054] 10C , if the extension direction of the image 93 of the linear spot 91 is inclined with respect to the extension direction of the light receiving surface of the line sensor 50, a speckle pattern waveform with a sufficient number of pixels cannot be observed by the line sensor 50. For this reason, it is desirable to provide spot position adjustment mechanisms 63 and 64 and a spot rotation adjustment mechanism 65.

[0055] An example of the spot position adjustment mechanism 63 is a moving stage for finely adjusting the focusing cylindrical lens 62 in the Y direction. If the position of the light ray incident from the center of the focusing cylindrical lens 62 is shifted in the Y direction, the beam emission direction is deflected. Therefore, by finely adjusting the focusing cylindrical lens 62 in the Y direction, the position of the beam focused by the imaging lens 61 can be shifted in the Y direction. In other words, the state of FIG. 10B can be adjusted to the state of FIG. 10A. Therefore, even if the Y direction position of the image 93 of the linear spot 91 is deviated from the optical axis position of the light receiving unit 40 in the initial state, the spot position adjustment mechanism 63 can adjust the image 93 of the linear spot 91 so that it can be received by the line sensor 50.

[0056] An example of the spot rotation adjustment mechanism 65 is a rotation stage for finely adjusting the focusing cylindrical lens 62 around the Z axis. When the focusing cylindrical lens 62 is rotated around the Z axis, the image 93 of the linear spot 91 rotates within the XY plane. By rotating the rotation stage of the spot rotation adjustment mechanism 65 while observing the output waveform from the line sensor 50 so that an output is obtained from all pixels, the state shown in FIG. 10C can be adjusted to the state shown in FIG. 10A.

[0057] The method for adjusting the position of the image 93 of the linear spot 91 is not limited to the above example. For example, by adjusting the tilt of the entire light receiving unit 40 around the X axis, the image 93 of the linear spot 91 can be moved in the Y direction on the light receiving surface of the line sensor 50. Alternatively, a stage may be provided that shifts the line sensor 50 in the Y direction relative to the imaging lens 61.

[0058] Similarly, the method of spot rotation adjustment is not limited to the above example. For example, a mechanism for rotating the entire light receiving unit 40 around the Z axis may be provided. Alternatively, a rotation stage for adjusting the rotation of the line sensor 50 around the Z axis relative to the imaging lens 61 may be provided.

[0059] After proper alignment has been performed and the light-receiving pixels of the line sensor 50 are superimposed on the image 93 of the linear spot 91 as shown in FIG. 10A, vibration sensing is started.

[0060] 11 is a diagram showing an example of the hardware configuration of an image processing device 70 of the vibration sensor 1. The image processing device 70 has a processor 101, a memory 102, a storage device 103, and an interface 104. The image processing device 70 may be a computer. The processor 101 is a CPU (Central Processing Unit) or the like. The memory 102 is a volatile semiconductor memory such as a RAM (Random Access Memory). The storage device 103 is a non-volatile storage device such as a hard disk drive (HDD) or a solid state drive (SSD). A detection signal from the line sensor 50 is input to the interface 104.

[0061] Each function of the image processing device 70 may be realized by a processing circuit. The processing circuit may be dedicated hardware, or may be a processor 101 that executes a program stored in a memory 102. The processor 101 may be any of a processing device, an arithmetic unit, a microprocessor, a microcomputer, and a DSP (Digital Signal Processor).

[0062] Where the processing circuitry is dedicated hardware, the processing circuitry may be, for example, a single circuit, a multiple circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or any combination thereof.

[0063] When the processing circuit is the processor 101, the program is realized by software, firmware, or a combination of software and firmware. The software and firmware are written as programs and stored in the memory 102. The processor 101 can realize the functions of the image processing device 70 shown in FIG. 1 by reading and executing the programs stored in the memory 102.

[0064] <Effects> As described above, by using the vibration sensor 1 according to the present embodiment, it is possible to receive light more efficiently than in the prior art by using the light-receiving unit 40 that focuses the linear spot 91 of laser light emitted from the light-projecting unit 10 in the Y direction on the object 90 and focuses the reflected and scattered light of the linear spot 91 on the object 90 only in the Y direction and receives it with the line sensor 50. Therefore, even when a laser light with a low power is used, it is possible to quickly acquire the one-dimensional waveform of the speckle pattern and detect vibrations.

[0065] Furthermore, when the extension direction of the line sensor 50 is aligned with the X direction in which the light-projecting unit 10 and the light-receiving unit 40 are aligned, even if the distance L shown in FIG. 12 changes, the image 93 of the line-shaped spot 91 of the speckle pattern is unlikely to deviate from the light-receiving surface of the line sensor 50, as shown in FIGS. 13(A) and 13(B).

[0066] Furthermore, since the size of the line spot 91 of the light-projecting unit 10 on the object 90 can be changed only in the extension direction, the size of the speckle pattern waveform received by the line sensor 50 can be changed. As a result, as shown in Figures 9(A) to (C), the size of the speckle pattern can be optimized according to the magnitude of the vibration, thereby improving the accuracy of vibration detection.

[0067] Furthermore, when the structure for converting the laser light from the light projecting unit 10 into a line beam is configured with a set of two cylindrical lenses 32 and 33, the size of the speckle pattern can be changed without changing the width of the line beam in the focusing direction.

[0068] Furthermore, since the non-axisymmetric optical system of the light receiving unit 40 includes a cylindrical concave lens, the size of the speckle pattern can be changed without changing the width of the line-shaped beam in the focusing direction.

[0069] <<Variation 1>> When the object 90 is a vibration source that emits sound or an object that vibrates due to sound waves, the vibration sensor 1 can reconstruct sound from the vibration. In this case, the vibration sensor 1 is a sound reconstruction device and is called a laser microphone. The audible range used in everyday conversation is approximately 4 kHz. The minimum frame rate required for the line sensor 50 based on the sampling theorem is twice that, or 8 kHz. However, as already explained, a faster frame rate is required to accurately capture vibration waveforms even when the vibration amplitude is large. Therefore, it is desirable for the line sensor 50 to have a frame rate of 20 kHz to 40 kHz. Since the exposure time of the line sensor 50 cannot be longer than the frame period, increasing the frame rate shortens the exposure time (i.e., the image becomes darker). According to this embodiment, the light receiving efficiency is improved, allowing the frame rate to be increased, and the line sensor 50 can also be used as a laser microphone to reconstruct sounds that require a high frame rate. Furthermore, since the laser microphone can remotely measure only the vibrations at the point on the object where the laser is irradiated, without passing through air compression waves, it can obtain the necessary sound information even in a noisy environment.

[0070] In the above description, the extension direction of the image 93 of the linear spot 91 and the extension direction of the line sensor 50 are both the X direction, but the extension directions do not necessarily have to be the X direction as long as both extension directions are facing the same direction. However, when the extension direction is the X direction, there are the following advantages.

[0071] Fig. 12 is a schematic plan view showing image displacement when the distance changes in the vibration sensor 1 according to the embodiment. Fig. 13(A) to (D) are schematic plan views showing image displacement when the distance changes in the vibration sensor 1 according to the embodiment.

[0072] As shown in FIG. 12, assume that the distance L to the object 90 changes from a state where the optical axis Z' and the optical axis Z intersect on the object 90 (a state where L=L0) to a state where L≠L0.

[0073] Both image 93 of line-shaped spot 91 and the light-receiving surface of line sensor 50 extend in the X direction, and when L = L0, as shown in Fig. 13A, image 93 of line-shaped spot 91 overlaps with the light-receiving surface of line sensor 50. Even if the distance L is slightly shifted and L ≠ L0, image 93 of line-shaped spot 91 is long in the X direction, so that the overlap between image 93 of line-shaped spot 91 and the light-receiving surface of line sensor 50 remains ensured, as shown in Fig. 13B.

[0074] When the image 93 of the linear spot 91 and the light receiving surface of the line sensor 50 are inclined with respect to the X direction and L = L0, the image 93 of the linear spot 91 and the light receiving surface of the line sensor 50 can be properly superimposed as shown in Figure 13(C), and there is no problem with the function of the vibration sensor 1. However, if the distance L in Figure 13(C) is slightly shifted so that L ≠ L0, the overlap between the image 93 of the linear spot 91 and the light receiving surface of the line sensor 50 is shifted as shown in Figure 13(D). In the case of Figure 13(D), the light receiving surface of the line sensor 50 and the image 93 of the linear spot 91 do not overlap, so the line sensor 50 cannot observe the image 93 of the linear spot 91. Therefore, it is necessary to make another adjustment to superimpose the two, as shown in Figures 10(A) to 10(C).

[0075] As described above, it is desirable that the extension direction of the image 93 of the linear spot 91 and the extension direction of the light receiving surface of the line sensor 50 are both the X direction.

[0076] REFERENCE SIGNS LIST 1 vibration sensor, 2 light-emitting / receiving head, 10 light-emitting unit, 11 first optical axis, 20 laser light source, 30 light-emitting optical system, 31 collimator lens, 31a adjustment mechanism, 32 concave lens, 33 convex lens, 34 X-direction beam width conversion element, 40 light-receiving unit, 41 second optical axis, 50 line sensor (line camera), 60 light-receiving optical system, 61 imaging lens, 62 condensing cylindrical lens, 70 image processing device, 90 object, 91 line-shaped spot, 93 image of line-shaped spot, X first direction, Y second direction, α tilt angle, δX shift amount (movement amount).

Claims

1. A light-projecting unit having a laser light source and a light-projecting optical system that projects the laser light emitted from the laser light source as a line spot onto an object; a light-receiving unit having a line sensor and a light-receiving optical system that focuses the reflected and scattered light of the laser light irradiated onto the object onto the line sensor; and an image processing device that calculates the amount of movement between frames of a speckle pattern generated in the reflected and scattered light received by the line sensor and estimates vibration of the object based on the amount of movement, wherein a first optical axis that is the optical axis of the light-projecting unit and a second optical axis that is the optical axis of the light-receiving unit intersect on the object, a first direction in which the line sensor extends is the same as the direction in which the line spot extends, the light-receiving optical system is an optical system that is axially asymmetric with respect to the first direction and a second direction perpendicular to the first direction, and in the first direction, an image of the object formed by the light-receiving optical system is not focused on the light-receiving surface of the line sensor, A vibration sensor according to claim 1, wherein in the second direction, an image of the object formed by the light receiving optical system is focused on the light receiving surface of the line sensor.

2. A vibration sensor as described in claim 1, characterized in that in the light receiving optical system, the object-side focal plane that focuses the line sensor in the first direction is located between the light receiving optical system and the target object.

3. A vibration sensor according to claim 1 or 2, characterized in that the light-emitting portion and the light-receiving portion are arranged side by side in the first direction.

4. A vibration sensor according to any one of claims 1 to 3, characterized in that the light projection optical system includes an adjustment mechanism for changing the size of the linear spot in the first direction.

5. A vibration sensor according to any one of claims 1 to 4, characterized in that the light projection optical system includes a pair of cylindrical lenses that form the linear spot.

6. A vibration sensor according to any one of claims 1 to 5, characterized in that the light receiving optical system includes a cylindrical lens, which is a concave lens, as a non-axisymmetric optical system.

7. The vibration sensor according to any one of claims 1 to 6, wherein the image processing device reconstructs sound based on the vibration of the object.

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