Force sensor and method for measuring force

Abstract

To provide a force sensor capable of performing three axis force measurement and in-plane force distribution measurement with high time resolution.SOLUTION: To provide a force sensor and a force measuring method for three dimensionally imaging a pattern on the surface of a plate by a line scan camera through a prism and analyzing its displacement.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a force sensor and a force measurement method using the same. [Background technology]

[0002] Conventional force sensors include force plates and pressure sensor sheets. These are used, for example, to measure the reaction force of the human sole. Measuring the reaction force of the human sole is important in, for example, sports science, prosthetic and orthotic development, rehabilitation, and robotics.

[0003] As an example of a conventional force sensor, Patent Document 1 (Patent Document 1) describes a force plate equipped with force sensors at each of its four corners. This force plate detects force using triaxial load cells installed at the four corners of the plate, enabling it to measure the total plantar reaction force (pressure and shear forces). However, this force plate's spatial resolution is not necessarily sufficient. Another method for measuring plantar reaction force is the array pressure sensor sheet. However, while this can measure pressure distribution, it has the drawback of being fundamentally unable to measure shear force, which is important for human locomotion. Measuring shear force is important in sports science, prosthetic and orthotic development, rehabilitation, robotics, and other fields.

[0004] Patent Document 2 describes a balance training device. Patent Documents 3 and 4 describe balance training systems. Patent Document 5 describes a displacement measurement device and a displacement measurement method, which can measure the displacement of a measurement point while reflecting the displacement of the imaging device itself.

[0005] Conventional force plates have difficulty measuring microorganisms and micromachines due to their complex mechanical structure and the arrangement of sensor elements such as strain gauges. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2020-003275 A (Patent No. 7106065) [Patent Document 2] JP 2020-146330 A (Patent No. 7081540) [Patent Document 3] Japanese Patent Publication No. 2022-032091 [Patent Document 4] Japanese Patent Publication No. 2022-038343 [Patent Document 5] Japanese Patent Publication No. 2023-128044 Summary of the Invention [Problem to be solved by the invention]

[0007] In one embodiment, an object of the present invention is to provide a force sensor capable of measuring three-axis forces with high time resolution. In another embodiment, an object of the present invention is to provide a force sensor that can be miniaturized and has high time resolution. [Means for solving the problem]

[0008] As a result of extensive research into solving the above-mentioned problems, the inventors discovered that, as an example, the above-mentioned problems can be solved by capturing images of at least two inclined patterns on the surface of the object to be measured using a line scan camera via an optical element that splits one light beam into two or more beams, and analyzing the displacement, and completed the present invention, which incorporates this as one embodiment.

[0009] The present invention includes the following embodiments. [1] A force sensor comprising: (i) a plate; (ii) a spring portion; (iii) an optical element that splits one light beam into two or more beams; and (iv) a line scan camera, the plate has a pattern on its surface, the pattern having at least two symmetrical oblique stripes; The spring portion is arranged to exert an elastic force when a force is applied to the plate, The force sensor is arranged so that the line scan camera can capture images of the at least two symmetrical inclined stripe patterns applied to the plate from two or more directions through the optical element. [2] The force sensor according to embodiment 1, wherein the optical element is a prism. [3] The force sensor according to embodiment 1, further comprising a mirror (v) between the optical element (iii) and the line scan camera (iv) on the optical path. [4] The optical element is further connected to a computer, and the computer has an algorithm or software that can analyze the displacement of the at least two symmetrical inclined striped patterns attached to the surface of the plate, which are imaged by the line scan camera through the optical element, and calculate the force applied to the plate from the spring constant of the spring part and the displacement of the patterns. 2. A force sensor as described in embodiment 1. [5] The force sensor according to embodiment 1, wherein the line scan camera is a line scan camera capable of capturing images at a frequency of 100 frames per second or more. [6] (i) A step of capturing an image of a pattern on a surface of a plate placed on a spring portion with a line scan camera via an optical element that splits a single beam of light into two or more beams at a first point in time before applying a force to the plate and at a second point in time when applying a force to the plate, wherein the pattern has at least two symmetrical oblique stripes. (ii) superimposing the at least two symmetrical oblique stripe patterns applied to the surface of the plate at a first time point and the at least two symmetrical oblique stripe patterns applied to the surface of the plate at a second time point, and analyzing the displacement of the patterns applied to the surface of the plate between the first time point and the second time point; (iii) calculating the force applied to the plate from the displacement of the at least two symmetrical oblique striped patterns between a first time point and a second time point and the spring constant of the spring portion; Methods for measuring force, including: [7] The method of embodiment 6, wherein the optical element is a prism. [8] The method of embodiment 6, wherein a mirror is disposed between the optical element in the optical path and the line scan camera. [9] The method of embodiment 6, wherein steps (ii) and (iii) are performed by a computer, algorithm, or software.

[10] The method of embodiment 6, wherein imaging is performed at a frequency of 100 frames per second or more. [Effects of the Invention]

[0010] In one embodiment, the present invention has the advantage of being able to measure not only in-plane but also out-of-plane displacement of a plate with high time resolution. In one embodiment, the present invention has the advantage of realizing a small force sensor with high time resolution. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is an overall view of a force sensor according to the present invention, in which the mirror is not essential and may be omitted. [Figure 2] This shows the displacement of pattern 2 when the plate is displaced. Plate 1 is shown in A. The arrow indicates the direction of the applied force. B is an image of pattern 2 branched via optical element 3. C is pattern 2 captured by line scan camera 5. When a vertical force is applied to plate 1, the left and right halves of each striped pattern are displaced in directions that move closer to the center. [Figure 3] This shows the displacement of pattern 2 when the plate is displaced. Plate 1 is shown in A. The arrow indicates the direction of the applied force. B is an image of pattern 2 branched via optical element 3. C is pattern 2 captured by line scan camera 5. When a lateral force is applied to plate 1, pattern 2 also displaces laterally. [Figure 4] This shows the displacement of pattern 2 when the plate is displaced. Plate 1 is shown in A. The arrow indicates the direction of the applied force. B is an image of pattern 2 branched via optical element 3. C is pattern 2 captured by line scan camera 5. When a load is applied to the vertical surface of plate 1, patterns 2 move toward or away from each other. [Figure 5] This is a schematic diagram of measuring the butterfly, Butterfly leuconoe. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will now be described in detail with reference to the drawings.

[0013] The force sensor and measurement method using the same of the present invention use a line scan camera. A line scan camera is a camera that uses a long, narrow sensor with one or several horizontal rows of light-receiving elements that detect light. The number of light-receiving elements can be, for example, several thousand to tens of thousands. A line scan camera is also called a line camera or a line sensor camera. These terms are interchangeable in this specification. A typical camera is also called an area camera, and has a surface sensor with light-receiving elements arranged vertically and horizontally. When comparing a line scan camera and an area camera that have the same number of light-receiving elements horizontally, the line scan camera has a higher image processing speed, can increase the scan rate, and therefore has a higher time resolution.

[0014] In certain embodiments, the scan rate of the line scan camera can be 100 fps or more, 200 fps or more, 300 fps or more, 400 fps or more, 500 fps or more, 600 fps or more, 700 fps or more, 800 fps or more, 900 fps or more, 1000 fps or more, 1100 fps or more, 1200 fps or more, 1300 fps or more, 1400 fps or more, 1500 fps or more, 1600 fps or more, 1700 fps or more, 1800 fps or more, 1900 fps or more, 2000 fps or more, 3000 fps or more, 4000 fps or more, or 5000 fps or more. In some embodiments, the scan rate of the line scan camera may be 5000 fps or less, 4000 fps or less, 3000 fps or less, 2000 fps or less, 1500 fps or less, 1000 fps or less, for example 500 fps or less.

[0015] In the present invention, a line scan camera is used to capture an image of a stripe pattern. Figure 1 illustrates the principle of the present invention. A pattern 2 is pre-applied to the surface of plate 1 (the downward-facing surface in Figure 1), which is the object of measurement. Next, the pattern 2 on the surface of the plate is captured by line scan camera 5 via optical element 3 (e.g., a prism). First, the pattern 2 is captured with no force applied to plate 1. Then, a force is applied to plate 1, the pattern 2 is captured, and the displacement is analyzed. This enables measurement of displacement not only in the in-plane direction but also out-of-plane direction. Optionally, a mirror 4 may be placed between line scan camera 5 and optical element 3 on optical path 8. For convenience of explanation, the object of measurement in this specification is referred to as a plate, but the object of measurement may have any shape. A stripe pattern may also be simply referred to as a pattern.

[0016] Pattern 2 is sometimes referred to herein as a grating. A grating is a collection of identical, parallel, elongated elements arranged at regular intervals. Unless otherwise specified, a grating contains a single type of identical, parallel, elongated elements. Unless otherwise specified, the elongated elements are straight lines.

[0017] The striped pattern is symmetrical with respect to the center. The left and right striped patterns may each be tilted. The tilt angle may be, for example, 30 to 60 degrees, for example, 40 to 55 degrees, and preferably 45 degrees. Here, the tilt angle is defined as 0 degrees for a line parallel to the (imaginary) center line of the center, and 90 degrees for a line perpendicular to the (imaginary) center line of the center.

[0018] When a force is applied to plate 1, plate 1 deforms. In the present invention, this deformation is captured as a displacement of pattern 2 captured by line scan camera 5. When the displacement of pattern 2 when force is applied to plate 1 is captured via optical element 3, the image of pattern 2 is divided into two, and each is captured as a one-dimensional image. In this case, for example, if a triangular prism with a 45-degree tilt angle (a triangular prism having a right-angled isosceles triangle at its base and top) is used, the two images correspond to images observed from directions tilted ±45 degrees in the y direction, and the striped pattern becomes a continuous black and white one-dimensional image. For example, when the plate is displaced in the x direction, the two striped patterns move equally in the x direction. In the camera image, the left and right halves of the stripes in the images corresponding to the two prism faces move toward each other's center. When the plate is displaced in the y direction, the striped pattern images on the two faces move by 1 / √2 of the displacement in the y direction, and the striped pattern images also move in the one-dimensional camera image. When the plate is displaced in the z direction, the stripe pattern images on the two surfaces move in the opposite y direction by a magnitude of 1 / √2 of the displacement, and the pixels of the two images also move closer together in the camera image. The moiré pattern created by the stripe pattern images captured on these two surfaces and the initial image captured in advance is analyzed to derive the displacement. This sampling moiré method allows these displacements to be resolved to 1 / 1000 of the stripe pattern pitch and detected with high precision. Therefore, by measuring the displacements in three directions and multiplying them by the spring constants in each direction, the three-axis force can be calculated with high resolution.

[0019] In one embodiment, the displacement of the pattern 2 can be analyzed using a sampling moiré method. The sampling moiré method is a technique for measuring the displacement of a measurement object by applying a pattern having elongated elements at known intervals to the measurement object and photographing the pattern with a camera. The sampling moiré method utilizes moiré fringes that are generated by superimposing photographs of the pattern before and after deformation, allowing for highly accurate measurement of displacement. Moiré fringes here refer to stripe patterns that are generated due to a shift in the periodicity of a regular pattern. Using the sampling moiré method, the displacement of each plate 1 can be resolved to 1 / 1000 of the spacing (pitch) of the pattern elements and detected with high accuracy. For more information on the sampling moiré method, see, for example, References 1 and 2.

[0020] Examples of the line scan camera 5 include a camera with a CCD image sensor and a digital camera. Moiré fringes can be analyzed using an algorithm or software, such as, but not limited to, commercially available sampling moiré offline analysis software DSMC-10A (Kyowa Electronics Co., Ltd.).

[0021] The optical element 3 may be, but is not limited to, a prism. The optical element may have any shape as long as it can simultaneously measure light from two or more directions. Examples of prisms include, but are not limited to, a triangular prism, an Amici prism, a Pellin-Broca prism, an Abbe prism, a Fery prism, and a Porro prism. Examples of triangular prisms include a triangular prism having a right-angled isosceles triangle at the base and apex. Materials for the optical element 3 include, but are not limited to, glass, quartz, organic glass, transparent plastic, diamond, or other transparent materials.

[0022] The principle of the displacement of pattern 2 in response to the displacement of plate 1 is shown in Figures 2, 3, and 4. In Figures 2, 3, and 4, A represents plate 1. The arrow indicates the direction of the applied force. B is an image of pattern 2 branched through optical element 3. C is pattern 2 captured by line scan camera 5. We will explain the case where a triangular prism is used as optical element 3. The in-plane direction of the plate, which is perpendicular to the triangular faces of the triangular prism, is defined as the x-direction. When force is applied from the x-direction, the two stripes move equally in the x-direction. In the camera image, the left and right halves of the stripes in the images corresponding to the two prism faces move toward each other's centers. When the plate is displaced by d in the x-direction, the left and right halves of the two pattern 2 images are displaced by ±d / (√2), either toward or away from the center of each pattern. The in-plane direction of the plate, parallel to the triangular faces of the prism, is defined as the y-direction. When force is applied from the y-direction, both patterns 2 move in the y-direction. When the plate is displaced by an amount d in the y direction, the two images of pattern 2 are displaced by an amount d / (√2) in the y direction. The direction perpendicular to the surface of the plate is defined as the z direction. When force is applied from the z direction, patterns 2 move closer to or farther away from each other. When the plate is displaced by an amount d in the z direction, the two images of pattern 2 are displaced by ±d / (√2) in opposite directions in the y direction. Displacement in three directions can be measured using the sampling moiré method, and the displacement can be converted to force by multiplying the spring constants in each direction.

[0023] In one embodiment, the present invention provides a force sensor. In this specification, the force sensor may be referred to as a displacement measurement system. This force sensor includes an optical element 3 that splits a single light beam into two or more beams. By splitting a single light beam into two or more beams, the optical element 3 enables simultaneous measurement of the light beam from two or more directions. The force sensor of the present invention may optionally include a mirror 4. By inserting the mirror 4 between the optical element 3 and the optical path 8 of the line scan camera 5, the size of the force sensor in the z direction shown in FIG. 1 can be reduced. Unless otherwise specified, in this specification, the pattern 2 on the plate 1 is considered to be part of the force sensor. In certain limited embodiments, the pattern 2 on the plate 1 is not part of the force sensor, but the force sensor can be used in combination with a pattern previously applied to the plate 1. Unless otherwise specified, in this specification, the line scan camera 5 is considered to be part of the force sensor. In certain limited embodiments, the line scan camera 5 is not part of the force sensor, but the force sensor can be used in combination with a line scan camera.

[0024] The force sensor may be connected to a computer 9. The computer 9 may have an algorithm or software for analyzing the Moiré fringes. The algorithm or software may be stored in a memory or storage medium, and the memory or storage medium may be stored in the computer 9. The force sensor may be directly connected to the computer 9, or may be indirectly connected. An indirect connection also includes a remote connection via a communication means. For example, the force sensor may transmit imaging data via a conventional communication means (transmitting means), and the transmitted data may be received by the computer 9 via a receiving means. In one embodiment, the computer 9 is not a part of the force sensor, but the force sensor can be connected to the computer 9 to measure force. A force sensor connected to the computer 9 may be referred to as a force sensor system in this specification.

[0025] The force sensor may have a spring portion 7. Unless otherwise specified, in the force sensor, the spring portion 7 is arranged so as to exert an elastic force when a force is applied to the plate 1. The spring portion 7 may have any shape. In one embodiment, the spring portion 7 has a shape that has elastic forces in three axial directions. In this case, in the force sensor, the spring portion 7 is arranged so as to exert an elastic force in three axial directions when a force is applied to the plate 1. The three axial directions here refer to two directions within the plane of the plate (x direction, y direction) and one direction out of the plane (z direction). In one embodiment, the spring portion 7 may be spring coils arranged at the four corners of the plate.

[0026] In one embodiment, the computer 9 has an algorithm or software that can analyze the displacement of the pattern 2 on the surface of the plate 1, which is imaged by the line scan camera 5 via the optical element 3, and calculate the force applied to the plate 1 from the spring constant of the spring portion 7 and the displacement of the pattern 2. In another embodiment, the computer 9 has an algorithm or software that can analyze the displacement of the pattern 2 on the surface of each of the two or more plates 1 included in the set, which is imaged by the line scan camera 5 via the optical element 3, and calculate the force applied to each of the plates 1 from the spring constant of the spring portion 7 and the displacement of the pattern 2, and measure the distribution of the force applied to each of the two or more plates 1 included in the set. In one embodiment, the analysis and calculation of the force applied to the plate 1 can be performed by an algorithm or software. In one embodiment, the analysis, calculation of the force applied to each plate 1, and measurement of the force distribution can be performed by an algorithm or software. In one embodiment, the analysis of the displacement of the pattern 2 on the surface of the plate 1 can be performed by a sampling moiré method.

[0027] The sampling moiré method is known to have applications at construction sites, such as measuring strain on pillars and bridges (References 1, 2, etc.). When the object to be measured is large, multiple patterns are often attached to the object and measurements are performed. However, capturing multiple patterns requires a long shooting distance. This also narrows the angle of view and reduces resolution. Therefore, these conventional methods are not suitable for miniaturization.

[0028] In the present invention, one pattern 2 is imaged by a line scan camera 5 via an optical element 3, thereby obtaining two or more images, for example, two images, from one pattern 2. The displacement of the pattern 2 in the two images is then analyzed using an analysis method such as the sampling moiré method. This allows the entire system to be made smaller. It also prevents narrowing of the angle of view and a decrease in resolution. Furthermore, by using a line scan camera, it is possible to perform high-frequency imaging even in a short period of time, thereby increasing the temporal resolution. This makes it possible to analyze complex movements such as the takeoff movement of insects.

[0029] The present invention will be described in more detail in the following embodiments, but the present invention is not limited thereto.

[0030] Here, we design and manufacture a force plate to measure the takeoff of the large butterfly, Idea leuconoe. The takeoff motion of insects is a complex movement in which two forces act in combination: the aerodynamic force caused by the flapping of the wings and the jumping force of the legs. From the movement trajectory alone, it is impossible to determine the influence of each force and to what extent, making it an indeterminate problem. Among these, it is predicted that the aerodynamic force and jumping force of large butterflies are of similar magnitude, and their unique movement characteristics have attracted attention in the field of biomechanics.

[0031] The required specifications for the force plate are a force resolution of 0.1 mN, which is 1 / 50 of body weight, and a sampling rate of 1 kHz or higher. Simulations are used for the design, and the plate structure, including the spring structure, is fabricated using a 3D printer or machine. The plate dimensions are 50 mm x 50 mm, with spring constants on the order of 100 N / m in the x, y, and z directions. A MEMS film mask capable of accurately printing lines with widths of less than 0.1 mm is used for the stripe pattern. A right-angle prism made of N-SF11 glass with a refractive index of 1.785 and a 41-degree tilt angle is used for the prism. The prism is positioned directly below the force plate, and a setup is constructed so that it can be imaged from the side by a camera via a mirror. A line-scan camera capable of observing the stripe pattern with high resolution and high temporal resolution is used for the camera, and a telecentric lens is used for observation using parallel light. The first time point is when nothing is placed on the plate. The second time point is when an object 6, such as a butterfly, is placed on the plate. At least two symmetrical oblique striped patterns applied to the surface of the plate at a first time point are superimposed on at least two symmetrical oblique striped patterns applied to the surface of the plate at a second time point, and the displacement of the patterns applied to the surface of the plate between the first and second time points is analyzed. Next, the force applied to the plate is calculated from the displacement of the at least two symmetrical oblique striped patterns between the first and second time points and the spring constant of the spring unit. The state in which a measured object 6, for example, an Butterfly butterfly is placed on the plate is the state in which a force is applied to the plate. [Industrial Applicability]

[0032] The present invention not only introduces a revolutionary measurement principle to the force plate, but also, by combining it with a line scan camera, realizes a three-axial force measurement method that enables analysis with high time resolution.

[0033] References 1. Tuladhar et al., Buildings 2022, 12(11), 1778 2. Nakajima et al., Mem. Grad. Eng. Univ. Fukui, Vol. 66(March 2018)

[0034] All documents mentioned herein are incorporated herein by reference in their entirety. [Explanation of symbols]

[0035] 1 plate 2. Stripes 3 Optical Elements 4. Mirror 5 Line scan cameras 6 Object to be measured 7 Spring part 8 light path 9. Computer d displacement

Claims

1. A force sensor comprising: (i) a plate; (ii) a spring portion; (iii) an optical element that splits one light into two or more; and (iv) a line scan camera, the plate has a pattern on its surface, the pattern having at least two symmetrical oblique stripes; The spring portion is arranged to exert an elastic force when a force is applied to the plate, The force sensor is arranged so that the line scan camera can capture images of the at least two symmetrical inclined stripe patterns applied to the plate from two or more directions through the optical element.

2. The force sensor of claim 1 , wherein the optical element is a prism.

3. The force sensor according to claim 1 , further comprising: (v) a mirror between (iii) the optical element and (iv) the line scan camera on the optical path.

4. The optical element is further connected to a computer, and the computer has an algorithm or software that can analyze the displacement of the at least two symmetrical inclined striped patterns attached to the surface of the plate, which are imaged by the line scan camera through the optical element, and calculate the force applied to the plate from the spring constant of the spring part and the displacement of the patterns. The force sensor of claim 1 .

5. 2. The force sensor according to claim 1, wherein the line scan camera is a line scan camera capable of capturing images at a frequency of 100 frames per second or more.

6. (i) a step of capturing an image of a pattern on a surface of a plate placed on a spring portion with a line scan camera via an optical element that splits a single beam of light into two or more beams at a first point in time before a force is applied to the plate and at a second point in time when a force is applied to the plate, wherein the pattern has at least two symmetrical oblique stripe patterns; (ii) superimposing the at least two symmetrical oblique stripe patterns applied to the surface of the plate at a first time point and the at least two symmetrical oblique stripe patterns applied to the surface of the plate at a second time point, and analyzing the displacement of the patterns applied to the surface of the plate between the first time point and the second time point; and (iii) calculating the force applied to the plate from the displacement of the at least two symmetrical oblique striped patterns between a first time point and a second time point and the spring constant of the spring portion; Methods for measuring force, including:

7. The method of claim 6 , wherein the optical element is a prism.

8. The method of claim 6 , wherein a mirror is disposed in the optical path between the optical element and the line scan camera.

9. 7. The method of claim 6, wherein steps (ii) and (iii) are performed by a computer, algorithm, or software.

10. 7. The method of claim 6, wherein imaging is performed at a frequency of 100 frames per second or greater.

Images