Ball type FBG tactile sensor

Abstract

The invention discloses a ball type FBG tactile sensor, and belongs to the technical field of intelligent monitoring and control, the tactile sensor is provided with an FBG, a flexible material block, a ball and a ball container, the FBG is packaged in the flexible material block, the packaging depth is half of the thickness of the flexible material block, the ball is placed in the ball container, and the flexible material block is arranged in the ball container. The ball container is divided into an inner shell and an outer shell, the inner shell is provided with hollow parts with two diameters, the hollow parts are slightly larger than and slightly smaller than the diameters of the balls respectively, so that the balls can roll freely and do not fall off, the outer shell is a cylinder with a hollow inner part, and the bottom of the ball container is fixed on the surface of the flexible material block and needs to cover a grid region of the FBG. The device has the advantages of being small in size, high in sensitivity and resistant to electromagnetic interference, and can be used for touch sensing of the fingertips of the intelligent robot.

Description

Technical Field

[0001] The invention belongs to the technical field of intelligent monitoring and control, and in particular relates to a ball-type FBG tactile sensor. Background Art

[0002] In today's rapidly developing intelligent era, tactile sensors, as key components in human-machine interaction, robot perception, and intelligent device control, are increasingly showing their irreplaceable importance. Tactile sensors can simulate and surpass the perception function of human skin, accurately capture and identify information such as object texture, shape, and texture, and provide a richer and more delicate perception dimension for machine intelligence. How to develop tactile sensors with high precision, small size, and strong robustness is the focus of current research on tactile perception technology.

[0003] At present, researchers have developed a variety of tactile sensors for texture recognition using different principles, mainly including piezoresistive, capacitive, and piezoelectric. However, sensors developed based on traditional electromagnetic theory generally have problems such as complex structure, susceptibility to electromagnetic interference, and high hysteresis. In recent years, considering the advantages of fiber optic sensors such as small size, high sensitivity, and resistance to electromagnetic interference, some researchers have developed fiber optic tactile sensors, especially fiber optic tactile sensors based on FBG. Fiber Bragg grating (FBG) sensors not only have the general advantages of optical waveguide sensors, but also have the advantages of small structure, light weight, high sensitivity, corrosion resistance, and no electromagnetic interference. They are relatively soft, easy to implant, and suitable for node-type quasi-distributed sensing. The central wavelength of the fiber grating sensor is affected by the force, and the value changes. The relationship between strain and central wavelength can be analyzed.

[0004] Although the fiber optic tactile sensor developed based on FBG has advantages such as high sensitivity and anti-electromagnetic interference, the current fiber optic tactile sensor has some defects for some complex tactile perceptions such as texture recognition. The current fiber optic sensors are mainly oriented to the recognition of object shape and surface roughness, and fiber optic tactile sensors for texture recognition are rarely proposed. This is because the application of fiber optic tactile sensors in texture recognition needs to consider the influence of friction during contact on texture signals. Objects of different materials have different frictions, and the same object will also produce different frictions due to differences in temperature, humidity and other conditions, which makes texture recognition more challenging. At the same time, miniaturization is also an important factor that fiber optic tactile sensors should consider so that they can be integrated into the fingertips of intelligent robots to achieve intelligent perception. Summary of the invention

[0005] The purpose of the present invention is to provide a ball-type FBG tactile sensor with small size, high sensitivity and anti-electromagnetic interference characteristics, which can meet the occasions of intelligent tactile perception. The rolling sensing method is adopted to effectively reduce the influence of the heat generated by friction on the FBG wavelength deviation, and also make the sensing process smoother.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A ball-type FBG tactile sensor comprises an FBG, a flexible material block, a ball and a ball container, wherein the FBG is encapsulated inside the flexible material block, the ball is placed in the ball container, the top of the ball container is fixed to the surface of the flexible material block and covers the gate area of ​​the FBG, the ball rolls on the surface of an object to sense information, and outputs the surface information of the object through the central wavelength of the FBG.

[0008] Furthermore, the packaging depth of the FBG in the flexible material block is half of the thickness of the flexible material block.

[0009] Furthermore, the outer surface of the inner shell has 8 symmetrically distributed grooves. During assembly, the ball is first placed in the outer shell, and then the inner shell is buckled. Fixing agent is injected through the grooves on the outer surface of the inner shell to make the inner and outer shells adhere together.

[0010] Furthermore, the flexible material block is formed by pouring polydimethylsiloxane liquid in a mold and then curing it.

[0011] Furthermore, the surface features of different objects are mapped by the size of the FBG central wavelength offset and the time point when the offset occurs.

[0012] Furthermore, the magnitude of the FBG center wavelength offset is determined by the degree of deformation caused by the ball squeezing the flexible material block. The greater the fluctuation of the surface texture of the object, the greater the fluctuation of the ball, and thus the greater the FBG center wavelength offset.

[0013] Furthermore, different textures are distinguished according to the amplitude of the FBG central wavelength fluctuation.

[0014] Beneficial effects of the present invention:

[0015] 1. The present invention uses fiber Bragg grating (FBG) as a sensing element, which has the advantages of anti-electromagnetic interference and high sensitivity.

[0016] 2. The present invention designs a ball and ball container device, which obtains tactile information in a rolling manner. Compared with the existing public technology, it has the following advantages: reducing the friction in the tactile perception process, making the perception process smoother; avoiding the occurrence of FBG thermal offset in the tactile perception process; small size, easy to integrate into the fingers of a humanoid robot; FBG does not directly participate in tactile perception, reducing the loss of sensor elements and having a longer service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below in conjunction with the accompanying drawings.

[0018] Figure 1 It is a structural diagram of the ball-type FBG tactile sensor of the present invention;

[0019] Figure 2 FIG. 1 is a diagram of a ball container of a ball-type FBG tactile sensor of the present invention;

[0020] Figure 3 This is a sensing principle diagram of the ball-type FBG tactile sensor of the present invention;

[0021] Figure 4 This is a diagram of the tactile perception object of the ball-type FBG tactile sensor of the present invention;

[0022] Figure 5 This is a diagram showing the tactile perception effect of the ball-type FBG tactile sensor of the present invention at different speeds;

[0023] Figure 6 This is a diagram showing the tactile perception effects of the ball-type FBG tactile sensor of the present invention on different textures;

[0024] Figure 7 This is a diagram showing the tactile perception effect of the ball-type FBG tactile sensor of the present invention with different contact forces.

[0025] In the figure: 1. FBG; 2. flexible material block; 3. ball; 4. ball container; 5. ball container outer shell; 6. ball container inner shell. DETAILED DESCRIPTION

[0026] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] like Figure 1 , Figure 2A ball-type FBG tactile sensor is shown, comprising a FBG (fiber Bragg grating) 1, a flexible material block 2, a ball 3 and a ball container 4, wherein the FBG 1 is packaged inside the flexible material block 2, and the packaging depth of the FBG 1 in the flexible material block 2 is half of the thickness of the flexible material block 2;

[0028] The ball 3 is placed in the ball container 4, and under the condition that there is no interference from external forces other than gravity, the ball 3 will partially protrude from the ball container 4. The ball container 4 is divided into an outer shell 5 and an inner shell 6, and the outer wall of the inner shell 6 is fixed to the inner wall of the outer shell 5;

[0029] Specifically, the outer surface of the inner shell 6 has 8 symmetrically distributed grooves. During the assembly process, the ball 3 is first placed in the outer shell 5, and then the inner shell 6 is buckled. The fixing agent is injected through the grooves on the outer surface of the inner shell 6 so that the inner shell 6 and the outer shell 5 are adhered together;

[0030] The interior of the inner shell 6 includes hollow parts with different inner diameters connected end to end, the diameter of the part close to the flexible material block 2 of the two hollow parts is slightly larger than the diameter of the ball 3, and the diameter of the other hollow part is slightly smaller than the diameter of the ball 3, so that the ball 3 can roll freely and not fall off;

[0031] The housing 5 is a partially hollow cylinder, the lower surface of the flexible material block 2 is in close contact with and fixed to the top of the ball container 4, and the top of the ball container 4 should cover the grid area of ​​the FBG1. The ball 3 rolls on the surface of the object to sense information, and outputs the surface information of the object through the central wavelength of the FBG1.

[0032] Specifically, the flexible material block 2 is entirely made of polydimethylsiloxane (PDMS) material, with a size of 20×30×4mm, which can effectively protect the optical fiber and make the FBG1 more sensitive to external stress changes. The ball 3 and the ball container 4 are made of polyamide (PA) material through 3D printing technology. The ball container 4 is in the shape of a cylinder with a bottom radius of 6.5mm and a height of 8mm. The height of the protruding part of the ball 3 is 2.4mm. The diameter of the hollow part of the outer shell 5 is 12.4mm and the height is 6mm. The diameter of the hollow part of the inner shell 6 is 9.8mm and 8.4mm respectively, and the height is 4.3mm and 1.7mm respectively. The spherical radius of the ball 3 is 4.2mm.

[0033] like Figure 3The sensing principle of the ball-type FBG tactile sensor shown in the figure, when the ball-type tactile sensor performs tactile sensing, the flexible material block 2 should be located above the ball container 4, and the side that is not in contact with the ball container 4 should be fixed. The tactile sensor senses the surface texture of the object through the ball 3 and relies on FBG1 to transmit texture information. FBG1 is an optical sensing element with a grating structure with periodic refractive index changes inside its core. Based on the Fresnel principle, when a beam of broadband light source is emitted from a medium with a high refractive index to a medium with a low refractive index, reflection and transmission will occur. Only light signals that meet a specific wavelength will be reflected back, and light signals of other wavelengths will be transmitted. The central wavelength λ of the reflected light B Determined by the characteristics of FBG, the following conditions are met:

[0034] λ B =2n eff Λ

[0035] Among them, the reflected light wavelength λ B The effective refractive index n of the core eff and the grating period Λ. When the FBG is affected by external stress and temperature, the effective refractive index of the core and the grating period will change, causing the central wavelength of the reflected light to shift. This process can be expressed as:

[0036]

[0037] Among them, p 11 and p 12 is the effective elastic-optic coefficient; α is the thermal expansion coefficient, ξ is the thermo-optic coefficient; Δε and ΔT represent the strain and temperature change of the FBG respectively; v represents the Poisson of the optical fiber. Since the texture signal acquisition in this paper is carried out in a constant temperature environment and a rolling sensing method is adopted, the influence of the ambient temperature change and the heat generated by friction on the FBG can be ignored. So the simplified formula is:

[0038]

[0039] When the ball 3 performs tactile perception, it will cause different degrees of deformation to the flexible material block 2. Figure 3 The relative position diagram of the top of the ball when in contact with the concave-convex textures of different spacings is shown. When the distance between the texture protrusions is reduced from d0 to d1, the maximum descending distance s0 of the ball 3 is also reduced to s1. When the present invention senses the texture of the object at a uniform contact position, the strain of the flexible material block 2 in each direction can be expressed by the following formula according to Hooke's law:

[0040]

[0041] Where σ is the stress on the flexible material block 2, ε z ,εx ,ε y are the strains in each direction, E,v PDMS are the elastic modulus and Poisson's ratio of the flexible material block 2. The smaller the distance that the ball 3 can descend, the greater the reaction force on the flexible material block 2, while the elastic modulus and Poisson's ratio are both constants, then the greater the strain of the flexible material block 2 in the axial direction of FBG1, the central wavelength offset of FBG1 is related to the axial strain caused by the stress on the optical fiber, so different textures will lead to different degrees of fluctuation range of the central wavelength of FBG1.

[0042] The fiber optic demodulator was used to record the change of FBG central wavelength, and the performance of the ball-type FBG tactile sensor was evaluated by sensing different speeds, different textures, and different contact forces.

[0043] Before the measurement, a static calibration experiment can be performed on the ball-type FBG tactile sensor of the present invention, including applying a positive pressure in the range of [1N: 1N: 10N] to the manufactured ball-type FBG tactile sensor and analyzing the relationship between the wavelength offset and the pressure.

[0044] like Figure 4 The ball-type FBG tactile sensor shown in the figure is an experimental object for tactile perception. Specifically, three concave-convex texture plates are designed. The length, width and height of the three texture plates are all 200×80×7mm. The difference is that the protrusion spacing of the texture is different, and the spacing d is 2, 3, and 4mm respectively. The texture plate is modeled using COMSOL Multiphysics and manufactured using polylactic acid (PLA) material through 3D printing technology.

[0045] like Figure 5 The ball-type FBG tactile sensor shown in the figure performs tactile perception at different speeds. Specifically, it senses three texture plates at speeds of 5, 10, and 15 mm / s, respectively. Figure 5 The output signal of the perceived d=4mm texture plate. It can be seen that when the same texture is perceived at different speeds, the fluctuation range of the signal waveform is the same. The difference is that the faster the speed, the tighter the signal waveform.

[0046] like Figure 6 The ball-type FBG tactile sensor shown in FIG. 1 performs tactile perception on different textures. Specifically, the ball-type FBG tactile sensor senses texture plates with spacings of d=2, 3, and 4 mm at a speed of 15 mm / s. The results are shown in FIG. Figure 6As shown in the figure, it can be seen that the larger the texture spacing d is, the larger the fluctuation range of the signal waveform of the ball-type FBG tactile sensor is. The texture spacing is calculated based on the time span between adjacent envelopes. The calculation results for textures with actual spacing d of 2, 3, and 4 mm are 1.95, 3.09, and 4.07 mm, respectively. The errors are 2.5%, 3.0%, and 1.8%, respectively, reflecting that the present invention has good performance in measuring textures with different spacings.

[0047] like Figure 7 The ball-type FBG tactile sensor shown in the figure performs tactile perception with different contact forces. Specifically, it perceives textures with different spacings at a speed of 15 mm / s. Taking the texture d=4 mm as an example, the output signal of the ball-type FBG tactile sensor is as follows: Figure 7 As shown, different textures are distinguished according to the value of the product of speed and time. The texture spacing calculated from the waveform is 4.08, 4.08, and 4.09 mm, and the error rates are 2.0%, 2.0%, and 2.3%, respectively.

[0048] The above contents are merely examples and explanations of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in a similar manner. As long as they do not deviate from the invention or exceed the scope defined by the claims, they shall all fall within the protection scope of the present invention.

Claims

1. A ball-type FBG tactile sensor, characterized in that: The invention comprises an FBG, a flexible material block, a ball and a ball container, wherein the FBG is encapsulated inside the flexible material block, the ball is placed in the ball container, the top of the ball container is fixed on the surface of the flexible material block and covers the gate area of ​​the FBG, the ball rolls on the surface of an object to sense information, and outputs the surface information of the object through the central wavelength of the FBG.

2. A ball-type FBG tactile sensor according to claim 1, characterized in that: The packaging depth of the FBG in the flexible material block is half of the thickness of the flexible material block.

3. The ball-type FBG tactile sensor according to claim 1, characterized in that: The outer surface of the inner shell has 8 symmetrically distributed grooves. During assembly, the ball is first placed in the outer shell, and then the inner shell is buckled. Fixing agent is injected through the grooves on the outer surface of the inner shell to make the inner and outer shells adhere together.

4. The ball-type FBG tactile sensor according to claim 1, characterized in that: The flexible material block is formed by pouring polydimethylsiloxane liquid into a mold and then curing the liquid.

5. The ball-type FBG tactile sensor according to claim 1, characterized in that: The surface features of different objects are mapped by the size of the FBG central wavelength offset and the time point when the offset occurs.

6. The ball-type FBG tactile sensor according to claim 5, characterized in that: The magnitude of the FBG center wavelength offset is determined by the degree of deformation caused by the ball squeezing the flexible material block. The greater the fluctuation of the surface texture of the object, the greater the fluctuation of the ball, and thus the greater the FBG center wavelength offset.

7. The ball-type FBG tactile sensor according to claim 6, characterized in that: Different textures are distinguished according to the amplitude of the FBG central wavelength fluctuation.

Images