Hand perception sensor based on FP and FBG optical fiber sensing and preparation method thereof

Abstract

The invention provides a hand perception sensor based on FP and FBG optical fiber sensing, which comprises a Fabry-Perot cavity (FP), a single-mode optical fiber and at least one Bragg grating (FBG), the Fabry-Perot cavity is positioned at one end of the single-mode optical fiber, and the Bragg grating is embedded in the single-mode optical fiber. The FP optical fiber sensor and the FBG optical fiber sensor are adopted, tiny strain changes can be detected by measuring input light and through changes of reflection wavelengths of optical fibers, and the soft robot shape sensing device has the advantages of being high in sensitivity, large in elasticity, high in resolution ratio, free of electromagnetic interference and the like in the aspect of sensing the shape of a soft robot. The sensing system is integrated into the soft finger and used for detecting bending and deformation behaviors of the joint of the soft finger, pressure monitoring is carried out on the touch sense of the tail end of the finger, the working performance is stable, the integrity is high, and the application potential in real-time monitoring of the force position of a soft mechanical arm is huge.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a design and preparation method of an embedded soft hand sensing sensor based on Fabry-Perot cavity (FP) and Bragg grating (FBG) optical fiber sensing. Background Art

[0002] With the development of robots, soft robots have become a major trend in the development of robotics. The special structure and materials of soft robots make them flexible and can achieve bending, twisting, stretching and other movements. They have continuity and good interactivity in operation, which significantly improves the adaptability and safety of robots in actual operations. However, due to the continuity and large ductility of soft robots, traditional sensors such as encoders and strain gauges cannot be used.

[0003] At present, soft sensors for soft robots mainly include resistive and capacitive thin film and array sensors. Although they have certain flexibility, the former two are susceptible to electromagnetic interference, and the latter has low spatial resolution. In addition, the nonlinear and large deformation characteristics of soft robots make the monitoring and calibration of sensors very difficult. Summary of the invention

[0004] The objectives of the present invention are achieved through the following technical solutions.

[0005] The present invention proposes an embedded soft hand perception sensor based on Fabry-Perot cavity (FP) and Bragg grating (FBG) optical fiber sensing.

[0006] Specifically, the present invention provides a hand perception sensor based on FP and FBG optical fiber sensing, including a Fabry-Perot cavity (FP), a single-mode optical fiber and a Bragg grating (FBG), wherein the Fabry-Perot cavity is located at one end of the single-mode optical fiber, and the Bragg grating includes at least one and is embedded in the single-mode optical fiber.

[0007] Furthermore, the Bragg gratings include three, the spacing distances are 25 mm and 30 mm, the distance between the Bragg grating and the Fabry-Perot cavity is 20 mm, and the length of the Bragg grating is 5 mm.

[0008] Furthermore, the preparation process of the Fabry-Perot cavity (FP) comprises the following steps:

[0009] S11, adjusting the parameters of the fiber fusion splicer, and axially fusion splicing the single-mode fiber and the hollow-core fiber; placing the fused fiber on a precision adjustable two-dimensional micro-displacement platform, adjusting the platform X and Y axes so that the single-mode fiber and the hollow-core fiber are located within the microscope field of view, and using the fiber cutting platform to cut the hollow-core fiber, leaving the hollow-core fiber length at 200 μm;

[0010] S12, using a fiber fusion splicer to seal the end of the prepared hollow-core optical fiber, translate the entire optical fiber toward the hollow-core optical fiber, ensure that the electrode is located 180 μm away from the fusion point of the hollow-core optical fiber and the single-mode optical fiber, and then adjust the arc discharge current to 65 mA to taper and seal the hollow-core optical fiber to ensure that the gas in the microcavity does not flow out. The high temperature generated by the high current causes a part of the hollow-core optical fiber to collapse and form a bubble at the end of the single-mode optical fiber. The initial bubble has an irregular shape and an inner diameter of about 60 μm.

[0011] S13. The position of the electrode is continuously adjusted according to the shape of the bubble, and the discharge current is reduced to 40 mA. Multiple discharges make the volume of the bubble gradually increase, and the side wall becomes thinner and thinner. The final spherical bubble is located at the end of the entire optical fiber structure and has an inner diameter of 130 μm.

[0012] Furthermore, the core diameter of the single-mode optical fiber is 9 μm, and the cladding diameter is 125 μm; the air core diameter of the hollow optical fiber is 80 μm, and the cladding diameter is 125 μm.

[0013] Furthermore, the preparation process of the Bragg grating (FBG) comprises the following steps:

[0014] S21, using an ultraviolet light source to irradiate the single-mode optical fiber through a phase mask to generate a periodic refractive index change in the optical fiber; during the micro-hole processing, a charge coupled camera (CCD) is used to obtain an optical fiber image and monitor the etching condition of the optical fiber;

[0015] S22. Repeat the preparation of three FBGs with intervals of 25 mm and 30 mm, a distance between FBG and FP of 20 mm, and a length of FBG of 5 mm.

[0016] Furthermore, the ultraviolet light source is an excimer laser, and its power, central wavelength, frequency and repetition rate are 20mJ, 248nm, 100Hz and 50Hz respectively.

[0017] Furthermore, the single-mode optical fiber is a four-core optical fiber.

[0018] Furthermore, the four-core optical fiber includes a central core and an equilateral triangle three-core optical fiber.

[0019] Furthermore, the preparation process of the four-core optical fiber includes:

[0020] (a) drilling four holes in the cladding preform rod and inserting the core preform rod into the corresponding positions to form a multi-core optical fiber preform rod;

[0021] (b) The multi-core optical fiber preform is placed in a clamping and rotating mechanism, passes through a heating furnace, a laser diameter gauge, an online coating device, a coating curing lamp, a capstan, and finally is wound into a finished product.

[0022] The advantages of the present invention are that the FP and FBG optical fiber sensors used in the present invention can detect tiny strain changes by measuring the input light and the change of the reflected wavelength of the optical fiber. In terms of sensing the shape of soft robots, they have the advantages of high sensitivity, high elasticity, high resolution, and no electromagnetic interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0024] Figure 1 A schematic diagram of the structure of an embedded soft hand force position sensing sensor based on Fabry-Perot cavity (FP) and Bragg grating (FBG) optical fiber sensing according to an embodiment of the present invention is shown.

[0025] Figure 2 A process flow chart of manufacturing a Fabry-Perot cavity (FP) according to an embodiment of the present invention is shown.

[0026] Figure 3 A process flow chart of fabricating a Bragg grating (FBG) according to an embodiment of the present invention is shown.

[0027] Figure 4 A schematic diagram of an application scenario of a hand perception sensor according to an embodiment of the present invention is shown.

[0028] Figure 5 A schematic diagram of the strain of a measuring point and the neutral layer strain according to an embodiment of the present invention is shown.

[0029] Figure 6 A process flow chart of preparing a quad-core optical fiber according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0031] In order to solve the technical problems existing in the background technology, the present invention proposes an embedded soft hand force position sensing sensor based on Fabry-Perot cavity (FP) and Bragg grating (FBG) optical fiber sensing. Figure 1As shown, it includes a Fabry-Perot cavity (FP), a single-mode optical fiber and a Bragg grating (FBG), wherein the Fabry-Perot cavity is located at one end of the single-mode optical fiber, and the Bragg grating includes at least one and is embedded in the single-mode optical fiber. Preferably, it includes three FBGs, with intervals of 25 mm and 30 mm, the distance between the FBG and the FP is 20 mm, and the length of the FBG is 5 mm. The single-mode optical fiber is a four-core optical fiber.

[0032] The invention proposes a novel soft hand force position sensing optical fiber sensor, the main principle of which is: in the air-filled optical fiber microbubble, since the inner walls of the upper and lower surfaces of the optical fiber microbubble are smooth, the upper and lower surfaces form reflection surfaces M1 and M2 respectively, and the reflection surfaces M1 and M2 form a double-beam Fabry-Perot interferometer (FPI) structure, such as Figure 1 As shown in the figure, the light I0 transmitted in the optical fiber is reflected by the reflection surfaces M1 and M2, and then re-enters the optical fiber to form double-beam interference. Assume that the distance between the upper and lower sides of the optical fiber bubble is l, and the refractive index of the gas medium in the bubble is n e The central wavelength corresponding to the trough position of the spectrum can be expressed as Where m is the number of Bragg gratings (FBG). When the bubble is subjected to pressure, the length l of the bubble cavity changes, resulting in the movement of the interference peak; different pressures lead to different changes in the interference peak, and the interference peak is monitored by a spectrometer and converted into a change in flow rate after demodulation.

[0033] Among them, Figure 2 As shown, the preparation process of the Fabry-Perot cavity (FP) includes the following steps:

[0034] S11. Adjust the parameters of the fiber fusion splicer to axially fusion-splice the single-mode fiber and the hollow-core fiber; place the fused fiber on a precision adjustable two-dimensional micro-displacement platform, adjust the platform's X and Y axes so that the single-mode fiber and the hollow-core fiber are within the microscope's field of view, and use a high-precision fiber cutting platform to cut the hollow-core fiber, leaving the hollow-core fiber length at 200μm. The core diameter of the single-mode fiber is 9μm, and the cladding diameter is 125μm; the air core diameter of the thick hollow-core fiber is 80μm, and the cladding diameter is 125μm.

[0035] S12, using a fiber fusion splicer to seal the end of the prepared hollow-core optical fiber, and moving the entire optical fiber toward the hollow-core optical fiber, ensuring that the electrode is located 180 μm away from the fusion point of the hollow-core optical fiber and the single-mode optical fiber, and then adjusting the arc discharge current to 65 mA to perform cone-shaped sealing on the hollow-core optical fiber to ensure that the gas in the microcavity does not flow out. The high temperature generated by the high current causes a part of the hollow-core optical fiber to collapse and form a small bubble at the end of the single-mode optical fiber. The initial bubble shape is irregular and the inner diameter is about 60 μm;

[0036] S13. The position of the electrode is continuously adjusted according to the shape of the bubble, and the discharge current is reduced to 40 mA. Multiple discharges make the volume of the bubble gradually increase, and the side wall becomes thinner and thinner. The final spherical bubble is located at the end of the entire optical fiber structure and has an inner diameter of 130 μm.

[0037] like Figure 3 As shown, the preparation process of the Bragg grating (FBG) includes the following steps:

[0038] S21. Use an ultraviolet light source (such as an excimer laser) to irradiate a single-mode optical fiber through a phase mask to produce a periodic refractive index change in the optical fiber. During the microhole processing, a charge coupled camera (CCD) is used to obtain an optical fiber image and monitor the etching of the optical fiber. The power, central wavelength, frequency and repetition rate of the excimer laser are 20mJ, 248nm, 100Hz and 50Hz respectively.

[0039] S22. Repeat the preparation of three FBGs with intervals of 25 mm and 30 mm, a distance between FBG and FP of 20 mm, and a length of FBG of 5 mm.

[0040] Figure 4 A schematic diagram of the application scenario of the hand sensing sensor according to an embodiment of the present invention is shown. First, the optical fiber sensor is firmly bonded to the finger using cyanoacrylate glue. The low viscosity of the glue allows the glue to flow quickly between the optical fiber sensor and the finger surface and cure at room temperature for 12 hours. During this process, the FP should be placed at the tip of the finger before bonding, and the FBG should correspond to the joint bend.

[0041] Then, under broadband light excitation, each FBG reflects a narrow band centered on the FBG-specific wavelength λ. The reflected wavelength λ varies proportionally to the strain experienced by the sensor, with a sensitivity to axial strain as high as about 1.2pm at a wavelength of 1550nm, and a typical resolution of strain detection on the order of 0.1με or higher.

[0042] The present invention proposes a new type of soft hand force position sensing optical fiber sensor. The main principle is: FBG is a grating with a spatial phase formed by the photosensitivity of the optical fiber material and the periodic change of the incident light in the optical fiber core. Its essence is to form a narrowband filter in the optical fiber core. When broadband light passes through the optical fiber, light of a specific wavelength will be reflected, and the remaining light waves will be projected out by the grating. By measuring the wavelength reflected by the grating, it can be converted into the strain of the object being measured. The central wavelength of the Bragg grating is:

[0043] λ B =2n eff Λ(1)

[0044] Among them, n effis the effective refractive index of the optical fiber, and Λ is the grating period. Keeping the temperature constant and ignoring the temperature coefficient, the wavelength change can be expressed as:

[0045] △λ=△εK ε =λ B (1-E f )△ε(2)where E f is the elastic modulus of the optical fiber, K ε is the gauge coefficient of FBG, the difference between the strain ε1 at the measuring point and the strain ε0 at the neutral layer, where r is the radius of the optical fiber, as Figure 5 As shown. Substituting into formula (2), we can get the relationship between the drift and curvature of the central wavelength of the i-th fiber core:

[0046] △λ i =λ i (1-E f )kr i (3)

[0047] in, ρ is the radius of curvature, θ i is the azimuth angle of the fiber core i, and θ is the azimuth angle of the optical fiber.

[0048] In this way, the sensor of the present invention is placed at the end of the finger of the soft manipulator as a tactile sensor. When the finger touches an object, the cavity microbubble produces a slight deformation, the distance of the FP cavity changes, causing the Fabry-Perot interference peak to move, and the tactile pressure is monitored after demodulation. Three Bragg fibers are placed at the finger joints as bending sensors. When the finger is bent, the grating period and the effective refractive index change, thereby causing the center wavelength of the Bragg grating reflection spectrum to drift. After demodulation, high-precision and high-efficiency monitoring of the bending angle is achieved. The bending and tactile information of five fingers are integrated to achieve continuous motion detection, action posture and object recognition. The sensor probe is small in size, compact in structure, flexible in use, and highly sensitive, and can be used for force and position monitoring of soft manipulators.

[0049] The sensing system of the present invention is integrated into a soft finger to detect the bending and deformation behavior of the soft finger joints and to monitor the pressure of the tactile sensation at the finger tip. It has stable working performance and strong integrity, and has great application potential in real-time force and position monitoring of soft manipulators.

[0050] The four-core optical fiber used in the present invention includes a central core and an equilateral triangle three-core optical fiber, wherein the central core is used as a temperature compensation to eliminate the influence of ambient temperature, and the equilateral triangle three-core optical fiber is used for bending and torsion measurement. In the four-core optical fiber used, the spacing between the equilateral three cores is 100 μm. Figure 6 The process flow chart of preparing a quad-core optical fiber according to an embodiment of the present invention is shown. Specifically, it includes:

[0051] (a) drilling four holes in the cladding preform rod and inserting the core preform rod into the corresponding positions to form a multi-core optical fiber preform rod;

[0052] (b) The multi-core optical fiber preform is placed in a clamping and rotating mechanism, passes through a heating furnace, a laser diameter gauge, an online coating device, a coating curing lamp, a capstan, and finally is wound into a finished product.

[0053] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A hand sensing sensor based on FP and FBG optical fiber sensing, characterized in that: It comprises a Fabry-Perot cavity (FP), a single-mode optical fiber and a Bragg grating (FBG), wherein the Fabry-Perot cavity is located at one end of the single-mode optical fiber, and the Bragg grating comprises at least one and is embedded in the single-mode optical fiber.

2. The hand sensing sensor according to claim 1, characterized in that: The Bragg gratings include three, with spacings of 25 mm and 30 mm, the distance between the Bragg grating and the Fabry-Perot cavity is 20 mm, and the length of each Bragg grating is 5 mm.

3. The hand sensing sensor according to claim 1, characterized in that: The preparation process of the Fabry-Perot cavity (FP) comprises the following steps: S11, adjusting the parameters of the fiber fusion splicer, and axially fusion splicing the single-mode fiber and the hollow-core fiber; placing the fused fiber on a precision adjustable two-dimensional micro-displacement platform, adjusting the platform X and Y axes so that the single-mode fiber and the hollow-core fiber are located within the microscope field of view, and using the fiber cutting platform to cut the hollow-core fiber, leaving the hollow-core fiber length at 200 μm; S12, using a fiber fusion splicer to seal the end of the prepared hollow-core optical fiber, and moving the cut optical fiber as a whole toward the direction of the hollow-core optical fiber, ensuring that the electrode is located 180 μm away from the fusion point of the hollow-core optical fiber and the single-mode optical fiber, and then adjusting the arc discharge current to 65 mA to taper and seal the hollow-core optical fiber to ensure that the gas in the microcavity does not flow out. The high temperature generated by the high current causes a part of the hollow-core optical fiber to collapse and form a bubble at the end of the single-mode optical fiber. The initial bubble has an irregular shape and an inner diameter of about 60 μm; S13. The position of the electrode is continuously adjusted according to the shape of the bubble, and the discharge current is reduced to 40 mA. Multiple discharges make the volume of the bubble gradually increase, and the side wall becomes thinner and thinner. The final spherical bubble is located at the end of the entire optical fiber structure and has an inner diameter of 130 μm.

4. The hand sensing sensor according to claim 3, characterized in that: The core diameter of the single-mode optical fiber is 9 μm, and the cladding diameter is 125 μm; the air core diameter of the hollow optical fiber is 80 μm, and the cladding diameter is 125 μm.

5. The hand sensing sensor according to any one of claims 1 to 4, characterized in that: The preparation process of the Bragg grating (FBG) comprises the following steps: S21, using an ultraviolet light source to irradiate the single-mode optical fiber through a phase mask to generate a periodic refractive index change in the optical fiber; during the micro-hole processing, a charge coupled camera (CCD) is used to obtain an optical fiber image and monitor the etching condition of the optical fiber; S22. Repeat the preparation of three FBGs with intervals of 25 mm and 30 mm, a distance between FBG and FP of 20 mm, and a length of FBG of 5 mm.

6. The hand sensing sensor according to claim 5, characterized in that: The ultraviolet light source is an excimer laser, and its power, central wavelength, frequency and repetition rate are 20 mJ, 248 nm, 100 Hz and 50 Hz respectively.

7. The hand sensing sensor according to claim 1, characterized in that: The single-mode optical fiber is a four-core optical fiber.

8. The hand sensing sensor according to claim 7, characterized in that: The four-core optical fiber includes a central core and an equilateral triangle three-core optical fiber.

9. The hand sensing sensor according to claim 7 or 8, characterized in that: The preparation process of the four-core optical fiber includes: (a) drilling four holes in the cladding preform rod and inserting the core preform rod into the corresponding positions to form a multi-core optical fiber preform rod; (b) The multi-core optical fiber preform is placed in a clamping and rotating mechanism, passes through a heating furnace, a laser diameter gauge, an online coating device, a coating curing lamp, a capstan, and finally is wound into a finished product.

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