Multifunctional tactile sensor based on double-tapered plastic optical fiber and its preparation method

Abstract

The present invention provides a multifunctional tactile sensor based on a double-tapered plastic optical fiber and a preparation method thereof. The sensor is a sensor in which the middle portion of a PMMA plastic optical fiber is formed into a cone shape by using a melt-tap technology, and after taper, an input optical fiber, a first taper region, a micro optical fiber, a second taper region, and an output optical fiber are formed, which are sequentially connected from left to right. The periphery of the second taper region is provided with a coating layer of polyester resin NOA 144, and the peripheries of the first taper region, the micro optical fiber, and the second taper region are also encapsulated in an elastic cladding of PDMS. The sensor described in the present invention can perform force measurement and sliding direction identification. The micro optical fiber diameter and the taper region length can directly change the square distribution of the sensing area, thereby significantly affecting the sensitivity of the sensor and the accuracy of pressure measurement. The taper region is obtained by using a melt-tap technology, which has a simple manufacturing process, low manufacturing cost, high device integration, and strong stability. It has great competitiveness and application prospects in the fields of tactile sensing, medical operations, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and in particular relates to a multifunctional tactile sensor based on a double-tapered plastic optical fiber and a preparation method thereof. Background Art

[0002] In recent years, the rapid development of technologies such as intelligent robotics, medical health monitoring, and virtual reality has placed higher demands on flexible tactile sensors: they must possess high sensitivity, multi-parameter detection capabilities (such as dynamic sliding and static pressure), and adaptability to complex environments. As a non-electrical sensing technology, fiber optic sensors offer unique advantages in the field of flexible sensing due to their resistance to electromagnetic interference, inherent safety, and potential for miniaturization. Based on their sensing mechanism, fiber optic sensors can be categorized as light intensity modulation, phase modulation, and wavelength modulation. Their sensing media are primarily divided into two main types: silica fiber (quartz fiber) and polymer fiber (plastic fiber).

[0003] In traditional applications, quartz optical fiber has long dominated the fields of communications and industrial sensing thanks to its low transmission loss and high mechanical strength (tensile strength ≥5GPa). However, its application in flexible tactile sensing faces significant bottlenecks. Quartz optical fiber lacks mechanical flexibility, and its brittle nature (fracture strain <5%) makes it difficult to withstand repeated bending or impact loads, limiting its integration into wearable devices or robotic joints. Furthermore, its packaging compatibility is poor, and its interfacial adhesion to flexible substrates (such as PDMS) is weak, making it susceptible to deformation and peeling, leading to signal drift. In contrast, polymer optical fibers (such as PMMA core / fluoropolymer cladding structures) are ideal carriers for flexible tactile sensors due to their high fracture toughness (fracture strain >50%), low elastic modulus (2-3GPa), and excellent deformation recovery capabilities. Its technical advantages are reflected in: large diameter (0.25-1mm) and high numerical aperture, which makes optical coupling easier and can simplify the design of peripheral optical systems; adjustable refractive index, which can achieve precise control of the core-cladding refractive index difference through material modification to optimize sensing sensitivity; good biocompatibility, suitable for direct contact with human tissue in medical scenarios.

[0004] Despite the significant potential of polymer optical fibers in the field of flexible sensing, existing technologies still suffer from the following key drawbacks: Single functionality: Most sensors can only detect a single parameter, pressure or strain, and are unable to simultaneously capture dynamic sliding direction and pressure changes, lacking the ability to collaboratively identify sliding direction; Low integration: Existing integrated sensors, such as capacitive and resistive sensors, experience severe signal coupling when simultaneously detecting sliding and pressure, resulting in reduced accuracy; High-precision sensors rely on complex circuits or multi-layer material stacking, resulting in high manufacturing costs and difficulty in miniaturization, limiting their application in flexible electronics and wearable devices; Poor anti-interference performance: When operating in special environments, they are susceptible to electromagnetic interference, resulting in signal distortion and inaccurate measurements; Arrays cannot be arranged densely to improve spatial resolution; achieving multimodal sensing requires array measurement, as in patents. For example, patent CN113551831A proposes a polymer optical fiber knot sensor that only measures pressure; Patent CN202410378704.6 proposes a fiber array dynamic tactile sensing system that improves spatial resolution and enables dynamic tactile sensing, but still cannot achieve multi-point measurement with a single sensor. Therefore, polymer optical fiber sensors need to be further optimized and improved in the above aspects. Summary of the Invention

[0005] In view of this, the present invention aims to propose a multifunctional tactile sensor based on double-tapered plastic optical fiber and a preparation method thereof, which can realize dual-modal detection of sliding direction recognition and pressure measurement, and is suitable for fields such as robot tactile feedback, human-computer interaction interface, minimally invasive surgical instruments and smart wearable devices.

[0006] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0007] A multifunctional tactile sensor based on a double-tapered plastic optical fiber. The sensor is formed by forming a cone in the middle of a PMMA plastic optical fiber using a melt-tapering technique. After tapering, an input optical fiber, a first taper region, a micro optical fiber, a second taper region, and an output optical fiber are sequentially connected from left to right. The outer periphery of the second taper region is provided with a coating layer of polyester resin NOA144, and the outer peripheries of the first taper region, the micro optical fiber, and the second taper region are also encapsulated in an elastic cladding of PDMS. The second taper region comprises, from inside to outside, a high-refractive-index core layer of PMMA, a medium-to-low-refractive-index coating layer of polyester resin NOA144, and a low-refractive-index elastic cladding of PDMS. The first taper region and the micro optical fiber comprise, from inside to outside, a high-refractive-index core layer of PMMA and a low-refractive-index elastic cladding of PDMS. After entering the first taper region from the input optical fiber, the signal is split into two parts. One part is emitted from the first taper region due to mode mismatch, and the other part continues to be transmitted along the micro optical fiber to the second taper region and the output optical fiber.

[0008] Furthermore, the length L of the first tapered region and the second tapered region are both 1000-2000 μm, and the tapered angle α is 5.5-10°.

[0009] Furthermore, the diameter of the micro optical fiber is 40-50 μm.

[0010] Furthermore, the thickness of the coating layer is 50-100 μm, and the refractive index n of the coating layer is 1.45-1.47.

[0011] Furthermore, the elastic cover has a size of 10 mm×5 mm×3 mm.

[0012] Furthermore, the diameter of the PMMA plastic optical fiber is 250 μm.

[0013] The present invention also provides a method for preparing the multifunctional tactile sensor based on the double-tapered plastic optical fiber as described above, the method comprising the following steps:

[0014] 1) Turn on the optical fiber fusion taper equipment, fix the optical fiber with a clamp, set the temperature of the temperature control equipment and the drawing speed, wait for the optical fiber to be heated to the molten state, start the control program, and draw the optical fiber;

[0015] 2) Stretch the tapered optical fiber to the set length, completing the tapering process;

[0016] 3) Fix the tapered optical fiber vertically with a clamp, apply polyester resin to the second tapered area using a dispensing process, and wait for gravity to evenly distribute the cladding on the surface of the tapered area;

[0017] 4) Irradiate the second cone area with a UV lamp to cure the polyester resin coating;

[0018] 5) Remove the cured optical fiber and fix it in the center of the rectangular mold, and fill the mold with the treated PDMS precursor solution;

[0019] 6) Fix the mold and optical fiber on a heating table and heat and cure;

[0020] 7) Demolding to form a multifunctional tactile fiber optic sensor.

[0021] Furthermore, in step 1), the temperature of the temperature control device is set at 100-120° C., the stretching speed is 350-450 μm / s, and the stretching length is set at 1000-1500 μm.

[0022] Furthermore, in step 4), the wavelength of the ultraviolet light is 365-395nm and the intensity is 10-30mW / cm 2 , irradiation area 30-60s.

[0023] Furthermore, in step 6), the heating temperature is 70-80° C., the curing time is 30-40 minutes, and the mold is removed and allowed to stand for 5-10 minutes.

[0024] Compared with the prior art, the multifunctional tactile sensor based on double-tapered plastic optical fiber described in the present invention has the following advantages:

[0025] (1) The multifunctional tactile sensor based on a dual-taper plastic optical fiber described in this invention achieves linear pressure measurement at multiple points in the first and second taper regions, as well as in the microfiber, with a maximum sensitivity of 0.1914 / N. Furthermore, by enhancing the asymmetry of the sensing structure, it enables the sliding direction identification of a single optical fiber, effectively reducing the complexity of the dynamic tactile sensor.

[0026] (2) The present invention introduces polyester resin as a refractive index matching medium to improve light transmission efficiency and suppress environmental interference.

[0027] (3) The present invention adopts PDMS elastomer integrated packaging and enhances adhesion through interfacial chemical modification.

[0028] (4) The sensor described in the present invention adopts the fused cone technology to obtain the sensing area, which has a simple manufacturing process, low manufacturing cost, high device integration and strong stability. It has great competitiveness and application prospects in the fields of tactile sensing, medical operation, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0030] Figure 1 This is the overall structure of the optical fiber after tapering;

[0031] Figure 2 is a schematic diagram of the cone angle α;

[0032] Figure 3 This is the structural diagram after the second cone area is encapsulated with polyester resin;

[0033] Figure 4 This is the overall structure diagram of the optical fiber sensor after elastic cladding encapsulation;

[0034] Figure 5 This is a schematic diagram of the experimental device connection;

[0035] Figure 6 The experimental results of force measurement in the first cone region, micro-fiber, and second cone region are shown;

[0036] Figure 7 The diagram shows the light intensity measurement results in different sliding directions;

[0037] Figure 8 A comparison chart of the output light intensity of different micro-fiber diameters;

[0038] Figure 9This is the effect of different cone lengths (cone lengths) on sensitivity.

[0039] Description of reference numerals:

[0040] 1. Input optical fiber; 2. First cone region; 3. Micro optical fiber; 4. Second cone region; 5. Output optical fiber; 6. Coating layer; 7. Elastic cladding; 8. LED coupling light source; 9. Pressure applying device; 10. Detector; 11. PC terminal. DETAILED DESCRIPTION

[0041] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features therein may be combined with each other.

[0042] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0043] Example 1 Multifunctional tactile sensor

[0044] like Figure 1-4 Figure 1 shows a multifunctional tactile sensor based on a double-tapered plastic optical fiber. The sensor is constructed by forming a tapered middle portion of a PMMA plastic optical fiber using a melt-tapering technique. The tapering process forms, from left to right, an input optical fiber 1, a first tapered region 2, a microfiber 3, a second tapered region 4, and an output optical fiber 5. The outer periphery of the second tapered region 4 is provided with a coating layer 6 of polyester resin NOA144. The outer peripheries of the first tapered region 2, the microfiber 3, and the second tapered region 4 are also encapsulated in an elastic cladding 7 of PDMS. From the inside out, the second tapered region 4 comprises a high-refractive-index core layer of PMMA, a medium-to-low-refractive-index coating layer 6 of polyester resin NOA144, and a low-refractive-index elastic cladding 7 of PDMS. From the inside out, the first tapered region 2 and the microfiber 3 comprise a high-refractive-index core layer of PMMA and a low-refractive-index elastic cladding 7 of PDMS. Therefore, the sensor has an asymmetric structure. After the signal from the input optical fiber 1 enters the first tapered region 2, it is split into two parts. One part is emitted from the first tapered region 2 due to mode mismatch, while the other part continues to transmit along the microfiber 3 to the second tapered region 4 and the output optical fiber 5.

[0045] When pressure acts on the sensor, PDMS, as a force-conducting medium, deforms, causing tiny bends in the first cone region 2, micro-fiber 3, and second cone region 4. At this time, high-order modes do not meet the total reflection conditions and leak, resulting in light intensity attenuation, which is used for pressure measurement.

[0046] Specifically, the diameter r1 of the micro-optical fiber 3 is 50 μm, and the diameter r2 of the PMMA plastic optical fiber is 250 μm, that is, the optical fiber is gradually tapered from 250 μm to a smaller diameter of 50 μm, and the cone area forms a diameter gradient change. The length L of the first cone area 2 and the second cone area 4 are both 1500 μm, the cone angle α is 7.62°, and the micro-optical fiber area is 1500 μm long.

[0047] Cone angle α Figure 2 As shown, the cone angle α is calculated as follows:

[0048] tanα=(r2-r1) / L.

[0049] The thickness of the coating layer 6 is 50 μm, and the refractive index of the coating layer 6 is n≈1.461.

[0050] The dimensions of the elastic cover 7 are 10 mm×5 mm×3 mm.

[0051] The sensor is manufactured by using a fusion taper technique to taper a PMMA optical fiber with a diameter of 250 μm into a tapered optical fiber, including the following steps:

[0052] (1) Turn on the optical fiber fusion taper equipment, use a clamp to fix the optical fiber, set the temperature of the temperature control device to 120℃, the stretching speed to 400μm / s, and the stretching length to 1500μm. After the optical fiber is heated to the molten state, start the control program and slowly stretch the optical fiber;

[0053] (2) Stretch the tapered optical fiber to the set length to end the tapering process; Figure 1 The optical fiber shown is gradually tapered from 250 μm to a smaller diameter of 50 μm. The diameter gradient changes in the tapered area. The length of the tapered area is 1500 μm, the cone angle α is 7.62°, and a micro-fiber area is reserved in the middle. The micro-fiber area is 1500 μm long.

[0054] (3) Fix the tapered optical fiber vertically with a clamp, apply polyester resin NOA144 to the second cone region 4 using a dispensing process, and let it stand for 10 minutes until gravity causes the cladding to be evenly distributed on the surface of the cone region. The thickness is 50 μm, and the refractive index n≈1.461. The coating layer 6 obtained in this way is relatively thin to avoid affecting the hardness of the second cone region 4 due to excessive hardness. Excessive hardness will lead to a significant decrease in sensitivity.

[0055] (4) Use a wavelength of 365 nm and an intensity of 30 mW / cm 2 The ultraviolet light is irradiated on the second cone area for 30-60 seconds to cure the polyester resin coating layer 6, and the optical fiber structure is cured. Figure 3 As shown;

[0056] (5) Remove the cured tapered optical fiber and fix it in the center of a 10 mm × 5 mm × 3 mm rectangular mold. Pour the PDMS precursor solution into the mold after sufficient stirring and vacuum degassing. The mass ratio of curing agent to PDMS is 1:10.

[0057] (6) Fix the mold and optical fiber on a heating table and adjust the temperature to 70 degrees Celsius. Heat and cure for 40 minutes, then remove the mold and let it stand for 5 minutes.

[0058] (7) After the mold is completely cooled, use tweezers to demould it and obtain Figure 4 The sensor shown has an elastic sheath 7 measuring 10 mm x 5 mm x 3 mm, encased in PDMS. The dimensions of the elastic sheath 7 determine the force transmission efficiency of the elastomer, which indirectly affects sensitivity and measurement range as a force measurement intermediate. Therefore, the dimensions of the elastic sheath 7 are carefully considered to balance sensitivity and measurement range without compromising mechanical strength.

[0059] The working principle of this sensor is:

[0060] When the light source signal enters the first cone region 2 from the input optical fiber 1, it is divided into two parts. One part is emitted from the first cone region 2 due to mode mismatch, and the other part continues to be transmitted along the micro-fiber 3 to the second cone region 4 and the output optical fiber 5. When pressure acts on the sensor, PDMS, as a force transmission medium, deforms and causes a slight bend in the first cone region 2, micro-fiber 3, and second cone region 4 of the optical fiber. At this time, the high-order mode does not meet the total reflection condition and leaks, resulting in light intensity attenuation, which is used for pressure measurement. The gradual diameter of the cone region after tapering can lead to stress concentration and enhance local deformation. Therefore, the force measurement sensitivity of the tapered optical fiber can be significantly increased. The force measurement experimental results are shown in Figure 2. Figure 6 The force measurement sensitivity of the first cone region 2 in the range of 0-3N (i.e. the absolute value of the slope of the curve) can reach 0.1914 / N, and the linearity R 2 >0.989; the force measurement sensitivity of micro-fiber 3 in 0-3N can reach 0.0985 / N, and the linearity R 2 >0.928; the force measurement sensitivity of the second cone area 4 in 0-3N can reach 0.1007 / N, and the linearity R 2 >0.977.

[0061] The second cone region 4 is coated with polyester resin material to reduce the photosensitivity of the second cone region 4, increase the stiffness of the second cone region 4, and enhance the asymmetry of the sensing structure. When the external force slides along the axial direction of the optical fiber, it passes through the low-rigidity first cone region 2, the micro-fiber 3, and the high-rigidity second cone region 4 in sequence. The difference in sensitivity causes significant changes in the intensity response of the three sections. By comparing the intensity time domain image features, the sliding direction can be determined, such as Figure 7As shown, the light intensity after passing through the first cone region 2 and the second cone region 4 is different, from which the sliding direction can be known.

[0062] The experimental device of this system is as follows Figure 5 As shown, it includes an LED coupled light source 8, a pressure applying device 9, a detector 10, and a PC terminal 11. The 650nm LED coupled light source 8 emits a light signal that enters the sensing area through the input optical fiber 1. The light signal modulated by the sensor is connected to the detector 10 through the output optical fiber 5 for data signal collection. The collected data is collected and displayed by the PC terminal.

[0063] In the experiment, as the pressure changes, the light intensity signal detected by the detector 10 changes accordingly, and the pressure and the light intensity signal are linearly related, thereby achieving pressure measurement. Figure 7 As shown in the figure, when the same object slides from left to right, the pressure remains unchanged, but the light loss caused by passing through the first cone area 2, micro-fiber 3, and second cone area 4 in sequence is different, which is reflected as small-medium-large in the intensity time domain image. When sliding from right to left, the intensity time domain image is reflected as large-medium-small, thereby realizing the demodulation of the sliding direction.

[0064] Comparative Example 1 Comparison of microfiber diameters

[0065] Five groups of comparative experiments were conducted on the micro-fiber diameters of 40μm, 50μm, 60μm, 80μm, and 100μm. The results are as follows Figure 8 As shown in the image, the output light intensity shows a linear downward trend at 4μm and 50μm, fluctuates more gently within a certain range at 60μm and 100μm, and then increases at 80μm. The larger the radius of microfiber 3, the smaller the light divergence angle, and the more concentrated the first emitted light. Pressing down changes the direction of light emission from the first cone, causing the light from the first cone to be received by the second cone. When the loss light intensity is less than the received light intensity, the output light intensity increases. When the microfiber radius decreases, the divergence angle increases, and the emitted light becomes less concentrated. Therefore, when the light loss caused by pressing the cone is much greater than the gain of the light received in the second cone, the light intensity shows a linear downward trend. Therefore, the microfiber radius should be between 40μm and 50μm. While 40μm has higher sensitivity, the sensor has lower mechanical strength and is easily damaged. Therefore, 50μm is the preferred sensor size.

[0066] Comparison of cone length in Example 2

[0067] The length of the cone area is also an important factor affecting the taper and divergence angle. Through simulation and comparison, sensors with cone lengths of 1000μm-3000μm are compared. The results are as follows Figure 9As shown, the sensitivity of sensors with a cone length of 1000-2000 μm shows a slow decline as the cone length increases, but the overall impact is small. Above 2000 μm, the sensitivity fluctuates significantly and becomes even lower, so the cone length is selected to be between 1000-2000 μm, preferably 1500 μm.

[0068] By comparing the data given in Examples 1 and 2, the importance of the diameter of the microfiber and the length of the tapered region is illustrated.

[0069] In the sensor described herein, the microfiber diameter and tapered length directly alter the light field distribution in the sensing area, significantly affecting the sensor's sensitivity and pressure measurement accuracy. Experimental results show that a microfiber diameter of 40-50 μm and a tapered length of 1000-2000 μm achieve better sensor performance across multiple dimensions, including sensitivity, stability, and mechanical strength.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A multifunctional tactile sensor based on a double-tapered plastic optical fiber, characterized by: The sensor is a PMMA plastic optical fiber formed into a cone shape at its middle portion by melt taper technology. After taper, an input optical fiber, a first taper region, a micro optical fiber, a second taper region, and an output optical fiber are sequentially connected from left to right. The outer periphery of the second taper region is provided with a coating layer of polyester resin NOA144. The outer peripheries of the first taper region, the micro optical fiber, and the second taper region are also encapsulated in an elastic cladding of PDMS. The second taper region is composed of a high-refractive-index core layer of PMMA, a medium- and low-refractive-index coating layer of polyester resin NOA144, and a low-refractive-index elastic cladding of PDMS from the inside to the outside. The first taper region and the micro optical fiber are composed of a high-refractive-index core layer of PMMA and a low-refractive-index elastic cladding of PDMS from the inside to the outside. After the signal enters the first taper region from the input optical fiber, it is divided into two parts. One part is emitted from the first taper region due to mode mismatch, and the other part continues to be transmitted along the micro optical fiber to the second taper region and the output optical fiber.

2. The multifunctional tactile sensor based on double-tapered plastic optical fiber according to claim 1, characterized in that: The length L of the first cone region and the second cone region are both 1000-2000 μm, and the cone angle α is 5.5-10°.

3. The multifunctional tactile sensor based on double-tapered plastic optical fiber according to claim 1, characterized in that: The diameter of the microfiber is 40-50 μm.

4. The multifunctional tactile sensor based on double-tapered plastic optical fiber according to claim 1, characterized in that: The thickness of the coating layer is 50-100 μm, and the refractive index n of the coating layer is 1.45-1.

47.

5. The multifunctional tactile sensor based on double-tapered plastic optical fiber according to claim 1, characterized in that: The dimensions of the elastic envelope are 10 mm x 5 mm x 3 mm.

6. The multifunctional tactile sensor based on double-tapered plastic optical fiber according to claim 1, characterized in that: The diameter of PMMA plastic optical fiber is 250μm.

7. A method for preparing a multifunctional tactile sensor based on a double-tapered plastic optical fiber according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: 1) Turn on the optical fiber fusion taper equipment, fix the optical fiber with a clamp, set the temperature of the temperature control equipment and the drawing speed, wait for the optical fiber to be heated to the molten state, start the control program, and draw the optical fiber; 2) Stretch the tapered optical fiber to the set length, completing the tapering process; 3) Fix the tapered optical fiber vertically with a clamp, apply polyester resin to the second tapered area using a dispensing process, and wait for gravity to evenly distribute the cladding on the surface of the tapered area; 4) Irradiate the second cone area with a UV lamp to cure the polyester resin coating; 5) Remove the cured optical fiber and fix it in the center of the rectangular mold, and fill the mold with the treated PDMS precursor solution; 6) Fix the mold and optical fiber on a heating table and heat and cure; 7) Demolding to form a multifunctional tactile fiber optic sensor.

8. The method for preparing a multifunctional tactile sensor based on a double-tapered plastic optical fiber according to claim 7, characterized in that: In step 1), the temperature of the temperature control device is set at 100-120° C., the stretching speed is 350-450 μm / s, and the stretching length is set to 1000-1500 μm.

9. The method for preparing a multifunctional tactile sensor based on a double-tapered plastic optical fiber according to claim 7, characterized in that: In step 4), the wavelength of the UV lamp is 365-395nm and the intensity is 10-30mW / cm 2 , irradiation area 30-60s.

10. The method for preparing a multifunctional tactile sensor based on a double-tapered plastic optical fiber according to claim 7, characterized in that: In step 6), the heating temperature is 70-80°C, and the curing time is 30-40 minutes. The mold is removed and allowed to stand for 5-10 minutes.

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