Torsion identification sensor based on liquid crystal filled grapefruit type photonic crystal fiber spiral winding structure

Abstract

The invention belongs to the field of optical fiber sensing, and provides a torsion identification sensor based on a liquid crystal filled grapefruit type photonic crystal fiber spiral winding structure. The optical fiber is characterized in that bionic pomelo petal-shaped air holes of the pomelo type photonic crystal fiber 2 are all filled with nematic liquid crystals, and intrinsic birefringence of the optical fiber is enhanced by using birefringence characteristics of the liquid crystals; the left end of the grapefruit type photonic crystal fiber 2 is sequentially connected with the coreless fiber 3 and the single-mode fiber 4; the single-mode optical fiber 1 is wound on the surface at a precise screw pitch to form a stress coupling interface, and birefringence change caused by torsion is detected through UV glue packaging in combination with an OFDR technology, so that high-sensitivity torsion identification and direction discrimination are realized. The cost of the optical fiber shape sensor is reduced, and the optical fiber shape sensor can be applied to the fields of medical endoscope navigation, interventional operation catheter monitoring, intelligent robot posture control, rock-soil structure deformation monitoring and the like.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure. It aims to achieve high sensitivity and high precision in distinguishing between forward and reverse torque measurements. It is suitable for scenarios with stringent accuracy requirements, such as joint control in surgical robots, main shaft monitoring of wind power equipment, precision mechanical systems, aerospace, intelligent robots, and structural health monitoring, among many other application areas. Background Technology

[0002] Fiber optic sensing technology, with its unique advantages, is profoundly changing the landscape of industrial monitoring. Its inherent safety and explosion-proof characteristics completely eliminate the risk of electrical sparks in hazardous environments such as petrochemical plants and mines, providing unparalleled safety for high-risk environments. Simultaneously, fiber optic sensors possess strong resistance to electromagnetic interference, enabling reliable operation in complex electromagnetic environments such as ultra-high-voltage substations and medical MRI equipment, demonstrating irreplaceable advantages. More importantly, a single optical fiber can simultaneously demodulate multi-dimensional parameters such as temperature, strain, and vibration, significantly reducing the complexity and cost of system deployment. These inherent characteristics have greatly promoted the rapid penetration of fiber optic sensing technology in core areas such as energy equipment monitoring, medical robot navigation, and infrastructure health diagnosis, forming a huge market.

[0003] Despite the numerous advantages of fiber optic sensing technology, it still faces significant technical bottlenecks and challenges in the field of high-precision torsion monitoring. Existing solutions for torsion identification generally rely on complex algorithm compensation or multi-optical-path structures, which not only drastically increases system complexity but also makes it difficult to meet the real-time response requirements of multi-degree-of-freedom mechanical systems. For example, traditional electrical and magnetic torsion sensors are highly susceptible to interference and have complex fabrication processes. In the fiber optic field, although various torsion fiber optic sensors have been proposed, such as those based on long-period fiber gratings (LPFG), polarization-preserving photonic crystal fiber (PM-PCF), hollow-core fiber (HCF), core-biased dual-mode fiber (TMF), irregular spiral ultra-long-period fiber gratings, triangular FBG sensor arrays, and polished fiber optic sensors, while achieving high sensitivity in some aspects, they generally suffer from complex manufacturing processes, insufficiently compact structures, or inadequate sensitivity to meet the requirements of micro-torque scenarios.

[0004] Grapefruit-shaped photonic crystal fibers, through a biomimetic arrangement of grapefruit-petal-shaped symmetrical air holes (typically six-fold symmetry), can break the circular symmetry of optical fibers and generate ultra-high birefringence. However, current research mainly focuses on temperature and refractive index sensing, and has not yet effectively solved the problem of high-sensitivity direct discrimination of torsion. These technical shortcomings are particularly prominent in scenarios with extremely stringent precision requirements, such as minimally invasive surgical robots, severely limiting the further application of fiber optic sensing technology. Therefore, there is an urgent need to develop a simple, easy-to-fabricate, and highly sensitive torsion recognition fiber optic sensor.

[0005] Birefringence is the phenomenon where light propagating in an anisotropic material is split into two beams that travel along different refractive directions. In a non-homogeneous medium, incident light is split into two polarized beams with unequal refractive indices, perpendicular vibration directions, and different propagation speeds—this is the birefringence phenomenon. Liquid crystals are anisotropic materials with an asymmetric molecular structure, similar to the birefringence of uniaxial crystals. Filling liquid crystals into grapefruit-shaped photonic crystal fibers can alter their transmission characteristics and phase modulation capabilities, showing broad application prospects.

[0006] This invention fully fills the air holes of a grapefruit-shaped photonic crystal fiber with liquid crystal. The biomimetic grapefruit-petal-shaped air hole array of the grapefruit-shaped photonic crystal fiber provides stronger constraint on the fully filled liquid crystal, resulting in more uniform molecular deflection under stress and more significant birefringence changes. Furthermore, a unique encapsulation method is employed, where a single-mode fiber is spirally wound onto the surface of the grapefruit-shaped photonic crystal fiber. By optimizing the pitch and the distance between the spiral single-mode fiber and the sensor's centerline, the sensitivity of torsion measurement can be significantly improved. This orientation recognition mechanism overcomes the problems of low accuracy and difficult manufacturing of traditional multi-core fiber optic sensors, achieving precise measurement and compensation of torsional strain and torsion, ensuring high-precision shape reconstruction. Summary of the Invention

[0007] This invention aims to provide a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure, which solves the problems of insufficient recognition capability, low sensitivity and high cost of existing fiber optic torsion sensors, and realizes the recognition of torsion and high-precision measurement of torsion parameters to meet the application needs of multiple fields such as medical, robotics, and geotechnical monitoring.

[0008] This invention discloses a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure, comprising a grapefruit-shaped photonic crystal fiber, a single-mode fiber, and a coreless fiber. The grapefruit-shaped photonic crystal fiber serves as the core sensing element, its biomimetic grapefruit-petal-shaped symmetrical air hole array entirely filled with nematic liquid crystal, utilizing the strong birefringence of the liquid crystal to enhance the intrinsic birefringence of the fiber. The single-mode fiber is spirally wound around the surface of the grapefruit-shaped photonic crystal fiber with a pitch of 0.1-0.5 mm, maintaining a tension of 0.5-1.0 N. The left end of the single-mode fiber serves as the optical signal input end, and the left end of the grapefruit-shaped photonic crystal fiber is sequentially connected to the coreless fiber and the single-mode fiber, with the single-mode fiber serving as the bundle output end. The right ends of both fibers are sealed with UV-curable adhesive to ensure structural stability and liquid crystal sealing.

[0009] This groundbreaking helical wound stress-coupled structure establishes a deterministic correspondence between strain direction and spectral shift in principle. Specifically:

[0010] When torsional force When applied to the sensor, according to the circular axis torsion theory of materials mechanics, the distance from the center of the cross-section of the grapefruit-shaped photonic crystal fiber is... Shear stress at the point The distribution is as follows:

[0011] in Let be the polar moment of inertia of the grapefruit-shaped photonic crystal fiber. Due to the unique biomimetic grapefruit-petal-shaped symmetrical air hole array structure of the grapefruit-shaped photonic crystal fiber, shear stress will generate non-uniform stress coupling at the interface between the grapefruit-shaped photonic crystal fiber and the helically wound single-mode fiber. This causes the shear stress to induce shear strain γ in the cladding of the grapefruit-shaped photonic crystal fiber. According to Hooke's law:

[0012] Where G is the shear modulus of the material, the shear strain distribution is obtained by combining (1) and (2):

[0013] Fully filled nematic liquid crystal molecules are initially aligned along the fiber axis and subjected to shear strain. When applied, it deflects, and the deflection angle is... Based on the birefringence properties of liquid crystals, the birefringence index... Follow The changes satisfy:

[0014] in The ordinary light refractive index of liquid crystal is This is an unusual optical refractive index. Simultaneously, the photoelastic effect in the grapefruit-shaped photonic crystal fiber leads to a change in refractive index:

[0015] in , The elastic coefficient is 1. For axial strain. The change in total birefringence is a superposition of liquid crystal deflection and elasto-optic effect:

[0016] Phase difference of light propagating in an optical fiber The relationship with birefringence is as follows:

[0017] in The effective sensing length of the sensor. The incident light wavelength was measured. The optimal performance is achieved at 1550 nm. Shear strain With torsion angle The geometric relationship is as follows:

[0018] Joint (3) (7) (8), eliminated and Finally, the mapping formula between the torsion angle and the phase change is obtained:

[0019] Phase difference between the reference set and the measurement set was acquired using OFDR. Substituting into formula (9), the torsion angle can be directly calculated. The grapefruit-shaped photonic crystal fiber with fully filled liquid crystal amplifies the birefringence change through formula (4), thus reducing the phase difference. More sensitive to minute torsions, and combined with stress coupling in helical single-mode fiber, distributed phase data acquired via OFDR shows that if the phase shift curve of a certain fiber segment exhibits a monotonically increasing trend over time, i.e. If the torque is 0, it is determined to be clockwise rotation; otherwise, it is counterclockwise rotation, thus achieving high-precision torque measurement. Attached Figure Description

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Figure 1 This is a schematic diagram of a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber helical winding structure. It shows the grapefruit-shaped air hole array structure of the grapefruit-shaped photonic crystal fiber and the winding layer of the single-mode fiber, clearly presenting the overall structure of the sensor.

[0022] Figure 2This is a schematic diagram of a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure applied to a torsion testing device.

[0023] Figure 3 This is a cross-sectional schematic diagram of a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure, clearly showing the biomimetic grapefruit petal-shaped air hole array of the grapefruit-shaped photonic crystal fiber and the initial molecular alignment direction of the fully filled liquid crystal. Detailed Implementation

[0024] The following specific embodiments will further illustrate how the present invention performs detection.

[0025] Please see Figure 1 , Figure 1 This is a schematic diagram of a preferred embodiment of a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure.

[0026] This embodiment uses a grapefruit-shaped photonic crystal fiber with fully filled E7 nematic liquid crystal as the core sensing element, and combines a torsion testing system and OFDR technology to achieve torsion identification, such as Figure 2 As shown, the specific steps are as follows:

[0027] A grapefruit-shaped photonic crystal fiber 2 with a cladding diameter of 125 μm and an effective sensing length of 1.5 cm was selected. A capillary action method was used to fully fill the E7 liquid crystal. Specifically, one end of the grapefruit-shaped photonic crystal fiber was immersed in the E7 liquid crystal, and the capillary effect of the air holes allowed the liquid crystal to naturally fill all the air holes. The filling length was consistent with the sensing length. The optical parameters of the E7 liquid crystal at 20℃ were: ordinary refractive index... unusual optical refractive index birefringence Initially, the molecules are aligned along the fiber axis. After filling, the right end face is sealed with UV-curable adhesive to prevent liquid crystal leakage and ensure the stability of the fully filled structure. The left end is sequentially connected to a coreless fiber 3 and a single-mode fiber 4, with the single-mode fiber 4 serving as the bundle output end. Following the standard parameters of 0.3mm spiral pitch and 0.8N winding tension, the single-mode fiber 1 is tightly spirally wound onto its surface. High-precision winding equipment is used to control the winding parameters to ensure the stability of the stress coupling interface. After winding, the entire structure is encapsulated with UV-curable adhesive.

[0028] Next, two identical miniature lead screw slides 5 and 6 are symmetrically installed on a horizontal plate. Screws and other fasteners are used to ensure a secure connection. The integrated axial tension control module of the miniature lead screw slides enables linear motion adjustment along the x-axis and stable control of the fiber optic axial tension. The fiber optic clamp 7 is fixed to the left miniature lead screw slide 5 to clamp one end of the sensor. The torsion system 8 is fixed to the right miniature lead screw slide 6. This system consists of a hollow shaft rotary motor, a bare wire adapter, and ferrules, serving as a rotating fiber optic clamp to achieve unobstructed optical path coupling and high-precision torque loading control in the entire circumference.

[0029] The left end of the sensor is connected to an optical switch 10. The optical switch selects the transmission path of the optical signal according to control commands, allowing the optical signal to enter the OFDR 11. The OFDR is used to collect distributed strain data from the sensor, and it transmits the collected data to the data acquisition and processing device 12 for processing. The interactive system 9 is connected to the main control chip STM32F103RCT6 via a serial screen UART transmission protocol, and also connects to the host computer to realize data interaction and command transmission between the host computer and the main control chip. After installation, according to the material elastic deformation formula... The axial tension control module was calibrated to establish the correspondence between tension and lead screw displacement. The speed and angle of the hollow shaft rotary motor were calibrated using speed and angle measuring instruments to ensure that its speed was controlled between 1-10° / s and the angle resolution reached 0.01°.

[0030] In addition, a high-precision harmonic reducer is used in the transmission structure design of the torsion testing device. Its reduction ratio is positively correlated with the torque amplification factor. By reducing the speed and increasing the torque, the power of the hollow shaft rotary motor is stably transmitted to the sensor testing end, reducing energy loss and torque fluctuation during the transmission process, and providing a stable and reliable torsional excitation environment for the sensor.

[0031] The host computer sets the torsion detection conditions in the interactive system to determine parameters such as the torsion angle range, rotational speed, and torque loading gradient. Based on the set parameters, the main control chip uses a PID control algorithm (8) to coordinate the operation of the micro lead screw slide and the hollow shaft rotary motor.

[0032] in , , These are the proportional, integral, and differential coefficients, respectively. This is the error.

[0033] One end of the prepared sensor is fixed to the left fiber clamp 7, ensuring a secure fixation without damaging the fiber; the other end is fixed to the torsion system 8 of the torsion system. The clamp is adjusted to allow the sensor to be in a naturally straight state, avoiding initial stress. The reference group phase signal is acquired using OFDR to record the initial birefringence phase difference of the grapefruit-shaped photonic crystal fiber. .

[0034] The hollow shaft rotary motor is started to apply torsional force to the sensor. Torsional force Transmitted through a helical single-mode fiber to a grapefruit-shaped photonic crystal fiber, shear stress is generated in its cross-section. (Formula 1) induces shear strain (Formula 3). E7 liquid crystal molecules deflect under shear strain, and the deflection angle is... The birefringence increases with increasing torsion angle, leading to... It changes according to formula (4).

[0035] Phase difference detected by OFDR It is determined by both the change in liquid crystal birefringence and the photoelastic effect (Equation 7). When a +5° twist is applied... When a -5° torsion is applied, The phase difference changes linearly with stress.

[0036] Will Substituting into formula (9), and combining the parameters quartz shear modulus G and the polar moment of inertia I of the grapefruit-shaped photonic crystal fiber... P The calculated torsion angle verified the effectiveness of the formula.

[0037] The interactive system transmits the collected data to the host computer, and analyzes the OFDR phase shift trend. Determining the direction of twist: If the phase shift of a certain fiber optic segment increases over time, it is determined to be a clockwise twist; if it decreases, it is determined to be a counterclockwise twist.

[0038] In summary, a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber helical winding structure is disclosed. This sensor fills the biomimetic grapefruit-petal-shaped air holes of the grapefruit-shaped photonic crystal fiber with liquid crystal possessing strong birefringence properties, and combines this with a precise helical winding structure of a single-mode fiber to form a highly efficient stress coupling interface. Distributed phase information is acquired using OFDR technology, and based on the correlation between liquid crystal molecule deflection and birefringence changes, high-precision detection of the phase difference caused by torsion is achieved. Furthermore, through the mapping formula between the torsion angle and phase change, the torsion angle can be accurately calculated, and clockwise / counterclockwise direction can be accurately distinguished based on the phase shift trend. This fully verifies the effectiveness and reliability of this invention in achieving high-sensitivity torsion recognition and high-precision parameter measurement in the field of fiber optic sensing, providing an innovative and practical technical solution for applications in multiple fields such as medicine, robotics, and geotechnical monitoring.

[0039] The specific embodiments described herein are preferred embodiments of the invention, but the scope of protection of this invention is not limited to the listed examples. Any equivalent improvements to the shape or structure designed based on this invention, as long as they do not depart from the core technical solution of this invention, should be covered within the scope of protection of the claims of this invention.

Claims

1. A torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber helical winding structure, characterized in that: It consists of a grapefruit-shaped photonic crystal fiber 2 and a single-mode fiber 1. The grapefruit-shaped photonic crystal fiber 2 serves as the sensing substrate, and its biomimetic grapefruit-shaped symmetrical air hole array is completely filled with nematic liquid crystal. The single-mode fiber 1 is spirally wound around its surface with a precise pitch to form a tight stress coupling interface. The left end of the single-mode fiber 1 is the optical signal input end. The left end of the grapefruit-shaped photonic crystal fiber 2 is connected in sequence to the coreless fiber 3 and the single-mode fiber 4. The single-mode fiber 4 serves as the bundle output end. The right ends of the two fibers and the entire spiral structure are sealed with UV-curable adhesive.

2. The torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure according to claim 1, characterized in that: The grapefruit-shaped photonic crystal fiber 2 features a biomimetic grapefruit-petal-shaped symmetrical air hole array structure with a cladding diameter of 125 μm and an effective sensing length of 1.5 cm. The fully filled nematic liquid crystal molecules are initially aligned along the fiber axis. Light entering the liquid crystal is decomposed into ordinary ray (O-ray) and extraordinary ray (E-ray). Since the ordinary and extraordinary rays have different refractive indices and propagate at different speeds within the liquid crystal, their relative positions change when they exit from the other side of the crystal, causing the original single beam of light to be decomposed into two beams after passing through the liquid crystal.

3. The torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure according to claim 1, characterized in that: The fully filled liquid crystal of the grapefruit-shaped photonic crystal fiber 2 is prepared by capillary action, specifically: S1. One end of a grapefruit-shaped photonic crystal fiber is immersed in a nematic liquid crystal. The adhesion between liquid molecules and the container wall is less than the adhesion between liquid molecules, so that the liquid crystal naturally fills all the biomimetic grapefruit-shaped air holes. The filling length is consistent with the effective sensing length of the sensor. S2. After filling, seal the end face with UV-cured adhesive to prevent liquid crystal leakage.

4. The torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure according to claim 1, characterized in that: The spiral winding pitch of the single-mode optical fiber 1 is controlled with an accuracy of 0.1-0.5 mm, and the winding tension is maintained at 0.5-1.0 N to ensure the linear response of the stress coupling interface.

5. A torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure according to claim 1, characterized in that: The coreless optical fiber 3 has a length of 1-2 mm and is used to match the mode field distribution of the grapefruit-shaped photonic crystal fiber 2 and the single-mode fiber 4 to reduce optical signal transmission loss.

6. A torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber spiral winding structure according to claim 1, characterized in that: The single-mode fiber 4 is matched with the fundamental mode field diameter of the grapefruit-shaped photonic crystal fiber 2, and serves as the bundle output end.

7. A torsion recognition method for a torsion recognition sensor based on a liquid crystal-filled grapefruit-shaped photonic crystal fiber helical winding structure, comprising the following steps: S1. When the fully liquid crystal-filled grapefruit-shaped photonic crystal fiber sensor is in a straight state, the distributed Rayleigh scattering phase information of the spiral single-mode fiber and the grapefruit-shaped photonic crystal fiber is collected by OFDR as a reference group signal; when the sensor is bent or twisted, the distributed phase information of the spiral single-mode fiber and the grapefruit-shaped photonic crystal fiber is also collected as a measurement group signal. S2. The phase demodulation method is used to extract the phase change curve of the spiral single-mode fiber and the birefringence phase difference curve of the grapefruit-shaped photonic crystal fiber, and then convert them into the strain distribution curves of the two. S3. From the total strain curve of the helical single-mode fiber, combined with the birefringence strain curve caused by the deflection of liquid crystal molecules in the grapefruit-shaped photonic crystal fiber, the pure torsional strain component is separated to obtain the torsional strain distribution of the helical single-mode fiber. S4. Based on the torsional strain distribution of helical single-mode fiber, and combined with the mapping relationship between torsional strain and torsional angle, the torsional angle distribution of helical single-mode fiber is calculated.

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