A polymer optical fiber synchronous multipoint displacement sensor based on bending loss coupling of helical bending structure

Abstract

The present application belongs to the technical field of sensor, and in particular to a polymer optical fiber synchronous multipoint displacement sensor based on spiral bending structure bending loss coupling, comprising: a main optical fiber fixed into at least one spiral bending structure extending along the axial direction; a light source connected with one end of the main optical fiber and injecting measuring light into the main optical fiber; at least one secondary optical fiber arranged outside the spiral bending structure and capable of linear displacement along the spiral axis relative to the main optical fiber; the light power output by the photodetector changes with the position of the secondary optical fiber relative to the spiral bending structure, thereby realizing displacement measurement, and when multiple secondary optical fibers or multiple spiral units are arranged, each measurement channel is independent of each other through the respective radiation coupling path and can work synchronously without signal decoupling. The coupling of the radiation loss generated by the spiral bending of the main optical fiber and the secondary optical fiber realizes independent work of multiple measurement channels without additional signal decoupling.

Description

Technical Field

[0001] This invention belongs to the field of sensor technology, specifically relating to a polymer optical fiber synchronous multi-point displacement sensor based on bending loss coupling of a helical bending structure. Background Technology

[0002] With the development of fields such as structural health monitoring, precision motion control of robots, and biomedical displacement detection, the demand for multi-point displacement measurement is increasing. Traditional displacement sensors, such as resistance strain gauges, capacitance sensors, or laser displacement sensors, while meeting the accuracy requirements for some applications, have the following drawbacks: Multi-point measurements are difficult to synchronize: multiple measurement points require separate placement of independent sensing elements, which complicates signal processing and makes them susceptible to mutual interference; Complex structure or difficult installation: Traditional sensors require additional mechanical support or complex wiring, limiting their application in space-constrained or curved structures; Poor environmental adaptability: Some sensors are sensitive to electromagnetic interference or are prone to drift under temperature changes, affecting measurement stability.

[0003] To overcome the aforementioned problems, fiber optic displacement sensing technology has emerged as a potential solution. Utilizing the bending loss characteristics of optical fibers, non-contact, distributed, multi-point synchronous measurements can be achieved; however, existing technologies still have the following shortcomings: Multi-point measurement systems often require complex signal decoupling algorithms; Traditional fiber optic deployments make it difficult to achieve independent coupling of multiple channels, which limits the sensitivity and reliability of sensors. The lack of optimization in the coupling and sensing area design of polymer optical fibers makes it difficult to balance measurement range and sensitivity.

[0004] Therefore, there is a need for a fiber optic displacement sensor that is simple in structure, can realize multi-point synchronous displacement measurement, has independent channels, and is easy to deploy, in order to meet the application needs of industrial, robotics, and biomedical fields. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a polymer fiber synchronous multi-point displacement sensor based on bending loss coupling of a helical bending structure.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a polymer optical fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling, comprising: The main optical fiber is fixed into at least one helical bend structure extending along the axial direction; A light source is connected to one end of the main optical fiber and injects measurement light into the main optical fiber; At least one secondary optical fiber is arranged outside the helical bend structure and can be linearly displaced relative to the main optical fiber along the helical axis; At least one photodetector is connected to the output end of the secondary optical fiber; In this system, the radiation loss generated at multiple bends of the helical bend is captured by the secondary optical fiber in the coupling region between it and the main optical fiber. The optical power output by the photodetector varies with the position of the secondary optical fiber relative to the helical bend structure, thereby realizing displacement measurement. When multiple secondary optical fibers or multiple helical units are set, each measurement channel is independent of each other through its own radiation coupling path and can work synchronously without signal decoupling.

[0007] Preferably, the number of the spiral bending structures on the main optical fiber is one, the number of the secondary optical fibers and the number of the photodetectors are both three, and at least three independent secondary optical fibers are arranged in their circumference to achieve multi-point synchronous measurement. Multiple spiral sensing units are connected in series along a single main optical fiber, and an independent secondary optical fiber is arranged adjacent to the bending area of ​​each spiral unit and connected to an independent photodetector.

[0008] Preferably, the number of helical bend structures on the main optical fiber is three, and the number of secondary optical fibers and photodetectors is also three.

[0009] Preferred options also include: At least one cylindrical substrate is provided for supporting at least one of the helical bend structures on the main optical fiber; A spiral groove is provided on the cylindrical substrate and is spirally arranged in the main optical fiber embedded in the spiral groove.

[0010] Preferred options also include: The support is fixedly connected to both ends of the main optical fiber; The bracket is detachably connected to at least one of the photodetectors and is capable of adjusting the height of at least one of the photodetectors.

[0011] Preferably, it also includes a pre-tightening mechanism; The pre-tightening mechanism can tighten the helical bending structure wound on the main optical fiber.

[0012] Preferably, the pretensioning mechanism includes a knob, a slide, a support frame, at least two fixing rods, and at least two springs; The knob is slidably connected to the outer wall of the cylindrical base, the knob is connected to the slide rail, the slide rail is slidably connected to the support frame, at least two of the fixed rods are slidably connected to the support frame, the spring is sleeved on the fixed rod, and the two ends of the spring are respectively connected to the support frame and the fixed rod; At least two of the fixing rods are symmetrically arranged and the main optical fiber is held by the elastic deformation of the spring; The sliding connection between the slide rail and the support frame can compensate for the height difference generated when the main optical fiber is tightened.

[0013] Preferably, it also includes a fixing component, which includes at least two connecting rods, a base, at least two snap-fit ​​parts, at least two snap-fit ​​grooves, and a fixing seat; At least two of the connecting rods are rotatably connected to the base, the base is connected to the inner wall of the cylindrical base, the snap-fit ​​part is provided on the connecting rod, and the snap-fit ​​part can snap into the snap-fit ​​groove provided on the fixed base.

[0014] Preferably, the fixing assembly further includes a tension spring connected to the two connecting rods.

[0015] Preferably, the fixing component further includes an inclined surface, and the pre-tightening mechanism further includes a contact element; The inclined surface is provided on the connecting rod, and the knob is connected to the contact element; When the contact and the knob move upward, the contact can contact the inclined surface on the connecting rod and bring the two connecting rods closer together, thereby causing the locking part to disengage from the locking groove.

[0016] Compared with existing technologies, it has the following beneficial effects: Multi-point synchronous measurement is achieved by coupling the radiation loss generated by the spiral bending of the main optical fiber with the secondary optical fiber, enabling multiple measurement channels to work independently without the need for additional signal decoupling. Simple structure and easy to arrange: the main optical fiber is arranged along a spiral, and the secondary optical fiber can move linearly along the spiral axis, making it easy to install and suitable for space-constrained scenarios; Non-contact high-sensitivity measurement: Non-invasive displacement measurement is achieved by capturing the bending radiation loss of the main fiber through the secondary fiber. The sensitivity can be improved by adjusting the helical bending radius or the core diameter of the secondary fiber. High independence of multiple channels: Each measurement channel works independently through its own radiation coupling path, without interfering with each other, ensuring measurement accuracy and stability; High environmental adaptability: Polymer optical fibers are not sensitive to electromagnetic interference and can work stably within a certain temperature range, making them suitable for industrial, robotics and biomedical applications. Attached Figure Description

[0017] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1This is a schematic diagram of the overall structure of the present invention; Figure 2 This is a schematic diagram of the structure from another perspective in this invention; Figure 3 This is a schematic diagram of the connection structure between the main optical fiber and the spiral groove in this invention. Figure 4 This is an exploded view of the pre-tightening mechanism and fixing components in this invention; Figure 5 This is a schematic diagram of the planar connection structure of the pre-tightening mechanism and the fixing component in this invention; Figure 6 This is a schematic diagram of a partial connection structure of the pre-tightening mechanism in this invention; Figure 7 This is a schematic diagram of the connection structure of the fixing component in this invention; Figure 8 This is a schematic diagram of the connection structure between the contact element and the knob in this invention; Figure 9 Schematic diagrams are shown of the displacement sensor response (a) using a 2 mm diameter optical fiber and (b) using a 1 mm optical fiber; Figure 10 The schematic diagrams show the sensor's response to increases and decreases in displacement (a) with a 1 mm fiber and (b) with a 2 mm fiber. Figure 11 A schematic diagram showing the use of two fiber optic displacement sensors is presented. Figure 12 A schematic diagram of the sensor's response under different bending radii i is shown; Figure 13 A schematic diagram of the response using a multi-point displacement sensor with the first configuration is shown; Figure 14 A schematic diagram of the response of a multi-point displacement sensor with a single helical unit is shown.

[0018] Explanation of reference numerals in the attached figures: 1-Support, 10-Bracket, 2-Light source, 3-Cylindrical base, 4-Main optical fiber, 5-Spiral groove, 6-Photodetector, 7-Secondary optical fiber, 8-Pre-tightening mechanism, 801-Contact, 802-Knob, 803-Slide rail, 804-Support frame, 805-Fixing rod, 806-Spring, 9-Fixing assembly, 901-Seat body, 902-Connecting rod, 903-Inclined surface, 904-Tension spring, 905-Snap-fit ​​part, 906-Fixing seat, 907-Snap-fit ​​groove. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] like Figures 1 to 5 As shown: A polymer fiber synchronous multi-point displacement sensor based on bending loss coupling of a helical bending structure has the following structure: The sensor includes a main optical fiber 4, a light source 2, at least one secondary optical fiber 7, at least one photodetector 6, a cylindrical base 3, a spiral groove 5, a support 1, and a bracket 10.

[0021] The main optical fiber 4 is a polymer optical fiber with a core diameter of approximately 980 μm, an allowable deviation of approximately ±60 μm, and a numerical aperture of approximately 0.5. The main optical fiber 4 is fixedly mounted on the support 1 and extends axially. The main optical fiber 4 is arranged in a helical bend structure and embedded in a helical groove 5 on the outer surface of the cylindrical substrate 3 to achieve stable support and constraint. A light source 2, preferably a light-emitting diode with a center wavelength of approximately 660 nm, is connected to one end of the main optical fiber 4 and is used to inject measurement light into the main optical fiber 4.

[0022] A secondary fiber 7 is arranged outside the helical bend structure of the primary fiber 4. This secondary fiber is also a point-of-freedom (POF) fiber, with a core diameter of approximately 1 mm and / or approximately 2 mm. The secondary fiber 7 is arranged along the helical length of the primary fiber 4 and forms a loose coupling relationship with minimal spacing, thereby collecting the radiation loss power generated at the bend of the primary fiber 4. To constrain the movement of the secondary fiber 7, a three-dimensional guide groove can be provided on the cylindrical substrate 3, allowing the secondary fiber 7 to move linearly only in a direction perpendicular to the helical axis, thus reducing measurement errors introduced by axial offset.

[0023] The photodetector 6 is connected to the output end of the secondary optical fiber 7. It can be equipped with an optical power meter and its matching photodetector module to detect the intensity of the optical signal and output displacement-related electrical signals. The photodetector 6 is mounted on the bracket 10. The bracket 10 and the photodetector 6 are detachably connected, and the height of the photodetector 6 can be adjusted to adapt to different installation requirements.

[0024] In one embodiment, the main optical fiber 4 is provided with a helical bend structure, and at least three independent secondary optical fibers 7 are arranged along the circumference of the main optical fiber 4, and each is connected to three independent photodetectors 6. In this way, synchronous displacement measurement at three different positions can be achieved, and each measurement channel operates independently without the need for additional signal decoupling.

[0025] In another embodiment, multiple helical sensing units are connected in series on the main optical fiber 4. Each helical unit is adjacent to an independent secondary optical fiber 7 and connected to an independent photodetector 6. For example, when three helical units are set on the main optical fiber 4, three secondary optical fibers 7 and three photodetectors 6 are configured accordingly, thereby realizing multi-point distributed synchronous displacement measurement.

[0026] In the above embodiments, the effective sensing length of each helical sensing unit is approximately 8 mm, and the helical bending radius can be selected from 10 mm to 25 mm. By reducing the bending radius, the coupling efficiency between optical fibers and the displacement sensitivity can be improved. Furthermore, using a secondary optical fiber with a core diameter of 2 mm provides higher sensitivity in the 0–3 mm measurement range compared to a 1 mm core diameter.

[0027] The sensor described in this invention can operate stably within an ambient temperature range of 20 °C to 55 °C, and the different channels are independent of each other, with no cross-sensitivity issues. The main optical fiber 4 is fixed within the sensing area, and displacement is caused only by the relative movement of the secondary optical fiber 7, thereby achieving non-invasive displacement measurement through changes in coupling strength.

[0028] In use, the main polymer fiber is fixed into a helical bend structure; measurement light is injected into the main fiber 4 through the light source 2; at least one secondary fiber 7 is driven to move relative to the helical axis; the optical power at the output end of the secondary fiber 7 is collected, and the correspondence between optical power and displacement is established to obtain the displacement value.

[0029] In practical applications, linear calibration can be performed in the two quasi-linear regions of 0–3 mm and 7–14 mm respectively, and the displacement value can be calculated accordingly. Multi-point synchronous measurement can be achieved by using a single spiral with multiple secondary optical fibers; multi-point sequential measurement can be achieved by connecting multiple spiral units in series along the same main optical fiber and configuring secondary optical fibers respectively, and the channels do not interfere with each other and do not require decoupling.

[0030] The sensor described in this invention can be widely used in applications requiring multi-point synchronous displacement monitoring, such as structural health monitoring, robot motion measurement, and biomedical displacement detection. It has advantages such as simple structure, high sensitivity, and strong anti-interference ability.

[0031] like Figures 6 to 8 As shown: It also includes a pre-tensioning mechanism 8 for tensioning and fixing the main optical fiber 4. The pre-tensioning mechanism 8 includes a knob 802, a slide rail 803, a support frame 804, at least two fixing rods 805, at least two springs 806, and a contact element 801.

[0032] The knob 802 is slidably connected to the outer wall of the cylindrical base 3 and contacts the inclined surface 903 on the connecting rod 902 through the contact element 801; The slide rail 803 is slidably connected to the support frame 804 to compensate for the height difference generated during the tensioning of the main optical fiber 4; At least two fixed rods 805 are slidably connected to the support frame 804 and are fitted with springs 806, with the two ends of the springs 806 connected to the fixed rods 805 and the support frame 804 respectively; The fixing rods 805 are symmetrically arranged, and the main optical fiber 4 is clamped by the elastic deformation of the spring 806 to achieve stable fixation.

[0033] When the pre-tightening mechanism 8 drives the contact 801 to move upward via the knob 802, the contact 801 acts on the inclined surface 903 on the connecting rod 902, bringing the two connecting rods 902 closer together, thereby disengaging the snap-fit ​​part 905 from the snap-fit ​​groove 907, which facilitates the adjustment or release of the main optical fiber 4.

[0034] The fixing assembly 9 includes a base 901, at least two connecting rods 902, a snap-fit ​​part 905, a snap-fit ​​groove 907, a fixing seat 906, a tension spring 904, and an inclined surface 903. The base 901 is fixed to the inner wall of the cylindrical base 3, and the connecting rods 902 are rotatably connected to the base 901; the snap-fit ​​part 905 is provided on the connecting rod 902 and can snap into the snap-fit ​​groove 907 on the fixing seat 906; the tension spring 904 is connected to the two connecting rods 902 and is used to provide a restoring force; the inclined surface 903 is provided on the connecting rod 902 and is used to cooperate with the contact member 801 to realize the disassembly operation of the cylindrical base 3.

[0035] In practical use, the knob 802 operates the contact 801, which, through the action of the inclined plane, moves the connecting rod 902 closer to or further away, thereby clamping or releasing the cylindrical base 3. Users can replace the cylindrical base 3 with different specifications of spiral grooves 5 as needed. The sliding connection between the fixing rod 805 and the spring 806, along with the slide rail 803 and the support frame 804, ensures the stability and tension uniformity of the cylindrical base 3 during use.

[0036] Please refer to Figures 9 to 14 In the early stages of research, we systematically analyzed and optimized the key parameters of the single-helix sensing unit, including radius, fiber tension, and coupling distance. Simultaneously, we tested the secondary fiber using two different specifications (1 mm and 2 mm). Based on the insights gained from the single-unit characterization, we extended this technical solution to a dual-configuration scheme, successfully developing a multi-point displacement sensing system.

[0037] A comparison of 2 mm and 1 mm diameter fiber-optic couplers (POFs) showed that, within the same displacement range, the 2 mm fiber could couple more power. This is attributed to its larger surface area and higher energy harvesting efficiency, allowing the 2 mm fiber to maintain a consistently stronger power coupling capability. To investigate the characteristics of the coupling-based sensing mechanism, we conducted controlled displacement experiments using a fixed helical structure in conjunction with a vertically movable secondary fiber. The secondary fiber was mounted on a translation platform and scanned bidirectionally along the helical axis. Setting-1: Move the secondary fiber from top to bottom, starting with the first spiral bend and continuing until the last one.

[0038] Setting-2: Move the secondary fiber from the bottom upwards, starting from the last bend and ending at the first bend.

[0039] The experiment demonstrates the optical power injected into the secondary fiber in each experiment, using a 2 mm diameter POF as the receiver. In setup 1, the coupling power decreases with increasing displacement. This phenomenon is not due to a decrease in curvature at subsequent bends, but rather because each helical bend radiates a different intensity of optical power. Conversely, in setup 2, the coupling power increases with increasing displacement—this experiment progresses from the last bend to the first. This is because the sensor gradually enters the region of the optical core that retains more optical power, resulting in a stronger bending radiation effect and thus achieving a better coupling effect.

[0040] When the secondary fiber is moved back and forth (i.e., from the first bend to the last and then in the opposite direction), the optical power response exhibits good repeatability. Notably, the experiments show repeatable coupling behavior during both forward and reverse movements, with stable optical power levels observed at specific distances (e.g., 0 mm and 14 mm), indicating the existence of displacement-insensitive points. Furthermore, the response is unaffected by displacement velocity, demonstrating the robustness and consistency of the coupling-based sensing mechanism. This indicates stable sensor performance, as consistent results are obtained regardless of the speed at which the secondary fiber moves.

[0041] To further investigate the response characteristics of the sensor in bidirectional displacement, all subsequent experiments were conducted using setting 1. Figure 10 The results shown illustrate the optical power response curves when the displacement varies bidirectionally from 0 mm to 14 mm. Figure 10As shown, the coupled optical power continuously decreases as the displacement increases (i.e., the secondary fiber moves from the first bend to the last bend). This is because the optical power gradually attenuates due to previous radiation losses at each successive bend, causing the coupling effect of the fiber to gradually weaken in the helical path. Conversely, when the displacement decreases (i.e., the secondary fiber moves from the last bend to the first bend), the coupled optical power increases because the sensor approaches the high-power region near the initial bend. These regions generate more radiated light due to the higher residual power within the fiber, thus enhancing the coupling effect. The near-symmetrical characteristics of the curves in both directions highlight the reversibility of the sensing mechanism, confirming that the sensor possesses bidirectional cyclic displacement tracking capability and maintains stable optical response characteristics.

[0042] Figure 11 The sensor's displacement response characteristics were demonstrated, showcasing its ability to monitor a full 14 mm displacement range. However, due to the nonlinear effect of optical power coupling caused by the helical bending structure, the sensor did not exhibit a completely linear response throughout the measurement range. Therefore, we divided the response curve into two quasi-linear intervals: In region I (0-3 mm), the optical power showed a steep and relatively linear upward trend; in region II (7-14 mm), it exhibited a relatively flat but still linear change. Through linear fitting analysis, we quantified the sensor's performance in the two quasi-linear intervals. Table 3 summarizes the linear regression equations and coefficients of determination (R²) for 1 mm and 2 mm POF configurations. 2 ) and sensitivity value (unit: µW / mm).

[0043] In the 0-3 mm range, both optical fibers exhibit excellent linearity: the linearity coefficient R of the 1 mm fiber is... 2 The linearity coefficient is 0.9889, while that of 2 mm fiber reaches 0.9966. Notably, the 2 mm fiber exhibits a sensitivity of 2.301 µW / mm, significantly superior to the 1.3966 µW / mm of the 1 mm fiber. This is primarily due to its larger surface area and more efficient light collection capabilities. In the 7-14 mm range, although the linearity coefficient remains relatively high (R0...), the linearity coefficient... 2 The sensitivity value was > 0.98, but due to the reduced optical power at subsequent bends, the sensitivity value decreased significantly. The sensitivity performance of 2 mm fiber once again surpassed that of 1 mm fiber (0.2128 µW / mm and 0.0608 µW / mm, respectively), fully demonstrating its applicability in long-distance displacement detection.

[0044]

[0045] Each bend in the helical structure results in varying power loss, primarily because the optical power within the fiber gradually attenuates along the helical path. At the first bend, the fiber carries the highest input power, thus generating a stronger bending radiation effect and achieving higher secondary coupling efficiency. As the light travels along the helical path, the optical power at each previous bend has been lost. Therefore, while the second bend still radiates a significant amount of optical power, its radiation is slightly lower than at the first bend due to reduced remaining energy. This trend continues along the helical structure, with the loss gradually decreasing at each subsequent bend—not because the bending force weakens, but because the available optical power for loss decreases. Ultimately, the initial power at each bend differs, resulting in a non-uniform power loss distribution along the helical structure.

[0046] Since the sensor structure generates radiation loss through helical bending, it is crucial to assess the impact of changes in the bending radius on coupling characteristics. Figure 12 Experimental data visually demonstrates this relationship: the sensor structure is wound around three cylindrical rods with bending radii of 10 mm, 15 mm, and 25 mm, respectively. The results show that a smaller bending radius leads to higher radiation loss, thus improving the coupling efficiency of the secondary fiber. Specifically, the helical structure wound around the 10 mm rod exhibits the highest coupled optical power, thanks to a more significant mode leakage effect caused by bending in the main fiber. In contrast, the structure with a bending radius of 25 mm exhibits the lowest radiation and coupling power because its curvature is gentler and its optical leakage effect is minimal. This phenomenon is consistent with bending loss theory—tighter bending disrupts total internal reflection, leading to enhanced evanescent fields and exacerbating radiation loss. Therefore, a smaller bending radius improves sensor sensitivity, while a larger bending radius provides greater stability while reducing sensitivity to small displacements. Sensitivity data for different bending radii are detailed in Table 4.

[0047]

[0048] Figure 3 An experimental setup for a cascaded multi-point displacement sensor was demonstrated. Three independent helical units were arranged sequentially along a single optical fiber, equivalent to a single sensor with three independent sensing points. Related experimental results are as follows: Figure 14 As shown, where Figure 14 The response curves obtained using a 1 mm optical fiber are shown. Figure 14 The response of a 2 mm optical fiber is presented. The sensing length of each helical unit is 8 mm. The optical signal is first input to sensor 1, and then propagates through sensor 2 and sensor 3 in sequence.

[0049] As previously observed in the single-helix unit, the initial bends, due to their higher residual core energy, allow for the coupling of more optical power. A similar pattern emerges in the cascaded structure: sensor 1 exhibits strong coupling at the first two bends, with the response gradually becoming smoother and linear after the fourth bend. In this cascaded sequence, the first bend of sensor 2 corresponds to the 9th bend, while the first bend of sensor 3 corresponds to the 17th bend, leading to power loss accumulated at the upstream bends, which reduces the coupling strength. If... Figure 14 In setup 2 (where light enters from sensor 3 to sensor 1), the response of sensor 3 will be mirror-symmetrical with that of sensor 1, with a more significant coupling effect at the initial bend. However, while this configuration is suitable for multi-point distributed sensing, it has inherent limitations due to power attenuation issues. Therefore, we propose an alternative (configuration 2) that uses a single-helix structure with multiple sets of secondary optical fibers to extend the sensing range and improve overall efficiency.

[0050] The second multi-point displacement sensor (single spiral shape, with multiple receivers) The second configuration offers a simpler and more compact solution for multi-point displacement sensing because it eliminates the need for multiple helical units. Once a single helical structure is formed, the loss effect caused by bending ensures that each bend radiates optical power uniformly across a 360° range. In this design, multiple secondary (receiver) fibers are strategically distributed at different locations within the helical structure to independently capture the radiated power. This arrangement allows for a highly efficient multi-point sensor from a single helical light source, significantly reducing manufacturing complexity. Figure 10 As shown, each receiving fiber of sensors 1, 2, and 3 exhibits a slightly different coupling response, attributed to the non-uniform power distribution in the bends. Since each bend radiates different amounts of power depending on its location and remaining energy, subtle differences in coupling effects exist between different receiving points. Despite this variation, the overall sensing characteristics of all receiving fibers remain consistent, confirming the feasibility of this simplified structure in multi-point displacement sensing.

[0051] In both designs, the receiving fiber can independently couple the radiated power of the main helical structure, enabling simultaneous measurement of multiple displacement points. The core advantage of this approach is that it eliminates the need for complex decoupling techniques—techniques typically used in multiplexed sensor systems to separate overlapping signals. In our design, each sensing unit (whether an independent helical structure or a single receiving fiber) operates autonomously, with no optical interference between channels.

[0052] Table 5 summarizes the configuration schemes of the multi-point displacement sensor. Temperature influence must be carefully considered during sensor deployment. Both configurations utilize polymer optical fibers, which operate stably within a temperature range of 20°C to 55°C. This operating temperature range is consistent with the conclusions of several previous studies [39,40], which indicate that 20°C to 55°C is the ideal temperature range for maintaining the mechanical flexibility and optical performance of POF-based sensors. Therefore, the designed sensor operates stably within this temperature range, exhibiting neither significant thermal degradation nor cross-sensitivity issues.

[0053] This study comprehensively and systematically investigates fiber optic displacement sensors based on helical structures, focusing on their application in multi-point sensing using bending-induced coupling mechanisms. Experimental results verify the theory that controlled helical bending can generate measurable radiated optical power, which can be efficiently coupled to secondary fibers near the bending region. By analyzing key parameters such as fiber diameter, bending radius, and displacement direction, the influence of these parameters on coupling strength and sensor linearity is revealed. Two different multi-point displacement sensing configuration schemes are proposed: the first scheme uses three helical sensing units connected in series along a single optical path, with the optical signal transmitted sequentially through each unit. While this achieves spatially distributed sensing, the detection range is limited due to accumulated power loss. The second scheme uses a single helical structure paired with multiple sets of receiving fibers at different positions, achieving multi-point synchronous detection without the need to repeatedly construct helical units. This design offers advantages in terms of compactness and ease of fabrication.

[0054] Notably, the receiving optical fibers in both configurations can be independently coupled to optical signals, enabling synchronous displacement monitoring without decoupling techniques. This characteristic highlights the scalability and practicality of this design in real-time distributed sensing applications. Furthermore, the sensor operates stably within a temperature range of 20°C to 55°C, perfectly matching the typical operating conditions of polymer optical fibers reported in the literature. The proposed helical multi-point sensing structure exhibits high sensitivity, repeatability, and flexibility. The sensing resolution and spatial distribution can be flexibly adjusted through structural design and coupling geometry, making this scheme a highly promising candidate for applications in structural health monitoring, robotics, and biomedical sensing.

[0055] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A polymer fiber synchronous multi-point displacement sensor based on bending loss coupling of a helical bending structure, characterized in that, include: The main optical fiber (4) is fixed into at least one helical bend structure extending along the axial direction; A light source (2) is connected to one end of the main optical fiber (4) and injects measurement light into the main optical fiber (4); Multiple secondary optical fibers (7) are arranged outside the helical bending structure and can be linearly displaced along the helical axis relative to the main optical fiber (4); Multiple photodetectors (6) are connected to the output ends of the multiple secondary optical fibers (7); In this process, the radiation loss generated at multiple bends of the helical bend is captured by the secondary optical fiber (7) in the coupling region between it and the main optical fiber (4). The optical power output by the photodetector (6) varies with the position of the secondary optical fiber (7) relative to the helical bend structure, thereby realizing displacement measurement. When multiple secondary optical fibers (7) or multiple helical sensing units are set, each measurement channel is independent of each other through its own radiation coupling path and can work synchronously without signal decoupling. When the number of the spiral bending structures on the main optical fiber (4) is one, multiple independent secondary optical fibers (7) are set in its circumference to achieve multi-point synchronous measurement. When multiple spiral sensing units are connected in series along a single main optical fiber (4), an independent secondary optical fiber (7) is arranged adjacent to the bending area of ​​each spiral sensing unit and connected to an independent photodetector (6).

2. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 1, characterized in that: The number of helical bend structures on the main optical fiber (4) is three, and the number of secondary optical fibers (7) and photodetectors (6) is three each.

3. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to any one of claims 1 or 2, characterized in that: Also includes: At least one cylindrical substrate (3) is used to support at least one of the spiral bend structures on the main optical fiber (4); A spiral groove (5) is provided on the cylindrical substrate (3) and is spirally arranged in the main optical fiber (4) embedded in the spiral groove (5).

4. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 3, characterized in that: Also includes: The support (1) is fixedly connected to both ends of the main optical fiber (4); The bracket (10) is detachably connected to the photodetector (6) and the height of the photodetector (6) can be adjusted.

5. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 4, characterized in that: It also includes a pre-tensioning mechanism (8); The pre-tightening mechanism (8) can tighten the spiral bending structure wound on the main optical fiber (4).

6. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 5, characterized in that: The pretensioning mechanism (8) includes a knob (802), a slide (803), a support frame (804), at least two fixing rods (805) and at least two springs (806). The knob (802) is slidably connected to the outer wall of the cylindrical base (3), the knob (802) is connected to the slide rail (803), the slide rail (803) is slidably connected to the support frame (804), at least two of the fixed rods (805) are slidably connected to the support frame (804), the spring (806) is sleeved on the fixed rod (805), and the two ends of the spring (806) are respectively connected to the support frame (804) and the fixed rod (805); At least two of the fixing rods (805) are symmetrically arranged and the main optical fiber (4) is clamped by the elastic deformation of the spring (806). The sliding connection between the slide rail (803) and the support frame (804) can compensate for the height difference generated when the main optical fiber (4) is tightened.

7. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 6, characterized in that: It also includes a fixing component (9), which includes at least two connecting rods (902), a seat (901), at least two snap-fit ​​parts (905), at least two snap-fit ​​grooves (907) and a fixing seat (906). At least two of the connecting rods (902) are rotatably connected to the seat (901), the seat (901) is connected to the inner wall of the cylindrical base (3), the snap-fit ​​part (905) is provided on the connecting rod (902), and the snap-fit ​​part (905) can snap into the snap-fit ​​groove (907) provided on the fixed seat (906).

8. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 7, characterized in that: The fixing component (9) also includes a tension spring (904) which is connected to the two connecting rods (902).

9. The polymer fiber synchronous multi-point displacement sensor based on helical bending structure bending loss coupling according to claim 7, characterized in that: The fixing component (9) further includes a ramp (903), and the pre-tightening mechanism (8) further includes a contact element (801); The inclined surface (903) is disposed on the connecting rod (902), and the knob (802) is connected to the contact element (801); When the contact (801) and the knob (802) move upward, the contact (801) can contact the inclined surface (903) on the connecting rod (902) and bring the two connecting rods (902) closer together, thereby causing the snap-fit ​​part (905) to disengage from the snap-fit ​​groove (907).

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