A three-dimensional fiber Bragg grating wrist force sensor with separation elastic structure

A 3-D force sensor based on fiber Bragg grating (FBG) is proposed in this article. A spatially layered elastomer structure is designed, and a single-mode fiber inscribed with four FBGs is laid at each place of the structure to realize the 3-D force sensing of the robot and compensate for temperature interference. The principle of FBG sensing and force measurement is investigated to reveal the mapping relationship between the wavelength drift and force of this 3-D force sensor. Based on the finite-element simulation analysis, the FBG package position is optimized, and the sensor prototype is tested. Experimental results show that the linearity of the 3-D force sensor force sensing is good, and its sensitivity is 3.104, 3.077, and 1.890 pm/N. Based on sparrow search algorithm optimization extreme learning machine (SSA-ELM), a nonlinear decoupling algorithm is used to construct the neural network model, and the SSA is used to optimize the model and obtain the best initial weights and thresholds of the network. The maximum I error after decoupling is 1.58%, the maximum II error is 0.93%, and the decoupling training time is 1.44 s. The algorithm can construct the interdimensional coupling relationship of 3-D force more effectively and improve the sensor measurement accuracy. In addition, the sensor has strong resistance to electromagnetic interference and noise, fewer sensitive components, a simple lineup, a small structure and lightweight, and easy processing, and meets the requirements of miniaturization, lightweight, and refinement of multidimensional force sensors for collaborative robots.

During retinal microsurgery, excessive interaction force between surgical instruments and intraocular tissue can cause serious accidents such as tissue injury, irreversible retinal damage, and even vision loss. It is essential to accurately sense the micro tool-tissue interaction force, especially for the Ophthalmic Microsurgery Robot. In this study, a fiber Bragg grating (FBG) three-dimensional (3-D) micro-force sensor for micro-forceps is proposed, which is integrated with the drive module as an end-effector and can be conveniently mounted onto the ophthalmic surgical robot. An innovative axial force sensitivity-enhancing structure is proposed based on the principles of flexure-hinge and flexible levers to overcome the low sensitivity of axial force measurement. A dual-grating temperature compensation method is adopted for axial force measurement, which considers the differential temperature sensitivity of the two FBGs. Three FBGs are arranged along the circumference of the guide tube in this study to measure transverse forces and compensate for effects caused by changes in temperature. The experimental results demonstrate that the micro-forceps designed in this study achieved a resolution of 0.13 mN for transverse force and 0.30 mN for axial force. The temperature compensation experiments show that the 3-D micro-force sensor can simultaneously compensate for temperature effects in axial and transverse force measurement.

Thermoacoustic Imaging (TAI) combines the high contrast of microwave imaging with the high resolution of ultrasound imaging, establishing itself as a novel, non-invasive medical diagnostic technique. However, the high-power (peak power: 5∼100kW; pulse width: 10∼1000ns) used in TAI often cause remarkable interference with thermoacoustic signal acquisition systems and traditional piezoelectric ultrasound transducers. This interference leads to degraded thermoacoustic image quality and a pronounced "near-field dead zone" (NFDZ), which severely restricts the clinical application and development of TAI. To address these challenges, this paper proposes using fiber Bragg grating (FBG) for thermoacoustic signal detection. The high sensitivity and strong resistance to electromagnetic interference offered by FBG are leveraged to achieve interference-resistant signal acquisition. Additionally, a photoelectric conversion synchronous triggering method is adopted to prevent strong coupling of spatially distributed pulse microwave signals to the data acquisition system due to excessively long coaxial cables. This approach ensures more reliable and interference-resistant data acquisition for TAI. Utilizing FBG with their high sensitivity and electromagnetic interference resistance, the system replaces traditional piezoelectric transducers for thermoacoustic signal detection. Additionally, a photoelectric conversion triggering method was implemented to avoid microwave pulse coupling interference caused by long coaxial cables. Experimental validation included comparisons between FBGs and traditional transducers in terms of signal-to-noise ratio (SNR), as well as noise measurements using both coaxial bayonet nut connector (BNC) cable and photoelectric trigger configurations to contrast test noise levels. Furthermore, the distance between the microwave antenna and data acquisition system was adjusted to validate the attenuation pattern of electromagnetic interference. TAI experiments used soy sauce tubes for their controllable composition/morphology. Systems imaging the tube also apply to human arms and ex vivo porcine liver (similar microwave absorption to tumors). This standardized, cost-efficient, and ethical approach provides a reliable basis for biological tissue imaging research. Experimental results show FBG with photoelectric triggering achieves SNR values of 28.32 (70.2% improvement) and 30.74dB (66.3% improvement) compared to traditional transducers at 16.64 and 18.48dB, respectively, representing an overall SNR enhancement of 64.0%∼88.0%. Furthermore, the photoelectric conversion triggering method reduces noise levels, which means shortening the NFDZ, leading to improved imaging quality and an extended effective detection range. Additionally, it was found that increasing the distance between the microwave antenna and the data acquisition system effectively reduced the noise amplitude, with the attenuation of direct spatial coupling interference conforming to the electric field intensity attenuation law (1/r attenuation). This study establishes that the FBG-based thermoacoustic imaging system, leveraging FBG's high sensitivity and electromagnetic interference resistance combined with photoelectric triggering, enhances signal quality and effectively suppresses near-field artifacts, enabling reliable clinical imaging. Validation through noise, signal, and spatial analyses confirms FBG's utility in advancing TAI. In summary, the method proposed in this article can be used for thermoacoustic endoscopy (especially for intravascular thermoacoustic endoscopy), thyroid and breast ablation TAI monitoring, as well as multimodal imaging combined with MRI and TAI.

Proximal force signals are widely used benefiting from the advantage of easy collection and are critical for enhancing the safety of the catheter surgical robot. This article presented two kinds of 3-D printable fiber Bragg grating (FBG)-based sensors for clamping force and axial force sensing. The clamping force sensor can realize the clamping–sensing function integration based on a 45° inclined rod and an arc hinge, while the axial force sensor adopts two symmetrical lamina emergent torsional (LET) joints to enhance the sensitivity and hysteresis characteristic. A data and theoretical model-driven method has been proposed to derive sensitivity models of sensors, and the relative errors compared with the experimental sensitivities are all within 10%. The calibration experiment results demonstrate that the designed sensor has obvious benefits in resolution (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ &lt; 2.5 \text {mN}$ </tex-math></inline-formula>) and sensitivity (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$480 \text {pm/N}$ </tex-math></inline-formula>) compared with other similar sensors. In addition, dynamic and durability experiments have been implemented to validate the reliability of the designed sensors. To achieve direct measurement of proximal force and reduce the influence of parasitic movement, the integration mode of two sensors is also carefully considered. A wireless technology-based primary–secondary control system and a fuzzy proportional–integral–derivative (PID) control strategy with overshoot compensation have been constructed, leading to the achievement of automatic adjustment of catheter movement and clamping force. Furthermore, several in vitro experiments have been implemented to verify the vascular protection, primary–secondary hand switching, and grading force control functions of the system. Such merits indicate that the designed proximal force sensors have huge potential in the field of catheter surgical robots.

Optical fiber Bragg grating (FBG) displacement sensors play an important role in various areas due to the high sensitivity to displacement. However, it becomes a serious problem of FBG cross-sensitivity of temperature and displacement in applications with FBG displacement sensing. This paper presents a method of temperature insensitive measurement of displacement via using an appropriate layout of the sensor. A displacement sensor is constructed with two FBGs mounted on the opposite surface of a cantilever beam. The wavelengths of the FBGs shift with a horizontal direction displacement acting on the cantilever beam. Displacement measurement can be achieved by demodulating the wavelengths difference of the two FBGs. In this case, the difference of the two FBGs’ wavelengths can be taken in order to compensate for the temperature effects. Four cantilever beams with different shapes are designed and the FBG strain distribution is quite different from each other. The deformation and strain distribution of cantilever beams are simulated by using finite element analysis, which is used to optimize the layout of the FBG displacement sensor. Experimental results show that an obvious increase in the sensitivity of this change on the displacement is obtained while temperature dependence greatly reduced. A change in the wavelength can be found with the increase of displacement from 0 to 10mm for a cantilever beam. The physical size of the FBG displacement sensor head can be adjusted to meet the need of different applications, such as structure health monitoring, smart material sensing, aerospace, etc.

Design and testing of a new microinjector with FBG force sensor for robot-assisted single cell microinjection

A robot body is not only an active unit but also a sensing unit. This paper describes a humanoid that integrates stereo vision, full-body tactile sensor, sound sensors and wrist force sensors into a sensor image. The sensor image reflects the current state of the environment and is regarded as all the input to the robot brain in this system. The brain program processes a series of sensor images and controls its body using them. The robot is remote-brained and designed to be a testbed to do research on sensor based behaviors of a full-body humanoid. This paper describes the design concept and the details of the system.

Fiber Bragg Grating (FBG) sensor is a new type of optical sensor that is light in quality, small in volume, easy to achieve distributed measurement, and easy to embed. When FBG sensor technology is introduced to the damage detection of the CFRP shaft, the shortcomings of conventional damage detection technologies, including ultrasonic testing and radiographic testing, can be effectively overcome. Considering the working load, operating environment and life requirements of the CFRP shaft , combined with the relevant national and industry standards for composite material testing, Prepare three CFRP shaft with embedded fiber gratings (non-destructive CFRP shaft, single hole CFRP shaft , double hole CFRP shaft), the hole diameter is 1mm, and the layering of CFRP shaft is ([0 / 90 / 45 / 0 / 0 / 45 / -45 / 0 / -45 / 0 / 90 / 90 / 0 / -45 / 0 / -45 / 45 / 0 / 0 / 45 / 90 / 0 ]). FBG is embedded between 18 and 19 layers, pasted on the surface of CFRP shaft with 10 FBG sensors (named FBG1, FBG2... FBG10). The strain information of the CFRP shaft is obtained through fiber grating to obtain the deformation of the CFRP shaft under a three-point bending load. Each CFRP shaft is loaded 12 times (A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, D3). Through the BP neural network method, the mapping relationship between strain distribution and damage model was established. In the model, the FBG sensor measures the strain of the CFRP shaft and forms the input of the neural network. Combined with the damage situation of the composite material, the learning samples of the network formed. After learning and training, the neural network can damage recognition. To better verify the accuracy of the neural network to distinguish and identify the CFRP shaft non-destructive, single-hole, and double-hole damage patterns. The finite element analysis was used to simulate the damage of three CFRP shafts under the same experimental conditions, and extract the eigenvalues at the same position to form the input of the network, combined with the damage situation of the composite material, constitutes a learning sample of the network. The experiment shows that the CFRP shaft experiment based on BP neural network verifies the reliability of the CFRP shaft damage recognition method. The finite element analysis is used to extract the strain information of CFRP shaft measuring points with different damage modes. The damage position information and strain information were input into the BP neural network. The experimental results are in good agreement with the simulation results, which verifies the accuracy of the BP neural network to distinguish the CFRP shaft non-destructive, single-hole, and doublehole damage patterns.

A fiber Bragg grating (FBG) accelerometer using transverse forces is more sensitive than one using axial forces with the same mass of the inertial object, because a barely stretched FBG fixed at its two ends is much more sensitive to transverse forces than axial ones. The spring-mass theory, with the assumption that the axial force changes little during the vibration, cannot accurately predict its sensitivity and resonant frequency in the gravitational direction because the assumption does not hold due to the fact that the FBG is barely prestretched. It was modified but still required experimental verification due to the limitations in the original experiments, such as the (1) friction between the inertial object and shell; (2) errors involved in estimating the time-domain records; (3) limited data; and (4) large interval ~5 Hz between the tested frequencies in the frequency-response experiments. The experiments presented here have verified the modified theory by overcoming those limitations. On the frequency responses, it is observed that the optimal condition for simultaneously achieving high sensitivity and resonant frequency is at the infinitesimal prestretch. On the sensitivity at the same frequency, the experimental sensitivities of the FBG accelerometer with a 5.71 gram inertial object at 6 Hz (1.29, 1.19, 0.88, 0.64, and 0.31 nm/g at the 0.03, 0.69, 1.41, 1.93, and 3.16 nm prestretches, respectively) agree with the static sensitivities predicted (1.25, 1.14, 0.83, 0.61, and 0.29 nm/g, correspondingly). On the resonant frequency, (1) its assumption that the resonant frequencies in the forced and free vibrations are similar is experimentally verified; (2) its dependence on the distance between the FBG's fixed ends is examined, showing it to be independent; (3) the predictions of the spring-mass theory and modified theory are compared with the experimental results, showing that the modified theory predicts more accurately. The modified theory can be used more confidently in guiding its design by predicting its static sensitivity and resonant frequency, and may have applications in other fields for the scenario where the spring-mass theory fails.

Hull monitoring system with Fiber Bragg Grating (FBG) sensors increasingly receives people’s attentions. However, for the ship hull monitoring, the deformation of hull girder changes a lot as is subjected to a huge temperature variation. Therefore, the compensation method with only FBG temperature self-correction is not suitable for the hull monitoring sensors because no material thermal expansion effects are reasonably included. In this paper, the new compensation method of hull monitoring FBG sensor based on the sensor theory with both FBG temperature self-correction and steel thermal expansion effects correction is studied. The coupled compensation method suitable for hull monitoring sensor is obtained by theoretical derivation. As the comparison, the coupled compensation experiment was carried out. The results show that the relative error under the temperature compensation method is large in the case of drastic strain and temperature changes, and the correction results of the tested method will be closer to the true level.

Axial flux magnetic-geared double-rotor machines (AMGDRMs) are characterized as a potential core component of the power splitting system in hybrid electric vehicles (HEVs). Conventional AMGDRMs suffer from low mechanical strength due to high axial magnetic force. In this paper, an interior-modulating-rotor axial flux magnetic-geared double-rotor machine (IMR-AMGDRM) is proposed to solve this problem. An analytical model of the IMR-AMGDRM is proposed to predict the magnetic field distribution and electromagnetic performances. Since analytical model is based on 2-D geometry, a 2-D approximation of IMR-AMGDRM which has similar output performance to 3-D finite element analysis (FEA) is presented based on the permanent magnet (PM) magnetic energy equivalent principle. Utilizing subdomain method, the magnetic field distribution and electromagnetic performances are calculated. Finally, the accuracy of the proposed analytical model is validated by 3-D FEA.Index Terms—Analytical model, axial flux, magnetic-geared machine, double-rotor machine.I. IntroductionAxial flux magnetic-geared double-rotor machines (AMGDRMs), which enable high torque density, have been considered a promising candidate for power splitting in hybrid electric vehicles (HEVs) [1]. However, conventional AMGDRMs suffer from low mechanical strength due to high axial magnetic force. To enhance mechanical strength, an interior-modulating-rotor axial flux magnetic-geared double-rotor machine (IMR-AMGDRM) which can reduce axial magnetic force dramatically is proposed in this paper. Fig. 1 (a) and (b) illustrate the configuration of the proposed IMR-AMGDRM.As axial flux type machine, IMR-AMGDRMs have complex 3-D magnetic circuits. 3-D finite element analysis (FEA) can offer accurate results by taking into account the nonlinear factors such as magnetic saturation and flux leakage. But the major drawback of 3-D FEA is serious time cost. Analytical model which is much lower time-consuming provides an alternative for electromagnetic performance prediction in the initial design procedure.In this paper, an analytical model of the IMR-AMGDRM is proposed. The geometry of analytical model is derived based on PM magnetic energy equivalent principle to guarantee similar output performance to 3-D FEA. The solution to analytical model is calculated by subdomain method. To evaluate the accuracy of the proposed analytical model, the results computed by analytical model and that of 3-D FEA are compared.II. Configuration of IMR-AMGDRMIn conventional configuration, the inner and outer rotors are the permanent magnet (PM) rotor and the modulating rotor, respectively. Since the modulating rotor is closer to the PM rotor than stator, the modulating rotor suffers from high axial magnetic force. Furthermore, the modulating rotor is hollow and thus the low mechanical strength of modulating rotor becomes the major drawback of conventional configuration [1]. To solve this problem, the IMR-AMGDRM which switches positions of the PM rotor and the modulating rotor is proposed. In interior-modulating-rotor configuration, as shown in Fig. 1 (a) and (b), the PM rotor is between the modulating rotor and stator, i.e. the inner and outer rotors are the modulating rotor and the PM rotor, respectively. Owing to that the PM rotor suffers from reverse axial magnetic forces from both sides simultaneously, the total axial magnetic force is dramatically reduced. Hence, although the outer rotor is hollow in both configurations, the axial magnetic force of interior-modulating-rotor configuration is significantly decreased, leading to higher mechanical strength and reliability.III.Geometry of 2-D ApproximationGenerally, the unrolled section of axial flux machine at mean radius is used as 2-D approximation. However, few references are found to evaluate the electromagnetic performance equivalence of such approximation. To make the electromagnetic performance of 3-D model and 2-D approximation equal, the amount of magnetic energy of the PM should remain unchanged after approximation, i.e. PM magnetic energy equivalent principle [2]. A 2-D approximation of IMR-AMGDRM which has identical output performance is presented by using PM magnetic energy equivalent principle.IV. Analytical Calculation of IMR-AMGDRMThe proposed analytical model is illustrated in Fig. 1 (c). It can be seen that the model consists of four subdomains. Region i, I, II and III represent the ith slot of the modulating rotor, the inner air gap, the PM rotor and the outer air gap, respectively. Utilizing subdomain method, the magnetic field distributions in the inner and outer air gaps are obtained by the solution of Laplace’s and Poisson’s equations. Then, the electromagnetic performances such as no-load back EMF, torque and axial force can be derived from the calculated magnetic field distributions.Comparisons of computation accuracy between analytical model and 3-D FEA is shown in Fig. 2. As can be observed, the results of analytical model are well consistent with that of 3-D FEA, validating the proposed method. **

The lack of force information in surgical robots introduces new risks, such as injuries caused by too much force. This article presents a novel real-time temperature-compensated force sensor for surgical robot, which can provide real-time multidimensional force information without changing the structure of the surgical tool, but only installing one sensing fiber with four fiber Bragg gratings (FBGs). The finite element simulation shows that each FBG is subjected to different temperature changes under the influence of heat source. In order to achieve more accurate temperature compensation (TC), the sensitivity of reflectivity of FBG to strain and temperature is analyzed by theory and experiment, and TC is also realized with the dual parameter of reflectivity and wavelength shift. In addition, a zero-phase filter based on single-sided extension of fitting point is further designed to process the force signals, which eliminates the time delay introduced by traditional filters while filtering out interference signals. Finally, the effectiveness of this study is demonstrated by comparing the results with commercial force sensor.

Electromagnetic interference (EMI) immune and light-weight, fiber-optic sensor based Structural Health Monitoring (SHM) will find increasing application in aerospace structures ranging from aircraft wings to jet engine vanes. Intelligent Fiber Optic Systems Corporation (IFOS) has been developing multi-functional fiber Bragg grating (FBG) sensor systems including parallel processing FBG interrogators combined with advanced signal processing for SHM, structural state sensing and load monitoring applications. This paper reports work with Auburn University on embedding and testing FBG sensor arrays in a quarter scale model of a T38 composite wing. The wing was designed and manufactured using fabric reinforced polymer matrix composites. FBG sensors were embedded under the top layer of the composite. Their positions were chosen based on strain maps determined by finite element analysis. Static and dynamic testing confirmed expected response from the FBGs. The demonstrated technology has the potential to be further developed into an autonomous onboard system to perform load monitoring, SHM and Non-Destructive Evaluation (NDE) of composite aerospace structures (wings and rotorcraft blades). This platform technology could also be applied to flight testing of morphing and aero-elastic control surfaces.

A corrosion sensor utilizing fiber optics and the magnetic attraction force is proposed. The sensor aims to detect the internal corrosion of pipelines that are made of ferromagnetic materials. Its components include a beam made of a non-magnetic material, a strong permanent magnet, and a Fiber Bragg Grating (FBG) sensor that is very sensitive to strain changes. The sensor is based on the assumption that the magnetic attraction force generated between a magnet and a ferromagnetic material decreases if the thickness of the ferromagnetic material is decreased. To generate this force between the sensor and the pipe, the beam is positioned in a way that the magnet is only few millimeters away from the pipe. The internal corrosion causes a reduction in the thickness of the interior pipe wall, which according to the assumption should reduce the attraction force. As a result, the strain measured by the optical fiber will be affected as it is directly related to the variations in force. We present an initial numerical investigation of the feasibility of the proposed working principle utilizing a Finite Element Analysis (FEA) simulation tool. Simulation results show that the attraction force first increases then saturates with the increase in wall thickness. The change in force becomes significant once the thickness reduces to a threshold value. We also investigate the effect of changing the magnet size, magnetic permeability of pipe material, separation distance between pipe and magnet, and the magnetic flux density of the magnet.

In this study, a down-scaled wind turbine blade was designed and fabricated using glass and carbon fiber materials for the skin and stiffener, respectively. In the course of its fabrication, an array of FBG (fiber Bragg grating) sensors was embedded in the composite laminates. The embedded FBG sensor array was used to measure the residual strain in the stiffener before and after the curing process, and after fabrication of the blade, the FBG array was used to monitor the structural conditions, including structural dynamic behavior during the structural testing of the blade. The results of the tests showed that the FBG sensor array effectively measured the residual strain distribution of the blade stiffener before and after the curing process. And the measured natural frequencies and mode shapes by the FBG array matched the results obtained from the FE analysis and conventional accelerometers.