Wide-Range Fiber Bragg Grating Tilt Sensor Based on a Cam Structure

Fiber Bragg grating (FBG) sensors play an increasingly important role in structural health monitoring, and tilt sensors are one of the most commonly used among them. Traditional FBG tilt sensors are easily affected by external vibration, and to overcome such a problem, this work proposes a novel FBG tilt sensor with a vibration damping feature. Based on the classical equal strength beam structure, a vibration-isolating spring is used as the vibration-damping connection between the beam and the mass block, while a damping fluid is filled inside the sensor housing as the vibration-damping material. After elaborating on the details of sensor structural design and measurement theory, five sensors were set up for relevant performance tests, and the controlled experiments were carried out for the FBG sensors with and without the vibration isolation springs. Meanwhile, for sensors with vibration isolation springs, the viscosity of filled damping fluid was also varied in the experiments, to better reveal its influence on the sensor performance. The experimental results show that the proposed sensors have a high sensitivity of 112.743 pm/° within a range of ±15° and also have superior creep resistance as well as good temperature compensation capability. From controlled experiments, it is evident that the proposed sensor with a vibration damping structure offers better vibration stability and monitoring accuracy and can be effectively adapted to safety monitoring tasks in different engineering applications by simply adjusting the viscosity of filled damping fluid.

Distinguishable circumferential inclined direction tilt sensor based on fiber Bragg grating with wide measuring range and high accuracy

A new temperature-insensitive inclinometer was fabricated using a 3-D printing method based on fiber Bragg grating (FBG) sensors. This new tilt sensor is characterized by its light weight, immunity to electromagnetic interference (EMI) small size, quick prototyping, and cost effectiveness. FBG sensors were directly inscribed into polylactic acid, which was also used for fabricating all different parts of tilt sensors. Calibration tests of this new tilt sensor were performed in laboratory to examine its measurement performance. Measurement sensitivity of 0.012 nm/° and a maximum tilt angle of 25° of the FBG inclinometer was approached in the calibration experiment. The measured tilt angles were validated by theoretical angles calculated in terms of all physical parameters of internal functional parts of the inclinometer. A monitoring test of a sandy slope model using this new tilt sensor was conducted in laboratory. Finite element analysis was also used to examine the mechanical behavior of the slope model under different loading levels. Monitoring test results indicate that the FBG-based tilt sensor presented no obvious change before a surcharge load approaching 0.245 kN but exhibited step-by-step rise in tilt angle afterward, achieving a maximum tilt angle of around 6°. Horizontal displacement, which was calculated in terms of the tilt sensor at different loading levels, was found to agree fairly well with the displacement data from both finite element analysis and displacement transducers.

A fiber-optic anemometer based on fiber Bragg gratings (FBGs) is presented. A short section of cobalt-doped fiber was utilized to make a fiber-based "hot wire" for wind speed measurement. Fiber Bragg gratings (FBGs) were fabricated in the cobalt-doped fiber using 193 nm laser pulses to serve as localized temperature sensors. A miniature all-optical fiber anemometer is constructed by using two FBGs to determine the dynamic thermal equilibrium between the laser heating and air flow cooling through monitoring the FBGs' central wavelengths. It was demonstrated that the sensitivity of the sensor can be adjusted through the power of pump laser or the coating on the FBG. Experimental results reveal that the proposed FBG-based anemometer exhibits very good performance for wind speed measurement. The resolution of the FBG-based anemometer is about 0.012 m/s for wind speed range between 2.0 m/s and 8.0 m/s.

A novel bidirectional center wavelength tuning method of Fiber Bragg Grating (FBG) was put forward in this paper. It is based on bidirectional temperature control unit, including semiconductor cooler/heater, negative temperature coefficient (NTC) resistor and peripheral circuits. The cooler/heater, NTC and a FBG were integrated in a DIL metal package like common laserdiodes. Wavelength tuning experiments were carried out at three different environment temperature (0°C, 20°C and 40°C), and the temperature tuning range is from -20°C to 60°C with a step of 10°C. The experimental results show that center wavelength of FBG changes linearly along with temperature variation, and the linearity is better than 0.9986, moreover, environment temperature has little influence on FBG's center wavelength.

A novel demodulation method for tilted fiber Bragg grating (TFBG) refractometry is proposed and experimentally demonstrated. This “integrated differential area” method consists of calculating the difference between measured spectral transmission over a selected bandwidth and a reference spectrum and then integrating the absolute value of the result. This has the effect of multiplying the effect of the individual cladding mode resonance shifts relative to the background noise. The method further allows the use of spectrum analyzers with low resolution, including portable ones with spectral resolution of the order of 150-200 pm without loss of sensitivity. In practical measurements, a reflection configuration is demonstrated using a TFBG cascaded by a chirped-fiber Bragg grating with bandwidth of 19 nm. A refractive index sensitivity of 7.19% transmission change per refractive index unit is achieved for refractive indices ranging from 1.32 to 1.38, along with a limit of detection of 0.00026 refractive index units.

A tilt measurement structure and signal detection method is proposed based on a self-demodulated fiber-Bragg grating (FBG) sensor, which consists of a couple of matched FBGs and a cantilever-based pendulum clinometer. Compared by the typical matched FBGs demodulating method, of which a sensing FBG and a demodulating filter FBG is placed separately, both matched FBGs of this tilt sensor are attached on the upper and lower surface of only one single pendulum-type cantilever element for simultaneous sensing and demodulating. So the received light power will change due to the split of the two reflected spectrum of FBGs, which is corresponding to the tilt angle. In addition, the cross-sensitivity effect of the FBG-based sensor is automatically solved due to a differential signal process method. Experimental results indicate the feasibility of the proposed idea.

This study suggests an improved bonding method for a surface mounted fiber Grating strain sensor to significantly reduce the mechanical influences from the surrounding cement of a glued fiber grating in the conventional bonding method. The linear relationship between the voltage signals and the average strains within the glue-free fiber grating is the advantage of the improved bonding method over the conventional bonding method (distributing cement along the full fiber grating). The numerical computations based on the coupled-mode theory were conducted to obtain the reflection spectra of a fiber grating, that are induced from different strain fields. The one-dimensional strain fields, that vary linearly within a fiber grating, were considered in this research. In addition, the filtered spectral power interrogation system to obtain the voltage signals from an optical fiber framework was taken in consideration. The filtered spectral power interrogation system is economic, compact, and suitable for on site measurements compared with other systems such as the optical spectrum analyzer. For a glued fiber grating, results show that the distortions in shape and the dilations in area of its reflection spectrum are because of the varied strain gradients within the grating during a strain history. Also, the central wavelength shifts of a reflection spectrum are related to the average strains within a fiber grating. In applications using the improved bonding fiber grating strain sensor, its linear relationship between the voltage signals and the average strains shows the feasibility of obtaining calibrations by using a commonly used resistance strain gauge.

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.

The authors describe a detailed investigation on tilted fiber Bragg grating (TFBG) structures with tilted angles exceeding 45deg. In contrast to the backward mode coupling mechanism of Bragg gratings with normal and small tilting structures, the ex-45deg TFBGs facilitate the light coupling to the forward-propagating cladding modes. The authors have also theoretically and experimentally examined the mode coupling transition of TFBGs with small, medium, and large tilt angles. In particular, experiments are conducted to investigate the spectra and far-field distribution, as well as temperature, strain, and refractive-index sensitivities of ex-45deg devices. It has been revealed that these ex-45deg gratings exhibit ultralow thermal sensitivity. As in-fiber devices, they may be superior to conventional Bragg and long-period gratings when the low thermal cross sensitivity is required

We present optical fiber sensors to measurement strain and vibration. The sensors are based on fiber Bragg gratings (FBG). We prepared construction of strain sensors with respect to its implementation on the outer surface of concrete structures and with compensation of potential temperature drifts. These sensors are projected with look forward to maximal elongation and strength which can be applied to the sensor. Each sensor contains two optical fibers with FBGs. One FBG is glued into the sensor in points of fixation which are in the line with mounting holes. This FBG is prestressed to half of measurement range, than the stretching and pressing can be measured simultaneously by one FBG. The second FBG is placed inside the sensor without fixation to measure temperature drifts. The sensor can be used to structure health monitoring. The sensors to measurement vibration are based on tilted fiber Bragg grating (TFBG) with fiber taper. The sensor uses the TFBG as a cladding modes reflector and fiber taper as a bend-sensitive recoupling member. The lower cladding modes (ghost), reflected from TFBG, is recoupled back into the fiber core via tapered fiber section. We focused on optimization of TFBG tilt angle to reach maximum reflection of the ghost and taper parameters. In this article we present complete set-up, optical and mechanical parameters of both types of sensors.

Chapter 1 - Fiber Gratings

A novel fiber Bragg grating (FBG) current sensor with temperature compensation has been proposed. The fiber Bragg grating is glued on the surface of an isosceles triangle cantilever beam, which has a step thickness along the beam axis. Due to the electromagnetic force created by a solenoid and a permanent magnet mounted on the top of the beam, a step strain is applied on the fiber Bragg grating. The change of the electric current in the solenoid makes the spectrum of the fiber Bragg grating split. By monitoring the shift difference of the two split center wavelengths, which is related to the electric current in the solenoid, a current sensor with temperature compensation is obtained. The test range of 0–400 mA is achieved. The experimental results also show that the relationship between the shift difference of the two split center wavelengths of the FBG and the electric current has a linearity of 0.9937, and the sensitivity is about 2.64 nm/A; the test results are independent of the temperature, so the cross sensitive problem is solved.

Reliable structural health monitoring (SHM) including nondestructive evaluation (NDE) is essential for safe operation of infrastructure systems. Effective monitoring of the rock components of civil infrastructures such as tunnels and caverns remains challenging. The feasibility of employing smart optical fibre sensor (OFS) and piezoelectric impedance sensor made up of lead zirconate titanate (PZT) for comprehensive health monitoring of rocks, covering load history monitoring/retrieval as well as damage assessment is presented in this paper. The rock specimens are subjected to cyclic loading and their conditions are continuously monitored using OFS and PZT sensors. OFS based multiplexed fibre Bragg grating (FBG) sensors are surface bonded on the rock specimens. Their strain sensing performance is compared with the conventional electric strain gauges (ESGs). In addition, PZT patches are also bonded on the specimens to study the damage pattern during different loading cycles. Unlike the FBGs or ESGs, PZT patches are used as bi-functional sensors and actuators, enabling them to be efficient detectors of incipient damages using the principle of electromechanical impedance. The experimental study demonstrated superior performance of these smart FBG and PZT impedance sensors. This work is expected to be useful for SHM based NDE application of rock structures such as caverns and tunnels.

We propose and demonstrate a referencing scheme for dual-wavelength differential detection of tilted fiber Bragg grating sensors that compensates for drift in the laser diode wavelength and dynamic chirp, and results in 40 fm rms stability, and a refractive index change limit of detection of $3 \times 10 ^{-6}$.