Compact FBG strain sensor for accurate measurement method with wide-range temperature compensation

Demand for compact and simple sensor with high accuracy in the production line is increasing. A compact sensor using a self-coupling effect of the semiconductor laser has been studied. A measurement method which can detect self-coupling effect from change in terminal voltage of semiconductor laser without photodiode is proposed. It is confirmed that this sensor using terminal voltage change can measure a distance as same as sensor with photodiode. The measurable distance of this sensor is shorter than that with the photodiode. However, this sensor can be used as a shape measuring sensor with high accuracy by closely arranging semiconductor laser in a two-dimensional array.

This project presents on the analysis of strain sensing performance through software simulation and experimental setup. The bare and FBG strain sensors are tested by applying strain at the grating region of the FBG and the output of the experiment is displayed on the optical spectrum analyzer (OSA) in terms of power (dBm) and wavelength (nm). The performance of the FBGs has been evaluated by comparing the strain sensitivity and wavelength shift obtained. The FBG strain sensor has a strain sensitivity of 38.25 pm/N, which is higher than the bare FBG's strain sensitivity of 12.49 pm/N. The FBG strain sensor's experimental and simulation performance is also investigated. OptiSystem software is used to simulate the strain sensor performance. The overall results indicate the ability of FBG to be utilized as a strain sensor as the Bragg wavelength is shifted by the combination effect of effective refractive index and grating period influenced by the changes of strain

Mid-infrared spectral analysis has long been recognized as the most accurate noninvasive blood glucose measurement method, yet no practical compact mid-infrared blood glucose sensor has ever passed the accuracy benchmark set by the USA Food and Drug Administration (FDA): to substitute for the finger-pricking glucometers in the market, a new sensor must first show that 95% of their glucose measurements have errors below 15% of these glucometers. Although recent innovative exploitations of the well-established Fourier-transform infrared (FTIR) spectroscopy have reached such FDA accuracy benchmarks, an FTIR spectrometer is too bulky. The advancements of quantum cascade lasers (QCLs) can lead to FTIR spectrometers of reduced size, but compact QCL-based noninvasive blood glucose sensors are not yet available. This work reports on two compact sensor system designs, both reaching the FDA accuracy benchmark. Each design commonly comprises a mid-infrared QCL for emission, a multiple attenuation total reflection prism (MATR) for data acquisition, and a computer-controlled infrared detector for data analysis. The first design translates the comb-like signals into conventional spectra, and then data-mines the resultant spectra to yield blood glucose concentrations. When a pressure actuator is employed to press the patient's hypothenar against the MATR, the sensor accuracy is considered to reach the FDA accuracy benchmark. The second design abandons the data processing step of translating combs-to-spectra and directly data-mines the "first-hand" comb signal. Beyond increasing the measurement accuracy to the FDA accuracy benchmark, even without a pressure actuator, direct comb data-mining upgrades the sensor system with speed and data integrity, which can impact the healthcare of diabetic patients. Specifically, the sensor performance is validated with 492 glucose absorption scans in the time domain, each with 20 million datapoints measured from four subjects with glucose concentrations of 3.9-7.9 mM. The sensor data-mines 164 sets of critical singularity strengths, each comprising 4 critical singularity strengths directly from the 9840 million raw signal datapoints, and the 656 critical singularity strengths are subjected to a machine-learning regression model analysis, which yields 164 glucose concentrations. These concentrations are correlated with those measured with a standard finger-pricking glucometer. An accuracy of 99.6% is confirmed from the 164 measurements with errors not more than 15% from the reference of the standard glucometer.

We present in this paper the results of monitoring the construction process of a steel incrementally launched bridge located at the Kadagua Valley in Bilbao (Spain) with FBG sensors. The installation of FBG strain and temperature sensors was done in order to obtain deformation and temperature variations during the launching operation. The deflection recovery process was also monitored. The setup carried out in the sensors installation process consists of five optical channels (one for each cross section monitored) and a multiplexed structure of nine strain sensor in each optical channel. Temperature sensors were also installed in order to measure temperature variation of the steel structure but also for thermal compensation for the FBG strain sensors. The installation of the optical sensors is explained in detail including cleaning, bonding and connection of the almost fifty sensors installed in this structure. We also are going to explain the behaviour of the steel structure by presenting several figures showing the strain values for each sensor taken in real time during the launching of the bridge.

Fiber Bragg grating is sensitive to temperature and strain, which lead to the temperature compensation must be considered when the FBG strain sensor is applied. Discussed several typical temperature compensation scheme for the applicability of the FBG strain sensor, analyzed the important factors which affected the compensation accuracy of FBG strain sensor. Based on the reference grating temperature compensation scheme, we designed an encapsulation structure of FBG strain sensor with built-in temperature compensation grating. This structure is simple, practical; also calibration test and outdoor test have proved that it has a good temperature compensation effect.

FBG strain sensors are more and more accepted in the area of structural health monitoring, especially in civil infrastructures. However, due to the fact that FBG senses both strain and temperature simultaneously. And moreover the wavelength shift induced by 1°C is almost equivalent to that induced by 10μe. So temperature compensation for FBG strain sensors of long-term structural monitoring is indispensable. In this paper, based on the FBG's strain and temperature sensing principles, the techniques of dual FBGs temperature compensation and the environmental temperature compensation are brought forward. The experiment of cement paste curing monitroed by dual FBGs was carried out. And the advantage of the technique of dual FBGs temperature compensation are proved, which is proper for long-term structureal monitoring system for civil infrastructures.

Realization of temperature compensation and high sensitivity is a challenging task in the field of FBG based strain sensor. A unique scheme of high sensitive temperature compensated interrogation technique for FBG strain sensors is proposed. Two chirped FBGs are designed in such a way that effect of temperature on one FBG is compensated by the other FBG. The strain sensitivity is enhanced by a factor of sixteen compared to the signal reflected from the edge of the FBG using only third order Four wave mixing (FWM) product. The third order FWM product generated in the $1^{st}$ stage FWM process is filtered and utilized in the $2^{nd}$ stage FWM process to enhance the sensitivity. The proposed scheme attains high sensitivity of 0.312 dBm/ ${\mu \varepsilon }$ with strain resolution of $0.032 {\mu \varepsilon }$ using only $3^{rd}$ order FWM product.

A differential fiber Bragg grating strain sensor is developed, thereinto, the strain of a gage rod is translated into the deflection of the cantilever beam, on which the fiber Bragg gratings suffer the strain and shift their Bragg wavelengths. In this scheme, temperature compensation is achieved by the differential operation between the Bragg wavelength shifts of sensing gratings mounted on the top and bottom surfaces of the beam. The loading experiment indicates that the least-square linearity between the strain of the gage rod and the difference of the Bragg wavelength shifts of the sensing gratings is 0.3% in the range of the strain -1500 to 1500 με, the maximum error is 20 με, and the measure precision is 0.007. According to the flaws of the second lining of Shan Xin-Po Tunnel explored by the geological radar, these differential fiber Bragg grating strain sensors are installed on the lining. During the backfill period of 53 days and the operation period of 341 days, the strain survey results that the strains are related to the distribution of back cavity in the backfill period; however, the strains became gradually stable in the operation period.

In the paper a new implementation of a compact smart resistive sensor based on a microcontroller with internal ADCs is proposed and analysed. The solution is based only on a (already existing in the system) microcontroller and a simple sensor interface circuit working as a voltage divider consisting of a reference resistor and a resistive sensor connected in parallel with an interference suppression capacitor. The measurement method is based on stimulation of the sensor interface circuit by a single square voltage pulse and on sampling the resulting voltage on the resistive sensor. The proposed solution is illustrated by a complete application of the compact smart resistive sensor used for temperature measurements, based on an 8-bit ATxmega32A4 microcontroller with a 12-bit ADC and a Pt100 resistive sensor. The results of experimental research confirm that the compact smart resistive sensor has 1°C resolution of temperature measurement for the whole range of changes of measured temperatures.

A procedure to embed fibre Bragg grating strain sensors into GFRP sandwich structures

The dynamic response of offshore oil platform under seismic excitation is the coupling response of liquid and solid vibration. In recent years, the computation of dynamic responses and design of offshore platform have attracted the attention of many researchers. This paper presents a shaking table test of offshore oil platform model scaled down an actual one. Fiber Bragg grating is a new measurement technology with its superior ability of explosion proof, immunity to electromagnetic interference and high accuracy. In this paper, FBG sensors are used to monitor the dynamic response of offshore oil platform model on line. Ten FBG sensors are installed, one of which is temperature sensor, two of which are acceleration sensors and the others of which are strain sensors. One FBG accelerometer is placed on the surface of the shaking table; and another one is placed on the top surface of the offshore oil platform model. FBG strain sensors are placed on the key parts of the platform model. Some traditional strain gauges are installed in parallel with FBG strain sensors. In this experiment, electromagnetic interference of strain gauge is very big, while the FBG strain sensor has not this phenomenon. Based on the experiments results, it can be concluded that FBG sensor is superior to strain gauge.

Accurate measurement of strain variation and effective prediction of failure within models has been a major objective for strain sensors in dam model tests. In this paper, a FBG strain sensor with enhanced strain sensitivity and packaged by two gripper tubes is presented and applied in the seismic tests of a small scale dam model. This paper discusses the principle of enhanced sensitivity of the FBG strain sensor. Calibration experiments were conducted to evaluate the sensor's strain transferring characteristics on plates of different material. This paper also investigates the applicability of the FBG strain sensors in seismic tests of a dam model by conducting a comparison between the test measurements of FBG sensors and analytical predictions, monitoring the failure progress and predicting the cracking inside the dam model. Results of the dam model tests prove that this FBG strain sensor has the advantages of small size, high precision and embedability, demonstrate promising potential in cracking and failure monitoring and in identification of the dam model.

Resistive strain and bending sensors offer a versatile platform for sensing various physical parameters with relatively little effort and budget. The lightweight, robust and compact sensors are extensively used in manifold low-power applications. Recently, scribed and flexible laser-induced graphene sensors have shown potent capabilities for a variety of measurements, including flow, deflection, and force. Achieving a high sensitivity to various stimuli remains a challenge due to limited change in relative resistance. In this paper, we report a multifunctional LIG sensor with widely tunable properties and significantly enhanced electromechanical performance. A method of repeated laser writing is used to increase the porosity, the uniform carbonization degree and, most importantly, the sensitivity of the LIG sensors. A gauge factor of 91.2 is achieved after three-times laser writing at low power, which is an increase of 750% to one-time laser writing and 720% higher than the ones previously reported for LIG strain sensors. The increase is attributed to a more porous surface morphology that provides more overlapping area and displacement of the graphene layers. A homogeneous bidirectional response was obtained by scribing the electrodes on both faces of the substrate. Parylene C-coating is used to protect the LIG sensors from environmental effects. Coated sensors were packaged to a PCB assembly for easy integration into various applications. An example is a LIG bending sensor customized for velocity profile monitoring of Unmanned Aerial Vehicles in the outdoor environment.

We summarize various sensors based on photonic crystal fiber loop mirrors proposed by our research team, including strain sensors, temperature sensors and curvature sensors. Both of highly birefringent and lowly birefringent photonic crystal fibers (PCFs) have been inserted into fiber loop mirrors (FLMs) as the sensing elements. A temperature-insensitive strain sensor with highly birefringent PCF based FLM has been realized and it has a strain sensitivity of 0.23 pm/µe in the range of 0–30 me. For a low-cost, a demodulation is proposed by using a distributed-feedback (DFB) laser as the light source and an optical power meter instead of an expensive OSA. Strain and curvature sensors based on FLMs are also demonstrated by using a lowly birefringent PCF, which has a strain sensitivity of 0.457 pm/µe and a curvature sensitivity of 0.337 nm/ m −1 , respectively. Further, a novel and compact temperature sensor based on a FLM combined with an alcohol-filled highly birefringent PCF is proposed and experimentally demonstrated and it has a high sensitivity as high as 6.6 nm/°C.

Application of fiber-optic strain sensing technology in high-precision load prediction of aircraft landing gear