Method of simultaneous measurement of two direction force and temperature using FBG sensor head

Abstract

This paper presents a method for measuring two components of bending force and temperature using one sensor head. Indirect inference based on the spectra of two fiber Bragg gratings (FBGs) placed on a cantilever beam is used. The method was developed during work on the inverse problem of determining a nonuniform stress distribution based on FBG spectra. A gradient in the FBG stress profile results in a characteristic shape of its reflective spectrum. The simultaneous measurements of force and temperature were possible through the use of an appropriate layout of the sensor head. The spectral characteristics of the sensor's gratings do not retain full symmetry, which is due to the geometry of the sensor's head and the related difference in the distribution of the axial stress of the gratings. In the proposed approach, the change in width of the sum of the normalized transmission spectra was used to determine the value of the applied force. In the presented method, an increase in the sensitivity of this change to the force is obtained relative to the other known systems. A change in the spectral width was observed for an increase in bending forces from 0 to 150 N. The sensitivity coefficient of the spectral width to force, defined as the ratio of the change of the spectral half-width to the change in force was 2.6e-3 nm/N for the first grating and 1.2e-3 nm/N for the second grating. However, the sensitivity of the whole sensor system was 5.8e-3 nm/N, which is greater than the sum of the sensitivities of the individual gratings. For the purpose of this work, a station with a thermal chamber has been designed with a bracket on which fiber optic transducers have been mounted for use in further measurements. The sensor head in this experiment is considered to be a universal device with potential applications in other types of optical sensors, and it can be treated as a module for development through its multiplication on a single optical fiber.

The Reusable Load Cell with Protection Applied for Online Monitoring of Overhead Transmission Lines Based on Fiber Bragg Grating

“…Here, the wavelength separation shift of two FBGs is proportional to the imposed force while the cross-sensitivity problem of strain and temperature in force sensing is solved. Other temperature insensitive methods can be found in literatures [ 16 , 17 , 18 ]. Some of them use only one FBG to obtain temperature insensitivity, or use two FBG allows to determine temperature and two direction force.…”

Research of Parameters of Fiber-Optical Measuring Systems

“…Values of matrix parameters T might be used for defining characteristics of both reflected (10), and transmitted (11) waves. Further the indexed components T ij indicate the elements values under the i-column and j-line of transmission matrix.…”

“…There are a lot of methods for specifying the Bragg wave length, as well, allowing its defining, based on the spectra with big noise [2,14]. Linear processing of the value having been measured upon transmitting the wave's standard length setoff, called the Bragg wave's central length, makes them natural transformers of physical values, such as power [3,5,10], temperature or deformation [11,13].…”

Analysis of Metrological Properties Fiber Bragg Gratings With a Constant and Variable Period

“…Simple batch structures such as homogeneous Bragg gratings, thanks to the properties of linear processing of magnitude measured on the Bragg wave shift, are great transducers of physical quantities such as temperature and stress [24,35], displacement [48] or force [10]. These structures are also widely used in the study of many physical quantities simultaneously, eg: elongation and temperature [25], or strength and temperature [22].…”