Investigation of the method of moving object weight measurement based on quasi-distributed fiber Bragg gratings with temperature compensation

Weight sensors are widely used in the freight transportation industry. In the systems for weighing vehicles while moving, ceramic, polymer, quartz piezoelectric sensors, load or hydraulic cells, strain gauges are used as sensitive elements. However, most electric sensors are susceptible to electromagnetic interference. Currently, fiber-optic sensors are most actively developed and put into operation due to their relatively low cost, small weight and size parameters, high measurement accuracy and complete passivity to electromagnetic disturbances. Fiber-optic sensors are most often implemented using fiber Bragg gratings, taking into account the convenience of their multiplexing. Mechanical deformations lead to a shift in the wavelength of the Bragg resonance of the grating. At the same time, the problem of using fiber gratings is associated with their sensitivity to temperature. To achieve high accuracy in measuring the deformation value, and accordingly, the weight characteristics of the object, it is necessary to eliminate or compensate for the effect of the sensor temperature on its readings. Most modern studies describe sensors that either operate in laboratory conditions or involve the use of an additional sensor that complicates the circuit for measuring temperature. The paper proposes a method for solving the problem of cross-sensitivity of a fiber diffraction structure to temperature and deformation. The method is based on the use of a pair of closely spaced gratings in the sensitive element of the sensor. One of the gratings has a constant period along the length, and the other has a variable one. The design of the sensitive element ensures the transfer of mechanical load only to the fiber grating with a constant period, and the temperature change equally affects both diffraction structures. A design solution for the sensitive element is proposed that allows for temperature effects compensation without using additional elements. A mathematical model of temperature effects is presented, allowing estimating the dependence of the temperature gradient on time for different thicknesses of the sensitive element. Modeling has showed that for a sample 0.95 cm thick, the temperature gradient inside the substrate is insignificant. With a sharp change in temperature, the equalization of the temperature field within the substrate at a level of 90 % occurs in no more than 2.5 s. The mechanical load on the sensitive element can pass relative to the fiber grating at different angles in connection with which the value of the shift of the central wavelength of the Bragg resonance was studied in detail depending on the point of application and direction of the load. The proposed technology may be of interest in the development and operation of automatic weight and size control systems with temperature compensation without the use of additional sensors. The proposed system is easy to operate and it has a low cost.