Our case of study is to use an FBG sensors with a micromegas (an abbreviation for “micro mesh gaseous structure” (MM)), that is a micro patter gas detector. One mandatory issue for the correct operation of the MM detector is a precise monitoring of its panels’ flatness/deformation. The basic idea, proposed here for the first time, relies with the development of a deformation monitoring system based on FBG technology, where several FBG strain sensors are surface attached to both detector support panels. Measuring strain on both sides of the panel it is possible to obtain its local curvature that is related to the second derivatives of the shape described by the bent surface. Actually, if a planar surface is bent, a positive strain (tensile) is induced on a side and a negative strain (compressive) is induced on the other one: curvature is proportional to the their relative difference. On the contrary, if a same sign strain is measured on the two sides of the surface, a longitudinal deformation of the surface can be derived.
Following this idea, the whole surface of the support panels can be subdivide into a proper number of elements that undergo simple deformation. Each element will be monitored by using FBGs attached to the surface of the panel. A mechanical Finite Element Method analysis of a full size support panel is necessary in order to determine the number and the dimension of these elements, taking into account the mechanical properties of the panel and performance requirements.
The sensing principle of a fiber Bragg grating can be expressed as ∆λ_B/λ_B =S_ε ε+S_T ∆T where λ_B is the original Bragg wavelength under strain free and initial temperature condition, Δλ_B is the variation in Bragg wavelength due to applied strain ɛ and temperature variation ΔT, and S_ɛ and S_T are the sensitivity coefficients to strain and temperature, respectively. It is worth noting that in order to measure the strain of an host material, temperature compensation of the FBG sensor is required. The resolution of these technology is of the order of 1 με , suitable for the physical requirements on the detector geometry.
The obtained results prove that the proposed approach has the potentialities to permit a continue monitoring of the deformation and bending of the detector.