As a sensing cell, Fiber Bragg grating (FBG) can transduce physical quantities like strain, temperature, etc, having
attractive merits of being small and light, resistance to corrosion and immunity to electromagnetic interference, etc.
Commercial FBG strain sensor has a sensing range of no more than 9000 με (0.9%), however, larger-range strain sensor
is demanded in industry such as heavy structural distortion and crack-happening. A new kind of large strain sensor based
on FBG is studied here. The sensing element has a metal trapezoidal frame. The two feet of the frame can sense a large
strain of the body, which is converted to a small strain on the surface of the frame' beam. The attached FBG senses this
small strain, and then the body's strain can be known from the FBG's wavelength shift. The trapezoidal frame is taken
theoretically analysis adopting the 'unit load method' and numerical simulation by finite element method. The
sensitivity model of the sensor between the body's strain and the FBG's wavelength shift is deduced and verified. Real
large strain sensors are homemade, with verifying sizes. The large strain is controlled by a motorized translation stage,
and the FBG's wavelength shift is interrogated by MOI sm125 interrogator. The experimental results show an
outstanding large-strain sensing ability of the sensors, having the sensing range of -20~40%, with the linearity of less
than 1%, the hysterisis error of less than 1% and the repeatability of less than 0.9%.