Optical fiber Bragg Grating（FBG）displacement sensor has many advantages such as high sensitivity

intrinsic safety and easy multiplexing

etc.However

it is difficult to obtain a large range FBG displacement sensor due to the limitations of sensitive materials.In this paper

in order to solve this problem

a large range FBG displacement sensor is proposed and demonstrated.The planetary gear train and cantilever beam composite structure are utilized in this sensor.The FBG is pasted on the cantilever beam.Through changing the rotating ratio of the planetary gear train

different range of the sensors are achieved.Finite element analysis method is used to optimize the material and structure of the cantilever.The output characteristics of the sensor in different range are experimentally studied.The results show that the sensor has a very good linear relationship between output wavelengths and displacements and its maxima range can reach 2 000 mm.