# Operation State Evaluation of Miter Gate Based on On-Line Monitoring and Finite Element Analysis
## 2 Monitoring Systems
### On-Line Monitoring System for Gates  
The system adopts large-scale, multi-parameter, and fiberoptic grating sensors to monitor all critical components and stress concentration areas of the gate and to realize the dynamic monitoring of the stress strain and the cracks of the gate in a full-time and total size. Fiber Bragg grating (FBG) technology is formed by using ultraviolet exposure technology to cause periodic changes in the refractive index in the fiber core. The FBG stress sensor is usually attached to an elastic body and protected with packaging. The wavelength of reflected light is very sensitive to temperature, stress, and strain. When the flexible body is under pressure, the fiber Bragg grating and the elastic body will strain together, resulting in the shift of the peak wavelength of the reflected light of the fiber Bragg grating. The temperature, stress, and strain can be sensed by measuring the wavelength shift. The FBG sensor has a strong anti-electromagnetic interference ability, electrical insulation, corrosion resistance, and material safety compared with traditional electrical sensors. It can be transmitted without forming a loop and maintain good working characteristics in harsh environments.  
The system consists of signal detection, data acquisition, data processing, data presentation, remote supervision, and other modules, as shown in Figure 2. The signal monitoring module includes sensors, signal transmitters, and a signal pre-processing board, using FBG stress sensors to collect the stress signal at different locations on the gate and an FBG crack sensor to collect the gate crack state.  
During the overhaul process of the Gezhouba No. 1 to No. 3 lock miter gates, some cracks were found on the node plate of the bottom layer, the connection of the lower flange plate of the main transverse beam of the gate body, as well as on the stiffening plate of the gate shaft column end, etc., [22; 23; 24]. Combined with the faults and the location of the cracks in the gate body during the historical overhaul of No. 2 locks, the FBG stress and crack sensors were chosen for application. The FBG crack sensors are installed on the door shaft,  
Figure 2: On-line monitoring system for the miter gate.  
door partition, diagonal joint column, and door seam. We installed the FBG crack sensors on the back ties, rear flange plate, and inner web of the 23 rd to 27 th layers of the door body, where there are connection holes between adjacent measurement points in the horizontal and vertical directions. The fiberoptic cables are installed to connect the network using these holes, as shown in Figure 3. Waterproof and long-life FBG strain gauge GBC-1108 (produced by Wuhan Polytechnic Optical Co., Ltd., Wuhan, China) with the tube package and the exceptional support (Q345 steel) is used for stress monitoring. The FBG crack displacement gauge GYC-1102 (produced by Wuhan Polytechnic Optical Co., Ltd.) with box package is installed by the special brackets (Q345) in series on the cracks repaired by welding on the door body for real-time monitoring. All sensors' sampling frequency is 1 Hz.