Application of optical fiber Bragg grating strain gauge to cable force monitoring of FAST

To monitor the cable force of the cable-net supporting structure of the Five-hundred-meter Aperture Spherical radio Telescope(FAST)and to avoid the effect of traditional measuring methods on the cable-net supporting structure and the electromagnetic interference,this paper presents a method to obtain the cable force values indirectly.The method uses Fiber Bragg Grating(FBG)sensors to monitor the cable connector strain to measure the cable force indirectly.In the measurement,a FBG strain gauge was installed at the linear strain region of the cable connector axially by using a dedicated welded base,by which the adverse effect of welding directly on the performance of cable connector structure was avoided.A temperature compensation method was used to compensate the effect of temperature changes on the FBG stain gauge measurement.The correlation coefficients were acquired by the calibration of the cable prestressing process.The cable force monitored then was calculated fromthe calibrated relation in actual engineering applications.Test results indicate that the fitting linearity of the cable force and the cable connector strain is up to 0.98 and the Root Mean Square(RMS)error in practical measurement is 1.38 t.The relative error is better than 3% within the range of the cable force for the FAST.The method is characterized by no electromagnetic interference,simple wiring,good reliability and long duration.It has been successfully applied to the cable force monitoring of 316 typical cables of the supporting cable-net in the FAST project.