Study On On-line Stress Monitoring Of Multi-bolt Of Wind Turbine Based On Ultrasonic

The high-strength bolts are important fastening connectors for the pitch bearing of wind turbines.In the process of use,the loads they receive are very complex,and accidents such as bolt connection failures and even fractures occur from time to time,resulting in huge economic losses.Therefore,the realization of on-line monitoring of connection state of high-strength bolts of pitch bearing in the operation of pitch system of wind turbine,and the detection and identification of possible bolt connection failure faults have a very important role and significance for ensuring the safe operation of wind turbine,providing against possible trouble,avoiding the occurrence of major safety accidents,and protecting the safety of property and life.Based on this,this thesis intends to study a method and system suitable for on-line monitoring of axial stress of multi-bolt in pitch bearing connection structure of wind turbine.According to the characteristics of high-strength bolts of wind turbine,the ultrasonic detection method based on acoustic elastic effect is determined as the axial stress detection method of wind turbine high-strength bolts after analyzing and comparing the advantages and disadvantages of the current main bolt stress detection methods.In this thesis,theoretical modeling and finite element simulation analysis were combined to study the stress of pitch bearing and its high-strength bolt connection structure.The results show that wind turbines are subjected to various complex alternating loads in the operation,which will increase the axial stress of bolts.When the bolt stress value is too large for a long time,the bolt failure accident is very easy to occur.On the basis of mechanical analysis,according to the functional and index requirements of the monitoring system,the overall scheme of the multi-bolt stress monitoring system of wind turbines was determined,and the hardware design and software design of the monitoring system were completed.The system hardware consists of three parts: ultrasonic transducer,embedded device base on ZYNQ and remote monitoring computer;the software includes driver and control program of multi-channel ultrasonic signal excitation and acquisition,core program of embedded ARM and remote monitoring computer program.The monitoring system can perform online stress monitoring on 24 or more high-strength bolts in pitch bearing connection structure of wind turbines,and detect and identify the failure state of the bolts.In order to improve the accuracy of bolt stress detection,this thesis analyzed and compared the advantages and disadvantages of several different ultrasonic signal preprocessing and timeof-flight calculation methods.According to the simulation analysis results and the characteristics of ultrasonic detection signal,the wavelet threshold denoising method is selected as the ultrasonic signal preprocessing method of the monitoring system,and the level comparison method and the maximum peak-zero cross point method are used to calculate the ultrasonic timeof-flight in the bolt.According to the actual stress condition of the flange of the wind turbine pitch bearing,the thesis designed and established a flange connection multi-bolt stress monitoring test system,and carried out the finite element simulation analysis and actual monitoring test on the beach for simulating loading of the monitoring test system.The test results shows that the multi-bolt stress monitoring method and system studied in this thesis can effectively monitor the axial stress of multiple bolts under different loads at the same time.The system works stably,and can detect the failure state of the bolt simulated by the test.The monitoring system is suitable for on-line monitoring of the multi-bolt stress of the pitch bearing of the wind turbine.