Low-frequency Dynamic Characteristics Of Bridge Deflection Monitoring System Based On Communicating Pipes

Deflection is one of important parameters to monitor and evaluate bridge safety. In order to detect whether the bridge structure has mutation or abnormity, multipoint synchronous and dynamic monitoring of deflection is quite necessary. Communicating pipes measurement has many advantages which other deflection measurements do not have, such as multipoint synchronous monitoring, high precision, wide adaptation, small environmental interference, wide adaptation and high ratio of performance to Cost. It has great economic and theoretical value to develop the potential of communicating pipe measurement, combine the static and dynamic measurement system together and use communicating pipes to realize static and dynamic measurement of deflection. Communicating pipes dynamic measurement uses the inside liquid as medium to measure dynamic displacement response, and has something with the movement characteristics of inside liquid. So it is quite necessary to carry out the research in this filed.On the basis of static measurement principle of communicating pipes , this thesis gives a conclusion that the inside liquid of communicating pipes is unstable and deduces its energy and continuous equation. Because the inside liquid of multiple communicating pipes couples together, it is quite complex. So it simplifies it into triply communicating pipes with a reference barrel. The movement differential equations of the inside liquid are deduced using energy equation and continuous equation On this basis, dynamic characteristic is analyzed in time domain and frequency domain. Dynamic characteristic index in time domain is calculated. The influence of parameters to liquid movement and natural frequency is analyzed, such as diameter, length and liquid highness in the communicating pipe, viscous coefficient of liquid, and the sectional area ratio of reference barrel to communicating pipe. Frequency response characteristic of liquid system in communicating pipe is analyzed.Based on the theoretical analysis, according to the bridge feature when it is stimulated, this paper puts forward the experiment method of step response and continuous vibration. The experiment system is made up of tee communicating pipes, vibration system and automatic liquid level measurement system. It uses a transducer, electromotor, reduction box, link mechanism and guide wall to design a vibrating system. It chooses the liquid level automatic measurement system made up of opto-electronic liquid level sensor which is developed by Chongqing University.Opto-electronic liquid level sensor is designed for measurement of static liquid level, and the quality of dynamic signal that collected by it is poor and the frequency is low, so it can't meet the experimental requirements. Analyzing element of liquid level measurement system and principle of sensor, deformation is found in dimensional measurement of moving floater in photoelectric image method. Dynamic measurement models are built up to analyze problems. The conclusion is reached that practical highness of acquisition floater is a function of liquid viscous influence coefficient, the velocity of vibration table and scanning speed of sensor. And the influence quantity is linked with the vibrating direction. For the problem that the former circuit hardware is complex and inefficient, the whole circuit is improved, and multipoint synchronous dynamic acquisition is completed by multi-communication design and expanded data memorizer. The acquisition and processing program and working way in software are designed considering the analysis of deformation model and the characteristic of acquisition waveform. It achieves multipoint synchronous dynamic acquisition.Experiments of step response and continuous vibration are done, dynamic characteristic index in time domain of communicating pipe system is measured, and parameters influence on the system is also analyzed. On the basis of experiment data, the characteristic curve between amplitude and frequency under the condition of low frequency vibration stimulation is gained. And some phenomena are found from the curve, for example the frequency range without system response distortion is narrow, and amplitude distortion appears when the frequency increases, but the frequency distortion is small. At last according to data characteristics of ultra-low frequency dynamic displacement measurement, it proposes to use improved wavelet threshold analytical method to get rid of noises, and experiment results are good.According to the theoretical analysis and experiments of dynamic characteristics of communicating pipes system, ultra-low frequency displacement signals can be measured by communicating pipes. It has poor amplitude response when communicating pipes system is used under high oscillation pump-frequency, but its frequency response distortion is less. So it can be a new method to measure oscillation signals, which has important values for modal measurement.