I. Measuring principle of magnetic flux sensor
When ferromagnetic materials are subjected to external force, mechanical stress or strain is generated inside, which correspondingly causes the change of magnetization, that is, the magnetoelastic effect is produced. By establishing the relationship between magnetization and stress, the stress in ferromagnetic materials can be detected. The feasible stress detection method is to wind two coils, a primary coil and a secondary coil, on the tested component, and take the tested material as the core of the coil for measurement. If an AC excitation signal is applied across the primary coil, an alternating magnetic field that changes with time will be generated. According to Faraday's law of electromagnetic induction, induced electromotive force will be generated in the secondary coil:
Vind(t)=-N( 1)
The magnetic flux passing through the coil is along the direction of the measured part. During the test, the measured part may not completely fill the coil, so the total magnetic flux consists of two parts: the magnetic flux passing through the air and the magnetic flux passing through the specimen. The induced voltage is:
Vind(t)=-NH((2)
Where, and are the coil surface area occupied by air and specimen respectively. Is the permeability of air. If the induced voltage is integrated with time, the average value of the output voltage with time is:
Vout=-inddt=[+ds](3)
Where h and b are the changes of magnetic field strength and magnetic flux density in time interval (t2-t 1) respectively, and the current increases from 0 to Ia. The magnetic field intensity corresponding to Ia is Ha, and the magnetic permeability should be measured when the magnetic field intensity is Ha. If the number of turns of the coil is large and closely arranged, the magnetic field in the coil is almost uniform even if there is an iron core. Therefore, Equation (3) can be simplified as:
Vout=()](4)
Where S0 is the total cross-sectional area of the coil, Sf is the cross-sectional area of the sample, and t is the time constant of the RC circuit. When no test piece is placed in the coil, the integral of the output voltage changing with time is:
V0=(5)
According to equation (4) and equation (5)
=[ 1+(- 1)](6)
Then the relative permeability.
= 1+(- 1)(7)
Fitting the relationship between relative permeability and cable force (or stress) can be used to measure the internal tension of ferromagnetic materials.
The magnetic flux sensor made by the above principle has a structure as shown in figure 1, and consists of a primary coil and a secondary coil. When the pulse current acts on the primary coil, the ferromagnetic material is magnetized, which will generate a pulse magnetic field in the longitudinal direction of the steel core sample. Due to mutual induction, an induced voltage is generated in the secondary coil, which is proportional to the applied magnetic flux. For any ferromagnetic material, the relationship between permeability change and structural stress and temperature can be established by carrying out multiple groups of tests under stress and temperature in the laboratory, which can be used to measure the internal force of components made of this material.
2. Application design of magnetic flux sensor of Haihe Bridge
The self-weight of the main girder and the vehicle load of Haihe Bridge are borne by cables, which are particularly prone to fatigue and corrosion damage, and their service life is often shorter than that of other bridge components. However, cable is an important component in the bridge, which plays the role of pulling and starting the whole body, so it is very important to master the internal force and its changing characteristics accurately and timely.
In order to monitor the changes in the station, when the stay cable is damaged, it can be monitored in time by installing the sensor of the cable force monitoring system. In this system, some stay cables are selected to monitor long, medium and short cables.
The cable force can be measured by dynamic method, fiber grating intelligent cable and magnetic flux sensor, among which vibration method is widely used, with good accuracy for long cables, but large error for short cables. The magnetic flux technology has high test precision, but high cost, complex data acquisition system and high requirements for transmission lines. Fiber Bragg grating smart cable can accurately measure the strain of cable, but it has high requirements for the production and transportation of finished cable. In this scheme, the magnetic flux sensor is used to monitor the cable tension, thus directly evaluating the safety of the cable. There are 74 stay cables in the whole bridge, among which sensors are arranged in 24 stay cables.
Cable force analysis: it mainly includes cable force identification based on vibration method, cable force extreme value analysis and fatigue damage analysis based on cable force strain monitoring. According to the extreme value of cable force and fatigue damage, early warning is carried out.
Third, the data acquisition system
The data acquisition system adopts two ways: threshold and timing. This design is as follows: (1) At the initial stage of bridge structure operation, the strategy of 24-hour continuous adoption is adopted; After 30 days of operation, the data are analyzed to reveal the actual stress characteristics and laws of the bridge structure. According to the actual stress characteristics and laws of the bridge structure, the threshold of triggering the acquisition system and the specific time period of timing acquisition are determined.
The design of data acquisition system considers the overall architecture, software, hardware and data acquisition strategy of data acquisition system. The bridge is not very long and the signal attenuation is not obvious, so a data acquisition station is used for centralized data acquisition. The junction of tower and beam of data acquisition station is located on the bridge deck of main girder. In order to ensure the normal operation of the monitoring instrument, it is necessary to maintain 24-hour uninterrupted power supply.
Data acquisition strategies are divided into dynamic acquisition and static acquisition. The data acquisition system needs to adopt threshold and timing, and the sampling frequency will be determined according to the calculation results of the bridge structure, but the data should be guaranteed to correspond in real time.
The data processing and control subsystem completes the verification, structured storage, management and visualization of monitoring data and the control of monitoring sampling. Data processing and control subsystem plays an important role in the whole system. It has the following items: (1) Verification of monitoring data; Preliminary analysis of data; Structure, storage, query and visualization of data; Be able to respond to data requests of subsequent functional modules; The sampling of the sensor subsystem can be controlled.
Four. conclusion
Haihe Bridge health inspection, 38 groups, each group has 2 stay cables, 24 stay cables are equipped with magnetic flux sensors, all cable force sensors are installed under the anchor, and the magnetic flux sensors are installed in the cable factory. The flux sensor is installed in the lower embedded pipe of the cable, and the whole flux sensor is selected and directly sheathed outside the cable. The magnetic flux sensor can effectively monitor the safety of the sling. The data collected by the system can be transmitted to the monitoring room of Haihe Bridge through optical fiber, which greatly reduces the burden of bridge managers and provides data for bridge management. After receiving the real-time data, the monitoring center performs hierarchical alarm monitoring according to the set early warning threshold.
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