Study On Key Technique Of Electromagnetic Detection For Metal

In the production of steel strip, the temperature distribution and cooling rates have a significant effect on the steel microstructure and hence on final material properties, such as yield strength, tensile strength and ductility. Consequently, demands for higher product consistency and a greater diversity of sophisticated steel grades have increased the requirements for a new instrument to monitor the microstructure of steel during controlled cooling on a hot strip mill and hence improved instrumentation. It is well know that the electromagnetic (EM) properties of steel change significantly as the material progresses through phase transformation. And electromagnetic induction measurement technique and Electromagnetic Tomography (EMT) can be used to obtain the distribution of conductivity and / or permeability of objects and possesses the advantages of being simple in structure, low in cost, and non-intrusive to the flow medium. Thus electromagnetic induction detection techniques will be widely used in steel production industry.In order to apply EMT technique to achieve the monitoring the microstructure of steel on-line, the link between the electromagnetic properties of metal and sensor output was explored and a digital electromagnetic induction measurement system was also described. Some main works and conclusions are presented as following:1. The key to the successful application of EM techniques is to analyze the complex relation in the transduction chain from microstructure to the EM properties, and to the sensor output. Theoretical treatment of the relationship of them was given and the completed analytical solutions have been obtained for an axially symmetric coil above a single infinite plane conductor.2. A digital electromagnetic measurement system based on Field Programmable Gate Arrays (FPGA) was designed and implemented. The system consists of a flexible multi-frequency excitation source and a high-precision dual-channel voltage/current measurement unit. The sensing voltage and the excitation current can be measured synchronously to acquire the induction impedance. The digital demodulation technology under FPGA support with flexible structure and high speed was used to obtain the amplitude and phase information of the measured signal.3. A series of experiments were done on the basic of the hardware system. First, Resistors and capacitors with known value were used to evaluate the system. Metal plate detection experiments were then carried out to verify the impact of conductivity, permeability and thickness of a thin metal plate and the lift-off (the distance between the probe and the test sample) on sensor output.Finally, some improvement suggestions are presented at the end of the thesis.