Research On An FPGA Based Electromagnetic Measurement System And Its Application

Electromagnetic Non-Destructive Testing (NDT) is a non-destructive testing method based on electromagnetic induction. It can be used to test surface and sub-surface cracks in the conducting materials. This technique has several advantages, such as non-contact and automated measurement, easy operation and fast response. It can be used widely in the non-destructive testing of conducting materials.After investigation on electromagnetic NDT principle and application, a new electromagnetic NDT system is presented in this paper. The system consists of a Data Acquisition (DAQ) system based on FPGA and a magnetic field measurement unit based on Giant MagnetoResistance (GMR) sensor. The main work is as follows:1. Coil is used as magnetic field detection sensor in the ordinary electromagnetic NDT system. Coil detects the change rate of the magnetic field, so it is limited in the application of low frequency excitation. GMR sensor detects the magnetic field directly, thus it has a good frequency response characteristic. And the small package of GMR sensor will benefit the design of sensor array and simplify the implementation of signal processing circuit.2. A DAQ system based on Xilinx XC3S400 FPGA is presented. The system can generate sinusoid voltage, muti-frequency voltage and pulse voltage so it can be used for different excitation mode. Two signal acquisition channels were designed to sample the induction voltage and excitation current respectively. A USB communication circuit based on CY7C68013A was designed to communicate with the host computer. The system runs well. And it can also be used as a general signal generator and DAQ system.3. The modules in FPGA were programmed in Verilog HDL and PicoBlaze soft core was used as the controller. The combination of the parallel characteristic of Verilog HDL and serial characteristic of PicoBlaze assembly guarantees the operation of the whole system.4. Simulation experiments of orthogonal sequence demodulation were conducted. Simulation of orthogonal sequence demodulation based on MAC IP core was carried out using ModelSim. The relationship between the demodulation accuracy and number of accumulated data, the amplitude & phase of the signal was analyzed. A principle of reducing error was proposed.5. The performance of the system was tested and some explorative experiments were done. It was proved that GMR sensor can detect the crack.