Research On FPGA-Based Coherent Electrical Capacitance Tomography System

Electrical capacitance tomography (ECT) technique is a widely studied technique for the measurement of two-phase flow parameters. Owing to the advantages of low-cost, high-speed capability, non-invasion, wide applications and reliability, ECT has been applied in many fields. However, it is still in the stage of laboratory research due to a few problems and difficulties.This dissertation is based on the study of 12-electrode electrical capacitance tomography system for oil-water two-phase measurement. It builds a coherent AC detecting ECT system which is based on FPGA. The main problems researched and achievements are as follows: Based on explaining the important significance of process tomography technology in scientific research and production and describing the current status and developing trend of the future process tomography, the principle and constitution of electrical capacitance tomography system are analyzed and mathematical model of sensitivity field is proposed. The 12-electrode embedded capacitance transducer is designed based on the influence of structure parameters and design ruler which inhibits the effects of stray capacitance coming from coaxial cable effectively. A switch controlling circuit of transducer electrode array is designed, so that all the electrodes can use the same excitation source. A FPGA is used in the control circuit of transducer electrode array to make the scalability strong. Phase-locking technology and synchronous detector technology are used to avoid the phenomenon of phase shift instability brought by stray capacitance and improve the system's sensitivity. The influence of coupling capacitance to the measurement system is analyzed. A model of coupling capacitance compensation is proposed, and is built using software of real time compensation. A FPGA based data acquiring system has been developed, which can advance the speed of data acquisition and processing. The common ECT image reconstruction algorithms are analyzed and also the RBF neural network image reconstruction algorithm with parameter optimization is proposed.The system tests have achieved satisfactory results, so that the small capacitance measurement problem is solved and the system's anti-noise ability is improved.