Research On The Front End Of ECT System With High Frequency Excitation

Electrical Capacitance Tomography(ECT)is one of the hot spots in the research of electrical detection technology(CT)and has been widely used to measure the distribution of dielectric constants in cross-sections since the 1980 s because of its non-invasive,fast response time and no radiation.The data processing system converts the capacitance data into voltage signals and then transmits them to the computer imaging system to visualize the measured area inside the pipe using image reconstruction algorithms.The sensor and data acquisition and processing unit of the ECT system are the most critical aspects of the system hardware design,which directly determine the accuracy of the measured internal capacitance value of the pipe and restrict the accuracy of the image reconstruction during the ECT inverse problem.Starting from two perspectives of low-noise sensor and impedance matching-based ECT system front-end,this project proposes a design scheme of ECT system front-end with high-frequency excitation in conjunction with practical applications.Based on this,the design of the hardware platform is completed and relevant comparison experiments are conducted.The main work of the thesis is as follows.(1)To reduce the dynamic range of the capacitance to be measured and improve the channel signal-to-noise ratio,a differential sensor and a differential C/V circuit are designed.The differential sensor reduces the influence of internal parasitic devices,and the differential C/V circuit has good suppression of external interference signals.The fully symmetrical layout design enables the front-end of the ECT system to show better signal characteristics under high frequency excitation.(2)In order to improve the adaptability of ECT system sensors to pipes with different outside diameters,a differential sensor with controllable pole spacing is designed so that a single sensor can complete detection in a variety of application environments and improve portability.(3)The front-end circuit of ECT system based on impedance matching is proposed.ECT system is mostly used in complex industrial environments,so the sensor and front-end circuit are connected by a certain length of coaxial cable.For high-frequency excitation,transmission line effects such as standing waves,reflections and impedance mutations can occur,resulting in uncertain noise signals in the measurement system and causing significant measurement errors.(4)Based on the above general scheme,the hardware design of ECT system differential sensor,high-frequency excitation signal source,differential C/V circuit,and switching selector module was completed.The impedance transformation and phase shift characteristics of the front-end circuit of the high-frequency excitation ECT system were measured by a network analyzer,and the front-end design of the ECT system based on impedance matching was completed.