Design Of Direct Three-dimensional ECT Measuring System Based On Digital Measurement Techniques

Electrical Capacitance Tomography(ECT)is an automated detection technology to obtain the two-dimensional or three-dimensional distribution of the measured object in a closed space by measuring the capacitance distribution between plates.Compared with X-ray tomography and video surveillance,ECT technology has many advantages,such as low cost,high security and non-destructive.Due to the inadequate ability of two-dimensional ECT measurement system to acquire spatial information,the application of this technology in industrial field is limited.Therefore,three-dimensional ECT has attracted more and more attention because of its stronger spatial information acquisition capability.The direct three-dimensional ECT excitation method is the main direction of 3D ECT technology because of its remarkable effect.Among the research directions of direct three-dimensional ECT,the direct threedimensional ECT measurement system is the key to transform the technology from theoretical research to practical system.Compared with the two-dimensional capacitance sensor,the sensor used in the direct three-dimensional ECT technology has lower inter-plate capacitance,more measurement information and wider dynamic range of the object.Therefore,the design of measurement system needs to focus on sensor optimization,micro capacitance measurement circuit design,control circuit design and signal generation and acquisition circuit design to improve the measurement ability of the system.On this purpose,following works have been done in this thesis:(1)Sensor structure optimization and design.Based on the measurement characteristics of direct three-dimensional ECT,the effect of sensor shielding plate array and plate size characteristics are simulated and analyzed,and the sensor is designed and fabricated.(2)Theoretical and Experimental Research on micro capacitance measurement circuit.Three kinds of circuit configurations for capacitance measurement are derived theoretically and simulated numerically.The existing circuit models in the laboratory are used to test,and the signal characteristics and the source of circuit errors are analyzed.Based on the analog demodulation principle of AC capacitance measurement circuit,a discrete digital phase sensitive demodulation algorithm is proposed.(3)Hardware implementation of a direct 3D ECT measurement system.The measurement circuit of compensation sensor is designed and manufactured for the pre-research of system hardware design.Aiming at the problems of com pensation sensor circuit,the discrete signal acquisition scheme and zero switch control logic are proposed.According to the measurement requirement,the circuit of the modular direct three-dimensional ECT measurement system is designed and debugged,and the hardware of the measurement system is built.(4)Software implementation of direct 3D ECT measurement system.The compilation and debugging of communication software of compensation sensor measurement system and host computer database interaction software are completed;The compilation and debugging of state machine of measurement control of direct three-dimensional ECT measurement system based on FPGA are completed;Communication interface between FPGA and STM32 is designed to realize data transmission between two processors and the upper monitor acquisition program of direct 3D ECT measurement system is compiled and debugged.This thesis finally completes the overall construction of the direct three-dimensional ECT measurement system.Through the digital demodulation algorithm,zero-hold switch control logic and the corresponding system circuit design,the measurement signal-to-noise ratio of 23.8dB and the capacitance resolution of 0.035pF of the direct three-dimensional ECT measurement system are realized.By adjusting the gain of the input signal conditioning circuit,the sampling signal is obtained.The input dynamic range of the signal is reduced to 66%of the dynamic range of the measurement signal.Meeting the system requirements of direct three dimensional ECT measurement technology,a reliable modular hardware measurement platform is built for subsequent imaging algorithm research.