Research Of LowCapacitance Detection Method Based On CDC And Its Application In ECT

Capacitance digital converter is designed for excitation capacitance sensor with low power consumption,high integration density,high accuracy,which determines its leading place in consumer electronics industry.Because CDC can only detect the changes in single electrode,so how to apply CDC into tiny capacitance measurement circuit with several electrodes attracts interests from various countries scientists.Capacitance tomography(ECT)technology is a kind of process tomography technology,which can rebuild the image by measuring tiny capacitances.So it is meaningful to research how to apply CDC into ECT data acquisition system.Combined with the feature of capacitancesand limited range,ECT data acquisition system especially for tiny capacitances with AD7747 has been designed.The main works are shown as follow:(1)The fundamental of capacitance acquisition system has been studied and combined with the condition of ECT system.Capacitance detection circuit based on CDC was also designed.(2)The output characteristics of active protected electrodes based on ANSYS has been studied.And the parameters of capacitance sensor was optimized to get high sensitivity.(3)The principle of CDC technology has been studied.The capacitance parallel acquisition circuit and serial acquisition circuit with AD7747 was designed,which contains expansion circuit,electrodes switch circuit,capacitance detection and processing circuit,communication circuit.The circuit board was made and debugged.(4)The principle of ECT imaging has been studied and the corresponding collection procedures were written to accomplish switching capacitance electrodes,AD7747 communications and data acquisition.(5)A real-time data processing system based on LabVIEW in host computer has been designed,which was expected to accomplish data transmission,the establishment of the initial data bytes,the processing of digital to analog,normalization,threshold setting and ECT imaging.(6)ECT image rebuilding system has been constructed based on CDC.Under the laboratory conditions,ECT capacitance tomographic imaging experiments research has been done.The experimental results showed that the resolution of the system cross-section was up to 15%.The occupation ratio error of the cross-section was no greater than 20%for plaster stick which occupation ratio of the cross-section is larger than 20%.