Design And Implementation Of COD Online Monitoring Instrument Based On DsPIC And MCGS

Chemical oxygen demand (COD) is one of the control indicators of the standards for water quality, effuent and wastewater discharge in China. With the increasingly of water pollution and the efforts of the environmental protection department to environmental monitoring, to improve the water quality monitoring level, developmenting of modern COD on-line monitoring instrument is necessary. There are many rivers, lakes, wide water surface and sewage outlets of cities and factories in China,while the fund for monitoring water quality is very lack. The high price of the high detection precision COD on-line monitoring instrument of foreign is not conducive to promotion in China. Therefor it’s very necessary to developing a COD on-line monitoring instrument which suitable for the national conditions of China.In this dissertation, the current situation of COD-line monitoring techniques at home and abroad was analyzed, and the common COD detection methods was also discussed, sealing catalytic digestion-spectrophotometry which is one of improved potassium dichromate methods was chosen. A new type of COD on-line monitoring instrument was designed based on dsPIC (Digital Signal Controller) and MCGS (configuration software) technique.Firstly, the COD online automatic monitor system hardware was designed, including the measuring system, the digestion system, the control system and the touch-screen. The measuring system is composed by the Multi-level high-precision optical sensor and peristaltic pump; The digestion system includes sealed digestion tube、the temperature control module and the optical detection module, the optical fiber which could eliminate the effects of temperature is used in the optical detection module. The control system circuit was designed according to the composition and working principle of the measuring and digestion system, including the dsPIC minimum system, switch output circuits, analog signal acquisition circuit, communication interface circuit design, and the anti-jamming measures of PCB board was introduced.Secondly, the COD on-line monitor system software was designed. It includes the control system program and touch-screen applied program two parts, which adopted Modbus communication protocol. The control system program realized automatic detection, PID temperature control, and communication with the touch screen and so on; used MCGS as a touch screen developed platform, the master window, the device window, the user window, the operation strategy window and the real-time database was designed. Function interfaces such as customer operations, equipment status, equipment commissioning, history reports, parameter setting and abnormal information was realized. Especially the3D flow chart interface in which the liquid flow diagram, peristaltic pump and solenoid valve working condition with instrument synchronization is intuitive and accurate.Finally, linear fitting least squares algorithm the calibration method of the COD value was introduced, and program was designed. The measuring basic error and repeatability error of COD monitoring instrument was measured and the results was respectively less than5%and3%. It shows that this instrument could be used for COD on-line monitoring, and its system was reliable and stable, and meets the technical requirements of COD analyzer.