The PROFIBUS-Based Sewage-Treatment Automatic Control System

Fieldbus is a digital, two-way transmission and multi-branch communication network which connects intelligent working equipment with automatic control system. It is an integrated product of computer technology, communication technology and control technology. It's a new open and distributive pattern. Fieldbus is now one of focuses in automation research and development.Nowadays, in China, over 85 percent sewage without any treatment is drained away into waterbody, which does harm to both human health and the industrial and agricultural production. Therefore, building modern sewage- treatment plants is a task, which brooks no delay.This paper analyses the status quo of fieldbus and its trends, introduces fieldbus network structure as well as communication protocol, probe into the PROFIBUS failure-safe communication technology, from PROFIsafe main features, communication principles and failure-safety precautions to message structure. On the basis of the computerized technological process control of sewage treatment in a urban sewage treatment plant in Jiangxi, the author brings forward a plan of PROFIBUS-based automatic control system for sewage treatment, designs the automatic control system diagram and the PROFIBUS network diagram for sewage treatment plant. At the same time, the author also makes a study of techniques to control the sewage aeration, which is the most difficult problem in sewage treatment process. The fuzzy controller can indicate the changes of the water quality in treated sewage, effectively control the volume of aeration and then cut down the operation costs.This design of automatic sewage-treatment control system in the PROFIBUS-based fieldbus makes use of advanced Karrussel oxidizing-groove aeration technology and discards traditional A/O technology and contact-oxidizing technology. The application of PROFIBUS technology sets up communication between the dominating process controller IPC, aeration process PLC, mud-draining system PLC and sewage-pumping PLC in the principal stations. It also establishes communication between the above-mentioned main stations, soft starters of sewage pumping, frequency converters in aeration process, and intelligent sensors in technological application and outside I/O equipment in subordinate stations.The automatic control system was put to use in June 2004. Water quality of treatedsewage comes up to the designed requirements. It has produced good economic and social benefits. The operation of the automatic control system shows that the research and development of the project is of great importance.