| With the maturity of computer,communication and network control technology,PLC plays an increasingly important role in the field of industry control.The control mode is changing from traditional single PLC control/network communication toward large scale multi-PLC network control system.Affected by wiring,environment,and cost factors,field devices from different suppliers with different protocols have to be integrated into one system.At the same time,in current PLC network control system,developers have to understand the topology of the system,program for each controller respectively,and scatter the control logic into each device.Moreover the network communication should also be manually coordinated to achieve the synchronization of variables and control logic between controllers.It will become a hard work when the number of controllers is large and the control logic is complex.With the growth of the system scale,this kind of device-orient programming mode which focus on single controller adversely affects the development efficiency and quality of the PLC control program.So how to achieve unified device operation and management in a PLC control system which consists multi-types of fieldbuses,and how to unified the control network as one object,and achieve network-orient programming,compiling and downloading has important research significance and application value.The concrete research contents of this paper are as following.For the coexistence of multi-fieldbuses in industrial applications,and the reliance of protocol specific software for current device integration techniques,a unified operation and management method for multi-fieldbus devices based on description has been proposed.The description models of protocol,device resource and variable are proposed,with which the system can achieve unified configuration,management and data access based on corresponding description files.This method improves the situation that specific software needs to be developed or purchased to operate each separate device.For the problems,such as low communication efficiency,heavy network load and uncontrollable QoS,which arise from the practical use of the description based unified device management method,communication optimization and improved priority scheduling strategies are put forward.A weighted digraph which describes the relationship between periodic variables,communication packets and network load is built and packets generated according to protocol description are optimized by a path planning algorithm of the digraph model to reduce the network load.Further more,the transmission of periodic and burst real time packets are controlled to ensure the the schedule delay of each kind of messages.For the development of PLC control system,a network-orient programming method is proposed.Regarding the control system as one object,the engineers can program the whole system using all of the resources in the network directly.And a modelling and compilation method is also put forward to decompose the control logic and disperses the program into parallel tasks.These tasks are distributed into corresponding PLC controllers according to the device performance and communication cost.A share variable region is established and communication instructions are added into each controller to achieve the variable and logic synchronization,so the control logic can be executed appropriately.This method can effectively improve the develope efficiency of large scale PLC control network,and reduce the difficulty of system maintenance.Finally,to illustrate how these methods are applied in practical applications,the methods are implemented in PLC_Config software,and verified in a power monitoring project and a platform of integrated monitoring system.The result demonstrated that the information required by variable and device management and the communication with different fieldbus devices can be provided by the proposed protocol,device and variable description methods.The packet optimization and scheduling methods can reduce the network load effectively and meet the scheduling delay requirement of different types of communication.The network-oriented programming and compiling method can regard the control system as a whole object,and disperses the program into appropriate controllers according to the control logic and device topology.These methods have a certain application value of the design,development and maintenance of the network PLC control system. |
