Research On Ethernet-based Real-time Communication Of Distributed Numerical Control System

With the great advantages of excellent stability, good compatibility, high bandwidthand low cost, the Ethernet technology has been a more popular solution for datatransparent transmission and low-level transmission speed enhancement of distributednumerical control system (DNCs). However, Ethernet takes non-determinism tocommunication, due to the media access mechanism of CSMA/CD. Thus, it is a difficultyto research on the topic"how to realize the time synchronization and hard real-timecommunication in the motion control level by the Ethernet technology", and the research isoutspread around the topic in this paper.Based on the hierarchical architecture of DNC, the architecture of software andhardware of Ethernet-based DNCs is analyzed and the hierarchical network topology ofreal-time and synchronized communication network is given.Analyzing the communication features of motion control level, the paper designs ahard real-time scheduling mechanism based on CSMA/CD, which gives a process ofprecision time synchronization and real-time scheduling, in order to avoid the randomicityof data transmission under CSMA/CD, before the transmitted data reach the MAC layer.For the procedure of precision time synchronization and real-time communicationunder the hard real-time scheduling mechanism, an optimization method is proposed tooperate time synchronization, data transmission and real-time traffic control parallelly, andthen the network efficiency can be improved while network load caused by timesynchronization to real-time communication can be reduced.In one communication cycle of the motion control level, there are mainly three kindsof data to send, which are cyclic real-time data, acyclic burst data and acyclicnon-real-time data, but not to satisfy the real-time and efficient transmission by onesingle-scheduling method. To solve the above problem, the paper presents a real-timetransmitting scheduling method to cope with multi-phase and multi-type task.Based on the realization and optimization of hard real-time scheduling mechanism, areal-time control protocol is designed. By this protocol, the real-time andtime-synchronized communication under the real-time transmitting scheduling method isresearched: according to the time determinism of cyclic data, a polling scheduling based ontime slot mechanism is designed, by time triggering and event triggering, to providedeterminism and reliability to cyclic real-time data transmission. And also, according to the deadline character of acyclic data, a real-time traffic control model is built to shape thenetwork traffic, in order that the acyclic burst data transmission can be completed in thedeadline of a real-time communication cycle.An experiment platform for testing hard real-time communication is put up based onARM development board and embedded Linux system. Real-time control protocol is addedinto original network architecture in Linux by modifying core file, and the experimentresult is shown finally.