| As the key technology integrated with microelectronic, computer, information pro-cessing, etc. in manufacturing, CNC system is always a hotspot in the field of industrial control and advanced manufacturing. Along with the fast development of computer science, automation, network and communication, CNC system is also on rapid devel-opment, reducing its giant system scale to be highly functional, miniaturized, and more intelligent. Now CNC system has developed into a multi-task control system with hard real-time requirement, in which real time is one of key performance indicators, and re-quires to process the data effectively and make responses in a timely manner by raising new tough demands in synchronization precision and task execution efficiency.From what has been mentioned above, the real-time issue is problematic in two re-spects。First, the synchronicity of communication between the control device of CNC system and field devices should be guaranteed. As a hard real-time system, CNC sys-tem is sensitive to lags, that’s why synchronous characteristics of CNC system appear to be pretty important in real-time performance. In the runtime, control device in the system manages instruction and data transmission to field devices, and machine tools and servos have to execute the commands and provide status feedback simultaneously. So to guarantee the real-time performance of CNC system in real-time communication, the synchronization of devices in industrial control network has to be maintained. Sec-ond, to achieve the development objective of high speed and precision, CNC system has to process enormous amounts of complicated operation tasks effectively. Tasks in the system can be generally divided into two groups in functionality:data management and motion control. The former mainly consists of tasks like data display, fault diagnosing, parameter setting, code input and edit, etc. There is no need for these tasks with little requirement of real-time performance to be executed immediately. In contrast, the latter primarily comprises hard real-time tasks like decoding, tool compensation, interpola-tion, position control and so on. Aimed at various types of tasks, a lot of methods and schemes have been proposed to resolve scheduling problem. However, as CNC system further develops, the coexistence of different types of tasks with real-time requirements that differ from each other has been a chief issue of scheduling in the system. It brings new challenges to the study on real time of CNC system.According the analysis above, this paper studies the problems of real-time in CNC system, covering both time synchronization and real-time task scheduling. Key tech-nologies involved includes real-time communication and precise time synchronization in CNC system, real-time scheduling strategy for hybrid task system, and so on. The main contributions of the dissertation are described as following:1. A real-time time synchronization method for dual ring fieldbus in the CNC sys-tem is designed in this paper to support the synchronization control of stations in the CNC system. In this method a synchronizing message transmission scheme based on dual ring architecture and the synchronization algorithm between master and secondary stations are integrated. In the scheme, the clock models of both master and secondary stations are optimized by functional module partition and the stamp transmission based on the dual ring fieldbus is devised exclusively that improves transmitting efficiency. All the secondary stations can accomplish the consistent state with master station by updating clock discrepancy information in one communication cycle, it takes the ad-vantage of two-way transmitting and makes the best use of dual ring structure, so the real-time performance of the system can be promoted while retaining synchronization precision. To evaluate the performance, the costs of the method and errors during syn-chronizing are noted and analyzed in runtime environment. The results show that this method reduces communication cost and ensures the smoothness of synchronization process with low lag effects that will better support the synchronous control of CNC system.2. A synchronization scheme applying to CNC system is proposed based on the intermediate-level modeling for initial synchronization and periodical resynchroniza-tions with basic concurrency control and tight bounds to meet rising demands of syn-chronization precision in distributed nodes in LAN network. In this model, functional modules are partitioned by the varied requirements in synchronization process to sup-port instant response to syncing tasks. Several representative threads are designed and well scheduled to create a fast and efficient circumstance for communication between nodes that prevents the system from access collision. An experimental realization based on RT Linux kernel and corresponding measurements are accomplished. The results indicate that in a small-scale distributed environment, the nodes are able to be well syn-chronized during in the long run with hundreds of resynchronizations and the deviations are significantly limited in a reasonable range that increases the real-time performance of CNC system.3. A hierarchical scheduling framework for CNC system is constructed to manage different types of RT tasks uniformly based on round-piece scheduling algorithm. In hybrid real-time system, scheduling approach is required to ensure all hard RT tasks to be finished before the deadline with respect to real-time constraints but improve the quality of service of soft RT and non-RT tasks as well. The concept of Rotation Period is proposed to resolve this issue by forcibly assign time slices to non hard real-time tasks to prevent them from missing deadlines. The hierarchy of the framework is illustrated in detail and the issues of QoS control and overload protection are also in discussion. The feasibility of rotation period is proved and the schedulability of framework in different cases was validated, theoretical analysis and related proofs are given as well. To testify the effectiveness and evaluate the scheduling performance of this framework, a concur-rent runtime environment is designed to measure both overloads of major functions and satisfaction of time constraints of real-time tasks in comparison with existing methods. The results show that different types of tasks can be well scheduled under the execution demands in CNC system that effectively strengthen the real time and reliability of the system.4. An optimal scheduling algorithm is presented with the capability of synthesizing a multitude of types of tasks based on shortest-path strategy. In this work, the model of hybrid task system is introduced and the detail of the algorithm based on shortest path strategy is described based on the features of different machining tasks and cor-responding constraints. By transforming the question of task execution to the shortest path problem in states of space tree, the optimal scheduling algorithm is able to obtain the optimized sequence of hybrid tasks in CNC system. To more fully testify the effect, the algorithm is tested in both simulation and practical machining environment, and the scheduling result is compared with common RTOS. The results of tests show that op-timal scheduling algorithm can achieve better performance whatever on the stability of CNC system or the efficiency of task scheduling that satisfies the real-time requirement of CNC system.Based on contributions above, the synchronicity of communication between con-trol device and field devices has been enhanced, and the time accuracy in data transmis-sion and task execution also get improved. Through the research on characteristics of hybrid tasks in CNC system, real-time scheduling strategies for different types of tasks have been effectively utilized to promote task-handling ability of CNC system in com-plicated runtime environment on the premise of meeting various constraints of tasks. Thus, the real-time requirement of CNC system is well satisfied. |
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