Real-time performance evaluation of urgent aperiodic messages in FF communication and its improvement☆
Introduction
The increasing complexity of control systems, as well as the large dimension of applications, such as process control, factory automation, space vehicle system, etc., has lead to the development of distributed control system (DCS), where control commands and state information are exchanged through networks. Within DCS, it is very necessary to execute computation tasks timely, which usually reside in different nodes and communicate with one another to accomplish a common goal. It is difficult to ensure timely results of tasks in a DCS without a network that supports the timely inter-task messages [1], [2], [3]. Therefore the temporal property of the underlying network is important, and special-purposed real-time industrial networks are in need.
Foundation Fieldbus (FF), one of eight IEC international Fieldbus standards proposed by Fieldbus Foundation, is just a kind of special-purposed real-time industrial networks. FF-liked networks are generally characterized by the obligation to respect stringent temporal constraint, which must be met to guarantee the correctness and safety of field devices [4], [5], [6]. In order to achieve above object, centralized Media Access Control (MAC) is utilized by FF to support periodic messages, and distributed MAC to support aperiodic messages.
Concerning periodic messages, the main problem is constructing schedule table and schedule algorithm. Similar researches can be found in [7], [8], [9], [10], [11], [12], [13], [14]. As for aperiodic messages, FF utilizes the mechanism of Pass Token (PT) priority and PT circulation period to meet its temporal constraint. At the aspect of guaranteeing important aperiodic messages, there are only few mechanisms, such as in [15], [16]. Moreover, these researches are not detailed enough. This paper not only analyzes the effect of PT circulation period on Worst-Case Response Time (WCRT) of aperiodic messages in detail, but also points out main cause of current mechanism's deficiencies. Furthermore, this paper proposes an improved one and validates it with simulation.
The remainders of this paper are organized as follows: Section 2 describes the primary transfer procedures of periodic and aperiodic messages in FF and corresponding models. Then in Section 3, the effect of Setting PT Circulation Period (SPTCP) and Actual PT Circulation Period (APTCP) on real-time communication of aperiodic messages is evaluated from different aspects of view, such as guaranteeing urgent aperiodic messages, high level utilization of FF, effectiveness of FF on best-effort transfer of normal aperiodic messages, etc. In Section 4, an improved APTCP computing mechanism and then a method of setting SPTCP are proposed. The simulation results for the proposed mechanism are presented and analyzed in Section 5. Finally, in Section 6 some conclusions are given.
Section snippets
Network and message models
Consider a DCS where there are nn nodes interconnected by a FF network. Assume there are nP periodic messages MP=(CP, TP, DP)(i∈[1, nP]) and nA aperiodic messages MAi=(CAi, TAi, DAi)(i∈[l, nA]) within the DCS. Wherein, TPi, DPi and CPi correspond to periodicity, deadline and transaction duration of periodic message MPi, respectively. TAi, DAi and CAi correspond to periodicity, deadline and transaction duration of aperiodic messages MAi, respectively. For aperiodic messages, their arrival
The effect of SPTCP and APTCP on real-time communication of aperiodic messages
In the above section, the real-time capability of aperiodic messages is evaluated under the assumption that the instant of PT changing priority is known. However, the effect of SPTCP and APTCP is neglected actually. In this section, how to set SPTCP is investigated in detail. The following analysis is done by guaranteeing the real-time requirement of urgent aperiodic messages and by meeting the real-time requirement of normal aperiodic messages with best efforts (Just for convenience, normal
Impact of various PT priority states on the response time of urgent and normal aperiodic messages
The analysis in previous sections indicates that the WCRT of urgent and normal aperiodic messages varies significantly under different conditions, and temporal constraints of these messages cannot be effectively guaranteed by current mechanism utilizing PT priority and PT circulation period. In current mechanism, PT priority changes only at node 1 within a new PT cycle if there is enough difference between previous APTCP and SPTCP. Obviously, this mechanism is too simple to effectively adapt to
Performance evaluation of improved mechanism
A simulation model has been constructed using OPNET Modeler 8.0. The main objective of the simulation study is to gain insight into the performance of the proposed APTCP computing mechanism. It is assumed that eight nodes are connected using FF. Each node produces two aperiodic message streams: one is a Poisson stream with urgent priority, packet length of 60 bits and arrival rate of 10 packet/s, the other is a Poisson stream with normal priority, packet length of 120 bits and arrival rate of
Conclusions
Supporting real-time traffic of critical aperiodic messages using FF is a complicated issue since it is concerned with ST, APTCP, SPTCP and PT priority, etc. This paper first proposes an integrated message transmission model, which integrates periodic messages with aperiodic messages together. Then formulas for the response time of urgent and normal aperiodic messages are given. Through the formula, deficiency of current APTCP computing mechanism in meeting real-time traffic of urgent and normal
Zhi Wang, born in 1969, received his MS degree from Southeast University in 1997 and PhD degree from Shenyang Institute of Automation of Chinese Academy of Science in 2000. He has been doing Post-Doctoral Research in Zhejiang University and National Polytechnique Institute de Lorraine from 2000 to Sep. 2002. He was a senior Visitor and Researcher in LORIA TRIO from July 2002 to Aug. 2002 and from Sep. 2003 to Nov. 2003. In July 2002, he was promoted to Associate Professor. At present, he is the
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Zhi Wang, born in 1969, received his MS degree from Southeast University in 1997 and PhD degree from Shenyang Institute of Automation of Chinese Academy of Science in 2000. He has been doing Post-Doctoral Research in Zhejiang University and National Polytechnique Institute de Lorraine from 2000 to Sep. 2002. He was a senior Visitor and Researcher in LORIA TRIO from July 2002 to Aug. 2002 and from Sep. 2003 to Nov. 2003. In July 2002, he was promoted to Associate Professor. At present, he is the leader of RoDaC (Open Realtime Distributed Control Architecture) group and mainly researchers networked control and embedded system, including discrete event system, realtime schedule, IP networks, sensor network and open distributed control system based on IEC Function block.
Xingfa Shen received his B.S. degree in Electrical Engineering from Zhejiang University, China, in 2000. He is currently a PhD student in College of Information Science and Engineering of University of Zhejiang University, China. His current research interests include realtime network scheduling, network modeling and wireless sensor networks protocols.
Jiming Chen received his B.S. degree in Electrical Engineering from Zhejiang University, China, in 2000. He is currently a PhD student in College of Information Science and Engineering of University of Zhejiang University, China. His current research interests include real-time schedule, weakly real-time analysis, Real-time QoS of industrial Ethernet, fieldbus-based distributed control system.
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This work is supported by National Natural Science Foundation of China (NSFC 60203030, 60084001), National Basic Research Program of China(2002CB312200) and Advance Research Program of France-China(PRA SI03-02).