原文:PLC Communication using PROFINET: ExperimentalResults and AnalysisAbstractPROFINET is the Industrial Ethernet Standard devised by PROFIBUS International for “Ethernet on the plant floor”. PROFINET allows to implement a comprehensive communications solution on Ethernet which includes peer-to-peer communication between controllers, distributed I/O, machine safety, motion control and data acquisition. In this paper an analysis is conducted on the peer-to-peer interlocking performance based on PROFINET specification. Tests were performed to determine the performance of the peer-to-peer communication mechanism, to evaluate the impact of switches on the system, and to measure the impact of data size on peer-to-peer communication performance. The paper summarizes the test results. 1.IntroductionAlthough a wide variety of networks and fieldbuse s have been used in the manufacturing industry over the past decade [1], the widespread adoption of Ethernet as a de facto standard in other domains (e.g., the internet) has made it an attractive option to consider. The increased network speed and the reduced cost of devices has further heightened interest. The introduction of switched Ethernet has allowed formore deterministic behavior and alleviated many of the concerns about unbounded delays [2, 3, 4]. Ethernet is already being widely used as a diagnostic network in manufacturing systems and is making inroads into the control networking domain [5, 6].However, standard Ethernet (IEEE 802.3) is not a deterministic protocol, and network quality of service cannot be guaranteed. To address this inherent nondeterminism, different “flavors” of Ethernet have been proposed for use in industrial automation. Several of these add layers on top of standard Ethernet or on top of the TCP/IP protocol suite to enable the behavior of Ethernet to be moredeterministic [7]. However, the network solutions may no longer be “Ethernet” other than at the physical layer.Since time delay is an important issue in control systems, there have been a number of projects devoted to analyzing and experimentally testing network performance for use in control systems. It has been shown that the largest component of the time delay in sending messages from one node to another is typically not on the network itself, but rather the application layer that interfaces to the network [8, 9]. Experimental analyses have been carried out to specifically address the issue of delays in switched Ethernet [10, 4]. However, due to the relatively recent introduction of commercial devices that implement the new industrial Ethernet protocols, there have been only a few published accounts of their actual performance [11, 12].Over the past six months, our group at the University of Michigan has undertaken an industrial Ethernet testing project [13]. The goal of the project was to evaluate the suitability of real-time Ethernet for peer-to-peer communication between PLCs on a factory floor. The purpose of this paper is to summarize the results of our tests on PROFINET, and discuss our findings.The outline of the paper is as follows. In Section II, we summarize how PROFINET enables real-time communication over Ethernet. In Section III, we describe the tests that were performed. Section IV presents the results of those tests, and conclusions are given in Section V.2.PROFINET CBA with Real-Time Channel Communication PROFINET distinguishes two views: PROFINET IO for integration of distributed I/O and PROFINET CBA (Component Based Automation) for creation of peer-to-peer communication and interlocking between controllers in modular plants (Figure 1)All other PROFINET applications such as safety, motion control, and HMI (Human Machine Interface) are based on these communication modes. PROFINET communication is scalable in three levels: PROFINET TCP/IP Communication (NRT) enables cycle times as low as 100 ms, PROFINET Real-Time Communication (RT) enables cycle times up to 1-10 ms and Isochronous Real-Time Communication (IRT) enables cycle times up to 1 ms with Jitter less than 1µs.Component based communication is realized through PROFINET CBA which uses selectively the TCP/IP or the Real-Time (RT) channel. Communication for distributed I/O is implemented through PROFINET I/O which uses Real-Time and Isochronous Real-Time (IRT) communication.PROFINET Real-Time Channel The PROFINET Real Time Channel is a cyclic communication path used by individual stations to exchange time critical data at periodic intervals specified by the programmer. It is based on the IEEE and IEC definition s [14], which only permit a limited time for execution of Real-Time services within a bus cycle. Real-Time data are handled with higher priority than Non-Real-Time (NRT) data. The tightness of the window depends on the Real-Time characteristics. The Real-Time mechanism is based on Layer 2 of the OSI model and several protocol layers are omitted. Thus the communication overhead associated with preparing data, transferring it and making it available to the overlying application for use are reduced. Using Ethereal it was found that the total overhead associated with Cyclic Real Time communication is 56 bytes.3.Tests PerformedThe following tests were designed to measure the impact of system parameters on peer-to-peer interlocking performance using PROFINET CBA with RTcommunication method. The system parameters include data size and number of switches. The tests are vendor neutral so that any implementation can be configured to undergo each test. Connection failures or errors are not included in this test plan. To perform tests the following equipment was used: one computer with Matlab and the protocol analyzer Ethereal, SIMATIC iMap and STEP7 as configuration software, five switches from Hirschmann and two Siemens SIMATIC PLCs (Programmable Logic Controllers). The PLCs were configured using the factory defaults for processor and communication allocation options. The Hirschmann switches (100Mbps) were configured for port speed auto negotiation. Due to the fact that PROFINET is based on Unicast communication the Multic ast functionality was not configured in the switches.3.1 PerformanceMetricsThe performance metrics analyzed are PLC1 Packet Time Interval and Round Trip Time Interval.PLC1 Packet Time Interval is the time between two successive transmittals of packets from PLC1. Ideally, the PLC1 Packet Time Interval is always exactly the same as the configured update interval in the PLC. However, in practice there is some variability associated with this interval. The experimental results that follow summarize the average (mean value) and the jitter (standard deviation) of the PLC1 packet time interval. These metrics (mean and standard deviation) are important, as they give ameasure of the determinismthat can be obtained for realtime control using PROFINET.Round Trip Time Interval is defined as the Time Interval needed for a packet from PLC1 to reach PLC2, be echoed and come back to PLC1. Consider a test where PLC1 generates data and PLC2 echoes themback to PLC1 through a switch.Figure 2 shows the timing chart for the communication between PLC1 and PLC2 where PLC1 sends messages at T1, T2, T3,. . . and PLC2 echoes at t1, t2, t3,. . . . PLC1 Packet Time Interval should be equal to the configured update interval on PLC1, and PLC2 Packet Time Interval should be equal to configured update interval on PLC2. If the echo from PLC2 arrives before T2, then the round trip counter getsincremented and the new value is transmitted from PLC1 at T2. Since the increment of the round trip counter is taken for calculation of the Round Trip Time Interval, in this case it should be equal to the PLC1 Packet Time Interval. Consider the case when t1 shifts relative to T2. Then the echo fromPLC2 is received after T2, and the round trip counter is not incremented in themessage transmitted from PLC1 at T2. Hence, the Round Trip Time Interval becomes twice the PLC1 Packet Time Interval.Figure 2. Timing chartFrom the above observations it is noticed that Round Trip Time interval mean and standard deviation are also important as measures of the degree of synchronization for real-time control using PROFINET.3.2 Test DescriptionsTest1: Benchmark Test1 is the benchmark test. The other tests are compared to Test1. In this test PLC1 generates eight bytes of data and PLC2 echoes it back to PLC1 through a switch. PLC1 uses the last 4 bytes (dint) of the data for a new data received counter. PLC1 increments this counter as discussed in section 3.1.To perform measurements, a PC running Ethereal was connected to the managed switch which connects to the PLCs. All packets going to and from PLC2 and theirrespective timestamps were mirrored onto this port.Test2: Network Switches The objective of Test2 is to evaluate the impact that switches introduce to the system. The number of switches between two PLCs is the test variable. The same variables are measured as in Test1. We will consider the case of three and five switches between the PLCs.Test3: Size of Data The objective of Test3 is to measure the impact of data size on peer-to-peer communication performance. The test variable is the data size. Measurements are performed as described in Test1. We will consider two cases. In the first case 216 bytes of unused data, in the second 440 bytes of unused data.4.Test ResultsIn performing the tests and analyzing the results a data capture of 5000 packets per PLC is considered in order to assess the timing performance. The average and standard deviation values of PLC1 Packet Time Interval and Round Trip Time Interval are measured in milliseconds and rounded off to th ree significant digits after the decimal point. All tests are performed with an update time of 8ms which is typical for these applications in the factory. Figures 3, 4 and 5 show the benchmark test results, PLC1 Packet Time Interval, Round Trip Time Interval histogram, and Round Trip Time Interval scattering diagram respectively. We can notice the highly deterministic behavior of the network. Since we are using the PROFINET RT protocol a similar behavior is expected also from the other tests.Figure 3. Test1 PLC1 Packet Time Interval histogram4.1 Network SwitchesTo evaluate the impact that switches introduce to the system, data results from Test1 will be compared to those obtained from Test2. Tables 1 and 2 show that, in the case of three or five switche s between two PLCs, there are no significant changes between the two tests. PLCs Packet Time Interval and Round Trip Time Interval present the same average value and similar standard deviation. Figure 6 shows the histogram of round trip time interval for Test2 which is close to that of Test1 (Figure 4). As expected the switches do not alter the performance metrics. Similar resultswere found in [10].Figure 4. Test1 Round Trip Time Interval histogramFigure 5. Test1 Round Trip Time Interval scattering diagramFigure 6. Test2 Round Trip Time Interval histogram, case with 3 switches4.2 Size of DataBy comparing the results of Test1 and Test3 we will measure the impact of data size on peer-to-peer communication performance. As observed in Tables 1 and 2, PLC1 packet and Round Trip Time Interval average values are the same. In both PLC1 packet and Round Trip Time Intervals there is a decrease of value in standard deviation. Figure 7 shows the Round Trip Time Interval of Test3 with three switches which behaves like Test1 round trip interval (Figure 4). From the results obtained (Tables 1 and 2) we can conclude that data size does not impact Packet and RoundTrip Time Interval.Figure 7. Test3 Round Trip Time Interval histogram, case with 216 bytes 5.ConclusionsTo measure the impact of data size carried by a packet and switches on a PROFINET CBA with RT communication based network three tests were designed. Test1, represented by a simple network made of two PLCs and one switch, was used as benchmark. Figures 3, 4 and 5 showed the deterministic behavior of the network. Test2 is similar to Test1 but instead of one switch, three to five have been used. Test3 is also similar to Test1 but, instead of using 8 bytes data per packet, 216 and 440 bytes were used. To investigate the delay introduced by the switches Test1 and Test2 results were compared. The impact of data size was analyzed by comparing Test1 and Test3. Results show that PLC1 Packet Time Interval and Round Trip Time Interval are unaffected by data size per packet and number of switches. AcknowledgementsThis work was supported in part by the Engineering Research Center for Reconfigurable Manufacturing Systems of the National Science Foundation under Award Number EEC-9529125. The authors would also like to acknowledge the support received from General Motors Powertrain, Siemens and Hirschmann in thecompletion of the tests.References[1] J.-P. Thomesse, “Fieldbus Technology in Industrial Automation”, Proc. of theIEEE, vol. 93, no. 6, 2005.[2] J. M oyne and F. Lian, “Design considerations for a sensor bus system insemiconductor manufacturing”, in International SEMATECH AEC/APC Workshop XII, 2000.[3] P. G. Otanez, J. T. Parrott, J. R.Moyne, and D. M. Tilbury, “The Implications ofEthernet as a Co ntrol Network”, in Proc. of the Global Powertrain Congress, 2002.[4] K. C. Lee and S. Lee, “Performance evaluation of switched Ethernet fornetworked control systems”, in Proc. of IEEE Conf. of the Industrial Electronics Society, volume 4, 2002.[5] J.-D. 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