INTEGRATION OF FIELDBUS SYSTEMS AND TELECOMMUNICATION SYSTEMS IN THE FIELD OF INDUSTRIAL AUTOMATION
Peter Neumann ifak Institut fuer Automation und Kommunikation Magdeburg Steinfeldstrasse 3 D-39179 Barleben Phone: +49 39203 81010 Fax: +49 39203 81100 e-mail:
[email protected] http://www.ifak.fhg.de
Abstract: In the last 20 years, the digital communications within both the industrial automation and office automation influenced, to a large extent, the architecture and components of the information systems. Nowadays, the digital communication solutions of these different application fields are merging. Thus, the requirements, features and use conditions of the digital communication systems within these fields are particularly under consideration. This report deals with recent activities in the field of industrial communications (using radio technology, switched Ethernet, saving hard real-time behaviour) and with additional application functions (object-oriented approaches, auto-configuration management, web-based management) to improve the run-time behaviour and the engineering overhead.
Keywords: Communications; Distributed Computer Control Systems; Fieldbus; Networks; Protocols; Telecommunication;
1.
INTRODUCTION
Digital communications have been important driving forces of the computer control systems for the last 25 years. In particularly over the last 15 years, fieldbus systems required a lot of work in the area of definition, specification, implementation and market dissemination (IEC 2001a, IEC 2001 b). Nowadays, the fieldbus systems and the Ethernet-TCP/IP-oriented local area networks are standardised and widely used within the industrial automation and the office automation. To realise the access to data in various layers of an enterprise information system by different users there is a need to merge the different digital communication systems within the plant level, control level, and device level of an enterprise network. On these different levels, there are different requirements (figure 1) dictated by the nature and type of information being exchanged. Network physical size, the number of devices supported, network speed, response time, frequency of exchange and payload size are a few of
the performance characteristics used to classify and group specific network technologies. Response Time min
Payload Size
Frequency
MBytes
rare
Plant
Control
Device ms
Bits
often
Figure 1. Layer Model of Factory Communication Plant level networks span an entire production facility. They interconnect large numbers of computer systems supporting office, engineering and production applications. Today Ethernet network supporting the TCP/IP Suite of networking protocols are used at this level.
Control level networks interconnect equipment controllers. A high speed, reliable, and constant exchange of time-sensitive control signal information is needed. Control level networks typically interconnect moderate numbers of specialised computing devices over a moderately large area. Device level networks tend to be smaller, moderate speed networks which interconnect large numbers of limited function devices operating in the harsh plant floor environment. Their primary purpose is to communicate limited amounts of sensor and actuator data in a reliable, timely and cost-effective fashion. The real-time requirements depend on the type of messages to be transmitted: meeting deadlines for data transmission, restricting jitter for audio and video stream transmission. Otherwise, the offered resources at the various network levels are different. At the device level, there are extremely limited resources (hardware, communications), but at the plant level there are powerful computers allowing comfortable software and memory consumption. Another requirement is the local flexibility of equipment / devices. One can distinguish a device with a flexible location but fixed during the operation and a mobile device. In both cases a radio-based communication network is suitable. In some application scenarios, safe communication networks are necessary. There are two main solutions to improve safety: measures within the protocol stack (provided by the majority of fieldbus systems but not adequate); measures within the applications (additional parts of PDU containing safety-relevant code), e. g. PROFISafe (PNO 1999). For some applications, fast and strictly synchronous communication systems are indispensable, especially within the motion control area. There are time-triggered solutions, e.g. TTP, Interbus, hardware-based synchronisation application-triggered solutions, e.g. PROFIBusMotion Control (PNO 1999) combined with a high baud rate of the fieldbus system. Additional requirements are caused by the need for effective engineering of distributed multi-vendor control systems. The remote access of special information systems (e. g. commissioning tools, diagnosis applications, maintenance tools) to contained parameters of Function Blocks and/ or
devices and their partial setting (parameterisation) require special features within the devices and network components (gateways, routers, link devices, segment couplers, etc.) and special security mechanisms. Otherwise, the cost-intensive commissioning of industrial communication networks has to be optimised by auto configuration mechanisms. The following section deals with recent activities in some of the fields mentioned to meet the requirements.
2.
SOME RECENT ACTIVITIES IN INDUSTRIAL COMMUNICATIONS
2.1 Completion of the fieldbus transmission technologies by radio technology Twenty years ago, industrial communication systems has been based on (often even doubled) shielded coaxial cable. Ten years ago, coaxial cables were replaced by shielded twisted pair cables. Now we are about to replace these cables by radio techniques. This is motivated by the use of mobile equipment (carriages, shelf storage, cranes), contactless applications (any equipment on wheels) or by simple cable replacement needs. What are the requirements to be fulfilled? real-time communication (equal or more than 2 Mbit/s) integration into existing network architecture guarantee that no message is lost mobile applications (speed of equal or smaller than 20 km/h) handoff (handover) functionality (in equal or smaller than 2 sec) without disconnecting the communication industrial multimedia communication security (encryption & authentication) safety applications (emergency stop) explosion protection low power solutions mid range area (100 m x 100 m) with 30 devices in maximum short range area (10 m x 10 m) with 10 devices in maximum remote connection up to 1000 m control of multipath propagation behaviour and harsh noise environment network configuration support (ad hoc network support, plug & play), it means the radio
communication system has to support the automatic device recognition and accessibility. To fulfil these requirements, the following objectives have to be reached: specification and validation of radio technologies for usage in industrial communication systems replacing of wire-line with a similar reliability and availability by radio in mobile or standard applications system planning of a whole radio network. Haehniche (2001) describes the available radio transmission solutions and compares them from the point of view of their suitability in industrial communications. See also Wireless LAN (1999), HIPERLAN (1996), HIPERLAN (2000), Bluetooth (1999), RFieldbus (1999), Mock et al. (2000), Bilstrup & Wilberg (2000), Hähniche et al. (2001), Hähniche (2001), Rauchhaupt (1999). Haehniche and Rauchhaupt (2000) reported about the first measurement results within the European RFieldbus project using existing radio components including the actual principles to be implemented within the UMTS technology. The results were very optimistic, therefore one can conclude that the above mentioned requirements can be met, see also Coston et al (2000). What are the consequences for the network architecture and the protocol stack of the field devices?
M3 LS1
S1
S2
LS2
S4
M1
BS1
M2 S3
S5 S6
LS3
M : Master
LS : Link Station
L B S : Linking Base Station
S : Slave
BS : Base Station
B M : Beacon Master
BM
LBS1 S7
M4
Figure 2. Radio Fieldbus Network Architecture Figure 2 depicts a typical mixed wired and wireless network architecture. This network is a mixture of wired parts (given fieldbus segments) and wireless device clusters. This system works as an homogeneous fieldbus system. Figure 3 shows the needed protocol suite consisting of traditional wired fieldbus functions and additional
radio-based functions, multimedia-related management-related functions. Multi Media Application (Client or Server) PROFIBUS-DP Application (Master or Slave)
and
Systems Management Application
TCP/IP Stack IP Mapper
PROFIBUS-DPV1 AL IP ACS DP Mapper Station Management DP/IP Dispatcher PROFIBUS DL Data Communication Equipment Independent Sublayer (DIS) Data Communication Equipment (DCE) ACS Admission Control and Scheduling DLX Data Link Layer Extension TCP Transmission Control Protocol
DLX Data Communication Equipment Independent Sublayer (DIS) Data Communication Equipment (DCE) AL Application Layer DP Decentralized Periphery
DL Data Link Layer IP Internet Protocol
Figure 3. Protocol Suite of a Radio-based Field Device 2.2 Using switched Ethernet and TCP/IPoriented protocol stack In the past, the main disadvantage of Ethernet according to the real-time behaviour was the random CSMA/CD bus arbitration. To eliminate that drawback, the latest Ethernet switch technology can be used instead of a hub-based infrastructure. Switch technology divides collision domains into simple point-to-point connections between network components and stations. Collisions no longer occur and the random back off algorithm is no longer required. The concept of switching or of media access control bridging, which was introduced in IEEE 802.1 in 1993, was expanded upon in 1998 by the definition of additional capabilities in bridged LAN’s (see VBLAN 1998, MAC Bridges 1998). The additional traffic capabilities support the transmission of timecritical information in a LAN environment. The standard describes also the prioritisation of MAC frames. The up to 8 available output queues per port inside the switch are called traffic classes. The scheduling strategy inside the switch (in case of more than one queue per port) is not determined by the standard. By this reason, it is necessary to investigate the different scheduling algorithms, e.g. first-comefirst-served (FCFS) and non- pre-emptive priority queuing (PQ). The PQ scheme assigns a strict priority to important traffic. It flexibly prioritises traffic according to network protocol, the incoming interface, the packet size, the source/ destination address, and so on. It is suitable to assign the priority level according to the different message types (or applications).
Jasperneite (2001) investigates the real-time behaviour of a switched Ethernet network used for a distributed computer control system, see also Jasperneite and Neumann (2000), Heger and Watson (1982), Cseh at al (1999), Cseh and Jasperneite (1998), Torab (2000), Chitra Venkatramani (1996). The investigations show the influence of the topology (star topology, bus-like topology) and of the scheduling algorithm for 4 different message types: MT1 - cyclic send and request process data objects with reply (confirmed service): 80 % of frames (short), priority 5 MT2 - acyclic transmission of events or process data objects (e. g. alarms, event notification) (unconfirmed service): 10 % of frames (short), priority 6 MT3 –acyclic transmission for network control (confirmed service): 9,5 % of frames (medium) highest priority 7 MT4 – acyclic transmission of file or program segments (confirmed service): 0,5 % of frames (long for response) priority 4. Star Topology
Transaction Time [s]
1,E-03
MT 4 PRIQ FCFS 1,E-04
MT 3 MT 1 MT 2 1,E-05 0
0,2
0,4
0,6
0,8
1
1,2
Load
Figure 4. Transaction Time of the four Message Types as a Function of the Resulting Load in a Star Topology Bus-like Topology
Figure 5. Transaction Time of the four Message Types as a Function of the Resulting Load in a Bus-like Topology Figures 4 and 5 depict the transaction times of the 4 message types as a function of the resulting load in a star topology or bus-like topology respectively. Up to a load of 50 %, the results of the corresponding mean values of TRA (response time, round-trip-time, consisting of client-server TETE and back time as well as the thinking time TTH, for confirmed services) and of TETE (end-to-end delay, one-way delay for unconfirmed services) respectively for both packet scheduling strategies within a topology are almost identical. For higher load, the higher prioritised message types MT2 and MT3 are not influenced by any load enlargement. In the case of loads larger than 100 %, MT4 with its large payload is no longer served. Thus, a message with a high priority seldom needs to wait until a message with a lower priority has been served. Under the same load conditions, the transaction times in a bus-like topology are larger than that in a star topology to more than an order of magnitude. This is due to the much larger number of traversed switches in the bus-like topology for the message. It is not surprising that the star topology is the best choice, but causes the highest costs. The bus-like topology meets also the present real-time requirements, but can use existing cable installation. Generally, one can say that nowadays the dwell time of today’s control devices are the significant part of overall transaction time. This “technological gap” between the properties of the network and the devices can be diminished or even eliminated by optimising the protocol stack and the hardware (e. g. processor) performance. Summarising one can conclude that the switched Ethernet technology can be used for real-time data transmission in many use cases within a shop floor automation.
1,E-02
Transaction Time [s]
MT 4
M T3
PRIQ
1,E-03
FCFS
M T1
M T2
1,E-04 0
0,2
0,4
0,6
Load
0,8
1
1,2
2.3 Meeting hard real-time requirements using combined stacks with Ethernet, TCP/IP and fieldbus functions A lot of recent activities are directed to the usage of Ethernet and TCP/IP (or UDP) by special application layers within the automation domain, see Kweon et al (1999), Furrer (1998). Thus, the Fieldbus Foundation specified the functionality HSE, the ODVA developed the Control and Information Protocol CIP, both mapped to the UDP/TCP sublayer, see Van Gompel (2001), IEC (2001a), IEC (2001b). CIP provides (soft)
real-time and peer-to-peer messaging. CIP can also be used by ControlNet and DeviceNet networks. Another approach is the usage of certain middleware concepts mapping special application layer services to TCP/ UDP. Thus, PROFInet uses DCOM concept from Microsoft (see PROFInet 2001, Biehler et al 2001, Feld et al 2001) while “Interface for Distributed Automation” IDA uses NDDS from RTI (see IDA 2001). The main problem in the automation domain is to meet the real-time requirements. Real-time means seconds for archival, download etc. 100 milliseconds for visualisation etc. 10 milliseconds for I/O data flow (cycles) < 5 milliseconds for synchronisation cycles < 20 microseconds for jitter within a cycle. To meet these requirements, a mixture of proven technologies is needed. To combine the advantages of both the fieldbus systems (real-time behaviour) and the Internet technology using TCP/IP or UDP protocol suite (remote web-based access to process data and device parameters) there are various opportunities: IP tunneling via fieldbus systems integrated services for IP and hard real-time.
requirements is shown in figure 7, see Poeschmann (1999).
Office Standard FTP, HTTP, ...
TCP
Automation Standard (PROFINet , IDA, ...)
TCP
RT- I/O image via Fieldbus AL
UDP
IP
FAL
Open IP Tunneling API
UDP
RT- I/O image via Fieldbus AL
PEAC- Socket API
IP Predictable Ethernet Access Control (PEAC) Ethernet NIC
Figure 7. Integration of Hard Real-time Mechanisms into Ethernet-TCP/IP Protocol Suite A special Predictable Ethernet Access Control PEAC controls the access of TCP (UDP)/IP-oriented services (soft real-time, non real-time) via Berkeley sockets on the one hand and of Fieldbus AL Services (hard realtime) via PEAC sockets on the other hand. The PEAC function inserts the requests into scheduled slots. The scheduling strategy influences the time behaviour of the supported services (see also section 2.2)
3. Office Standard FTP, HTTP, ...
Automation Standard (PROFINet , IDA ...)
ADDITIONAL APPLICATION FUNCTIONS TO GAIN THE COMFORT OF NETWORKS
The first phase introducing industrial communications into practice was determined by the definition, specification, implementation and market dissemination of various network concepts (Fieldbus Systems, Ethernet-TCP/IP-oriented networks).
Fieldbus IEC61158 NIC
Figure 6. Integration of a Tunneling Concept into a Control Device´s Protocol Suite Figure 6 depicts the integration of a tunneling concept into a control device’s protocol suite. An additional router (separated or integrated in a control device) connects the fieldbus transmission and the Ethernetbased transmission. The open IP Tunneling API mentioned in figure 6 is part of a network device driver within the implementation architecture of a device. The other opportunity to meet (hard) real-time
The second phase focusing on computer-aided engineering is characterised by specification and implementation of additional application functions to gain the comfort of the network installation, commissioning and maintenance. Many installation and configuration tools are on the market. These tools mostly support the process of installation and commissioning. Nowadays, some activities are directed to saving manpower within the mentioned areas. The following section deals with two important problems: Auto configuration management to gain the commissioning and the restart of fieldbus systems web-based management of networks
3.1
Auto Configuration Management
To operate an industrial communication network, some management domains have to be considered (Figure 8). The Auto Configuration Management (ACFG) domain is discussed in more detail. Organisational Aspects Management Domains
Configuration Mgt
Notification of communication faults
Passive/ Active Device Data
Active Device
Network Management : Starting the network Detection of equipment on the network Communication Configuration Up-/Download System Management: Identification
Dependability/Fault Mgt
General Approach: central Manager - Agent principle (redundancy of the central manager is possible)
Remote ACFG manager role
System MIB
ACFG management agent agent role role Fieldbus Communi cations
Device MIB
Communication objects
Notification Operations
other devices ...
Fieldbus Auto Configuration Management (ACFG Management) Security Mgt
Read and modify device status from the system standpoint, maintain system global state, strategy
Parameter Optimisation (optional)
Accounting Mgt.
Performance Mgt.
Systems Integrity
Figure 8. Management Domains The system behaviour can be modelled as shown in figure 9. The main states are: Power-Down (system switched off), System Setup (installation/ first commissioning), Operation (normal operation mode), and Restart (repeated switch on with existing configuration). PON means Power On. System Setup m ste N Sy P O alid a t i o n v r no f i g u n Co Normal Check OK and / or Human OK
Power-Down
System Reset or non compatible Strategy arrived
Operation Power Down
PO va N l ex i d S ist y ing s t e m
Dynamic System Expansion or compatible new Strategy
Fast Check OK, Human OK
Co
nf
ig
ur
at
io
n
Restart
Figure 9 System Behaviour Model
Figure 10 depicts the general ACFG Management model.
Figure 10. General Auto Configuration Management Model Mandatory ACFG functions are ACFG Network Strategy Function: control and observation of the system’s state; interface to the user programmes ACFG Network Enumeration Function: automatic baud rate adjusting; address assignment and management of address conflicts ACFG Knowledge Discovery Function: acquisition of device information (formal described, see Simon and Demartini 1999, PNO 2000a); device identification. Optional ACFG functions are ACFG Performance Enhancement Function ACFG Manager Role Election Function. Using these functions, the following features are possible for PROFIBUS: Fully compatible to existing devices and plants actual system state indication in all devices with ACFG functions integration of the ACFG functions only by software update address conflict management in devices with ACFG functions without address area reduction. The ACFG mechanism accepts given addresses of devices without ACFG functions. According to PROFIBUS, the following add-ons within the devices are needed (figure 11 for Master class 1, figure 12 for Slave with DPV1 functions).
Data transfer in Operational Phase
DPV1 ACFG MANAGER
Application Strategie, Rückmeldung der Teilnehmer
Data Transport Service DPV1 ACFG Knowledge Discovery Function DPV1 Master Class 2 MSAC2M
System MIB Http Tunnelling (optional)
DPV1 ACFG Network Performance Optimisation Function MSAC2S MSRM2S
Master Class 1 DP or DPV1 USIF und DDLM
DPV1 ACFG Network Strategy Function
DPV1 ACFG Network Enumeration Function
FDL / FMA 1/2
Figure 11. ACFG Management Architecture in PROFIBUS Master Class 1 Data transfer in Operational Phase
DPV1 ACFG AGENT
Application Identification of linked Periphery
Data Transport Service
DPV1 Slave USIF, MSAC1S, Alarm and DDLM
Embedded WWW Server
Device MIB
DPV1 ACFG Knowledge Discovery Function
MSAC2S MSRM2S
DPV1 ACFG Network Enumeration Function
FDL / FMA 1/2
Figure 12. ACFG Management Architecture in PROFIBUS-DPV1 Slaves Using these extended functions for the commissioning (or restart) of installed PROFIBUS networks, the commissioning obtains more and more plug & play features. The drawback is that the assortment of devices (especially of the market leader) on the market have to extend their functionality by the ACFG Management functions. 3.2 Web-based management of industrial communication networks Industrial communication networks pass the phases engineering, configuration and implementation, operation and service, diagnosis and maintenance. The different phases can be characterised by different tasks that have to be performed. These tasks are influenced by the capabilities which Web technologies are providing. A direct influence means that technologies such as HTTP/HTML are used as an integrated part of an application. An indirect influence is to use technologies like XML which are connected with the Web technology. Web-based
management means that management-relevant information from the devices (from the system) have to be mapped to interactive web-pages acting as frontends for the management client. The client itself can be a browser combined with some specific code. Usually a web server is used for an integrating instance implementing the mapping of the fieldbusrelated data, and web-related information, and the storage of the data. Web integration can be performed using a framework which implements a set of generic interfaces to applications and technologies. Important is a generic description of data definitions, using XML, and interaction schemes between applications describing a content model with structure and access methods. Wollschlaeger (1999), Wollschlaeger (1999a), Wollschlaeger (2000), Wollschlaeger (2001), Wollschlaeger et al (2001) and Bangemann (2001) offer a deeper view to the framework and experiences made by the European Project NOAH, see also Doebrich et al (1999), Moss (1998), Korsakas and Moyne (2000).
4.
CONCLUSION
Wired fieldbus systems, wireless public telecommunication systems, and Ethernet-TCP/IPbased local area and wide area networks have been successfully introduced into the market.. Their specific domains have continued to increase. Thus, an overlapping of these domains occurred in the past. Nowadays, this overlapping can be used to develop hierarchical enterprise information systems with integrated communication services. However, one of the most important aims is to meet the domain-specific requirements, e. g. real-time behaviour, local flexibility, remote data access, usage of web-based technologies. Thus, it is a logical decision to use the radio technology completing the wired fieldbus transmission technologies, to use the fast Ethernet technology within an industrial communications system meeting hard and soft real-time requirements by eventually used additional functions of the fieldbus technology. There is however a wide span between the developed solutions in the field of fieldbuses and used local area network technologies. To reduce this span, one can merge Ethernet-TCP/IP oriented approaches (PROFInet, IDA, CIP) and fieldbuses using proxy concepts or one can develop a special mechanism to exchange data between different hierarchical systems in both directions using the OPCDX that will be specified and implemented (prototype) by the end of this year. However, all combined industrial communication networks have to be engineered, installed, configured, operated and
maintained. Thus, auto configuration management and other web-based management approaches are needed. There is still a lot to do in this area accompanied by excellent research work.
5.
Döbrich, U. and P. Noury (1999). ESPRIT Project NOAH. Introduction. in D. Dietrich, P. Neumann, H. Schweinzer (Eds.). Fieldbus Technology. Proc of FET’99 Magdeburg, Springer Wien, New York, pp. 414-422
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