Fieldbus

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A field bus is a bus system that connects field devices such as measuring probes ( sensors ) and actuators ( actuators ) in a plant for the purpose of communication with an automation device . If several communication participants send their messages over the same line, it must be specified who (identifier) ​​says what (measured value, command) when (initiative). There are standardized protocols for this .

The first generation of fieldbus technology was developed in the 1980s in order to replace the parallel wiring of binary signals and analog signal transmission with digital transmission technology, which was common up until then . Today there are many different fieldbus systems with different properties established on the market. Fieldbuses have been standardized worldwide in the IEC 61158 standard ( Digital data communication for measurement and control - Fieldbus for use in industrial control systems ) since 1999 . The second generation of fieldbus technology is based on real-time Ethernet .

description

For the regulation or control of a system more are to many sensors and actuators necessary.

If the automation is electrical, the question arises as to how the sensors and actuators should be connected to the automation device. Two basic variants are possible:

  1. A pair of cables is drawn from the automation device to each sensor and actuator (parallel wiring, star topology).
  2. Only a single pair of cables is drawn from the automation device: The pair of cables is routed to each sensor and actuator (serial wiring, bus topology).

With the increasing degree of automation of a system or machine, the cabling effort increases with parallel wiring due to the larger number of input / output points. This is associated with great effort in planning , installation, commissioning and maintenance.

The demands on the cables are often high, e.g. B. special cables must be used for the transmission of analog values.

The parallel field wiring becomes a serious cost and time factor in automation technology . In comparison to this, the serial networking of the components in the field area using so-called field bus systems is much cheaper.

The fieldbus replaces the parallel bundle of cables with a single bus cable and connects all levels, from the field to the control level. Regardless of the type of automation device, e.g. B. Programmable logic controllers (PLC) from different manufacturers or PC-based controllers, the transmission medium of the fieldbus networks the components in the field.

A bus interface card is used instead of several I / O cards . This reduces the space required in the control cabinet .

advantages

The advantages of a field bus:

  • less cabling saves time in planning and installation
  • Cables, distribution boxes and the size of the control cabinet are reduced
  • Self-diagnosis by the system possible
  • Higher reliability and better availability thanks to short signal paths
  • Protection against interference increases, especially with analog values.
  • Open fieldbuses standardize data transmission and device connection across manufacturers. Components from different manufacturers are easier to exchange, at least with regard to basic communication.
  • Extensions or changes are easy to carry out and guarantee flexibility and thus future security.
  • It is not necessary to define measuring ranges for transmitters. The (visual) display scale in the control system can be changed at any time.

disadvantage

The disadvantages of a fieldbus:

  • more complex system - more qualified employees necessary
  • higher price of components with fieldbus functionality
  • complex measuring devices
  • slightly longer response time
  • The smallest exchangeable unit becomes more expensive.
  • Due to the large number of different field buses, sensor / actuator manufacturers are forced to support several field buses, which causes additional costs. In addition, it is very difficult to predict which fieldbuses will gain or lose importance in the future. As a result, there is a risk of bad investments in the development of fieldbus connections.
  • Due to the central connection principle, the control system can be cut off from all sensors and actuators in the event of a bus fault. Therefore redundant bus systems may be required.

Different topologies

Tree topology or extended star
Bus topology
Ring topology
Star topology

standardization

Fieldbuses for industrial applications have been standardized worldwide in the IEC 61158 standard ( Digital data communication for measurement and control - Fieldbus for use in industrial control systems ) since 1999 . The individual field buses are listed in the IEC 61784-1 standard as Communication Profile Families (CPF). The new real-time capable Ethernet- based fieldbuses are compiled in the IEC 61784-2 standard. Each protocol family can define further fieldbuses. The following protocol families are listed in the standard:

family version Brand name
CPF1 FOUNDATION Fieldbus (FF)
CPF1 / 1 FF-H1 (Low Speed)
CPF1 / 2 FF-HSE (High Speed ​​Ethernet)
CPF1 / 3 FF-H2 (high speed)
CPF2 CIP ( Common Industrial Protocol )
CPF2 / 1 ControlNet
CPF2 / 2 EtherNet / IP
CPF2 / 3 DeviceNet
CPF3 PROFIBUS and PROFINET
CPF3 / 1 PROFIBUS DP
CPF3 / 2 PROFIBUS PA
CPF3 / 3 PROFINET CBA
CPF3 / 4 PROFINET IO Conformance Class A
CPF3 / 5 PROFINET IO Conformance Class B
CPF3 / 6 PROFINET IO Conformance Class C
CPF4 P-NET
CPF5 WorldFIP
CPF6 INTERBUS
CPF7 SwiftNet
CPF8 CC-Link
CPF9 HARD
CPF10 VNET / IP
CPF11 TCnet
CPF12 EtherCAT
CPF13 Ethernet POWERLINK
CPF14 EPA (Ethernet for Plant Automation)
CPF15 Modbus
CPF15 / 1 Modbus / TCP
CPF15 / 2 RTPS
CPF16 SERCOS
CPF16 / 1 SERCOS I
CPF16 / 2 SERCOS II
CPF16 / 3 SERCOS III
CPF17 RAPIEnet
CPF18 SafetyNet p
CPF19 MECHATROLINK

Common field buses

Safety properties of field buses

If field bus systems are to be used in systems that have to withstand a test according to safety standards such as IEC 61508 or EN 954-1 , the bus system has to meet certain special requirements. These requirements are satisfied, for example, through the redundant structure of software and hardware in the end devices and, depending on the bus protocol, measures such as running counters, CRCs , acknowledgments, timeouts , identifiers for sender and receiver or redundancy with cross-comparison. See also safety requirement level , safety concept , safety . The internationally valid international standard IEC 61784-3: Industrial communication networks - Profiles - Part 3: Functional safety fieldbuses defines nine different protocols for safety-related fieldbuses in the CPF 1, 2, 3, 6, 8, 12, 13 and 14 families .

Another safety function of field buses relates to occupational safety if people are threatened by dangerous movements. For this purpose, emergency stop actuations, locking of safety doors for machines and robots, light grids and light curtains as well as optical scanners and the like are used. a. deployed and networked. Such facilities are subject to acceptance (e.g. Institute for Occupational Safety and Health of the German Social Accident Insurance in St. Augustin and TÜV ). The realization takes place without additional conventional wiring increasingly through innovative fieldbus solutions that do without the redundancy concepts mentioned above and can be implemented on a normal sensor-actuator fieldbus. More information about a system with "Safety at Work" and its creation can be found in the articles AS-Interface , Horst Saalbach and Werner Kriesel .

literature

  • Udo Enste, Jochen Müller: Data communication in the process industry . Oldenbourg Industrieverlag, Munich 2007, ISBN 978-3-8356-3116-8 .
  • NP Mahalik: Fieldbus Technology . Springer Verlag, Berlin 2003, ISBN 978-3-540-40183-4 .
  • Gerhard Schnell: Bus systems in automation and process technology . Basics, systems and trends in industrial communication. Ed .: Bernhard Wiedemann. 7th edition. Vieweg + Teubner Verlag, Wiesbaden 2008, ISBN 978-3-8348-0425-9 .
  • Werner Kriesel , Tilo Heimbold , Dietmar Telschow: Bus technologies for automation . Networking, selection and application of communication systems. 2nd Edition. Hüthig, Heidelberg 2000, ISBN 3-7785-2778-9 .
  • Michael Lupik: Bus systems in automation and process technology . Basics, systems and trends in industrial communication. Ed .: Gerhard Schnell. 5th edition. Vieweg, Braunschweig, Wiesbaden 2003, ISBN 3-528-46569-7 .
  • Jürgen Jasperneite: Real-time Ethernet at a glance In: Automation technology practice (atp) , No. 3, 2005, pp. 29–34, ISSN  0178-2320
  • Frithjof Klasen, Volker Oestreich, Michael Volz (eds.): Industrial communication with fieldbus and Ethernet . VDE Verlag, Berlin, Offenbach 2010, ISBN 978-3-8007-3297-5 .
  • Werner Zimmermann, Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition. Springer Vieweg, 2014, ISBN 978-3-658-02418-5 .
  • Werner Kriesel , Frank Sokollik , Peter Helm and Ralph Seela: KNX / EIB for building system technology in residential and functional buildings. Hüthig Jehle Rehm Verlag, Heidelberg, 5th edition 2009, ISBN 978-3-7785-4054-1 .