Fieldbus

A fieldbus is a bus system in a plant field devices such as sensors ( sensors) and actuators (actuators ) to a control device for the purpose of communication. If multiple communication devices transmit their messages over the same line, then you must define who does what (measured value, command ) when ( Initiative) says ( ID). For this, there are standardized protocols.

The first generation of Fieldbus technology was developed in the 1980s to replace the hitherto conventional parallel wiring of binary signals and the analog signals via digital transmission technology. Today, many different field bus systems with different properties are established in the market. Since 1999, field buses in the IEC 61158 standard (Digital data communication for measurement and control - Fieldbus for use in industrial control systems ) are standardized worldwide. The second generation of the fieldbus technology is based on real-time Ethernet.

Description

Usually several sensors and actuators are required for the control of a system. If the control is electric, the question arises how the sensors and actuators to be connected to the control unit. Two basic options are possible:

With increasing degree of automation of a plant or machine wiring with parallel wiring increases due to the larger number of Ein-/Ausgabepunkte. This is a huge hassle for design, installation, commissioning and maintenance.

The requirements for the cables are often high, eg, special lines for the transmission of analog values ​​must be used. Thus, the parallel field wiring is at a serious cost and time factor in automation technology. In comparison, the serial networking components in the field area using the so-called field bus is much cheaper.

The fieldbus replaces the parallel line bundle by a single bus cable and connects all levels, from the field level to the control level. Regardless of the type of the automation equipment, such as programmable logic controllers ( PLC) from different manufacturers, or PC-based controllers, the transmission medium of the field bus networks the components in the field. Instead of several I / O boards a bus interface card is inserted. Thereby, the space requirement is reduced in the control cabinet.

Benefits

The advantages of a fieldbus compared to the parallel wiring:

• Lower cabling saves time during planning and installation
• Cable, Patch Panels and dimensions of the cabinet are reduced
• Self-diagnosis by the possible system
• Higher reliability and higher availability through short signal paths
• Especially with analog values ​​increase the protection against interference.
• Open fieldbuses unify vendor-independent data transfer and device connection. Components from different manufacturers are easily interchangeable at least in terms of basic communication.
• Extensions or alterations are easy to perform and guarantee flexibility and thus future-proofing.
• The setting ranges for transmitters is not required. The ( visual ) display scale in the control system can be changed any time.

Disadvantages

The disadvantages of a fieldbus compared to the parallel wiring:

• Complex system - more qualified staff necessary
• Higher price of components with fieldbus functionality
• Complex instruments
• Somewhat longer reaction time
• The smallest replaceable unit is more expensive.
• Due to the variety of fieldbuses Sensor-/Aktor-Hersteller are forced to support multiple fieldbuses, which results in additional costs. Moreover, predicting which fieldbuses will gain or lose importance in the future, very difficult. There is the risk of bad investments in the development of field bus connections.
• The central principle of connection may be cut off at a bus fault, the control system of all sensors and actuators. Therefore, possibly redundant bus systems are required.

Fieldbus topologies

Standardization

Since 1999 fieldbuses for industrial applications in the IEC 61158 standard (Digital data communication for measurement and control - Fieldbus for use in industrial control systems ) are standardized worldwide. The individual field buses will be performed in the standard IEC 61784-1 as a Communication Profile Families (CPF ). The new real-time Ethernet -based fieldbuses are presented in the standard IEC 61784-2. Each protocol family can define other field buses. The following protocol families are listed in the standard:

Common fieldbuses

• ARCNET Deterministic, of real-time field bus, used in the automotive, industrial automation (especially printing machines) and medical
• ARINC 629 avionics bus Fast, the company Arinc, used in the Boeing 777
• AS- Interface for connection of sensors and actuators
• BACnet Building Automation and Control Networks used for building control, but also partly down to the field level
• BITBUS
• CAN, for example, in the automotive sector
• CANopen ( CAN-based, higher-level protocol ) standard for elevator technology, automation technology, vehicle bodies, medical, marine electronics CAN in Automation ( CiA)
• ControlNet
• DALI for lighting in building automation
• DeviceNet ( CAN-based, higher-level protocol )
• EIB European Installation Mainly domestic installation, predecessor of KNX
• EtherCAT Ethernet -based fieldbus in automation technology
• Ethernet Powerlink Ethernet -based fieldbus for machine and plant construction
• EtherNet / IP ( Ethernet-based, higher-level protocol ), at least the former, especially in the United States
• FAIS bus, a Japanese standard fieldbus
• Foundation Fieldbus ( FF ) of the Fieldbus Foundation (Process Automation)
• FIP bus, French and Italian fieldbus standard competitor to Profibus
• FlexRay bus in the automotive sector (X -by-wire )
• HART Communication for industrial field devices
• INTERBUS engineering, plant construction in special design for safety technology
• KNX standard for building automation, successor of EIB
• LCN Local Control Network Universal Building Management System
• LIN bus in the automotive sector
• Loconet for model railways
• LON mainly for building automation
• M-Bus ( Fieldbus )
• MIL -STD -1553 mainly in military aviation
• Modbus industry
• MOST bus in the automotive multimedia area
• P -NET The P -NET fieldbus
• PROFIBUS (variants: DP & PA), PROFINET: robot, mechanical engineering, plant engineering, process automation
• SafetyBUS p safety-critical applications
• SERCOS interface motion control, CNC, robotics, mechanical engineering, plant
• SmallCAN Integrative low-cost/low-power system, mainly for building automation (as well as general automation technology)
• SMI Standard Motor Interface for control of electronic drives, eg for blinds or shutters
• SpaceWire
• T bus used mainly in agriculture, irrigation, technology and environmental monitoring
• Time-Triggered Protocol ( TTP)
• VARAN Ethernet -based fieldbus for automation of machinery and equipment

Security properties of fieldbuses

If field bus systems are used in systems that must withstand a test according to safety standards such as IEC 61508 or EN 954-1, are imposed on the bus system some special requirements. These requirements are satisfied for instance by the redundant design of software and hardware of the devices and, depending on the bus protocol measures such as running counter, CRCs, acknowledgments, timeouts, identifiers for the sender and receiver or redundancy with cross comparison. See also safety integrity level, safety concept, safety. In the current international international standard IEC 61784-3: Industrial communication networks - Profiles - Part 3: Functional safety Fieldbuses nine different protocols for safety fieldbuses the CPF families are defined 1,2,3,6,8,12,13 and 14.

Another safety feature of fieldbuses refers to the work safety, if people are threatened by dangerous movements. For this emergency- operations, locks security doors for machines and robots, light grids and light curtains and optical scanners are used among other things and networked. Such devices are subject to scrutineering (eg Institute for Occupational Safety and Health of the German Social Accident Insurance in St. Augustine and TUV ). Implementation is under increasing need for additional conventional wiring through innovative fieldbus solutions that do not require above-mentioned redundancy concepts and on a normal sensor-actuator fieldbus can be realized. For more information on a system with " Safety at Work" and its formation can be found in Articles AS-Interface, Horst Saalbach and Werner Kriesel.