Fieldbus

Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, standardized as IEC 61158.

A complex automated industrial system — such as manufacturing assembly line — usually needs a distributed control system—an organized hierarchy of controller systems—to function. In this hierarchy, there is usually a Human Machine Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). At the bottom of the control chain is the fieldbus that links the PLCs to the components that actually do the work, such as sensors, actuators, electric motors, console lights, switches, valves and contactors.

Description

Fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments in a manufacturing plant. Fieldbus works on a network structure which typically allows daisy-chain, star, ring, branch, and tree network topologies. Previously, computers were connected using RS-232 (serial connections) by which only two devices could communicate. This would be the equivalent of the currently used 4-20 mA communication scheme which requires that each device have its own communication point at the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require only one communication point at the controller level and allow multiple (hundreds) of analog and digital points to be connected at the same time. This reduces both the length of the cable required and the number of cables required. Furthermore, since devices that communicate through fieldbus require a microprocessor, multiple points are typically provided by the same device. Some fieldbus devices now support control schemes such as PID control on the device side instead of forcing the controller to do the processing.

History

Bitbus

The oldest commonly used field bus technology is Bitbus. Bitbus was created by Intel Corporation to enhance use of Multibus systems in industrial systems by separating slow i/o functions from faster memory access. In 1983, Intel created the 8044 Bitbus microcontroller by adding field bus firmware to its existing 8051 microcontroller. Bitbus uses EIA-485 at the physical layer, with two twisted pairs - one for data and the other for clocking and signals. Use of SDLC at the data link layer permits 250 nodes on one segment with a total distance of 13.2 km. Bitbus has one master node and multiple slaves, with slaves only responding to requests from the master. Bitbus does not define routing at the network layer. The 8044 permits only a relatively small data packet (13 bytes), but embeds an efficient set of RAC (remote access and control) tasks and the ability to develop custom RAC tasks. In 1990, the IEEE adopted Bitbus as the Microcontroller System Serial Control Bus (IEEE-1118).[1][2]

Today BITBUS is maintained by the BEUG - BITBUS European Users Group.[3]

Standardization

Although fieldbus technology has been around since 1988, with the completion of the ISA S50.02 standard, the development of the international standard took many years. In 1999, the IEC SC65C/WG6 standards committee met to resolve difference in the draft IEC fieldbus standard. The result of this meeting was the initial form of the IEC 61158 standard with eight different protocol sets called "Types" as follows:

This form of standard was first developed for the European Common Market, concentrates less on commonality, and achieves its primary purpose—elimination of restraint of trade between nations. Issues of commonality are now left to the international consortia that support each of the fieldbus standard types. Almost as soon as it was approved, the IEC standards development work ceased and the committee was dissolved. A new IEC committee SC65C/MT-9 was formed to resolve the conflicts in form and substance within the more than 4000 pages of IEC 61158. The work on the above protocol types is substantially complete. New protocols, such as for safety fieldbuses or real-time Ethernet-fieldbuses are being accepted into the definition of the international fieldbus standard during a typical 5-year maintenance cycle.

Both Foundation Fieldbus and Profibus technologies are now commonly implemented within the process control field, both for new developments and major refits. In 2006, China saw the largest FF (Foundation Fieldbus) systems installations at NanHai and SECCO, each with around 15000 fieldbus devices connected.

IEC 61158 specification

There were many competing technologies for fieldbus and the original hope for one single unified communications mechanism has not been realized. This should not be unexpected since fieldbus technology needs to be implemented differently in different applications; automotive fieldbus is functionally different from process plant control. The final edition of IEC standard IEC 61158 allows 8 technologies. This are the some hierarchic layer of the automation protocols.

IEC 61158 consists of the following parts, under the general title Digital data communications for measurement and control – Fieldbus for use in industrial control systems:

Standards

There are a wide variety of competing fieldbus standards. Some of the most widely used ones include:

See List of automation protocols for more examples.

Cost advantage

The amount of cabling required is much lower in Fieldbus than in 4-20 mA installations. This is because many devices share the same set of cables in a multi-dropped fashion rather than requiring a dedicated set of cables per device as in the case of 4-20 mA devices. Moreover, several parameters can be communicated per device in a Fieldbus network whereas only one parameter can be transmitted on a 4-20 mA connection. Fieldbus also provides a good foundation for the creation of a predictive and proactive maintenance strategy. The diagnostics available from fieldbus devices can be used to address issues with devices before they become critical problems.[4]

Networking

With the exception of ARCNET, which was conceived as early as 1975 for office connectivity and later found uses in industry, the majority of fieldbus standards were developed in the 1980s and became fully established in the marketplace during the mid-1990s. In the United States, Allen-Bradley developed standards that eventually grew into DeviceNet and ControlNet; in Europe, Siemens and other manufacturers developed a protocol which evolved into PROFIBUS.

During the 1980s, to solve communication problems between different control systems in cars, the German company Robert Bosch GmbH first developed the Controller Area Network (CAN). The concept of CAN was that every device can be connected by a single set of wires, and every device that is connected can freely exchange data with any other device. CAN soon migrated into the factory automation marketplace (with many others).

Despite each technology sharing the generic name of fieldbus the various fieldbus are not readily interchangeable. The differences between them are so profound that they cannot be easily connected to each other.[5] To understand the differences among fieldbus standards, it is necessary to understand how fieldbus networks are designed. With reference to the OSI model, fieldbus standards are determined by the physical media of the cabling, and layers one, two and seven of the reference model.

For each technology the physical medium and the physical layer standards fully describe, in detail, the implementation of bit timing, synchronization, encoding/decoding, band rate, bus length and the physical connection of the transceiver to the communication wires. The data link layer standard is responsible for fully specifying how messages are assembled ready for transmission by the physical layer, error handling, message-filtering and bus arbitration and how these standards are to be implemented in hardware. The application layer standard, in general defines how the data communication layers are interfaced to the application that wishes to communicate. It describes message specifications, network management implementations and response to the request from the application of services. Layers three to six are not described in fieldbus standards.[6]

Technical committees, with representatives of many different companies, have been responsible for turning the original specifications into international ISO standards. Bury, among others, reports that work is underway to implement a common fieldbus protocol.[7] This will entail a common set of application-layer services that can be provided regardless of the lower-layer implementation details. Although very much in its infancy, it is expected that this protocol may become reality by 2010. Whether designed for low-level sensor communications or high-level machine connectivity (or both), a fieldbus is an important enabling technology for an open architecture controller.[8]

Features

Different fieldbuses offer different sets of features and performance. It is difficult to make a general comparison of fieldbus performance because of fundamental differences in data transfer methodology. In the comparison table below it is simply noted if the fieldbus in question typically supports data update cycles of 1 millisecond or faster.

Fieldbus Bus power Cabling redundancy Max devices Synchronisation Sub millisecond cycle
AFDX No Yes Almost unlimited No Yes
AS-Interface Yes No 62 No No
CANopen No No 127 Yes No
CompoNet Yes No 384 No Yes
ControlNet No Yes 99 No No
CC-Link No No 64 No No
DeviceNet Yes No 64 No No
EtherCAT No Yes 65,536 Yes Yes
Ethernet Powerlink No Optional 240 Yes Yes
EtherNet/IP No Optional Almost unlimited Yes Yes
Interbus No No 511 No No
LonWorks No No 32,000 No No
Modbus No No 246 No No
PROFIBUS DP No Optional 126 Yes No
PROFIBUS PA Yes No 126 No No
PROFINET IO No Optional Almost unlimited No No
PROFINET IRT No Optional Almost unlimited Yes Yes
SERCOS III No Yes 511 Yes Yes
SERCOS interface No No 254 Yes Yes
Foundation Fieldbus H1 Yes No 240 Yes No
Foundation Fieldbus HSE No Yes Almost unlimited Yes No
RAPIEnet No Yes 256 Under Development Conditional
Fieldbus Bus power Cabling redundancy Max devices Synchronisation Sub millisecond cycle

Process Fieldbus vs. Device Networks

It should be noted that requirements of fieldbus networks for process automation applications (flowmeters, pressure transmitters, and other measurement devices and control valves in industries such as hydrocarbon processing and power generation) are different from the requirements of fieldbus networks found in discrete manufacturing applications such as automotive manufacturing, where large numbers of discrete sensors are used including motion sensors, position sensors, and so on. Discrete fieldbus networks are often referred to as "device networks".[9]

Ethernet and Fieldbus

Recently a number of Ethernet-based industrial communication systems have been established, most of them with extensions for real-time communication. These have the potential to replace the traditional fieldbuses in the long term.

Here is a partial list of the new Ethernet-based industrial communication systems:

For details, see the article on Industrial Ethernet.

Safety

Fieldbus can be used for systems which must meet safety-relevant standards like IEC 61508 or EN 954-1. Depending on the actual protocol, fieldbus can provide measures like counters, CRCs, echo, timeout, unique sender and receiver IDs or cross check. Ethernet/IP and SERCOS III both use the CIP Safety protocol,[10] Ethernet Powerlink uses openSAFETY, while FOUNDATION Fieldbus and Profibus (PROFIsafe) can address SIL 2 and SIL 3 process safety applications.

In January 2006, the Fieldbus Foundation announced that TÜV Rheinland Industrie Service GmbH, Automation, Software and Information Technology, a global, independent and accredited testing agency, had granted Protocol Type Approval for its Safety Specifications. The Foundation Technical Specifications - Safety Instrumented Functions are in compliance with International Electrotechnical Commission (IEC) 61508 standard (functional safety of electrical/electronic/programmable electronic safety-related systems) requirements up to, and including, Safety Integrity Level 3 (SIL 3).[11]

Market

In process control systems, the market is dominated by FOUNDATION fieldbus and PROFIBUS PA.[12] Both technologies use the same physical layer (2-wire manchester-encoded current modulation at 31.25 kHz) but are not interchangeable. As a general guide, applications which are controlled and monitored by PLCs (programmable logic controllers) tend towards PROFIBUS, and applications which are controlled and monitored by a DCS (digital/distributed control system) tend towards FOUNDATION Fieldbus. PROFIBUS technology is made available through Profibus International with headquarters in Karlsruhe, Germany. FOUNDATION Fieldbus technology is owned and distributed by the Fieldbus Foundation of Austin, Texas.

See also

Notes

  1. ↑ Hunziker, Robin; Schreier, Paul G. (August 1993). "Field buses compete for engineers' attention, start gaining commercial support". Personal Engineering & Instrumentation News (Rye, NH: PEC Inc.) 10 (8): 35–37. ISSN 0748-0016.
  2. ↑ Zurawski, Richard, ed. (2005). Industrial Communication Technology Handbook. Industrial Technology Series 1. Boca Raton, FL: CRC Press. pp. 7–10. ISBN 0849330777. LCCN 2004057922. Retrieved 4 Feb 2013.
  3. ↑ Bitbus/fieldbus community site.
  4. ↑ http://www.controlglobal.com/articles/2007/217.html
  5. ↑ Bury (1999)
  6. ↑ Farsi & Barbosa 2000
  7. ↑ Bury 1999
  8. ↑ Pfeiffer & Komischke 1987
  9. ↑ http://www.isadenver.org/docs/fieldbus.pps
  10. ↑ "CIP Safety on SERCOS Specification". Retrieved 2010-02-05.
  11. ↑ http://www.fieldbus.org/images/stories/enduserresources/technicalreferences/documents/wp_arc_ff-sif_908.pdf
  12. ↑ http://www.fieldbus.org/images/stories/fieldbus_report/FieldbusReport_Apr08.pdf

References

Bibliography

External links

Foundation Fieldbus End User Councils
Independent organizations
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