Fieldbus

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

A complex automated industrial system — say a manufacturing assembly line — usually needs 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 which links the PLCs to the components which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and contactors.

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[edit] What is fieldbus?

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 has 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 at the controller level and allow multiple (100's) 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.

[edit] History

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 this "8-headed monster" 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 realtime ethernet-based fieldbuses are being accepted into the definition of the international fieldbus standard during a typical 5-year maintenance cycle.

Recent additions or planned additions to IEC 61158 include but are not limited to:[citation needed]

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 systems installations at NanHai and SECCO, each with around 15000 fieldbus devices connected.[citation needed]

[edit] IEC 61158 specification

There were many competing technologies for fieldbus and the original hope for one single unified communications mechanism has not been realised. 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.

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:

  • Part 1: Overview and guidance for the IEC 61158 series
  • Part 2: Physical Layer specification and service definition
  • Part 3: Data Link Service definition
  • Part 4: Data Link Protocol specification
  • Part 5: Application Layer Service definition
  • Part 6: Application Layer Protocol specification

[edit] Standards

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

See List of automation protocols for more examples.

[edit] Cost advantage of fieldbus

A major advantage of fieldbus implementation is the capital expenditure (CAPEX) savings associated with cable elimination; multiple devices share wire-pairs in order to communicate over the bus network and savings are also available through speedier commissioning.[citation needed]

Users have now found that ongoing maintenance and process control system performance are also very significantly enhanced through adopting fieldbus systems, which results in operations expense savings (OPEX).[citation needed]

[edit] Fieldbus networking

The US-based Fieldbus Foundation defines fieldbus as:

"...an all-digital, serial, two-way communication system that interconnects measurement and control equipment such as sensors, actuators and controllers. At the base level in the hierarchy of plant networks, it serves as a Local Area Network (LAN) for instruments used in process control and manufacturing automation applications and has a built-in capability to distribute the control application across the network. Furthermore, a Fieldbus must be an open system that is supported by several vendors, and not tied to a single technology."[citation needed]

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.[1] 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.[2].

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.[3] 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.[4]

[edit] Disadvantages of fieldbus

There are disadvantages to using fieldbus compared to the 4-20 mA analog signal standard (or to 4-20 mA with HART):

  • Fieldbus systems are more complex, so users need to be more extensively trained or more highly qualified
  • The price of fieldbus components is higher
  • Fieldbus test devices are more complex compared to a (high-spec) multimeter that can be used to read and simulate analog 4-20 mA signals
  • Slightly longer reaction times with fieldbus, depending on the system
  • Device manufacturers have to offer different versions of their devices (e.g. sensors, actuators) due to the number of different (incompatible) fieldbus standards. This can add to the cost of the devices and to the difficultly of device selection and availability.
  • One or more fieldbus standards may predominate in future and others may become obsolete. This increases the investment risk when implementing fieldbus.

[edit] Current developments

In recent years 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 field buses in the long term. Currently the issue stopping most Ethernet fieldbus implementations is the availability of device power. Most industrial measurement & control devices need to be powered from the bus and Power-Over-Ethernet (PoE) does not deliver enough.

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

[edit] Fieldbus safety

Fieldbus can be used for systems which must meet safety-relevant standards like IEC 61508 or EN 954-1. Depending of the actual protocol, fieldbus can provide measures like counters, CRC's, echo, timeout, unique sender and receiver ID's or cross check. Both FOUNDATION Fieldbus and Profibus (PROFIsafe) have varieties of their communications protocol which are compatible with safety systems.

[edit] Market

In process control systems, the market is dominated by FOUNDATION fieldbus and PROFIBUS PA.[citation needed] 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 Internatonal with headquarters in Karlsruhe, Germany. FOUNDATION Fieldbus technology is owned and distributed by the Fieldbus Foundation of Austin, Texas.

[edit] References

  1. ^ Bury (1999)
  2. ^ Farsi & Barbosa 2000
  3. ^ Bury 1999
  4. ^ Pfeiffer & Komischke 1987
  • Chatha, Andrew. (1994). Fieldbus: The Foundation for Field Control Systems Control Engineering, May, 47–50.
  • Furness, Harry. (1994). Digital Communications Provides... Control Engineering, January, 23–25.
  • Furness, Harry. (1994). Fieldbus: The Differences Start From the Bottom Up Control Engineering, March, 49–51.
  • Fouhy, Ken. (1993). Fieldbus Hits The Road Chemical Engineering, September, 37–41.
  • Johnson, Dick. (1994). The Future of Fieldbus At Milestone 1995 Control Engineering, December, 49–52.
  • Loose, Graham. (1994). When Can The Process Industry Use Fieldbus? Control and Instrumentation, May, 63–65.
  • Spear, Mike. (1993). Fieldbus Faces Up To First Trials Process Engineering, March, p36.
  • Lasher, Richard J. (1994). Fieldbus Advancements and Their Implications Control Engineering, July , 33–35.
  • Pierson, Lynda L. (1994). Broader Fieldbus Standards Will Improve System Functionality Control Engineering, November, 38–39.
  • O'Neill, Mike (2007). Advances in Fieldbus, Process Industry Informer, January, 36–37.

[edit] Bibliography

  • Babb, Michael. (1994). Will Maintenance Learn To Love Fieldbus? Control Engineering, January, 19.
  • Babb, Micahel. (1994). Summer, 1994: Another Fieldbus Delay, Schneider's DPV, and Open Systems Control Engineering, July , 29.
  • Gokorsch, Steve. (1994). Another Scenario: Maintenance Will Learn to Love Fieldbus Control Engineering, June, 112–114.
  • Gunnel, Jeff. (1994). Analyser Links Can Use Fieldbus Control and Instrumentation, March, 33–35.
  • Hodgkinson, Geoff. (1994). Communications Are We Listening? Process Engineering, Instrumentation Supplement 1994, s19–s21.
  • Jones, Jeremy. (1992). Can Fieldbus Survive? Control and Instrumentation, August, 25–26.
  • Kerridge, Brian. (1994). Network Vendors Aganize Over Fieldbus StandardEDN, April 28th, 45–46.
  • Rathje, J. (1994). Namur Says Yes To Fieldbus Technology and the Promise of Reduces Costs Control and Instrumentation, September, 33–34.
  • Reeve, Alan. (1993). Fieldbus — Are Users Involved? Control and Instrumentation, August, 25–26.
  • Spear, Mike. (1994). A Plant View of Fieldbus In Use Process Engineering, April, 38–39.
  • Spear, Mike. (1994). Fieldbus Ready To Start The Last Lap? Process Engineering, April, 37.

[edit] External links