Fieldbus

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A Fieldbus is an industrial network system for real-time distributed control.

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 comminications 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 and contactors.

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[edit] Standards

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

[edit] What is Fieldbus?

Fieldbus is a generic-term which describes a modern industrial digital communications network intended to replace the existing 4-20mA analogue signal standard. The network is a digital, bi-directional, multidrop, serial-bus, communications network used to link isolated field devices, such as controllers, transducers, actuators and sensors. Each field device has low cost computing power installed in it, making each device a "smart" device. Each device is able to execute simple functions on its own such as diagnostic, control, and maintenance functions as well as providing bi-directional communication capabilities. With these devices not only is the engineer able to access the field devices, but they are also able to communicate with other field devices.

Potentially a fieldbus can replace centralized process control with distributed-control networks. Therefore fieldbus is much more than a replacement for the 4–20 mA analogue standard. The fieldbus technology promises to improve quality, reduce costs and boost efficiency. These promises made by the fieldbus technology are derived partly from the fact that information that a field device is required to transmit or receive can be transmitted digitally. This removes the requirement for D-A conversion in the field, and then A-D conversion at the controller. This can be a great deal more accurate. Each field device can also be a "smart" device and can carry out its own control, maintenance and diagnostic functions. As a result it can report if there is a failure of the device or manual calibration is required. This increases the efficiency of the system and reduces the amount of maintenance required.

Fieldbus devices are more flexible than older devices due to the inclusion of a CPU. One fieldbus device could be used to replace a number of devices using the 4–20 mA analogue standard. For example, a pressure transducer can measure process pressure, atmospheric pressure, and also process temperature and supply all three via the fieldbus. Other major cost savings from using fieldbus are due to wiring and installation — the 4–20 mA analogue signal standard requires each device to have its own pair of wires and its own analog connection point at the controller level. Fieldbus eliminates this need by requiring one communication point at the controller level to connect to multiple (100's) of analog and digital points, while at the same time reducing the length of cable runs by connecting to the field devices in a daisy-chain, star, ring, branch, tree style network topology.

[edit] The international debate

Although fieldbus technology has been around since 1988, the development of the international standard took many years. In 2000, a set of interested parties converged to create the IEC fieldbus standard, IEC 61158 with eight (8) different protocol sets called "Types" as follows:

  • Type 1 — FOUNDATION Fieldbus H1
  • Type 2 — ControlNet
  • Type 3 — PROFIBUS
  • Type 4 — P-Net
  • Type 5 — FOUNDATION Fieldbus HSE (High Speed Ethernet)
  • Type 6 — Interbus
  • Type 7 — SwiftNet (a protocol developed for Boeing, since withdrawn)
  • Type 8 — WorldFIP

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 consortiums 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, like for safety fieldbusses or realtime ethernet based fieldbusses are being accepted into the definition of the international fieldbus standard only in form of a typical 5 year maintainance cylce.

[edit] History

In the 1940s, process instrumentation relied upon pressure signals of 3-15psi for the monitoring of control devices. In the 1960s, the 4-20mA analogue signal standard was introduced for instrumentation. Despite this standard, various signal levels were used to suit many instruments which were not designed to the standards specification. The development of digital processors in the 1970s sparked the use of computers to monitor and control a system of instruments from a central point. The specific nature of the tasks to be controlled called for instruments and control methods to be custom designed. In the 1980s smart sensors began to be developed and implemented in a digital control, microprocessor environment. This prompted the need to integrate the various types of digital instrumentation into field networks to optimise system performance. While the "if it works then use it" mentality progressed, it became obvious that a fieldbus standard was required to formalise the control of smart instruments.

[edit] Ongoing standard

The decision to provide an international standard saw the Instrument Society of America (ISA), the International Electrotechnical Commission (IEC), Profibus (German national standard) and FIP (French national standard), form the IEC/ISA SP50 Fieldbus committee. The standard to be developed must integrate the enormous range of control instruments, provide them with interfaces to operate various devices simultaneously, and set a communication protocol to support them all. This daunting task was perceived by many to be moving too slow, a problem compounded by companies world wide pushing to have their own product ideas standardised. With the diversity in products and methods of implementation, there was no one direct solution for the standard to be set to. In 1992, two groups, each consisting many major companies world wide, emerged to lead the market in a fieldbus solution. The ISP (Interoperable Systems Project) and WorldFIP (Factory Instrumentation Protocol) both share differing views on the implementation of fieldbus, but they claim they will alter their products to conform to the ISA's SP50 standard when it is formalised.

Also see sections Other Advantages and Fieldbus Organizations

The SP50 committee decided to concentrate on four layers for the fieldbus solution:

  1. Physical Layer: This defines the media that communication occurs over and could be viewed as the 4–20 mA standard replacement.
  2. Data Link Layer: This monitors the communications taking place among the various devices and detects errors.
  3. Application Layer: This formats the data into messages which all devices connected to the network can understand and provides the services for process control, supplying them to the user layer.
  4. User Layer: This connects the individual plant areas and provides an environment for applications. It is implemented using high level control functions.

Of these layers, the ISA S50.02 Part 2 Physical Layer was approved in September 1992. The Data Link Layer is expected to be drafted into an IEC standard by mid 1996. The Application and User Layers are in committee with balloting due to be resolved early and late this year respectively. The System and Network Management is expected to be completed by mid 1996. (Dick Johnson, December 1994) In September of 1994, WorldFIP and ISP, joined forces to become Fieldbus Foundation (FF), in an effort to speed up the process of completing the fieldbus standard.

[edit] Fieldbus testing

For several years now companies world wide have been engaged in the testing of the evolving fieldbus standard through implementation in small areas of already operational plants. The aims of these companies is undoubtedly to test the suitability of fieldbus in their operating environments. This real life testing is the best way to examine the reliability of a fieldbus system, and to determine whether fieldbus will live up to the process industry's high expectations.

In early 1993, all eyes were turned towards the BP Research site in Sunbury, where the first tests on a fieldbus system were coming to conclusion. Large numbers of instruments were set up in a flow test rig to perform various tests in line with the currently evolving fieldbus standard. This first phase test examined the physical layer set down by the SP50 committee with the intent of verifying its operation. Successful tests confirmed physical layer layout and demonstrated that the cable limits suggested could be safely exceeded by 50%.

The latest international standard defining the physical layer for fieldbus in IEC61158-2 3rd edition 2003-05, incorporating all fieldbus technologies required for automotive systems, manufacturing & machinery control and continuous/batch process plants.

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.

[edit] Advantages of Fieldbus

The fieldbus has a multitude of advantages from which end users can benefit. At first glance, 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. 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).

[edit] Initial savings

One of the main features of the fieldbus is its significant reduction in wiring costs. For example, FF and Profibus PA segments support up to 32 devices on one twisted pair cable rather than 32 individual wire pairs. The fieldbus system requires less labour to install than conventional bus systems, and saves money due to a reduction in materials needed for the installation.

The simpler system design implies that fewer system drawings will be needed in order to develop a fieldbus system. This also has the advantage that the simpler design will result in less complex and faster bus systems.

[edit] Maintenance savings

The fact that the fieldbus system is less complex than conventional bus systems implies that there will be less overall need for maintenance. The simplification of systems means that the long term reliability of the bus system is increased.

With the fieldbus system, it is possible for the operators to easily see all of the devices included in the system and to also easily interpret the interaction between the individual devices. This will make discovering the source of any problems and carrying out maintenance much simpler, and thus will reduce the overall debugging time.

The debugging and maintenance of the system will also be enhanced due to the fact that fieldbus enables online diagnostics to be carried out on individual field devices. The online diagnostics include functions such as open wire detection and predictive maintenance and simplify tasks such as device calibration.

[edit] Improved Systems Performance

Fieldbus allows the user increased flexibility in the design of the bus system. Some algorithms and control procedures that with conventional bus systems must be contained in control programs can now reside in the individual field devices, reducing the overall size of the main control system. This reduces the overall systems cost and makes future expansion a simpler prospect. System performance is enhanced with the use of fieldbus technology due to the simplification of the collection of information from field devices. Measurement and device values will be available to all field and control devices in engineering units. This eliminates the need to convert raw data into the required units and will free the control system for other more important tasks. The reduction in information complication will allow the development of better and more effective process control systems.

With fieldbus technology, two-way communication between field devices and the control system is made possible. System performance is enhanced due to the ability to communicate directly between two field devices rather than via the control system. This also enables several related field devices to be combined into one device.

With fieldbus technology, field instruments can be calibrated, initialised, operated and repaired faster than most conventional analog instrumentation. this leads to an overall reduction in time required to operate the fieldbus system.

[edit] Other Advantages

As well as the cost advantages that fieldbus technology embodies, there are many other miscellaneous advantages that are included in the fieldbus package. Although it is a major challenge trying to develop a single worldwide protocol for process control, there are currently only two predominant protocols for fieldbus: FOUNDATION Fieldbus (largely in USA & Japan) and PROFIBUS (largely in Germany). While there are still two protocols rather than a world standard, it is better than a possible many. Work is being done to merge these two protocols into one standard, which will be a major advantage. The fact that eventually all fieldbus equipment will be standardised will mean that expansion of a system or addition of field devices will be extremely simple, requiring no interfaces or converters.

Also see sections Ongoing standard and Fieldbus Organizations

The fieldbus protocol involves only four layers and a set of management services. Fieldbus has the advantage that the user should not have to be concerned with the Data Link layer or the Application layer. As far as the end user is concerned, they should simply work. The user will only be required to have a limited knowledge of the management services, because some of the information generated by them will be needed if a problem occurs in the system. In fact, it should only be necessary for the user to concern themselves with the Physical and User layers.

[edit] Disadvantages of Fieldbus

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

  • Analog current signals are generally more immune to electomagnetic noise (e.g. from poor shielding and long runs) than digital signals such as fieldbus.
  • 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 4-20mA 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 availabililty.
  • One or more fieldbus standards may predominate in future and others may become obsolete. This increases the investment risk when implementing fieldbus.

[edit] Realtime-Ethernet

During recent years a number of Ethernet based industrial communication system have been established, most of them with extensions for real-time communication. These have the potential to replace the traditional field busses 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.

  • EtherCAT
  • Ethernet-Powerlink
  • SERCOS III
  • PROFINET
  • ETHERNET/IP
  • VARAN
  • SafetyNET p

see as well: Industrial Ethernet

A common property of all of these systems seems to be that they are supported by only one PLC/DCS manufacturer for their central logic, and hardly any are compatible with any other.

[edit] Safety properties of field busses

If a fieldbus is to be used for systems which must meet safety relevant standards like IEC 61508 or EN 954-1, then the field bus must meet special requirements. These requirements can be met, for example, by redundant soft- and hardware of the field devices and, depending of the actual protocol, measures like counters, CRC's, echo, timeout, unique sender and receiver ID's or cross check. Both FOUNDATION Fieldbus and Profibus have varieties of their communications protocol which are compatible with safety systems. In addition there exist specialised field busses for safety like SafetyBUS p. See as well Safety level, Safety concept, Safety.

[edit] Fieldbus Organizations

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 is required to be implemented differently in different applications; automotive fieldbus is functionaly different to process plant control. The final edition of IEC standard IEC61158 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

In process control systems, the market is dominated by FOUNDATION Fieldbus and PROFIBUS. Both technologies use the same physical layer (2-wire manchester-encoded current modulation at 31.25KHz) 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.

Also see sections Ongoing standard and Other Advantages

[edit] References

  • 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

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