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 central computer at the top, where an operator can monitor or program the whole system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a bus 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.

Contents

[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 new 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.

In essence fieldbus will replace centralised control networks with distributed-control networks. Therefore fieldbus is much more than a replacement for the 4–20mA 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 is a great deal more accurate than transmitting using analogue methods which were used previously. Each field device is also 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 4–20mA analogue standard. Other major cost savings from using fieldbus are due to wiring and installation - the 4–20mA analogue signal standard requires each device to have its own set of wires and its own connection point. Fieldbus eliminates this need so only a single twisted pair wiring scheme is required.

[edit] The international debate

Although fieldbus technology has been around since 1988, the development of the international standard took many years. In 2000, all of the interested parties converged to create a the IEC fieldbus standard, 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 61158. This work is substantially complete and several new protocols are being accepted into the definition of the international fieldbus standard.

[edit] History

In the 1940s, process instrumentation relied upon pressure signals of 3-15 psi for the monitoring of control devices. In the 1960s, the 4-20 mA 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.

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 4-20mA 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 layed out and demonstrated that the cable limits suggested could be safely exceeded by 50%.

The second phase of fieldbus testing is taking place in Norway on an Esso Norway refinery implementing and testing digital control systems using the protocols proposed by WorldFIP and ISP. Results of the testing of these two protocols are expected by the middle of this year. Both of these organisations have working implementations of their fieldbus systems in many companies, as the ongoing standard develops, attesting to the promise that fieldbus offers to industry

[edit] Advantages of Fieldbus

The fieldbus has a multitude of advantages that the end users will benefit from. The major advantage of the fieldbus, and the one that is most attractive to the end user is its reduction in capital costs. The savings attained by the user stem from three main areas, initial savings, maintenance savings, and savings due to improved systems performance.

[edit] Initial savings

One of the main features of the fieldbus is its significant reduction in wiring. Each process cell requires only one wire to be run to the main cable, with a varying number of cells available. The cost of installing field equipment in a fieldbus system is thus significantly reduced. Installation costs are further reduced due to the fact that the fieldbus it is a multi-drop rather than point-to-point system and the multidrop network can offer a 5:1 reduction in field wiring expense. The price of equipment is reduced significantly in a fieldbus system, with savings of approximately $50 per field device possible.

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 real protocols for fieldbus, being ISP and WorldFIP and while there is still two protocols rather than a world standard, it is better than a possible many. Work is being done to merge the two protocols into a 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.

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

Compared to the standard 4-20 mA application (or 4-20 mA application with HART) there are some disadvantages to using fieldbus. Current signals can be expected to be more immune to electomagnetic noise from poor shielding and long runs than a serial signal. 4-20 mA signals can be read and simulated with a (high-spec) multimeter.

[edit] Fieldbus organisations

This section establishes who have been the major instigators of fieldbus development over the past several years. A brief summary of the developing standard is also covered. The major players in the fieldbus area were previously dominated by two major groups, WorldFIP (World Factory Instrumentation Protocol) and ISP (Interoperable Systems Project). However, recently, these two groups have joined together to form the Fieldbus Foundation (FF). The Fieldbus Foundation and another organisation known as Profibus-ISP are now competing for market dominance.

Two standards bodies known as the IEC (International Electrotechnical Commission) and the ISA (Industry Society of America ) are currently working on an international standard known as SP50. This standard will hopefully allow the manufacturers of fieldbus equipment all around the world to produce compatible instruments for industrial applications. WorldFIP, ISP and FF have pledged that they will eventually evolve their products to meet the standard when it arrives. However, when the standard finally does arrive, users of existing non-conforming equipment will run the risk of having obsolete equipment or having to purchase new systems at an excessive cost.

At the time of writing, information regarding actual market share for the Fieldbus Foundation and Profibus-ISP was not available, but Process Engineering’s Instrumentation Supplement for 1994, predicts that the Fieldbus Foundation will take the greater market share.

[edit] World Factory Instrumentation Protocol

The World Factory Instrumentation Protocol (WorldFIP) was developed from an earlier French National Standard known as NFC 46-600, or more commonly as FIP. It is a consortium of companies producing field bus instruments that use a messaging system. Time critical options are supposedly guaranteed in a WorldFIP implementation. WorldFIP plans to add a device description tool, known as the WorldFIP Device Builder. The Device Builder will automatically inform the control system what features and parameters each instrument connected to the bus has. Interestingly , WorldFIP is divisional in nature with a UK, European and North American division. Each division is motivated by similar goals and similar implementations, but each operates almost autonomously from the others.

Some of the major members of WorldFIP include:

  • Honeywell (Arizona)
  • Bailey Controls (Ohio)
  • Cegelec (Paris)
  • Allen Bradley Corporation (Ohio)
  • Telemecanique (Paris)
  • Ronan Engineering Co. (California)
  • Square D
  • Electricite de France (France)
  • Elf (France)

[edit] Interoperable Systems Project

The Interoperable Systems Project (ISP) implementation is based on the German National Standard DIN STD19245, also known as Process Field Bus, or Profibus. Profibus is similar to the token passing network commonly implemented on many networks today. The ISP extension to Profibus is the Device Description Language (DDL). DDL allows an instrument added to the bus system to communicate to a master control what its functions and capabilities are. Some of the major members of ISP include:

  • Siemens (Germany)
  • The Rosemount Group (Minnesota)
  • Fisher Controls, Inc. (Texas)
  • Foxoboro Co. (Massachusetts)
  • ABB Co. (Sweden)
  • Yokogawa Electric Corporation (Tokyo)

[edit] Fieldbus Foundation

On a positive note ISP and WorldFIP (North American division) have been working together since late 1993 on a possible merger of their technology. A single solution has been what industry has needed for a long time, so in June of 1994, the Fieldbus Foundation (FF) was set up between ISP and WorldFIP (NA). However, at least 1 to 2 years of delay is expected before a complete product can be produced.

[edit] Profibus-ISP

Effectively a breakaway group of the Profibus and ISP organisations, this group effectively announced to the world that they will have their own fieldbus communications system ready in approximately June/July 1994. Profibus-ISP is derived from the Profibus and ISP products, and hence has the features of both with some small additions. At the time of writing, little information on Profibus-ISP and the Fieldbus Foundation was available.

[edit] IEC/ISA SP50

The ISA/IEC are developing a standard with the working name of SP50. The standard will follow the ISO/OSI seven layer model for data communications with an additional eighth layer which focuses on the product interoperablility.

Current progress on the SP50 is as follows

Physical - Completed. Specification includes

  • 31.25 kbit/sec, 1 Mbit/sec and 2.5 Mbit/sec data transfer rates.
  • Requirements for fieldbus component parts.
  • Media and network configuration requirements for data integrity and interoperability between devices.
  • Data Link - Balloting 1995. Draft standard by 1996.
  • Application - Balloting in 1995.
  • User - In Committee now. Balloting late 1995.
  • System & Network Management - expect to be completed mid 1996.

[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 Eningeering, 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.

[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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