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2007:010 CIV

M A S T E R ' S T H E S I S

A Comparison between Fieldbusesand Remote I/O for Instruments

in the Process Industry

Lars Persson

Lule University of Technology

MSc Programmes in Engineering

Electrical EngineeringDepartment of Computer Science and Electrical EngineeringDivision of EISLAB

2007:010 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--07/010--SE

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Preface

This Masters Thesis is the final part required for my Master of Science degree in ElectricalEngineering at Lule University of Technology. The work has been carried out during thewinter 2005/2006 at Outokumpu Technology AB in Skellefte. The aim with this Thesis is toidentify advantages and disadvantages with fieldbus technology in Industrial applications.

I would like to thank the following persons who have helped me with this Thesis: my supervi-sor Pr Norman at Outokumpu Technology for his time, support and ideas; Leif Nyberg,Manager of the Electrical and automation department at Outokumpu Technology and PerLindgren at LTU for allowing me to do this Thesis; Leif Karlsson from ABB who has an-swered a lot of my questions; Jan stensson, Mats Ntsaari and Jan Malmstrm at Husumand Magnus Normell at Eurocon for the interviews; my sweet girlfriend Freja for proofread-ing and correcting my English; and finally the rest of the staff at Outokumpu Technology for

help and support throughout the work on this Thesis.

Skellefte, 17th December 2006

Lars Persson

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Table of content1 Introduction........................................................................................................................... 1

1.1 Background ...................................................................................................................... 11.2 Purpose............................................................................................................................. 11.3 Delimitations ....................................................................................................................11.4 Outokumpu Technology AB, Skellefte .......................................................................... 1

2 Method.................................................................................................................................... 3 2.1 Literature review .............................................................................................................. 32.2 Interview........................................................................................................................... 32.3 Test system....................................................................................................................... 3

3 Literature review...................................................................................................................5 3.1 Communication levels in a plant...................................................................................... 53.2 History.............................................................................................................................. 5

3.3 Fieldbus in general ........................................................................................................... 73.4 4-20 mA vs. fieldbus instrument technology ................................................................. 103.5 Wireless fieldbus ............................................................................................................ 113.6 Description of Profibus, Foundation Fieldbus, DeviceNet and Interbus ....................... 133.7 Practical experience from fieldbus installations............................................................. 22

4 Interview .............................................................................................................................. 27 4.1 Background .................................................................................................................... 274.2 Result.............................................................................................................................. 27

5 Test system ........................................................................................................................... 31 5.1 Background and problem ............................................................................................... 31

5.2 Material .......................................................................................................................... 325.3 Method ........................................................................................................................... 345.4 Result.............................................................................................................................. 345.5 Problems......................................................................................................................... 41

6 Result .................................................................................................................................... 43 6.1 Comparing fieldbuses with remote I/O ..........................................................................436.2 Requirement of knowledge ............................................................................................ 436.3 Differences between Foundation Fieldbus H1 and Profibus PA.................................... 436.4 Wireless fieldbuses......................................................................................................... 44

7 Discussion and Conclusion ................................................................................................. 45

8 Future work ......................................................................................................................... 47 References ............................................................................................................................... 49 Appendix A: AS-Interface (Actuator-Sensor Interface) Appendix B: Test system specification Appendix C: GSD file Appendix D: Interview questions Appendix E: Abbreviations

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1 Introduction

1.1 BackgroundThe society is constantly developing and changing and therefore we have to adapt to thechanges. The automation of metal process industry is no exception. The 4-20 mA analoguestandard for instrumentation is still used at most plants in Sweden, but the standard has beenaround since the 1960s. However, the time of change seems to have reached the process in-dustry as well. In the early 1990s, digital communication between instruments and the con-trol system began to develop. Still, new developments, improvements and changes are madeevery year, making it problematic to decide when to invest time and money into the new sys-tems. It has to be taken into consideration that a control system for the process industry runsfor at least 15 years. The research question of this thesis is: is it time for new technology totake over?

1.2 PurposeOutokumpu Technology AB in Skellefte (OTSk) is currently trying to decide whether thecompany should learn how to use fieldbuses and recommend it to their costumers. A fieldbusis like a data bus where several devices are connected to the same cable and communicatethrough digital messages. If the answer to the previous question is yes, which fieldbus should be used? Should OTSk recommend different fieldbuses depending on for example where inthe world the installation is taking place? To facilitate the decision, OTSk wants to knowwhich advantages fieldbuses can give regarding installation, operation and maintenance. Ad-ditionally to this, this thesis includes a limited search of wireless fieldbus solutions.

The thesis will

1. Identify advantages and disadvantages with fieldbuses compared to remote I/O2. Describe the differences between the fieldbuses3. Identify advantages and disadvantages with wireless fieldbuses and make a study of

wireless products and their manufacturers.

The purpose is not to give a recommendation, but to provide data for decision-making.

1.3 DelimitationsDue to time limits, the thesis will only look at four different fieldbuses for field devices.These are DeviceNet, Foundation Fieldbus H1, Interbus and Profibus PA. This thesis willthen focus on two of them, Foundation Fieldbus H1 and Profibus PA, since these are operat-ing at the instrument level.

To know more about how a fieldbus functions in a plant, visits will be arranged to Husum andETEK. Interviews will be performed with installation, maintenance and operation personnelat both plants. They are both located near rnskldsvik in Sweden.

A system was provided by OTSk to evaluate Profibus PA. It included an ABB AC 800F con-trol system and selected instruments from different manufacturers.

1.4 Outokumpu Technology AB, Skellefte

Outokumpu Oy is an international company specialized in stainless steel and technology. Thecompany has about 13 000 employees and operates in over 40 countries. Outokumpu Tech-

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nology is a part of the Outokumpu group, which mainly sells technology for the metal andmineral process industry. This includes designing, developing and supplying tailored plants, processes and equipment.

Outokumpu Technology in Skellefte, Sweden was formerly owned by Boliden and was

known as Boliden Contech AB. The company provide engineering and project services inSweden and the rest of the world. The contact between OTSk and Boliden is still strong andBoliden is the major customer. With time new customers have been acquired and Outokumpunow have customers worldwide. China has fast become an important market due to the rap-idly growing metal process industry. Outokumpu has an area of knowledge stretching frommining to melting and chemical activities. Around 100 employees are located in Skellefteand about 25 of these works at the Electrical and Automation department.

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2 Method

2.1 Literature reviewThe Thesis began with a literature review of fieldbuses and their advantages compared to re-mote I/O, which is currently used. The information was mainly taken from technical descrip-tions and from the standard-makers official homepages. Articles discussing advantages anddisadvantages were also searched for on the Internet.

The information about wireless fieldbus instruments has been found on the Internet. Mainlywww.google.com and homepages of instrument manufacturers, likewww.abb.com, have been used. In addition to the two above Internet sources,www.scholar.google.com and ieeex- plore.ieee.org, have been used to find general information about fieldbuses and informationabout fieldbuses in practice.

2.2 InterviewA number of questions were prepared in advance and used as base for the interviews. Thequestion sheet was constructed in cooperation with my supervisor and sent to the intervieweesin advance.

2.3 Test systemThe AC 800F control system was new to me and the manual was used to learn how to use it.Parts of the personnel at Outokumpu Technology in Skellefte had used the AC 800F controlsystem before and helped when the manual was not enough. However, information about howto use Profibus in the AC 800F control system was not known at Outokumpu Technology inSkellefte and personnel at ABB was used as an information source.

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3 Literature review

3.1 Communication levels in a plantThe amount of data sent in a plant is increasing. Not only are the plants growing larger andthe number of instruments increasing, the size of data retrieved from each device is also in-creasing. This puts higher demands on the communication. Industrial communication can bedivided into three levels: Cell, Field and Sensor/Actuator level, see Figure 1. The communica-tion can be both vertical and sometimes pass a level as well as horizontal. Binary signals aresent on the Sensor/Actuator level and the devices are usually powered and communicating onthe same cable. At the Field level lie the distributed devices such as transmitters, drive unitsand I/O modules. Some devices at the Field level like a transmitter send a limited amount ofdata and can be powered by the bus while others like a driver have external power but send alot of data. At Cell level we have the process stations which usually send lots of data. How-ever, the data is not as time critical as the data sent at Field and Sensor/Actuator level. All

fieldbuses studied in this Thesis communicate at the field level.

Figure 1. The different communication levels in a plant.

3.2 HistoryIn a conventional DCS (Distributed Control System) each device is connected with two wires,see Figure 2. A device can be an instrument for measuring temperature, pressure etc. or anactuator that act on the system like a valve. In conventional communication, an instrumentsends its measured values with an analog signal. In a similar way, an actuator is controlled by

varying the current sent to it. If the device does not have large power consumption, it can befed through the cable. The instrument sends its measured value by varying the current it uses between 4 and 20 mA. It is a standard that will be referred to as 4-20 mA in this Thesis. If 4-20 mA instruments are used in a large plant, lots of cables have to be installed. Even if severaldevices are located at the same place each one has its own cable, containing two wires.

Cell Level

Field Level

Sensor/ActuatorLevel

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Figure 2. The conventional way to connect devices. Every device has its own cable to the DCS.

One step towards fieldbuses and digital communication came with remote I/O. It was mainlyused to reduce the cables that have to be installed. In remote I/O, the I/O cards are movedfrom the DCS and placed closer to the instrument, see Figure 3. These I/O cabinets are then

connected to the control system with a single cable. To be able to send several measured val-ues across this cable the communication is made digital.

Figure 3. Remote I/O. Devices located close to each other are connected to the same remote I/O cabinet.The cabinet is then connected to the DCS with a bus cable.

10110110

DCS

Remote I/O cabinet

DCS

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However, the remote I/O did not change the communication between the instrument andDCS. The devices still gave an analogue value that had to be interpreted. And no data could be sent to the device, which is needed to configure it out in the field. Another step towardsdigital communication came with the HART protocol.

Highway Addressable Remote Transducer (HART) was a first step towards intelligent in-struments. It uses the conventional 4-20 mA to send the process value but it also sends a digi-tal message in the same cable. Information can be sent in both directions and be used for di-agnostics or to calibrate the device. It sends a low frequency, FM (Frequency Modulated)sinusoidal signal superimposed on the analog signal, see Figure 4. It has to use a point-to- point connection, and has a speed of 1200 bps. It is a widely used technology and can be usedwith new remote I/O systems. Before it was supported by remote I/O, a handheld device wasused to communicate with the instrument. [1]

Figure 4. HART communication superimposed on an analog signal. (Source:http://www.romilly.co.uk/hartwave.gif Accessed 2005-10-07)

By sending the process value as an analog signal, the device is still compatible to older sys-tems and whether to use the advantages or not is up to the user. A problem is that special in-struments have to be bought to communicate with HART devices, and the operator had to goout on the field and connect it to the cable for the device. [1], [2], [3]

3.3 Fieldbus in generalIn the 1980s, a new way to communicate between the devices and the control system began todevelop, called fieldbus. The fieldbus is an all-digital way of communicating, which givesnew possibilities for intelligent devices and new solutions.

A fieldbus is constructed as a bus, which means that several devices share the same cable, seeFigure 5. Because of digital communication, more than one variable can be measured in one

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device. These reductions in cable and number of instruments can reduce installation time andcost. Fieldbus devices can also be made more advanced, like a thermal camera, which meas-ures several points. The devices can also signal when they are about to break down or if thereis something else wrong, for example a valve that can not close. Since all devices share thesame cable, it enables direct communication between the devices. This can be used to move

the control from the central system and put it out in the field. By doing this, important seg-ments can work properly, even if process station stops or a cable breaks between the deviceand process station. The data can also be shared to operators by connecting the host to an or-dinary intranet (Ethernet) or even Internet. In this way the electrician does not have to be atthe site to do for example diagnostics.

Figure 5. On a fieldbus, all devices are connected to the same cable.

The cable can be of different types depending on desired speed, electromagnetic interference,cable length etc. Copper, fibre optics or even radio links can be used in some cases. Somefieldbuses are bound to use one type while other supports several.

Fieldbus gives the opportunity to use multivariable devices, which is devices that can measuremore than one process variable. One example is the multivariable transmitter 2100T fromABB that measures mass flow with absolute pressure and temperature compensation. Theadditional information can be used both to get a more accurate reading, by compensating fortemperature, and also send more information to the operator thus reducing the number of de-vices. [2], [4]

3.3.1 Geographical differencesWhen the fieldbuses began to develop, each manufacturer made their own bus with their owncommunication protocol. The problem was that only devices from that manufacturer could beconnected to that bus. To avoid this problem, the busses where made independent from themanufacturer by creating organizations which was open for everyone. These organizationsthen tried to make their communication a standard, so that it would be used everywhere. Itusually began at a national level where it was rather easy to be accepted as standard. It hap- pened simultaneously in different countries and the problems began to arise when these wheretrying to become international standards.

BSi British Standards describe a standard as a published document that contains a techni-cal specification or other precise criteria designed to be used consistently as a rule, guideline,or definition. (British Standards (2006); What is a standard? accessed 2006-02-09, availableonline at http://www.bsi-global.com/British_Standards/Standardization/what.xalter). The ideais that if everyone follows the standard, all instruments would be compatible with all systems.The problem was that there were two promising fieldbus solutions in Europe, GermanProfibus and French FIP. They had different approaches and both had spent too much time

10110110Controller T

Fieldbus

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and money developing their fieldbus to give it up. Several articles, like [5] and [6], have stud-ied what could be called the fieldbus war. It all resulted in a compromise standard (IEC61158), which included 8 fieldbuses, all of them incompatible with each other. The fieldbusesare Foundation Fieldbus H1, Controlnet, Profibus, P-net, Foundation Fieldbus HSE, Swiftnet,World-FIP and Interbus-S. [7]

The different fieldbuses where supported in different parts of the world and therefore, thereare geographical differences between them. Below are the geographical locations of the fourfieldbuses studied in this Thesis. [5], [6]

Profibus originates from Germany (Siemens) and it is therefore natural that it has a stronghold in Germany and the whole of Europe. It is growing in the rest of the world as well. It hasabout 20% of the worlds fieldbus market and is used in a wide range of industries, such asautomotive, production machinery and metal processing. PNO (PROFIBUS Nutzer Organiza-tion) is the organization that controls Profibus.

DeviceNet comes from the US (Allen-Bradley), which is also its main market. It is especiallydeveloped for the factory automation and is competing with fieldbuses like Profibus DP andInterbus. It is used in general production machinery, automotives etc. It is also found in facto-ries in Asia.

Interbus originates from Germany (Phoenix Contact) with Europe as the main market. Inter- bus was developed in 1990 and was the first fieldbus that was manufacturer independent. It iscontrolled by Interbus Club. The fieldbus is mainly used by automotive manufacturers.

Interoperable System Project (ISP) and WorldFIP North America created Fieldbus Founda-tion, as a result of that the Europeans could not agree to one standard. It is a rather new field- bus and therefore not as large as the others, but they are growing rapidly. The main market is North America. For the process industry, for which Foundation Fieldbus was specified, theyhave equal shares with Profibus PA in Asia. [8], [9], [10]

3.3.2 Diagnostics and predictive maintenanceAccording to [11], one of the main costs for industries is maintenance. Therefore, managersoften try to reduce costs in this area. The maintenance today, is mainly based on time limits ora consumption trigger, like the number of full strokes for a valve. This is exactly what we dowith our cars, when specific parts are changed after a number of kilometers. It is called pre-ventive maintenance. If the intervals are short, the risk for failure gets lower, but the costs get

higher. Even when set at a reasonable level, one problem still exist. Preventive maintenancedoes not avoid failure only reduce the probability. Another maintenance strategy is mainte-nance on fault, which basically means that the instrument is used until it breaks down and isthen replaced. This strategy is not acceptable for devices that the process depends on and afault would cause a standstill, since they cost a lot of money. There is a third maintenancestrategy, called predictive maintenance. As the name suggests, the aim is to predict when afault will occur, and hopefully be able to predict it just in time to save money and avoid faults.[11]

However, predictive maintenance is not as easy as it sounds. Very little is known about whichdata that is useful or how to use the data to predict faults. It also depends on what kind of de-

vice and fault it is. Measure instrument faults are hard to predict because it mostly consist ofelectronic components, while valve positioners are easier since they have a relevant mechani-

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cal part. Motors are other devices where faults can be and has been predicted. Motors, how-ever, sends lots of data and is usually not connected to Profibus PA or Foundation FieldbusH1. Since these are the fieldbuses focused on in this thesis no more effort will be put into pre-dictive maintenance for motors. [11], [12]

In theory, the fieldbus is not a requirement for diagnostics of intelligent devices. HART de-vices are fully capable of sending diagnostics, but the problem is to get the information intothe control system. The usual way to get diagnostics from a HART device it to connect ahandheld device directly to the analog line connecting the I/O and device. It can however besent to the operator or engineer from the I/O cabinet by using fieldbuses like Profibus DP[13]. With fieldbus connected instruments, the diagnostic data is available to the operator orengineer with little or none work, which is not the case for HART devices. [14]

Two articles have been found, each describing a plant that practice predictive maintenance[15], [14]. The first one, Cargill Vitamin E Plant in Eddyville, Iowa has been using predictivemaintenance since 2001. They have documented savings through improved maintenancewhich resulted in better reliability. The software they are using is the AMS Suit from Emer-son, which is capable of instrument commissioning, configuration and trouble shooting. Allmaintenance activities are also automatically documented, like which changes are made in aninstrument configuration. This reduces documentation time and makes it easier to trackchanges. [15]

An example, where diagnostics were used, was a travel deviation alert from a control valvesent to the engineer. From the control room it seemed to work properly. When the technicianschecked out the valve, they found that the air supply line of plastic was melted, because it wastoo close to a steam line. It resulted in a collapse of the line, which limited the air supply tothe valve, which caused it to respond very slowly. This increased the variability of the proc-ess, without the operators knowing why. [15]

In [15] the importance of knowledge is told. They recommend that one person should be assetmanager, with good knowledge about the process, instruments and the software used. Anotherrecommendation is not to expect immediate result, because it takes time to learn the system,software and how to work with predictive maintenance.

The Total Solvants' Oudalle plant in the Normandy region of France is another example of a plant that uses predictive maintenance. Like the Vitamin E Plant, this plant uses AMS fromEmerson for diagnostics. They have also found that the documentation is easier since the

software documents all changes of the device settings. Before they used predictive mainte-nance, all of their 100 control valves were checked annually. Today, only 10 valves have to be checked each year, saving 90% of the maintenance work on control valves. [14]

3.4 4-20 mA vs. fieldbus instrument technologyThere are plenty of advantages with fieldbus. One feature is the ability to use more advancedinstruments like a temperature-measuring camera that measures more than one point. Thefieldbus can also increase the accuracy and reduce noise. The value sent from a fieldbus de-vice is not altered on its way to the process station, while the analog signal is subjected tonoise and interference. The fieldbus also has only one analog to digital conversion while the4-20 mA has two and one additional D/A conversion, see Figure 6. These are all subjected toquantization errors. The deviations that show up in a fieldbus system are deviations in the

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process itself and not some noise picked up on the way. This gives the opportunity to analysethe process itself, with greater extend.

Figure 6. Comparison of communication link between the sensor and control system with 4-20mA (left)

and fieldbus (right).

The diagnostic functions of a fieldbus device can warn the operator before an accident or a breakdown occurs that could cause an unplanned standstill. The factories are growing biggerand so is the cost for a standstill. In case of a breakdown, either a cable or a device, a fieldbuscan in some cases make the error detection easier. In hazardous areas, the enhanced diagnosticabilities mean fewer visits to devices in hazardous areas thus minimizing personnel risks.

Some fieldbuses have the ability to distribute the control out to the devices instead of using acentralized control. By doing that, the central system only needs to monitor the process. Thesystem becomes less dependent on a single computer or PLC and by creating independent

subsections, each of these can be shut down and maintenance can be preformed individually,if the process allows it.

However, there are drawbacks as well. 4-20 mA is an industrial standard that is used every-where. It is simple to understand and easy to use. Even if the support groups of differentfieldbuses states that they are an international standard, there are several of them. Currently,they do not seem to come together to form one united standard, but instead make their busmore unique to push it forward. [3]

3.5 Wireless fieldbusBelow is a short literature review of advantages and disadvantages with wireless fieldbusesand after that, an overview of wireless products, mainly for fieldbus usage.

UnidirectionalBidirectional

4-20 mA

Controlsystem

A/D

D/A

Microprocessor

A/D

Sensor

D/A

Microprocessor

A/D

Sensor

A/D

Microprocessor

A/D

Sensor

Microprocessor

A/D

Sensor

Fieldbus

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3.5.1 Advantages and disadvantages with wirelessSome of the advantages with wireless connections are reduction in cost and time for installa-tion and maintenance of the signal cable. It is also useful for connecting instruments frommobile units or other places, where chemicals or vibrating parts, are likely to wear of the ca- bles. It will also make the system more dynamic since instruments are easier to move and themaintenance personnel can use a handheld wireless device to configure or get diagnostics.[16], [17]

Fieldbuses are dependent on deterministic behavior to live up to hard real-time demands forcyclic polling of data. If a part of, or the entire system, is built on wireless links, these have tolive up to the same demands. Wireless connections are more affected by noise and interfer-ence then a copper wire, messages can be lost and the message has to be re-sent. This makes ithard for wireless links to meet real-time demands. Security is another issue. It is easy toeavesdrop, send malicious packets or disturb the transmission. Energy supply is an importantissue, mainly since field devices are usually fed with power by the signal cable. And if a cableis needed it is not truly wireless. [16], [17]

These issues have to be solved before wireless can become the industries first choice. Even ifit might not be ready to install in every part of any plant in any industry, it is already used inmore specialized areas where cables are likely to break or cables are difficult to install. [18]shows that an aluminum plant, with large mobile units, could reduce the number of produc-tion stops due to wire wear.

3.5.2 A product overviewAlthough wireless technology has been known for a long time, it is not widely used for devicecommunication in the process industry. One reason can be that they still need a power supply.

A separate power cable can be used, but then most advantages are lost. Batteries are anotheroption, but usually instruments are placed in tight and not central areas and to change batterieswould be a hard job. Instead manufacturers are looking for new alternative power sources.One that is already in use is induction. ABB has a wireless proximity switch, which is pow-ered by an electric field. The field is generated by an alternating current in a power loop and acoil inside the instrument that draws power from the field. [19]

There are products available to send fieldbus data across the network. One example forProfibus DP is an optical link from HIRSCHMANN. It provides a wireless link, 0.5 to 15 m, between two or more segments. Since it is an optical link, line-of-sight is needed. One limita-tion is that masters are only allowed on one of the segments. Supported speeds are 9.6 kbit/sto 1.5 Mbit/s. Each or these segments still have to be supplied with power. [20]

Omricon has a similar product for DeviceNet. It uses radio transmission and has a range of upto 60 m in indoor environments. The system has master and slave modems, and several mas-ters can operate in the same area. The speed is however limited to 100 kbit/s. One advantageis that the usual six meters limits for spur cables can be overcome. The power supply is unfor-tunately still needed for each subnet. [21]

ELPRO Technologies has a wireless modem for longer ranges. It uses radio frequency forcommunication and if it has line-of-sight it can work for up to 20 km. The usual workingrange however is about 1-2 km. The range can be extended with up to four repeaters. To beable to transmit this range, it uses more power than the other wireless links described in thisThesis. It has support for and can connect to several fieldbuses / protocols for example

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Profibus, DeviceNet and Ethernet. Each modem also has eight discrete I/O that can be config-ured as either input or output. The modems support the same transfer rates as the fieldbus.[22]

3.6 Description of Profibus, Foundation Fieldbus, DeviceNet and InterbusA fieldbus is usually designed for one kind of industry. Figure 7 shows which are best suitedfor production or process industry. The fieldbuses designed for process industry are Founda-tion Fieldbus, Profibus PA, HART and AS-Interface. HART is not a fieldbus but merely adigital extension of the analog devices. However, it plays an important role in development ofintelligent field instruments. AS-Interface, described in Appendix A, is a fieldbus, but is verylimited. It is constructed for binary devices with a maximum bus length of 300 m. It is usuallyused in combination with an ordinary fieldbus. To extend the knowledge about fieldbuses,and not only for the process industry, a short description of DeviceNet and Interbus is alsoincluded in this section.

Figure 7. Area of usage for the different fieldbuses. Source: [23], page 15, figure 6

3.6.1 ProfibusProfibus consists of three different busses, Profibus PA, Profibus DP and PROFInet. Profibus

PA and Profibus DP are used at the field level. Profibus PA has a lower speed but instead itcan supply power in the cable. Profibus DP is more suited to send lots of data from for exam- ple drivers. PROFInet is used to connect the different networks and can be used to give theoperators information about the system on their PC through the local network (Ethernet). Thisalso gives the opportunity for the electrician to configure the devices from any PC connectedto the intranet. [24], [25]

PROFInetPROFInet is a wide automation concept that has been developed due to the increased use ofmodular, decentralized control. PROFInet is both a specification and an open, system inde- pendent software that handles the run time communication.

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A Profibus DP or Profibus PA section can be connected to PROFInet by using a proxy. Withthese, all devices can be accessed directly from the PROFInet network. PROFInet usesEthernet to communicate between devices. [26]

Profibus DP (Decentralized Periphery)

The Profibus DP bus is mainly used to communicate between the PLC (Programmable LogicController) and the decentralized periphery. Profibus DP can connect segments of ProfibusPA and remote I/O cabinets with its high-speed bus. It supports DP-V2, which is described below. Profibus DP communicates through RS 485 and has a maximum speed that dependson the length of the bus, see table 1. Profibus DP can be used in hazardous areas if RS 485-ISand Profibus DP-Ex barriers are used. With RS 485, one segment can have up to nine repeat-ers. If fibre optic cables are used, any number of repeaters can be used. The maximum numberof devices that can be connected to a Profibus DP bus is 126, due to limitations in addresses.That limit is however theoretical. In practise about 40 60 devices are maximum for ProfibusDP since the response time would become too high otherwise. [24]

Table 1. Maximum segment length at different transmission speeds for Profibus DP with copper cable.Transmission

speed(kbit/s)

Maximumsegment length

(m)3 000 12 000 1001 500 200500 400187.5 100093,75 9,6 1200

Profibus PA (Process Automation)The Profibus PA bus was developed for the process industry. The bus communicates througha two-wire cable and corresponds to the MBP (Manchester coded, Bus Powered) standard.This standard is used in process automation because it can power the devices connected to it.As Profibus DP, Profibus PA also has an intrinsically safe version, where MBP-IS is used andcan therefore be used in hazardous areas like chemical plants. The specification states that the bus cable should be a twisted, preferably shielded two-wire cable and must be terminated at both ends to avoid the signals from reflecting back. The bus speed is 31.25 kbit/s and up to 32devices can be connected on one bus segment. This number can be reduced because the de-vices demand too much current or the cycle time becomes to long. To send 1 or 0 on the bus,a device increases or decreases its power consumption by 9 mA. It will increase or decrease

the voltage of the bus with 0.5 V which can be detected by the other devices connected to the bus. A special power source has to be used so that it does not try to compensate for this loss.The signals are Manchester coded which means that the signals are read at the middle of a bitcell, see. A rising edge is a 1 while a trailing edge is a 0. This coding is self-clocking, whichmeans that the receiver can determine the clock rate of the transmitter from the signal. In each bit cell the signal is high for half of it and low for the other half, which means that the mean bus voltage is independent of the number of ones and zeros that are sent. MBP supports all bus topologies and have a maximum length of 1900 m.

Repeaters can be used on a Profibus PA segment to extend the number of devices or themaximum cable length. Up to four repeaters are allowed and give a total of 126 devices and amaximum length of 9500 m. Profibus PA allows cyclical communication, DP-V0, to send

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process values and acyclical communication, DP-V1, for diagnostic and configuration values.[24], [27]

Figure 8. Manchester coded data.

Communication protocol DP

Profibus uses the communication protocol DP to communicate between the devices and the process station. There are three different versions, DP-V0, DP-V1 and DP-V2. They are com- patible with each other and can be used on the same bus. Profibus DP can use either of themwhile Profibus PA uses DP-V0 and DP-V1. A higher version supports all functions from theversions below.

The functions are:

DP-V0

Cyclic data transfer Diagnostic Configuration via GSD files

DP-V1

Acyclic data transfer 1 Alarm handling FDT/DTM and EDD device management Function blocks acc. IEC 61131-3 PROFIsafe

DP-V2

Broadcast communication (one to one/many) Time and time stamp Isochronous mode2 Up- and download functions Hart on DP Redundancy

[26]

1 Acyclic data transfer enables data to be transferred in between the cyclic data. By using it, setting parameters

and calibration of instruments are possible in runtime.2 With isochronous mode, highly precise positioning process with less than one microsecond in clock deviationis possible. This works independent of the busload.

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As there are several devices sharing the same bus, rules are needed to control which one isallowed to send. Otherwise, conflicts can occur if two devices send at the same time. It issolved by using a MAC (Medium Access Control) protocol. Profibus usually works on a mas-ter/slave system where the master asks a slave for data and the slave immediately respond.The slave does not initiate any communication without being allowed by a master. Profibus

allows several masters on the same bus and therefore need to control which master that is al-lowed to send. It is controlled by a timed token that is passed between the masters. Only onetoken exists on the bus and the master that has it is allowed to pull data from its slaves. Whenall slaves have been pulled for data or the maximum allowed time has elapsed the token is passed to the next master. [26]

Process value diagnosticsProcess values in a Profibus system are sent as a 32-bit floating-point number (IEEE 754).The value is calculated as follows

Process value = (-1) sign * 2( E -127) * (1 + F )

Where sign , E and F are in the bits shown in Figure 9.

Bit 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16Sign Exponent ( E ) Fraction ( F )

27 26 25 24 23 22 21 20 2-1 2-2 2-3 2-4 2-5 2-6 2-7 Bit 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Fraction ( F )2-8 2-9 2-10 2-11 2-12 2-13 2-14 2-15 2-16 2-17 2-18 2-19 2-20 2-21 2-22 2-23

Figure 9. The value of each bit in an IEEE 754 floating-point number.

Together with the process value, a status code is sent which is one byte. This diagnostic bytegives an easy access to the status of the device, especially for transmitters with limited diag-nostics. Valves have more diagnostics where this byte is not enough. Examples of useful in-formation sent with the byte are; the process value has exceeded a limit, the thermo element isnot connected or the device is out of service. The first two bits, 0 and 1, represent the quality, bit 2 to bit 5 is sub-quality and bit 6 and 7 shows limit status. The errors that are reported de- pend on the device. Some status codes are the same for all devices likeGood value and Out of service while other are device specific like lead breakage of the sensor for the temperaturetransmitter. Table 2 shows some common status codes.

Table 2. Common status codes for ProfibusStatus Description0x00 Bad value0x1F Device Out of Service0x80 Good value0x89 Good, LOW_LIM alarm active0x8A Good, HI_LIM alarm active0x8D Good, LOW_LOW_LIM alarm active0x8E Good, HI_HI_LIM alarm active

PROFIsafe

Tasks with high demands on security like emergency stop buttons usually have to use a spe-cial bus or conventional technology. To be able to connect these devices to Profibus DP,

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PROFISafe was developed. It defines how security devices should communicate with eachother on the Profibus so that it can be used for secure automation. PROFISafe checks for sev-eral errors that can occur on a serial link like delays, lost or repeat of data, data in the wrongorder, wrongly addressed data or corrupt data.

PROFISafe is a software that put itself on top of the Profibus DP protocol, in the ISO/OSImodel. It can work with devices that do not use PROFISafe without disturbing the rest of thedevices. It uses acyclic transmission to communicate and works on MBP, RS485 and fibreoptic cable. [24]

Interaction with HARTBecause of the large number of HART devices installed in the industry today, it has beenmade possible to connect it to Profibus. The HART software is implemented into the slaveand master and can be send on the Profibus. The HART devices can be connected to theProfibus via a HMD (HART Master Device). Several HART devices can be connected to oneHMD. [26]

Device configurationTo be able to communicate with different devices, a GSD (General Station Data) file is usedto specify the communication properties for a device. For the application functions of a de-vice, like configuration of parameters and variable ranges, EDDL (Electronic Device Descrip-tion Language) can be used. For more complex applications, FDT/DTM is used.

GSD is a plain text file with information about the communication with a specific device.There are both required data, like Vendor_Name, and optional data depending on if it is amaster or a slave. Other information that can be stored in a GSD file is parameters, with datatype and range, and product identification number. Diagnostic messages can also be set in thisfile. The GSD files belong to a product and should come with the device. There are also gen-eral GSD files, which makes it possible to connect for example a pressure transmitter fromany manufacturer to the bus. GSD is not suitable for application related parameters and func-tions for field devices. Therefore EDDL is used for a more precise description of the device.The main reason for developing EDDL is its ability to describe functions for a fieldbus.

EDDL and GDS are limited in use and are not useful for describing complex devices and notstandardized, specialized properties in intelligent devices. With FDT (Field Device Tool), aDTM (Device Type Manager) is written by the manufacturer and can contain specific deviceinformation. DTM can be compared with a PC driver, like a driver for a printer. Once in-

stalled, the PC can communicate directly to the printer and also use its special features. In thesame way, the electrician can access all parameters and diagnostic data from the device, if thecorrect DTM is used. The DTM also specifies the Human Machine Interface (HMI) for thedevice. The FDT program is needed to use the DTM files and connects the device with theelectrician. [26], [28]

3.6.2 Foundation FieldbusFoundation Fieldbus has two different busses. H1 is a slow and, if needed, intrinsically safe bus that can supply power to the devices connected to it. High Speed Ethernet (HSE) is thesecond one used for communication between process stations.

On a Foundation Fieldbus the control can be placed inside the actuator instead of in the proc-ess station. It puts higher demands on the actuators and more hardware in the devices, but at

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the same time reduces the work for the process station. One advantage of putting the controlin each actuator is that the entire system does not have to go down if the process station loosecontact with the segment.

H1

The H1 bus, like Profibus PA is based on MBP, see chapter 3.5.1 for details. The followingapplies to the Foundation Fieldbus H1 bus.

Each device should have at least nine Volts to work properly. Without repeaters, the maxi-mum length of a one segment is 1900m. With up to four repeaters, five segments can be con-nected can reach 9500m. Maximum numbers of devices per segment is 32. By using repeat-ers, 240 devices can be connected to the network. But there can be no more than four repeat-ers between two devices. The bus speed is 31.25 kbit/s. [29], [30]

Foundation Fieldbus supports both scheduled and unscheduled transmission. Real-time appli-cations that are time critical, like the control of valves, uses scheduled transmission to assurethey are executed in time. Tasks like parameterization and diagnostics, which are less timecritical uses unscheduled communication services and uses the time between the scheduledonce.

As stated earlier the control can be done directly by the actuators. It can contain a PD (Propor-tional/Derivative) or PID (Proportional/Integral/Derivative) function block that can be con-nected to an instrument. If they are on the same bus, actuators can read the instruments when-ever they send their data. However, there is still need for controlling access to the bus. A LAS(Link Active Scheduler) is used to control the transmission. It is a device that has a scheduleof all scheduled transmission and controls the communication by polling information from theinstruments.

There are two types of devices on a H1 bus, basic devices and Link Masters. Link Master de-vices are devices that have the potential to become a LAS. Therefore, each Link Master isconfigured with the same schedule. If the active LAS goes down another Link Master takesover. This makes the bus less dependent on one device. To synchronize the time of all devicesthe LAS sends out a Time Distribution on the bus.

When the engineer configures the system, a schedule is made for each device. It states whenscheduled tasks should be preformed. For an instrument it could be 1: reading the Analog In(AI) and 2: sending the information. The second task should be executed a fixed time after the

first to ensure that the value has been received. From the configuration of all devices a sched-ule is created, specifying when a device is allowed to send.

The LAS also polls for unassigned device addresses in between the cyclical communication,which makes it possible to connect devices during operation and integrate them into the sys-tem. [30], [31]

Example of a scheduled data transmissionTo give an example of how a scheduled data transmission is handled by the H1 bus, one cycleof the system in Figure 10 will be described. Assume we have two instruments and one actua-tor that depend on the instruments. The tasks should be executed at time offset in Table 3.

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Figure 10. Example of a Foundation Fieldbus system.

Table 3. A time scheme for devices.Device Type Action Offset

S1 Instrument Read AITransmit read data

030

S2 Instrument Read AITransmit read data

040

A1 Actuator Execute PIDSet AO

5079

The following is happening:At time 0, both instruments read their value from AI (Analog In).At time 30, the LAS send a message to S1 to broadcast its data. A1 is dependent on S1and when S1 broadcasts its value A1 stores it.At time 40, S2 sends its data after permission from LAS. No devices can write to the bus unless the LAS permit it. It is done to avoid conflicts on the bus.At time 50, A1 has received both values and can therefore execute the PID controller.At time 79, the PID has calculated the desired value of AO and it can be set.After a fixed time, the loop begins from the top again.

HSE (High Speed Ethernet)HSE can create a control backbone for all devices in the plant. It provides a wide networkwhere the electrician can manage functions like calibration or diagnostics throughout the plant. Users can connect different segments for basic control, emergency shutdown etc. Byusing a LD (Linking Device), data from one H1 segment can be send directly on the HSEnetwork.

HSE uses standard Ethernet technology and provides peer-to-peer communication, removingthe need for a central computer. Ethernet is available at low cost and widely used. It has highspeed (100Mbit/s), which makes it possible to send lots of data. The drawback is that Ethernetuses random bus access and can therefore not be used for time critical tasks.

The same function blocks are used in H1 and HSE and therefore the same programming lan-guage can be used for the entire system. [30]

3.6.3 DeviceNetDeviceNet is an open standard that is controlled by Open DeviceNet Vendor Association,ODVA. It uses the CAN (Controller Area Network) bus to send information across the net-work and CIP (Common Industrial Protocol) for interpretation of the data. As with Profibusand Foundation Fieldbus, several manufacturers supply the same product and these are inter-

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changeable. Interchangeable means that a device from one manufacturer can be replaced withone from another.

DeviceNet supports multidrop topology and can provide power through the cable. But it hasfour wires, two for power and two for sending signals. The communication over the bus is

controlled by CAN, which uses a priority system to avoid several devices to send messages onthe bus at the same time. The system can use either master-slave or a peer-to-peer if the con-trol is distributed throughout the system. Multimaster and change-of-state, which is an eventdriven message, are also allowed. Several devices, independent on which buss communicationis used can receive one message.

Devices can be added or removed from the network and new data paths can be added whilethe system is on-line. Power taps can be added at any point on the network and can thereforegive redundant power supply. The maximum speed depends on the length of the bus and thedrop lines, which is shown in Table 4.

Table 4. The end-to-end network distance varies with data rate and cable thickness. (Source:http://www.odva.org/10_2/05_tech/PUB00026R1.pdf, page 2, table 2)

Data Rates 125 KBPS 250 KBPS 500 KBPSThick trunk length (m) 500 250 100Thin trunk length (m) 100 100 100Flat trunk cable (m) 380 200 75Maximum drop length (m) 6 6 6Cumulative drop length (m) 156 78 39

According to the CAN specification the bus can be in two states: dominant (logic 0) or reces-sive (logic 1). Any device can put the bus in dominant state, while recessive is only possiblewhen no device is in dominant state. All devices listens on the bus, even when they are send-ing, and if the state they receive is not the same as it sends, another device is sending at thesame time. This is used to make a priority system that makes sure there are no collisions onthe bus. An 11-bit identifier is being sent at the beginning of each transmission and that iswhere the priority is put. Each packet contains 0-8 bytes of data, which is enough for mostdevices. But if longer messages are needed, DeviceNet has a fragmentation protocol that han-dles larger data amount. At the end of the frame, a CRC field is used to check for transmissionerrors. There is also an ACK bit that is used by the receiving devices to acknowledge that themessage has been received.

The data is sent with the CIP which is object oriented. An object has attributes, services and behaviour. In common devices there are a standardized set of objects. These devices can then be exchanged with devices from different manufactures without changing the programming.CIP is not dependent on a specific transport network, like a CAN bus or Ethernet, and cantherefore travel between these.

Even if DeviceNet fundamentally uses peer-to-peer communication, it also has a communica-tion schedule for Master/Slave connections. This can be used when the data communicationlinks are known at power-up. The data can be either polled, where one or several can receivethe message, cyclic, where the data is produced at a predefined rate or change-of-state, where

the data is sent when it changes. The last option also has settings for minimum and maximumtime between transmission, to give an alive signal and avoid flooding the bus. [32]

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3.6.4 InterbusInterbus is a fieldbus that has well developed diagnostic abilities. It is always build with a ringtopology. This is however not always shown out in the field, because the bus passes the de-vices twice, on its way out and on the way back. Because of this, the communication is virtu-ally bidirectional. Smaller loops, often containing simple devices, can use a true ring topol-ogy. Figure 11 shown what an Interbus connection can look like.

Figure 11. An example of an Interbus fieldbus segment. (Source:http://www.interbusclub.com/en/doku/pdf/interbus_basics_en.pdf page 8, figure 4)

Because of the ring topology, the devices have point-to-point connections with theirneighbors, which means it can cover long distances. Interbus is a mono master system andone frame send through the entire system contains information to and from all devices. Theaddressing of the devices is automatically assigned, by using their position in the ring. Theengineer is able to assign aliases for the device addresses, which makes it easier to add or re-move devices without re-addressing existing devices.

The system is divided into three different structure types, Remote Bus, Local Bus and Loop.The controller board is the master in the system and controls the data traffic. It is the connec-tion between Interbus and a higher-level network like Ethernet. It also handles diagnosticmessages and if it has a display it can display these messages there. The controller board isconnected to the Remote Bus. The data can be transmitted using several medias, copper (RS-485), fiber optics, infrared etc. It can also supply current to devices connected to it. Thetransmission speed is 500 kbps and the maximum distance between Remote Bus devices is400m. This can be achieved since each device works as a repeater and allows a total maxi-

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mum bus length of 13 km. If fiber optic cable is used instead, the maximum length is 80 km.Bus terminals are connected on the Remote Bus and can branch out Local Buses. These alsodivide the system into subsystems, which can be individually closed. Decentralized instru-ments and actuators are connected by a loop. These have to be connected as a ring and the busdoes not pass it twice. They are connected by a two wire cable which both supply power and

transports data. 63 devices can be connected in the loop at a maximum distance of 20 m be-tween two of them and a total distance of 200 m.

To integrate planning, configuration, diagnostics etc. into one tool Interbus uses CMD (Con-figuration, Monitoring and Diagnostic). After the system is finally connected, it starts upautomatically and can be configured with the CMD tool. Each subsystem can be tested sepa-rately using the monitor function. The controller board can be programmed to do limited processing by using IEC 61131 programming language, Function Block Diagram. It can beused to do time critical control tasks and reduce the load of the control system. To con-nect/configure a device most manufacturers provide electronic device description. There arealso standard profiles for these that make it possible to change a device from one manufactureto another without changing the programming.

When a bus error occur, like a broken wire or device, the ring structure makes it possible to both localize the error and continue to run the part not effected by the error. This is done byletting the device closes to the error send the information back instead of sending it to thefaulty link. A CRC is used on each point-to-point transmission between devices to make surethe received information is correct. By looking at the error statistics from these, parts withlarge disturbances caused by for example a weary slip ring can be detected and replaced intime. [33], [34]

3.7 Practical experience from fieldbus installationsAfter searching the Internet for articles describing fieldbus installation in practice, I found thatthat I am not the only one trying to find out how the fieldbuses work in practise. Three docu-ments [35], [36] and [37] were found useful and reliable enough to use.

The first article [35] comes from vrmeforsk, an organisation where heat and power producersmeet to share experience and form research groups. The author visited four installations inSweden. The first one was an energy supply central with a heat pump and two refrigeratingmachinery at Bo01, which is a residence trade fair in Malm. The second one was Barse- bcksverket waste disposal, which has a system for handling of radioactive waste. The thirdone was Nimrod in Stockholm, a district refrigeration plant with four refrigeration units. And

the last visited plant was Scanraff, a Propane plant.The second article [36] tries to estimate the economical differences in installing a fieldbusinstead of remote I/O. A new plant was also constructed based on the report.

The third document [37] was found on the Foundation Fieldbus web server. It describes theexperience from implementing a fieldbus in Western Australia at a sodium cyanide plant. It isthe least reliable of these articles but it shows that knowledge and experience is importantfactors when it comes to fieldbuses.

3.7.1 Reduction in time and moneyThe largest saving was found at Nimrod where the refrigeration plant saved as much as 50%of the project time and an estimate of 30% cut in costs. The major cuts where made in com-

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missioning. One reason for the large cuts can be that the plant has a sister plant that wascommissioned one year before. The experience from that was used in constructing the newone. However, it reveals that there are huge potential savings if fieldbuses are used with ex- perience. The electronic installation gave a cut of 25%. The cut was not larger because thefieldbus has to be protected from disturbance, grounded correctly and a more expensive cable

has to be used. The documentation is another area where a lot of time was saved, because thenumber of circuit diagrams was reduced.

At Bo01 the commission was a part that saved a lot of time. Mainly because when one devicewas configured, that information could be reused for the other devices of the same sort. Thefieldbus is therefore more profitable the larger the plant is. There where some problems, but itwas not worse than if conventional I/O would have been used. The fieldbus devices are a littlemore expensive but the reduction in commissioning time makes the fieldbus installationcheaper than remote I/O.

At Barsebcksverket the modernisation would be taking place during operation. Another con-dition was that a lot of the existing equipment should be used, for example cables and instru-ments. Because of it, the savings in commissioning was very small, since the fieldbus had to be adapted to the existing installation. Barsebcksverket estimate that the cost was slightlyhigher compared to a conventional installation.

For Scanraff, the instrumentation went faster than conventional technology, while the time formeetings and work with construction, configuration and layout took more time.

The theoretical calculations conducted in FuRIOS estimate the cost reduction to 4.2%. Theystate that a reduction of up to 20% suggested in former studies is due to the fact that theycompare to conventional wiring and not remote I/O. The cost reduction when changing fromconventional wiring to remote I/O is therefore roughly 15%. The greatest cut in cost wherethe I/O system, with a reduction of 18.1%. This was mainly because the I/O cabinets and their power supply were replaced with fieldbus barriers. The fieldbus barriers are light, small andeasy to install compared to remote I/O cabinets that have to be carefully assembled. Thefieldbus is more dynamic since it does not have to be decided in advance where each devicehas to be connected. The barriers are more decentralized which can make the spurs to the de-vices shorter. The cost for fieldbus devices where slightly increased (+0.6%). However, thedevices where replaced by an equal one, and if multivariable devices where used instead, itcould have lowered the cost. The use of standards, reduction of error sources and easier faultdiagnostics due to transparency have the potential to make commissioning much faster. It is

not necessary to make a loop check. The device just has to be connected and checked whetherit shows a sign of life. For a 12-18 month project, the FuRIOS report estimate the reductionin time to 10 days. This report uses prudent calculations and it is fair to assume that some ar-eas can save more time and money.

3.7.2 Practical experience installation and commissioningAt Bo01 they have established that the usage of fieldbus advantages in devices differs a lot between the manufacturers. While some uses fieldbus to its full extend, others just support thenecessary basic functions. While installing, the data cables where continuously checked withrespect to impedances to assure it was as resistant toward interference as possible. A wrong-fully grounded cable can give random errors that are hard to find the source of. To make theProfibus DP bus more resistant towards interference, the communication speed was set to halfthe maximum speed. When configuring and connecting the devices to the control system

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some problems occurred. One GSD file did not work in DeltaV, which is a Digital Automa-tion System from Emerson Process Management. It was also problematic to find correct in-formation about the memory mapping, which defines where in the memory different parame-ters are stored.

Barsebcksverket experienced minor problems during commissioning but no worse then ifconventional technology would have been used. It is important to have the right version of theGSD file and suppliers were unfortunately not good at either giving a GSD file nor give theright one. They also found that a segment can work properly, even without termination, butintermittent errors can occur that are hard to trace.

Since the signal cable is more sensitive to interference, Nimrod decided to use a separate ca- ble ladder for fieldbus cables. They also made it easy to identify and follow a buss segment byusing different colors for the different segments.

At Scanraff there were problems with finding the driver for the devices. There have also been problems with the first generation DeltaV H1 cards that were not stable. It stopped to respondand had to be restarted. Upgrading the software solved the problem.

The sodium cyanide plant in Australia experienced a lot of problems, mainly because theycould not find a contractor with fieldbus experience. During installation, they made sure thescreening was properly connected since they thought it to be a possible issue during commis-sioning. When commissioning the host vendors pressure and differential pressure transmit-ters, it was fairly straightforward. However, the third party devices, for example valves fromthree different vendors, took longer time. This was mainly a result from lack of experiencewith third party Foundation Fieldbus devices. They where also more advanced devices wheredata was sent in both directions. While the commissioning of host transmitters took minutes,it took on average one day to commission each valve.

In FuRIOS 2, the plant build on the FuRIOS report confirmed that there are saving potentialsin commissioning. They also found that the installation gets more flexible since it does nothave to be decided in advance, exactly which and where the devices should be placed.

3.7.3 Practical experience maintenance and running a fieldbus systemAt Bo01, the personnel are generally positive to the usage of fieldbuses. A lot of informationabout the devices is presented which makes the fault finding easier. Diagnostic tools have forexample been used to find fault on Foundation Fieldbus transmitters.

Barsebcksverket has not experienced any major problems with the bus. The only problemwith the bus itself was that the communication heads on a few valves stopped to work becausethey had old versions.

Nimrod uses the extended diagnostics that the fieldbus provide. One example is alarm frommalfunctioning devices and communication errors. The configuration software that is used toget diagnostics from the devices is an integrated part of the software packet PCS7 from Sie-mens. A new type of error that can occur on a fieldbus is consequence alarms. This error can be triggered by a short circuit or a device that interrupt the communication on the segment,which result in alarm from practically every device on the segment. All alarms can make itvery hard to identify the error source.

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Another communication disturbance occurred at Scanraff when they got moisture in an outlet,which stopped the communication on that segment. This short circuit was not enough totrigger the short circuit protection but it disturbed the communication. All devices can be con-figured to send alarms, but to avoid a lot of alarms showing up at the operator station, onlycritical alarms and sum alarms are presented at the operator station. More specific alarms and

diagnostics can then be reached from the engineering station.3.7.4 Demands on knowledge Nimrod has found that finding errors and using the fieldbus diagnostics needs both educationand new work procedures. At a typical plant there are usually one electronic, one instrumentand one control department with different assigned tasks. With the introduction of intelligentdevices these tasks can move between the departments, where the knowledge is not present.This problem has to be taken into consideration and either educate the personnel or changethe organisation. Education is an important issue that has to be looked into, otherwise it mightcause problems. Another problem might be oppositions from the personnel, they might benegative to changes or unwilling to learn a new system.

At Scanraff, the personnel made major parts of the fieldbus installation, programming of op-erator stations and control system, with support from Emerson. Three workers have been edu-cated for three weeks and the remaining operators got 2 + 2 days of education. Besides theeducation, the involved personnel had learned them selves. New changes in the system comesso quickly that not even the supplier can keep up with how to solve certain problems.

At the constructed plant in the FuRIOS 2 article, the managers of the report admit that theyunderestimated the demands on training. However, they agree with the FuRIOS report that itis less training compared to remote I/O because once trained with Profibus, little more train-ing is needed, while new devices in remote I/O systems has different operation philosophieswhich requires new training. They also state the importance of teaching everybody, installa-tion personnel, engineers and managers about the fieldbus, in order to take advantages of thenew technology and also know its limits.

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4 Interview

4.1 BackgroundThe literature review, of sites with fieldbuses installed, provided information about the instal-lation of a fieldbus. However, the information about operation of a fieldbus system was insuf-ficient. To complement this with more information, two sites in rnskldsvik, Sweden, werevisited. The sites were ETEK, a pilot plant for ethanol, which has been running for about ayear, and Husum, a paper mill that has been using fieldbuses for several years. The interviewswhere conducted in November 2005.

4.2 Result

4.2.1 ETEKETEK is a pilot plant for ethanol production with forest residues as raw material. They use

DeltaV from Emerson as control system. Foundation Fieldbus H1 is used to connect instru-ments and valves, AS-I for discrete valves and motor on/off and Profibus DP for AC driversand interlocking controllers. ETEKs reasons for choosing to use a fieldbus were easy changesand less cable, compared to conventional I/O. It reduced the number of cable ladders by halfand also made the cross-connection rooms unnecessary. Since ETEK is a pilot plant they con-stantly change the process and therefore have to add, move or remove instruments and valves.The system is build to be dynamic and all segments have spare outlets to be able to add de-vices while online. At ETEK they are satisfied with the fieldbus and think that the fieldbushave lived up to its expectations.

There are two areas where a fieldbus could not be used. The first one is security applications,which has high demands on reliability. The other is the small Ex area where it would be tooexpensive to use Foundation Fieldbus H1 Ex-barriers on only a few devices (about 10 de-vices).

On the Foundation Fieldbus H1 bus ETEK connects up to 16 devices, but no more than 4 can be valves. This is because the load of the bus would otherwise be too great, resulting in longcycle times. The devices are connected to the bus through a multibarrier box, which has over-load protection. This is used to avoid a shortcut or electrical fault to make the bus unusable.

ETEK has experienced some problems with commissioning, where some devices were lostfrom the system after the bus had been without power. They found that these devices had losttheir address. This was due to different initiation procedures for different manufacturers,which resulted in that devices did not store the given address. Another drawback was that thecost for programming the system got somewhat higher than planned.

Finding faults on the Foundation Fieldbus H1 bus is done in two different ways. The first iswith diagnostics that is sent from the device, through the bus and into the engineering station.The second is with a hand held device Field communicator connected to a specific device.It is mainly used when a device is in the workshop for inspection. Although some diagnosticswhere available before, with HARTS, it is being more used now with the fieldbus.

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4.2.2 HUSUMHusum is a paper mill in the north of Sweden that began to be built in 1915. The mill pro-duces both market pulp and paper, coated and uncoated. 1450 people are employed at themill.

The personnel at Husum have always been curious about new control technology and have been using fieldbuses for several years. They have both upgraded from 4-20 mA to fieldbus aswell as built a new fieldbus installation. The control systems used for the fieldbuses are Free-lance 2000 from ABB and Siemens PCS7. As fieldbus advantages they see additional diag-nostics, faster commissioning and easier configuration. An I/O card has to be set up individu-ally and mistakes can be done because the signal has to be converted from analog to digital,and the range has to be set both on the instrument and the I/O card. With a Profibus PA de-vice, a 32 bit value is sent to the operator station and does not have to be interpreted. As soonas a connection with the device is established, there is usually no problem with commission-ing.

The personnel have found that it is not always possible to exchange an instrument whileonline. With some systems it is possible to replace it online if the devices are of the same typeand manufacturer. However, if the configuration (DTM) file has to be changed, the systemmust be reprogrammed. The maintenance personnel experiences fewer small errors (like sen-sor drifting), but in total it is not that much of a difference.

One good thing with fieldbus instruments is that the devices can show the actual value, andnot only values inside their calibration range. A pressure transmitter can have a physical range0 200 kPa but a working range of 20-40 kPa. While the 4-20 mA transmitter would onlyshow values within this range and the accuracy depends on the width of the calibration range,

a fieldbus can show the entire physical range at very high accuracy.Husum uses Fieldcare to diagnose and configure their devices. One reason is that DTMs, fromother manufacturers than ABB, has problems working properly in the Freelance control sys-tem. Therefore the system uses GSD files. A new segment does however use DTM in Free-lance as well. A diagnose device is connected to the Profibus DP bus and can, if the system istransparent, be used to diagnose several Profibus PA segments. When an instrument has beenexchanged, a configuration file can be downloaded to commission it as fast as possible. Tomake it easier to find errors and configure devices, they always order the devices with dis- plays.

A maximum of twelve devices on each segment is used to keep the cycle time low, about 500ms. Even if the cyclic data exchange takes about half of it, the rest is used for acyclic ex-change like alarms. When an alarm occurs it is not unusual with consequence alarms, whichthen cause a high load on the bus. To minimize the risk of disturbing the Profibus PA buswhen adding, moving, removing and even when commissioned, multibarrier boxes are used.All Profibus PA segments have the same cycle time although some of them could haveshorter. Husums reason for using the same cycle time is that they want to be able to connectnew devices to any segment, without having to check which cycle time it has. Up to 4Profibus PA segment are connected to one Profibus DP/PA segment coupler.

As for education, the operators got none while the maintenance personnel got 3 to 4 days. Theerrors occur seldom resulting in the personnel forgetting how to find the errors.

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The devices themselves are robust and have very few errors. The reason for changing a deviceis usually wear, vibrations or other unplanned physical accidents with the device. These errorsoccur both on fieldbus devices and 4-20 mA in about the same amount. However, the buscommunication can cause problems. When a 4-20 mA system was replaced with fieldbus, thegrounding was not done correct, which caused devices to temporary loose connection and

send an alarm. There were also sections where the bus cable was placed close to the voltagecable for motors, when for example passing through a hole in the wall. To avoid this, the in-staller should have the knowledge about how to install a fieldbus and inspect the bus prior tocommission. Husum recommend that a good cable is used, with well-made connections, andnot reuse old 4-20 mA, unless shielded.

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5 Test system

5.1 Background and problemThe aim for this system test is to test a number of possible scenarios that might occur wheninstalling and maintaining a Profibus PA system. The result can be used to compare withsimilar situations when conventional I/O is used instead. The test should also try to find and prepare system engineers for problems that might occur. The test consists of several tasksdivided into two groups, installation and maintenance. The result will describe how to accom- plish these tasks in the specified control system with the specified software. Parts of the resultshould therefore not be used to draw conclusions about fieldbuses in general. However, it canshow problems that might occur in other control systems as well.

These tasks where defined after a meeting with Per Norman, my supervisor. The first twotasks are related to installation and commissioning of fieldbus devices and the remaining are

related to maintenance and function of fieldbus devices. These are the tasks5.1.1 Connecting and installing devices on the fieldbus:Installation of fieldbus connected devices can be done in different ways. Therefore performthe following:

Describe in detail, how configuration and installation of an instrument with a DTMfile is done.

Describe in detail, how configuration and installation of an instrument with a GSD fileis done.

Summarize the difference between DTM and GSD files.

5.1.2 Configuration and commissioning of a control loop:The task is to program a traditional control loop consisting of one positioner, one pressuretransmitter and a tank. The operator station view should also be programmed. The systemshould be controlled by a PI-controller. There is no physical system and therefore a simulated process should be programmed. The intension with the control loop is to use it for the mainte-nance tasks, of this test.The operator view should display process values, enable the operator to change set points anddisplay some alarms and diagnostics. The status and settings of the devices should be dis- played on the process values faceplate. The regulator should support two modes, automaticand manual.

Describe how configuration, installation and commissioning of the devices are madeand how the control loop is programmed (including the simulated process).

5.1.3 Exchange of pressure transmitter (ABB) of the same type:The test system has lost contact with the pressure transmitter. The operator station shows theerror Slave not existent. A quick check out in the field shows that the transmitter is notworking at all. There is a pressure transmitter of the same type in storage, which should beused to replace it. The settings from the old device should be used in the new one. The onlydocumented information, except information stored in CBF, is the node number on the bus.All information from the engineering station can of course be used.

Describe how the faulty device is replaced with a new one.

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5.1.4 Exchange of pressure transmitter of different type:Same task as above but with one exception, the stored pressure transmitter is equivalent butnot of the same type.

Describe how the faulty device is replaced with a new one.

5.1.5 Disturbance of a positioner:The positioner of the control valve has problems with the compressed air. In the high windsthere is a ladder that squeezes the air tube, which finally blocks the air supply totally. The positioner is no longer responding to control, which can be observed from the operator sta-tion.

Describe how the error finding is conducted. Assume that the problem with the ladderis not known.

Describe if there are any diagnostics or alarms that occur before the air supply is blocked, which could alert the operator before the final block.

5.1.6 Safety position of the positioner:When problems occur in a process, which causes it not to work properly, positioners canmove to a safety position. A power failure would for example make controlling impossible.For these cases the valves usually have a default position that is mechanically constructed. Aspring valve would become either fully opened or closed. One case that can occur when usingfieldbuses are that the bus and positioner is powered but the communication with the processstation is lost.

Describe in detail, how to configure the positioner to freeze in the current positionwhen communication with the process station is lost.

5.1.7 Alarm and diagnostics:The fieldbus devices have extended diagnostics compared to analog devices (without HART).A temperature transmitter (ABB) is showing an error. Simulate the error by removing the sen-sor from the transmitter.

Describe in detail, how the error is presented to the operator and engineer. Describe how to configure which value should be shown/sent when the error occurs.

5.2 MaterialTo conduct the test, a system was borrowed from ABB. It was an AC 800F control system,which consists of one process station and a DP/PA segment coupler. A positioner, a tempera-ture transmitter and a pressure transmitter were also borrowed from ABB. To test instrumentsfrom other manufacturers, a temperature transmitter was borrowed from ALNAB and a pres-sure transmitter from Endress+Hauser. The hardware was set up as shown in Figure 12

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Figure 12. The hardware set-up of the test system.

A PC was used both as engineering and operator station, and the software used consisted ofCBF (Control Builder F 8.1) and DigiVis, an operator station software. CBF runs on Win-dows XP and therefore it was installed on the PC. The PC was connected to the process sta-tion with Ethernet. The process station had one Profibus DP card, which was connected to thesegment coupler.

This test will not evaluate the performance of the bus or how it can be connected. Thereforethe connections can be made as simple as possible. All devices were connected in parallel tothe same point, where a terminal block with screw connections where used, to make it easy to

add or remove devices on the bus. No terminators were included in the test system and there-fore none were used on the Profibus PA bus. The limited length of the bus and number of de-vices connected on it made it possible to use without termination. No lost of devices or com-munication has been noticed during the tests.

To conduct the test, a GSD and DTM file is needed for each device. With most of the devicescame a GSD file, but the DTM had to be downloaded from the Internet. For the ABB devices,the DTMs could not be downloaded separately and the file was large, about 350 MB. It alsoincluded ABBs configuration and diagnostic program SMART VISION. A similar file had to be downloaded from Endress+Hauser, but it was not available for everyone. The device fromALNAB did not actually have a DTM, but after talking to them over the phone, they sent a

beta version of the DTM for their transmitter.

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5.3 MethodTo be able to conduct these tests, knowledge of both software and hardware is needed. Mostof the information was learned from manuals, data sheets and similar documentation. CBFhas been used by two employers at Outokumpu (but not with Profibus PA), which were askedwhen specific problems occur. Leif Karlsson at ABB had experience from both DTMs andGSD files in CBF.

5.4 Result

5.4.1 Connecting and installing devices on the fieldbus:

How to install and configure a device with a GSD fileTo be able to communicate with a Profibus device the master has to have a GSD file, which islike a driver for the device. These are the same for both Profibus DP and Profibus PA. A GSDfile is a plain text file that describes how to communicate with the device. Each row has one

variable and a value for that variable, see Figure 13. Examples of variables are Vendor_Nameand Model_Name. A variable of importance is the Ident_Number. These numbers are given by PNO when a company wants to have a device validated for Profibus. If the Ident numberof the GSD file and device are not the same, the wrong GSD file is used and no communica-tion will be set up. The GSD file also specifies the speed supported by the device. SinceProfibus PA only supports the lowest speed, 31.25 kbit/s, therefore these variables are onlyuseful for Profibus DP devices. The GSD file specifies what cyclic data that can be receivedfrom it and what the process station should send to retrieve it. The meaning of the differentdiagnostic bits are also described in the file.

Figure 13. Selected parts of a GSD file. Complete file in appendix C.

;GSD File for Profibus DP (EN 50170)#Profibus_DPVendor_Name = "ABB Automation";Model_Name = "TF12 Temperature Transmitter";OrderNumber = "ABB, TF12"Ident_Number = 0x04c493.75_supp = 131.25_supp = 1Bitmap_Device = "TF12___N"Unit_Diag_Bit( 0) = "Hardware failure electronics"Unit_Diag_Bit( 4) = "Memory error"Unit_Diag_Bit( 5) = "Measurement failure"

; Module DefinitionModule = "Temperature 1" 0x00, 0x42, 0x84, 0x08, 0x05, 0x002Info_Text = "Secondary Variable 1 (Channel 1)"EndModuleModule = "Temperature 1 & Temperature 2" 0x00, 0x42, 0x84, 0x08, 0x05, 0x42, 0x84, 0x08, 0x055Info_Text = "Secondary Variable 1 (Channel 1) + Secondary Variable 2 (Channel 2)"EndModule

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The GSD file only specifies how the communication is done, which cyclic data that can beretrieved and description of diagnostic bits. Therefore, when using GSD files, there are otherways to configure the device with acyclic data through the bus. DPV1 is the acyclic service of

Profibus and here variables can be read or written to in different blocks of the device.CBF has its own way of reading and writing the acyclical data. The user can specify a slot andindex for a variable, which is a memory position, and the data type. This variable can then beread and, if allowed, written to. This is however not as easy as it sounds. The table that speci-fies slot and index is long and it is not easy to find which variable to change. It dependsmostly on the vendor and how well documented their device is. The pressure device fromEndress+Hauser was well documented and the memory table was send with the manual. Asfor the ABB devices, none came with the devices and it could not be found on their home- page. However, after a call to Lars Forslund, ABB technical support Sweden I got memorymaps for the ABB devices as well. Although it seem to work well with acyclic communica-

tion with CBF it is time consuming and hard to find the interesting variables. A good thing isthat once the variables has been identified and set up for one device, the configuration can beexported and used on other devices. It can also be exported between different PCs. These filescould not be downloaded from the ABB website