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Session 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 12Fieldbus in the Process Control Laboratory Its Time Has ComeJames A. Rehg, William H. Swain, Brian P. YangulaPenn State AltoonaAltoona, PA 16601Steven WheatmanPrincipal EngineerHoneywell Industrial Automation & ControlFort Washington, PA 19034Abstract - Industrial process control has evolved followingthe path of Direct Digital Control (DDC-1962),Programmable Logic Controllers (PLC-1972), DistributedControl Systems (DCS-1976), and Field Control Systems(FCS-1994). The latest Field Control System evolution thatis taking place is the implementation of Foundation fieldbus(FF) into the manufacturing environment. This articledescribes Foundation fieldbus, its role in a plant networkhierarchy, and a comparison between Foundation fieldbusand the older DCS model. In addition, this article discussesinstructional issues associated with teaching Foundationfieldbus in a system control course and laboratory, alongwith an overview of the hardware and software necessary tobuild a network of Foundation fieldbus devices throughintegration of FF on existing process trainers.IntroductionFoundation fieldbus is a digital control network that inter-links smart sensors and actuators in a manufacturing envir-onment. It is the latest technology used to automate thecapture of process data and the control of productionsystems. The evolution of the system architecture fromDirect Digital Control (DDC) to Distributed ControlSystems (DCS) and now to Field Control Systems (FCS) isillustrated in Figure 1. In every step of the evolution, thecontrol of the process has moved closer to the sensors andactuators.Figure 1 Control Evolution 1Figure 2 illustrates the shift of the proportional-integral-differential (PID) function from the primary system compu-ter to the sensors and actuators at the point of measurementand control. This movement of the control process reduceswiring, aids in troubleshooting, and decreases maintenancecosts of the industrial control network. It also allowsfieldbus devices to be controlled by any Fieldbus hostcomputer on an existing plant Local Area Network (LAN)with the appropriate interface to the fieldbus system.Figure 2 Control Architecture 1Another feature of the FF system is the capability ofadding fieldbus devices to an existing fieldbus process whileit is operational. This makes the implementation of newfieldbus networks into an existing system a less complexprocess and does not require that the process be shutdownwhen sensors are replaced or new field devices are added.Fieldbus devices have the ability to run process controlloops internally without having any need for processingpower from a central computer or digital processor on thenetwork. In addition to the data networking function, the FFtwisted-pair network cable can supply the power required torun all the sensors and actuators on the network. Thestandard requires that up to 32 devices can be on a singlesegment. By using repeaters, as many as 240 fieldbusdevices can be on a single network.The FF standard of interoperability supports “Plug andPlay” architecture. This allows field devices from differentvendors o be mixed in a working fieldbus model and theaddition of new field devices without the need for majorLAN reconfiguration.ntsSession 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 13Fieldbus Systems and StandardsThe number of network control techniques, using the termfieldbus to describe their operation, is numerous. As a result,a considerable level of confusion exists in the selection anddesign of fieldbus driven control system. The chart in Figure3 provides an overview of the current network protocolFigure 3 Fieldbus LAN Options 2choices available to the design engineer. The fieldbus LANsare divided into the two broad application categories ofdiscrete and process . The level of automation availablefurther differentiates the choices from bit-level sensor oper-ation to support for the process control and interfaces intothe business unit for databases management and inventorycontrol. In the discrete area, the Profibus DP, CAN,Devicenet, and SDS protocols have good vendor support. Onthe process control side, two protocols predominate: theFieldbus Foundation and Profibus PA.Development of the fieldbus standard started in the mid-1980s when the Instrument Society of America (ISA)formed the SP50 fieldbus committee. In 1992, the number ofvariations in the standard narrowed when Fisher,Rosemount, Yokogawa, and Siemens created the Inter-operable Systems Project (ISP) and the other major SP50companies, including Honeywell, Allen Bradley, and othersformed the WorldFIP standard group 2. Further consoli-dation occurred in 1993 when the ISP and WorldFIP joinedto form the Fieldbus Foundation (FF). As a result, twoprotocols have evolved for LAN based process controlapplications: the Fieldbus Foundation, a standard supportedin the United States and Asia, while the Profibus PAstandard is popular in Europe.Fieldbus Control ArchitectureThe architecture used with Foundation fieldbus (FF)configurations includes two LAN types, called H1 and H2.The H1 segment is a 31.25-kbit/sec bus structure used to linkFF devices together. The H1 bus, illustrated in Figure 4, canbe point to point, bus with spurs or multi-drop, daisy chain,and tree. Type A shielded twisted pair wire is the preferredwire for H1 connections that is specified in the IEC/ISAphysical layer standard 3. A The maximum length 1900Figure 4 Possible Fieldbus Topologies 3meters for H1 cabling is also specified in the same standard.While new installations would use this wire, most currentimplementations could convert to FF technology usingexisting instrumentation wiring in most situations.The expansion block or junction box, illustrated in Figure5, is used to create the tree or star/chicken-foot topology andto create multiple spurs of the H1 bus at some distance fromthe FF interface. A typical configuration for expansionblocks is shown in Figure 5. Note that the initialFigure 5 Expansion Blocks 4block is a power conditioner plus field device terminator,while subsequent blocks are only used for termination offield devices. The initial block also provides a connectionntsSession 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 14point for the host computer. The H1 bus must also haveterminators (a series resistor and capacitor) placed at bothends of the bus to improve network data transmission. Someterminator blocks have terminators built into the interfaces.A second FF LAN, called H2, is a high-speed fieldbuscommunications mode, which serves as a backbone for theH1 segments. The H2 backbone can operate at 1, 2.5, or 100Mbits/s. A typical configuration for an H1 and H2 LANusing FF protocol is shown in Figure 6.Figure 6 H1 and H2 LAN Structure 4The H2 network speeds are useful for transferring data be-tween the smart field devices and other production hardwarelike programmable logic controllers (PLCs) and processanalyzers. The H2 LAN permits access to the fieldbusstructure from any computer on a plant intranet, and givesprocess engineers and production planners direct access toprocess data and the ability to program the system fromremote locations.Comparison of FF and DCSThe Distributed Control Systems (DCS) is the currentstandard for large process control applications. In DCS, acentral processor, Figure 7, controls all parameters. Fieldbusmoves data filtering, conversion, tuning constants, andalarms into the field to be accomplished directly within thefieldbus sensor or actuator (Figure 2). This greatly reducesthe need for central processing capability in a fieldbussystem. Configuring all device settings, including processparameters, process variables, and set points is possible inthe fieldbus system using available configuration tools.Also, the parameter notation has been standardized acrossFF device manufacturers. This ensures the interoperabilitybetween components from different manufacturers andallows the devices to be used in a “Plug and Play” fashion.6In comparison, DCSs have a single process control com-puter to which all of the sensors and actuators are connected.While the sensors and actuators can come from manydifferent vendors, each device has to be configured to workwith the process computer selected. The major advantage ofthe FF over the DCS is the information and rich data setavailable to the network from the field device. In the DCSsetup, the sensor supplies only values related to the processvariable. In contrast, each FF field device has the capabilityto provide all of the data that would come from the DCScentral process computer.Figure 7 - DCS Process Control ModelFoundation fieldbus systems can coexist with DCSsystems. This ensures that existing investments in DCSsystems are protected. However, care must be taken incombining the two systems since no existing DCS systemcan match the functional characteristics of FF. Thedifferences between FF and DCS characteristics can poten-tially cause confusion and undesirable system operation ifdevices for one system are used within the network design ofthe other. 7Introduction of Fieldbus into the LaboratoryThe first step in creating a Foundation fieldbus system is toselect the process to control. Implementations of the fieldbusnetwork in the control laboratory can include a number ofprocess systems, both new and existing. Most processcontrol laboratories in colleges and universities have processtrainers for teaching control of temperature, pressure, flow,and level. The 4-20 mA control systems and standalone PIDcontrollers in these simulators can be supplemented byfieldbus devices without losing the 4-20 ma control option.The hardware and software needed to implement a FFsystem includes: A computer with a FF interface card and FF configur-ation software.ntsSession 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 15 A power supply with a dc voltage of 9-32 volts andcurrent capacity of about 20 mA per attached fielddevice. The wiring for the 31.25 kHz H1 fieldbus network witha resistance of 100 /ft and an attenuation of 3dB/kmmaximum. The cabling must be 16-26 gauge twistedshielded pair. The recommended color code is orangeor white (+) and blue or black (-). 8,9 A terminator composed of a capacitor ( 1F) and aresistor (100 ) to avoid reflection signals in the wire.Terminator blocks with the proper electronics can bepurchased or constructed from available components.8 A power conditioner made from an inductor (5mH) anda resistor (50 ) must be placed between the powersupply and the network. 8Field devices, such as sensors and actuators, can bepurchased from a multitude of companies. FieldbusFoundation lists all of the companies with Foundationfieldbus standard equipment on their web page 9. Since thetechnology is new, relatively few components are currentlyavailable compared with the availability of the older 4-20mA devices. The basic components are temperature sensors,pressure sensors, flow sensors, valve positioners, fieldbus-to-current (4 to 20 mA) converters, and current (4 to 20mA)-to-fieldbus converters. Additionally, “Round Cards”are available to connect existing 4-20 mA devices intoFieldbus networks.Modifying an Existing Process Flow TrainerThe fieldbus network and control system, designed for theprocess control laboratory at Penn State Altoona, utilizes anexisting flow process trainer shown in Figure 8. The originalunit had a differential pressure transmitter connected acrossan orifice plate, Figure 9, with the 4 to 20 ma output of theDP unit connected to a variable speed motor controller,Figure 10. The motor controller changes the pumpingcapacity of a pump to control the fluid flow through thesystem diagramed in Figure 11. A study of the system layoutindicates that it is a straight forward control problem with atank, single flow loop, pressure orifice, pump, pump motorspeed control, and inline visual flow indicator.In the fieldbus solution the system consists of the samevariable speed pump controlled by a Smar fieldbus to currentconverter (FI-302) and a Honeywell differential pressuresensor (ST3000 ) illustrated in Figure 12. The network isshown in Figure 13. These components are programmed andconfigured through a PC with a National Instruments AT-FBUS interface card and the National InstrumentsConfigurator software package. This trainer was designedto teach/demonstrate how process flow system componentsand control software can be configured to maintain acontinuous fluid flow, even when a processFigure 8 Existing Process Flow Trainerdisturbance is presented to the system. In the originalsystem, the Proportional Integral Differential (PID) controlalgorithm was implemented in a standalone digitalcontroller. In the fieldbus implementation the differentialFigure 9 Orifice Platepressure sensor includes the (PID) controller with internalelectronics and software that monitors and controls the fluidflow in the system to a level designated by the user.The Honeywell DP field device in the fieldbus solutionincludes a Link Active Scheduler (LAS) program, which isntsSession 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 16factory installed in their differential pressure sensor. TheLAS allows the computer, which is used to configure andmonitor the system, to be disconnected from the processnetwork without creating any disruption in the processcontrol or disturbance in the flow control.Figure 10 Variable Speed DriveThe network configuration for the fieldbus system uses aSmar fieldbus to current converter (FI-302) so that thecontrol parameters from the DP sensors can be interfacedinto the existing 4 to 20 ma control of the Seco AC adjus-table drive.All of the laboratory exercises previously performed withthe original 4 to 20 ma DP are directly applicable with thefieldbus system. One variation is that students must con-figure the Honeywell ST3000 differential pressure trans-mitter, Figure 12, for proper control parameters instead oftuning the digital controller. In addition, students learn to usethe Foundation fieldbus configuration program for fielddevices and to display a much broader range of process datathan was available with the standard DP unit.Figure 11 System Flow SchematicFigure 12 Honeywell ST-3000Differential Pressure TransmitterFuture Expansion PlansBased on the success of the initial FF inst allation, threeadditional process trainers will be modified to incorporateFoundation fieldbus technology. These three include atemperature trainer, a liquid level trainer, and a pressuretrainer. All four trainers will be linked together utilizing anH2 high-speed (100 MHz) bus. The network to be created isdepicted in Figure 14. Additionally, the main computer onthe H2 bus can be located at an instructors station in the labto monitor the progress of students work on the FF systems.The central computer can be used by students to test theremote monitoring and control possible with Fieldbussystems. Another advantage of the H2 bus and centralcomputer is the ability of the teacher to inject problems orfailures into the local networks, enabling students totroubleshoot real-time failures.Figure 13 Fieldbus Network for Flow TrainerntsSession 13b40-7803-5643-8/99/$10.00 1999 IEEE November 10 - 13, 1999 San Juan, Puerto Rico29 th ASEE/IEEE Frontiers in Education Conference13b4- 17The implementation of FF on the three additional trainersis similar to the efforts used to convert the Flow Processtrainer. The major difference is in selection of theappropriate FF transmitters and actuators. Presently, alltechnology is available to retrofit each trainer with anindependent Foundation fieldbus control system and to linkthe systems with H2 high-speed bus.ConclusionsFieldbus is the next technology that will find broad use inmanufacturing control for the following reasons: it isrelatively easy to reduce costs by allowing a user to startwith a small system and expand as necessary and financialabilities allow, field wiring is reduced, and troubleshootingof system problems is enhanced. Also, since Foundationfieldbus components are all compatible between companies,anyone can create components for use in Foundationfieldbus systems allowing for greater competition in themarket.Because of the anticipated benefits of usi
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