Making fieldbus projects as painless as possible
Designing and implementing a successful—and painless—fieldbus project requires mapping out network requirements as well as understanding the plant’s information, diagnostic, and maintenance needs.
Larry O'Brien, Fieldbus Foundation
The engineering and design phase of a FOUNDATION fieldbus installation can be relatively quick and economical if you understand the technology (see Figure 1). Failing to realize that FOUNDATION fieldbus is a digital technology can increase engineering hours and project cost. Without an understanding of digital networks or the proper degree of training, embarking on a fieldbus project could be a painful learning experience.
Fortunately, first-time users can avoid common mistakes by taking advantage of numerous resources that the Fieldbus Foundation has developed. This article offers important guidance and presents essential information to know before embarking on a fieldbus project.
Qualified engineering partner is essential
The Fieldbus Foundation strongly recommends having a knowledgeable and experienced engineering partner with training on how to implement a fieldbus network. It should be emphasized that without an engineering partner with these skills, the cost of your project will be greater. Many engineering firms believe that there are no great differences between 4-20 mA analog and fieldbus installations. If you do not know what you are doing, you can end up in the weeds pretty quickly.
The Fieldbus Foundation offers certified training courses through the FOUNDATION Certified Training Program (FCTP), which has eight certified training institutions around the world. Inquire about the types and training levels of your engineering partner’s staff engineers. Although they may not have taken certified training courses, they still may have produced successful reference projects. With nearly 20,000 FOUNDATION fieldbus systems installed around the world, the pool of engineers with fieldbus experience is growing. Taking just a little time to check engineers’ references, expertise, and training will provide a quick payoff.
Not surprisingly, engineering partners that do not have experience in fieldbus installations will try to steer potential clients away from the technology. Don’t allow engineering companies to dictate your automation strategy. Instead, form a coherent strategy beforehand, and then find a partner that can fulfill your requirements. Many engineering service providers are available and are quite capable of doing a fine job. Also consider retaining the services of a fieldbus consultant. A good consultant can offer valuable guidance add the benefit of years of experience to your project.
Understand project requirements
Having a good understanding of project requirements is fundamental. The “AG 181 System Engineering Guide” is a free resource provided by the Fieldbus Foundation and can be downloaded at www.fieldbus.org/technicalguides. Starting with project requirements, AG 181 provides plenty of valuable guidance throughout the project lifecycle. The guide points out five fundamental differences between conventional projects and FOUNDATION fieldbus projects. Understanding the following differences can help ensure a successful project:
- With FOUNDATION fieldbus, field devices and segments become an integral part of the DCS. This requires an integrated configuration, data management, and a system architecture approach to field network design. You can no longer treat devices and process automation systems as separate islands of automation.
- The system integration aspects of FOUNDATION fieldbus require some design activities to be performed earlier and with greater detail in the project lifecycle.
- You can achieve more advanced functions with FOUNDATION fieldbus designs compared to traditional technologies. These advanced functions offer considerable opportunities for operational cost savings, and improved commissioning and start-up.
- System, field device, and segment component-testing processes are more advanced than for simple analog connections.
- Traditional lines of authority between crafts and legacy-derived practices should be reviewed to determine if they are suitable for FOUNDATION fieldbus. In other words, this technology will require different work processes than you may be used to—especially if you want to practice true predictive and proactive maintenance, which is where a lot of your operational cost savings will occur.
Use registered devices, hosts
It may seem obvious, but it is always preferable to use a host system or device that has gone through the Fieldbus Foundation testing and registration process. The Fieldbus Foundation check-mark symbol indicates that a product has been tested and registered. However, you should investigate beyond the check mark to determine the version of the interoperability test kit (ITK) or host profile registration process to which the host or device conforms.
The Fieldbus Foundation has greatly strengthened its testing and registration requirements. Devices tested under the latest Version 6.0 of the ITK will provide more advanced diagnostic functionality, including the NAMUR NE 107 recommendations for presenting diagnostic data. The Fieldbus Foundation’s latest iteration of host profile registration has made many previously optional requirements mandatory, such as support of enhanced function blocks and DD V5.1 device-level access.
A good plant asset management system is essential
It does not matter which process fieldbus you are installing. If you don’t have some kind of plant asset management (PAM) application to deal with all the new diagnostic data you are going to get from your intelligent devices, you will not realize the significant operational and maintenance cost benefits that come with the technology. A PAM system will help you manage your data and will be a valuable tool for maintenance personnel.
Examples of current PAM systems include Emerson’s AMS Suite, Honeywell Field Device Manager, and Yokogawa PRM. The PAM system is the core application that will allow you to develop a predictive and proactive maintenance strategy. Remember the earlier statement that some aspects of the fieldbus project must be planned earlier in the lifecycle? The PAM system is one of these items. You should start early in the front-end engineering and design phase to determine how the PAM system will be used and who will need to use it.
A word about work processes
You should define any new or revised work processes that may be required because of the use of FOUNDATION fieldbus. This is one of the most often overlooked aspects of a fieldbus project. If you do not pay attention to proper work processes, the people that need the data from fieldbus devices may not be able to get it, or you could remain stuck in traditional maintenance processes that can be transformed by fieldbus.
Fieldbus is essentially a data pipeline, and users must plan where that flood of data is going and how it will be transformed into useful information. In most cases, this is a secondary consideration that is addressed only after the system is up and running. Having a team in place that consists of personnel from affected areas of the plant will expedite this process. What information do your maintenance personnel need? What about data storage and validation concerns for companies that must undergo regulatory compliance such as those in the pharmaceuticals, and food and beverage industries?
The information flow from the device level to the associated applications that need it must be mapped out prior to start-up to achieve maximum benefits. Intelligent field devices are really data servers, and much of their information is not appropriate for the control system operator. If you are dealing with a supplier that has an effective fieldbus configuration tool integrated with a good PAM application, the effort to get the right information to the right people at the right time will be minimized.
Many end users may lack sufficient knowledge to implement these work processes. A new ISA standards effort is underway to develop recommended work processes for managing the data from your intelligent devices. The ISA 108 standard will define work processes for different roles in the organization and provide best practices for the project lifecycle.
FOUNDATION fieldbus and redundancy
FOUNDATION fieldbus is used in critical applications and can support key functions for demanding applications, including redundancy. FOUNDATION fieldbus supports redundancy on many levels, if that is one of your requirements. Redundant power supplies, redundant H1 (device level) fieldbus interface cards, and redundant controllers are the most common ways to do this. With redundant H1 cards, it is possible to replace either H1 card under power with no impact on the H1 segments associated with the redundant cards. It is also possible to upgrade H1 card software/firmware without affecting communications on the related H1 segments.
FOUNDATION fieldbus is comprised of the H1 network at the device level. The HSE standard Ethernet-based network addresses high-speed applications and devices such as motor-operated valves, drives, and remote I/O. FOUNDATION HSE is based on unmodified IEEE 802.3 Ethernet and therefore is compatible with standard Ethernet equipment. Unlike mere ring topology, FOUNDATION HSE provides complete DCS-style redundancy with redundant network switches, redundant devices, and redundant communication ports for extremely high availability.
Wireless device, remote I/O integration
The HSE network also provides integration with wireless networks such as ISA 100.11a and WirelessHART through FOUNDATION for remote operations management (ROM) program development (see Figure 2). HSE can also serve as a wireless or wired backhaul network for remote applications such as pipeline SCADA and offshore platform automation.
FOUNDATION for ROM provides direct access to information and diagnostics in wireless and remote I/O devices. Conversely, FOUNDATION for ROM can take data from those devices and place them into the FOUNDATION fieldbus environment for data management and quality.
FOUNDATION fieldbus function blocks
A key consideration at the device level—aside from buying products that have been tested and registered—is the support of advanced diagnostic functions and function blocks. The support of function blocks is unique to FOUNDATION fieldbus and is the enabler for control in the field and many of the advanced diagnostic functions the technology offers.
Function blocks were really the first instances of object-oriented programming. They enable FOUNDATION fieldbus to take a comprehensive, object-oriented approach to process control. FOUNDATION fieldbus supports standard function blocks, advanced function blocks, and additional function blocks (See Figure 3). The 10 standard function blocks include:
- AI = analog input
- AO = analog output
- B = bias
- CS = control selector
- DI = discrete input
- DO = discrete output
- ML = manual loader
- PD = proportional/derivative control
- PID = proportional/integral/derivative control
- RA = ratio.
Advanced function blocks include:
- Device control
- Setpoint ramp
- Input selector
- Signal characterizer
- Dead time
- Analog alarm.
Additional function blocks include:
- Multiple analog input
- Multiple analog output
- Multiple discrete input
- Multiple discrete output.
Not all function block types are suitable or available for all instruments. Therefore, it is essential to make a considered choice when specifying the function blocks to be included in various field device types. Although it is appropriate to host most of these blocks in controllers, their use in field devices on H1 networks/segments may be limited (due to the availability of devices) to the following blocks: AI for transmitters, AO and PID for valves, and DI/DO for discrete devices.
Additional function blocks are likely to be added in the future. Therefore, it is advisable to check function block availability with the instrument manufacturer to ensure that the host system will allow the use of the desired function blocks at the time of purchase. It should be noted that there is seldom a need to have all function blocks available in all field devices. Users should also verify host and field device compatibility, should it be necessary to revise the functionality location.
User application blocks
Where function blocks allow the user to create control strategies, user application blocks allow the user to configure devices and applications. The resource block describes characteristics of the FOUNDATION device, such as the device name, manufacturer, and serial number. The transducer block (TB) contains information such as calibration date and sensor type.
Field devices require at least one TB to make the device useful. TBs decouple function blocks from the local I/O functions required to read sensors and command output hardware. This is also where parameterization, calibration, and diagnostics for the device are carried out.
Power requirements for fieldbus are different than those for analog systems. FOUNDATION fieldbus devices may be either powered from the segment (bus) or locally powered, depending on the device design. Bus-powered devices typically require 10-30 mA of current at between 9 Vdc and 32 Vdc. Any network/segment designed to operate at less than 2 V above the minimum-required voltage at any device or wiring component should normally carry a warning about additional loads in the network documentation. Minimum network/segment voltage should always be shown in the network documentation. The total current draw from all devices on the network must not exceed the rating of the FOUNDATION fieldbus power supply. The network/segment design must take the following into account:
- Total maximum device quiescent current draw
- One spur short-circuit fault (around 50 mA additional current draw)
- From 15% to 25% of additional current load above the two previous requirements (for inrush current and expansion)
- Current consumption of wiring components
- Test equipment (typically 12 mA per device).
There is a wide range of registered FOUNDATION fieldbus cable choices. In November 2007, the Fieldbus Foundation released its H1 Cable Test Specification (Document FF-844) for use by manufacturers developing cables for Foundation fieldbus H1 (31.25 kbit/sec) installations. The specification test cases—based on the IEC 61158-2:2003 standard for Type-A cable—include (but are not limited to) impedance, capacitive unbalance, connector pin-outs, and attenuation. Cable manufacturers submit their test reports to the foundation, which provides official cable registration.
There are many choices for registered cable, but there are also ways to test your installed instrument cable for compatibility with a FOUNDATION fieldbus installation. Some wiring-component and test-equipment suppliers offer automated tools that can test your existing cable.
Components of fieldbus segments can be connected together in various topologies. The topology selected is often, though not always, driven by the physical device location in order to reduce installation costs. Hence, control narratives, plot plans, and risk management considerations are used in addition to P&IDs and instrument indexes in the design of a fieldbus segment. FOUNDATION fieldbus installations use the tree, spur, or combination topologies (see Figure 4). Other topologies are not recommended.
Point-to-point: Point-to-point topology consists of a network having a maximum of only two devices. The network could be entirely in the field (e.g., a transmitter and valve, with no connection beyond the two) or it could be a field device connected to a host system (doing control or monitoring). This topology should not be used because it is typically not an economical design concept.
Tree: Tree topology consists of a single fieldbus segment connected to a common junction box to form a network. This topology can be used at the end of a home-run cable and is practical if the devices on the same segment are well separated but in the general area of the junction box.
Spur: Spur topology consists of fieldbus devices connected to a multi-drop bus segment through a length of cable called a spur. This technology is technically acceptable but generally not a good economical choice when there are many devices.
Combinations of these topologies must follow the rules for maximum fieldbus network/segment length and include the length of spurs in the total-length calculation. These topologies are preferred for designs using bricks with tray cable. Spurs are permitted to extend only from trunk lines and not from other spur lines.
What is a segment?
A FOUNDATION fieldbus segment is a network or part of a network that serves as the primary communication highway for the connected fieldbus devices. It includes the fieldbus network card interface, power supply, and power conditioner (see Figure 5). The cable serves as the principal link between the fieldbus network and fieldbus devices. It can be a part of the fieldbus network wiring that is electrically independent from other parts and may or may not have an associated power supply (as in the use of repeaters). Trunks, spurs, and terminators are fundamental parts of a FOUNDATION fieldbus installation.
Trunk: A trunk is the main communication highway acting as a source for other lines (spurs) from the control room. Generally, the trunk is the longest cable path between any two devices on a segment.
Spur: A spur is a fieldbus branch line that connects to the segment. A spur does not have terminators at either end of the spur cable. A spur can vary in length from 3.28 ft to 394 ft, depending on the number of devices on the spur.
Terminator: A terminator is an impedance-matching module used at each end of a segment. A terminator is required at each end of the longest cable path between any two devices on a segment to prevent distortion and signal errors. Only two terminators can be used on a single fieldbus segment.
Segment isolation: Segment isolation is a basic requirement for FOUNDATION fieldbus design. Where devices are installed in areas that are considered to be at a high risk of lightning strikes, segment segregation must be used as an additional measure of protection. High-risk applications include tank farms where transmitters are located on top of tanks, remote areas in open spaces, devices requiring more than two wires (temperature), unprotected structures with little or no surrounding steel, and transmitters that are located on the top of columns or in the open on top of structures.
Number of devices per segment: Many users are under the misconception that only a small number of devices can be placed on a fieldbus segment. As a rule of thumb, the AG 181: System Engineering Guide recommends that the maximum number of devices on any segment (including future devices) should be limited to 12, but there are situations where more than 12 is possible. Students at some of the Fieldbus Foundation’s training facilities routinely overload segments with as many as 16 devices and they continue to function. Of course, overloading segments is not recommended. AG-181 recommends that project staff should set a design standard for the maximum number of devices per segment for that project if less than 12. This design standard should be used as the basis for voltage-drop calculations and should include spare capacity for future expansion.
Designing and implementing a fieldbus project can be an intimidating task. However, taking the time to understand FOUNDATION fieldbus technology and to map out the project requirements can take much of the pain out of the process.
Larry O’Brien is the global marketing manager of Fieldbus Foundation. He joined Fieldbus Foundation in April 2011 after serving as research director for process automation at ARC Advisory Group for 18 years.
For more information
- The AG-181: System Engineering Guide is available for free download from the Technical References section of the Fieldbus Foundation web site www.fieldbus.org.
- Fieldbus Foundation also offers a free automated segment design tool called DesignMate, which can also be downloaded from the Technical References section. DesignMate automatically audits segment layouts for conformance with the Foundation physical layer specification, which is based on the international IEC 61158-2 (Type 1) standard. This assures end users that their fieldbus infrastructure will work with desired parameters such as cable length, number, type of installed devices, and selected power supplies.
- In addition to FCTP-certified training partners, the Fieldbus Foundation offers many free end user seminars.
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