FF HSE: The Best Protocol for Process Automation?
Monday, April 03, 2006
Many different industrial protocols use Ethernet as the media and IP for data transport. Providing a completely interoperable network solution requires a single application protocol. Different protocols have characteristics targeted towards process control, manufacturing automation, and motion control, etc. The protocol arguably best suited to the requirements of process automation is Foundation Fieldbus HSE. Jonas Berge argues this claim by examining a process automation technology demonstrator installation.
The world-class International Specialty Products plant manufacturing the chemical 1,4-butanediol (BDO) in Lima, Ohio, USA joined with automation suppliers in a technology demonstrator for FF HSE and associated hardware. Lima BDO was a joint project initiated by BP Amoco Chemicals and the Fieldbus Foundation. ISP, through one of its subsidiaries, acquired BP's BDO operations in March 2005. The Lima plant has an annual capacity of 60,000 tonnes of BDO, a key building block for specialty chemicals products used in pharmaceutical, personal care, food, beverage, coatings, oil field and other applications.
The BDO plant has used Foundation Fieldbus H1 (31.25kbps) technology for several years. HSE networking and controllers was later added on one of the filter sections of the plant. The process controllers run HSE while the configuration tools were provided by SMAR and Softing. Additionally, device management software based on HSE was provided by Yokogawa. This was integrated with the existing DeltaV control system, plus some fieldbus instruments using OPC.
Open process automation
Industrial Ethernet in general and FF HSE in particular has three application tiers: control-level networking, remote-I/O bus, and horizontal integration of subsystems. There are several advantages to using this technology, many of which were illustrated at the Lima plant.
"Few users will choose to configure a hodgepodge of different vendors' process control equipment for their next project; most of us still place great value on single-point accountability," said John Rezabek, lead controls engineer for the ISP Lima facility.
"Anyone who has had to perform 'mapping' between dissimilar proprietary devices will welcome the FF HSE standard. While OPC is easier than Modbus, the task of relating signals and statuses between systems remains tedious, time consuming, prone to errors, and challenging to validate, troubleshoot, and maintain. The elegance of H1, where communication of measurements, signal statuses, engineering units, and alarms is uniform and vendor-independent, is now available at the HSE process control network level."
H1 is a device-level bus technology great for field instrumentation such as transmitters, analysers, control valve positioners, and signal conditioners. However, the H1 technology is not suitable for the remote-I/O level and control-level networks, and therefore, never found its way into products such as variable speed drives, motor control centres, remote-I/O subsystems, and flow computers. There was always a need to add another bus technology such as DeviceNet or Profibus DP for variable speed drives and remote- I/O, such as for high volumes of discrete I/O. Mixing bus technologies is possible in most control systems. However, it is always more complex to support multiple bus technologies rather than just one.
Rezabek believes the use of multiple bus technologies not only creates support issues for end users, but also introduces complexity during engineering and commissioning. He put it this way: "In a multiple bus environment, maintenance technicians have to understand different topology rules, configuration tools, test tools, power supply schemes, termination schemes, wiring schemes, and device replacement procedures of every protocol used."
Although most agree that the diagnostics available through bus technologies simplifies maintenance, adding more than one bus technology has diminishing returns and results in a Tower of Babel phenomenon. It is better to use one protocol family rather than mixing several incompatible protocols.
HSE has resolved this problem and now complements H1 with a solution suitable for variable speed drives, remote-I/O subsystems for discrete I/O bulk, flow computers, etc. Many of these products have already become available since HSE was first introduced. By combining H1 and HSE in the system is it possible to eliminate the need for other protocols. We contend that you don't have to choose an inferior product to get the right protocol, or to select another protocol to get a superior product. Manufacturers will make devices available with different Ethernet protocols just like they today make transmitters available with different fieldbus protocols. This is, of course, the whole reason why the fieldbus standardisation effort started in the first place, and now the same thing is happening with industrial Ethernet.
One bus technology for all tasks
Discrete devices such as proximity switches and on/off valves are typically wired in groups to remote-I/O subsystems each handling multiple points. The reason is that devices such as solenoids typically don't have much diagnostics to justify digital communication, although there are exceptions in which discrete I/O devices use H1 communication. Large numbers of such simple discrete sensors and actuators are hardwired to remote-I/O subsystems mounted in panels in or near the field. H1 is not suitable for large point count remote-I/O devices. In the past, discrete I/O for logic and sequence for interlocks was typically a task for other bus technologies such as Profibus DP or proprietary technologies. Many systems did not even support remote I/O. Again, there was a real need for multiple bus technologies - and the associated problems.
We would claim that HSE has resolved this and now complements H1 with a solution suitable for large amounts of discrete I/O for use with logic and sequence as well as interlocks and permissives as part of the control strategy. These products are already available, and also handle large amount of 4-20mA signals where necessary. Ethernet provides a breadth of media options: optical fibre for remote I/O, and industrial wireless for SCADA.
The benefits of using networks based on RS485 and coax, and even proprietary solutions on Ethernet media, are quickly diminishing. The higher level of the system architecture needs an open protocol on Ethernet. H1 technology provided an unprecedented level of interoperability for device-level instruments such as transmitters and valve positioners. However, a similar level of interoperability has previously not been available for remote I/O-level buses and control-level networks. Although most current control systems use both Ethernet and TCP/IP as the control-level network, and sometimes for remote-I/O, invariably the application layer protocols are proprietary, ultimately resulting in a proprietary network technology. This is the reason why a controller card from one DCS does not work in another-even though they all use Ethernet and TCP/IP. There is another curiosity. Although I/O modules from hundreds of suppliers can sit on the same Ethernet network without conflict, they cannot talk to each other. Lack of peer-to-peer communications prevent sophisticated control schemes so forcing the use of gateways and drivers for wider integration.
HSE has resolved this problem and now complements H1 with a solution suitable for the higher-level networks. Link devices, controllers, remote I/O and flow computers from different suppliers can be connected to the same network, be configured from the same single tool, and exchange data peer-to-peer.
'Package units' is one of the trickier parts of control system integration. Transferring all the relevant data, including asset management information for the underlying instrumentation, and time stamping of alarms has been impossible. (See article Industrial Ethernet compatibility: Do we really need or want it? in IEB29.)
HSE has resolved the package unit integration conundrum: package boilers, metering skids, and other packages using controllers based on HSE integrate with the main control system regardless of supplier thanks to the interoperability of the technology. Flexible Function Block technology ensures that the package unit integrator can "hide" proprietary technology from the main system contractor if necessary, and at the same time the main contractor need not deal with those intricacies.
OPC servers are already available for the HSE protocol and were used where other systems don't support HSE, such as integrating with the existing control system in the Lima BDO plant.
No plant is pure regulatory process control. There is always an input loading end and the plant output often produces tasks more discrete in nature such as crushing or chipping of raw materials, and bagging or bottling of the final product.
The complementary H1 with HSE protocols offer a single-bus approach for all the plant's needs, both regulatory and discrete. All of this can now be handled with a single bus technology if both H1 and HSE are used, eliminating the need for other bus technologies or proprietary networks. There is no longer a need to separate process devices from discrete devices on a second bus technology. H1 and HSE cover "hybrid" plants end to end. Discrete logic and sequencing are implemented using Flexible Function Blocks programmed in the IEC 61131-3 languages including ladder diagram, such as in the Lima BDO plant.
According to Rezabek, the fieldbus demonstration project at the ISP Lima facility provided valuable, real-world experience for end-users seeking to replace proprietary control systems with an open, interoperable fieldbus architecture enabling robust, reliable control at the field level: "Having implemented the demonstration project in a mission-critical part of an operating process, we feel confident that we have passed a milestone in the development of fieldbus HSE and truly distributed control," said Rezabek. "The distinct advantages of implementing H1 and HSE in lieu of proprietary networks will become increasingly apparent, as our leading suppliers develop products to make this technology accessible to their customers everywhere."
Clearly, Ethernet is replacing RS485 and proprietary coax networking at the control- network level. Users must avoid a multitude of proprietary Ethernet solutions, and instead select a single, open industrial application layer for their industrial Ethernet LAN and use it throughout the plant. Ethernet is used at the control-level and above, as well as in remote-I/O and drives, not at the device-level. Perhaps Ethernet will also be used at the devicelevel in the future. You should already start building on standard application protocols to prepare for this. When Ethernet instruments become available they will not be using proprietary application layer protocols; likely as not they will use FF-HSE.
Fieldbuses for Process Control - Engineering, Operation and Maintenance, Jonas Berge, The Instrumentation, Systems and Automation Society (ISA)
Jonas Berge works for SMAR