Because the definitions of control architectures are blurring, it is getting more difficult every day to make clear distinctions. “In my conversations with users of process automation systems, there still seems to be a strong preference with traditional architectures,” says Chris Martin, manager process control for GE Fanuc Automation. “The differences in the first two options mentioned above are becoming blurred, and I believe most users are starting to think of them as one. Advantages of the traditional architectures are less reliance on PC-based operating systems, familiarity and lower overall cost with hardware redundancy. Also, separation of I/O networks from enterprise networks is still preferred.”
Carr of CH2M Hill thinks control system architectures will use all of the above. “This includes PLCs or RTUs for field I/O and control, Windows-based PCs for HMI and all data handling functions, and standard network hardware,” he says. “All devices are connected by Ethernet LANs and WANs. This works for everything from a small processing facility, to a campus environment, to a hydro-electric system spanning multiple dams across several states. Bigger systems don't need bigger parts, just more of them.”
Some users believe in standalone systems. “PLCs are the best option for anything but a large-scale chemical, petrochemical or maybe some pharmaceutical plants, where a batch may cost more than $1 million,” says Speedy. “Advantages are that the control schemes, while not as mature as those of the DCS world, are very, very capable and are available at a fraction of the cost. Another huge advantage to these systems is the vast support resources available.”
"Vendors are not promoting server technology. Maybe they think you are not ready for it."
Jorge Cano, a process control engineer at MetMex Peñoles S. A. de C. V. in Torreon Coah, Mexico, disagrees. “PLC equipment loses capability when the process grows,” he says. “If you install a PLC in a process that has a certain amount of processing and data requirements, it may be OK at the beginning, but if you grow your process maybe the PLC will not be able to handle the increased processing and communication load. Therefore, a PLC may be the best solution only for equipment and process units that you're sure will not grow.”
“The only drawback I see to these systems is the lack of true redundancy, unless you are dealing with an expensive system such as Triconex,” counters Speedy. “Also, while a PLC is capable of some complex control, the programmer must be well-versed in how to configure the system.”
David Crump, marketing manager for Opto 22
says a PLC system can be expanded when needed. “Expanding a PLC-based control system without degrading performance often requires deployment of a separate micro PLC,” he says. “Expansion of the PLC-based control system can also be accomplished using various remote I/O processors, which act as slaves or ‘dumb’ I/O units to the master PLC.”
Even so, with servers on the horizon, Cano’s comments about communications limitations may be spot-on. On the other hand, many PLC-based controls have PC-based HMIs, which are certainly capable of handling advanced communications.Speaking Up(ward)
Robert Jackson, PXI product manager for National Instruments
says modern PC-based systems make it easy to talk to servers. “Often times it is difficult to integrate traditional standalone systems into a server based infrastructure, such as the IT infrastructure of a plant,” he says. “However, the latest generation of standalone processors, such as PACs, offer this integration because they are based on PC technology. Challenges that must be overcome when connecting standalone processors to a server-based system include security, deterministic communications, and the availability of a real-time OS and supported hardware.”
Security is certainly an issue when talking to remote systems. We don’t have room to discuss cybersecurity in depth here, but suffice it to say that technology exists to provide all the security anyone needs. For example, completely isolating a control system from outside networks and only allowing an external “Shadow Server” to communicate with the world (see “What’s in Your Server?,” CONTROL, March 2004
) is an excellent way to protect your system.
In today’s world, even standalone systems must deal with the enterprise, as Steve McGeorge, Manager, Product Marketing, at Honeywell Process Solutions
points out: “As customers seek to optimize their supply chain, control systems need to be connected to Enterprise systems, and the various plant floor systems themselves need to be integrated. About 8-10 years ago, some users were attempting to craft their own architectures out of a mix of systems and components. These users have typically changed course since then, after experiencing the cost of integration, maintenance and support and general lack of ultimate functionality.”
A small DCS can fit into the standalone market. “At the smaller end of the market, where single loop and now multi-loop controllers are used on isolated process units, PLCs can run loops and are used in some situations,” notes McGeorge. “Honeywell serves this market with the HC900 integrated with Experion Vista system-level software. This configuration can be as small as a single PC and controller and be used, for example, to run a furnace.”