Control system architectures: Remote control

ControlGlobal.com

Keywords: ERP, fieldbus, OPC, remote process control, process automation, process safety management, automation engineering, process automation technologies and industrial automation systems

By using fieldbus, OPC links, secure data transmission systems, appropriate software and modern server technologies, you, too, can build your own server-based architecture. That is, if you are ready for it.

By Rich Merritt, Senior Technical Editor

WE'VE BEEN PREDICTING it for years: Server technology is coming. Instead of having millions of dollars worth of asset management, process historian, supply chain management and other high level software in your plant, it will be centrally located, perhaps at your corporate headquarters, and reached by high speed networks.

Or you may be renting high-level software, such as loop tuning and ERP programs that are running on secure servers thousands of miles away. In other words, instead of spending $6 million for ERP software, you’ll just pay for the time you use. Siemens already makes this service available in Germany.

Soon, you may be running your HMI/SCADA systems on servers at your corporate headquarters, instead of in the plants. As server technology grows and expands, more of the non-real-time control functions will leave plants and reside in central servers. Soon, some local control systems will be a thing of the past, because remote and unmanned plants won’t need them (See Figure 1).

ADVERTISEMENT FIGURE 1: REMOTE CONTROL

This offshore platform for Shell Malamplaya in the Philippines can operate completely unmanned, thanks to a server-based control system architecture and a remote DCS. Source: Emerson Process Management.

As you select a new control system architecture for your latest project, keep in mind that you probably will be connecting it to a server-based system sometime soon. Make sure that whatever you buy will have hooks to the future.

Shhh…Servers are Here Already
The concept of client-server systems in a plant is not new. Server technology has been around for several years, and it has been used in process control for quite a while. What’s new is that vendors are able to put advanced control and support functions in a server computer that can be located anywhere in the world – in remote server farms, in a vendor’s regional service center, or even at an end user’s central engineering office.  Vendors just don’t like to talk about it, apparently because the whole idea of outsourcing control support functions is anathema to the user community.

I scanned about a dozen recent issues of CONTROL, examining all the ads by process control vendors, and did not find a single mention of remote server capability, even though almost all of them can do it. Why keep it a big secret?

Jim Carr, a control engineer at CH2M Hill, a consulting engineering company in Corvallis, Oregon, works on municipal water and wastewater control systems, and he tells the typical user reaction: “I have to say that most of the clients we work with would have very little interest in having any entity that was not under their direct control having any part in the high level operation of their system.”

Since this attitude is widespread, no wonder the vendors are playing down their server capabilities.

Nevertheless, the secret is getting out. Cliff Speedy, a controls engineer at C&I Engineering sums up what you probably already know or have heard rumors about: “It seems that the DCS market is in a huge state of flux,” notes Speedy. “Honeywell is in the process of migration from the old TDC3000 to the newer Experion, Yokogawa and Delta V are now emerging with quality systems that seem to be easier to configure and maintain and employ open Ethernet-based communication systems. YCA and Honeywell both are employing remote servers to perform complex control functions.”

I searched the archives of other control magazines for any stories on Shell Malampaya – the most well-known example of a remote server application (see below)— and found only a single news story, with no mention of the servers. Clearly, vendors are not promoting server technology. Maybe they think you are not ready for it.

It is important to note up front that server-based systems do not perform real-time control, safety shutdowns, process logic, or any other function that requires fast, deterministic, reliable control. Those functions will always be in the plants, under the direct control of PLCs, PCs, hybrid controllers, distributed control systems, fieldbus systems, smart valves and similar products. Server-based systems monitor, supervise, analyze, and support those real-time plant systems, but they don’t actually control the plant on a real-time basis. Not yet, anyway. 

Achitecturally Speaking
We see three basic kinds of control system architectures available today:

• Standalone System: A standalone control system is completely self-sufficient and self-contained. Typically based on a PLC, industrial PC, hybrid PLC, or PAC (programmable automation controller), it includes I/O, a controller, an HMI, and some means of connecting its various components via a network, fieldbus, hard wiring or a bus system. Standalones are typically used to control individual unit operations, packaged systems, small processes, skid-mounted systems, etc. Although it may take direction from a higher-level system (such as a command from a supervisory system, “make batch recipe 123 today”), a standalone system carries out its task without direct supervision. A standalone system can be linked to other control elements in a plant if necessary, but if the communications are severed for some reason, the standalone system carries on by itself.
• Distributed control system: A DCS ranges from the traditional distributed process control systems that we've been using since 1975, up to the modern process automation system (PAS) of today. A DCS consists of multiple direct control elements, HMI workstations, SCADA systems, control processors, logic processors, I/O processors, servers, process historians, and high level software packages, all linked by networks such as fieldbus or Ethernet. A DCS is truly “distributed,” with various tasks being carried out in widely dispersed devices.  In general, a DCS is often used to control large processes, and it often incorporates high-level software packages, such as asset management. Oddly enough, none of the major control system vendors that made DCSes so popular in the past call their systems DCSes any more. They are now called every euphemism in the book except DCSes when they are, in fact, even more distributed than the original DCSes of the 1970s.
• Server-based system: In a traditional standalone control system or a DCS, as described above, all the various control and monitoring devices, networks, HMIs and other equipment are located on the plant’s premises. In a server-based system, certain non-critical parts of the control system can be located on or off the premises. While all I/O, critical controls, shutdown systems, and other real-time functions are kept in the plant, remote servers can perform all the advanced DCS-type supervisory control, SCADA, asset management, ERP, loop tuning and similar functions from afar. Such servers can be located anywhere in the world, and can be reached by secure and non-secure communication links, such as microwave, satellite, virtual private networks, the Web, dial-up modems and cell phones. Both standalones and DCSes can make use of server-based support systems.