PC-based control infiltrates process plants

As PC-based control becomes more widespread in process plants, often via purchased machines and process skids, companies are finding it's a short step to implementing it for control over critical processes.

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Dan Hebert, PEBy Dan Hebert, PE, Senior Technical Editor

PC-BASED control has been slow to catch on in the process industries. Many process end users have no qualms about using Intel-based controllers, but shy away from trusting control of critical processes to Windows, Mac, or Linux operating systems. In an ironic twist, however, performance and features specified by process plants are forcing their machine and process-skid suppliers to use PC-based controls.

Process-plant machines range from extruders to packaging equipment to inspection systems. End users buy process skids for water treatment, air handling, and specialized chemical processes such as fermentation. Process plants often need their machines and process skids to have a sophisticated local operator interface, connectivity to the enterprise, and remote monitoring capabilities. High throughput and extreme precision are typical requirements for these machines. Process skids may need sophisticated, advanced control algorithms to fulfill specifications.

To meet these requirements, many machine and process skid builders are turning to PC-based control. Our sister publication, CONTROL DESIGN, focuses on automation of machines and process skids. We find that about half of CONTROL DESIGN’s readers use PC-based control on at least some of their machines and process skids.

Initially, many builders used PCs to meet customer demands for a sophisticated local operator interface, connectivity to the enterprise, and remote monitoring capabilities. However, these machine and process skid builders would typically use a PLC for real-time control because they didn’t trust PCs for critical control.

This trend is rapidly changing because of lower costs, increased reliability, and increasing ease of use. Many of these factors were demonstrated by PC-based control suppliers at the recent National Manufacturing Week (NMW) 2006 trade show in Chicago.

Lower costs occur in two ways. One, it’s much cheaper to combine operator interface and control into one PC-based system than using a PC and a PLC. Not only are purchased costs lower, but integration between the PC and the PLC is eliminated. The second main cost saving occurs because PCs can outperform similarly priced PLCs.

For example, Axiomtek and National Instruments displayed panel PCs at NMW with 6.4-in. LCD screens. A panel PC looks like an inexpensive, low-end operator interface panel, but it performs like the PC that it is. Just add a couple of inches in panel depth and a few hundred dollars, and a graphics panel gains real-time control capabilities. And, because the graphics panel has a PC-based engine, off-the-shelf HMI software packages optimized for limited resource targets like panel PCs can be used. InduSoft was demonstrating this type of software at NMW.

Of course, a control system isn’t just a controller and an operator interface. It also needs I/O, a reliable real-time operating system, and an easy way to program discrete and process control applications.

Let’s start with I/O. Open standards aren’t perfect, but they’ve had a big positive impact on I/O for PC-based controllers. These typically are supplied with an Ethernet port, and many vendors make Ethernet I/O. Just make sure to match the controller and the I/O Ethernet protocols, because Ethernet by itself is not a complete standard.

Similarly, the operating system has long been a bane of PC-based control. Microsoft has addressed many of these concerns with its embedded CE and XP operating systems. Linux is also a popular choice. For ultimate reliability, hard, real-time operating systems are available from vendors such as Green Hills Software and TenAsys. All of these operating systems are real-time and reliable enough for most process control applications, and they aren’t memory hogs. Because memory requirements are reduced, RAM costs are cut, and flash disks can be used instead of less reliable hard disks.

The last hurdle for many is programming the PC to perform real-time control. C++ and other high-level computing languages used to be the only option, but the rise of Visual Basic has given many programmers another and often better option.

In addition, many suppliers now provide graphical programming software conforming to the IEC 61131-3 standard. These software packages allow programming of PC-based controllers with a standard methodology. IEC 61131-3 includes the Sequential Function Chart language used to structure the internal organization of a program, and four interoperable programming languages: Instruction List, Ladder Diagram, Function Block Diagram, and Structured Text.

As a result, PC-based control is becoming more widespread in process plants, often via purchased machines and process skids. Once automation professionals become comfortable with PC-based control through purchased systems, it’s a short step to implementing PC-based control for control of their own critical processes.

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