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Weíve gotten along pretty well with PLC-based machine controls. We supply machines to the contract liquid-filling industry, where we or our customers havenít had to deal very much with enterprise connectivity issues until now. Weíre having some difficulty sorting the competing vendor arguments that 1) we should just augment our PLC controls with a PC-based HMI to obtain the needed connectivity, or 2) we need to convert entirely to PC-based control to eliminate integration and support issues. Any advice?
From August 2006 Control Design
Use Only Big Name PC Systems
I have done numerous service calls on all types and brands of machine tool controls including Fanuc, GE/Fanuc, Control Technologies, and MDSI OpenCNC. Iím qualified to rate these controls with longevity in mind, which is a concept that seems to be lacking in the articles in your magazine. Many of the controls I work on are more than 10 years old, and some have been more than 25 years old.
When I refer to PC-based control, I mean the mish-mash of several manufacturersí hardware products, specifically those manufacturers that are small in size or havenít proven their longevity yet.
Most, if not all, of the PC-based controls I service have obsolete, irreplaceable parts in them. These usually are DAQ or I/O boards, especially counter I/O boards designed for encoders. It seems the manufacturers often re-design these boards, and donít offer repair services for their obsolete boards. Board repair often is impossible without the schematic diagrams and part specifications on a circuit board.
Iíve found that many engineers who completed PC-based installations no longer work for the company that installed the system. Often the company that installed the PC-based controls is defunct. I could make changes to the software configuration to allow newer hardware replacements, except for software ďCatch-22s.Ē Sometimes the software packages are turned over to the machine owner, but itís more likely the engineer keeps the packages. Whatís more, the software often canít be replaced without reengineering the entire control software configuration, due to version compatibility problems. There are the compatibility issues with the OS, DirectX, PC hardware drivers, machine I/O board drivers, and the proprietary CNC software package versions. All these versions must be of certain ranges in order to make a working package. Even if I had the proprietary software, it might require an older version of Windows to run properly, and the customer seldom has the original OS discs. If they did, MS Windows 98 updates are no longer supported by Microsoft, so we couldnít get some of the old hardware drivers. There would be no way to return the system to its original state. If it were an older OS such as Windows 95 or DOS-based, that would be even worse. Those issues wonít get any easier over the next 10 years.
Customers donít want a control that must be retrofitted every five to 10 years, and the machine downtime for such retrofits can be a real business-killer. However, most customers Iíve dealt with donít yet know the drawbacks of PC-based controls, and I feel an obligation to tell them when the subject comes up.
There will be growing negative sentiment toward PC-based controls among the managers who buy PC-based machinery over the next 10-15 years due to service problems.
As for augmenting our PLC controls with a PC, I believe this to be a perfectly acceptable alternative. I also would point out that some PLC manufacturers make PC processors that plug-in to existing PLCs. They include integrated Ethernet ports and would make a good solution if many PC components external to the processor module arenít needed.
Johnnie Alderson, president, Mainely Controls, Wayne, Me.
PLCs and PCs Can Co-Exist, ButÖ
For an already installed PLC-based system, you shouldnít need to replace everything with PC-based control. Itís possible to share data with PC-based systems via OPC. We achieved just such coordination a few years ago with A-B PLC systems and about two dozen PC-based systems. The two system types coexisted happily. However, integration and support issues did require having access to two technical experts instead of just one.
Harmony aside, PC-based control has evolved beyond just the PC. With PC-based automation vendors developing more products that are real-time OS and FPGA-based, the old complaint that PC-based equals Windows-based no longer holds true.
Yes, developing control systems on Windows machines has its share of headachesólike when IT unilaterally decides to override local machine settings regarding automatic updates, and pushes through an XP SP2 update that breaks network connectivity. Such headaches are overcome by moving to real-time platforms that Windows updates canít reach and which are deterministic by design.
So, while itís possible for the two types of systems to coexist, the more common question is whether PLC-based systemsówith their inherent lack of performance expansionóneed to be deployed at all, save for the sake of nostalgia.
Chris White, certified LabView architect, ThinkG Consulting LLC, Oceanside, Calif.
PC Controls Must Be Robust
Many people believe that having a separate PC simplifies the replacement of the PC in the event of a hardware failure. This only is true if youíre using an industrial PC where you can get product continuity over a long period of time. You also should consider using a UPS on the PC to reduce the risk of files being corrupted during power failures and improper machine shutdowns. I highly recommend Microsoft Windows XP Embedded as the operating system for the PC because this version of Windows is designed with our industry in mind. Itís simple to configure XPe, so that it boots from a read-only partition, which guarantees that Windows starts the same way every time. XPe also is very small in size, so it can boot from compact Flash, eliminating the need for a hard disk. The final thing you should do is restrict access to the standard Windows environment (desktop and start menu, and donít leave the machine logged in as administrator).
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