Best practices in control system migration

Of course, someone has to be first to adopt new software, and it may be you. A fair trade-off would be to get the vendor to provide free on-site support in consideration for use of your process automation system as a beta site. The key is to do your homework going in, so that you know if the new software is proven and stable.

Stable and bug-free vendor software is crucial, and so is the application software that will control and monitor the system. Making the right decision with respect to your application software is vital.

Reuse Your Existing Software?
Your existing control system has been controlling your processes for years, maybe decades. There are many lines of code, multiple HMI screens and a plethora of miscellaneous software programs. Some of the vendors claim to have conversion programs that will automatically convert existing software to run on your new system. But is this a good idea?

“I definitely owe the job’s success to the decision to throw out all of the old code and start from scratch with a competent integrator that had a broad background of S88 batch experience with both PLC/HMI and hybrid DCS platforms,” says Burgman of Sun Chemical.

“I could have had all of the control-module and equipment-mode code imported into the new system, and at first glance this seemed like an excellent way to save on programming costs. But this would have meant importing all the old mistakes, band-aid work-arounds and abandoned code of the old system. We also would have not been able to use current practice programming techniques with features like libraries for S88-phase and equipment-module logic,” concludes Burgman.

A system integrator had success using a third-party utility to convert code. “On a job for International Paper/Cantonment, the existing Allen-Bradley PLC3 had thousands of lines of code,” says Murray of FeedForward. “Instead of interpreting and manually recoding, we used software from Javelin Technologies to automatically convert the PLC3 code to ControlLogix. We tested representative points and found the conversion to be accurate.”

Migrate in Phases
Breaking a large migration project into smaller and more manageable phases reduces risk and downtime. “Our migration had to be completed with minimum downtime, so we used the phased replacement approach. The first phase targeted the operator stations, the second phase replaced the controllers and I/O, and the third phase was a combination of the first two phases in a different process area,” reports Yancey of Konica Minolta.

A system integrator relates its approach to phased migration. “For a large-scale retrofit, it’s best to use a phased approach,” claims Jim Zelazny, automation systems manager at systems integrator Integrated Mill Systems. “Phased migration eliminates risk by narrowing the focus incrementally while providing a fall-back to the old system. This approach requires communication with the existing system for interim phase-in, physical coexistence with the old equipment to avoid burning bridges, and the ability to switch quickly and easily between old and new signals for testing/tuning purposes. We leveraged the high bandwidth of GE Fanuc’s Control Memory Xchange to bridge to the old system. This and innovative techniques to quickly switch hard-wired signals, made it possible to successfully migrate without interrupting production,” concludes Zelazny.

Phased migration does have its drawbacks. “We did our Yokogawa migration in three phases and stretched it over a four-year period,” says Peppers of Hexcel. “After completing the detailed scope for the final phase, a cost comparison study was made. The three-part plan was great for lowering the annual capital required and the magnitude of the tasks, but I would have preferred one all-inclusive project plan. The three phases turned out to be three individual projects for Hexcel. Because of the extended time frame involved and other things, such as administrative/startup costs, scope increases, inflation, taxes, labor and loss of single-project efficiencies, a significant overall cost increase was realized.”

Phased migration will cost more and take more time, but it is a lower-risk approach with less downtime. Further risk and downtime reduction can be achieved by simulating the new system prior to installation.