9. I/O interface
This strategy enables the new controller to use an existing I/O subsystem (field devices, racks, terminations and I/O modules) and, so, saves considerable rewiring costs (Figure 9). The legacy I/O stays in place and the new controller interfaces with it. The old controller is removed.
Figure 9. Use of existing field devices, racks, terminations and I/O modules avoids significant rewiring costs.
Hurdle: Similar to the I/O replacement solution, the I/O interface strategy requires a special board that fits into the old controller backplane. The good news is that only one board per controller is needed.
Suggestion: In most cases, the incumbent vendor is the first place to turn for this kind of solution.
10. Field termination assemblies
Saving the field termination assembly (FTA) lives at the very bottom of the food chain of migration solutions because it comes closest to complete rip-out and replace (Figure 10). This hardware solution preserves existing wiring by providing a 1:1 replacement of existing terminations and connection to new I/O modules via a new FTA (with the same form/fit and function).
Figure 10. Replacing legacy FTAs with ones for the new controller that use the same connectors preserves existing wiring.
Hurdle: Ideally, an FTA should be the same size as the original terminations (so that no additional cabinet space is required) and use the existing connectors of the field wiring (so that no rewiring is needed), saving considerable labor costs.
Suggestion: The incumbent vendor may have an FTA for its legacy equipment similar to a small patch panel for cable translation between the old and new terminations.
One of the keys to a successful migration is to evaluate your existing system to determine which assets should be kept and which should be replaced. It is common to focus on preservation of field devices and installed wiring. However, in some cases, the value of the intellectual property dwarfs that of the hardware. Significant process expertise from years of continually optimizing the process is tied up in the controller program and HMI application. Some vendors have developed tools that automatically convert process graphics, controller code and historical information.
Be wary of one-off solutions. Some vendors create application patches before fully productizing the solution. This could result in long-term maintainability and supportability issues. Your best bet is a vendor that fully productizes its offerings and backs them with the same level of technical support available with its mainstream products.
First consider an HMI replacement solution. This gives operators the longest time with the old system. You want the HMI to be accepted by your operators, so it should have a similar look and feel if possible. You also want the HMI to gather at least the same amount of data as the old HMI, so look for a solution that has a similar throughput.
Another small step you can make is to add new I/O modules to an existing system using a fieldbus like Profibus.
Tools are available to redeploy controller code, but are never 100% complete. An experienced engineer must look at the code and complete the conversion.
The ideal strategy is to invest a few weeks working on site with the vendor doing the conversion; this ensures it stays on your project and allows you to give immediate feedback.
Chances are that you wont want the same graphic that you had in 1985. The best idea is to reconfigure the graphics yourself (after attending the vendors configuration class). Youll get the most up-to-date look and feel while taking advantage of all the latest features of the new graphics.
The bottom line
Each plant situation is unique. The optimum migration strategy depends upon numerous factors, including the cost of downtime, spare parts, maintenance, the age of existing system components, and the value that some assets (hardware and software) retain. Make sure your vendor offers you different approaches (such as HMI migration, communication gateways, I/O connectivity, termination replacements) so that you can decide what is best
The bottom line with any change is that someone has to go first. Swallow hard and take the first step. Chances are you will achieve a significantly better solution that puts you well ahead of your competition. And thats a reward that makes it worth facing your migration fears.
Ken Keiser is a migration marketing specialist for Siemens Energy and Automation, Spring House, Pa. E-mail him at firstname.lastname@example.org.