The temperature controller of a 20-year-old vacuum furnace had gone completely dark. This furnace was a critical piece of equipment for the owner, a local aircraft plant. The original manufacturer of the controller no longer supported it — but they would be happy to custom build a new controller and ship it out in six weeks. This was unacceptable to the owner; they needed the furnace ASAP, so they called ACES.
In this case, it was obvious: The controller was obsolete and a direct replacement was unavailable. The challenge was how to shed light on the inner workings of the controls.
The first thing the ACES Control Systems Investigators did was to request documentation. The wiring offered a few clues but was misleading, showing analog wiring applied to DC switch contacts. To make matters worse, the PLC was an outdated unit that required a DOS-based PC to communicate. Consequently the CSIs arrived on the scene armed with nothing more than parts and a blank hard drive; they installed DOS 6.22 on the drive as well as software which allowed them to look into the old ladder logic program. The furnace’s 38 recipes also offered more clues to the functions.
Based on the recipes and the PLC program, the ACES CSI team drew a picture of everything that was missing in the middle. With some code sleuthing they reverse engineered the old programming, reprogrammed all the controls, performed a tuning survey and had the furnace back online in four days — saving the customer six weeks and potential lost business. ACES is now quoting a PLC replacement for the old system to prevent a similar recurrence and further downtime.
ACES CSIs are experienced in casting light on dark and mysterious legacy controls. For another example, see The Case of the Obsolete PLCs.