Changing of the Guard: The Programmable-Logic Controller
Long the mainstay of industrial process controls, the programmable-logic controller (PLC) is now emerging as a superior control device for life-safety systems...
David F. Sereno, P.E., and Glenn Grasso, Affiliated Engineers, Inc., Madison, Wis.
Long the mainstay of industrial process controls, the programmable-logic controller (PLC) is now emerging as a superior control device for life-safety systems.
Safety-system designers have long relied upon hardwired logic panels-a myriad of relays, lights and alarms-to effect a series of yes/no decisions and annunciations. While building-automation systems have embraced the digital age such that front-end deployment of electrical-mechanical devices is rare, safety systems have just recently begun a similar migration to digital systems.
Reasons for the recent migration are twofold. First, insurance carriers have recognized that the reliability of a PLC, set up correctly, is much greater than that of a PC. Secondly, the PLC proactively "learns" the personality of a system and can anticipate a hazardous incident developing and react to prevent it. Consequently, decision-makers in building projects are extremely receptive to the anticipatory capabilities of a "smart PLC," especially when compared to the largely reactive capabilities of a hardwire-based system.
At the same time, insurance carriers and code officials have not accepted the PLC without qualifications. Namely, a minimum number of hardwired safeties must be maintained. This is typically specified as "safety-hardened PLCs with redundant I/O wiring for critical safety devices." By specifying emergency power in addition to the PLC, this now provides system designers with the best of all worlds: the ability to harness the power of a computer; to anticipate, read and thereby prevent a life safety incident; and also a fail-safe reliability status with respect to insurance carriers and code requirements.
Additional benefits of PLC technology are worth noting:
Now that PLCs are a mature technology, technician familiarity and parts support are no longer impediments. Additionally, process-data acquisition systems and vendors of other devices have interfaced with PLCs on a regular basis such that their interface software has been written, eliminating any roadblocks or surcharges to interface with a PLC.
Relative to hardwired alternatives, the PLC can easily reduce cabinet footprints and their associated service clearances by an order of magnitude.
A PLC solution greatly reduces the amount of wiring and moving parts and, unlike hardwired devices, does not rely on relays with springs that wear and contacts that stick.
Costs of PLCs, like those for the rest of the digital world, have dropped dramatically, such that the price associated with a dedicated PLC is easily justified.
PLCs can better handle analog I/O, which is central to allowing recognition of an impending event.