NOV-DEC 2017

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48 INTECH NOVEMBER/DECEMBER 2017 WWW.ISA.ORG By Michael Whitt O ne challenge faced by all control sys- tems integrators (CSIs) is the prospect of testing their software before de - ployment. CSIs often work remotely, without having the actual equipment to manipulate, or even a full complement of I/O racks and modules. Programmable logic controller (PLC) software development may be done complete - ly offline (static), or in a lab setting with only a PLC rack, a CPU, a power supply, and some kind of communications port. The CSI often develops software in the lab and implements it in the field. The vast majority of projects executed by CSIs are small, with a fast turnaround and limited budget, resources, and schedule. Although most modern PLC software has tools to simulate I/O, to force/clear internal bits, and to set analog val - ues through animation tables, employing those techniques to perform a realistic test of the PLC program can be unwieldy and time consuming. In addition, from a validation point of view, the tests are not repeatable, as they depend on the user to input values and responses. One alternative is to procure and configure simulation software to emulate field devices and complex control schemes. This software can be prohibitively expensive, requiring a very steep learning curve and having associated maintenance fees and recurring costs. To many, process simulation is simply a luxury. Why simulate? To our knowledge, no formal study has tested the actual time saved by making a software correction in the lab versus making that same correction in the field, but there is ample evidence to suggest those savings can be im - mense. This savings in time not only feeds the bottom line, but also mitigates the risk associ - ated with a poor startup, which negatively af- fects prestige—a more important parameter than cost. This effect presents itself not only on huge process control projects with plentiful resources and visibility, but also on small tasks with minimal or no additional equipment or infrastructure. So, how can a CSI address the testing dilemma if the budget does not include a simulation software package? Passive loopback process simulation Process control logic Device control logic PLC Commanded state CONTACTOR MOTOR A B C C M A DO B C CMD DI AUX Status feedback Figure 1. Typical motor control scheme

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