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DCS vs. PLC Analog Signal Inputs

October 11, 2010
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David W. Spitzer

Doing something and doing it right can often be two different things. Consider the case of converting an analog signal from a flowmeter so that it can be displayed and/or totalized. Years ago, the analog signal was recorded on a circular chart recorder or strip chart recorder that may have included an integral totalizer. As an alternative, the analog signal could also be fed to a separate flow totalizer.

With the advent of distributed control systems (DCS) and programmable logic systems (PLC), the analog signals could be connected to a DCS or PLC input that would convert the signal to a digital number for display and/or totalization. Remembering the development of DCS and PLC technologies had their roots in analog and discrete control respectively, so it would not be surprising to find different that DCS and PLC analog inputs are of different quality.

On a recent project, the accuracy of two PLC analog input cards from the same manufacturer used in the same application were 0.1 and 0.35 percent of full scale. This is a wide divergence in performance. Using the latter specification, the error attributable to the analog input is greater than the error attributable to a flowmeter with 0.5 percent of rate accuracy operating below approximately 70 percent of full-scale flow. It might seem counterintuitive that the analog input card would contribute more error than the flowmeter at such a high flowrate, but check the math for yourself.

In contrast, a DCS analog input card might exhibit an accuracy of 0.03 percent of span. Assuming this performance is typical of PLC and DCS analog inputs, it is not surprising the DCS (with a legacy of continuous analog control) exhibits better analog performance than the PLC (with a legacy of discrete control). Conversely, given these legacies, one would expect to find that PLCs have better digital capabilities than a DCS.? What is the quality of your analog input?

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular "Consumer Guide" series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.

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