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APC Background

The papers provided in this section have been written by prominent specialists in the field of APC and its application to industrial processes and submitted to APC-network.com for publication. They provide the users with a background knowledge of the relevant technology. The copyright for these papers belong to the authors and any authorisation to quote or use of the contents should be addressed direcly to the authors.


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APC Background
Inferenial Control
Y.Zak Friedman, PhD, Principal consultant

What is inferential Control? Some say it is the type of control scheme which tries to control product qualities without an on-stream analyzer. Traditionally people implemented inferential models measuring simple signals, such as a tray temperature on a column, in an attempt to avoid analyzers. The premise is: if we are able to maintain that tray temperature at a constant value, then the product quality is constant. No inferential controller can be very successful unless it models the influence of all pertinent variables. What then is a reasonable inferential controller? This paper proposes an approach which adheres to four principle: 1- The correlations must be based on fundamental laws or at least on established engineering procedures, such as API methods. 2- All predicted plant variables have to agree with process measurements. 3- There are operator inputs in the form of entering numbers into the program. These tend to be unreliable in time and in value, and they are best avoided. 4- There must be a simple way to calibrate the model. Further the models are verifiable and tunable via a few simple sets of steady state data. The paper proceeds to describe four examples of working inferential control schemes.

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