A Unified PID Control Methodology

PID control is the workhorse of process automation. At this week’s AIChE 2013 Spring Meeting and 9th Global Congress on Process Safety, Control Talk blog‘s Greg McMillan and Eastman Chemical’s Héctor Torres presented their paper, A Unified PID Control Methodology to Meet Plant Objectives.

Here is the presentation:

Greg and Héctor describe this unified approach:

A unified approach to tuning has been found that enables a common and simplified method for setting PID tuning parameters. Key features can be used to eliminate the need for retuning to deal with different dynamics and objectives. This paper shows a methodology that integrates a unified tuning approach and key features that minimizes implementation and maintenance efforts. A step by step method will be used that will address the myriad of dynamics and objectives in PID applications.

They explain the characteristics of the four types of process responses: dead time dominant self-regulating response, lag dominant self-regulating response, integrating response, and runaway response. The tuning requirements are so different for each type of response that process automation professionals can get confused.

The authors note that automation systems including sensors, final control elements and controllers can introduce excessive discontinuities, lag, noise, and dead time. For example, a control valve may have excessive friction causing a time delay in opening or closing.

They highlight the unified tuning approach:

The key breakthrough in thinking is to use the lambda tuning rules for integrating processes for lag dominant self-regulating and runaway processes. The lambda tuning rules automatically prevent the product of the PID gain and reset time from being less than the twice the inverse of the integrating process gain that would result in slow rolling oscillations. The main decision is whether to maximize the transfer of variability or maximize the absorption of the variability. Key features are used rather than tuning to address the automation system difficulties and to meet different process objectives.

You’ll want to read the paper to understand the considerations in the decision to transfer or absorb variability, understand the key features in PID to address these decisions, and the step-by-step procedure through the unified methodology.

Greg and Héctor conclude:

The use of key PID features, a unified PID tuning method, and an adaptive tuner enables an effective application of PID control for wide spectrum of process responses. Drastically different sources of variability, automation system difficulties and changing process objectives do not require PID retuning. PID features can be used to tailor the PID response to different application requirements on a much more understandable basis by the user. The role of the PID can be expanded to include process optimization besides regulation.

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