Educational Resource for Tuning and Control Loop Performance

Process Automation Hall of Fame member, Greg McMillan

Greg McMillan
ISA Fellow

Process automation hall of fame member Greg McMillan has nearly completed an updated version of the book, Tuning and Control Loop Performance: A Practitioner’s Guide. It had last been updated back in 1994. You may be familiar with Greg through his blog posts on the Control Talk and ISA Interchange blogs.

He shared with me that this 4th edition is nearly a 1000 pages long and is a complete rewrite of the 1994 3rd edition. Greg noted that a lot has happened and he has learned a lot in the last 20 years. The book shows how lambda tuning and external reset feedback (e.g. DeltaV Dynamic Reset Limit) can be used to overcome challenges associated with lag-dominance, cascade control, slow valves, interactions, nonlinearities, resonance, and valve position control.

This book will be available this fall and I will update the post when it is available for ordering. If you are interested in knowing when it will be available follow Greg in Twitter and/or connect with him in LinkedIn and he’ll share the news with you.

Here is the current draft of the preface from the book to better understand the scope of the changes:

Most plants depend entirely upon PID control. Even plants with model predictive control still depend on the PID to deal with valve nonlinearities and provide fast control of flows and pressures. Yet most of the capability of the PID is not effectively used due to disagreements as to PID tuning rules and a lack of guidance on how to meet different objectives and deal with process and automation system dynamics. This book seeks to provide an intensive and extensive view of what is needed to get the most out of the PID based on 44 years of experience in applying PID control in the process industry. New tools for analyzing and tuning loops offer automatic identification of loop dynamics, PID settings, and statistics on variability. This book seeks to take advantage of the new ability of tools today to know the dynamics and choose tuning rules and factors to free up the individual to focus more on selecting PID options and setting parameters to meet the challenges and objectives of the application.

The 1st edition in 1984 focused on the tuning methods prevalent at the time with the objective of minimizing the peak and integrated error from unmeasured load disturbances at the process input. Tuning techniques to minimize these errors were aggressive. In practice the PID gain was cut in half to provide more robustness (e.g. gain margin increased from 2 to 4). While minimizing these errors is still important for preventing activation of safety instrumentation systems, relief devices, preventing environment violations, and preventing off-spec product in columns, crystallizers, evaporators, and reactors, there are many other considerations. The book has been almost completely rewritten in the 4th edition to meet other objectives such as minimizing overshoot in the setpoint response, minimizing the propagation of oscillations due to interactions and resonance, maximization of variability absorption for surge tank level, and maximizing the coordination of loop responses to reduce transients in the material and energy balances. Also provided is the recognition of when measurement, valve, and variable frequency drive (VFD) dynamics and resolution limits present a problem and the use of PID tuning and options to minimize the consequences until the source of the problem is fixed. Tuning is not meant to be a cover up but a symptom of a system problem. Dynamics and tuning and can be used to find how to improve the equipment, piping, measurement, PID, valve, or VFD design and installation.

The 4th edition is exceptionally long because the topic is so huge and practical expertise that plays such a key role in the success largely remains in the brains of senior specialists, consultants and technologists. The principles, simple algebraic equations, recommendations, examples, test results, and key points presented seek to make the information more accessible. While some of the foundation is derived from the frequency domain, the details and understanding is based on relationships are detailed in the time domain since this is what is seen on trend recordings and can deal with discontinuities from analyzers and valve backlash and stick-slip. The analysis in the time domain is more consistent with the thought process of practitioners opening up a dialog between process control, operations and process design. This is not to discount the value of frequency domain analysis and power spectrums available in new tools that can build upon what is learned in university courses on control theory.

If I had to pick chapters to read first, I would recommend Chapters 1-3 to provide the basis and overview of the total solution. Subsequent chapters provide more of the details for a spectrum of application considerations, problems and solutions. The appendices provide significant technical support.

Chapter 1 starts out with the basics and provides an overview of tuning rules that are representative of the more than 100 rules documented in the book PI and PID Controller Tuning Rules by O’Dwyer. The 4th edition seeks to show how many of these rules converge to provide the same PID gain and reset time with slightly different factors when the objective is the minimization of errors from load disturbances. The author apologizes if a favorite tuning method is not shown. The important point is that the loop should be tuned and the egos behind particular rules should take a back seat to the solution. A good practitioner will get good results by adjusting factors and using PID features. The 4th edition seeks to document the practical expertise that is more important than the rule. Test results are used in most of the chapters to explore, discover and verify concepts.

Chapter 2 offers a comprehensive overview of the total system solution. A unified methodology is presented to tie together the whole solution. Chapter 3 provides simple equations to estimate the effect of tuning settings on peak and integrated errors for load response and the ability to get to a new setpoint quickly. The relationships between tuning and performance seen in the equations are more important than the actual use of the equations to predict particular values.

The effect of process and mechanical design is analyzed in Chapter 4 and the effect of automation system components are extensively discussed including tables of typical dynamics in Chapters 5-7. The effect of disturbances, nonlinearities and interactions are detailed in Chapters 8-10. The book concludes with a discussion in Chapters 11-15 of how to get the most out of cascade control, advanced regulatory control, process control improvement, auto tuners and adaptive control, and batch optimization.

Greg leads a Mentoring Engineers discussion group in the Emerson Exchange 365 community. Consider joining and participating!

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