PRACTICAL PROCESS CONTROL : TUNING AND TROUBLESHOOTING – CECIL L. SMITH

Practical Process Control (loop tuning and troubleshooting). This book differs from others on the market in several respects. First, the presentation is totally in the time domain (the word LaPlace is nowhere to be found). The focus of the book is actually troubleshooting, not tuning. If a controller is tunable , the tuning procedure will be straightforward and uneventful. But if a loop is untunable , difficulties will be experienced, usually early in the tuning effort. The nature of any difficulty provides valuable clues to what is rendering the loop untunable . For example, if reducing the controller gain leads to increased oscillations, one should look for possible interaction with one or more other loops. Tuning difficulties are always symptoms of other problems; effective troubleshooting involves recognizing the clues, identifying the root cause of the problem, and making corrections. Furthermore, most loops are rendered untunable due to some aspect of the steady-state behavior of the process. Consequently, the book focuses more on the relationship of process control to steady-state process characteristics than to dynamic process characteristics. One prerequisite to effective troubleshooting is to demystify some of the characteristics of the PID control equations. One unique aspect of this book is that it explains in the time domain all aspects of the PID control equation (including as the difference between the parallel and series forms of the PID, the reset feedback form of the PID equation, reset windup protection, etc.) The book stresses an appropriate P I (process and instrumentation) diagram as critical to successful tuning. If the P I is not right, tuning difficulties are inevitable. Developing and analyzing P I diagrams is a critical aspect of troubleshooting.