PID Controllers - Theory Design and Tuning 翻译.doc

IntroductionThe PID controller has several important functions: it provides feedback; it has the ability to eliminate steady state offsets through integral action; it can anticipate the future through derivative action.PID controllers are sufficient for many control problems, particularlywhen process dynamics are benign and the performance requirementsare modest. PID controllers are found in large numbers in all industries. The controllers come in many different forms. There are standalone systems in boxes for one or a few loops, which are manufactured by the hundred thousands yearly. PID control is an importantingredient of a distributed control system. The controllers are alsoembedded in many special-purpose control systems. In process control, more than 95% of the control loops are of PID type, most loopsare actually PI control. Many useful features of PID control have notbeen widely disseminated because they have been considered tradesecrets. Typical examples are techniques for mode switches and antiwindup.PID control is often combined with logic, sequential machines, selectors, and simple function blocks to build the complicated automation systems used for energy production, transportation, and manufacturing. Many sophisticated control strategies, such as model predictive control, are also organized hierarchically. PID control is usedat the lowest level; the multivariable controller gives the setpoints tothe controllers at the lower level. The PID controller can thus be saidto be the "bread and butter" of control engineering. It is an importantcomponent in every control engineers toolbox.PID controllers have survived many changes in technology ranging from pneumatics to microprocessors via electronic tubes, transistors, integrated circuits. The microprocessor has had a dramatic2 Chapter 1 Introductioninfluence on the PID controller. Practically all PID controllers madetoday are based on microprocessors. This has given opportunities toprovide additional features like automatic tuning, gain scheduling,and continuous adaptation. The terminology in these areas is notwell-established. For purposes of this book, auto-tuning means thatthe controller parameters are tuned automatically on demand froman operator or an external signal, and adaptation means that theparameters of a controller are continuously updated. Practically allnew PID controllers that are announced today have some capabilityfor automatic tuning. Tuning and adaptation can be done in manydifferent ways. The simple controller has in fact become a test benchfor many new ideas in control.The emergence of the fieldbus is another important development.This will drastically influence the architecture of future distributedcontrol systems. The PID controller is an important ingredient ofthe fieldbus concept. It may also be standardized as a result of thefieldbus development.A large cadre of instrument and process engineers are familiarwith PID control. There is a well-established practice of installing,tuning, and using the controllers. In spite of this there are substantialpotentials for improving PID control. Evidence for this can be foundin the control rooms of any industry. Many controllers are put in manual mode, and among those controllers that are in automatic mode,derivative action is frequently switched off for the simple reason thatit is difficult to tune properly. The key reasons for poor performance isequipment problems in valves and sensors, and bad tuning practice.The valve problems include wrong sizing, hysteresis, and stiction.The measurement problems include: poor or no anti-aliasing filters;excessive filtering in "smart" sensors, excessive noise and impropercalibration. Substantial improvements can be made. The incentive forimprovement is emphasized by demands for improved quality, whichis manifested by standards such as ISO 9000. Knowledge and understanding are the key elements for improving performance of thecontrol loop. Specific process knowledge is required as well as knowledge about PID control.Based on our experience, we believe that a new era of PID controlis emerging. This book will take stock of the development, assess itspotential, and try to speed up the development by sharing our experiences in this exciting and useful field of automatic control. The goalof the book is to provide the technical background for understandingPID control. Such knowledge can directly contribute to better productquality.Process dynamics is a key for understanding any control problem.Chapter 2 presents different ways to model process dynamics thatare useful for PID control. Methods based on step tests are discussedChapter 1 Introduction 3together with techniques based on frequency response. It is attemptedto provide a good understanding of the relations between the differentappr。