PREFACE
This book is a text for students learning automatic control systems and a reference for those pursuing careers in industry. When readers select a measuring sensor or a control valve, this book will refresh them about the important details that must be considered. When they interconnect the sensor and control valve into a communicating network, this book will remind them of the important methods for minimizing interference from electrical and magnetic fields. When they tune a controller, it will guide them toward establishing a desirable control loop response to a disturbance.
The concepts discussed in this book focus on a classic process control loop with its four major blocks: the measuring sensor block, the controller block, the process adjustment block, and the process block. Design procedures include the selection of measuring sensors and control valves. The sensors concentrate on the measurement of temperature, pressure, flow rate, and level, and analysis of humidity and electrolytic conductivity. Also covered is the design of network wiring to minimize interference.
Operational procedures show the presentation of process information for safely monitoring the trajectory of a changing process (e.g., trend display and alarm status), and they describe control loop responses to disturbances. The financial investment in this kind of equipment is also reviewed.
Maintenance procedures emphasize calibration of measuring sensors. Maintenance of a network to minimize interference from inadvertent grounds on the network is also described. Additionally, the tuning of controllers to achieve a desirable response to disturbances is emphasized.
Each chapter begins with objectives that present the concepts covered. At the end of each chapter are sets of questions and problems, suggested practical lab assignments, and references that provide additional resources on the concepts covered in this book. Selected problems have answers in Appendix C.
Chapter 1 introduces continuous automatic control and differentiates it conceptually from discrete control. The control loop is emphasized as the basic concept for continuous control of a process variable. The four blocks of every control loop are described, and their input and output signals are shown on a block diagram. The Instrument Society of America symbols are presented for describing control systems on a Process and Instrument Diagram (P&ID).
Chapter 2 begins with instrument calibration concepts and documents. Instruments for sensing temperature, pressure, flow, level, humidity, and electrolytic conductivity are described, and many of the device limitations for these measurements are mentioned. These limitations are shown as important elements involved in the selection of a device for a successful application.
The presentation of process control information for human operators and for machines is introduced in Chapter 3. The overview display, process group display, and the point display are described. Additional displays include trend, alarm summary, and hourly averages.
Chapter 4 presents techniques to achieve reliable, secure data communications around the control loop. Comparison of pneumatic and electric communications are made to show the speed of the electrical technique and its potential hazards. Problems due to interference from magnetic and electric fields and from improper electrical grounding are described, along with proper techniques to minimize these problems. The concepts of networks are introduced.
Chapter 5 emphasizes the application of control valves for process adjustment. The sizing of control valves for liquid and gas flow adjustment follows ISA standard S75.01.
Chapter 6 introduces the proportional, integral, and derivative (PID) controller and explains its functions and the ultimate cycle method for its tuning. Ratio control and cascade control of a process are also explained.
The appendixes describe ISA symbols, display blank calibration documents, display photos of control valves, list answers to selected problems, list ASCII symbols and codes, describe calculations for liquid pressure drops in piping, and list a table of thermocouple millivolts versus temperature and a table of RTD resistance versus temperature.
Students and instructors should review the Study Procedures on page vii to assess whether they have the prior training required to rapidly absorb the information in this book. The estimated time required for students to learn the material, study the theory, answer questions and problems, and perform the assignments is also given. The instructor's manual available for use with this text provides an outline of study and further information on how to successfully teach classes in process measurement and control.
Acknowledgments
I wish to thank Humber College for supporting me in the preparation of this book. Colleagues from the Electrical/Control Systems program who encouraged me include Jeff Dixon, Mike Birmingham, and Tarsem Sharma. The ISA-Toronto section was very helpful, especially Len Klochek, who made many suggestions for improvements. I would also like to thank the following reviewers for their valuable feedback: J. Tim Coppinger, Texas A&M University-Corpus Christi; Mohan Kim, San Jose State University; Xiaoli Ma; David Pacey, Kansas State University; William Reeves, Ohio University; Wahija Shireen, University of Houston; and Gang Tao, University of Virginia. My thanks to Alex Wolf of Prentice Hall and Tim Flem and Jim Reidel for significantly improving this book. I wish to thank Charles E. Stewart of Prentice Hall for his confident support.
Roy Fraser
Humber College of Applied Arts and Technology