TABLE OF CONTENTS Until recently, the majority of valves and actuators were used as part of analog systems. Today, as the process industry enters a new millennium, the face of process control is changing such that smart technology is quickly overtaking those antiquated analog systems, which were once so prevalent. Smart final control elements such as intelligent systems mounted on valves or digital positioners used with actuators have fewer or no moving parts to fail, and the performance associated with digital communications is far and away better than the 4-to 20-mA signal found with I/P analog systems. Plus, today s smart final control elements offer a whole host of new functionalities once thought futuristic such as automatic loop tuning, self-diagnostics, information processing, planned maintenance, and warning/alarm management.
To understand the terminology and abilities of smart products, a number of common instrumentation and control principles and terms must be generally understood.
A wide majority of control systems that link process sensors and final control elements, such as control valves and actuators, use controllers or distributive control systems to provide intelligence in the control loop. A controller is a microprocessor that receives input from a process sensor such as a pressure or temperature sensor or flow meter and compares that signal against a predetermined value. After the comparison is made, it sends a correcting signal to a final control element until the predetermined value is reached. A common controller seen...
