Configurations of Process Control Systems

Configurations of Process Control Systems

Centralized System

A central control system is especially useful in a large interdependent process plant where many different processes must be controlled for efficient use of the facilities and the raw materials. It also provides a central point where alarms can be monitored and processes can be changed without the need to constantly travel throughout the plant. Construction of this type of system using analog controllers requires that each sensor and actuator be wired back to the central control site so that the information from each remote site can be individually displayed.

Limitations of this type of control are the inability to transmit analog signals over long distances without adding noise to the signal. This reduces the overall resolution and accuracy of the system.

Hybrid Control

Hybrid control is a mixture of analog control loop processing and digital signal transmission to take advantage of the ease of use of the analog control and the advantage of noise immunity and reduced wiring expense of digital serial data transmission. In this configuration, local process control is handled by the analog control elements, and a digital computer- based central control system provides the ability to enter set points into each controller from one location. Each controller is interfaced through a digital transmission link.

Distributed Digital Control

Distributed digital control permits the distribution of the processing for each task among several control elements. Instead of just one computer located at a central control point doing all the processing, each local loop controller has digital processing capability. All of the local loop information can be passed to the central control computer so that it can be monitored easily by the operator. At the same time, each individual control loop operates independently from the others so that any communications failure or central computer overload does not affect the operation of any of the real-time process control loops. The only effect would be the inability to monitor the loops, or to pass information from one loop to another.

Each digital processor solves the control equations for its local loop under software control. Analog sensors and analog actuators are interfaced to the system with A/D and D/A converters. Many times other logic functions can be added easily and new requirements can be added by reprogramming one or all of the digital processors. As a result, digitally-based controllers represent the direction in the design of continuous control.