TABLE OF CONTENTS The lowest level of the control hierarchy is the control module. Control modules perform two primary functions: they provide an interface with the physical devices, and they contain basic control algorithms.
Control modules encapsulate basic control algorithms and the I/O interface to the actual physical devices.
A control module is a collection of basic control logic, sensors, actuators, control modules, and associated processing equipment that is operated as a single entity. Control modules are where the real work is done, in the form of basic control. Control modules only execute basic control and minimal coordination control. They perform no procedural control functions. The basic control includes state-oriented control (on/off, open/ closed, etc.), algorithm-based control (PID, mathematical model, fuzzy logic, etc.), and reactive control (perform action on event). The most common method of programming basic control are DCS function blocks or any of the IEC 61131-3 programming languages, such as Ladder Diagram, Function Block Diagram, Sequential Function Chart, Instruction List, and Structured Text.
| Note | The IEC 61131-3 Sequential Function Chart language is often used for equipment module procedural logic, and is not used in control modules. |
Control modules usually make up the majority of control system code in a batch application, but they are also the mechanism for defining significant amounts of reusable code. Some examples of control modules from the ISA 88 standard are
a regulating device consisting of a transmitter, a controller, and a control valve that is operated via the set point of the device.
