Control Module
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The lowest level grouping of equipment in the physical model that can carry out basic control.

NOTE - This term applies to both the physical equipment and the equipment entity.

A control module is typically a collection of sensors, actuators, other control modules, and associated processing equipment that, from the point of view of control, is operated as a single entity.  A control module can also be made up of other control modules. For example, a header control module could be defined as a combination of several on/off automatic block valve control modules.

Some examples of control modules are

- a regulating device consisting of a transmitter, a controller, and a control valve that is operated via the set point of the device;

- a state-oriented device that consists of an on/off automatic block valve with position feedback switches, that is operated via the set point of the device; or

- a header that contains several on/off automatic block valves and that coordinates the valves to direct flow to one or several destinations based upon the set point directed to the header control module.

User Requirements specifications for many Control Modules are covered by the P&ID however often the level of detail on a P&ID is insufficient and additional loop diagrams, or function block diagrams may be needed.

Also, there are some aspect of control modules and indeed new ones that arise during the design of the system.

A detailed functional requirements model captures all of these.

Many Automation Systems provide various types of Control Module as standard modules.

But beware, sometimes what are supposed to be standard turn out otherwise, and some 'standard modules' turn out to be not what you really wanted.

In general a URS should not specify the detailed functions for these, but it should specify the minimum requirements and of course list them. Or for a very high level URS list where they are to be found.