PLC & Control

The DCS is a control system which collects the data from the field and decides what to do with them. Data from the field can either be stored for future reference, used for simple process control, use in conjunction with data from another part of the plant for advanced control strategies. What must be in the DCS for it to be able to do so much?

Operator Console

These are like the monitors of our computers. They provide us with the feedback of what they are doing in the plant as well as the command we issue to the control system. These are also the places where operators issue commands to the field instruments.

Engineering Station

These are stations for engineers to configure the system and also to implement control algorithms.

History Module

This is like the harddisk of our PCs. They store the configurations of the DCS as well as the configurations of all the points in the plant. They also store the graphic files that are shown in the console and in most systems these days they are able to store some plant operating data.

Data Historian

These are usually extra pieces of software that are dedicated to store process variables, set points and output values. They are usually of higher scanning rates than that available in the history module.

Control Modules

These are like the brains of the DCS. Specially customized blocks are found here. These are customized to do control functions like PID control, ratio control, simple arithmetic and dynamic compensation. These days, advanced control features can also be found in them.

I/O

These manage the input and output of the DCS. Input and output can be digital or analogues. Digital I/Os are those like on/off, start/stop signals. Most of the process measurements and controller outputs are considered analogue. These are the points where the field instruments are hard-wired to. All above mentioned elements are connected by using a network, nowadays very often used is Ethernet. The practical and technological boundaries between a Distributed Control System DCS, Programmable Logic Controller PLC and Personal Computer PC control are blurring. Systems traditionally associated with process control are being used in discrete applications. Likewise, traditionally discrete solutions are used increasingly in both batch and continuous process control. Today's control hardware are constructed from many of the same standard industry components such as Intel processors. Therefore the only real difference between control systems is at the software level.

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