There are three basic types of Industrial Automation Controllers.
digital (e.g. microprocessor or computer based),
The following guidelines may be applied in selecting the appropriate controller:
For plants with many control loops use digital type of controllers.
For the expansion of existing controls within a plant use the existing technology. This may be either digital, analog electronic, or pneumatic controllers.
Use pneumatic controllers in hazardous areas.
Avoid use of pneumatic controllers in the control room.
When using digital control avoid depending on a single or a few control devices for the entire plant without having a backup. A redundant controller might not be required if only a single controller controls one loop.
However, if a single controller controls a large number of loops then provide redundancy so that if the controller fails another controller will automatically take over.
Use the type of controller that is the most economical and reliable.