The Choice of Controllers

Controllers for decentralized control and regulation are usually positioned lo­cally to the process and connected to the control room.

They attend to and control different technical plants and systems, and they have to deal with all processes required. In data-based plants, controllers usually communicate with one or several local or central computers.

If there are special requirements for communication at all technical data lev­els, the local controller must still be chosen from the same factory as the other technical operating programs in the central computer or PC. If not, there will be a considerable reduction in the amount of data sent to and from the controller.

In addition to controlling and regulating HVAC plants, DDC controllers have considerable internal memories for the storage of important system data and trend tables. Other important requirements are the interface for using and attending. If the controller is equipped with a graphic dynamic display, service selectors (key­board) or light diodes for input and output modules, separated operation selectors, and operating or error lights in the electrical distribution may be ignored. The costs for both the new system and future maintenance can be considerably reduced.

The market consists of a considerable assortment of PLC and DDC con­trollers. Most of the factory types are capable of controlling separate technical plants in a satisfactory manner.

The PLC controller mentioned earlier is used basically for regulating industrial processes or to replace relay controllers (latching) in electrical

Processes. Plants with local or central computers are used for operating and controlling energy requirements of HVAC plants. It is impossible to state that the PLC is capable of giving the required results, especially when they are connected in a network and function as self-operating controllers.

Many PLCs can have few and simple data programs, together with a limited internal memory capacity. Due to this fact, it is necessary in many cases to provide extra software and greater memory capacity in the centra! data station than for more specialized and autonomous DDC controllers. Another reason for selecting the DDC controller is that the skills of the firms who are producing and using DDC controllers are probably better adjusted to the special processes in the HVAC plants which are generally installed in industry and buildings.