Until about 1995 mainly the analog controller was used for controlling an HVAC process. Since then, the digital and data-based controller has taken over (Fig. 9.54). In principle there are no major control technical differences between an analog and similar digital controller. The digital data-based con­troller has the following advantages:

• Small size, requiring a relatively small installation area

• Accurate reading of the measuring signals

• Keyboard for accurate adjustment of different parameters

• Signals for internal defects and external status and alarms

• A possibility for communication with the computer system



FIGURE 9.54 The controlling loop with sensors, actuators, and controller.

Depending on the control system and components selected, the process variable from the sensors and signals to the actuator control unit is connected either directly or through transducers. Normally, the input and output are ad­justed from pneumatic sensors and control units. The signals produced then are transformed into standard electronic values before connecting to the con­troller’s input and output modules.

Data-based controllers are used for controlling energy, lights, cooling, heat, and ventilation in buildings and industrial plants. These are known as DDC controllers. The term DDC was used at the beginning of the computer age and includes the original data-based controllers. The term DDC derives from Amer­ican English and is an abbreviation of direct digital control. This term was used to separate the original simple controller PLC (program logic control) from more developed and specialized DDC controllers used for HVAC processes.

The modern DDC controller has only the control function PID. PLC con­trollers used in process installations may contain more complex regulation functions, for example, the fuzzy or auto-tuning of PID functions. Most DDC controllers are self-sufficient and independent of the controllers or computer programs that are used for system configuration.

They are suited for the technical (HVAC) processes in buildings and in­clude installed and tested computer programs, which cover the most common forms of controlling and regulating in a typical HVAC plant, such as

• Frost protection of heating coil (where water is used as an energy carrier for heating or cooling)

• Functions for selecting an average of the lowest or highest of different measuring values

• Calculations and mathematical functions

• PLC functions for controlling lights, electrical motors, heating coil, boilers, refrigerating machines, etc.

• Controlling from internal time programs (daily, weekly, or monthly clock with extra program for vacations)

• Optimally starting and stopping for the override of heating, cooling, and ventilation

• Protector for storage of hour rating and measuring values (trend logs)

• Theft-proof admission cards and alarms for water leaks, etc.

• Integrated network for connecting to BAS systems (central operation control)

Analog inputs and outputs in data-based (DDC) controllers are usually standardized and record the signals from sensors or transducers as 0(2)—10 VDC or 0(4)-20 mA. The inputs module can also be standardized for resis­tances, as Pt 100 DIN, Pt 1000 or Ni 1000 DIN (Pt = platinum, Ni = nickel).

Data-based (DDC) or programmable (PLC) controllers with universal in­puts and outputs can be used. It is essential that they are configured before use. In some cases the input may be used only for temperature measurement from special types of thermistors. (Thermistors are constructed from semicon­ductor materials where the resistance changes reversibly proportional to the temperature, i. e., a negative temperature coefficient.)