Control System Types for Building Automation
Building control systems largely fall into one of two categories:
Direct Digital
Controls (DDC)
These systems use electronic signals via computer to process data
for direct system control. Although most BASs use electronically powered
actuators, today a small proportion of BASs in older buildings use DDCs with
outdated pneumatically powered actuators, which use compressed air to control
valves and dampers.
Stand-Alone Controls
Buildings without BASs typically rely on the controls built into
individual pieces of large equipment, such as packaged rooftop units and
chillers, to maintain comfort conditions and manage energy consumption.
Some of these stand-alone controllers are quite sophisticated and
are capable of implementing many of the control strategies described above.
However, they fall short of being able to appropriately manage the entire
building unless they are "tied-in" to the BAS using a common
communications protocol.
Advantages of DDC
Systems Over Stand Alone Systems
In general, full DDC systems offer many more benefits over
stand-alone systems that are not tied in to the BAS.
- Better building feedback
DDC systems typically provide much more
information feedback from the building than stand-alone systems. For example, a
DDC system could reset variable air volume (VAV) static pressure by scanning all
the VAV damper positions and gradually changing the static pressure until only
one damper was completely open. At that level of static pressure the fan draws
the least amount of power required to distribute sufficient air to all the
boxes. Stand-alone controllers usually cannot reset static pressure in this
manner, because they typically have no way of sensing VAV damper position.
-
Centralized control
DDC
systems allow for building control and information flows to be centralized at a
single location (whereas stand-alone systems don't have the feedback mechanisms
necessary for this capability). As a result, operators can readily view and
control all building systems from a single computer terminal instead of having
to keep track of a variety of different control locations throughout the
building.
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