Civil Engineering Reference
In-Depth Information
Another contribution of a DDC system in addition to controlling the HVAC
system operation is that a DDC system can also facilitate controlling non-HVAC
systems in the building such as lighting system.
Generally the building DDC system is a combination of software and hardware
(micro-processor-based controllers and electronic sensors) that is capable of
continuously monitoring and maintaining different spaces desired temperatures,
pressures, humidity, air quality, energy management, etc. inside the building. A col-
lection of binary (on/off), analog (variable) or pulse accumulation signals from the
sensors throughout the building and its systems are delivered to the DDC system.
These delivered data to the direct digital control system are known as DDC system
inputs. These signals then will be compared against the preprogrammed values for
each space or location conditions through Proportional, Integral, Derivative (PID)
algorithms and the binary, analog or tri-state output signals then will be sent to the
actuators at each part of the HVAC system (or other building systems) to change and
adjust the current conditions and make the desired conditions at each space or
location available.
Generally the most important control components of a DDC system structure are
building controllers, custom application controllers, application specifi c controllers,
communication gateways, system integrator, routers, I(nput)/O(utput) devices,
auxiliary devices, and operator interfaces.
Typically the direct digital control network structure or architecture is the way
that network components have been arranged in respect to each other and interact
together. Communication between the network elements is done by wires (physical
media) and communication buses (data link layer). In building automatic controls,
communication buses have responsibility to connect workstations together, control-
lers to workstations and fi nally lower level controllers to each other. At the top of the
structure the building controller is located that is the main programmable general
purpose controller. This main controller then communicates with a network of lower
level controllers to manage the building systems operations. The lower level con-
trollers are known as either custom application or application specifi c controllers.
A custom application controller is a controller that generally controls specifi c com-
plex equipment such as air handling units or chillers, and an application-specifi c
controller is generally a manufacturer's preprogrammed controller that is used for
controlling less complicated equipment such as VAV (variable air volume) boxes
and other simple equipment.
A proprietary network is designed and built by a single manufacture; therefore
when the size of the DDC system is not large and there is no future plan for the
system expansion, performs the daily tasks more easily and less costly. On the other
hand, an open network can be arranged to work with different provider's systems,
and therefore more complex systems can be designed and installed. Current pre-
vailing trend of control design is using open systems due to its higher fl exibility. As
a result the most common case of integration between these two networking
systems occurs when the existing system is an old system working with a proprie-
tary system, and the new expansion is designed to work with an open network. In
this case a communication gateway shall be installed to relate two networks together.
