Specifications

102 Control and Automation Solutions Guide
Hierarchy
As mentioned at the beginning of this
chapter, BASs are truly DCSs made up
of a networked collection of controllers,
some specialized to a particular task
near the bottom of the hierarchy while
others, usually at the upper end of the
hierarchy, are less specialized and more
generically capable of handling trac
over standard industrial interfaces. The
similarity to factory control systems
is more than coincidental. Due to the
wide use of standard equipment and
interfaces in industry, the less specialized
equipment is very readily available and
experts in their programming and use
are also widely available. Taking the
comparison a bit further, the primary
bus shown in the diagram below is
analogous and potentially identical to
the Industrial Ethernet backbone in a
factory automation system. For BASs,
standard Ethernet may suce since
the time determinism of Industrial
Ethernet may not be needed. The
secondary buses shown are analogous
to the eldbus. The main dierences
are that the these secondary buses
are most often implemented using
BACnet, a data communication protocol
for building automation and control
networks, and LonTalk
®
from Echelon
Corp., as the communications standard,
which are tailored to BASs as opposed
to other protocols found in factory
automation systems such as PROFIBUS,
Modbus
®
, ControlNET, etc. Note that
all these communication standards
can use RS-485 as the physical layer.
BACnet and LonTalk do not specify
what the physical layer must be.
Controllers
Because some controllers are very
generic, see the Programmable Logic
Controllers (PLCs) chapter for details
on these general-purpose controllers.
For this chapter’s discussion, much
of the equipment found in homes
and buildings have their own built-
in controllers with user interfaces for
manual control and communications
interfaces for connections to that
equipment’s controller as part of the
BAS. These individual controllers built
into the equipment do not require
much in the way of big-picture control.
They can simply focus on the basic
running of that machine, leaving
the BAS (through its controllers for
that particular system to modify the
machine’s behavior) to implement more
sophisticated control algorithms.
For example, the BAS is instructed by the
building manager (a person) looking at
his schedule for the day to start warming
a conference room in preparation for a
meeting two hours before it starts. He
knows he can reduce his heating energy
bill by implementing a slow heating
cycle, and this is easy to do because the
BAS has energy-savings algorithms built
in so that it can be instructed to slowly
increment the temperature set point for
the room up to that temperature over a
two-hour period, instead of immediately
setting it to the nal temperature. The
room is temperature-controlled from a
VAV unit that supplies heated air to the
room through an automated terminal
unit (TU) that has a motor-controlled
damper. (This VAV may be relying on a
steam feed from the boiler for its heat
source, so the boiler must also be up
and running in advance, but this is likely
Control Systems
ELEVATOR,
ESCALATOR
CONTROLLER
FIRE
ALARM
CONTROLLER
SECURITY
ACCESS
SYSTEM
CONTROLLER
LIGHTING
CONTROLLER
CENTRAL
PLANT
CONTROLLER
CENTRAL
PLANT
CONTROLLER
BOILER
CONTROLLER
CHILLER
CONTROLLER
BACnet
DEVICE
SECONDARY BUSSECONDARY BUS
PRIMARY BUS
PROGRAMMABLE
LOGIC
CONTROLLER
PROGRAMMABLE
LOGIC
CONTROLLER
PROGRAMMABLE
LOGIC
CONTROLLER
PROGRAMMABLE
LOGIC
CONTROLLER
WATER
SUPPLY
SYSTEM
CONTROL
VAV
BOX
VAV
BOX
VAV
BOX
VAV
BOX
LonTALK
DEVICE
WEB
SERVER
HUMAN INTERFACE DEVICE
COMPUTER WORKSTATION
ENERGY
MANAGEMENT
SYSTEM
CONTROL
BACKUP
POWER
SYSTEM
CONTROL
Diagram outlining the control structure for a modern BAS.