Table of Contents
Advertisement
MICROPROCESSOR-BASED/DDC FUNDAMENTALS
Performance and reliability of temperature control
applications can be enhanced by using a single 12-bit A/D
converter for all controller multiplexed inputs, and simple
two-wire high resistance RTDs as inputs.
A/D converters for DDC applications normally range from 8
to 12 bits depending on the application. An 8-bit A/D converter
provides a resolution of one count in 256. A 12-bit A/D con-
verter provides a resolution of one count in 4096. If the A/D
converter is set up to provide a binary coded decimal (BCD)
output, a 12-bit converter can provide values from 0 to 999, 0
to 99.9, or 0 to 9.99 depending on the decimal placement. This
TYPES OF CONTROLLERS
Microprocessor-based controllers operate at two levels in
commercial buildings: the zone level and the system level. See
Figure 4.
SYSTEM-LEVEL
CONTROLLERS
AND ZONE
CONTROLLER
MANAGERS
ZONE-LEVEL
CONTROLLERS
Fig. 4. Zone- and System-Level Controllers.
ZONE-LEVEL CONTROLLERS
Zone-level controllers typically control HVAC terminal
units that supply heating and cooling energy to occupied
spaces and other areas in the building. They can control
VAV terminal units, fan coil units, unit ventilators, heat
pumps, space pressurization equipment, laboratory fume
hoods, and any other zone control or terminal unit device.
Design of a zone-level controller is usually dictated by
the specific requirements of the application. For example,
the controller for a VAV box is frequently packaged with
an integral damper actuator and has only the I/O capacity
necessary to meet this specific application. On the other
hand, a zone-level controller for a packaged heating/
cooling unit might have the controller packaged in the
thermostat housing (referred to as a smart thermostat or
smart controller). Zone level control functions may also
be accomplished with bus-connected intelligent sensors
and actuators.
IAQ CONTROL
AIR HANDLER TEMPERATURE
CONTROL
AIR HANDLER PRESSURE
CONTROL
CENTRAL PLANT
CHILLER/BOILER CONTROL
ENERGY MANAGEMENT
FUNCTIONS
BUILDING MANAGEMENT
FUNCTIONS
ZONE COMFORT CONTROL
ZONE ENERGY MANAGEMENT
LABORATORY AIRFLOW
SPACE PRESSURIZATION
EXHAUST FAN/RELIEF DAMPER
CONTROL
C2418
136
range of outputs adequately covers normal control and display
ranges for most HVAC control applications. D/A converters
generally range from 6 to 10 bits.
The output multiplexer (Fig. 3) provides the reverse operation
from the input multiplexer. It takes a serial string of output
values from the D/A converter and routes them to the terminals
connected to a transducer or a valve or damper actuator.
The communication port (Fig. 3) allows interconnection of
controllers to each other, to a master controller, to a central
computer, or to local or portable terminals.
SYSTEM-LEVEL CONTROLLER
System-level controllers are more flexible than zone-level
controllers in application and have more capacity. Typically,
system-level controllers are applied to systems in equipment
rooms including VAV central supply systems, built-up air
handlers, and central chiller and boiler plants. Control sequences
vary and usually contain customized programs written to handle
the specific application requirements.
The number of inputs and outputs required for a system-level
controller is usually not predictable. The application of the
controller must allow both the number and mix of inputs and
outputs to be variable. Several different packaging approaches
have been used:
— Fixed I/O configuration.
— Universal I/O configuration.
— Card cage with plug-in function boards.
— Master/slave I/O modules.
Universal I/O allows software to define the function of each
set of terminals.
Zone- and system-level controllers should be equipped with
a communications port. This allows dynamic data, setpoints,
and parameters to be passed between a local operator terminal,
a central building management system, and/or other controllers.
Data passed to other controllers allows sensor values to be
shared and interaction between zone-level programs and
system-level programs to be coordinated. For example, night
setback and morning warmup can be implemented at the zone-
level controller based on operational mode information received
from the system-level controller.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Advertisement
Chapters
Table of Contents
