Honeywell AUTOMATIC CONTROL, Engineering Manual

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MICROPROCESSOR-BASED/DDC FUNDAMENTALS
136
ENGINEERING MANUAL OF AUTOMATIC CONTROL
All input signals, whether analog or digital, undergo
conditioning (Fig. 3) to eliminate the adverse affects of contact
bounce, induced voltage, or electrical transients. Time delay
circuits, electronic filters, and optical coupling are commonly
used for this purpose. Analog inputs must also be linearized,
scaled, and converted to digital values prior to entering the
microprocessor unit. Resistance sensor inputs can also be
compensated for leadwire resistance. For additional
information about electronic sensors see the Electronic Control
Fundamentals section.
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 con-
verter provides a resolution of one count in 256. A 12-bit A/D
converter 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
range of outputs adequately covers normal control and dis-
play ranges for most HVAC control applications. D/A con-
verters 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.
TYPES OF CONTROLLERS
Microprocessor-based controllers operate at two levels in
commercial buildings: the zone level and the system level. See
Figure 4.
Fig. 4. Zone- and System-Level Controllers.
ZONE-LEVEL CONTROLLER
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.
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.
SYSTEM-LEVEL
CONTROLLERS
AND ZONE
CONTROLLER
MANAGERS
ZONE-LEVEL
CONTROLLERS
C2418
ZONE COMFORT CONTROL
ZONE ENERGY MANAGEMENT
LABORATORY AIRFLOW
SPACE PRESSURIZATION
EXHAUST FAN/RELIEF DAMPER
CONTROL
IAQ CONTROL
AIR HANDLER TEMPERATURE
CONTROL
AIR HANDLER PRESSURE
CONTROL
CENTRAL PLANT
CHILLER/BOILER CONTROL
ENERGY MANAGEMENT
FUNCTIONS
BUILDING MANAGEMENT
FUNCTIONS