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CONTROL SYSTEM CHARACTERISTICS
Automatic controls are used wherever a variable condition
must be controlled. In HVAC systems, the most commonly
controlled conditions are pressure, temperature, humidity, and
rate of flow. Applications of automatic control systems range
from simple residential temperature regulation to precision
control of industrial processes.
CONTROLLED VARIABLES
Automatic control requires a system in which a controllable
variable exists. An automatic control system controls the
variable by manipulating a second variable. The second variable,
called the manipulated variable, causes the necessary changes
in the controlled variable.
In a room heated by air moving through a hot water coil, for
example, the thermostat measures the temperature (controlled
variable) of the room air (controlled medium) at a specified
location. As the room cools, the thermostat operates a valve
that regulates the flow (manipulated variable) of hot water
(control agent) through the coil. In this way, the coil furnishes
heat to warm the room air.
CONTROL LOOP
In an air conditioning system, the controlled variable is
maintained by varying the output of the mechanical equipment
by means of an automatic control loop. A control loop consists
of an input sensing element, such as a temperature sensor; a
controller that processes the input signal and produces an output
signal; and a final control element, such as a valve, that operates
according to the output signal.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIRFLOW
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SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK
Fig. 18. Electrostatic Filter.
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THEORETICAL
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C2714
The sensor can be separate from or part of the controller and
is located in the controlled medium. The sensor measures the
value of the controlled variable and sends the resulting signal
to the controller. The controller receives the sensor signal,
compares it to the desired value, or setpoint, and generates a
correction signal to direct the operation of the controlled device.
The controlled device varies the control agent to regulate the
output of the control equipment that produces the desired
condition.
HVAC applications use two types of control loops: open and
closed. An open-loop system assumes a fixed relationship
between a controlled condition and an external condition. An
example of open-loop control would be the control of perimeter
radiation heating based on an input from an outdoor air
temperature sensor. A circulating pump and boiler are energized
when an outdoor air temperature drops to a specified setting,
and the water temperature or flow is proportionally controlled
as a function of the outdoor temperature. An open-loop system
does not take into account changing space conditions from
internal heat gains, infiltration/exfiltration, solar gain, or other
changing variables in the building. Open-loop control alone
does not provide close control and may result in underheating
or overheating. For this reason, open-loop systems are not
common in residential or commercial applications.
A closed-loop system relies on measurement of the controlled
variable to vary the controller output. Figure 19 shows a block
diagram of a closed-loop system. An example of closed-loop
control would be the temperature of discharge air in a duct
determining the flow of hot water to the heating coils to maintain
the discharge temperature at a controller setpoint.
15
CONTROL FUNDAMENTALS
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