Table of Contents
Advertisement
ANALOG CONTROL SIGNAL
OPEN
FINAL
CONTROL
ELEMENT
POSITION
CLOSED
DIGITAL CONTROL SIGNAL
OPEN
FINAL
CONTROL
ELEMENT
POSITION
CLOSED
Fig. 20. Comparison of Analog
and Digital Control Signals.
CONTROL MODES
Control systems use different control modes to accomplish
their purposes. Control modes in commercial applications
include two-position, step, and floating control; proportional,
proportional-integral, and proportional-integral-derivative
control; and adaptive control.
TWO-POSITION CONTROL
General
In two-position control, the final control element occupies
one of two possible positions except for the brief period when
it is passing from one position to the other. Two-position control
is used in simple HVAC systems to start and stop electric motors
on unit heaters, fan coil units, and refrigeration machines, to
open water sprays for humidification, and to energize and
deenergize electric strip heaters.
In two-position control, two values of the controlled variable
(usually equated with on and off) determine the position of the
final control element. Between these values is a zone called the
"differential gap" or "differential" in which the controller cannot
initiate an action of the final control element. As the controlled
variable reaches one of the two values, the final control element
assumes the position that corresponds to the demands of the
controller, and remains there until the controlled variable
changes to the other value. The final control element moves to
the other position and remains there until the controlled variable
returns to the other limit.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
TIME
TIME
C2080
An example of differential gap would be in a cooling system
in which the controller is set to open a cooling valve when the
space temperature reaches 26 C, and to close the valve when
the temperature drops to 25 C. The difference between the two
temperatures (1 C or 1 Kelvin) is the differential gap. The
controlled variable fluctuates between the two temperatures.
Basic two-position control works well for many applications.
For close temperature control, however, the cycling must be
accelerated or timed.
Basic Two-Position Control
In basic two-position control, the controller and the final
control element interact without modification from a mechanical
or thermal source. The result is cyclical operation of the
controlled equipment and a condition in which the controlled
variable cycles back and forth between two values (the on and
off points) and is influenced by the lag in the system. The
controller cannot change the position of the final control element
until the controlled variable reaches one or the other of the two
limits of the differential. For that reason, the differential is the
minimum possible swing of the controlled variable. Figure 21
shows a typical heating system cycling pattern.
TEMPERATURE
( C)
23.5
23
22.5
OFF
22
ON
21.5
21
20.5
20
Fig. 21. Typical Operation of Basic Two-Position Control.
The overshoot and undershoot conditions shown in Figure
21 are caused by the lag in the system. When the heating system
is energized, it builds up heat which moves into the space to
warm the air, the contents of the space, and the thermostat. By
the time the thermostat temperature reaches the off point (e.g.,
22 C), the room air is already warmer than that temperature.
When the thermostat shuts off the heat, the heating system
dissipates its stored heat to heat the space even more, causing
overshoot. Undershoot is the same process in reverse.
In basic two-position control, the presence of lag causes the
controller to correct a condition that has already passed rather
than one that is taking place or is about to take place. Consequently,
basic two-position control is best used in systems with minimal
total system lag (including transfer, measuring, and final control
element lags) and where close control is not required.
17
CONTROL FUNDAMENTALS
OVERSHOOT
CONDTION
DIAL SETTING
DIFFERENTIAL
UNDERSHOOT
CONDTION
C3972
TIME
Advertisement
Chapters
Table of Contents
