Control Modes; Two-Position Control; General; Basic Two-Position Control - Honeywell AUTOMATIC CONTROL Engineering Manual

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.
TIME
TIME
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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 78F, and to close the valve when
the temperature drops to 76F. The difference between the two
temperatures (2 degrees F) 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
(°F)
75
74
73
OFF
72
ON
71
70
69
68
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.,
72F), 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 ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
OVERSHOOT
CONDTION
DIAL SETTING
DIFFERENTIAL
UNDERSHOOT
CONDTION
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TIME