Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CONTROL FUNDAMENTALS
18
Figure 22 shows a sample control loop for basic two-position
control: a thermostat turning a furnace burner on or off in
response to space temperature. Because the thermostat cannot
catch up with fluctuations in temperature, overshoot and
undershoot enable the temperature to vary, sometimes
considerably. Certain industrial processes and auxiliary
processes in air conditioning have small system lags and can
use two-position control satisfactorily.
Fig. 22. Basic Two-Position Control Loop.
TIMED TWO-POSITION CONTROL
GENERAL
The ideal method of controlling the temperature in a space
is to replace lost heat or displace gained heat in exactly the
amount needed. With basic two-position control, such exact
operation is impossible because the heating or cooling system
is either full on or full off and the delivery at any specific instant
is either too much or too little. Timed two-position control,
however, anticipates requirements and delivers measured
quantities of heating or cooling on a percentage on-time basis
to reduce control point fluctuations. The timing is accomplished
by a heat anticipator in electric controls and by a timer in
electronic and digital controls.
In timed two-position control, the basic interaction between
the controller and the final control element is the same as for
basic two-position control. However, the controller responds
to gradual changes in the average value of the controlled
variable rather than to cyclical fluctuations.
Overshoot and undershoot are reduced or eliminated because
the heat anticipation or time proportioning feature results in a
faster cycling rate of the mechanical equipment. The result is
closer control of the variable than is possible in basic two-
position control (Fig. 23).
Fig. 23. Comparison of Basic Two-Position and Timed
Two-Position Control.
HEAT ANTICIPATION
In electromechanical control, timed two-position control can
be achieved by adding a heat anticipator to a bimetal sensing
element. In a heating system, the heat anticipator is connected
so that it energizes whenever the bimetal element calls for heat.
On a drop in temperature, the sensing element acts to turn on
both the heating system and the heat anticipator. The heat
anticipator heats the bimetal element to its off point early and
deenergizes the heating system and the heat anticipator. As
the ambient temperature falls, the time required for the bimetal
element to heat to the off point increases, and the cooling time
decreases. Thus, the heat anticipator automatically changes
the ratio of on time to off time as a function of ambient
temperature.
Because the heat is supplied to the sensor only, the heat
anticipation feature lowers the control point as the heat
requirement increases. The lowered control point, called
“droop”, maintains a lower temperature at design conditions
and is discussed more thoroughly in the following paragraphs.
Energizing the heater during thermostat off periods
accomplishes anticipating action in cooling thermostats. In
either case, the percentage on-time varies in proportion to the
system load.
THERMOSTAT
FURNACE
SOLENOID
GAS VALVE
C2715
72
71
73
70
69
74
75
68
CONTROL
POINT
TIME
C2089
TIMED TWO-POSITION CONTROL
72
71
73
70
69
74
75
68
OFF
ON
TEMPERATURE
(
° F)
DIFFERENTIAL
DIAL SETTING
UNDERSHOOT
CONDITION
TIME
OVERSHOOT
CONDITION
BASIC TWO-POSITION CONTROL
TEMPERATURE
(
° F)
OFF
ON