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CONTROL FUNDAMENTALS
An example of offset would be the proportional control of a
chilled water coil used to cool a space. When the cooling load
is 50 percent, the controller is in the middle of its throttling
range, the properly sized coil valve is half-open, and there is
no offset. As the outdoor temperature increases, the room
temperature rises and more cooling is required to maintain the
space temperature. The coil valve must open wider to deliver
the required cooling and remain in that position as long as the
increased requirement exists. Because the position of the final
control element is proportional to the amount of deviation, the
temperature must deviate from the setpoint and sustain that
deviation to open the coil valve as far as required.
Figure 33 shows that when proportional control is used in a
heating application, as the load condition increases from 50
percent, offset increases toward cooler. As the load condition
decreases, offset increases toward warmer. The opposite occurs
in a cooling application.
WARMER
OFFSET
SETPOINT
0%
LOAD
COOLER
Fig. 33. Relationship of Offset to
Load (Heating Application).
The throttling range is the amount of change in the controlled
variable required for the controller to move the controlled device
through its full operating range. The amount of change is
expressed in degrees kelvins for temperature, in percentages
for relative humidity, and in pascals or kilopascals for pressure.
For some controllers, throttling range is referred to as
"proportional band". Proportional band is throttling range
expressed as a percentage of the controller sensor span:
Throttling Range
Proportional Band =
Sensor Span
"Gain" is a term often used in industrial control systems for
the change in the controlled variable. Gain is the reciprocal of
proportional band:
Gain =
Proportional Band
The output of the controller is proportional to the deviation
of the control point from setpoint. A proportional controller
can be mathematically described by:
V = KE + M
CONTROL POINT
OFFSET
100%
50%
LOAD
LOAD
C2096
x 100
100
Where:
V = output signal
K = proportionality constant (gain)
E = deviation (control point - setpoint)
M = value of the output when the deviation is
zero (Usually the output value at 50 percent
or the middle of the output range. The
generated control signal correction is added
to or subtracted from this value. Also called
"bias" or "manual reset".)
Although the control point in a proportional control system
is rarely at setpoint, the offset may be acceptable. Compensation,
which is the resetting of the setpoint to compensate for varying
load conditions, may also reduce the effect of proportional offset
for more accurate control. An example of compensation is
resetting boiler water temperature based on outdoor air
temperature. Compensation is also called "reset control" or
"cascade control".
Compensation Control
GENERAL
Compensation is a control technique available in proportional
control in which a secondary, or compensation, sensor resets
the setpoint of the primary sensor. An example of compensation
would be the outdoor temperature resetting the discharge
temperature of a fan system so that the discharge temperature
increases as the outdoor temperature decreases. The sample
reset schedule in Table 2 is shown graphically in Figure 34.
Figure 35 shows a control diagram for the sample reset system.
Table 2. Sample Reset Schedule.
Temperature
Condition
Outdoor design
temperature
Light load
40
(FULL RESET)
20
-20
(FULL
RESET)
OUTDOOR AIR TEMPERATURE ( C)
Fig. 34. Typical Reset Schedule for Discharge Air Control.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
22
Outdoor Air
Discharge Air
Temperature
( C)
( C)
–20
40
20
20
20
(RESET
START)
C3984
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