CLS200 Series User’s Guide
Chapter 7: Turning and Control
132
Heat Off
Heat On
Setpoint
Setpoint - Spread
Process
On
Heat Off
Output
Off
Variable
980
˚
F
1,000
˚
F
Figure 7.1
— On/Off Control
Proportional Control
Proportional control eliminates cycling by increasing or decreasing the output proportionally with the
process variable’s deviation from the setpoint.
The magnitude of proportional response is defined by the proportional band. Outside this band, the
output is either 100% or 0%. Within the proportional band the output power is proportional to the
process variable’s deviation from the setpoint.
For example, if the setpoint is 1,000°F and the proportional band is 20°F, the output is:
• 0% when the process variable is 1,000°F or above
• 50% when the process variable is 990°F
• 75% when the process variable is 985°F
• 100% when the process variable is 980°F or below
However, a process which uses only proportional control settles at a point above or below the
setpoint; it never reaches the setpoint by itself. This behavior is known as
offset
or
droop
.
Offset
Proportional
Setpoint
Process Variable
Band
Figure 7.2
— Proportional Control
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