Watlow CPC400 - Manual (Page 96)

Chapter 4: Tuning and Control CPC400 Series User’s Guide
82 Watlow Anafaze Doc. 0600-2900-2000
On/Off Control
On/off control is the simplest way to control a process. The
controller turns an output on or off when the process vari-
able reaches limits around the desired set point. This limit
is adjustable.
For example, if the set point is 1000°F and the control hys-
teresis is 20°F, the heat output switches on when the pro-
cess variable drops below 980°F and off when the process
rises above 1000°F. A process using on/off control cycles
around the set point. Figure 4.1 illustrates this example.
Figure 4.1 On/Off Control
Proportional Control (P)
Proportional control eliminates cycling by increasing or de-
creasing the output proportionally with the process vari-
able’s deviation from the set point.
The magnitude of proportional response is defined by the
proportional band. Outside this band, the output is either
100 percent or 0 percent. Within the proportional band the
output power is proportional to the process variable’s devi-
ation from the set point.
For example, if the set point is 1000°F and the proportional
band is 20°F, the output power is as follows:
• 0 percent when the process variable is 1000°F or above
• 50 percent when the process variable is 990°F
• 75 percent when the process variable is 985°F
• 100 percent when the process variable is 980°F or be-
low
However, a process that uses only proportional control set-
tles at a point above or below the set point; it never reaches
the set point. This behavior is known as offset or droop.
When using proportional control, configure the manual re-
set parameter for the power level required to maintain set
point.
Heat Off
Heat On
Set Point
Set Point - Hysteresis
Process
On
Heat Off
Output
Off
Variable
980 ° F
1000 ° F