Watlow CLS200 - Manual (Page 170)

Chapter 8: Tuning and Control CLS200 Series User’s Guide
154 Watlow Anafaze Doc.# 0600-3050-2000
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 setpoint. This limit
is adjustable; Watlow Anafaze controllers use an adjust-
able spread.
For example, if the setpoint is 1,000˚ F and the spread is
20˚ F, the heat output switches on when the process vari-
able drops below 980˚ F and off when the process rises
above 1,000˚ F. A process using on/off control cycles around
the setpoint. Figure 8.1 illustrates this example.
Figure 8.1 On/Off Control
Proportional Control
Proportional control eliminates cycling by increasing or de-
creasing the output proportionally with the process vari-
able’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 pow-
er 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 off-
set or droop.
Heat Off
Heat On
Setpoint
Setpoint - Spread
Process
On
Heat Off
Output
Off
Variable
980 ° F
1,000 ° F