Watlow MLS - Manual (Page 112)

100 MLS User’s Guide
Tuning and Control
The next diagram shows a process under On/Off control.
Proportional Control
A process using On/Off control frequently cycles around the setpoint.
When process variable cycling is unacceptable or the process or setpoint
are variable, use proportional control. Proportional control, or Gain,
eliminates cycling by increasing or decreasing the output proportional to
the process variable's distance from the setpoint.
The limits of proportional control are defined by the Proportional Band
(PB); outside this band of control, the output is either 100% or 0%. For
example--using the same values from the example above and a PB of
20
º ---the output is:
• 50% when the process variable is 990 º F
• 75% when the process variable is 985 º F
• 100% when the process variable is 980 º F or below.
However, a process which uses only Proportional control may settle at a
point above or below the setpoint; it may never reach the setpoint at all.
This behavior is known as “offset” or “droop”.
This diagram shows a process under proportional control only.