Watlow ANASOFT - Manual (Page 68)

58 ANASOFT User’s Guide
Appendix A: PID
On/Off Control
On/Off control is the simplest way to control a process; a controller
using On/Off control turns an output on or off when the process variable
reaches a certain limit above or below the desired setpoint. You can
adjust this limit, since Watlow-Anafaze controllers use an adjustable
spread. For example, if your setpoint is 1000
º F, and your limit (spread)
is 20 º F, the output switches On when the process variable goes below
980 º F and Off when the process goes above 1000 º F. (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.