Watlow PPC-2000 - Manual (Page 248)

Chapter 6: Tuning and Control PPC-2000 User’s Guide
220 Watlow Anafaze Doc.# 30002-00 Rev 2.3
Control Algorithms
The next sections explain the different methods for controlling
a loop.
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 limits around the desired setpoint.
This limit is adjustable; Watlow Anafaze controllers use an
adjustable spread.
For example, if the set point is 1000°F, and the spread is 20°F,
the heat output switches On when the process 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
6.1 illustrates this example.
Figure 6.1 On/Off Control
Heat off
Heat on
SP = 1000°F
SP-Spread = 980°F
PV
On
Heat off
Output
Off