Watlow PPC-2000 - Manual (Page 249)

PPC-2000 User’s Guide Chapter 6: Tuning and Control
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 221
Proportional Control
Proportional control eliminates cycling by increasing or
decreasing the output in proportion to the process variable’s
deviation from the setpoint.
The magnitude of proportional response is defined by the
Proportional Band (PB); outside this band, the output is either
100% or 0%. Within the proportional band the output power is
proportional to the PV’s difference from the setpoint.
For example, if the setpoint is 1000°F and the PB is 20°F, the
output is:
• 0% when the process variable is 1000°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 offset or droop.
Figure 6.2 Proportional Control
offset
PB
SP
PV