Instruction Manual

PPC-2000 User’s Guide Chapter 6: Tuning and Control

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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