Watlow CLS200 Bedienungsanleitung PDF herunterladen (Seite 172)

Chapter 8: Tuning and Control CLS200 Series User’s Guide

156 Watlow Anafaze Doc.# 0600-3050-2000

rapidly approaching the setpoint from below, derivative

control reduces the output, anticipating that the process

variable will reach setpoint. Use it to reduce overshoot and

oscillation of the process variable common to PI control.

Figure 8.4 shows a process under full PID control.

Figure 8.4 Proportional, Integral and Deriva-

tive Control

Heat and Cool Outputs

Each loop may have one or two outputs. Often a heater is

controlled according to the feedback from a thermocouple,

in which case only one output is needed.

In other applications, two outputs may be used for control

according to one input. For example, a system with a heater

and a proportional valve that controls cooling water flow

can be controlled according to feedback from one thermo-

couple.

In such systems, the control algorithm avoids switching too

frequently between heat and cool outputs. The on/off algo-

rithm uses the SPREAD parameter to prevent such oscilla-

tions (see Spread on page 92). When PID control is used for

one or both loop outputs, both the SPREAD parameter and

PID parameters determine when control switches between

heating and cooling.

Proportional

Setpoint

Process Variable

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Watlow CLS200 Bedienungsanleitung Seite 172