Watlow Series D8 - Manual (Page 105)

Series D8 User’s Guide Chapter 4: Operation and Setup
Doc. 0600-3120-2000 Watlow Anafaze 87
Setting Up Closed-Loop Control
Closed-loop control is used to control an output based on
feedback from a sensor or other signal.
Feedback
The controller receives electrical signals, or feedback, from a
sensor or other device. The input parameters determine how
the controller interprets the signal. The controller interprets or
scales the input signal in engineering units such as °C or °F.
Control Algorithm
When the controller is in automatic control mode and a set
point is supplied, the controller determines the appropriate
output signal.
The controller calculates the output signal based on the feed-
back and the control algorithm. Each loop may use either
on/off control or any combination of proportional, integral
and derivative (PID) control. See the Tuning and Control
chapter for information about these control modes.
Control Output Signal Forms
The output level calculated by the controller is represented by
a percentage (0 to 100 percent) of power to be applied. That
value is applied on a digital or analog output according to the
user-selected output type. See Heat/Cool Output Type on
page 139 for more information about the output types avail-
able.
Heat and Cool Outputs
In some applications, two outputs may be controlled accord-
ing to one input. For example, a loop with both heat and cool-
ing water flow might be controlled according to feedback
from one thermocouple.
In such systems, the control algorithm includes provisions to
avoid switching too frequently between the heat and cool out-
puts. The on/off algorithm uses a hysteresis parameter. The
PID algorithms use both a hysteresis parameter and the PID
parameters to determine when control switches between heat-
ing and cooling.