Differential Control; Differential Control Example: Thermoforming - Watlow CLS200 User Manual

CLS200 Series User's Guide
Chapter 5: Enhanced Features

Differential Control

Differential control is a simple application of the ratio control option, used to control one process
(ratio loop) at a differential, or offset, to another (master loop). To use differential control, set the ratio
value to 1.0 and enter the desired offset with the SP DIFF. parameter.

Differential Control Example: Thermoforming

A thermal forming application requires that the outside heaters operate at a higher temperature than
the center heaters. The differential control point is determined by the master loop which is using
infrared (IR) sensors for temperature feedback. Secondary loops use thermocouples for feedback.
The loop using the IR sensor as an input is assigned to the master loop in the SETUP LOOP RATIO
CONTROL menu. The secondary loop is the differential control loop. Setting the setpoint differential
(SP DIFF) to the desired offset will produce the desired offset between the secondary and master
loops.
For example, the master loop can be controlled at 325º F and the secondary loop at 375º F by using
a differential of 50º F.
Loop 1 must be set up for PID control of the setpoint at 325º F.
DISPLAY
02 RATIO CONTROL
MAX SP? 300.0
ALARM
02 RATIO CONTROL
CTRL RATIO? 1.0
ALARM
02 RATIO CONTROL
SP DIFF.? 50
ALARM
4. Press BACK several times until the normal loop display appears. The setpoint of loop 2 will
now be equal to the process variable of loop 1.
5. To complete the remote analog setpoint setup, loop 1 may be configured for outputs
and alarms. Likewise, loop 2 must be configured for inputs, outputs, and alarms. See
Chapter 4: Setup for information on loop setup.
USER INPUT
Enter the maximum ratio loop setpoint. For this example, we
will use 300.0˚ F as a maximum.
Press ENTER.
Enter the control ratio, which is the multiple applied to the
master process variable. In this example the ratio is 1.0.
Press ENTER.
Enter the setpoint differential (or offset). For this example we
have no offset requirement and will use 0. Press ENTER.
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