Watlow Series D8 - Manual (Page 125)

Series D8 User’s Guide Chapter 4: Operation and Setup
Doc. 0600-3120-2000 Watlow Anafaze 107
How to Set Up Differential Control
Set up differential control as you would set up ratio control.
Set the Control ratio parameter to 1.0, and enter the desired
set point differential (offset) at the Ratio SP diff parameter.
Differential Control Example: Thermoforming
A thermal forming application requires that the outer heaters
operate at temperature 50º F hotter than the center heaters.
The center heaters use infrared (IR) sensors for temperature
feedback. The outer heaters use thermocouples for feedback.
We can use differential control to control the outer heaters at
a 50º F differential to the central heaters. For example, if the
set point for the center heaters is 325º F, the set point of the
outer heaters will be 375º F.
In this application, the center heaters will be controlled by the
master loop (on loop 1), and the outer heaters will be con-
trolled by the ratio loop (on loop 2).
To set up this application, first set up the master loop (loop 1)
for PID control with a set point of 325º F. Then, for the ratio
loop (loop 2), set the parameters in the Ratio menu as shown
in Table 4.12.
Table 4.12 Parameter Settings for the Ratio
Loop (Loop 2) for the Example
To complete the differential control setup, loop 1 and loop 2
must be configured for inputs, outputs and alarms.
Setting Up Remote Analog Set Point
Remote analog set point allows external equipment, such as a
PLC or other control system, to change the set point of a loop.
Menu Parameter Value Comment
Ratio Ratio master loop 01
Loop 1 is the master loop.
Ratio Ratio low SP 300.0˚F
The lowest allowable set point for the ratio
loop. For this example, we’ll use 300.0.
Ratio Ratio high SP 400.0˚F
The highest allowable set point for the ratio
loop. For this example, we’ll use 400.0.
Ratio Control ratio 1.0
For differential control, always set this parame-
ter to 1.0.
Ratio Ratio SP diff 50˚F
The set point differential, or offset.