Watlow CPC400 - Manual (Page 90)

Chapter 3: Operation and Setup CPC400 Series User’s Guide
76 Watlow Anafaze Doc. 0600-2900-2000
Differential Control Example: Thermoforming
A thermal forming application requires that the outer heat-
ers operate at temperature 50ºF hotter than the center
heaters. The center heaters use infrared (IR) sensors for
temperature feedback. The outer heaters use thermocou-
ples for feedback.
We can use differential control to control the outer heaters
at a 50ºF differential to the central heaters. For example, if
the set point for the center heaters is 325ºF, the set point of
the outer heaters will be 375ºF.
In this application, the center heaters will be controlled by
the master loop (on loop 1), and the outer heaters will be
controlled 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 325ºF. Then, for the ra-
tio loop (loop 2), set the parameters in the Ratio menu as
shown in Table 3.13.
Table 3.13 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.
Typically, a voltage or current source is connected to an an-
alog input on the controller, and this input is configured as
the master loop for ratio control.
Proper scaling resistors must be installed on the input to
allow it to accept the analog input signal.
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.