D8 Series User’s Guide
Chapter 4: Operation and Setup
85
Differential Control Example: Thermoforming
A thermal forming application requires that the outer heaters run 50ºF 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 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 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
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
parameter to 1.0.
Ratio
Ratio SP diff
50.0° F
The set point differential, or offset.
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 analog 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.
How to Set Up a Remote Analog Set Point
1. For the master loop (the loop that accepts the input signal from the external device), set the
parameters in the
Input
menu.
2. For the ratio loop (the one whose set point is controlled by the external device), set the
parameters in the
Ratio
menu. Specify the loop that accepts the input signal as the
master loop.
