Watlow EZ-ZONE PM Integrated Controller - Manual (Page 153)

Watlow EZ-ZONE ® PM Integrated Controller • 150 • Chapter 10 Applications
Example 5: Differential
Requirements:
Two analog inputs and the enhanced software option are required and at least one output adjusts the controlled
part of the process.
Overview:
Differential control maintains one process at a difference to another process. When function is set for Differential,
the PV Function output equals Source A minus Source B. Control loop 1 will control Analog Input 1 difference to
Analog Input 2 based on Closed Loop Set Point 1.
Example 6: Cascade
Requirements:
Two loops of control, two inputs and at least 1 output and the enhanced software option.
Overview:
Cascade control can handle a difficult process with minimal overshoot, while reaching the set point quickly. This
minimizes damage to system components and allows for over sizing heaters for optimal heat-up rates. Heater
life is also extended by reducing thermal cycling of the heater. Systems with long lag times between the energy
source (heater, steam, etc.) and the measured process value cannot be controlled accurately or efficiently with a
single control loop, because a lot of energy can build up before a response is detected. This can cause the system
to overshoot the set point, which could damage the heater, product or heat transfer medium, such as a heat trans-
fer fluid.
The majority of the user configuration is done via the Math function. There are two user selectable settings
that will enable Cascade control, Deviation Scale or process Scale. When Process Scale is selected the remote set
point will be within the defined Range low/high and Scale low/high settings. As an example, the graph below
shows a heat/cool application where the temperature range is between 32º to 1200º. With the scaling set as shown
100% cool will equate to 32º, likewise when the control is calling for 100% heat the temperature equates to 1200º.
When Deviation Scale is selected the Closed Loop Set Point (CLSP) will not deviate beyond the specified settings.
With the settings as shown in the graph below the CLSP (500º) will not deviate beyond ± 25º.
Heat/Cool
T
emperature
Process Scale
Scale Low
-100 %
Scale High
+100 %
Range Low
32o
Range High
+1200 o
0o
Cool
Heat
0 %
Heat/Cool
Deviation Scale
Scale Low
-100 %
Scale High
+100 %
-25
+25
0 %
CLSP = 500
Calculated inner loop (2) set point ± deviation
®
Input 1
Input 2
Output 1
Control Loop 1
Differential
Closed Loop
Set P
oint
1
Function
Filt
er
Source A
Source A
Source B
Heat Power
Cool Power
0 to 100%
0 to 100%
Output
Analog Input 1
Analog Input 2
Heat
Cool
Output n
Output n
PV Function