Watlow ez-zone pm - Page 224

Watlow EZ-ZONE
®
PMI Controller • 220 • Chapter 10 Applications
The majority of the user conguration is done via the Math function. There are two user select-
able settings that will enable Cascade control, Deviation Scale or process Scale. When Process
Scale is selected the remote set point will be within the dened Range low/high and Scale low/
high settings. As an example, the graph below shows a heat/cool application where the tempera-
ture 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 (SP) will not deviate beyond the speci-
ed settings. With the settings as shown in the graph below the SP (500º) will not deviate beyond
± 25º.
The graph below illustrates a system with a long lag time and the advantages in using cascade
control. Curve A represents a single-control system with PID parameters that allow a maximum
heat-up rate. Too much energy is introduced and the set point is overshot. In most long-lag-time
systems the process value may never settle out to an acceptable error. Curve C represents a
single-control system tuned to minimize overshoot. This results in unacceptable heat-up rates,
with the nal value taking hours to reach. Curve B shows a cascade system that limits the energy
introduced into the system allowing an optimal heat-up rate with minimal overshoot.
Note:
When using cascade control, two loops of
control are required. Changing the control
mode in either loop will affect both loops
of control. In other words, if loop one is
changed to manual mode, loop two will also
be changed to manual mode automatically.
When the Math function is set for Process or
Deviation Scale and Source E is not connected
or false, cascade control is enabled.
Note:
When the Math function is set for Process or
Deviation Scale the PM automatically makes
the connections for each Control Loop as shown in the graphic below. Each loop, 1 (process)
and 2 (energy) outer and inner respectively, cannot be changed. If it is desired to display the
inner loop process variable and set point, the home page must be changed via the Factory
Page, Custom Menu.
Heat/Cool
Deviation Scale
Scale Low
-100 %
Scale High
+100 %
-25
+25
0 %
CLSP = 500
Calculated inner loop (2) set point ± deviation
Heat/Cool
Temperature
Process Scale
Scale Low
-100 %
Scale High
+100 %
Range Low
32
°
Range High
+1200 °
0
°
Cool
Heat
0 %
Time
Temperature
Cascade
Curve A (PID)
Set
Point
Curve B (Cascade)
Curve C (Single-control)