Watlow CLS200 - Manual (Page 140)

Chapter 6: Enhanced Features CLS200 Series User’s Guide
124 Watlow Anafaze Doc.# 0600-3050-2000
even though the temperature near the element may have
been at setpoint (150˚ F).
With proportional control, when loop 1 is at setpoint, its
output is 0%, and the setpoint of loop 2 is equal to the base
setpoint (150˚ F). If the temperature of loop 1 drops to 149˚
F, the deviation results in a proportional output of 10%.
This times the span of 40˚ F results in an increase in set-
point for loop 2 of 4˚ F. The loop 2 setpoint increases to 154˚
F. For every degree that loop 1 drops, loop 2 increases by 4˚
F until the output of loop 1 is 100% and the loop 2 setpoint
is 190˚ F. Any further drop in the loop 1 process variable
does not affect loop 2.
The PID parameters of loop 2 must be tuned to perform ef-
ficient control.
For two-zone cascade control systems, the PID settings for
both loops, the primary plus the secondary, must be opti-
mized for good temperature control.
See Chapter 4, Setup for information on tuning PID loops.
Ratio Control
Ratio control allows the process variable of one loop (mas-
ter loop), multiplied by a ratio, to be the setpoint of another
loop (ratio loop). You can assign any process variable to de-
termine the setpoint of a ratio loop.
By adjusting the ratio control parameters, you can adjust
the influence that the master loop process variable has on
the setpoint of the ratio loop.
Figure 6.7 Relationship Between the Master
Loop’s Process Variable and the
Ratio Loop’s Setpoint
Maximum SP
Minimum SP
Minimum Maximum
Master PV * Control Ratio + SP Diff
erential
Ratio Loop Setpoint
Master Loop Process Variable
SP Differential
SP: Setpoint
PV: Process Variable
PV PV