Watlow CLS200, Руководство пользователя

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CLS200 Series User’s Guide
Chapter 5: Enhanced Features
107
4. To complete the cascade setup, both loop 1 and loop 2 must be configured for inputs,
outputs, and alarms.
In addition, the PID parameters of loop 1 must be tuned to produce the desired effect for the
application on the setpoint of loop 2. For a cascade control application that uses the secondary loop
for PID control, loop 1 typically uses only proportional mode. This must be set for the amount of
change in the process variable to cause a 100% change in the output level.
The proportional band is selected so the setpoint of the secondary loop has the desired relationship
to the process variable of the primary loop. In this application, the proportional band (PB) of the
primary loop is set to 10°F and the integral and derivative are turned off.
190˚F
150˚F
0% 50% 100%
Seconda
ry Loop Setpoint
Process Variable 1
(SP1-PB1) (SP1)
170˚F
150˚F 145˚F 140˚F
Heat Output (%)
(BASE SP + HT SPAN)
(BASE SP)
(eng. units)
SP: Setpoint
PB: Proportional Band
(eng
. units)
Primary Loop Output
Figure 5.6 — Secondary Loop Setpoint Related to Primary Loop Output
As the temperature of loop 1 drops, the output of loop 1 goes up proportionally and the setpoint
of loop 2 goes up proportionally. Thus heat is added to the system at the element 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 setpoint 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 efficient control.
For two-zone cascade control systems, the PID settings for both loops, the primary plus the
secondary, must be optimized for good temperature control.
See
Chapter 4: Setup
for information on tuning PID loops.