Watlow CLS - Manual (Page 161)

Appendix B: Enhanced Process Control
153
Cascade Control
Cascade Control is used to control thermal systems with long lag times,
which cannot be as accurately controlled with a single control loop. The
output of the first (primary) loop is used to adjust the setpoint of the
second (secondary) loop. The secondary loop normally executes the
actual PID control.
In some applications, there are two zone cascade control systems where
the primary channel PID output is used for the primary heat control and
the secondary cascaded channel PID output is used for a heat boost in a
second zone. These are used in the metals market such as aluminum
casting industries. You can use the primary heat output for both control
and for determining the setpoint of the secondary loop.
A customer has a tank of water, which has an inner and outer
thermocouple. The inner thermocouple is located in the center of the
water. The outer thermocouple is located near the heating element. The
desired temperature of the water is 150ºF, which is measured at the
inner thermocouple. Using cascade, the inner thermocouple is used on
the primary loop (in this example, PID loop 1), and the outer
thermocouple is used on the secondary loop (PID loop 2). The heater
will be controlled by loop 2 with a SP range of 150-190 ºF..
Using the 4CLS Watlow Anafaze controller equipped with the
Enhanced Control Option firmware, the programming sequence is
described below.
W a ter
Loo p 1 Input PV
Loo p 1 = Prim ary C ascad e Loop
Loo p 2 = Second ary C ascade Loo p
Inne r T/C
O ute r T/C
Loo p 2 Input PV L oop 2 PID Output
Hea ter
150 F
C L S
Pow er C ontrol