Watlow F4P Series, User Manual, Page: 74 / 124

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F e a t u r e s Wa t l o w S e r i e s F 4 P
Cascade
Cascade control is a control strategy in which one control loop
provides the set point for another loop. It allows the process
or part temperature to be reached quickly while minimizing
overshoot. Cascade is used to optimize the performance of
thermal systems with long lag times.
This graph illustrates a thermal system with a long lag time.
Curve A represents a single-loop control system with PID pa-
rameters that allow a maximum heat-up rate. Too much ener-
gy is introduced and the set point is overshot. In most sys-
tems with long lag time, the process value may never settle
out to an acceptable error. Curve C represents a single-con-
trol system tuned to minimize overshoot. This results in un-
acceptable heat-up rates, taking hours to reach the final val-
ue. Curve B shows a cascade system that limits the energy
introduced into the system, allowing an optimal heat-up rate
with minimal overshoot.
Cascade control uses two control loops (outer and inner) to
control the process. The outer loop (analog input 3) monitors
the process or part temperature, which is then compared to
the set point. The result of the comparison, the error signal,
is acted on by the settings in a Cascade Outer Loop PID set
(1 to 5), which then generates a power level for the outer
loop. The set point for the inner loop is determined by the
outer-loop power level and the Cascade Low Range/Deviation
and the Cascade High Range/Deviation settings for analog in-
put 3.
The inner loop (analog input 1) monitors the energy source
(heating and cooling), which is compared to the inner loop set
point generated by the outer loop. The result of the compari-
son, the error signal, is acted on by the settings in a Cascade
Inner Loop PID set (1 to 5), which generates an output power
level between -100% to +100%. If the power level is positive
the heat will be on; if the power level is negative the cool will
come on.
In Series F4 controllers, cascade control is available on chan-
nel 1. Analog input 3 is used to measure the outer-loop pro-
cess while analog input 1, the inner loop, is used to measure
the energy source. Power from the energy sources are sup-
plied by outputs 1A and 1B.
To set up and tune a system for cascade control, see the Oper-
ations Chapter.
Location in software: Setup Page and Operations Page.To set
up and tune a system for cascade control, see the Operations
Chapter.
Location in software: Setup Page and Operations Page
Figure 7.16a -- Control Lag Times
Figure 7.16b -- Cascade Control
✔
NOTE
:
Cascade Low Range and Cascade High Range Set
Points for Input 1 (as shown above) are setup under Analog Input
3. Refer to Setup Chapter.
Figure 7.16c -- Cascade Example
input 1
input 3
output 1
limit
limit sensor
oil in
heater
oil out
inner-loop
thermocouple
Lube O il Ta nk
outer-loop
thermocouple
F 4
1
2
1B 1B
1A 1A
A
M
/
Error Signal
(Heat)
0 to 100%
(Cool)
0 to -100%
Percent Power
0% to 100%
Set Point
(Process Part)
Input 3
Outer Loop
(Process Part)
Input 1
Inner Loop
(Energy Source)
Control Outer Loop
PID Set 1-5
ES/PB x 100 = % Out
Cascade Inner Loop
PID Set 1-5
ES/PB x 100 = % Out
0% = Cascade Low Range/Deviation
100% = Cascade High Range/Deviation
Error Signal
+
-
+
-
Outer Loop
Inner Loop
Time
Temperature
Curve A (PID)
Set
Point
Curve B (Cascade)
Curve C (Single-control)