Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 35)

Optimizing Your Process System with the WATLOW Series 988
Enhanced Software Features
4.2
Cascade
Overview
Cascade control can handle a difficult process
with minimal overshoot, while reaching the set
point quickly. This minimizes damage to
system components and allows for oversizing
heaters for optimal heat-up rates.
Systems with long lag times between the energy
source (heater, steam, etc.) and the measured
process value cannot be controlled accurately
or efficiently with a single control loop, because
a lot of energy can build up before a response
is detected. This can cause the system to
overshoot the set point, which could damage
the heater, product or heat transfer medium,
such as a heat transfer fluid.
This graph illustrates a system with a long lag
time. 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 final
value taking hours to reach. Curve B shows a
cascade system that limits the energy intro-
duced into the system, allowing an optimal
heat-up rate with minimal overshoot.
This drawing shows two controllers configured
as a cascade system. The second controller
input 1 input 2
output 1
Outer-loop
Controller Inner-loop
Controller
%
out
%
int
In1
SP
In2
SP
int
The cascade feature allows the Series 988 to internalize the func-
tions of two controllers
generates the internal set point. The Series 988
effectively combines both controllers into a
single package.
The primary controller measures the process in
the outer, or primary, loop with input 1 and
compares the value to the desired set point.
The difference between the set point and the
process temperature generates an internal
percent output value for the second controller.
This value cannot be seen by the operator. This
internal percent (%
int
) output generates the
internal set point for the secondary, or inner
loop. The secondary loop uses this set point
and the value of input 2 (typically attached to
the heater sheath) to control the heater tem-
perature.
Algorithm
The following formulas show how the primary
control sends a set point (based on input 2
range-high and range-low values) to the sec-
ondary control. The secondary control uses this
set point (SP
int
) to generate a percent output
(%
out
) to the heater.
1.) %
int
= PID Set A[In1 - SP]
2.) SP
int
= (rH2 - rL2) * %
int
+ rL2
3.) %
out
= PID Set B[In2 - SP
int
]
The critical parameters are the range settings
for input 2 of the second controller. The range-
high value (rH2) is the maximum allowed set
point for the secondary loop. The range-low
value (rL2) is the minimum allowed set point.
A
B
C
time →
set point
Two controllers in one