Two popular control strategies for improved disturbance rejection
performance are cascade control and feed forward with feedback trim.
Improved performance comes at a
price. Both strategies require that additional
instrumentation be purchased, installed and maintained. Both also require
additional engineering time for strategy design, tuning and implementation.
The cascade architecture offers alluring additional benefits such as the
ability to address multiple disturbances to our process and to improve set
point response performance.
In contrast, the feed forward with feedback trim architecture is designed
to address a single measured disturbance and does not impact set point
response performance in any fashion (explored in a future article).
The Inner Secondary Loop
The dashed line in the block diagram below (click for a large view)
loop like we have discussed in dozens of articles on controlguru.com. The only difference is
that the words "inner secondary" have been added
to the block descriptions. The variable labels
also have a "2" after them.
SP2 = inner secondary set point
CO2 = inner secondary controller output signal
PV2 = inner secondary measured process variable signal
D2 = inner disturbance variable (often not measured or available as
FCE = final control element such as a valve, variable speed pump or
The Nested Cascade Architecture
To construct a cascade architecture, we literally nest the secondary control
loop inside a primary loop as shown in the block diagram below (click for a large view).
Note that outer primary PV1 is our process variable of interest in this
implementation. PV1 is the variable we would be measuring and controlling if we had chosen a
traditional single loop
architecture instead of a cascade.
Because we are willing to invest the additional effort and expense to
improve the performance response of PV1, it is reasonable to assume that it is
a variable important to process safety
and/or profitability. Otherwise, it does not make sense to add the
complexity of a cascade structure.
Like many things in the PID control world, vendor documentation is not
consistent. The most common naming conventions we see for cascade (also
called nested) loops
▪ secondary and primary
▪ inner and outer
▪ slave and master
In an attempt at clarity, we are somewhat repetitive in this article by using labels like
"inner secondary" and "outer primary."
Two PVs, Two Controllers, One Valve
Notice from the block diagrams that the cascade architecture has:
▪ two controllers (an inner secondary and outer primary
▪ two measured process variable sensors (an inner PV2 and outer PV1)
▪ only one final control element (FCE) such as a valve,
pump or compressor.
How can we have two controllers but only one FCE? Because as shown in the
diagram above, the controller output
signal from the outer primary controller, CO1, becomes the set point of the
inner secondary controller, SP2.
The outer loop literally commands the inner loop by adjusting its set
point. Functionally, the controllers are wired such that SP2 = CO1 (thus, the
master and slave terminology referenced above).
This is actually good news from an implementation viewpoint. If we can install
and maintain an
inner secondary sensor at reasonable cost, and if we are using a
where adding a controller is largely a software selection, then the task of
constructing a cascade control structure may be reasonably straightforward.
Early Warning is Basis for Success
As shown below (click for a large view),
an essential element for success in a cascade design is the measurement and
control of an "early
warning" process variable.
In the cascade architecture, inner secondary PV2 serves as this early
warning process variable. Given this, essential design characteristics for selecting
PV2 include that:
▪ it be measurable with a sensor,
▪ the same FCE (e.g., valve) used to manipulate PV1 also manipulates PV2,
▪ the same disturbances that are of concern for PV1 also disrupt PV2, and
▪ PV2 responds before PV1 to disturbances of concern and to FCE
Since PV2 sees the disruption first, it provides our "early warning" that
a disturbance has occurred and is heading toward PV1. The inner secondary
controller can begin corrective action immediately. And since PV2 responds
first to final control element (e.g., valve) manipulations, disturbance
rejection can be well underway even before primary variable PV1 has been
impacted by the disturbance.
With such a cascade architecture, the control of the outer primary process variable
benefits from the corrective actions applied to the upstream early warning measurement
Disturbance Must Impact Early Warning Variable PV2
As shown below (click for a large view),
even with a cascade structure, there will likely be disturbances that impact
PV1 but do not impact early warning variable PV2.
The inner secondary controller offers no "early action" benefit for these outer disturbances. They are
ultimately addressed by the outer primary controller as the disturbance
moves PV1 from set point.
On a positive note, a proper cascade can improve rejection performance
for any of a host of disturbances that directly impact PV2 before disrupting
An Illustrative Example
To illustrate the construction and value of a cascade architecture,
consider the liquid level control process shown below (click for a large view).
This is a variation on our
gravity drained tanks, so hopefully, the behavior of the process
below follows intuitively from our previous investigations.
As shown above, the tank is essentially a barrel with a hole punched in the
bottom. Liquid enters through a feed valve at the top of the tank. The exit
flow is liquid draining freely by the force of gravity out through the hole in the
The control objective is to maintain liquid level at set point (SP) in
spite of unmeasured disturbances. Given this objective, our measured
process variable (PV) is liquid level in the tank. We measure level with a
sensor and transmit the signal to a level controller (the LC inside the circle
in the diagram).
After comparing set point to measurement, the level controller (LC) computes
and transmits a controller output (CO) signal to the feed valve. As the feed
valve opens and closes, the liquid feed rate entering the top of the tank
increases and decreases to raise and lower the liquid level in the
This "measure, compute and act" procedure repeats every loop sample time,
T, as the controller works to maintain tank level at
The disturbance of concern is the pressure in the main liquid header. As
shown in the diagram above, the header supplies the liquid that feeds our tank.
It also supplies liquid to several other lines flowing to different process
units in the plant.
Whenever the flow rate of one of these other lines changes, the header
pressure can be impacted. If several line valves from the main header open at about the same time, for example, the header pressure will drop
until its own control system corrects the imbalance. If one of the line valves shuts in an
emergency action, the
header pressure will momentarily spike.
As the plant moves through the cycles and fluctuations of daily production, the header pressure rises and falls
in an unpredictable fashion. And
every time the header
pressure changes, the feed rate to our
tank is impacted.
Problem with Single Loop Control
The single loop architecture in the diagram above attempts to achieve
our control objective by
adjusting valve position in the liquid feed line. If the measured level
is higher than set point, the controller signals the valve to close by an
appropriate percentage with the expectation that this will decrease feed
flow rate accordingly.
But feed flow rate is a function of two
▪ feed valve position, and
the header pressure pushing the liquid through the valve (a disturbance).
To explore this, we conduct some thought experiments:
Thought Experiment #1: Assume that the main header pressure is
perfectly constant over
time. As the feed valve opens and closes, the feed flow rate and thus tank
level increases and decreases in a predictable fashion. In this case, a single
loop structure provides acceptable level control performance.
Thought Experiment #2: Assume that our feed valve is set in a
fixed position and the header pressure starts rising. Just like squeezing harder on a spray bottle, the valve
position can remain constant yet the rising pressure will cause the flow
rate through the fixed valve opening to increase.
Thought Experiment #3: Now assume that the header pressure
starts to rise at the same moment that the controller determines that
the liquid level in our tank is too high. The controller can be closing
the feed valve, but because header pressure is rising, the flow rate
through the valve can actually be increasing.
As presented in Thought Experiment #3, The changing header pressure (a
disturbance) can cause a contradictory outcome that can confound the
controller and degrade control performance.
A Cascade Control Solution
For high performance disturbance rejection, it is not valve position,
but rather, feed flow rate that must be adjusted to control liquid level.
Because header pressure changes, increasing feed flow rate by a precise
amount can sometimes mean opening the valve a lot, opening it a little, and
because of the changing header pressure, perhaps even closing the valve a bit.
Below is a classic level-to-flow cascade architecture (click for a large view).
As shown, an inner secondary sensor measures the feed flow rate. An
inner secondary controller receives this flow measurement and adjusts
the feed flow valve.
With this cascade structure, if liquid level is too high, the primary
level controller now calls for a decreased liquid feed flow rate rather than simply a
decrease in valve
opening. The flow controller then decides whether this means opening or
closing the valve and by how much.
Note in the diagram that, true to a
cascade, the level controller output signal (CO1) becomes the set point for
the flow controller (SP2).
Header pressure disturbances are quickly detected and addressed by the
secondary flow controller. This minimizes any disruption caused by changing header pressure to the benefit of our
primary level control process.
The Level-to-Flow Cascade Block Diagram
As shown in the block diagram below (click for a large view),
our level-to-flow cascade fits into our block diagram structure. As
required, there are:
Two controllers - the outer primary level controller (LC) and inner
secondary feed flow controller (FC)
Two measured process variable sensors - the outer primary liquid
level (PV1) and inner secondary feed flow rate (PV2)
One final control element (FCE) - the valve in the liquid feed stream
As required for a successful design, the inner
secondary flow control loop is nested inside the primary outer level control
loop. That is:
The feed flow rate (PV2) responds before the tank level
(PV1) when the header pressure disturbs the process or when the feed valve
The output of the primary controller, CO1, is wired such that it
becomes the set point of the secondary controller, SP2.
Ultimately, level measurement, PV1, is our process variable of
primary concern. Protecting PV1 from header pressure disturbances is
the goal of the cascade.
Design and Tuning
The inner secondary and outer primary controllers are
from the PID family of algorithms. We have explored the design and tuning
of these controllers in numerous articles, so as we will see, implementing a
cascade builds on many familiar tasks.
There are a number of issues to consider when selecting and tuning the
controllers for a cascade. We explore next an
implementation recipe for cascade control.
Return to the
Table of Contents to learn more.
Copyright © 2007 by Douglas J. Cooper. All Rights Reserved.