Cascade control uses two or more interconnected control loops to control a process variable. In a basic cascade control scheme, the output of the primary controller determines the set point of the secondary controller. The secondary controller then adjusts the control variable. This allows the secondary controller to respond quickly to disturbances while the primary controller responds more slowly. An example is given of using cascade control to maintain the temperature of a fluid heated by steam. A secondary flow controller loop would respond quickly to changes in steam flow, while the primary temperature controller loop would adjust more slowly to variations in fluid temperature. Cascade control in this case allows compensation for disturbances in both steam and fluid flow rates to better maintain the desired fluid temperature.

1. Cascade controllers
Introduction

2.  SISO involves a single loop control that uses only one measured signal
(input). This signal is then compared to a set point of the control
variable (output) before being sent to an actuator (i.e. pump or valve)
that adjusts accordingly to meet the set point. Cascade controls, in
contrast, make use of multiple control loops that involve multiple
signals for one manipulated variable. Utilizing cascade controls can
allow a system to be more responsive to disturbances
 Meaning of the terms 'manipulated variables', 'measured variables' and
'control variables' should be clarified. The definitions of these terms
commonly found in literature are often interchangeable; but, they
typically refer to either the input or output signal. For the purpose of this
article, 'control variables' will refer to inputs like flow rates, pressure
readings, and temperature readings. 'Manipulated variables' and
'measured variables' will refer to the output signals which are sent to the
actuator.
Single loop control

3.  The simplest cascade control scheme involves two control
loops that use two measurement signals to control one
primary variable. In such a control system, the output of
the primary controller determines the set point for the
secondary controller. The output of the secondary
controller is used to adjust the control variable. Generally,
the secondary controller changes quickly while the primary
controller changes slowly.
 Once cascade control is implemented, disturbances from
rapid changes of the secondary controller will not affect
the primary controller.
Introduction

4.  To illustrate how cascade control
works and why it is used, a typical
control system will be analyzed. This
control system is one that is used to
adjust the amount of steam used to
heat up a fluid stream in a heat
exchanger. Then an alternative
cascade control system for the same
process will be developed and
compared to the typical single loop
control. The figure below shows the
performance of cascade control vs.
single-loop control in CST heater
 Cascade control gives a much better
performance because the
disturbance in the flow is quickly
corrected.

5.  The above process, the fluid is to be heated up to a certain temperature by the
steam. This process is controlled by a temperature controller (TC1) which
measures the temperature of the exiting fluid and then adjusts the valve (V1) to
correct the amount of steam needed by the heat exchanger to maintain the
specified temperature. Figure 2 shows the flow of information to and from the
temperature controller. to the heat exchanger is solely dependent on opening
the valve to varying degrees.
 If the flow rate of the steam supply changes (i.e. pipeline leakage, clogging, drop in
boiler power), the controller will not be aware of it. The controller opens the valve
to the same degree expecting to get a certain flow rate of steam but will in fact be
getting less than expected. The single loop control system will be unable to
effectively maintain the fluid at the required temperature.
Example of Cascade Control

6.  Implementing cascade control will allow us to
correct for fluctuations in the flow rate of the
steam going into the heat exchanger as an inner
part of a grander scheme to control the
temperature of the process fluid coming out of
the heat exchanger. A basic cascade control uses
two control loops;
 one loop (the outer loop, or master loop, or
primary loop) consists of TC1 reading the fluid out
temperature, comparing it to TC1set (which will not
change in this example) and changing
FC1set accordingly. The other loop (the inner loop,
or slave loop, or secondary loop) consists of FC1
reading the steam flow, comparing it to
FC1set (which is controlled by the outer loop as
explained above), and changing the valve opening
as necessary.

7.  The main reason to use cascade control in this system is that
the temperature has to be maintained at a specific value. The
valve position does not directly affect the temperature
(consider an upset in the stream input; the flow rate will be
lower at the same valve setting). Thus, the steam flow rate is
the variable that is required to maintain the process
temperature.
 The inner loop is chosen because it is prone to higher
frequency variation. The rationale behind this example is that
the steam in flow can fluctuate, and if this happens, the flow
measured by FC1 will change faster than the temperature
measured by TC1, since it will take a finite amount of time for
heat transfer to occur through the heat exchanger. Since the
steam flow measured by FC1 changes at higher frequency, we
chose this to be the inner loop. This way, FC1 can control the
fluctuations in flow by opening and closing the valve, and TC1
can control the fluctuations in temperature by increasing or
decreasing FC1set
 Thus, the cascade control uses two inputs to control the valve
and allows the system to adjust to both variable fluid flow and
steam flow rates.

8.  In order to have a smooth flow of information throughout the control system, a hierarchy of information
must be maintained. In a double loop cascade system, the action of the secondary loop on the process
should be faster than that of the primary loop. This ensures that the changes made by the primary output
will be reflected quickly in the process and observed when the primary control variable is next measured.
This hierarchy of information can be preserved by applying the following conditions when setting up the
cascade controls.
1) There must be a clear relationship between the measured variables of the primary and secondary
loops.
2) The secondary loop must have influence over the primary loop.
3) Response period of the primary loop has to be at least 4 times larger than the response period of the
secondary loop.
4) The major disturbance to the system should act in the primary loop.
5) The primary loop should be able to have a large gain, Kc.
Cascade control is best when the inner loop is controlling something that happens at fairly high frequency.
Cascade control is designed to allow the master controller to respond to slow changes in the system, while the
slave controller controls disturbances that happen quickly. If set up in reverse order, there will be a large
propagation of error. Hence, it is important to maintain the hierarchy of information. In summary, the master
controller responds to SLOW changes in the system, while the slave controller responds to the high frequency,
or FAST changes in the system. This also requires that the inner control scheme be tuned TIGHTLY so error is not
allowed to build. Commonly, the inner loops controls a flow controller, which will reduce the effect of changes
such as fluctuations in steam pressure.
Conditions for cascade control

9. Advantages and Disadvantages of Cascade
Control

10. te
Temperature control of steam out from boilers
to turbine blades using TCV 151

11. Logic diagram of Temperature control valve
Primary controller
Secondary controller