process control chap 9 cascade control

8/21/2019 Process Control Chap 9 Cascade Control

1/20

Cascade Control


2/20

Cascade control• A disadvantage of conventional feedback control is that corrective action for

disturbances does not begin until after the controlled variable deviates from the set

point.

• Feedforward control offers large improvement over feedback control for

processes that have large time constants or t ime delays. However, feedforward

control requires that the disturbances be measured explici tly and that a model

be available to calculate the control ler output.

• An alternative approach, one that can significantly improve the dynamic response

to disturbances, employs a secondary measurement point and a secondary

feedback contro ller.

• The second measurement point is located so that it recognizes the upset

condition sooner than the controlled variable, but the disturbance is notnecessarily measured.

• This approach, cascade control, is widely used in the process industries and is

particularly useful when the disturbances are associated with the manipulated

variable or when the final control element exhibits nonlinear behaviour(Shinskey,

1996).


3/20

A furnace temperature control scheme that uses

conventional feedback control

Furnace

Hot oil

Cold oil

Stack gasTC

TT

Fuel gas

• Consider the natural draft furnace

temperature control .

• The conventional feedback

control system may do

satisfactory job of regulating the

hot oil temperature despite

disturbances in oil flow rate or

cold oil temperature.

• However, if a disturbance occursin the fuel gas supply pressure,

the fuel gas flow will change,

which upsets the furnace

operation and changes the hot oil

temperature.

• Only then will the temperature

controller (TC) begin to takecorrective action by adjusting the

fuel gas flow.

• Thus, we anticipate that

conventional feedback control

may result in very sluggish

responses to changes in fuel gas

supply pressure.


4/20

A furnace temperature control scheme using

cascade control

Furnace

Hot oil

Cold oil

Stack gasTC

TT

Fuel gas

PT

PC


5/20

• A cascade control configuration which consists of a primary controlloop (TT and TC) and a secondary control loop that controls the

pressure via PT and PC.

• The primary measurement is the hot oil temperature that is used bythe master controller (TC) to establish the set point for thesecondary (slave) loop controller (PC).

• If a disturbance in supply pressure occurs, the pressure controllerwill act quickly to hold the fuel gas pressure at its set point.

• The cascade control scheme provides improved performancebecause the control valve will be adjusted as soon as the change insupply pressure is detected.

• Because the pressure control loop responses rapidly, the supplypressure disturbance will have little effect on furnace operation andexit oil temperature


6/20

• The cascade control loop structure has two distinguishing features:

1. The output signal of the master controller serves as the set point for the slavecontroller

2.The two feedback control loops are nested, with the secondary control loop

(for the slave controller) locate inside the primary control loop (for the mastercontroller)

• Thus there are two controlled variables, two sensors, and one manipulatedvariable, whereas, the conventional control structure has one controlledvariable, one sensor, and one manipulated variable.

• The primary control loops can change the set point of the pressure control loopbased on deviations of the hot oil temperature from its set point.

• If the hot oil temperature is at its set point, the deviation variable for thepressure set point is also zero, which keeps the pressure at its desired steadystate value.


7/20

• The simplest cascade control scheme involves twocontrol loops that use two measurement signals tocontrol 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 thesecondary controller is used to adjust the controlvariable. Generally, the secondary controller changesquickly while the primary controller changes slowly.

Once cascade control is implemented, disturbancesfrom rapid changes of the secondary controller will notaffect the primary controller.


8/20

Single loop control and cascades

control of a heat exchanger

https://controls.engin.umich.edu/wiki/index.php/File:FlowOfInformationHeatEx2.jpghttps://controls.engin.umich.edu/wiki/index.php/File:HeatExDoubleLoop2.jpghttps://controls.engin.umich.edu/wiki/index.php/File:FlowOfInformationHeatEx1.jpghttps://controls.engin.umich.edu/wiki/index.php/File:HeatExSingleLoop1.jpg


9/20

Information flow of a two loop cascade

control

• there are two separate loops. Loop 1 is known as the primary loop, outerloop, or the master, whereas loop 2 is known as the secondary loop, innerloop, or the slave. To identify the primary and secondary loops, one mustidentify the control variable and the manipulated variable. In this case,

the control variable is the temperature and the reference variable is thesteam flow rate. Hence, the primary loop (loop 1) involves the controlvariable and the secondary loop (loop 2) involves the reference variable.Please note that the user sets the set point for loop 1 while the primarycontroller sets the set point for loop 2.

https://controls.engin.umich.edu/wiki/index.php/File:Twoloopcascadecontrol1.jpg


10/20

General cascade schematic

• The reactor below needs to be cooledduring continuous-feed operation of anexothermic reaction.

• The reactor has been equipped with acooling water jacket with the water flowrate being controlled by cold water valve.

• This valve is controlled by two separatetemperature controllers. An “inner-loop”or “slave” (highlighted in orange)temperature transmitter communicates tothe slave controller the measurement ofthe temperature of the jacket.

• The “outer-loop” or “master” (green)temperature controller uses a mastertemperature transmitter to measure thetemperature of the product within thereactor.

https://controls.engin.umich.edu/wiki/images/7/70/CascadeReactorEx.jpg


11/20

• The reactor temperature is affected by changes indisturbance variables such as reactant feed temperature orfeed composition.

• The simplest control strategy, adjust a control valve on thecooling water inlet stream.

• However, an increase in the inlet cooling watertemperature, may cause unsatisfactory performance

• Add a feedback controller for the jacket temperature,whose set point is determined by the reactor temperature

controller• The control system measures the jacket temperature,

compare it to a set point, and uses the resulting error signalas the input to a controller for the cooling water makeup.


12/20

• The principal advantage of the cascade control

strategy is that a second measure variable is

located to a potential disturbance and its

associated feedback loop can react quickly,

thus improving the close loop response.


13/20

https://controls.engin.umich.edu/wiki/index.php/File:Slave1.jpghttps://controls.engin.umich.edu/wiki/images/7/7d/Master.jpghttps://controls.engin.umich.edu/wiki/images/4/4c/Cascade.jpg


14/20

https://controls.engin.umich.edu/wiki/index.php/File:Solvey2.jpghttps://controls.engin.umich.edu/wiki/index.php/File:Step_2.jpg


15/20

https://controls.engin.umich.edu/wiki/index.php/File:Y1t.jpghttps://controls.engin.umich.edu/wiki/index.php/File:Step_3.jpg


16/20

• In order to have a smooth flow of information throughout the control system, a hierarchy ofinformation must be maintained. In a double loop cascade system, the action of the secondaryloop on the process should be faster than that of the primary loop. This ensures that thechanges 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 byapplying the following conditions when setting up the cascade controls.

-There must be a clear relationship between the measured variables of the primary andsecondary loops.

-The secondary loop must have influence over the primary loop.

-Response period of the primary loop has to be at least 4 times larger than the response periodof the secondary loop.

-The major disturbance to the system should act in the primary loop.

-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 highfrequency. Cascade control is designed to allow the master controller to respond to slowchanges 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 maintainthe hierarchy of information. In summary, the master controller responds to SLOW changes inthe system, while the slave controller responds to the high frequency, or FAST changes in thesystem. This also requires that the inner control scheme be tuned TIGHTLY so error is notallowed to build. Commonly, the inner loops controls a flow controller, which will reduce theeffect of changes such as fluctuations in steam pressure.


17/20

• Characteristic equation?

• If the inner loop is removed?


18/20

performance

• Cascade control can improve the response to a set pointchange by using an intermediate measurement and twofeedback controllers. However, its performance in thepresence of disturbances is usually the principal concern.

• When the slave loop responds faster than the master loop,the cascade control system will have improved stabilitycharacteristics and thus should allow larger values of Kc1 tobe used in the primary loop. Cascade control also makesthe close loop process less sensitive to errors in the processmodel used to design the controller


19/20


20/20

process control chap 9 cascade control

Documents