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Process ControlCascade Control

Cascade ControlCascade Control

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Process ControlCascade Control

Basis of Cascade ControlBasis of Cascade ControlBasis of Cascade ControlÆ A controlled process may in

some cases be considered astwo process in series in whichthe output of the first process ismeasured.

Æ When each process is subject tounmeasured disturbances, thecontrol performance can be

improved by an additionalfeedback loop.

Æ The outer loop output becomesthe setpoint of the inner loop.

Æ Disturbances to the inner loopare corrected for and thus havelittle effect on the controlledparameter

SecondaryController

Manipulated

Disturbance

Disturbance

Process 2Process 1

PrimaryController

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Process ControlCascade Control

Cascade Control in DeltaVCascade Control in DeltaVCascade Control in DeltaVÆ In Cascade mode, the

setpoint of the secondaryloop is changed by the

outer loop through theCAS_IN connection.

Æ Disturbances to the inner loop are corrected for andthus have little effect onthe controlled parameter.

Æ The BKCAL_INconnection is used for bumpless transfer andprevention of windupwhen the downstreamloop is limited.

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Slide 8 - 5Process ControlCascade Control

Cascade –Dynamic ResetCascadeCascade – – Dynamic ResetDynamic ResetÆ Select FRSIPID_OPTS for

Dynamic Reset Limit in theprimary of the cascade toautomatically compensatesfor poor response of thesecondary loop.

Æ The CONTROL_OPTS in thesecondary must be set for Use PV for BKCAL_OUT for Dynamic Reset Limit to

provide benefit.

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Slide 8 - 6Process ControlCascade Control

Cascade Control WorkshopCascade Control WorkshopStep 1. Open EXAMPLE_E and go to on-line operation in Control Studio.

Set the mode of the secondary loop (PID2) to manual. Make astep change in the OUT of PID2 and observe the response of the

two process outputs. What difference do you notice in theresponse of the two outputs.

Step 2. Place PID2 in Auto mode and change the setpoint to 50.Observe the process response and also the automatic trackingthat is done by the primary loop.

Step 3. Place the PID2 in Cascade mode. With PID1 in manual mode,change the output of the PID1 and observe the change in thesetpoint of PID2.

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Slide 8 - 7Process ControlCascade Control

Cascade Control workshop (Cont)Cascade Control workshop (Cont)

Step 4. Place PID1 in automatic mode and observe the response when achange is made in the setpoint. With PID2 in cascade mode andPID1 in automatic, make a step change in the load disturbance.

What impact was there on the secondary loop? Did the primaryloop change as a result of the change in load disturbance?

Step 5. Enable FRSI_OPTS, “Dynamic Reset Limit” in PID1. EnableCONTROL_OPTS “Use PV for BKCAL_OUT” on PID2 and makea change in PID1 setpoint. Did you notice any difference in the

response.Step 6. Enable BYPASS in PID 2 and observe the difference in response

when the setpoint of PID1 is changed.

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Slide 8 - 8Process ControlCascade Control

EXAMPLE_E EXAMPLE_E