cascade_control_se665

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EXPERIMENT 6

Cascade Control

6.1 Objective

(i) To demonstrate the characteristics of On/Off control

(ii) To demonstrate the characteristics of PID control

(iii) To demonstrate the characteristics of Proportional Band, Integral Action and Derivative Action

on a cascade process control loop.

(iv) To demonstrate the characteristics of cascade control

6.2 Introduction

TheCascade Control Modulehas been designed for students demonstration and to give them

better understanding on how open and closed tank level measurement and level control is done,

and to provide them with a hands on experience on manipulating various control parameters

using a microprocessor based controller. It can be coupled with a flow metering loop to enable

cascade control studies. The equipment consists of industrial grade instrumentation to exhibit a

realistic working environment of a true level process using various types of measuring

instruments and control strategy.

The equipment is self contained and constructed on an epoxy coated mild steel frame with four

lockable castor wheels. The pumps, tanks, instrumentation and valves are strategically located


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p p g y

6.2.1 Process Description

The SOLTEQ Model: SE 665 Cascade Control Trainer is a water process. Water from the sump

tank, T-601 is pumped by P-601 to the level tank, T-602. There are two discharge possibilities

from the T-602 back to the T-601. One is known as the self-regulatory mode using gravity flow

for the water to return to the sump tank. The other is the non self-regulatory mode, using a

pump P-602 to drain the water from the tank.

6.2.2 Control Strategy

Various types of instrumentations are installed in the process. There are two PID control loops in

the process controller that controls the process, namely FIC-601 and LIC-603. There is a switch

at the local control panel, which allows selection of either flow control or level control or a

combination of the two, called the flow/level cascade control.

6.2.3 Level Control Loop

The level transmitter LT-603 feed the signal to LIC-603, which controls the valve FCV-601.

6.2.4 Flow Control Loop

The orifice flow transmitter FT-601 feed the signal to FIC-601, which controls the valve FCV-601.


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6.2.6 Experimental Capabilities

PID Single Loop Flow Control

PID Single Loop Level Control

Self-Regulating

Non Self-Regulating

On/Off Control

PID Level and Flow Cascade Control

Table 6.1 Instrument function and capabilities

No. Instrument Tag No. Description Range

1 PID ControllerFIC-601

LIC-603

Microprocessor based PID

controller, controlling level / flow-

2 RecorderLR-603

FR-603

Continuous 2-pen chart

recorder-

3Orifice D/P

TransmitterFT-601

Primary flow measurement

device giving 4 20 mA output0 100 LPM

4 D/P Transmitter LT-603Level measurement using

differential pressure.

0-

1000mmH2O

5 Control Valve FCV-601

1 inch globe type valve with

Cv=4.8, linear characteristic

with I/P positioner and I/P1 100%


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11 RotameterFI-601

FI-602

Flowrate measurement forwaterFlowrate measurement for air

0 100 LPM

0 6 LPM

12 Pressure Indicator

PI-601

PI-602

PI-603

Dial gauge pressure indicator at

location.

0 4 bar

0 4 bar

0 4 bar

13 Side GlassSG-601

SG-602

Observation of water level in

tanks.-

6.3 Process Control

Student may refer to section 0.3.1 for theory of process control

6.3.1 On/Off Control

Student may refer to section 0.3.2 for theory of On/Off Control

6.3.2 Proportional Control

Student may refer to section 0.3.3 for theory of proportional control

6.3.3 Integral Control

Student may refer to section 0.3.4 for theory of integral control

6.3.4 PID Control


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6.4 Procedure

Table 6.2 Start up procedure

STEP ACTION REMARKS

1

Ensure that all valves are set according to Flow Control Initial

Valves Position outlined in Table 1

Table 1:Flow Control Initial Valve Positions

Open CloseHV612 HV611HV614 HV613HV615 HV616

HV617 HV618HV620

2 Fill in Sump Tank T-601 with water to about 80% full.

3 Switch on the panel.

4Set the Control Mode switch to Flowto engage the Flow

control loop.

5 Switch on Pump P-601

6 Put the Controller FIC-601 into manual mode, then set theoutput to 100%.


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3Tune the output gradually so that the flow measurement

matches the set point at 50%.

4Turn on the recorder, then put the control loop into "auto"

mode.

5Stimulate load change by closing HV615 for 3 seconds, then

open HV615 back.

6Once the measurement stabilizes, turn off the recorder. Put

the control loop back into "manual" mode.




mode.

9Stimulate a set point change by increasing the set point to

75%.

10Observe the response. Once the response stabilizes, turn off

the recorder. Put the control loop back into "manual" mode.

11

With the I and D values maintained, repeat step 2 to step 10with the following PB values


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mode.

5Stimulate load change by closing HV615 for 3 seconds, then open

HV615 back.

6Once the measurement stabilizes, turn off the recorder. Put the

control loop back into "manual" mode.

7Tune the output gradually so that the flow measurement matches

the set point at 50%.

8 Turn on the recorder, then put the control loop into "auto" mode.

9 Stimulate a set point change by increasing the set point to 75%.

10Once the measurement stabilizes, turn off the recorder. Put the

control loop back into "manual" mode.

11

With the PB and D values maintained, repeat step 2 to step 10 withthe following I values

I

15

62

Table 6.5 PID Flow Control


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5Stimulate load change by closing HV615 for 3 seconds, then

open HV615 back.


the control loop back into "manual" mode.




mode.


75%.

10 Once the measurement stabilizes, turn off the recorder. Putthe control loop back into "manual" mode.

11

With the PB and I values maintained, repeat step 2 to step 10with the following D values

D

510

Table 6.6 PI Flow Control Loop Tuning

STEP ACTION REMARKS


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values are as follows: -

PB

100252

5

Determine the natural period using the following method

Natural Period, min60SpeedTrend

DT

Where D = Distance in mm between successive crests or

valleys

6

Set the Integral Time to the Natural period, repeat Step 3 and

6. The period of oscillation should increase by 40%. If the

period is longer than this, increase the integral time. If the

period is shorter than this, decrease the Integral time.

7Finally, adjust (increase) the PB until the desire degree of

damping is achieved

6.4 Level Process Control

The Level Experiments can be run either in Self-Regulating mode or Non-Self-Regulating mode.

The Procedure of experiments in sections 3B to 6B are based on self-regulating (Flow Under

Head) operation mode.


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Table 6.7 Self-Regulating (Flow Under Head)

STEP ACTION REMARKS

1

Ensure that all valves are set according to Level Control

Initial Valve Positions (Self-Regulating) outlined in Table 2.

Table 2: Level Control Initial Valve Positions (Self-

Regulating)

Open Close

HV612 HV611

HV614 HV613HV615 HV616HV619 HV618

HV620 HV621HV624 HV622

HV623HV625

2 Fill in Sump Tank T-601.

3 Switch on the panel

4Set the Control Mode switch to Levelto engage the level

control loop.

5 Switch on Pump P-601.

6 Align the mark on the HV620 handle to about 45o.

7 Y d d h i


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Table 6.8 Non-Self-Regulating (Pump Discharge)

STEP ACTION REMARKS

1

Ensure that all valves are set according to Level Control

Initial Valve Positions (Non-Self-Regulating) outlined in Table

3

Table 3: Level Control Initial Valve Positions (Non-Self-

Regulating)

Open CloseHV612 HV611HV614 HV613HV615 HV616

HV621 HV618HV622 HV623

HV626 (30o) HV625

2 Fill in Sump Tank T-601.

3 Switch on the panel

4Set the Control Mode switch to Levelto engage the level

control loop.


6 You are now ready to proceed to the experiment.


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mode.

5

Stimulate load change by opening HV620 fully for 20

seconds. (Be careful not to empty the tank during the

process) and restore it to its original position.


the control loop back into "Manual" mode.

7Tune the output gradually so that the level measurement



mode.

9

Stimulate a set point change by increasing the set point to

75%.

10Once the measurement stabilizes, turn off the recorder. Putthe control loop back into "Manual" mode.

Table 6.10 Proportional only Level Control

STEP ACTION REMARKS

Enter PB value of 100, I value of 1000 seconds, and D value


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the control loop back into "Manual" mode.




mode.


75%.


the control loop back into "Manual" mode

11

With the I and D values maintained, repeat step 8 to step 16with the following PB values

PB

1062s

Table 6.11 Proportional and Integral Level Control

STEP ACTION REMARKS

1

Enter PB value of 100, I value of 60 seconds, and D value of

0 second


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mode.


75%.

10 Once the measurement stabilizes, turn off the recorder.

11

With the PB and D values maintained, repeat step 8 to step16 with the following I values

I

30155

Table 6.12 PID Level Control

STEP ACTION REMARKS

1

Enter PB value of 100, I value of 6 seconds, and D value of 0

second.

2Put the control loop into "manual" mode, then adjust the set


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mode.


75%.

10Once measurement stabilizes, turn off the recorder. Put the

control loop back into "Manual" mode.

11

With the PB and I values maintained, repeat step 2 to step 10with the following D values

D

1

61230

60

Table 6.13 PID Level Control Loop Tuning

STEP ACTION REMARKS

1

Enter a PB value of 100, I value of 1000 seconds and D

value of 0 second. (Setting the I to its maximum and D to its

minimum providing proportional only control)


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5

Determine the natural period using the following method:

Natural Period, min60TrendSpeed

DT

Where D = Distance in mm between successive crests or valle

6

Maintain the PB value. Set I to 0.4T and D to 0.15T. Repeat

Step 2 and 4 and observe the response. There should be a

25% decrease in period. If the new period of oscillation is

shorter than this, reduce derivative time, if the period is

longer increase integral time.

7Finally, adjust (increase) the PB until the desire degree of

damping is achieved

LEVEL AND FLOW CASCADE PROCESS CONTROL

Table 6.14 Start-Up Procedure

STEP ACTION REMARKS

1

Ensure that all valves are set according to Level and Flow

Cascade Control Initial Valve Position outlined in Table 4.

Table 4:Cascade Control Initial Valve Positions

Open Close

HV612 HV611HV614 HV613


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6Put the Controller FIC-601 into manual mode, then set the

output to 100%.

7 Adjust HV618 to set flow measurement FT-601 to 100%.

8 You are now ready to proceed to the experiment.

Table 6.15 PID Level and Flow Cascade Control

STEP ACTION REMARKS

1Enter PID values as obtained in Level loop tuning.

2 Enter PI values as obtained in Flow loop tuning.

3

Put the control loop into "Manual" mode. Adjust the output of

the controller gradually so that the level of tank matches the

set point of 50%.

4

Turn on the recorder, and then put the control loop into

"Auto" mode. Wait until measurement settle down and


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9Wait until the measurement stabilizes, then turn off the recorder. Put

the control loop back into manual mode.

10Compare the cascade chart with the chart obtained from Level PID

experiments.

Appendix

A ] FLOW CONTROL TUNING

P CONTROL PI CONTROL PID CONTROL

P 50 100 209.1 209.1 209.1 209.1 209.1 209.1 209.1

I 9999 9999 9999 2.4 7.9 20.9 2.4 2.4 2.4

D 0 0 0 0 0 0 2.0 4.0 5.0

B ] LEVEL CONTROL TUNING

P CONTROL PI CONTROL PID CONTROL

P 6.6 16.6 16.6 16.6 16.6 16.6 16.6 16.6 16.6I 9999 9999 9999 5.6 13.6 25.6 13.6 13.6 13.6


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Piping and Instrumentation Diagram


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