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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.
7Tune the output gradually so that the flow measurement
matches the set point at 50%.
8Turn on the recorder, then put the control loop into "auto"
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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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.
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.
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%.
8Turn on the recorder, then put the control loop into "auto"
mode.
9Stimulate a set point change by increasing the set point to
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.
5 Switch on Pump P-601.
6 You are now ready to proceed to the experiment.
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4Turn on the recorder, then put the control loop into "auto"
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.
6Once the measurement stabilizes, turn off the recorder. Put
the control loop back into "Manual" mode.
7Tune the output gradually so that the level measurement
matches the set point at 50%.
8Turn 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. 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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6Once the measurement stabilizes, turn off the recorder. Put
the control loop back into "Manual" mode.
7Tune the output gradually so that the level measurement
matches the set point at 50%.
8Turn on the recorder, then put the control loop into "auto"
mode.
9Stimulate 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 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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7Tune the output gradually so that the level measurement
matches the set point at 50%.
8Turn on the recorder, then put the control loop into "auto"
mode.
9Stimulate a set point change by increasing the set point to
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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7Tune the output gradually so that the level measurement
matches the set point at 50%.
8Turn on the recorder, then put the control loop into "auto"
mode.
9Stimulate a set point change by increasing the set point to
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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5 Switch on Pump P-601.
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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