Cascade Tuning
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1 Peter Thomas Control Specialists Ltd ExperTune User Conference – Austin Texas April 2007 How to Tune Cascade Loops www.controlspecialists.co.uk • Established 1989 - British Standards Institute (BSI) ISO9001:2000 Tick IT registered • “Certified Suppliers” of FDA / GAMP Validated Control Systems for AstraZeneca -international Pharmaceutical Manufacturer • Approved Solutions Partner (Sales, Support & Training) of ExperTune Inc. • Experience of working in Oil, Pharmaceuticals, Chemicals, Glass, Food and Water. Background
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Transcript of Cascade Tuning
1
Peter ThomasControl Specialists Ltd
ExperTune User Conference –Austin Texas April 2007
How to Tune Cascade Loops
www.controlspecialists.co.uk
• Established 1989 - British Standards Institute (BSI) ISO9001:2000 Tick IT registered
• “Certified Suppliers” of FDA / GAMP Validated Control Systems for AstraZeneca -international Pharmaceutical Manufacturer
• Approved Solutions Partner (Sales, Support & Training) of ExperTune Inc.
• Experience of working in Oil, Pharmaceuticals, Chemicals, Glass, Food and Water.
Background
2
How to Tune Cascade Loops
• An overview of Cascade Control.
• Tuning Cascade Control Loops.
• Case Study.
Cascade Control
In the right circumstances, Cascade Control can greatly improve the performance of a control system.
In the right circumstances, Cascade Control can greatly improve the performance of a control system.
3
Temperature Control LoopSteam Header
TT
TIC
Heat Exchanger
To other User
Temperature Control LoopSteam Header
TIC
Heat Exchanger
To other User
TIC FLOW TEMP
Process SchematicTT
4
Temperature Control LoopSteam Header
TT
TIC
Response to a Load Change
Heat Exchanger
Steam Header
Temperature Control LoopSteam Header
TT
TIC
Heat Exchanger
To other User
TIC FLOW TEMP
Process Schematic
5
To other User
Steam Header
TT
TIC
FICFT
Temperature – Flow Cascade
Temperature – Flow Cascade
To other User
Steam Header
TT
TIC (Master)
FIC(Slave)
FTTIC FLOW TEMP
Process Schematic
FIC
The Cascade Loop
6
What's The Inner Loop For?
• Reduces phase lag of inner process• Disturbances to the inner loop are
compensated for before they upset the outer loop
• Prevents non-linearities in the inner loop from reaching the outer loop
Temperature – Flow CascadeSteam Header
TT
TIC (Master)
FIC(Slave)
FT
The Cascade Loop
Response to a Load Change
7
Master – Cascade Signals
MASTERPV SLAVEOP REM SP
OP
PV
Master – Cascade Signals
MASTERPV SLAVE
OP REM SP
OP
PV
REMTRK
When slave is NOT in Remote/Auto modeMaster OP is to track Slave Setpoint.
8
Master – Cascade Signals
MASTERPV SLAVE
OP REM SP
OP
PV
REMTRK
Integral action in Master is to be Inhibited when OP limit reached in Slave
OP LIMITINT
Other Examples of Cascade
• Batch Reactor Temperature Control (Batch Temperature onto Jacket Temperature)
• Level – Flow Control
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Level Control
LT
Level loops are Integrating Processes
Level Control
LT
Level loops are Mass Balance Systems
IN > OUT: Level Rises
IN < OUT: Level Falls
IN = OUT: Level Holds
IN
OUT
10
Level Control Problems(Non Linear Responses)
LT LIC
Head Variations
IN
OUT
LT LIC
A failure of the control valve to reposition itself correctly following a reversal inthe control signal – level will cycle !!
Level Control Problems(Valve Hysteresis)
IN
OUT
11
Fixing the problemsLevel – Flow Cascade
LT LIC
FICFT
IN
OUT
How to Tune Cascade Loops
• An overview of Cascade Control.
• Tuning Cascade Control Loops.
• Case Study.
12
What happens when cascade loops are poorly tuned?
• Loops “fight” each other• Create oscillations• Neither variable is properly controlled• Operator puts loop in manual.
Tuning Cascade Loops1. Always check for measurement and
valve-related issues.2. Inner Loop Tuning - put slave into
Local Auto or Manual and tune the slave controller as a normal PID loop.
3. Outer Loop Tuning - put slave into Cascade and tune master controller as a normal PID loop.
4. Adjust outer loop tuning values to ensure that the RRT (Relative Response Time) of outer loop is 3-5 times slower than the inner loop.
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Test Data - Inner Loop
Inner loop PV
Inner loop CO
Outer loop PV
Inner loop SP Change
Inner loop PVInner loop CO
Outer loop PVOuter loop SP change
Test Data - Outer Loop
14
Relative Response Time – What is it ?
1. An indication of the closed loop response of a control loop.
2. Loops with short RRT’s are fast loops
3. Loops with long RRT’s are slow loops.
4. The RRT approximately defines theperiod of any load disturbance initiated oscillations when thecontroller is in automatic.
Relative Response Time andCascade Loops
1. The closed loop response (i.e its RRT) of the outer loop MUST be 3 –5 times slower than the inner loop.
2. After tuning, adjust outer loop PID values to achieve this.
3. Our example:Inner: 38 secondsOuter: 400 seconds
15
Determining the Relative Response Time
Relative Response Time (RRT)
1/0.0677 = 15s
16
How to Tune Cascade Loops
• An overview of Cascade Control.
• Tuning Cascade Control Loops.
• Case Study.
Cascade Control – A Case Study
Waste Water Industry -Dissolved Oxygen Control
17
Dissolved Oxygen Control – As Found
• Oxygen Concentration controlled by QIC adjusting the flow of air.• PID Control implemented in a Siemens S7 PLC.• PIC used to control blower speed.• Flow measurement for information only.• If the blower speed exceeded 95% the second blower started (at the same speed) to assist the duty blower.• Poor control.
CASP 1 DO
0
10
20
30
40
50
60
70
80
90
100
Time
Val
ve p
ositi
on
0
0.5
1
1.5
2
2.5
3
3.5
4
DO
(m
g/l)
AV302
QI503
• Control Specialists Ltd modified the software at the request of the end user.• Flow loop quickly rejects disturbances due to duty-standby boosters coming on and off line.• Blower operating costs reduced.• Control dramatically improved.
CASP 1 DO
0
10
20
30
40
50
60
70
80
90
100
Time
Val
ve p
osi
tion
0
0.5
1
1.5
2
2.5
3
3.5
4
DO
(m
g/l)
AV302
QI503
Dissolved Oxygen Control – As Left
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Bridging the gap between Industrial Processes and the Programmable Systems that
control them.
www.controlspecialists.co.uk
