How to Tune Cascade Loops
Peter Thomas
Control Specialists Ltd
ExperTune User Conference –
Austin Texas April 2007
www.controlspecialists.co.uk
Background
• 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.
1
How to Tune Cascade Loops
• An overview of Cascade Control.
• Tuning Cascade Control Loops.
• Case Study.
Cascade Control
In the
the right
right circumstances,
circumstances,
In
Cascade Control
Control can
can greatly
greatly improve
improve
Cascade
the performance
performance of
of aa control
control system.
system.
the
2
Temperature Control Loop
Steam
Header
To other User
TIC
TT
Heat Exchanger
Temperature Control Loop
Steam
Header
To other User
TIC
FLOW
TEMP
TIC
TT
Process Schematic
Heat Exchanger
3
Temperature Control Loop
Steam
Header
TIC
TT
Response to a Load Change
Heat Exchanger
Temperature Control Loop
Steam
Header
To other User
TIC
FLOW
TEMP
TIC
TT
Process Schematic
Heat Exchanger
4
Temperature – Flow Cascade
Steam
Header
To other User
FIC
FT
TIC
TT
Temperature – Flow Cascade
Steam
Header
To other User
FT
FIC
(Slave)
TIC
FIC
FLOW
TEMP
TIC
(Master)
TT
Process Schematic
The Cascade Loop
5
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 Cascade
Steam
Header
FT
FIC
(Slave)
TIC
(Master)
TT
Response to a Load Change
The Cascade Loop
6
Master – Cascade Signals
PV
OP
PV
REM SP
MASTER
SLAVE
OP
Master – Cascade Signals
PV
OP
TRK
PV
MASTER
REM SP
REM
SLAVE
OP
When slave is NOT in Remote/Auto mode
Master OP is to track Slave Setpoint.
7
Master – Cascade Signals
PV
OP
TRK
PV
REM SP
REM
MASTER
SLAVE
INT
OP LIMIT
OP
Integral action in Master is to be
Inhibited when OP limit reached in Slave
Other Examples of Cascade
• Batch Reactor Temperature Control
(Batch Temperature onto Jacket
Temperature)
• Level – Flow Control
8
Level Control
LT
Level loops are Integrating Processes
Level Control
IN > OUT: Level Rises
LT
IN < OUT: Level Falls
IN = OUT: Level Holds
IN
OUT
Level loops are Mass Balance Systems
9
Level Control Problems
(Non Linear Responses)
LT
LIC
IN
Head Variations
OUT
Level Control Problems
(Valve Hysteresis)
LT
LIC
IN
OUT
A failure of the control valve to reposition
itself correctly following a reversal in
the control signal – level will cycle !!
10
Fixing the problems
Level – Flow Cascade
LT
LIC
IN
FT
FIC
OUT
How to Tune Cascade Loops
• An overview of Cascade Control.
• Tuning Cascade Control Loops.
• Case Study.
11
What happens when cascade loops
are poorly tuned?
•
•
•
•
1.
2.
3.
4.
Loops “fight” each other
Create oscillations
Neither variable is properly controlled
Operator puts loop in manual.
Tuning Cascade Loops
Always check for measurement and
valve-related issues.
Inner Loop Tuning - put slave into
Local Auto or Manual and tune the
slave controller as a normal PID loop.
Outer Loop Tuning - put slave into
Cascade and tune master controller
as a normal PID loop.
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.
12
Test Data - Inner Loop
Outer loop PV
Inner loop SP
Change
Inner loop PV
Inner loop CO
Test Data - Outer Loop
Outer loop PV
Outer loop SP change
Inner loop PV
Inner loop CO
13
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 the
period of any load disturbance
initiated oscillations when the
controller is in automatic.
Relative Response Time and
Cascade 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 seconds
Outer: 400 seconds
14
Determining the
Relative Response Time
Relative Response Time (RRT)
1/0.0677 = 15s
15
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
16
Dissolved Oxygen Control – As Found
CASP 1 DO
100
4
90
3.5
3
70
60
2.5
50
2
40
1.5
30
DO (mg/l)
Valve position
80
1
20
0.5
10
0
0
AV302
QI503
Time
• 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.
Dissolved Oxygen Control – As Left
CASP 1 DO
100
4
90
3.5
3
70
60
2.5
50
2
40
1.5
30
DO (mg/l)
Valve position
80
1
20
0.5
10
0
0
Time
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.
17
Bridging the gap between
Industrial Processes and the
Programmable Systems that
control them.
www.controlspecialists.co.uk
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