10 Tips for Tuning of Pid Looops
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Transcript of 10 Tips for Tuning of Pid Looops
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10 Tips for:
Tuning of PID LoopsBy
Steve Mackay
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This 45 minute session will commence shortly!
RH
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Objectives
• Concept of tuning • Ziegler Nichols classical rules • Practical closed and open loop strategies • Cascade loops • Dead time issues
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Concept of Tuning
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1 - Minimize Integral of error
Objectives of Tuning (1)
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2 - Minimize Amplitude of error squared
Objectives of Tuning (2)
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3 - Minimize wear and tear
4 - No overshoot at Start Up
5 - Minimize effect of known disturbances
Other Aspects of Tuning (3,4,5)
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Ziegler Nichols and others
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Loop Controller and Process
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1. Put controller in manual mode
2. Make a step change to the OP value
3. Observe Reaction Curve
4. Calculate timing constants
Reaction curve
Steps of Open Loop Tuning (Reaction Curve)
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P - Control
PI - Control
PID - Control
OP
N L%
0 9
3
. %
int
OPN L
T L
12
20 5
. %
.int
OPN L
T LT Lder
KC
KC
KC
Reaction Curve Calculations
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• Find critical value of K for continuous oscillations• Do not use I or D - control• Observe critical frequency for 180° phase shift• Stabilize loop by reducing K and ensure that I & D• Control still gives a phase lead
Continuous Cycling - Ziegler & Nichols
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P-only for Continuous Cycling Method
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1. Put controller in P - control only2. P - control on ERR = (SP - PV)3. Controller in Auto4. Step change to SP5. Double value of K until continuous cycling6. Calculate tuning constants
Continuous Cycling Method
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P - Control
PI - Control
PID - Control
K KC U 0 5.
K K
T PC U
U
0 45
12
.
.int
K K
T P
T P
C U
U
derU
0 6
2
8
.
int
Calculation of Tuning Constants
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First Order Lag
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First Order Lag Response Curve
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Variation on continuous cycling ....to avoid danger to process ...but damped oscillations are acceptablerequires some subjective evaluation
Damped Cycling Method
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Damped Oscillation Decay Ratio
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1. Put controller in P - control2. P - control on ERR = (SP - PV) 3. Controller into Auto4. Double K until oscillations.5. Terminate immediately6. Calculate tuning constants
Steps of Damped Cycling Method
Estimate damping of oscillationConvert P into Pu using diagram
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Convert P into Pu using diagram
Estimate damping of oscillation
(d) ... 0.5 acceptable
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PI - Control
PID - Control
T
P Rd dint .
12
R Ppd
u
T
P R
TP R
d d
derd d
int
2
8
K determined by good judgement.....
Calculate Tuning Constants
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Tuning for NO Overshoot on Start-up
Variation of continuous cycling method
When no overshoot even for start-up is permitted (manual auto)
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Same as for continuous cyclingFormula for PID – control:
K K
T P
T P
C U
U
derU
0 2
3
2
.
int
Formula for NO Overshoot on Start-up
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Variation of continuous cycling method
No overshoot during normal modulating controlonly
Tuning for SOME Overshoot on Start-up
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Same as for continuous cyclingFormula for PID - control
K K
T P
T P
C U
U
derU
033
2
3
.
int
Formula for Some Overshoot on Start-up
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Cascade Control
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Single Loop Temperature Control
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Two Controller Basic Cascade Control
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Tuning of Cascade Loops (cont…)
• Cascade Control - Secondary Controllers– Mainly flow controller– No D - control as mainly stable– K < 1
• Cascade Control - Primary Controllers– Mainly PID controller– Careful stability considerations
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Long Process Deadtime in Closed Loop Control & The
Smith Predictor
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Process Deadtime
• Difficult problem to overcome long deadtime in feedback control loop
• Especially when deadtime more than 20% of total time taken for PV to settle to its new value after last setpoint change
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Example of Process Deadtime
Illustration of a long conveyor system giving an excessive deadtime to the control loop
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Overcoming Process Deadtime
• Depends on operating requirements of process
• Easiest solution: de-tune controller to slower response rate so that controller will not over compensate unless deadtime excessively long
• Integral mode of controller very sensitive to deadtime since during period of PV inactivity, integrator ramps output
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Level ControlMaintain flow as constant
as possible to downstream equipment
Medium response
Tune as slowly as possible without overfilling or emptyingthe tank
Choice of Tuning Constant (1)
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Flow ControlMaintain constant flowAim for fast responseSmall process lag in systemTune for fast as possible
response
Choice of Tuning Constants (2)
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Temperature ControlMaintain a steady temperatureMajor process lag and dead time in the systemTune as quickly as possible without causing
oscillations
TCPV
SP
T2
O utlet
Inlet
Choice of Tuning Constants (3)
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Thank You For Your Interest
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