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10 Tips for:
Tuning of PID Loops
By
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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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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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
%
( )= × ×
= ×
09
3
. %
int
OP
N L
T L
( )= × ×
= ×
= ×
12
2
05
. %
.
int
OP
N L
T L
T 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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1. Put controller in P - control only
2. P - control on ERR = (SP - PV)
3. Controller in Auto
4. Step change to SP
5. Double value of K until continuous cycling
6. Calculate tuning constants
Continuous Cycling Method
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P - Control
PI - Control
PID - Control
K KC U= ×05.
K K
T
P
C U
U
= ×
=
0 45
12
.
.int
K K
T
P
T
P
C U
U
der
U
= ×
=
=
0 6
2
8
.
int
Calculation of Tuning Constants
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Variation on continuous cycling ....
to avoid danger to process ...
but damped oscillations are acceptable
requires some subjective evaluation
Damped Cycling Method
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1. Put controller in P - control
2. P - control on ERR = (SP - PV)
3. Controller into Auto
4. Double K until oscillations.
5. Terminate immediately
6. Calculate tuning constants
Steps of Damped Cycling Method
Estimate damping of oscillation
Convert 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 d
int .=
×
12
R
P
pd
u
=
( )
( )
T
P R
T
P R
d d
der
d 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 cycling
Formula for PID – control:
K K
T
P
T
P
C U
U
der
U
= ×
=
=
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 control
only
Tuning for SOME Overshoot on Start-up
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Same as for continuous cycling
Formula for PID - control
K K
T
P
T
P
C U
U
der
U
= ×
=
=
033
2
3
.
int
Formula for Some Overshoot on Start-up
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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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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 Control
Maintain flow as constant
as possible to
downstream
equipment
Medium response
Tune as slowly as possible without overfilling or emptying
the tank
Choice of Tuning Constant (1)
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Flow Control
Maintain constant flow
Aim for fast response
Small process lag in system
Tune for fast as possible response
Choice of Tuning Constants (2)
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Temperature Control
Maintain a steady temperature
Major process lag and dead time in the system
Tune as quickly as possible without causing oscillations
TC
PV
SP
T2
O utlet
Inlet
Choice of Tuning Constants (3)
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Thank You For Your Interest
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