Technology Training that works

Tuning of PID Controllers in Tuning of PID Controllers in Both Open and Closed Loop Both Open and Closed Loop

Control SystemsControl Systems

Technology Training that works

z Why Tune ?z Open loop tuning procedurez Closed loop tuning procedurez Damped oscillation tuning methodz Other tuning conditionsz Fine tuning of practical control loops

Tuning of Closed Loop Control SystemsTuning of Closed Loop Control Systems

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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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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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Open Loop Tuning Method (2)Open Loop Tuning Method (2)

Technology Training that works

P - Control

PI - Control

PID - Control

( )= OP N L%

( )= = 09

3

. %

int

OPN L

T L

( )= = =

12

205

. %

.int

OPN L

T LT Lder

KC

KC

KC

Reaction Curve Calculations

Technology Training that works

Single Loop control with Real PID

Controller mode to manual (F3)

Establish PVE steady at 50% (1000)

Step output from 50% to 65%

Determine L-lag and N-slope values

Open Loop Tuning - Exercise #7

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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

Technology Training that works

Reaction Curve Method (Ziegler & Nichols)

If we know the exact value of the effective lag

of an industrial process, then we are in the position

to find the frequency matching with the 180 phase

shift (and thence optimum tuning constants).

Technology Training that works

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

Technology Training that works

P - Control

PI - Control

PID - Control

K KC U= 05.

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 LagFirst Order Lag

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First Order Lag Response CurveFirst Order Lag Response Curve

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Single Loop control with Real PID

Control to P-only - Tint = 999, Tder = 0

Adjust gain (K) for continuos oscillation

Step SPE from 1000 > 1100

Record gain-K and period for calculations

Closed Loop Tuning - Exercise #8

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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 RatioDamped 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 Puusing 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=

( )( )

TP R

TP R

d d

derd d

int =

= 2

8

K determined by good judgement.....

Calculate Tuning Constants

Technology Training that works

Variation of continuous cycling method

When no overshoot even for start-up is permitted (manual auto)

Tuning for NO 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

= ==

0 2

3

2

.

int

Formula for NO Overshoot on Start-up

Technology Training that works

Variation of continuous cycling method

No overshoot during normal modulating control only

Tuning for SOME Overshoot on Start-up

Technology Training that works

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

Technology Training that works

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)