10 tips for tuning of pid looops

Technology Training that Workswww.idc-online.com/slideshare

10 Tips for:

Tuning of PID LoopsBy

Steve Mackay

www.eit.edu.au Technology Training that Workswww.idc-online.com/slideshare

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This 45 minute session will commence shortly!

Objectives

• Concept of tuning • Ziegler Nichols classical rules • Practical closed and open loop strategies • Cascade loops • Dead time issues

Concept of Tuning

1 - Minimize Integral of error

Objectives of Tuning (1)

2 - Minimize Amplitude of error squared

Objectives of Tuning (2)

3 - Minimize wear and tear

4 - No overshoot at Start Up

5 - Minimize effect of known disturbances

Other Aspects of Tuning (3,4,5)

Ziegler Nichols and others

Loop Controller and Process

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)

P - Control

PI - Control

PID - Control

T LT Lder

Reaction Curve Calculations

• 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

P-only for Continuous Cycling Method

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

P - Control

PI - Control

PID - Control

K KC U 0 5.

T PC U

Calculation of Tuning Constants

First Order Lag

First Order Lag Response Curve

Variation on continuous cycling ....to avoid danger to process ...but damped oscillations are acceptablerequires some subjective evaluation

Damped Cycling Method

Damped Oscillation Decay Ratio

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

Convert P into Pu using diagram

Estimate damping of oscillation

(d) ... 0.5 acceptable

PI - Control

PID - Control

P Rd dint .

derd d

K determined by good judgement.....

Calculate Tuning Constants

Tuning for NO Overshoot on Start-up

Variation of continuous cycling method

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

Same as for continuous cyclingFormula for PID – control:

Formula for NO Overshoot on Start-up

Variation of continuous cycling method

No overshoot during normal modulating controlonly

Tuning for SOME Overshoot on Start-up

Same as for continuous cyclingFormula for PID - control

Formula for Some Overshoot on Start-up

Cascade Control

Single Loop Temperature Control

Two Controller Basic Cascade Control

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

Long Process Deadtime in Closed Loop Control & The

Smith Predictor

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

Example of Process Deadtime

Illustration of a long conveyor system giving an excessive deadtime to the control loop

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

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)

Flow ControlMaintain constant flowAim for fast responseSmall process lag in systemTune for fast as possible

response

Choice of Tuning Constants (2)

Temperature ControlMaintain a steady temperatureMajor process lag and dead time in the systemTune as quickly as possible without causing

oscillations

O utlet

Choice of Tuning Constants (3)

Thank You For Your Interest

If you are interested in further training, please visit:

IDC Technologies Short Courses:Two-day practical courses available to the public:

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