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OPTIMIZATION l TRAINING l CONSULTING

How to Tune Interacting Loops

George Buckbee, P.E.

Which Loops Interact?

F Pressure/Flow NetworksF Cascade LoopsF Related via the ProcessF Many others...

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Finding the Source ofOscillation

F Sine wave - Usually TuningF Square wave PV, Triangle Wave OP =

StictionF Others - Process/mechanical, etc.F Use of Power Spectrum Analysis

What is Cascade Control?

F One loops Output = Anothers Setpoint

PVInnerPID

OuterPID

InnerProcess

OuterProcess

SP

PV

OP -> SP

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Why Use Cascade?

F One loop focused on fast variationF Other Loop maintains key process

parameter

InnerPID

OuterPID

InnerProcess

OuterProcess

SP

PV

OP -> SP

Simple loop without cascade

LYLY

LTLT

LICLIC

A major disturbance in the pressureline will be sensed on the level before

the level controller could react.

A slow varying level loop may not beable to recover fast enough from adisturbance in the pressure line.

The relationship between flow outputfrom the valve and controller output

may not be linear.

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Loop with cascade

LTLT

LICLIC

FICFIC

FTFT

SPSP

With cascade arrangement a majorand/or fast disturbance is corrected by

a faster loop before it affects themain(slower) process loop !

Valve non-linearities are taken careof! Viewed from LICs output we now

have an almost perfect valve.

Typical Cascade ControlProblems

F OscillationF Inner Loop Never SettlesF Poor ResponseF Does not work well during transients

The Cau

se : Imp

roper Tu

ning!

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Tuning of Cascade Loops

F Tune Inner Loop AT LEAST 3-5 X faster.F Separates the control reactionF Reduces the interaction & oscillationF If outer loop is fast, do not do cascade.

Process for Tuning CascadeLoops

F 1. Both Loops in ManualF 2. Stiction, Hysteresis, Non-Linearity

checks for Inner Loop only.F 3. Bump& Tune Inner Loop, Record RRTF 4. With New Tuning, Inner Loop in Auto.

Bump Outer Loop (SP Change to Inner)F 5. Tune outer RRT AT LEAST 3-5X SlowerF 6. Both in Auto, Verify with SP Bump on

outer loop.

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Ratio Control

F Mixing ApplicationsF ReactionsF BurnersF Boilers

Ratio Control : MixingApplication

40% Blue 60% Yellow

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Tuning :

F All Inner (Flow) Loops tuned to sameClosed-Loop Response Time.

F Outer (Level, Master Flow, etc.) 3-5XSlower than the inner loops.

Process For Tuning

F Bump each inner loop, in Manual.F Determine the slowest response.F Tune the slowest loop aggressively.F Tune all other inner loops at the same

response time.F Tune the outer loop 3-5X Slower RRT.

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Ratio Control : Burners,Reactions

F Choose key control loop to be themaster.

F Ratio SP of other loops to PV of themaster.

F Inner loop should be fastF Outer loop 3-5X Slower.

Boiler Control :More Techniques

F Cross-Limiting of Air & FuelF Excess AirF Two-Element Feedwater Control

Steam Flow Drum Level

F Asymmetrical TuningF Again, Tune for Non-Interaction

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Other Interacting Loops

F Supply HeadersF Position & Speed ControlF Coarse & Fine ControlF Mid-Ranging Control

Summary

F When Bumping Loops, all related loopsin Manual.

F Tune Inner Loop first. Inner Loop is theFaster Loop.

F Tune Interacting Loops such that RRTsdiffer by a factor of AT LEAST 3-5 X.