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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 loop’s Output = Another’s 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 LIC’s 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 Cause : Improper Tuning!
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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
è Steam Flowè 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 RRT’sdiffer by a factor of AT LEAST 3-5 X.