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Control Performance Monitoring. Alf Isaksson, Alexander Horch ABB Corporate Research. PROST Seminar 22 January 200 2. Goal: detect and diagnose malfunctioning control loops. oscillation. or too high variance. Bad control manifests itself as. Methods needed to. detect oscillations

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control performance monitoring

Control Performance Monitoring

Alf Isaksson, Alexander Horch

ABB Corporate Research

PROST Seminar22 January 2002

methods needed to
Methods needed to
  • detect oscillations
  • diagnose oscillations
  • determine of variance is too large

Since there are hundreds of loops methods should beautomatic

oscillation detection
Oscillation detection
  • Hägglund (1995). Consider areas between zero crossings (count if large enough).
  • Stattin and Forsman (1998). Based on same idea, easier to use.
  • Seborg and Miao (1999). Damping ratio of auto-correlation function.
oscillation index
Oscillation index

0 = no oscillation, 1 = perfect osc.

0.88

0.25

Controller re-tuned

slide8

Valve IP converter replaced

Oscillation index trend plot

index

days

major advantage correlation analysis

1

0.8

0.6

0.4

0.2

0

0

0.2

0.4

0.6

0.8

1

Major advantage: correlation analysis

Conclusion:

The loops interact. One of them is likely to cause both oscillations

oscillation loop 2

oscillation loop 1

potential causes are

F

FC

Potential causes are...

cycling load

static friction

tight tuning

if the cause is stiction

process output

cross-correlation

control signal

If the cause is stiction...
slide13

Stiction diagnosis

  • New method by Horch (1999) which utilizes that
    • when stictionin valve, process variable and control signal have odd cross-correlation
    • when”not stiction” the signals are such that the cross-correlation is even (due to negative feedback)
example two coupled loops

QC

Q

F

FC

Example: two coupled loops

Stiction

water

pulp

O.K.

example cont d

Diagnosis:

stiction

no stiction

Example cont’d

concentration loop

flow loop

data

cross-corr.

important assumptions
Important assumptions
  • O

Self-regulating process

Oscillation detected

Cross-correlation method O.K.

Integral action

No compressible media

example ii integrating plant

stiction

Example II: integrating plant

two different level control loops

no stiction

no stiction

ccf method useless for integrating plants
CCF-method useless for integrating plants!

Integration destroys the specific correlation in the stiction case.

CCF is even, no matter if stiction or not.

Re-calculation (differentiation) does not solve the problem

level control loop

slide19

...

...

‘Second derivative is infinite’

Idea!

Look for discontinuities in the data!

1 differentiate the process output

Y

dy

dt

d2y

dt2

stiction

no stiction

1.) Differentiate the process output!
3a histogram ideally

d2y

d2y

dt2

dt2

stiction

3a.) Histogram (ideally)

no stiction

3b histogram noise filter

d2y

d2y

dt2

dt2

stiction

3b.) Histogram (noise & filter)

no stiction

level control with stiction

d2y

dt2

Level control with stiction

y(t)

stiction

MSE: 0.97 2.01

level control without stiction

d2y

dt2

Level control without stiction

y(t)

no stiction

MSE: 1.17 0.46

detect too large variance too large 2 sigma
Detect too large variance (too large 2-sigma)

Basic problem:

-2σ

Is this good or bad?

performance index
Performance index
  • Introduce a control performance measure:
  • Possible to calculate denominator from normal operating data given knowledge of process time delay (deadtime).
  • Proposed by Harris (1989).
  • Modification presented in Horch and Isaksson (1999)

Current variance

Ip =

Theoretically opt variance

slide28

Modified Index:

Before:

2.11

1.07

After:

commercial tools suppliers

LoopMD

ABB

'LATTS'

KCL-CoPA

LoopAnalyst

Commercial tools / suppliers ...

PROTUNER™

latts loop auditing and tuning tool suite
LATTS – Loop Auditing and Tuning Tool Suite
  • Process model identification
  • PID controller tuning
  • Loop auditing

Part of ABB Industrial IT concept and uses the new Aspect Integrator Platform (AIP).

Consists of three Aspects:

auditing aspect
Auditing Aspect
  • Computes 21 different quantities/indices. For example:
    • Control error standard deviation
    • Oscillation index
    • Stiction diagnosis (correlation)
    • Stiction diagnosis (histogram)
    • Modified Harris index
auditing aspect cont d
Auditing Aspect cont’d
  • Combines these indices to test a number of hypotheses, such as
    • Acceptable performance
    • Possible valve problem
    • Sluggish tuning

The result is summarized in a report, either as a text file or in Internet Explorer

conclusions
Conclusions
  • Methods exist for non-invasive
    • Oscillation detection
    • Stiction diagnosis
    • Minimum variance benchmark
  • New ABB Product LATTS under Beta testing right now. Product release approximately June 2002.
future work industrial as well as academic
Future work (industrial as well as academic)
  • detection and diagnosis of mill-wide oscillations
  • distinction of linearly and non-linearly caused oscillations
  • performance assessment based on full process model (event-triggered estimation)
  • application of multivariable performance index
  • performance monitoring of MPC loops