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Chemometric and signal processing methods for real time monitoring and modeling: applications in the pulp and paper industry
KTH, School of Chemical Science and Engineering (CHE), Chemistry.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

In the production of paper, the quality of the pulp is an important factor both for the productivity and for the final quality. Reliable real-time measurements of pulp quality are therefore needed. One way is to use acoustic or vibration sensors that give information-rich signals and place the sensors at suitable locations in a pulp production line. However, these sensors are not selective for the pulp properties of interest. Therefore, advanced signal processing and multivariate calibration are essential tools. The current work has been focused on the development of calibration routes for extraction of information from acoustic sensors and on signal processing algorithms for enhancing the information-selectivity for a specific pulp property or class of properties. Multivariate analysis methods like Principal Components Analysis (PCA), Partial Least Squares (PLS) and Orthogonal Signal Correction (OSC) have been used for visualization and calibration. Signal processing methods like Fast Fourier Transform (FFT), Fast Wavelet Transform (FWT) and Continuous Wavelet Transform (CWT) have been used in the development of novel signal processing algorithms for extraction of information from vibrationacoustic sensors.

It is shown that use of OSC combined with PLS for prediction of Canadian Standard Freeness (CSF) using FFT-spectra produced from vibration data on a Thermo Mechanical Pulping (TMP) process gives lower prediction errors and a more parsimonious model than PLS alone. The combination of FFT and PLS was also used for monitoring of beating of kraft pulp and for screen monitoring. When using regular FFT-spectra on process acoustic data the obtained information tend to overlap. To circumvent this two new signal processing methods were developed: Wavelet Transform Multi Resolution Spectra (WT-MRS) and Continuous Wavelet Transform Fibre Length Extraction (CWT-FLE). Applying WT-MRS gave PLS-models that were more parsimonious with lower prediction error for CSF than using regular FFT-Spectra. For a Medium Consistency (MC) pulp stream WT-MRS gave predictions errors comparable to the reference methods for CSF and Brightness. The CWT-FLE method was validated against a commercial fibre length analyzer and good agreement was obtained. The CWT-FLE-curves could therefore be used instead of other fibre distribution curves for process control. Further, the CWT-FLE curves were used for PLS modelling of tensile strength and optical parameters with good results.

In addition to the mentioned results a comprehensive overview of technologies used with acoustic sensors and related applications has been performed.

Abstract [sv]

Vid framställning av pappersprodukter är kvaliteten på massan en viktig faktor för produktiviteten och kvalitén på slutresultatet. Det är därför viktigt att ha tillgång till tillförlitliga mätningar av massakvalitet i realtid. En möjlighet är att använda akustik- eller vibrationssensorer i lämpliga positioner vid enhetsoperationer i massaprocessen. Selektiviteten hos dessa mätningar är emellertid relativt låg i synnerhet om mätningarna är passiva. Därför krävs avancerad signalbehandling och multivariat kalibrering. Det nu presenterade arbetet har varit fokuserat på kalibreringsmetoder för extraktion av information ur akustiska mätningar samt på algoritmer för signalbehandling som kan ge förbättrad informationsselektivitet. Multivariata metoder som Principal Component Analysis (PCA), Partial Least Squares (PLS) and Orthogonal Signal Correction (OSC) har använts för visualisering och kalibrering. Signalbehandlingsmetoderna Fast Fourier Transform (FFT), Fast Wavelet Transform (FWT) och Continuous Wavelet Transform (CWT) har använts i utvecklingen av nydanande metoder för signalbehandling anpassade till att extrahera information ur signaler från vibrations/akustiska sensorer.

En kombination av OSC och PLS applicerade på FFT-spektra från raffineringen i en Termo Mechnaical Pulping (TMP) process ger lägre prediktionsfel för Canadian Standard Freeness (CSF) än enbart PLS. Kombinationen av FFT och PLS har vidare använts för monitorering av malning av sulfatmassa och monitorering av silning. Ordinära FFT-spektra av t.ex. vibrationssignaler är delvis överlappande. För att komma runt detta har två signalbehandlingsmetoder utvecklats, Wavelet Transform Multi Resolution Spectra (WT-MRS) baserat på kombinationen av FWT och FFT samt Continuous Wavelet Transform Fibre Length Extraction (CWT-FLE) baserat på CWT. Tillämpning av WT-MRS gav enklare PLS-modeller med lägre prediktionsfel för CSF jämfört med att använda normala FFT-spektra. I en annan tillämpning på en massaström med relativt hög koncentration (Medium Consistency, MC) kunde prediktioner för CSF samt ljushet erhållas med prediktionsfel jämförbart med referensmetodernas fel. Metoden CWT-FLE validerades mot en kommersiell fiberlängdsmätare med god överensstämmelse. CWT-FLE-kurvorna skulle därför kunna användas i stället för andra fiberdistributionskurvor för processtyrning. Vidare användes CWT-FLE kurvor för PLS modellering av dragstyrka samt optiska egenskaper med goda resultat.

Utöver de nämnda resultaten har en omfattande litteratursammanställning gjorts över området och relaterade applikationer.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , xv, 113 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:33
Keyword [en]
Acoustic, Vibration, Multivariate, Chemometrics, PLS, OSC, PCA, FFT, Wavelets, WT-MRS, CWT-FLE, Pulp Quality, CSF, Brightness, Tensile Strength properties, Optical Properties, Measurement Technology, On-line, Non-invasive, Process Control, Process Analysis, Process Analytical Technology, PAT.
Keyword [sv]
Akustik, Vibrationer, Multivariat, Kemometri, PLS, OSC, PCA, FFT, Wavelets, WT-MRS, CWT-FLE, Massakvalitet, CSF, Ljushet, Dragstyrkeegenskaper, Optiska egenskaper, Mätteknik, On-line, Icke berörande mätning, Processtyrning, Processanalys, Processanalytisk Teknologi, PAT.
National Category
Analytical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4383ISBN: 978-91-7178-679-1 (print)OAI: oai:DiVA.org:kth-4383DiVA: diva2:12066
Public defence
2007-06-01, Sal K1, Teknikringen 56, Stockholm, 13:00
Opponent
Supervisors
Note
QC 20100629Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-06-29Bibliographically approved
List of papers
1. Applied Real-Time Acoustic Chemometrics for Measurements of Canadian Standard Freeness
Open this publication in new window or tab >>Applied Real-Time Acoustic Chemometrics for Measurements of Canadian Standard Freeness
2005 (English)In: Paperi ja puu, ISSN 0031-1243, Vol. 87, no 7, 452-457 p.Article in journal (Refereed) Published
Abstract [en]

In this paper we present a method for measuring Canadian Standard Freeness (CSF) on-line in real-time using acoustic measurements. The method can be used to present a new CSF value every second. The higher measurement rate and the reduced time-delay compared to conventional pulp sensors make it a suitable tool for improved refiner control.

The method was developed during full-scale trials on two separate Sunds Defibrator double disc refiners at SCA Ortviken, Sweden. Acoustic measurements were performed using an accelerometer affixed to the refiner blow line. Trial conditions have been varied over a wide range of pulp quality and disturbances in the vibration signal. CSF was for example varied in the range 62-453 ml and refiner-housing pressure was varied from 0,35 to 0,39 MPa.

The vibration signals collected during I second were treated to yield power spectra that were used for all further work. A median spectrum was calculated for each experimental point and these spectra were combined with reference values for freeness to make up the calibration matrices. The spectra were pre-treated using Orthogonal Signal Correction (OSC) and Subsequently Partial Least Squares Regression (PLSR) was used to build models between spectra and freeness. For model validation leave-one-out cross validation has been used.

Over the range 90-250 ml we can predict CSF from median power spectra with a Root Mean Square Error of Prediction (RMSEP) of 23 ml. Over the larger CSF range of 62-453 mt the RMSEP deteriorates to 39 mt. Data from two separate trials on different refiners and from different seasons were included in these models.

Keyword
pulp quality control; CSF; TMP; process sensor; sound; acoustics; chemometrics; digital signal processing
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7140 (URN)000233046600010 ()2-s2.0-27744580387 (Scopus ID)
Note
QC 20100629Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved
2. Pulp Quality And Performance Indicators For Pressure Screens Based On Process Acoustic Measurements
Open this publication in new window or tab >>Pulp Quality And Performance Indicators For Pressure Screens Based On Process Acoustic Measurements
(English)Manuscript (Other academic)
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7141 (URN)
Note
QC 20100629Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-06-29Bibliographically approved
3. Spectra of wavelet scale coefficients from process Acoustic Measurements as input for PLS modelling of pulp quality
Open this publication in new window or tab >>Spectra of wavelet scale coefficients from process Acoustic Measurements as input for PLS modelling of pulp quality
2002 (English)In: Journal of Chemometrics, ISSN 0886-9383, E-ISSN 1099-128X, Vol. 16, no 8-10, 521-528 p.Article in journal (Refereed) Published
Abstract [en]

Acoustic and vibration signals are captured by simple standard accelerometers. These can often be mounted directly on operative process equipment, creating a completely non-invasive measurement system. The signals from the accelerometer are then amplified, digitized by an analogue-to-digital converter and stored in some suitable format in a PC. The method most often used for signal processing of acoustic data has been to apply variants of fast Fourier transform (FFT) on sampled data to produce a frequency domain representation. An alternative way tried here is to use the fast wavelet transform (FWT) in combination with FFT. The FWT has the advantage that it produces time-resolved representations and, on each time scale, different features can be extracted. However, in this case, time resolution has no meaning, since the starting points for data acquisitions were not fixed. The wavelet step can be seen as a series of pre-filters and it is here followed by FFT on coefficients at each wavelet scale. The results are compared to those obtained after FFT on the complete time series. We have used spectra of wavelet scale coefficients in an attempt to model pulp quality with PLS. In this case the number of points in the resulting wavelet multiresolution spectrum (WT-MRS) can be limited to a low number, e.g. 255 compared to 1025 with direct FFT on the time series. In the PLS modelling step the advantage is that the first two components describe Y much better than when using the conventional approach, e.g. 72% explained Y variance compared to 40%. A second advantage is that the model requires fewer coefficients.

Keyword
acoustics; wavelets; FFT; PLS; pulp; wavelet multiresolution spectra
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7142 (URN)10.1002/cem.731 (DOI)000177761200017 ()
Note
QC 20100629Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-12-03Bibliographically approved
4. Predicting Pulp Quality from Process Acoustic Measurements on a Medium Concistency Pulp Stream
Open this publication in new window or tab >>Predicting Pulp Quality from Process Acoustic Measurements on a Medium Concistency Pulp Stream
2006 (English)In: JPAC, Journal of Process Analytical Chemistry., ISSN 1077-419X, Vol. 10, no 1, 1-5 p.Article in journal (Refereed) Published
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7143 (URN)
Note
QC 20100629Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2010-06-29Bibliographically approved
5. Extraction of Distribution Curves from Process Acoustic Measurements on a TMP-process
Open this publication in new window or tab >>Extraction of Distribution Curves from Process Acoustic Measurements on a TMP-process
2004 (English)In: Pulp & paper Canada, ISSN 0316-4004, Vol. 105, no 11, 42-46 p.Article in journal (Refereed) Published
Abstract [en]

We have developed a method to extract fibre length distributions from acoustic data recorded on pulp streaming from a TMP refiner. The method is based on the use of a Continuous Wavelet Transform, CWT. Measurements were performed using an accelerometer on the refiner blow-line during a full-scale trial with a double disc refiner. During trials conditions were varied over a wide range. The agreement is good between results from our method, CWT-FLE and from FiberMaster.

Keyword
TMP; fibre length distributions; vibration; acoustic; process measurements; continuous wavelet transformation; continuous wavelet; transform fibre length extraction; CWT-FLE; PCA
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7144 (URN)000225500800011 ()2-s2.0-10944271863 (Scopus ID)
Note
QC 20100629 QC 20110914Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2011-09-14Bibliographically approved
6. Modeling of pulp quality parameters from distribution curves extracted from process acoustic measurements on a thermo mechanical pulp (TMP) process
Open this publication in new window or tab >>Modeling of pulp quality parameters from distribution curves extracted from process acoustic measurements on a thermo mechanical pulp (TMP) process
2007 (English)In: Chemometrics and Intelligent Laboratory Systems, ISSN 0169-7439, E-ISSN 1873-3239, Vol. 85, no 1, 63-69 p.Article in journal (Refereed) Published
Abstract [en]

In this paper the feasibility of modeling strength and optical pulp properties from length distribution curves extracted from acoustic data using continuous wavelet transform-fiber length extraction, CWT-FLE (A Björk and L-G Danielsson, 'Extraction of Distribution Curves from Process Acoustic Measurements on a TMP-Process', Pulp and Paper Canada 105 No. 11 (2004), T260-T264) by use of Partial Least Squares (PLS) have been tested. The curves used have earlier been validated against length distribution curves obtained by analyzing pulp samples with a commercial analyzer (FiberMaster). The curves were extracted from acoustic data without any "calibration" against fiber length analyses. The acoustic measurements were performed using an accelerometer affixed to the refiner blow-line during a full-scale trial with a Sunds Defibrator double disc refiner at SCA Ortviken, Sweden. Pulp samples were collected concurrently with the acoustic measurements and extensive physical testing has been made on these samples. For each trial point three pulp samples were collected. PLS1 and PLS2 models were successfully made linking the distribution curves obtained using CWT-FLE to pulp tensile strength properties as well as optical properties. The resulting Root Mean Square Error of Prediction (RMSEP) for all parameters is comparable to what can be obtained by pooling the standard deviations of reference measurements from the different trial points. The results obtained are compared to FiberMaster data modeled in the same fashion, yielding lower prediction errors than the CWT-FLE data. However, this can be partly due to the five-year storage of pulp samples between pulp sampling/acoustic measurement and FiberMaster analyses/sheet testing. The acoustic method is fast and produces results without dead time and could constitute a new tool for improving process control and optimizing the fiber characteristics in a specific process and for a specific purpose. The technique could be implemented in a PC-environment at a fairly low cost.

Keyword
Acoustic, Continuous wavelet transform, CWT-FLE, Fiber length, On-line, Pulp quality
National Category
Analytical Chemistry
Identifiers
urn:nbn:se:kth:diva-7145 (URN)10.1016/j.chemolab.2006.04.007 (DOI)000243628800007 ()2-s2.0-33845581462 (Scopus ID)
Note

QC 20100629

Available from: 2007-05-16 Created: 2007-05-16 Last updated: 2014-11-12Bibliographically approved

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