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Limitation of a line-of-light online paper surface measurement system
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
Mid Sweden University, Faculty of Science, Technology and Media, Department of Electronics Design.
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2014 (English)In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 14, no 8, 2715-2724 p.Article in journal (Refereed) [Artistic work] Published
Abstract [en]

A new prototype device has been developed based on a laser triangulation principle to measure online surface topography in the paper and paperboard industries. It characterizes the surface in a wide spatial scale of topography from 0.09-10 mm. The prototype's technique projects a narrow line-of-light perpendicularly onto the moving paper-Web surface and scattered reflected light is collected at a low angle, low specular, and reduced coherent length onto the CCD sensors synchronized with the laser sources. The scattering phenomenon determines surface deviations in the z-direction. The full-width, at half-maximum of a laser line in cross section is sensitive in computation of the surface topography. The signal processing aspect of the image processing, for example, threshold and filtering algorithms are also sensitive in estimating the accurate surface features. Moreover, improper light illumination, intensity, reflection, occlusion, surface motion, and noise in the imaging sensor, and so forth, all contribute to deteriorate the measurements. Optical techniques measure the surface indirectly and, in general, an evaluation of the performance and the limitations of the technique are both essential and challenging. The paper describes the accuracy, uncertainty, and limitations of the developed technique in the raw profiles and in terms of the rms roughness. The achieved image subpixel resolution is 0.01 times a pixel. Statistically estimated uncertainty (2σ) in the laboratory environment was found 0.05 μm for a smooth sample, which provides a 95% confidence level in the rms roughness results. The depth of field of the prototype is ~2.4 mm.

Place, publisher, year, edition, pages
2014. Vol. 14, no 8, 2715-2724 p.
Keyword [en]
accuracy, calibration, Laser triangulation, limitation, online surface measurement, optical profiler, paper-web topography, uncertainty
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:miun:diva-20448DOI: 10.1109/JSEN.2014.2314753ISI: 000340103500004Scopus ID: 2-s2.0-84904362184Local ID: STCOAI: oai:DiVA.org:miun-20448DiVA: diva2:675722
Note

Correction to this article published: 

IEEE SENSORS JOURNAL, 14 (10):3726-3726; 10.1109/JSEN.2014.2345891 OCT 2014 

Scopus: 2-s2.0-84961153977

Available from: 2013-12-04 Created: 2013-12-04 Last updated: 2017-07-04Bibliographically approved
In thesis
1. Online optical method for real-time surface measurement using line-of-light triangulation
Open this publication in new window or tab >>Online optical method for real-time surface measurement using line-of-light triangulation
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Real time paper surface-web measurement is one of the challenging research fields. The traditional laboratory method has many limitations and is unable to measure the entire tambour during the manufacturing process. It has been necessary to develop an online technique that could measure the surface topography in real time. An optical technique was developed, based on laser triangulation, and is applied to develop a new prototype device, which characterizes high speed paper-web surfaces over a wide scale of spatial wavelengths spectrum and computes the surface roughness in real time. The used multi channel pulsed laser diode, source of illumination onto the paper-web, is of benefit due to its low coherence length and is capable to deliver a powerful burst of light beam over a 1 µs duration, which delivers energy of 100 µJ per pulse. The short exposure time avoids blurriness in the acquired images which could possible due to the high speed and vibrations on the paper-web.

The laser beam is shaped into a narrow line-of-light using cylindrical lenses and is projected onto a paper-web surface, which covers a physical length of about 210 mm. The created line-of-light cross section full width at half maximum, FWHM Gaussian distribution, is 2-3 pixels on the image. The line-of-light is projected onto the paper-web perpendicular to the plane of the surface. The low angled, low specular, reduced coherence length, scattered reflected laser line is captured by the 3 CCD sensors, which are synchronized with the laser source. The low specular light ensures to avoid saturation of the imaging sensors if the surface is very smooth, and obliquely captures the z-directional fine feature of the surface.

The scattered phenomenon of the reflected light is responsible for the surface irregularity measurements. The basic image processing algorithm is applied in order to remove noise and cropped the images widthwise so that only pixels above a preset threshold gray level can be processed, which enables efficient real time measurement. The image is transformed into a 1D array using the center of gravity, COG. The accuracy and precision of the COG depends on the line-of-light FWHM, which, in turn, is responsible for the accuracy, noise and the resolution of the developed technique. The image subpixel resolution achieved is 0.01 times a pixel and uuncertainty in the raw data is 0.43 µm while it is 0.05 µm in the rms roughness.

The signal processing steps combining the B-Spline filter and the filter in the spatial frequency domain were employed in order to separate roughness, waviness, and form and position error in the raw profile. The prototype is designed to measure online surface roughness and to characterize surface in a spatial wavelength spectrum from 0.09 to 30 mm, which is extendable to any required spatial range in order to cover a wide scale surface feature such as micro roughness, macro roughness and waviness. It is proven that exploitation of a simple laser triangulation technique could lead to an improvement in the overall quality and efficiency in the paper and paperboard industries and it can also be of potential interest for the other surface characterization problems.

Place, publisher, year, edition, pages
Sweden Sundsvall: Mid Sweden University, 2013. 151 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 171
National Category
Engineering and Technology
Identifiers
urn:nbn:se:miun:diva-20449 (URN)STC (Local ID)978-91-87557-17-0 (ISBN)STC (Archive number)STC (OAI)
Public defence
2013-12-16, L111, Mittuniversitetet, Sundsvall, 13:15 (English)
Supervisors
Available from: 2013-12-06 Created: 2013-12-04 Last updated: 2016-10-19Bibliographically approved

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