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Proceedings Paper

Low-noise moisture meter with high-speed LED techniques
Author(s): Rami Aikio; Hannu Lindström; Pekka Suopajärvi; Jouko Malinen; Markku Mäntylä
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Paper Abstract

Near infrared (NIR) spectroscopy can provide inexpensive, rapid and contact-free chemical content measurements for on-line, hand-held and laboratory applications. Traditionally multiwavelength NIR analyzers are based on incandescent lamp light sources with rotating filter wheels, even though designs relying on lamp technology and moving parts mean larger size, require frequent maintenance and eventually limit measurement speed of the system. Today, optical power and available wavelength range of LEDs enable their use in chemical content analyzers. In this publication, a paper moisture meter with high speed LED techniques is presented. A prototype developed at VTT utilizes an extended InGaAs detector to measure diffuse reflection at four NIR wavelengths ranging from 1.2 to 2.1 μm. Source LED currents are amplitude modulated with fixed sinusoidal frequencies. Optical signals at each wavelength are demodulated from the detector signal using real-time digital lock-in detection method on an FPGA. Moisture content is calculated and displayed on the embedded platform. The design allows very high speed operation, where the result is updated every 1 ms. Performance of the prototype system was studied by measuring a set of known sealed paper samples. Paper moisture measurement accuracy was 0.14, repeatability 0.01 and 2σ noise 0.04 moisture percent. Laboratory tests showed that channel crosstalk after detection is below background noise level. The measured signal-to-noise ratios per channel were 70 - 85 dB when all LEDs were on. The overall performance equals the level of incandescent lamp based on-line moisture meters currently in use in paper mill and process automation. The developed system forms a good basis also for other content measurements.

Paper Details

Date Published: 27 April 2010
PDF: 10 pages
Proc. SPIE 7680, Next-Generation Spectroscopic Technologies III, 768008 (27 April 2010); doi: 10.1117/12.850010
Show Author Affiliations
Rami Aikio, VTT Technical Research Ctr. of Finland (Finland)
Hannu Lindström, VTT Technical Research Ctr. of Finland (Finland)
Pekka Suopajärvi, VTT Technical Research Ctr. of Finland (Finland)
Jouko Malinen, VTT Technical Research Ctr. of Finland (Finland)
Markku Mäntylä, Metso Automation Oy (Finland)

Published in SPIE Proceedings Vol. 7680:
Next-Generation Spectroscopic Technologies III
Mark A. Druy; Christopher D. Brown; Richard A. Crocombe, Editor(s)

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