Online measurement of paper thickness profile is essential in paper production. For decades paper thickness has been
measured online with sensors that are contacting the web on both sides. In 2005 a new optical online paper thickness
gauge was introduced which only contacts the web on the other side. The sensor is based on a laser triangulation sensor
and a magnetic sensor, and it determines the paper thickness from the difference of the two measurements. For
calibration of the two sensors, a robust concept has been developed which utilizes the measured object and takes place in
the measuring environment so that the calibration is automatically adjusted to the current measuring circumstances. More
importantly, with the presented method the non-linearity of the laser sensor is cancelled enabling the measurement of the
thickness profile shape with an accuracy much better than that of the laser sensor. Profile accuracy of 0.5 μm (2σ) has
become normal while the measuring range is often several hundreds of microns and the measuring distance to the paper
web 1.0-1.5 mm with a laser sensor having linearity of ±2 μm.
A non-contacting online caliper measurement has been papermakers' dream
for over two decades. Currently, paper thickness is measured using buttons
contacting the paper web on both sides. In such a configuration, paper
thickness is assumed to be the distance between the contacting surfaces
and determined by a magnetic measurement principle. However, this
arrangement of contacting measurement has several disadvantages including
sheet marking, hole creation, dirt build-up on the contacting buttons,
wearing of the contacting surfaces, and even sheet breaks. Moreover, the
current trends in paper manufacturing, especially the increasing use of
recycled raw materials are necessitating the development of a more
reliable thickness measurement solution that is not affected by dirt and
other material on paper or board sheet surfaces.
So far, a non-destructive, on-line thickness measurement has not been
successfully applied in paper production environment. Recently, Metso
Automation has successfully piloted in several mills a caliper sensor that
does not contact the sheet on both sides and is able to measure paper
thickness with sub-micron accuracy. The new sensor is based on single
sided laser triangulation. This paper presents the measurement set-up and
discusses the challenges encountered. Measurement results obtained in
mill trials with various paper grades are reviewed and compared to those
made simultaneously with contacting, on-line sensors and off-line
laboratory results of the same sheet. Factors affecting the measurement
with conventional and optical thickness sensors are also discussed.
A light and fast 2-axial fine-pointing mirror has a number of space applications, especially in intersatellite optical links. The fine pointing of laser beams in optical links is currently realized with electromagnetic and piezoelectric actuators, which are relatively large and heavy. MEMS technology bears a high potential in space applications offering reduction of device size, mass and power consumption. Micro technology makes batch mode fabrication possible yielding a low cost per unit. VTT Automation has designed and partially tested a silicon micro machined electrostatically actuated 2-axial mirror, which can be controlled with a microradian accuracy and a large bandwidth over the angular range of +/- 3 mrad.
A Finnish-French group has proposed an imaging spectrometer- based instrument for the ENVISAT Earth observation satellite of ESA, which yields a global mapping of the vertical profile of ozone and other related atmospheric gases. The GOMOS instrument works by measuring the UV-visible spectrum of a star that is occulting behind the Earth's atmosphere. The prime contractor of GOMOS is Matra Marconi Space France. The focal plane optics are designed and manufactured by Spacebel Instrumentation S.A. and the holographic grating by Jobin-Yvon. VTT Automation, Measurement Technology has participated in the GOMOS studies since 1989 and is presently responsible for the verification tests of the imaging quality and opto-mechanical interfaces of the holographic imaging grating of GOMOS. The UVIS spectrometer of GOMOS consists of a holographic, aberration corrected grating and of a CCD detector. The alignment of the holographic grating needs as an input very accurate knowledge of the mechanical interfaces. VTT Automation has designed, built and tested a characterization system for the holographic grating. This system combines the accurate optical imaging measurements with the absolute knowledge of the geometrical parameters at the accuracy of plus or minus 10 micrometers which makes the system unique. The developed system has been used for two breadboard gratings and the qualification model grating. The imaging quality results and their analysis together with alignment procedure utilizing of the knowledge of mechanical interfaces is described.
Optical measurement of the density of ozone and other atmospheric species at night is possible by using stars as light sources. The Technical Research Centre of Finland (VTT) has built a star-pointing spectrometer, which records stellar spectra by a 2D CCD-array. The spectrometer has a 'slitless' design, so it can measure the absolute intensity level of a stellar spectrum attenuated by the atmosphere. A spectral inversion method designed for the satellite-based instrument GOMOS is applied for constituent retrieval form stellar spectra measured on ground. Analysis of simulated measurements shows that when averaging over one night the total ozone column can be measured by the VTT spectrometer at an accuracy of 2-3 percent.