The main research projects in our Laboratory have involved combined development in electronics, optics, thermal waves and ultrasonics.
An EU project concerning microbiological growth in "premade" food packages has been completed. It uses ultrasonics by monitoring the ultrasonic absorption and second harmonic generation in food stuffs. The results have been particularly encouraging in the case of packaged milk.
A BRITE-Euram EU project is to be commenced next year and concerns thermal waves in the measurement of surface hardening of steel without damaging the sample. A large European consortium has been set up for the initial feasibility study.
We are also developing a rather simple non-contact acoustic device to measure the thickness or surface density of papersheets or other similar foils. The resolution has been about one micrometer which is good for simple equipment. We have also co-operated with Savcor Inc. to study the air content of pulp mass using ultrasonics.
The optical confocal measurement principle has been utilized to measure the crystallization of certain polymers.
7.2. OPTICAL MEASUREMENT METHODS
FIBER OPTIC CONFOCAL LASER SCANNING MICROSCOPE FOCLSM
Pekka Raatikainen and Mauri Luukkala
In the Electronics Research Laboratory we have built a fiber optic confocal laser scanning microscope (FOCLSM). As a light source we use a semiconductor laser (wavelength 650 nm) which is coupled directly to a single mode optical fiber. The light emitted by the laser is guided to the object through a fiber coupler. The light reflected from the object is focused back into the fiber and guided to the detector. It is possible to scan the focusing objective in the axial direction using an electromagnetic scanner. The object can be moved laterally using stepper motors and axially using a piezo mover.
The FOCLSM is compact and very easy to adjust because of the fiber optic components used. It has been used mainly for measuring the thickness of plastic films and plates. The thickness of the object can range from 3 µm to several millimeters. Multilayer structures and rounded surfaces are allowed. The objects should preferably be clear and have smooth surfaces, but rough surfaces and opaque materials can also be measured. The FOCLSM can also be used for three dimensional imaging and surface profilometry using computer controlled scanning.
The fiber optic confocal laser scanning microscope has proven to be a very useful tool for obtaining images of high resolution in the axial direction since out-of-focus signals make only a weak contribution to the detected signal. The unique depth discrimination property makes the confocal microscope useful in noninvasive medical and industrial inspection and in many other applications.
PROFILOMETRY WITH A COHERENCE SCANNING MICROSCOPE
Pekka Raatikainen, Ivan Kassamakov* and Mauri Luukkala
The coherence scanning microscope is based on a white light interferometer where the object or the reference mirror is scanned in the longitudinal direction. Monitoring one point in the image during the scanning results in white light fringes. The coherence function is given by the envelope of these fringes. By determining the peak amplitude of the envelope the location of the surface of the object can be found with high accuracy. Preliminary studies on using the coherence scanning microscope in surface profilometry have been carried out.
* Bulgarian Academy of Science, Institute of Applied Physics, Plovdiv, Bulgaria
7.3. THERMAL NON-DESTRUCTIVE EVALUATION
DETECTION OF VERTICAL CRACKS IN CARBON FIBER COMPOSITES USING AN INFRARED LINE SCANNER
In infrared line scanning techniques the inspected sample is heated with a suitable line heat source and the resulting surface temperature variations are detected with an infrared line scanner aimed closely behind the heating line. In addition to the heat flow normal to the sample surface, the moving line heat source generates lateral heat flow, which can, in principle, be used for detecting cracks normal to the surface.
Crack detection was studied in a unidirectional carbon fiber composite, where the cracks were simulated with thin saw cuts. Measurements were performed using both laser and radio frequency induction heating. The former provided a highly localized heat source, the use of which is advantageous in crack detection. The latter utilized the disturbance caused by the cracks to the electrical properties of the carbon fibers.
DEFECT DETECTION IN COPPER PRODUCTS WITH AN INFRARED LINE SCANNER
Copper products are used in a variety of industrial applications like heat sinks, electrical components, etc. The specific parts are originally manufactured from copper by processes which produce long tubes or wires of various cross sectional shapes. The processes usually generate several hundreds of meters of e.g. wire, which is wound on a big drum. The drum is eventually sold, for example, to a manufacturer of electrical components. However, defects can also occur in the manufacturing process. The defects usually take the form of delaminations, and because of the fast drawing speeds of the tube or wire, the delaminations can be several meters long.
The delaminations can be detected with infrared techniques. In this study, defects were observed using an infrared line scanning technique. The line scanning method is especially suitable for inspecting materials that are moving on a conveyor belt or on a similar arrangement. The method was capable of resolving small air bubbles the size of few square millimetres. The expected long delaminations were easily observed and the prospects for industrial applications are promising.
7.4. ULTRASONICS AND ACOUSTIC MEASUREMENT METHODS
ULTRASOUND DETECTION OF MICROBE CONTAMINATION IN PREMADE FOOD
The second phase of a major study for assessing the possibilities to use ultrasound based methods to detect microbes in aseptically packed premade food is presented. The initial part of this phase, the selection round, deals with physical optimization of the measurement system, while the second part deals with improving the system performance using statistical data analysis.
This second phase is a logical continuation of the first phase of the study as the methods investigated are those which performed best in the selection round. These methods were a) a conventional absorption method (ABS) based on measuring changes in the absorption spectrum according to the L1-norm and b) a second harmonic generation method (G3) based on measuring the energy shift from the first to the second harmonic frequency.
The aims of the second phase were 1) to acquire estimates of detection thresholds in the chosen products, as well as estimates of the methods' capability to detect different contaminants in the chosen products, and 2) to formulate specifications for a prototype together with estimates of the potential of the developed concept.
For the research process a semiautomatic test system was constructed where only the sample positioning was manually operated. The research revealed the potential of ultrasound-based methods to detect contaminants in aseptically packed premade food packages. The estimate for the detection threshold for Escherichia coli in UHT milk, Nantua fish soup, and Pursoup vegetable soup is 103 cfu/ml, a contamination level reached approximately 6 hours after inoculation. Five MPC-t (method, product, contaminant, and measurement time) solutions with classification probabilities in excess of 80% were found. These were: [Milk, G3, Clostridium sporogenes, day 4], [Nantua, ABS, Clostridium sporogenes, day 1], [Nantua, ABS, Candida kefyr, day 2], [Pursoup, G3, Candida kefyr, day 4], [Pursoup, ABS, Pseu-domonas fluorescens, day 1].
These results are likely to herald in a completely new area of quality management in the food industry as they are, as far as the writer is aware, the first demonstration of the potential of nondestructive methods for assessing food packages.
Reference Hæggström, Edward. Ultrasound Detection of Microbe Contamination in Premade Food, Acta Polytechnica Scandinavica, Applied Physics Series No. 214, Helsinki 1997, 131 pp. Published by the Finnish Academy of Technology. ISBN 952-5148-32-7, ISSN 0355-2127
GENERATION AND DETECTION OF SURFACE ACOUSTIC WAVES USING LASERS
Juha Aaltonen, Jussi Varis and Jyrki Stor-Pellinen
Acoustic waves can be produced using high power pulsed lasers. The method does not require direct contact to the target being investigated. Detection of the optically generated waves can be accomplished using low power CW lasers. The detection methods are based on inter-ferometric equipment or, as in our case, more robust laser beam diffraction. These two methods, when combined, offer a totally remote method for generating and detecting acoustic waves. Non-contacting nondestructive testing NDT methods offer faster and though more productive way to control and supervise production in various industrial environments. The disadvantage of the optical detection method is the requirement for a highly reflective surface of the target. Some solutions to overcome this problem have been studied.
CHARACTERIZATION OF PAPERS AND PAPERBOARDS IN RESPECT TO THEIR WETTING PROPERTIES BY THE AIR ULTRASONIC LAMB WAVE METHOD
Jyrki Stor-Pellinen, Juhani Rintanen and Edward Hæggström
We have applied our earlier developments of air-coupled Lamb wave methods to the measurement of papers that have the special qualities of absorbing liquids. Usually, the results from the Lamb wave method are related to the elastic moduli of the sample. The elastic properties of papers and paperboards, on the other hand, depend the number of fibre couplings in the sample, and their mechanical quality. The structure of fibers and pores has an influence on the proc-ess of wetting. The measurement results also show a correlation with wetting properties.
EXPERIMENTAL INVESTIGATION OF THE MAGNETOACOUSTIC MEMORY PHENOMENON IN COMMERCIAL AND EXPERIMENTAL FERRITE MATERIALS
Vladimir Ermolov*, Jyrki Stor-Pellinen and Mauri Luukkala
This study is a continuation to the investigation of the acoustic memory phenomenon. The requirements for a good material for the observation of magnetoacoustic memory phenomenon are conflicting. The material should be easily magnetized but without saturation and it should remain in the new state of magnetization for a time long enough to allow experimentation, yet still be erasable with a reasonably high magnetic field so that an experiment can be repeatable. The acoustical precondition for the material is that there should be complete homogeneity but this conflicts with the desirable effect of grain stucture on the formation of magnetic domains. Novel amorphous and nanocrystalline ferrites and Terfenol are being studied. To date, the best demonstration of the phenomenon has been for polycrystalline Ni-Co and Ni-Cu-Co ferrites.
* Moscow Engineering Physics Institute, 115409 Moscow, Russia
DYNAMIC WETTING MEASUREMENT BY ULTRASOUND
Edward Hæggström and Jyrki Stor-Pellinen
The wetting of a porous plate-like sample can be monitored by the through transmission of ultrasound in the immersion tank. We designed a mechanism where the immersion of the sample happens in a few milliseconds, which facilitates the measurement of very thin samples. The measurements give a different image of the process depending on frequency which facilitates the study of the wetting phenomenon: the influences of porous structure, the viscosity of the wetting liquid and adhesion.
A NONCONTACT GRAMMAGE AND THICKNESS MEASUREMENT OF PAPER
The mass per unit area is the most frequently measured parameter in the paper industry. Normally, some pieces of the final paper lot are passed to the laboratory to be studied. On-line beta ray meters are used, but the necessity for permission to use radioactive sensors diminish the interest in increasing their use. The paper industry needs information on the profile of the paper during the manufacturing process and even the best laboratory analysis cannot give the grammage profile of the whole lot. The area to be measured in paper mills is very large, so the measurement has to be fast.
A new reliable and fast acoustic measurement system has been constructed. This system uses acoustic tone bursts of 6 kHz to 15 kHz. The optimum sound frequency is chosen depending on the sample grammage. More massive samples need a lower frequency and a bigger separation of transmitter and sample. The measurement system is a new modification of that devised last year where thin plastic foils were measured with ultrasonic pulses. The new system is capable of measuring the grammage of paper samples less than 230 g/m2. If the density of the paper is constant the thickness is also obtained. However, the density of paper is not spatially constant and only some average of the thickness is usually achieved. The tone burst system consists of a 16 piece loudspeaker array to generate plane waves and a pair of microphones integrated with a signal source and a digital oscilloscope. The whole system is controlled by a Macintosh II microcomputer.