7. ELECTRONICS AND INDUSTRIAL PHYSICS RESEARCH LABORATORY (atom.physics.helsinki.fi/~etlab/)

 

7.1. PREFACE

 

In our laboratory we have been developing new and innovative measurement methods using our extensive experience in ultrasonics, optics, electronics and thermal imaging. Our thermal image camera has been the catalyst to develop the infrared lumber quality scanner for VTT. The instrument can create an infrared picture of boards non-contact while the board is moving as fast as 5 m/s. With this instrument the grade of lumber and board can be evaluated more accurately.

Together with Vaisala we have been developing further the acoustic rain sensor which actually counts the raindrops. This new sensor has turned out to be more accurate than the conventional funnel-type rain collectors. Also this new sensor does not become overloaded in torrential tropic rains.

The EU Brite-Euram project about the surface hardening of steel has now finally ended and the final report is being prepared. We have also been able to create non-linear ultrasonic measurement methods in paper and also other materials. With non-linear ultrasound a much higher sensitivity and resolution can be obtained. We look forward to apply this method to a number of interesting applications.

Mauri Luukkala

 

7.2. THERMAL NON-DESTRUCTIVE EVALUATION

 

INFRARED LINE SCAN CAMERA FOR LUMBER INSPECTION

Juha Aaltonen, Pekka Raatikainen, Jussi Varis, Reijo Vuohelainen and Mauri Luukkala

VTT Building Technology chose us to build the infrared line scan camera for the multi-head inspection system (European project). The long-term work of our laboratory in the infrared scanning techniques and the knowledge gathered in the process convinced them that we were the best choice for the project.

The other instruments in the multi-head inspection system are the visual line scan camera, the X-ray imaging device, and the array of RF detectors. The system scans the moving lumber specimen at the maximum speed of five meters per second; the width of the specimen may extent to 500 millimeters.

In this project we successfully integrated the bilinear PbSe infrared detector with the digital frame grabber and the user software running on Windows NT platform. The dusty industrial environment called for the specially designed infrared optics and the camera housing which was build in our department’s workshop by Pekka Pihkala. The special electronics interfacing the detector equipment and the digital frame grabber were designed and manufactured in our laboratory. The infrared imaging system contained two separate software components: one for the frame grabbing procedure and the other for some further image analysis.

The infrared imaging system was constructed well within the time parameters and it fulfilled the specifications. This project demonstrated clearly that even with our laboratory’s reduced personnel we have the capability to design optics, electronics and computer-controlled systems.

 

DETERMINATION OF THE HARDNESS PROFILE IN CASE HARDENED STEELS USING INFRARED PHOTOTHERMAL RADIOMETRY

Atte Salmi, Jussi Varis* and Mauri Luukkala

The performance of surface hardened parts is a major issue in automotive and aerospace industries. There is, therefore, a strong need of hardening companies to improve the quality control of their products by introducing new measurement systems which allow for non-destructive, non-contacting hardness profile measurements as an alternative to the presently used destructive inspection methods. It is aim of this Brite-Euram project to develop a contact free, non-destructive photothermal measurement methodology.

During the second year in the network, case-hardened steel specimens with different hardening depths and surface hardnesses were investigated. Hardness inspections at our laboratory were performed using a frequency scanned laser or radio frequency induction heating. By temporal modulation of the heating, material properties were correlated with the propagation properties of so called thermal waves which correspond to critically damped harmonic solutions of the thermal diffusion equation. The depth of probing penetration was chosen by changing the excitation frequency, resulting in a maximum range of about 5 mm in-depth in the materials in question. The thermal wave field at the sample surface was measured by detecting the thermal radiation emitted from the sample by an IR detector.

* VTT Information Technology

 

 

7.3. ULTRASONICS AND ACOUSTIC MEASUREMENT METHODS

 

CAPACITIVE ULTRASONIC TRANSDUCER WITH NET BACKPLATE

P. Mattila*, Jyrki Stor-Pellinen, Janne Ignatius, J. Hietanen**,***, and Mauri Luukkala

Air-coupled capacitive ultrasonic transducers with a net backplate have been studied. Capasitive transducer is, in principle, a capacitor in which either an electrical force or an acoustical force changes the distance between the electrodes. One electrode of the transducer is a thin metal coating on a plastic membrane, and the other electrode is a backplate in this case is the net. The function of this transducer depends essentially on the shape of the backplate, the dimenstions and distribution of the resonant cavities against the membrane, and the electric field distribution.. A procedure for the calculation of the surface charge density distribution for transducers with net backplates was developed, and the influence on the transducer function studied.

* VTT Automation, Machine Automation, P.O. Box 1302, FIN-33101 Tampere, Finland

** Helsinki University of Technology, Laboratory of Computational Engineering, P.O. Box 9400, FIN-02015 HUT, Finland

*** Nokia Research Center, P.O. Box 100, FIN-33721 Tampere, Finland

 

THE USE OF AIR-COUPLED ULTRASOUND TO TEST PAPER

C.S. McIntyre*, D.A. Hutchins*, D.R. Billson* and Jyrki Stor-Pellinen

Capacitive transducers with a polymer membrane have been used to generate broad bandwidth ultrasonic signals in air, and these have been used to propagate signals through various paper products, including paper and cardboard of various thickness, with a range of additives, in order to measure properties of paper by a non-contact technique. Acoustic pulse signals with frequencies extending up to 1 MHz were applied at various angles of incidence. Resonances, at normal incidence, were clear in the thicker samples, which can be correlated to both the thickness of the paper sample for a given density and to an important parameter known as the liquid penetration coefficient (related to the rate of absorption of liquids). Air-coupled Lamb waves were also generated at oblique incidence, and their propagation was also seem to have a trend with some of the paper parameters. The moisture content of paper was seen to have effect on the velocity of ultrasound, and hence the frequency of the acoustic wave mode propagating through the sample.

* Dept. of Engineering, University of Warwick, Coventry, CV4 7AL, UK

 

THE EFFECTS OF HIGH-POWER ULTRASOUND ON SORPTION OF VISCOUS LIQUIDS INTO POROUS SAMPLES

Jyrki Stor-Pellinen and Mauri Luukkala

In liquids, high-power ultrasound induces effects such as cavitation and acoustic streaming, the former being a function of the acoustic power applied, partial pressures of the liquid components, and viscosity. As to acoustic streaming, the acoustic attenuation is important. Gas-liquid exchange constitutes liquid sorption into porous materials. By the effects of (high-power) ultrasound it is possible to force sorption process into the stationary state essentially faster than by the spontaneous wetting. Wetting process of thin porous sheets in liquids with 50 to 3000 cP viscosity exposed to ultrasonic field have been studied experimentally in order to model the gross effects and in order to study the potentials for applications.

 

METHOD FOR MEASUREMENT OF DYNAMIC WETTING PROCESSES OF PAPER EXPOSED TO AIRBORNE ULTRASOUND TRANSMISSION AND MIST GENERATION

Jyrki Stor-Pellinen, Edward Hæggström, Timo Karppinen and Mauri Luukkala

This investigation reports on development of a non-immersive wetting measurement. The system allows control of both the amount of wetting agent applied and of the droplet size of the mist pulse. Furthermore, it allows calibration and wetting measurements on the same samples. Office paper samples were examined by time-dependent attenuation and phase variation of 40 to 1600 kHz airborne ultrasound at an oblique angle. The samples were wetted with mists from mixtures of water and isopropanol. The transmitted ultrasonic signal was measured during 7 s with 15 ms intervals after application of mixture. The attenuation and phase change depends on the degree of absorption of liquid absorption into the paper sample, which gives a signature of the paper type and quality. The measurement results showed that at different frequencies, the attenuation and phase change by time was different, indicating different views of the development of wetting process. The measurement results also showed that at different angles of incidence, the attenuation and phase change by time is different indicating anisotropy in the development of wetting phenomena. By the results this concept was able to observe the development of the wetting process with 12 g/m2 of applied wetting agent.

 

ULTRASONIC MONITORING OF BEEF TEMPERATURE DURING ROASTING

Edward Hæggström and Mauri Luukkala

Ultrasound has appealing properties as a means to monitor quality of industrially manufactured food items. Changes in ultrasonic signal properties are associated with changes in raw material or process parameters. This paper introduces a concept based on an ultrasound transmission measurement which has shown ability to monitor the temperature of minced meat beef during simulated automated roasting. Commercially available beefs were roasted and the change in internal temperature of the beef was correlated to changes in the ultrasonic transmission signal. R2 values of 0.94 were obtained.

 

AN INSTRUMENT FOR MEASUREMENT OF PAPER-WETTING BY NONLINEAR ULTRASOUND TRANSMISSION

Edward Hæggström, Jyrki Stor-Pellinen and Mauri Luukkala

Understanding wetting processes in different paper qualities is important for both analysis and adjustment of paper production processes as well as for designing properties of printing paper. This investigation presents a dynamic transmission meter working with nonlinear ultrasound. Unsized base paper was examined by time-dependent multi-frequency nonlinear generation of ultrasound. Ultrasound was transmitted through paper during immersion in degassed isopropanol solutions.The nonlinearity coefficient, which is known to be sensitive to structural degradation, was monitored as a function of time. Generation of higher harmonics depends on the degree of liquid absorption into the paper sample, which gives a signature of the paper type and quality.

Instrument tests showed that at different frequencies, generation of 2nd harmonic by time was different, indicating the possibility of detecting different phases of wetting phenomena. A correlation between isopropanol concentration of the wetting liquid and nonlinear ultrasonic parameters analyzed was found.

 

MULTI-SPECTRAL ANALYSIS OF SKIN PHANTOM DURING UV EXPOSURE

Edward Hæggström and Mauri Luukkala

Due to ozone depletion the ultraviolet population dose dose has increased. At the same time there has been found a trend of increasing skin cancer occurrence. The correlation between these two findings has been found to be statistically significant. Knowledge of this fact has increased the amount of examinations performed. The examinations are currently being performed by dermatologists who use biopsies which are histologically examined by a pathologist. There is a need for a classifying instrument both during examination and surgery. Four potential areas for instrumentation development are: 1) Identification of prestages of skin cancer, 2) determination of surface boundaries of the tumor, and 3) determination of penetration depth of the tumor, 4) skin dose estimation prior to light therapy for psoriasis patients.

In this investigation the feasibility of a multispectral instrument was investigated with collagen samples. The samples were exposed to UV-A and UV-C and mechanical and optical parameters of the samples were monitored during the exposure. The mechanical properties were probed both by linear and nonlinear ultrasound in a reflection mode set-up. The optical parameters were probed both with a classic reflection spectrum set-up and with a polarometric confocal set-up. The preliminary results indicate that it is possible to monitor UV induced alterations in the collagen samples with the laboratory prototype.

 

DYNAMIC SURFACE ROUGHNESS MEASUREMENT OF PAPER DURING WETTING

Jyrki Stor-Pellinen, Edward Hæggström, Timo Karppinen and Mauri Luukkala

Temporal traces of a liquid penetration meter (LPT) shows the combined effects of many wetting processes. Segregating the different wetting processes exhibited in the temporal trace of a LPT requires separate measurements of i.e. surface roughing.

In this investigation, five different kinds of paper were examined by time-dependent reflectivity with 1.24 MHz airborne focussed ultrasound. Samples were wetted with mixtures of water and isopropanol, 12 g/m2 and 42 g/m2, and the reflected ultrasonic signal was measured during 10 s with 10 ms intervals after application of mixture. The reflectivity depends primarily on surface roughness of the sample and only partially on the effects of liquid absorbtion into the paper sample.

The results showed that, the reflectivity change by time is different with different paper qualities indicating different development of surface wetting phenomena. We suggest an interpretation that connects the measured temporal roughness changes with the effects and processes related to wetting of paper.