Drago Bračun

Triangulation model taking into account light sheet curvature

This paper examines the problem of systematic measurement errors in optical triangulation when some light sheets that illuminate the measured surface exhibit non-negligible bending (curvature). The problem is demonstrated experimentally by triangulation measurement of two reference bodies whose geometry reveals systematic measurement errors due to light sheet curvature. To correct these errors a triangulation model is developed which assumes parabolic light sheet shape and allows exact solution of system equations. Test measurements show that the model successfully compensates for systematic measurement errors originating from the curvature of light sheets.

The use of 3D laser imaging and a new breast replica cast as a method to optimize autologous breast reconstruction after mastectomy

Aesthetically pleasing and symmetrical breasts are the goal of reconstructive breast surgery. Sometimes, however, multiple procedures are needed to improve a reconstructed breasts symmetry and appearance. In order to avoid additional corrective procedures, we have developed a new method that uses a reverse engineering technique to produce what we call a new breast replica cast (NBRC). The NBRC is a mould of the contralateral healthy breast, designed according to preoperative laser 3D images. During surgery, the mould is used to help shape the new breast. With this method, we are able to achieve breast symmetry in terms of volume, projection, contour, and position on the chest wall more accurately, more quickly, and more safely than before.

A method for surface quality assessment of die-castings based on laser triangulation

This paper presents a new method for the surface quality assessment of safety-critical die-castings. We have developed a measurement system that measures the surface of a die-casting and provides quantitative surface quality assessment within a die-casting cycle of 70 s. The measurement system, based on the laser triangulation principle, has an asymmetrical measuring range and is capable of high-resolution measurements of the casting surface (0.02 mm). Geometry specific parameters (flatness of the particular surface region, average deviation of the measured points and height of the surface defects) are calculated from the acquired surface data and then checked whether/how they fit within the tolerances specified in a technical documentation. The method has been tested in the laboratory by examination of a sample of castings taken randomly from the production process. A comparison of the results obtained by this method and by a qualified operator has shown good agreement.

Pre-processing of point-data from contact and optical 3D digitization sensors.

Contemporary 3D digitization systems employed by reverse engineering (RE) feature ever-growing scanning speeds with the ability to generate large quantity of points in a unit of time. Although advantageous for the quality and efficiency of RE modelling, the huge number of point datas can turn into a serious practical problem, later on, when the CAD model is generated. In addition, 3D digitization processes are very often plagued by measuring errors, which can be attributed to the very nature of measuring systems, various characteristics of the digitized objects and subjective errors by the operator, which also contribute to problems in the CAD model generation process. This paper presents an integral system for the pre-processing of point data, i.e., filtering, smoothing and reduction, based on a cross-sectional RE approach. In the course of the proposed system development, major emphasis was placed on the module for point data reduction, which was designed according to a novel approach with integrated deviation analysis and fuzzy logic reasoning. The developed system was verified through its application on three case studies, on point data from objects of versatile geometries obtained by contact and laser 3D digitization systems. The obtained results demonstrate the effectiveness of the system.

Indentation shape parameters as indicators of spot weld quality

This contribution presents a laser-based method for three-dimensional (3D) measurements of the shape of an electrode indentation. The method is based on the illumination of the indentation with structured light and the detection of the image of the illuminated indentation by means of a digital camera. Image processing algorithms are employed to determine the 3D shape of the indentation. A batch of welds obtained from an automotive production process has been measured and analysed for characteristic geometrical indentation parameters such as diameter, depth, presence of cracks, splash, etc. The potential of indentation shape parameters to serve as indicators of spot quality is discussed.

3D Measurement of Electrode Contact Area in Resistance Spot Welding of Coated Steel

The document discusses the possibility of contact area estimation during resistance spot welding employing post-weld 3D measurement of electrode indentation. Experimental welding of a common type of mild zinc-coated steel was conducted using two different electrode tip types varying welding time, RMS current and number of successive welds. The other parameters (clamping force, voltage, pre- and post-weld time, weld-piece thickness) were constant. The results show that this approach gives valuable information on the conditions under which the weld-spot was formed.

Optodynamic measurement of a non-stationary temperature field in air by multiple laser-beam deflection

A new method for non-stationary temperature-field measurement is presented. Optoacoustic generation is used to emit short acoustic transients within the observed field. The transient deflects a probe laser beam, which is folded by two parallel mirrors in such a way that it passes through the observed field several times before it reaches a position-sensitive photo-detector. The timing of the acoustic wavefront transits at several distances from the source is obtained from a single oscilloscope trace of the photo-detector output. By measuring the acoustic wavefront transit times and the distances between the probe-beam segments, the velocity of sound is determined and from this we were able to calculate the local temperature.

Research of a Lensless Artificial Compound Eye.

One of the challenges in designing an artificial compound eye (ACE) is the manufacturing and assembly of the ommatidia lenses. The paper presents three lensless ACE designs based on light guiding structure and an array image detector. In the solid design, deep holes pointed to different directions in space act as artificial ommatidia. Low image resolution of the solid design is improved by the shell design, where the artificial ommatidia are created within the perforated shell. In the third design, artificial ommatidia are created between the pinhole and pixels on the image detector. The best image performance is achieved with the third pinhole design. An array of pinholes on a single image detector can improve the environment perception by combining the images and assessing the object distance based on image disparity. The proposed lensless design can be easily manufactured and miniaturised to the microchip scale, and finds potential application in small and lightweight autonomous robots.

Artificial Compound Eye and Synthetic Neural System for Motion Recognition

This paper presents modelling of a fruit fly’s visual neural system for motion recognition employing non-spiking Hodgkin-Huxley neurons. Motion detection operates based on the Hassenstein-Reichardt correlator principle. An array of motion detectors reveals the velocity field pattern, and an additional summation layer allows calculation of the vanishing point. The synthetic nervous system is successfully designed using the functional subnetwork approach. This allows the model to be scaled up to several hundred motion detectors according to the number of ommatidia. The output provides the abstraction of motion on a couple of exit neurons, which can be used in further implementation of control for the mobile robot. The simulation of operation on artificially generated input signals for different types of motion, and a summary of neuronal activities are given.

Use of Bifocal Objective Lens and Scanning Motion in Robotic Imaging Systems for Simultaneous Peripheral and High Resolution Observation of Objects

Imaging systems are widely used in robotic systems to detect features of their surroundings and their position in order to guide the robot’s motion. In the paper, a variant of an artificial imaging system based on the unique bifocal eye of sunburst diving beetle (Thermonectus marmoratus) larvae is proposed. The biologically inspired imaging system of a single sensor and a coaxial lens form a superposition of two focused narrow and wide view angle images. The output image contains a high resolution area of interest and its periphery. The scanning motion of the bifocal imaging system is also imitated and provides positional relations between objects. Acquired images are used for distance assessment. The intended use of the proposed imaging system is in a guidance system of an autonomously moving robot with biologically inspired locomotion.