234. Optical touch probe for the inspection of mechanical components
University of Pisa, Italy
Reverse Engineering (RE) techniques are widely used in all branches of modern manufacturing industry. In the field of mechanical engineering and industrial manufacturing, RE refers to the creation of geometrical documentation data from existing physical parts. When original drawings are not available, it is often required to reconstruct CAD models from the existing parts by exploiting digitization techniques. These models can be used for numerical analyses in order to improve the product effectiveness. Moreover, geometry inspection may be needed by manufacturers to check the components fulfillment of the given tolerances and specifications.
In general, the shape of an existing physical model can be retrieved by using contact or non-contact measuring devices. Traditional point-by-point systems, as mechanical probes, or full-field optical scanners may be adopted to acquire target surfaces characterized by complex geometries. Coordinate Measuring Machines (CMMs) with contact probes provide measurements with high accuracies, but on-site measurements are not allowed due to the bulky equipment. Articulated arms, characterized by 6 or 7 DoF, can be alternatively used. These systems, equipped with either a laser line scanner or a touch probe, can be manually moved with respect to the target object, resulting particularly effective for on-site measurements. Anyway, both CMMs and articulated arms only provide a limited number of sampling points and are not suitable if free-form shapes must be reconstructed. Among non-contact approaches, optical methods based on the triangulation principle are able to provide full-field measurements with minimal interaction with the operator. Laser line scanning and structured light scanning can be used to obtain dense point cloud data on the measured surfaces. Optical techniques allow the acquisition of visible surfaces, whereas the digitization of internal geometries (i.e., slot, holes) is subjected to partial or complete restrictions due to optical occlusions. For this reason, complete reconstructions providing visible and internal geometries should be obtained by integrating contact and non-contact methodologies.
In this paper, an automatic and versatile 3D measurement system has been developed by integrating tactile and optical probing. In particular, a hand-held tactile probe and a stereo structured light scanner are combined to perform reliable multi-sensor measurements of mechanical components. The tactile probe is optically tracked by the stereo camera system of the optical scanner by means of 3D measurements of a prismatic flag, rigidly connected to the probe, and equipped with multiple chessboard patterns differentiated by QR codes. The probe configuration has been designed to provide both versatility and adaptability to various applicative contexts. Moreover, a suitable calibration process has been developed to relate the probe tip with respect to the tracking flag. The passive stereo cameras system is further augmented with a multimedia light projector in order to compose a structured light scanner. Full-field measurements of visible surfaces (i.e., external shape of the impellers) can then be integrated with point-by-point measurements of non-visible surfaces performed by the tactile probe, thus providing complete reconstructions of industrial components having complex shapes. The effectiveness of the developed multi-sensor system has been finally tested in the surface reconstruction of mechanical parts.
235. Digital Image Correlation based on projected pattern for high frequency vibration measurements
University of Pisa, Italy
The vibrational response of mechanical components is a crucial issue in several industrial fields. In particular, rotating machineries represent an application subjected to high vibrational solicitations, and generally, the bladed wheels are the critical part of the machine. The characterization of the critical components is generally performed through numerical simulations, but also experimental validation is essential for safety issues. Experimental Modal Analysis and experimental Harmonic Response Analysis are then valuable tools for machine validation. Several contact techniques were developed to measure the dynamic response of the components such as extensimeters or accelerometers. Anyway, those contact sensors may influence the response of the component and have severe limitations in data transmission when the component is rotating. For this reason, non-contact techniques were developed. The most commonly adopted sensor is the Laser Doppler Vibrometer (LDV), which guarantees high sensitivity measurements in a wide frequency range. Even if LDV is able to perform fast surface scans, it is limited in the measurement orientation. Moreover, it only provides velocity measurements along the laser beam direction, thus giving 1D information. The present work is aimed at developing a full-field optical method for the measurement of vibrating machinery components. The final target is a 3D acquisition system based on a couple of stereo cameras, capable to acquire vibrational response of components in the range 1-10 kHz. However, in this work, preliminary results have been obtained by using a single standard camera (having a resolution of 2 Mp), thus determining only a 2D displacement field through Digital Image Correlation (DIC) algorithm applied to a projected pattern. This approach has been followed in order to validate the feasibility of the proposed methodology, providing, at the same time, information about the camera specifications considering the measured vibrational frequency. Expensive high-speed cameras were discarded due to their cost and limited resolution. Low frame rate cameras were instead selected, having a short exposure time (20 μs). The available frame rate, however, would not allow to measure high-speed vibrations due to the Nyquist-Shannon theorem. Anyway, the excitation can be arbitrarily imparted by an electromagnetic shaker. For this reason, a single sinusoidal component at a given (known) frequency has been used, thus allowing the reconstruction of high frequency phenomena with low frame rate acquisitions by properly triggering the camera. The more severe limitation is then represented by the exposure time, which must be much lower than the vibration period (100 times was chosen in the present paper), so that the measurement target appears still during the acquisition of the single frame. Preliminary tests showed encouraging results, so that future developments were planned to achieve 3D measurements by adopting a stereo camera pair. In particular, the proposed system hardware exploits two cameras assembled in a stereo configuration and a multimedia projector used to project a speckle pattern. Digital Image Correlation (DIC) techniques could then be adopted to achieve 3D displacement measurements of the vibrating component.
262. Fast and low cost acquisition and reconstruction system for human hand-wrist-arm anatomy
1Department of Industrial Engineering, Via di Santa Marta 3, Firenze 50139, Italy; 2Research, Innovation and International Relations Office, Meyer Children's Hospital, Viale Gaetano Pieraccini, 24, Firenze 50139, Italy
Dedicated 3D body scanners are paramount to deliver the exact measures of a human body to be used in a range of applications dealing with health, fashion and fitness as well as in several reverse engineering applications for robotics, automotive and computer vision in general. 3D human models obtained from 3D scanning foster personalized manufacturing in many applications, since they can be used to develop custom products perfectly tailored to the specific user.
Human oriented 3D scanners (the so-called body scanners) pose new challenges in the panorama of existing optical measurement systems; in fact the agile human nature, imposes the acquisition to be specifically fast to avoid movement artefacts. Nowadays very good results are possible with existing body scanners (both professionals and consumer devices); however, when focusing on relative complex shape of some body details (e.g. hand-wirst), obtainable results still lack completeness and accuracy.
In fact, human hand represents one of the most challenging parts to measure, yet it is required in a variety of applications.
Taking advantage from the emerging 3D depth cameras technologies, in this paper we present the design of a new, compact, low cost 3D dedicated hand-wirst-arm scanner system, able to deliver a full 3D point cloud in less than three seconds.
The system comprises 7 to 8 commercial devices, appropriately arranged on an adjustable structure made of two annular frames in order to allow the scanner to operate on body dimension rising from 4 years old to adult subjects.
The scanner is tested on a case study, represented by the design of custom orthosis. The semi-automatic reconstruction procedure, resulting in a parametric CAD model of the human wrist and thumb anatomy is also presented.
321. Enhancing porcelain whiteware quality assessment by means of Reverse Engineering-based procedures
1Università di Firenze, Italy; 2Richard Ginori srl, viale Giulio Cesare 50, Sesto Fiorentino, 50019, Italy
During sintering process, porcelain changes its chemical composition as well as its physical and mechanical properties. In fact, the raw materials – which are a mixture of about 50% kaolin, 25% feldspar and 25% quartz – are initially processed and reduced to fine powder. Successively the primitive shape of the artefact (named "green body") can be obtained in several alternative forming process such as slip casting, isostatic pressing and plastic forming.
Once the initial shape is formed, the artefact undergoes at least two subsequent thermal processes: the first one, called bisque-firing, allows to improve the handling capabilities of the material in order to proceed with glazing (and decorating if required) limiting the risk of fractures. The last one is the final firing, during which the sintering is completed and the final characteristics of the artefact are reached.
The most relevant effect of the transformations induced by above mentioned process is a significant change of shape, which is a combination of shrinkage and pyroplastic deformations (i.e. deformations due to gravity, caused by softening).
Both of these deformations are taken into account during the definition of the green body shape. However, due to the complexity of the transformations and the high number of factors that can influence them, some variability is expected during the production process. Eventually, this leads to a significant scatter among the obtainable geometries of the manufactured pieces: deformations that occur during the manufacturing of a specific artefact may significantly vary even among the same batch.
For this reason high quality porcelain production requires a severe control on the produced articles and in particular on the production of tableware (dishes). More in deep, three parameters are monitored among the final pieces: the drop of the well, the bending of the rim and – whether the artefact is axial-symmetric - the circularity of the artefact.
These three parameters are evaluated by means of calibres and comparators on a number of samples at the end of the production cycle. However, these procedures are affected by typical drawbacks of hand-made measurements such as, for instance, limited repeatability and inaccurate evaluation. With the aim of enhance the quality measurement accuracy, in the present work an alternative approach – based on 3D laser scanning and reverse engineering based methods- is proposed. The virtually reconstructed shape of fine porcelain products, obtained by using a 3D scanner, is processed to extract, and possibly to re-think, the three quality parameters and to redefine them as reverse engineering standard procedures. The devised procedure mainly relies on the unlimited accessibility to the virtual geometry, taking advantages from computer aided measurement, and allows a more deep capability of investigating the product quality. Tested against a number of case studies, the proposed procedure proves to be effective in providing accurate, reliable and repeatable quality parameter measurements.
322. AUTOMATIC FEATURES RECOGNITION FOR ANTHROPOMETRY
University of L'Aquila, Italy
The biological objects are morphologically-complex elements, which performs a particular physiological function. In many applications, geometric and dimensional parameters of these components of human body are analysed to gather some evidence, which may be useful in medicine, in anthropology and forensic investigations. In every of the previously described applications, measures are required to be accurate enough to discriminate the factor being investigated.
Generally speaking, the measurements are performed in-vitro or in-vivo. In the first case, manual measuring devices, such as sliding caliper and a goniometer are used. In-vivo, when the component of human body is available in the form of a 3D geometric model (as it is the case of CT-scans) its measurement is performed as point-to-point distance between points manually selected by the operator in a specific software. All these approaches are not structured since the measure is not associated to an ideal feature as prescribed by GPS standards. It is mainly for this reason that this kind of measures are affected by wide uncertainties.
At the purpose to reduce the measurement uncertainties, the authors presented a new automatic method to measure morphologically-complex objects, which takes advantage from the representation of the object in the form of 3D geometric model obtained from CT-scans or 3D scanning. In this work the method is verified in real cases and compared with the traditional approaches.