E. Cuesta

Application of a Force Sensor to Improve the Reliability of Measurement with Articulated Arm Coordinate Measuring Machines

A study of the operator contact force influence on the performance of Articulated Arm Coordinate Measuring Machines (AACMMs) is presented in this paper. After developing a sensor capable of measuring the contact force applied by an operator, a ring gauge has been used to analyse the relationship between the contact force and diameter and form errors measured with the AACMM. As a result, contact force has been proved as one of the main factors influencing the AACMM performance. A probe deflection model based on the Finite Element Method (FEM) has been also proposed in order to obtain the AACMM probe deflection caused by contact force. This allows measurement correction by comparing them with reference values, specifically, a ring gauge. Experimental test results show a significant measurement improvement that minimizes diameter error. Finally, an uncertainty evaluation for the contact force sensor and AACMM measurements with and without probe deflection model has been carried out in order to validate the ability of the sensor and the methodology followed.

Automatic determination of bending sequences for sheet metal parts with parallel bends

A method for solving the problem of bend sequencing in sheet metal manufacturing is presented. The algorithm developed divides the part into basic shapes (channels and spirals) and determines the partial sequences associated with them. The complete bending sequences associated with the complete part were obtained from the combination of these partial sequences. To make this combination possible, several strategies were defined to reduce the number of solutions and, therefore, the searching time. All sequences (partial and complete) were checked considering possible part-tool collisions and tolerance constraints. The last stage was to order the sequences by taking into account the total process time. To attain the required process time accuracy, a robot was used for the handling operations. Finally, the sequence associated with the lower process time was selected as the optimal solution.

A new concept of feature-based gauge for coordinate measuring arm evaluation

Articulated arm coordinate measuring machines (AACMM or CMA) have conquered a market share in the actual dimensional metrology field, overall when their role implies the inspection of geometrical and dimensional tolerances in an accurate 3D environment for medium-size parts. However, the unavoidable fact of AACMM manual operation constrains its reliability to a great extent, avoiding rigorous evaluation and casting doubt upon the usefulness of external calibration. In this research, a dimensional gauge especially aimed at AACMM evaluation has been developed. Furthermore, the operator skill will be revealed through the use of this gauge. A set of geometrical features, some of them oriented to evaluate the operator and others the equipment, have been collected for the gauge. The proposed evaluation methodology clearly distinguishes between dimensional and geometrical tolerances (with or without datum references), whereas actual verification standards only consider the former. Next, quality indicators deduced from the measurement results are proposed in order to compare AACMM versus coordinate measuring machine (CMM) performance, assuming that CMM possess the maximum accuracy that AACMM could reach, because CMM combines maximum contact accuracy with minimum operator influence. As a result, AACMM evaluation time could be significantly reduced since this gauge allows us to perform a customized evaluation of only those specific tolerances of interest to the user.

Feasibility Evaluation of Photogrammetry versus Coordinate Measuring Arms for the Assembly of Welded Structures

The present paper studies the technical feasibility of Photogrammetry and Coordinate Measuring Arms (AACMM or CMA) for the geometrical inspection of welded pins used for the assembly of medium-sized sheetmetal structures. As a result of this evaluation it was found that the deviations (measurement recorded by the device minus the nominal value of the measurement) obtained by means of Photogrammetry and the CMA were equivalent. Moreover, the number of measurements found to be out of tolerance by both methods was statistically equivalent. According to our results both methods are feasible for performing the dimensional inspection of the position of the welded cylinders of the structure as they are able to distinguish measurements inside and outside of tolerance and are equally capable of detecting the data variance.

Sensor Prototype to Evaluate the Contact Force in Measuring with Coordinate Measuring Arms

This paper describes the design, development and evaluation tests of an integrated force sensor prototype for portable Coordinate Measuring Arms (CMAs or AACMMs). The development is based on the use of strain gauges located on the surface of the CMAs’ hard probe. The strain gauges as well as their cables and connectors have been protected with a custom case, made by Additive Manufacturing techniques (Polyjet 3D). The same method has been selected to manufacture an ergonomic handle that includes trigger mechanics and the electronic components required for synchronizing the trigger signal when probing occurs. The paper also describes the monitoring software that reads the signals in real time, the calibration procedure of the prototype and the validation tests oriented towards increasing knowledge of the forces employed in manual probing. Several experiments read and record the force in real time comparing different ways of probing (discontinuous and continuous contact) and measuring different types of geometric features, from single planes to exterior cylinders, cones, or spheres, through interior features. The probing force is separated into two components allowing the influence of these strategies in probe deformation to be known. The final goal of this research is to improve the probing technique, for example by using an operator training programme, allowing extra-force peaks and bad contacts to be minimized or just to avoid bad measurements.

Influence of surface material on the quality of laser triangulation digitized point clouds for reverse engineering tasks

Laser triangulation systems (LTS) are one of the most popular non-contact inspection techniques. These systems are widely used in reverse engineering tasks as they allow for a fast acquisition of thousands of points that represent the geometry of the part in a virtual 3D model. The accuracy and repeatability of these systems are lower than the traditional contact inspection techniques, as they depend on the surface properties, the illumination conditions and the LTS configuration. Present work deals with the influence of surface material on the quality of the virtual model. The objective is to evaluate the behaviour of different materials and their suitability for being digitized.

Testing coordinate measuring arms with a geometric feature-based gauge: in situ field trials

This work describes in detail the definition of a procedure for calibrating and evaluating coordinate measuring arms (AACMMs or CMAs). CMAs are portable coordinate measuring machines that have been widely accepted in industry despite their sensitivity to the skill and experience of the operator in charge of the inspection task. The procedure proposed here is based on the use of a dimensional gauge that incorporates multiple geometric features, specifically designed for evaluating the measuring technique when CMAs are used, at company facilities (workshops or laboratories) and by the usual operators who handle these devices in their daily work. After establishing the procedure and manufacturing the feature-based gauge, the research project was complemented with diverse in situ field tests performed with the collaboration of companies that use these devices in their inspection tasks. Some of the results are presented here, not only comparing different operators but also comparing different companies. The knowledge extracted from these experiments has allowed the procedure to be validated, the defects of the methodologies currently used for in situ inspections to be detected, and substantial improvements for increasing the reliability of these portable instruments to be proposed.

Implementation of decision rules for CMM sampling in a KBE system

This work presents a methodology for setting the number and spatial distribution of inspection points in the automatic inspection of parts using Coordinate Measuring Machines. This methodology collects the existing knowledge about sampling rules in automatic inspection. The final purpose is to store this knowledge in a KBE platform oriented to the inspection planning process and based on the MOKA methodology.

Accessibility analysis for star probes in automatic inspection of rotational parts

This article describes a method of accessibility analysis based on the geometrical study of all possible interferences between the probe and the part to be inspected. The parts considered in the analysis are rotational parts and their recognition is automatically made from a feature-based design. The probe considered is a star type, defined by adjustable parameters that allow for the adaptation to different types and sizes. The system analyses the possible collision with the machine column, touch probe and probe stylus. Apart from determining the conditions of collision, the system also calculates any collision-free positions on each surface to be probed. These positions can be used to obtain inspection paths for communication to a CMM or to a global inspection planning system. The collision conditions calculated have been checked and validated using different rotational parts.

Influencia del Acabado Superficial en el Digitalizado con Sensores de Triangulación por Láser

The present work deals with the measurement of the influence that surface finishing has on the accuracy and quality of the scanning performed using laser triangulation sensors. Amongst the great number of parameters that influence the scanning quality, the effect of roughness is analysed. To achieve this, a procedure for constraining the variables while providing an excellent repeatability and reproducibility was proposed. This procedure has been applied to several manufacturing processes that produce parts with very different quality of surface finishing. As a result, certain correlations have been extracted that quantify the influence of the roughness on the quality of the cloud-point, the spatial distribution of the cloud-point dispersion and the optimal laser intensity for each manufacturing process.