J.A. Albajez

Modelling and Calibration Technique of Laser Triangulation Sensors for Integration in Robot Arms and Articulated Arm Coordinate Measuring Machines

A technique for intrinsic and extrinsic calibration of a laser triangulation sensor (LTS) integrated in an articulated arm coordinate measuring machine (AACMM) is presented in this paper. After applying a novel approach to the AACMM kinematic parameter identification problem, by means of a single calibration gauge object, a one-step calibration method to obtain both intrinsic—laser plane, CCD sensor and camera geometry—and extrinsic parameters related to the AACMM main frame has been developed. This allows the integration of LTS and AACMM mathematical models without the need of additional optimization methods after the prior sensor calibration, usually done in a coordinate measuring machine (CMM) before the assembly of the sensor in the arm. The experimental tests results for accuracy and repeatability show the suitable performance of this technique, resulting in a reliable, quick and friendly calibration method for the AACMM final user. The presented method is also valid for sensor integration in robot arms and CMMs.

Development and calibration of self-centring probes for assessing geometrical errors of machines

A new type of probe for calibration, verification or interim checking of machine tools as well as robots or parallel-kinematics machine tools is presented in this paper. This probe is part of a ball-artefact-based method to assess the geometrical errors of linear and angular axes in a quick and reliable way. The discussion about the best design concept for the self-centring probe, the mathematical modelling and design process of one of those concepts and the development of three different prototypes are shown. Different sensors and design options were developed to optimize the size and cost of the probe. The calibration of the probe prototypes by way of a kinematically coupled reference device to allow on-machine tests is shown. The uncertainties obtained were around 1 µm for one of the prototypes and below 5 µm for the other two. Finally, results of a geometrical verification of a machine tool are demonstrated, with values similar to the ones achieved using other methods, such as laser interferometry. Compared to those other methods, the new technique was shown to be less time consuming.

Design Optimization for the Measurement Accuracy Improvement of a Large Range Nanopositioning Stage

Both an accurate machine design and an adequate metrology loop definition are critical factors when precision positioning represents a key issue for the final system performance. This article discusses the error budget methodology as an advantageous technique to improve the measurement accuracy of a 2D-long range stage during its design phase. The nanopositioning platform NanoPla is here presented. Its specifications, e.g., XY-travel range of 50 mm × 50 mm and sub-micrometric accuracy; and some novel designed solutions, e.g., a three-layer and two-stage architecture are described. Once defined the prototype, an error analysis is performed to propose improvement design features. Then, the metrology loop of the system is mathematically modelled to define the propagation of the different sources. Several simplifications and design hypothesis are justified and validated, including the assumption of rigid body behavior, which is demonstrated after a finite element analysis verification. The different error sources and their estimated contributions are enumerated in order to conclude with the final error values obtained from the error budget. The measurement deviations obtained demonstrate the important influence of the working environmental conditions, the flatness error of the plane mirror reflectors and the accurate manufacture and assembly of the components forming the metrological loop. Thus, a temperature control of ±0.1 °C results in an acceptable maximum positioning error for the developed NanoPla stage, i.e., 41 nm, 36 nm and 48 nm in X-, Y- and Z-axis, respectively.

Procedure for geometrical errors identification in machine tool

This article presents a procedure to determine any of the 21 geometrical errors of a three axes machine tool (MT) taking into account the machine architecture. This verification technique is based on the utilisation of a 1D ball artefact and a self-centring probe, though it could be easily generalised to be used with a laser tracker. Most of the times, users are not interested in evaluating all the errors of their MTs but a subset. In this way, this procedure allows to any potential user the evaluation of any single error by providing a systematic method to obtain the needed equations and the experimental points to measure, as opposed to volumetric error compensation techniques where the work area is globally evaluated and the focus is shifted from the determination of each MT geometrical error to the reduction of the total error in every point. In that case, relevant information related with the physical values of each error is not preserved.

Geometrical Characterisation of a 2D Laser System and Calibration of a Cross-Grid Encoder by Means of a Self-Calibration Methodology

This article presents a self-calibration procedure and the experimental results for the geometrical characterisation of a 2D laser system operating along a large working range (50 mm × 50 mm) with submicrometre uncertainty. Its purpose is to correct the geometric errors of the 2D laser system setup generated when positioning the two laser heads and the plane mirrors used as reflectors. The non-calibrated artefact used in this procedure is a commercial grid encoder that is also a measuring instrument. Therefore, the self-calibration procedure also allows the determination of the geometrical errors of the grid encoder, including its squareness error. The precision of the proposed algorithm is tested using virtual data. Actual measurements are subsequently registered, and the algorithm is applied. Once the laser system is characterised, the error of the grid encoder is calculated along the working range, resulting in an expanded submicrometre calibration uncertainty (k = 2) for the X and Y axes. The results of the grid encoder calibration are comparable to the errors provided by the calibration certificate for its main central axes. It is, therefore, possible to confirm the suitability of the self-calibration methodology proposed in this article.

Development of the control strategy of a 2D nanopositioning long-range stage

In order to provide an effective positioning along a wide working range with nanometre resolution, a two-dimensional stage platform has been designed and manufactured. In view of the demanding metrological performance, the drive system and control strategy are being analysed and developed. Thus, this paper describes the preliminary modelling of the control when using four custom-made Halbach linear motors as a driving system. Firstly, the 1D control case is defined and validated by simulation. The different required blocks of the loop are presented and an initial controller solution is proposed to achieve the established positioning features. Secondly, the 2D scheme is presented to define all the needs of the whole control, including all sensors and actuators of the system. In addition, this work justifies the hardware and software selected, as an optimal low-cost solution for the developed long-range stage prototype.

General Procedure to Evaluate the Progress and Results of the Student: Teaching and Learning the Skills of Manufacturing Engineering

The results of this paper suggest that there is a serious problem in the curricula definition of some degrees. It is necessary to analyze the educational activities, teaching methodologies and evaluation systems for the Bachelor’s and Master’s degrees at the institutional level. This will allow for improvements in the evaluation of competence acquisition. Therefore, it is necessary to review the definition of the curricula while paying special attention to the relationship between competences and courses because the current study has detected notable discrepancies. The courses must be evaluated through competences or, more precisely, through the results of the learning process. This would considerably improve an evaluation model of the progress and results on the learning acquired by graduated students in terms of competences and would match what is established in the current regulation. When analysing the results for any two students, a simple comparison based on grades is not realistic, as it depends on the subjectivity of instructors and teachers during the grading process. By contrast, an evaluation based on competences is much more objective and comparable and would demonstrate which university reaches a better level of both general and specific competence achievement. The results of the aforementioned procedure are of interest to employers, enabling them to search and hire the best students in the desired field of study.

Homing Sensor System Design for a 2D Long Range Nanopositioning Moving Platform

The nanotechnology field has been developing strongly in recent years and ultra-precision measuring systems are nowadays required. A new two-dimensional moving platform with 50x50 mm range of travel, nanometer resolution and sub micrometer accuracy is being designed by the authors in order to be integrated with an Atomic Force Microscope (AFM). In this work the definition, design and experimental characterization of a homing sensor system for this 2D moving platform is presented. The homing sensor system will allow the generation of an absolute 2D reference for the platform (X-Y axis and θz rotation), defining an initial cero for the measuring system, which is based on laser encoders.

Development of a 1D‐displacement low‐cost sensor prototype based on the inverse square law

In this paper the design and development of a new prototype for measuring lineal displacements with micrometric resolution are presented. This device is based on the use of opto‐electronic sensors (photodiodes) with a mechanical displacement system. This generates a more compact and lower cost solution than the commercial ones (LVDT, optical encoders, etc.) usually utilized for measuring in a range of 10 mm with a resolution of tenths of micrometers. The mechanic system for the displacement comes from a relative movement between a linear guide and its slide unit. The final goal of this prototype is the calibration of machine tools with less expensive self‐centering probes than the nowadays available commercial ones. Firstly, the properties and behavior of the photodiodes have been analyzed in order to verify that they are adequate for this appliance. In the following tests carried out the fulfillment of the square law has been verified but the system repeatability has been severely affected by the tempera...