J.J. Aguilar

An Overview of Kinematic and Calibration Models Using Internal/External Sensors or Constraints to Improve the Behavior of Spatial Parallel Mechanisms

This paper presents an overview of the literature on kinematic and calibration models of parallel mechanisms, the influence of sensors in the mechanism accuracy and parallel mechanisms used as sensors. The most relevant classifications to obtain and solve kinematic models and to identify geometric and non-geometric parameters in the calibration of parallel robots are discussed, examining the advantages and disadvantages of each method, presenting new trends and identifying unsolved problems. This overview tries to answer and show the solutions developed by the most up-to-date research to some of the most frequent questions that appear in the modelling of a parallel mechanism, such as how to measure, the number of sensors and necessary configurations, the type and influence of errors or the number of necessary parameters.

Study of self-calibration and multilateration in machine tool volumetric verification for laser tracker error reduction:

This article aims to present the influence of the measurement uncertainty of a commercial laser tracker on the volumetric verification of a machine tool through the study of verification procedures that are affected by measurement uncertainty, multilateration and laser tracker self-calibration. Self-calibration provides relative positioning between measuring coordinate systems (laser trackers) and the reference system from the measured points of the same mesh. The measured points are affected by the noise of each laser tracker; therefore, they provide positions that are different from the real positions of the laser trackers. By applying the technique of multilateration and by knowing the positions of the laser trackers, the measurement noise can be reduced. The range of the measurement noise reduction is influenced by the radial measurement noise of the laser tracker, the distance between the laser tracker and the measured point and the techniques that multilateration and laser tracker self-calibration employs. This article presents different laser tracker self-calibration procedures, a least squares adjustment, trilateration and quadrilateration as well as the scope and appropriateness of each method relative to the laser tracker measurement noise. Moreover, the influences of radial laser tracker noise on the trilateration and quadrilateration techniques are described as well as the influence of the distance between the laser tracker and the measured point on multilateration.

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.

Robust Road Condition Detection System Using In-Vehicle Standard Sensors

The appearance of active safety systems, such as Anti-lock Braking System, Traction Control System, Stability Control System, etc., represents a major evolution in road safety. In the automotive sector, the term vehicle active safety systems refers to those whose goal is to help avoid a crash or to reduce the risk of having an accident. These systems safeguard us, being in continuous evolution and incorporating new capabilities continuously. In order for these systems and vehicles to work adequately, they need to know some fundamental information: the road condition on which the vehicle is circulating. This early road detection is intended to allow vehicle control systems to act faster and more suitably, thus obtaining a substantial advantage. In this work, we try to detect the road condition the vehicle is being driven on, using the standard sensors installed in commercial vehicles. Vehicle models were programmed in on-board systems to perform real-time estimations of the forces of contact between the wheel and road and the speed of the vehicle. Subsequently, a fuzzy logic block is used to obtain an index representing the road condition. Finally, an artificial neural network was used to provide the optimal slip for each surface. Simulations and experiments verified the proposed method.

Application of virtual distances methodology to laser tracker verification with an indexed metrology platform

High-range measuring equipment like laser trackers need large dimension calibrated reference artifacts in their calibration and verification procedures. In this paper, a new verification procedure for portable coordinate measuring instruments based on the generation and evaluation of virtual distances with an indexed metrology platform is developed. This methodology enables the definition of an unlimited number of reference distances without materializing them in a physical gauge to be used as a reference. The generation of the virtual points and reference lengths derived is linked to the concept of the indexed metrology platform and the knowledge of the relative position and orientation of its upper and lower platforms with high accuracy. It is the measuring instrument together with the indexed metrology platform one that remains still, rotating the virtual mesh around them. As a first step, the virtual distances technique is applied to a laser tracker in this work. The experimental verification procedure of the laser tracker with virtual distances is simulated and further compared with the conventional verification procedure of the laser tracker with the indexed metrology platform. The results obtained in terms of volumetric performance of the laser tracker proved the suitability of the virtual distances methodology in calibration and verification procedures for portable coordinate measuring instruments, broadening and expanding the possibilities for the definition of reference distances in these procedures.

Optimization of an Optical Test Bench for Tire Properties Measurement and Tread Defects Characterization

Tire characteristics and behavior are of great importance in vehicle dynamics since the forces transmitted in the tire-road contact are the main contributors to global vehicle performance. Several research groups have focused on the study and modeling of tires. Some of the most important factors that need to be known are tread characteristics and pressure distribution in the tire-ground contact patch. In this work, a test bench has been used to adequately determine the aforementioned factors. The measurement principle of the test bench is the frustration of total internal reflection (FTIR) of light. It makes use of a laterally illuminated glass on which the tire leans. An interposed plastic interface between them causes the reflection of light. Finally, a video camera captures the bright image formed through the glass. The brightness level in each pixel of the image is related to existing normal pressure. A study of the parameters that affect the test bench calibration such as type of interface material used, diffuse light, hysteresis, creep and transverse light absorption is performed. Experimental tests are conducted to relate tire inflation pressure and camber angle to the pressure distribution. Furthermore, the test bench is used to detect and evaluate the influence of defects in the tire on the contact pressures.

Empirical analysis of the efficient use of geometric error identification in a machine tool by tracking measurement techniques

Volumetric verification is becoming increasingly accepted as a suitable technique with which to improve machine tool accuracy. In the same way, the use of laser trackers to obtain machine error information using the new Active Target motorised retro-reflector allows the verification of all types of machine tool throughout their workspaces. Non-linear optimisation methods and machine tool kinematic models are the mainstays of this technique. Whereas the latter provide the relationship between the nominal coordinates, the geometric errors of the machine and laser tracker measurement, the former reduces the combined influence of geometric errors by obtaining their approximation functions. However, within these two procedures, several factors affect the scope of the produced verification results. The present paper focuses on the analysis of the adequacy of commercial measurement techniques using laser trackers and the new motorised retro-reflector in a real milling machine. An examination is also made regarding the influence of the optimisation sequence defined by the identification strategy, as well as the impact of the number of measured points in relation to the employed regression functions.

Calibration, non-geometric error modelling and correction methods for Articulated Arm Coordinate Measuring Machines

The different accuracy and repeatability requirements of articulated Arm Coordinate Measuring Machines (AACMM) and robots make it necessary to consider different kinematic parameters identification techniques covering the characteristic operational parameters in each case. This paper presents a data capture technique for subsequent identification of an AACMM kinematic model parameters, using nominal data reached by a ball bar and a kinematic coupling. Also this paper addresses the performance evaluation of the algorithm and objective functions used, based on a new approach, including terms regarding measurement accuracy and repeatability.

Metrovisionlab: A Matlab Tool for Learning Vision Camera Calibration

This paper describes the Metrovisionlab computer application implemented as a toolbox for the Matlab program. The application: 1) simulates a virtual camera, providing a simple and visual understanding of how the various characteristics of a camera influence the image that it captures; 2) generates the coordinates of synthetic calibration points, both in the world reference system and the image reference system, commonly used in camera calibration; and 3) can calibrate with the most important and widely‐used methods in the area of vision cameras, using coplanar (2D) or non‐coplanar (3D) calibration points.

Pant‐tilt Platform Design Based on Parallel Kinematics

A new long distance measurement system design, based on parallel kinematics, is presented in this paper. This system has two degrees of freedom for positioning and orientating two high precision cameras. In this document is presented the system design. Several configurations have been analyzed and the components needed such as actuators, linear captators (to measure the displacement), spherical ball‐and‐socket joints and universal joints, have been selected. The developed model allows us to obtain the kinematic joint variables, depending on geometric parameters, by means of the resolution of opened chains for each leg. The direct model allows us to obtain the platform position and orientation for a determined displacement values. Once the matrix transformation (which describes the coordinates of the platform reference system respect to the base reference system) is obtained, the actuators elongation can be found out through the inverse model with numeric or geometric methods. The design optimization, by m...