Alfonso Hernández

Synthesis and Design of a Novel 3T1R Fully-Parallel Manipulator

In this paper a new topology of four degrees-of-freedom 3T1R fully-parallel manipulator is presented, which is defined only using lower kinematic pairs. The paper starts with a complete type synthesis of different topologies of fully-parallel manipulators that can perform the so-called Schonflies motion, based on the Theory of Groups of Displacements. After imposing some practical requirements, the different possibilities of manipulators are reduced to only one topology of fully-parallel and fully-symmetrical parallel manipulator. Then, the kinematic analysis of the manipulator is shown, including the closed-form resolution of both forward and inverse position problems, the velocity and the singularity analysis. Finally, a prototype of the manipulator is presented, which is intended to be used in pick and place applications.

Workspaces associated to assembly modes of the 5r planar parallel manipulator

The aim of this paper is to show how it is possible to obtain for the 5R planar parallel manipulator the complete workspace associated with each solution of the direct kinematic problem or assembly mode. The workspaces associated with the different inverse kinematic problem solutions or working modes are joined and the robot moves from one to another without losing the control. An exhaustive analysis of the complete workspace and singular positions of the 5R planar parallel manipulator with two active joints is presented. Furthermore, application of these principles to path planning will be explained.

A practical procedure to analyze singular configurations in closed kinematic chains

The authors present a general method for the automated singularity analysis of any mechanism at a given configuration. The procedure uses a base of the motion space. This is obtained from a velocity equation characterized by a geometric matrix. This special form of Jacobian matrix has some advantages for automatic implementation. This approach provides the degree of freedom associated with the singularity, uncontrolled motion, and kinematic dependencies. It also facilitates the choice of actuators and redundant devices. The method has been implemented in a computer program for kinematic analysis.

Multiobjective Optimum Design of a Symmetric Parallel Schönflies-Motion Generator

The multiobjective optimization of a four-degree-of-freedom symmetric parallel manipulator for Schonflies-motion generation is the subject of this paper. First, the inverse positioning problem is discussed, along with its velocity analysis. The workspace is then analyzed, thereby deriving a closed-form expression of the workspace volume, which depends on the orientation of the moving platform. Next, the isotropic design of the manipulator is obtained. Then, the dexterity analysis of the manipulator is conducted, which is one objective of the optimum design of the manipulator, a second objective being a normalized workspace volume. Finally, the results of the optimization process are reported in terms of a set of Pareto-optimum pairs of the design parameters.

Transitions between Multiple Solutions of the Direct Kinematic Problem

The direct kinematic problem in parallel manipulators has multiple solutions that are traditionally called assembly modes. Non-singular transitions between some of these solutions have been detected and shown in the past. Cusp points have been defined as special points on the projection of the singularity curve onto the joint space that have the property of allowing such a non-singular transitions when encircling them. In this paper the authors will show that the condition for such a transition is more general. Authors also argue about the need for a differentiation between the concept of assembly mode and solution of the direct kinematic problem.

Educational software tools for the kinematic analysis of mechanisms

Educational software for the kinematic analysis of planar and spatial mechanisms is presented in this article. This general‐purpose kinematic software has been developed as a complement to Machine Theory lectures. The different modules integrated in the software compute and analyse various kinematic entities, which enable an advanced student to investigate the characteristics of a mechanism.

Judder vibration in disc brakes excited by thermoelastic instability

Friction vibrations and noises which are common in brakes, have attracted a great deal of attention lately. This paper analyses low frequency vibrations in disc brakes excited at high car speed. This vibration, called judder, has a frequency in the range 10 to 300 Hz and usually comes in association with hum noises. The dynamic phenomenon shows two principal components, one normal and the other one tangential to the disc brake surface. It is explained how variations of friction coefficient, and thermoelastic instability caused by the tangential component, contribute to the appearance of judder. A numerical analysis in 3D using the finite element method has been implemented combining both tangential and normal components, and solving the thermoelastic process. Special attention is dedicated to the simulation of the thermoelastic process showing the correlation with experiments.

A Parallelogram-Based Parallel Manipulator for Schonflies Motion

A parallelogram-based 4 degrees-of-freedom parallel manipulator is presented in this paper. The manipulator can generate the so-called Schonflies motion that allows the end effector to translate in all directions and rotate around an axis parallel to a fixed direction. The theory of group of displacements is applied in the synthesis of this manipulator, which employs parallelograms in every limb. The planar parallelogram kinematic chain provides a high rotational capability and an improved stiffness to the manipulator. This paper shows the kinematic analysis of the manipulator, including the closed-form resolution of the forward and inverse position problems, the velocity, and the singularity analysis. Finally, a prototype of the manipulator, adding some considerations about its singularity-free design, and some technical applications in which the manipulator can be used are presented.

A methodology for static stiffness mapping in lower mobility parallel manipulators with decoupled motions

In this paper a general methodology for obtaining static stiffness maps in lower mobility parallel manipulators is proposed. The main objective is to define a set of guidelines, which allow the experimental work to be optimized and computational time to be reduced. First, a two-degree-of-freedom (DOF) mechanism will be used for methodology validation, since it is the stiffness of the basic kinematic chain of the manipulator that is to be analysed. Two mathematical models of this mechanism and an experimental prototype will be considered for the validation. After that, the methodology will be applied to a lower mobility (4-DOF) parallel manipulator. In this paper, the experimental prototype and its set-up is highly important because some particular features of the experimental analysis will be defined. This paper introduces a key experimental tool: the preload, which allows the clearances and possible assembling errors to be considered. The added value from the application of this procedure is the obtaining of graphs that describe, in an intuitive and useful way, the behaviour of the manipulators stiffness inside its workspace as a function of the mobile platform position and orientation.

Diseño de un estudio poblacional del riesgo cardiovascular en Castilla y León a través de los equipos de atención primaria

Se describe el diseno y los indicadores de respuesta de un estudio transversal para estimar diferentes parametros del riesgo cardiovascular en la poblacion de Castilla y Leon. Se obtuvo una muestra de 4.950 personas de 15 y mas anos de edad en dos etapas: en la primera etapa se eligieron 198 medicos de atencion primaria, y en la segunda se selecciono una muestra de 25 personas del cupo de cada medico. Las tasas de respuesta de los medicos alcanzaron el 98%, y las de la poblacion superaron el 80%. Se encontraron diferencias estadisticamente significativas en la distribucion por edad del marco muestral y la muestra definitiva. Despues de realizar las ponderaciones derivadas del diseno, solo el grupo de edad de 35-64 anos se encuentra significativamente sobredimensionado. El acceso a la poblacion general en atencion primaria es un procedimiento factible y eficaz. La alta tasa de respuesta contribuye a la validez de la informacion.