A Framework to Illustrate Kinematic Behavior of Mechanisms by Haptic Feedback

The kinematic properties of mechanisms are well known by the researchers and teachers. The theory based on the study of Jacobian matrices allows us to explain, for example, the singular configuration. However, in many cases, the physical sense of such properties is difficult to explain to students. The aim of this article is to use haptic feedback to render to the user the signification of different kinematic indices. The framework uses a Phantom Omni and a serial and parallel mechanism with two degrees of freedom. The end-effector of both mechanisms can be moved either by classical mouse, or Phantom Omni with or without feedback

Investigation on the Effort Transmission in Planar Parallel Manipulators

International audienceIn the design of a mechanism, the quality of effort transmission is a key issue. Traditionally, the effort transmissivity of a mechanism is defined as the quantitative measure of the power flowing effectiveness from the input link(s) to the output link(s). Many researchers have focused their work on the study of the effort transmission in mechanisms and diverse indices have been defined. However, the developed indices have exclusively dealt with the studies of the ratio between the input and output powers and they do not seem to have been devoted to the studies of the admissible reactions in passive joints. However, the observations show that is possible for a mechanism to reach positions in which the transmission indices will have admissible values but the reaction(s) in passive joint(s) can reach excessively high values leading to the breakdown of the mechanism. In the present paper, a method is developed to ensure the admissible values of reactions in passive joints of planar parallel manipulators. It is shown that the increase of reactions in passive joints of a planar parallel manipulator depends not only on the transmission angle but also the position of the instantaneous centre of rotation of the platform. It allows the determination of the maximal reachable workspace of planar parallel manipulators taking into account the admissible reactions in its passive joints. The suggested method is illustrated vie a 5R planar parallel mechanism and a planar 3-RPR parallel manipulator. I Introduction Parallel manipulators have many advantages in terms of acceleration capacities and payload-to-weight ratio [1], but one of their main drawbacks concerns the presence of singularities [2]-[5]. It is known that in the neighbourhood of the singular positions the reactions in joints of a manipulator considerably grow up. In order to have a better understanding of this phenomenon, many researchers have focused their works on the analysis of the effort transmission in parallel manipulators. One of the evident criterions for evaluation of effort transmission is the transmission angle (or pressure angle which is equal to 90 degrees minus the transmission angle) [7]-[9]. The pressure angle is well known for characterizing the transmission quality in lower kinematic pairs, such as cams [10], but this idea was also used for effort transmission analysis in the parallel manipulators [7], [9]. To evaluate the effort transmission quality, several indexes have been introduced. One of the first attempts was proposed in [6]. This paper presents a criterion named the Transmission Index (TI) that is based on transmission wrench screw. The TI varies between 0 and 1. If it is equal to 0, the considered link is in a dead position, i.e. it cannot move anymore. If it is equal to 1, this link has the best static properties. In the same vein as [6], the study [11] generalizes the TI for spatial linkages and defines the Global TI (GTI). The authors also prove that the GTI is equal, for prismatic and revolute joints, to the cosine of the pressure angle. The conditioning index was also proposed [12] for characterizing the quality of transmission between the actuators and the end-effector. This index is based on the Jacobian matrix or its "norm", which relate the actuator velocities (efforts, resp.) to the platform twist (wrench, resp.) by the following relations:  t Jq  and T   w J τ , where J is the Jacobian matrix, t the platform twist, q  the input velocities,  the actuator efforts, and w the wrench applied on the platform. All these indices have been used in many works for design and analysis of parallel mechanisms [14]-[21]. However, it is also known that because of the non homogeneity of the terms of the Jacobian matrix, the conditioning index is not well appropriated for mechanisms having both translational and rotational degrees of freedom (DOF) [13]. Moreover, all the previously mentioned indices do not take into account the real characteristics of the actuators, i.e. the fact that their input efforts are bounded between [-max i  , max i  ] [13]. In order to solve this problem, in study [22] a numerical analysis method has been developed. It has been proposed to characterize the force workspace of robots taking into account a given fixed wrench applied on the platform and actuator efforts comprised in the boundary interval [-max i  , max i  ]. However, this workspace depends on the given direction and norm of the external force/moment and will change with the variation of the applied wrench. Moreover, for many robot applications, the external wrench direction is not known, contrary to its norm. Therefore, in [23], a way to compute the maximal workspac

Realistic Rendering of Kinetostatic Indices of Mechanisms

International audienceThe work presented in this paper is related to the use of a haptic device in an environment of robotic simulation. Such device introduces a new approach to feel and to understand the boundaries of the workspace of mechanisms as well as its kinetostatic properties. Indeed, these concepts are abstract and thus often difficult to understand for the end-users. To catch his attention, we propose to amplify the problems of the mechanisms in order to help him to take the good decisions

Obtenção dos mapas da capacidade de força isotrópica em manipuladores seriais planares com aplicações na definição da tarefa

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2017.A interação de robôs pode se dar por meio da ação de forças estáticas, ou seja, quando não existem movimentos significativos entre o robô e o meio, ou dinâmica, quando o robô está em movimento mantendo o contato com o meio. Quando a movimentação é relativamente lenta, é possível considerar a interação como quase-estática, pois os efeitos dinâmicos podem ser desprezados. Nas tarefas em que se exige contato a baixas velocidades, especificações e estudos sobre a capacidade de gerar forças e momentos (ou simplesmente capacidade de força) de robôs tornam-se interessantes. Em muitos casos, durante a execução da tarefa, a capacidade de força pode estar abaixo das forças requeridas pela tarefa em algum ponto da trajetória, o que pode acarretar a ocorrência de danos materiais ou ferimentos em pessoas durante o processo. A capacidade de força nos robôs depende basicamente do potencial de geração torques, ou forças em seus atuadores de revolução ou prismáticos respectivamente e das direções das forças aplicadas. Neste sentido surge o conceito de forças isotrópicas, que consiste na máxima magnitude de força que um robô pode aplicar ou suportar em todas as direções de acordo com uma postura. O objetivo principal deste trabalho é o desenvolvimento de um método, para obtenção dos mapas da capacidade de forças isotrópicas em manipuladores seriais planares em condições estáticas ou quase-estáticas. O estudo será realizado utilizando robôs com estruturas cinemática do tipo 3R, 4R, 5R e 6R. O método proposto tem como resultado um mapa de indicação das maiores forças isotrópicas que podem ser exercidas dentro da área de trabalho do robô. Os resultados obtidos permitem alocar uma tarefa na região com maior capacidade de força isotrópica, garantindo que a força mínima requerida pela execução da tarefa seja atendida. Neste trabalho, a modelagem estática é realizada através da teoria de helicóides e do método de Davies. A capacidade de força isotrópica é calculada através do método denominado fator de escala modificado. O método proposto é estudado por meio de casos e validado comparando os resultados com os métodos pesquisados no referencial teórico.Abstract: The interaction of robots can occur through the action of static forces, when there is no significant movement between the robot and the environment, or dynamics, when the robot is in motion while maintaining contact with the environment. When the movement is relatively slow, it is possible to consider the interaction as quasi-static, since the dynamic effects can be neglected. In tasks requiring contact at low speeds, specifications and studies on the robots ability to generate forces and moments (or simply force capability) become interesting. In many cases, during the execution of the task, the force capability may be below the forces required by the task at some point in the trajectory, which can lead to material damage or injury people during the process. The capability of force in the robots depends basically on the potential of generation of torques, or forces in their revolution or prismatic actuators, respectively and on the directions of the applied forces. In this sense arises the concept of isotropic forces, which consists of the maximum magnitude of force that a robot can apply or support in all directions according to a posture. The main objective of this work is the development of a method to obtain the isotropic forces maps for planar serial manipulators under static or quasi-static conditions. The study will be performed using robots with kinematic structures of 3R, 4R, 5R and 6R type. The proposed method results in a map indicating the greatest isotropic forces that can be exerted within the workspace of the robot. The obtained results allow to allocate a task in the region with greater capability of isotropic force, ensuring that the minimum force required for the execution of the task is met. In this work, static modeling is performed through the screw theory and the Davies method. The isotropic force capability is calculated using the method called the modified scale factor. The proposed method is will be applied to several cases studies and validated by comparing the results obtained from it with the methods found in the literature