Kinematics of Redundantly Actuated Closed Chains

The instantaneous kinematics of a hybrid manipulation system, which combines the traditional serial chain geometry with parallelism in actuation, and the problem of coordination is discussed. The indeterminacy and singularities in the inverse kinematics and statics equations and measures of kinematic performance are analyzed. Finally, coordination algorithms that maintain an optimal force distribution between the actuators while avoiding or exploiting singularities are presented

Parallel Manipulators

In recent years, parallel kinematics mechanisms have attracted a lot of attention from the academic and industrial communities due to potential applications not only as robot manipulators but also as machine tools. Generally, the criteria used to compare the performance of traditional serial robots and parallel robots are the workspace, the ratio between the payload and the robot mass, accuracy, and dynamic behaviour. In addition to the reduced coupling effect between joints, parallel robots bring the benefits of much higher payload-robot mass ratios, superior accuracy and greater stiffness; qualities which lead to better dynamic performance. The main drawback with parallel robots is the relatively small workspace. A great deal of research on parallel robots has been carried out worldwide, and a large number of parallel mechanism systems have been built for various applications, such as remote handling, machine tools, medical robots, simulators, micro-robots, and humanoid robots. This book opens a window to exceptional research and development work on parallel mechanisms contributed by authors from around the world. Through this window the reader can get a good view of current parallel robot research and applications

Elasto-geometrical modeling and calibration of redundantly actuated PKMs

International audienceRedundantlyactuated parallel kinematic machines (PKMs) offer a number of advantages compared to classical non-redundant PKMs. Particularly, they show a better stiffness thanks to singularity avoidance and they have an improved repeatability due to a better behavior against backlashes. The main problem with the calibration of these machines is that the redundancy leads to some mechanical strains in their structure. This makes it difficult to identify the geometrical errors of their structure without taking into account the effects of the elastic deformations. The main originality of this work is to propose an efficient elasto-geometrical and calibration method that allows the identification of both the geometrical and stiffness parameters of redundantlyactuated parallel mechanisms with slender links. The first part of the paper explains the proposed method through its application on a simple redundant planar mechanism. The second part deals with its experimental application to the redundant Scissors Kinematics machine

Wrench capability of planar manipulators

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2016.Robôs são amplamente utilizados em fábricas, e novas aplicações no espaço, nos oceanos, nas indústrias nucleares e em outros campos estão sendo ativamente desenvolvidas. A criação de robôs autônomos que podem aprender a agir em ambientes imprevisíveis têm sido um objetivo de longa data da robótica, da inteligência artificial, e das ciências cognitivas.Um passo importante para a autonomia dos robôs é a necessidade de dotá-los com um certo nível de independência, a fim de enfrentar as mudanças rápidas no ambiente circundante; para obter robôs que operem fora de ambientes rigidamente estruturados, tais como centros de investigação ou instalações de universidades e sem precisar da supervisão de engenheiros ou especialistas, é necessário enfrentar diferentes desafios tecnológicos, entre eles, o desenvolvimento de estratégias que permitam que os robôs interajam com o ambiente. Neste contexto, quando um contacto físico com o ambiente é estabelecido, uma força específica precisa de ser exercida e esta força tem de ser controlada em relação ao processo a fim de evitar a sobrecarga ou danificar o manipulador ou os objetos a serem manipulados.O principal objetivo deste trabalho é apresentar novas metodologias desenvolvidas para determinar a máxima carga que um mecanismo ou manipulador planar pode aplicar ou suportar (capacidade de carga), sejam eles paralelos, seriais ou híbridos e com redundância ou não. A fim de resolver o problema da capacidade de carga, neste trabalho foram propostas duas novas abordagens com base no método do fator de escala clássico e nos métodos clássicos de otimização. Essas novas abordagens deram como resultado um novo método chamado de método de fator de escala modificado utilizado para resolver a capacidade de carga em manipuladores seriais planares e quatro modelos matemáticos para resolver o problema de capacidade de carga em manipuladores paralelos planares com um grau líquido de restrição igual três, quatro, cinco ou seis (CN = 3, CN = 4, CN = 5 ou CN = 6).Abstract : Robots are now widely used in factories, and new applications of robots in space, the oceans, nuclear industries, and other fields are being actively developed. Creating autonomous robots that can learn to act in unpredictable environments has been a long-standing goal of robotics, artificial intelligence, and cognitive sciences.An important step towards the autonomy of robots is the need to provide them with a certain level of independence in order to face quick changes in the environment surrounding them; to get robots operating outside rigidly structured environments, such as research centres or universities facilities and beyond the supervision of engineers or experts, it is necessary to face different technological challenges, amongst them, the development of strategies that allow robots to interact with the environment. In this context, when a physical contact with the the environment is established, a process-specific force need to be exerted and this force has to be controlled in relation to the particular process in order to prevent overloading or damaging the manipulator or the objects to be manipulated.The main objective of this work is to present new methodologies developed for determining the maximum wrench that can be applied or sustained (wrench capability) in planar mechanisms and manipulators, whether it be serial parallel or hybrid and with redundancy or not. In order to solve the wrench capability problem, in this work two new approaches were proposed based in the classic scaling factor method and in classical optimization methods. These new approaches gave as result a new method called the modified scaling factor method used to solve the wrench capability in planar serial manipulators and four mathematical closed-form solutions to solve the wrench capability problem in planar parallel manipulators with a net degree of constraint equal to three, four, five or six (CN = 3, CN = 4, CN = 5 ou CN = 6)

Inverse Dynamics of a Redundantly Actuated Four-Bar Mechanism Using an Optimal Control Formulation

This paper presents an approach to estimating joint torques in a four-bar closed-chain mechanism with prescribed kinematics and redundant actuation, i.e., with more actuators than degrees of freedom. This problem has several applications in industrial robots, machine tools, and biomechanics. The inverse dynamics problem is formulated as an optimal control problem (OCP). The dynamical equations are derived for an open-chain mechanism, what keeps the formulation simple and straightforward. Sets of constraints are explored to force the three-link open-chain to behave as a four-bar mechanism with a crank rotating at a constant velocity. The controls calculated from the OCP are assumed to be the input joint torques. The standard case with one torque actuator is solved and compared to cases with two and three actuators. The case of two actuators presented the smallest peak and mean torques, using one specific set of constraints. Such torques were smaller than the solution obtained using an alternative method existing in literature that solves the redundancy problem by means of the pseudo-inverse matrix. Comparison with inverse dynamics solutions using well-established methods for the one-actuator closedloop four-bar were equal. Reconstructed kinematical trajectories from forward integration of the closed-loop mechanism with the OCP obtained torques were essentially similar. The results suggest that the adopted procedure is promising, giving solutions with lower torque requirements than the regularly actuated case and redundantly actuated computed with other approaches. The applicability of the method has been shown for the four-bar mechanism. Other classes of redundantly actuated, closed-loop mechanisms could be tested using a similar formulation. However, the numerical parameters of the OCP must be chosen carefully to achieve convergence

Optimization of Dynamic Forces in Mechanical Hands

Mechanical hands have become of greater interest in robotics due Introduction Mechanical hands offer certain advantages over conventional robotic grippers in tasks which require dextrous manipulation. However, they are also much more complex mechanical systems and their design and control must take into account the forces which can be generated during grasping. One problem which has been of recent interest is that of finding the finger/object contact forces and the actuator torques necessary to effect a prescribed motion of the hand/object system. However, when an object is grasped, the hand becomes redundantlyactuated and the dynamic force equations do not have a unique solution. Orin and Oh (1981) noted this problem while developing a technique to determine forces in a very similar problem: the design of walking machines. They proposed a linear-programming approach to obtain an optimum solution which minimized the energy consumption of the system. Kerr and Roth (1986) proposed a more conservative application of linear programming which maximized the "distance" from the solution to the inequality constraints. However, linear programming has certain disadvantages. It can produce solutions which are discontinuous for small changes in the constraints (typical of the motion of a hand during its task) when the constraints become parallel to the objective. As well, linear programming cannot be used to minimize the "internal forces" in the handobject system. These forces can be thought of as those which tend to crush the object or, alternatively, as those which do not contribute to the motion of the hand/object system