Two different visual servoing controls have been developed to govern a translating parallel manipulator with an eye-in-hand configuration, That is, a position-based and an image-based controller. The robot must be able to reach and grasp a target randomly positioned in the workspace; the control must be adaptive to compensate motions of the target in the 3D space. The trajectory planning strategy ensures the continuity of the velocity vector for both PBVS and IBVS controls, whereas a replanning event is needed. A comparison between the two approaches is given in terms of accuracy, fastness, and stability in relation to the robot peculiar characteristics.
Abstract. The article describes the dynamic modelling of I.Ca.Ro., a novel Cartesian parallel robot recently designed and prototyped by the robotics research group of the Polytechnic University of Marche. By means of screw theory and virtual work principle, a computationally efficient model has been built, with the final aim of realising advanced model based controllers. Then a dynamic analysis has been performed in order to point out possible model simplifications that could lead to a more efficient run time implementation.
The direct position analysis of parallel manipulators (PMs) brings to determine a finite number of platform poses compatible with an assigned\ud
set of actuated-joint variables’ values. Therefore, when, during functioning, the need to check the actual platform pose arises, the sensors usually located on the actuators are not sufficient and the additional pieces of information coming from ad-hoc-devised extra sensors are necessary. Here, for the first time, the actual implementation of extra sensors in underactuated parallel wrists (PWs) which contain a nonholonomic\ud
constraint is addressed. Differently from ‘‘ordinary’’ (i.e., non-underactuated) PWs, these PWs requires a continuous monitoring of the actual platform orientation to compensate the possible sliding in the nonholonomic constraint. Thus, in this case, the algorithms that interpret the pieces of information coming from the sensors must work in real time and must be integrated in the control software that manages the motion of the machine. All these special requirements are satisfied by the proposed algorithm. Moreover, the proposed hardware can also be implemented in some ordinary PWs with a cheap add-on kit
The article presents a prototypal design of a spherical parallel machine that can be used for the orientation of parts or devices: the peculiarity of this wrist is that, by exploiting non-holonomic constraints, it is able to bring the end-effector in any 3 d.o.f.'s orientation within its working space by only using 2 actuators. It is shown that, despite the higher complexity of the mechanical design, such architecture features good kinematic performances with respect to similar holonomic machines. Moreover, the use of 4 position sensors allows to obtain just one solution for the direct orientation problem, characterized by simple mathematical expressions. The kinematic performances of the wrist make it a suitable choice for the functional design of machines to be used for static tasks or point to point motions
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