On Type Synthesis, Kinematic Performances And Energy Harvesting Of 3-SPS Tensegrity Parallel Mechanisms

Parallel mechanisms have the advantages of high stiffness, low cumulative error, better control precision etc. They have been widely applied in the disciplines such as motion simulator, space docking and manufacturing.The development of mechanisms has benefited from their innovations. In the recent years, a kind of novel mechanisms named tensegrity parallel mechanisms has been proposed by the scholars. Unlike conventional parallel mechanisms, tensegrity parallel mechanisms use springs as tensile components. The use of springs allows tensegrity parallel mechanisms have the ability of under-actuated. Moreover, the gaps between the joints have been eliminated. The motion precision of tensegrity parallel mechanisms has thus been improved. This paper investigates the configuration evolution, direct and inverse position analysis, singularities, workspaces and applications of 3-SPS(Spherical joint-Prismatic joint- Spherical joint) tensegrity parallel mechanisms. The main contents of this paper are as follows:1) The initial configuration of 3-SPS tensegrity parallel is described. This mechanism is composed of a mobile platform, a fixed platform, three SPS limbs and three springs. It has three input variables. However, the number of output variables is six. Therefore, the output of the mechanism is indeterminate. Considering the constraints brought by springs, the output of the mechanism is determinate when it is in equilibrium. When the mechanism is not in equilibrium, passive limbs are needed to make the mechanism’s configuration determined. On the basis of screw theory, twenty mechanisms with different motion characteristics are obtained. The research objects are as follows: one is the tensegrity parallel mechanisms with passive limbs(non-under actuated tensegrity parallel mechanisms); the other is tensegrity parallel mechanisms with no passive limbs(underactuated tensegrity parallel mechanisms). For these two kinds of mechanisms, a method has been proposed to describe their structures.2) The direct and inverse position analysis of the two kinds of tensegrity parallel mechanisms is researched. A search method with variable steps is proposed to solve direct position analysis of the tensegrity parallel mechanisms with passive limbs. This method finds the solutions point by point with a certain step. If the solutions are not found, the step will become smaller and the solutions will be searched again. The model of this method is simple and it is not sensitive to initial values. It can thus be used to find the solutions to the direct position analysis of any mechanisms. Based on the energy method, the general equations of the tensegrity parallel mechanism with no passive limbs are developed. These equations are solved by using the search method with variable steps. The results indicate that the position analysis of tensegrity parallel mechanisms with passive limbs depends on their geometries. However, for the tensegrity parallel mechanism with no passive limbs, its position analysis depends on not only its geometry but also the forces in the springs.3) Based on the direct and inverse kinematic Jacobian matrices, the singularities of the tensegrity parallel mechanisms with passive limbs have been researched. The configurations corresponding to class 1, class 2 and class 3 singularities have been obtained and the singular curves have been mapped. Morever, the strategy for avoiding thsee singularies has been discussed.4) The workspaces of the two kinds of tensegrity parallel mechanisms have been researched. For tensegrity parallel mechanisms with passive limbs, the factors impacting their workspaces are researched. These factors are as follows: ① Limitations of active limbs;② Limitations of joint angles;③ Interferences between bars。The constraint equations for finding inside areas and boundaries of the workspace are developed. By using the numerical method, the volumes of the workspaces of the non-under-actuated tensegrity parallel mechanisms with different motion characteristics. The sensitive of the workspace to the pose parameters and the maximum of the allowable angle of the joints has been analysed. Considering the energy constraints, the method for computing the under-actuated tensegrity parallel mechanisms is proposed and the volume of its workspace is computed. The sensitive of its workspace to the maximum of the allowable angle of the joints has been also analysed.5) A novel energy harvesting device on the basis of 3-SPS tensegrity systems is proposed. The dynamic model of the proposed device is developed using the linear wave model. This dynamic model considers the coupling between the body dynamics and fluid dynamics. Moreover, this model is simulated by using the Longe-kuta method. The motion laws of the mobile platform have been revealed. The efficiency of electricity generating of the proposed device is compared with that of a conventional floating device. The results indicate that the spatial tensegrity-based harvester allows to harvest 20.85% more energy than a traditional floating system.