Towards cooperative manipulation using cable robots
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Cable robots form a class of parallel architecture robots with significant benefits including simplicity of construction, low cost, large workspace, significant payload to weight ratio and end-effector stiffness. In this work, we seek to extend the current capability of cable robots towards cooperative manipulation. Specifically, we first explore inclusion of mobility into the bases (in the form of gantries, and/or vehicle bases) which can significantly further enhance the capabilities of cable robots. However, this also introduces redundancy and complexity into the system which needs to be carefully analyzed and resolved. We propose a generalized modeling framework for systematic design and analysis of cooperative mobile cable robots, building upon knowledge base of multi-fingered grasping, and illustrate it with a case study of four cooperating gantry mounted cable robots transporting a planar payload. We then explore modifications on the payload attachment as alternate means to simplify the design and enable practical deployment. We examine analysis of the system and develop a virtual cable-subsystem formulation (which also facilitates subsumption into the previously developed mobile cable robot analysis framework). We also seek improvement of the tension distribution by utilizing configuration space redundancy to shape the tension nullspace. For true load sharing among multiple robots, we also investigate explicitly the compliance in cable robots and the modulation of task space stiffness of mobile cable robots. First the compliance is introduced via linear springs connected in series with non-extensible cables. The benefit of such series elastic cables include tension control without using force sensors and tension redistribution. We exploit the configuration redundancy in mobile cable robots to optimize certain desired task space stiffness criterion. Then we move on to variable stiffness modules instead of dealing with constant sitffness springs. Traditionally, most existing variable stiffness modules tend to be bulky by virtue of their use of solid components making them less suitable for mobile applications. In recent times, pretensioned cable-based modules have been proposed to reduce weight. While passive, these modules depend on significant internal tension to provide the desired stiffness and stiffness modulation capability tends to be limited. We present design, analysis and testing of a cable based active variable stiffness module that can be realized to achieve a large stiffness range with decoupled tension. A one degree-of-freedom (DOF) rotational joint is set up using two of these modules to evaluate the capability. We then present a planar 2DOF cable robot formed by three active variable stiffness modules. By controlling each module's stiffness, the overall Cartesian stiffness of the robot can be modulated. We show that this approach is more effective than by increasing internal tension only. It is also more efficient than varying configuration to achieve variable stiffness. Further, it is able to independently vary stiffness and internal tension achieving same Cartesian stiffness with much lower internal tension, which is more efficient.