Design and control of a cable-driven 6-DOF loading simulator

摘要

In this work, Cable-Driven Omnidirectional Loading Simulator (CabOLS) is designed and built to accurately control a 6-DOF wrench on a fixed or slow moving target. The CabOLS offers several important advantages: simplicity and efficiency of the mechanical structure and controller, precision in load simulation as well as the cost efficiency. The mechanical design of CabOLS is innovative in that it employs an accurate linear spring in each cable to estimate the tension in the cable instead of using a force sensor which adds complexity to the design. The spring also compensates for the nonlinear effect of backlash of the gearbox and thereby makes a simple control topology feasible.ududThe structure of the controller is managed to be as simple as possible without losing efficiency. In order to achieve accurate force control on the target object two levels of control in Cartesian and joint spaces were considered. Optimal projection of the tension in the cables i.e. redundancy resolution is examined in this work. It is proven that even though the redundancy resolution algorithm is nonlinear, the combination of the redundancy resolution algorithm and the model of the CabOLS is linear. Linearity makes it possible to apply robust method to simultaneously formulate the gains of the controller in both spaces. Moreover, the real-time redundancy resolution algorithm was successfully developed and utilized in closedloop control system.ududThe present work also demonstrates the application of the CabOLS for stiffness analysis of industrial robots. In an automated process the CabOLS is controlled to exert the desired wrench vector on an ABB robot and a laser tracker is employed to measure the related deflection. Simultaneously, the stiffness of joints is identified by means of the incoming data. In this work nonlinear and linear modeling of the joint stiffness are also formulated. CabOLS as a dynamic load simulator, makes it feasible to identify joint stiffness using either linear or nonlinear modeling. Moreover, the CabOLS makes it possible to validate the identified stiffness parameters.ududThis work also formulates the generalized, compact, and tractable closed-form of dynamics of cable-driven parallel manipulators. This formulation is innovative in that it employs Lagrangian variable mass analysis to exert the effect of mass streaming caused by cable elongation.