Analysis, realization and application of the tendon-based parallel robot SEGESTA
Parallel kinematic structures, like Stewart platforms, for manipulating objects offer advantages - compared to serial manipulators - in terms of force-to-weight ratio or positioning accuracy. Due to placing all actuators on the base of the manipulator, these do not need to support or drive the mass of the other actuators. Therefore, by using light-weight manipulators with small actuators, realized as parallel kinematic structures, one is able to generate high-dynamic motion. While in the classical concept of Stewart platforms the linear actuators are spindles or hydraulic/pneumatic cylinders, one is able to generate even higher accelerations by replacing the spindles or cylinders by tendons which are actuated by electric motors on the base of the manipulator. Since tendons can generate only tension and therefore, kinematically, have to be regarded as unilateral constraints, the kinematical and dynamical analysis of Tendon-Based Stewart Platforms (TBSP) is of special interest. Due to the complexity of its mechanical properties, together with the high requirements regarding actuation, sensing, and control, this is a typical example of a mechatronic system. Trajectory planning, based on a nonlinear control concept must consider various constraints like workspace limitations, actuator and tendon limits, as well as smooth motion requirements. As an experimental setup the prototype SEGESTA has been developed at the Chair of Mechatronics at the University of Duisburg- Essen. It has meanwhile been successfully applied for the testing of electrical control units (ECU) in passenger cars. At the time being, the SEGESTA test stand uses a basic control program which performs the desired continuous path trajectories based on simple control algorithms. To reach even higher velocities of the platform at the desired accuracy, more complex controllers are actually under development. To realize those modifications, a modular controller software is advantageous. MCA2 was chosen, as it offers a helpful program structure and even a graphical user interface (GUI). Using this GUI, the operation of the SEGESTA test stand will be more intuitive for the user. Running MCA2 on a Linux PC, costs for soft- and hardware are reduced while having a complete and comfortable standard operating system, which offers an extensive number of helpful applications. To provide a versatile interface for industrial standard components, CAN will be used both for optional bidirectional communication between PC and motor controllers on the one hand side and data exchange with external hardware (e.g. sensors) on the other. Furthermore, graphical editors for trajectory generation and workspace visualization will be developed and integrated to make the SEGESTA test stand ready for future technical applications.
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