A Study on Kinematics and Workspace Determination of a General 6-PUS Robot

Abstract

The 6-UPS parallel manipulator known as Stewart<br><br>platform is the most common type of 6-DOF parallel<br><br>robots. Most industrial applications such as the motion<br><br>simulators and applications requiring high load and high<br><br>workspace use this structure. However, the 6-UPS has its<br><br>disadvantages as the actuated joint is located in the second<br><br>place in the kinematic chain. The 6-PUS is another 6-DOF<br><br>structure and when compared with the 6-UPS, offers several<br><br>advantages by allowing to mount the heavy and the<br><br>vibration inducing actuated prismatic joint on the ground.<br><br>This offers the added benefit of transferring the majority<br><br>of the payload to the ground, resulting in lowering of<br><br>the overall costs. The question is: Can we find a 6-PUS<br><br>architecture that meets or exceeds the prescribed workspace<br><br>while meeting the same kinematics footprint requirement<br><br>of a given 6-UPS? In the present paper, a recently constructed<br><br>6-UPS mechanism for a metro station is selected<br><br>and using an optimization method based on a genetic algorithm<br><br>(GA), a 6-PUS mechanism that meets or exceeds the<br><br>required workspace is identified. To do this, first, the various architectures of the 6-PUS parallel robot are presented.<br><br>The inverse kinematics solution for a general 6-PUS architecture<br><br>is obtained. To fully define the architecture, three<br><br>additional kinematics parameters are selected and employed<br><br>to define the cost function used for the optimization. The<br><br>cost function acts as a penalizing mechanism which ignores<br><br>unwanted architectures of the 6-PUS. Finally, a new concept<br><br>for 6-DOF workspace visualization representation, called<br><br>workspace spheres, is presented. The new concept aids the<br><br>user by offering quantitatively data and visualization tool<br><br>for simultaneous description of rotational and translational<br><br>workspace.