Exploiting Higher Kinematic Performance - Using a 4-Legged Redundant PM Rather than Gough-Stewart Platforms

Abstract

It is believed that Gough and Whitehall (1962) first introduced parallel robots with an application in tire-testing equipments, followed by Stewart (1965) , who designed a parallel mechanism to be used in a flight simulator. With ever-increasing demand on the robot’s rigidity, redundant mechanisms, which are stiffer than their non-redundant counterparts, are attracting more attention. Actuation redundancy eliminates singularity, and greatly improves dexterity and manipulability. Redundant actuation increases the dynamical capability of a PM by increasing the load-carrying capacity and acceleration of motion, optimizing the load distribution among the actuators and reducing the energy consumption of the drivers. Moreover, it enhances the transmission characteristics by increasing the homogeneity of the force transmission and the manipulator stiffness (Yi et al., 1989). From a kinematic viewpoint, redundant actuation eliminates singularities (Ropponen & Nakamura, 1990) which enlarge the usable workspace, as well. The kinematic analysis on general redundantly actuated parallel mechanisms was investigated by Muller (2005). A number of redundantly full-actuated mechanisms have been proposed over the years and some of them which are more significant are listed in this section. The spatial octopod, which is a hexapod with 2 additional struts, is one of them (Tsai, 1999). A 5-DOF 3-legged mechanism was proposed by Lu et al. (2008), who studied its kinematics, statics, and workspace. Staicu (2009) introduced a new 3-DOF symmetric spherical 3-UPS/S parallel mechanism having three prismatic actuators. As another work of Lu et al. (2009), they introduced and used 2(SP+SPR+SPU) serial–parallel manipulators. Wang and Gosselin (2004) addressed an issue of singularity and designed three new types of kinematically redundant parallel mechanisms, including a new redundant 7-DOF Stewart platform. They concluded that such manipulators can be used to avoid singularities inside the workspace of non-redundant manipulators. Choi et al. (2010) developed a new 4-DOF parallel mechanism with three translational and one rotational movements and found this mechanism to be ideal for high-speed machining.