Reconfigurability of a class of parallel kinematics machines with lower mobility

The complexity of the kinematic models related to parallel kinematics machines (PKMs) with full mobility has directed recent studies toward the search for lower mobility PKMs. The latter generally have simpler analytic models at the expense of a reduced mobility. More recent works have shown that some kinematic architectures can have a degree of reconfigurability, namely they can change the mobility of their moving platform with local interventions on their mechanical structure [1],[2]. The reconfigurability is often obtained by means of lockable joints, which can be activated or deactivated in order to change leg kinematics. In particular the authors have found that a 3-CPU manipulator, which has three legs with a cylindrical-prismatic-universal sequence of joints, can change its mobility from pure translation to pure rotation if all the universal joints are reconfigurable [3]. In fact, the universal joints can be thought of as lockable spherical joints, which are obtained as a serial chain of three revolute joints where one axis can be locked. Two different configurations of the universal joints can be realized [4]: the first provides the robot with a translating behavior of its moving platform, whereas the second with a rotating one. The present paper extends such study to other kinematics architectures, inheriting the concept of lockable spherical joints. Mathematical tools, like screw theory and group theory, have allowed to identify new topologies of reconfigurable 3-DoF PKMs. All machines can be either translating or rotating PKMs alternately, according to the configuration of the spherical lockable joints. The reconfigurable joints can be activated manually or automatically by means of solenoids or simple brakes.