RCC characteristics of planar/spherical three degree-of-freedom parallel mechanisms with joint compliances

A three degree-of-freedom (DOF) planar parallel manipulator has been extensively studied as the fundamental example of parallel manipulators. In this work, we explicitly show that this mechanism possesses a completely decoupled compliance characteristic at the object space, which is the important operational requirement for a RCC device. As the first condition to have a RCC point, this mechanism should maintain symmetric configurations. As the second condition, the same magnitude of revolute joint compliance should be symmetrically placed at the same joint location of each chain. We also investigate the compliance characteristics of a spherical 3 DOF mechanism which has a similar kinematic structure to the planar mechanism through simulation. It turns out that the spherical mechanism also has a RCC point at the intersection point of all nine joint axes in its symmetric configuration. Further, more general output compliance model is derived for those mechanisms with redundant joint compliances. It is expected that these two parallel mechanisms not only can be used as excellent 3 DOF RCC devices, but also can be integrated into the design of a new six DOF RCC device.