Type Design of Decoupled Parallel Manipulators with Lower Mobility

Abstract

A typical parallel mechanism consists of a moving platform, a fixed base, and several kinematical chains (also called the legs or limbs) which connect the moving platform to its base. Only some kinematical pairs are actuated, whose number usually equals to the number of degrees of freedom (dofs) that the platform possesses with respect to the base. Frequently, the number of legs equals to that of dofs. This makes it possible to actuate only one pair per leg, allowing all motors to be mounted close to the base. Such mechanisms show desirable characteristics, such as large payload and weight ratio, large stiffness, low inertia, and high dynamic performance. However, compared with serial manipulators, the disadvantages include lower dexterity, smaller workspace, singularity, and more noticeable, coupled geometry, by which it is very difficult to determine the initial value of actuators while the end effector stands at its original position. In an engineering point of view, it is always important to develop a simple and efficient original position calibration method to determine initial values of all actuators. This calibration method usually becomes one of the key techniques that a type of mechanism can be simply and successfully used to the precision applications. Accordingly, few have been reported that the parallel manipulators being applied to high precision situations except micro-movement ones. The study of movement decoupling for parallel manipulators shows an opportunity to simply the original position calibration and to improve the precision of parallel manipulators in a handy way. One of the most important things in the study of movement decoupling of parallel manipulators is how to design a new type with decoupled geometry. Decoupled parallel manipulators with lower mobility (LM-DPMs) are parallel mechanisms with less than six dofs and with decoupled geometry. This type of manipulators has attracted more and more attention of academic researchers in recent years. Till now, it is difficult to design a decoupled parallel manipulator which has translational and rotational movement simultaneously (Zhang et al., 2006a, 2006b, 2006c). Nevertheless, under some rules, it is relatively easy to design a decoupled parallel manipulator which can produce pure translational (Baron & Bernier, 2001; Carricato, & Parenti-Castelli, 2001a; Gao et al., 2005; Herve, & Sparacino, 1992; Kim & Tsai, 2003; Kong & Gosselin, 2002; Li et al., 2005a, 2005b, 2006a; Tsai, 1996; Tsai et al., 1996; Zhao & Huang, 2000) or rotational (Carricato & Parenti-Castelli, 2001b, 2004; Gogu, 2005; Li et al., 2006b, 2007a, 2007b) movements.