Type Synthesis and Parasitic Motion Avoidance of Redundantly Actuated Parallel Mechanisms
|School||Beijing Jiaotong University|
|Course||Mechanical Design and Theory|
|Keywords||Redundantly actuated parallel mechanism Type synthesis independentparameters Parasitic motion Screw theory|
ABSTRACT:For the modernized requirements of major technical equipment, the redundantly actuated parallel mechanism is proposed to improve the wrench capabilities and fault tolerance.A systematic method is proposed for the type synthesis of redundantly actuated parallel mechanism in this thesis. By using the single loop or the multi-loop mechanism as the actuated unit, the possible constraints coupling can be performed in limb unit, and thereby the non-over constrained or the lower order over constrained redundantly actuated parallel mechanisms can be constructed. Based on the above synthesis method, the redundantly actuated parallel mechanisms with three rotational (3R), three translational (3T),2R1T,2T1R,4and5degrees of freedom (DOF) motions are synthesized.Based on the requirements of independent design parameters, the general evolutionary process of redundantly actuated parallel mechanism is presented. Without taking the DOF types into consideration, the position and orientation of a rigid body can be determined by three non-collinear points attached to the rigid body. After the forms of three non-collinear points are determined, the point in space, in planar or in line, which are realized by single loop mechanism or multi-loop mechanism, series of redundantly actuated parallel mechanisms can be obtained.The possible parasitic motions can be eliminated by adding proper redundantly actuated limbs. The typical3-UPU parallel mechanism is chosen as an example to perform the parasitic rotation evaluation and avoidance. Due to the effect of constraint errors, the reciprocal product of constraint wrenches and parasitic motions will not be zero in practice, namely the constraint wrenches act on the moving platform in such a way that it produce works while the moving platform is undergoing the parasitic motions. The norm expression of parasitic motion is obtained, which can be used to perform the evaluation of parasitic motion under the constraint errors. Finally, the redundantly actuated limb is used to eliminate the possible parasitic motions.A novel4-UPU redundantly actuated parallel mechanism with three translations is proposed. The2-UPU single loop mechanism is used as the basic configurational unit to avoid the possible parasitic motion of UPU type parallel mechanism. The forward kinematic of this4-UPU redundantly actuated parallel mechanism is very easy and there is only one structural singular configuration, which can be avoid completely. When there occurs failure of an active joint in one limb and the active joint is locked, the4-UPU parallel mechanism still possesses some spatial translations, and the fault-tolerant workspace is obtained.