Parasitic motion is an important kinematic phenomenon in mechanisms and a generic property of lower mobility parallel mechanisms (PMs), such as: 3-RPS PM (R denotes a rotational joint, P a prismatic joint, and S a spherical joint), Z3 head machine tool (3-PRS PM), Exechon mechanism (2-UPR-SPR, U denotes a universal joint) and so on. Parasitic motions refer to some dependent motions that accompany with other independent motions. It may introduce some undesired motion components which lead to lower manipulation accuracy/quality and more difficulties in calibration. On the other hand, it may perform a motion task by using a lower degree of freedom (DOF) PM that leads to a lower cost, lower complexity of kinematics and easier controls. We should utilize the advantages of parasitic motions and reduce/shield the disadvantages. However, the research on the parasitic motion is far from being adequate. This paper investigates the essence of the parasitic motion and gives a simple identification method. Firstly, the definitions and order of parasitic motions are discussed. Secondly, three classifications of parasitic motions are presented according to the property, amplitude and motion pattern. Then, an equation and corresponding method are detailed to identify the parasitic motion. Some examples are given to illustrate the method. Finally, the means are proposed to reduce/shield or utilize parasitic motion.

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