Velocity stability is one of the most important properties for a parallel tool head during machining process. However, due to the nonlinear kinematic transmission characteristics between the motion in joint space and the motion in task space of a parallel manipulator, there exists obvious rotational velocity fluctuation problem of a parallel tool head which couples two degrees of freedom and one translational degree of freedom (2R1T), and the kinematic performance would be heavily affected by the rotational velocity fluctuation problem. Thus, it is a core issue of how to ensure velocity stability of this type parallel tool heads under general numerical control (NC) system. In this paper, by taking a typical 2R1T parallel tool head as an object of study, a method for solving the rotational velocity fluctuation problem is investigated. First, the nonlinear kinematic characteristics of the 2R1T parallel tool head are analyzed to explain the reason of the rotational velocity fluctuation. Next, a completed optimization method of feed rate is proposed to reduce the rotational velocity fluctuation, and the different displacement increments and velocity mapping abilities of each driving shaft are considered. Finally, a comparison experiment is carried out to validate the effectiveness of the method. The result shows that the rotational velocity fluctuation of the 3-DOF parallel tool head is almost eliminated by setting appropriate feed rate.

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