Dynamics evaluation of 2UPU/SP parallel mechanism for a 5-DOF hybrid robot considering gravity
Introduction
Because of low density and high strength, the composites have been increasingly applied in many fields, especially in the aerospace field [1]. Most of the aerospace structural components bearing incredibly heavy load when the aerospace vehicle is launched are made of composite materials. Because of the anisotropic mechanical properties and the lower interlayer strength of composites, there may exist several defects during the processing, such as delamination, burrs, and carbon fiber tear. Robot processing is an effective method for machining composite materials [2], [3]. The large size and variety of these aerospace composite structural components raise higher demands on the stiffness and workspace of robots for machining during the drilling and milling processing [4]. The serial robot usually has positive dexterity and large workspace while the parallel robot has high stiffness and good force loading capacity [5], [6], [7], [8], [9]. The hybrid robot, which combines advantages of serial robot and parallel robot, is a better choice for machining composite material, especially the aerospace composite structural components [10].
Dynamic performance evaluation has attracted increasing attention since the parallel mechanisms are used in more and more machine tools and robots [11], [12], [13], [14]. The processing of aerospace composite structural components with complex shapes and multiple curved surfaces requires high acceleration/deceleration performance. Many evaluation indices are proposed to evaluate the dynamic performance. These performance indices can be mainly divided into ellipsoidal description methods and non-ellipsoidal description methods [15]. The GIE [16] and the DME [17] are two of the most famous ellipsoidal description methods. The GIE shows the easiness of generating arbitrary velocity of the end-effector by a given force, and the DME represents the easiness of generating arbitrary acceleration of the end-effector by a given set of forces. In addition, a number of non-ellipsoidal description methods have been proposed. Kim and Desa [18] proposed the acceleration sets, which use parallel polyhedrons to reflect the acceleration performance distribution of the mechanism. Bowing and Khatib [19] proposed dynamic capability equations (DCE) and used it to evaluate the dynamic performance of PUMA560 serial robot. Most of these evaluation methods always neglect the effect of the velocity term and gravity which are generally considered to play a small role in driving forces. However, the gravity has a non-negligible effect on the dynamics of a heavy parallel mechanism with certain configuration direction.
In this paper, a dynamic performance evaluation index considering gravitational effect is proposed to investigate the dynamics of a 2UPU/SP parallel mechanism in a 5-DOF hybrid robot for aerospace composite machining. Based on the dynamic performance index, the placed direction of 2UPU/SP mechanism in the 5-DOF hybrid robot is discussed. Considering that the 2UPU/SP mechanism is similar to the parallel mechanism in the well-known Tricept robot [20], the dynamic performance of the two mechanisms are also compared. The rest of this paper is organized as follows: Section 2 formulates the inverse dynamic model of the 2UPU/SP mechanism. Section 3 proposes a dynamic performance evaluation index considering gravity. Section 4 compares the influence of the 2UPU/SP mechanism placement direction on the dynamic performance and the dynamic characteristics of 2UPU/SP and the parallel mechanism in traditional Tricept robot. In Section 5, some conclusions of this paper are given.
Section snippets
Structure description
The 5-DOF hybrid robot shown in Fig. 1 is designed for processing aerospace structural components made of composites [21]. It is composed of a 3-DOF 2UPU/SP parallel mechanism and a 2-DOF RR rotating head. The 2UPU/SP parallel mechanism, whose schematic diagram is shown in Fig. 2, consists of a moving platform (MP), a fixed base and three limbs. Limb 1 is connected with the base and the moving platform respectively by universal (U) joints and driven by prismatic (P) joint. Limb 2 has the same
Dynamic performance indices considering gravity
As addressed in [23], the dynamic performance of a high-acceleration manipulator can be represented by the arbitrariness degree of changing the acceleration on the actuated joint force. Considering at the initial phase of motion and setting and , Eq. (41) can be rewritten as where .
Because the larger driving force could be regarded as the worse working situation, the maximum driving force under certain motion restrictions can be used as the evaluation index for the
Effect of 2UPU/SP mechanism configuration direction on dynamic performance
The influence of gravity on the dynamic performance varies with the configuration direction of the mechanism. Here, the effect of configuration direction on the dynamic performance of the 2UPU/SP mechanism is investigated. The geometric and inertial parameters of the mechanism are given in Appendix.
As for the 2UPU/SP mechanism studied in this paper, the three active limbs are all driven by servo motors through ball-screw systems. The maximum acceleration of the ball screw system is generally
Conclusions
In this paper, a dynamic performance evaluation of a 2UPU/SP parallel mechanism in a hybrid robot for aerospace composite machining is investigated. A dynamic performance index considering gravity is proposed and the influence of the 2UPU/SP mechanism arrangement direction on the dynamic performance is studied. Moreover, the dynamic performance of 2UPU/SP and the traditional Tricept mechanism are compared. The conclusions are drawn as follows:
(1) Considering the influence of gravity, the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by the National Key Research and Development Program of China-Intelligent Robotics (Grant No. 2017YFB1301904).
Xiaojian Wang received the B.S. degree in mechanical engineering from Tsinghua University, Beijing, China, in 2019. He is currently working toward the Ph.D. degree in the Department of Mechanical Engineering, Tsinghua University, Beijing, China.
His current research interests include dynamics and optimal design of parallel mechanism.
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Cited by (0)
Xiaojian Wang received the B.S. degree in mechanical engineering from Tsinghua University, Beijing, China, in 2019. He is currently working toward the Ph.D. degree in the Department of Mechanical Engineering, Tsinghua University, Beijing, China.
His current research interests include dynamics and optimal design of parallel mechanism.
Jun Wu received the B.S. and M.S. degrees in mechanical engineering from Tianjin University, Tianjin, China, in 2000 and 2003, respectively, and the Ph.D. degree in mechanical engineering from Tsinghua University, Beijing, China, in 2008. He is currently an Associate Professor in the Department of Mechanical Engineering, Tsinghua University, Beijing, China.
His current research interests include robot dynamics, dynamics and control of parallel kinematic machine, and optimal design of redundant parallel mechanism.
Yutian Wang received the B.S. degree in mechanical engineering from Changchun University of Science and Technology, Changchun, China, in 2018. He is currently a Ph.D. candidate in the Department of Mechanical Engineering, Tsinghua University, Beijing, China.
His current research interests include the dynamics and control of parallel mechanism.