Owing to the number of actuators is less than the degrees of freedom (DOFs), the underactuated hands own coupled-motion characteristic. Hence, underactuated hands provide a simple way to envelop an object with a reduced number of actuators and without complex control strategies. Self-adaptation, self-adaptive grasp process, and underactuated mechanism control are worthwhile research subjects in the field of underactuated robotic hand. The coupled-motion process of the finger before in contact with the grasped object is not further researched. However, motion strategy analysis at the pre-bending stage is a basic component of underactuated finger grasp research. The initial posture of the finger in contact with the grasped object will directly influence the self-adaptive grasp. The distributions of quasi-static balance postures guide the underactuated mechanism control. Therefore, the quasi-static motion strategy analysis at the pre-bending stage is a basic component of underactuated finger grasp research.This paper presents the pre-bending motion strategy and corresponding cable driving forces analysis of a 3-DOF underactuated finger comprising cable truss units, which uses a tendon pulley transmission and parallel four-linkage mechanism to realise grasps. The structure and the four motion strategies at the pre-bending stage are illustrated. On the basis of a situation where one or two cable driving forces drive the finger, the equivalent joint-driven model and quasi-static motion model are established. According to the virtual work principle, tendon pulley transmission is transformed into equivalent joint-driven system. On the basis of the constraints of maximum motion space of the finger, the joint spring stiffness distributions are discussed and the finger quasi-static motion space is analysed under the condition of one motor driving. The unique coupled motion process and corresponding cable driving force of the finger driven by a single motor are analysed. Furthermore, another three typical quasi-static motion strategies and corresponding cable driving forces are discussed.&nbsp;Valid simulation experiments are conducted to verify the accuracy of the quasi-static motion strategy analysis. Both the theoretical analysis and simulations illustrate that the UACT finger could regulate its posture to adapt to the shapes of the grasped objects by changing the two cable driving forces.This paper is a further in-depth study based on the previous researches and demonstrates that limited trajectory planning is also be carried out in the dual-cable-driven underactuated robotic hand. The analysis method could provide guidance and reference in cable-driven underactuated hands, and is the basic research for control of the couple-driven underactuated mechanisms.Keywords: motion strategy; quasi-static motion; equilibrium configuration; cable-driven finger; underactuated

Pre-Bending Motion Strategy Analysis of a 3-DOF UACT Robotic Finger

Paper Proceedings: https://asmedigitalcollection.asme.org/IDETC-CIE/IDETC-CIE2021/volume/85369

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