Research On The Friction And Collision Of Flexible Robot Fingers And Painlevé Paradox

Finger system is a kind of automatic operation device which can not only imitate the structure and function of human finger,but also can grasp and carry objects or use tools.Like rigid robotic finger,the frictional contact will happen inevitably when the flexible robotic finger carries out a complex mission.The frictional contact problem can be further divided into two simpler problems: 1)tangential friction problem with only tangential initial velocity;2)oblique impact with both of normal and tangential initial velocity.At present,there is no enough research on these two problems,and it is especially lack of researches on the Painlevé paradox in the first problem.Therefore,the frictional contact mechanism of flexible robotic finger is focused in this paper.The specific research contents and conclusions are as follows:(1)A novel hybrid computational model(HCM)and enent driven numerical method are presented.The HCM takes account of the normal and tangential compliance of the fingertip and global compliance of flexible robotic finger altogether.The deformation field and inertia filed of finger structure is discretized by absolute nodal coordinate formulation(ANCF).The normal and tangential compliance of the fingertip is described by a local contact model,and the calculation formula of contact force of normal compression-restitution state and tangential stick-slip state is given.The governing equations of finger system in different states are devired,and the switching criteria of these states are given.(2)The oblique impact of flexible robotic finger is studied by the present HCM.The feasibility of the proposed model is validated by comparing the HCM solution and the LS-DYNA solution.The numerical results show that the normal relative motion between the fingertip and the rough surface will experience 1-4 compression-restitution transitions when Young's Modulus is from 0.01 GPa to 1GPa or the slenderness ratio of phalanx is from 10.7 to 32.Furthermore,when the configuration of the flexible robotic finger controlled by driven moments is in tapping posture at the contact period,the fingertip will experience 3 slip-stick transitions in the tangential direction.The flexible finger even has a ‘reverse slip' phenomenon due to large tangential contact compliance.(3)Two models(i.e.uniform mass distribution model and lumped mass model)of rigid robotic figner for studying Painlevé paradox are proposed.Incorporating with linear complementarity problem,the influence of mass distribution on the critical coefficient of friction ?c and the paradox region of the Painlevé paradox is discussed.The numerical result shows that when the coefficient of friction is the same,the paradox region of uniform mass distribution model is larger than those of lumped mass model.(4)The effect of coefficient of friction and structure compliance on the dynamic self-locking(i.e.Painleve paradox of rigid finger)of flexible robotic finger is discussed by the use of HCM.The typical characteristics of the dynamic self-locking are summarized.The numerical results show that the dynamic self-locking of flexible robotic finger is always accompanied with stick state.Comparing with rigid robotic finger,the structure compliance of flexible finger leads to the expansion of dynamic self-locking region.