Finger Surface Following Control through Intrinsic Contact Sensing

A surface following finger is an indispensable mechanism to enable a dexterous robotic hand to recognize and study the surface properties of an object, namely, texture, material type and contours [1][2]. To follow an arbitrary surface geometry, a vision and proximity sensing based approach have been investigated in [3] and [4]. Force/tactile based methods are reported in [5] and [6]. However, to effectively follow a complex shape with rapidly changing surface contours is still a challenging problem. In this paper, we take advantage of instantaneous identification of normal and tangential force vectors provided by the force/torque based intrinsic contact sensing methods [7][8] and develop surface following control methods which allow a finger to follow a surface with rapidly changing geometry without prior knowledge of the surface shape. In this paper, two methods, velocity ratio control and position/impedance control, are introduced. I. SURFACE FOLLOWING USING VELOCITY CONTROL METHOD A case study was carried out using a two-joint finger. The direction of the fingertip’s motion can be decided by the velocity ratio between the two motors at the respective joints. Therefore, surface following can be achieved by controlling the velocity ratios. Further detail is available in [9]. A force control scheme for generating fingertip motion depending on the desired force vectors is shown in figure 1. Figure 1 Force control diagram with 2DOF finger In these control experiments, the fingertip follows the direction of the tangential and normal forces. These forces are generated by the velocity and position of the fingertip, so that the next position of the fingertip is dictated by the direction of a selected force. The current finger position in Cartesian space is denoted as p and the position at next time step as pt+1. The next position of the fingertip is calculated using: This work is supported by Grasp Stabilisation Control project funded by the TSB Technology Strategy Board, UK *indicates the correspond author, email{ hongbin.liu@kcl.ac. uk} The f denotes either the normal force fn or tangential force ft depending on the condition as shown in figure 2. k is a constant coefficient. Figure 2 surface following control system with velocity control Given the position in the next time step, the desired joint angles (θ1, . . , θn) are determined through inverse kinematics. The fingertip motion is controlled by calculated velocity ratios to allow for smooth motion, an efficient execution time and to be adaptive to a wide range of normal forces. Further details of the velocity ratio control are available in [9]. Figure 3 Normal force control result of velocity control (Blue : 0.2N to 0.4N, Black : 0.6N to 0.8N, and Red: 0.8N to 1N ) Figure 4 The trajectory results of surface following using velocity control Finger Surface Following Control through Intrinsic Contact Sensing Junghwan Back, 1 João Bimbo, Mark Addison, Ugo Cupcic, Gavin Cassidy, Rich Walker, Lakmal Seneviratne, Kaspar Althoefer and Hongbin Liu* 1 Centre for Robotics Research, King’s College London, UK, WC2R 2LS. 2 Shadow Robot Company Ltd, UK, N1 1LX