DESIGN AND FIRST EXPERIMENTS WITH A NEW UNDERACTUATED FINGER

This article discusses the design and testing of a new prototype of underactuated finger obtained with a topological optimization software. Many unforeseen issues have been encountered in the developpement of this prototype and are discussed here. The aim of this paper is to give a practical look at the design of underactuated fingers and report issues that are probably common to all the designers of such fingers while being rarely, if at all, reported in the literature. INTRODUCTION A simple task such as grasping an apple with a human hand requires many "hardware" components: up to 19 bones, 19 muscles and 17 joints can be used to complete this task [1]. Similarly, fully actuated robotic hands such as the DLR Hand II [2], the iLimb [3] or the Robonaut Hand [4] require several motors, electrical drives and sensors in addition to complex control techniques. This makes the hardware part of the hand usually bulky and/or expensive. The underactuation technique can overcome these problems by minimizing the number of actuators required to drive the finger while maintaining good overall grasping performances [5]. The downside of this approach is that it does not allow manipulation of the objects seized: since the number of actuators has been reduced, a complex task such as reorienting an object in the hand by moving one finger after the other is impossible. However, this problem can be solved by seizing the object using another hand or by putting it on a surface, and then, regrasping it with the same hand. Underactuation is achieved by a transmission linkage and passive elements such as mechanical limits and springs. The closing process usually goes as follow: the movement of the actuator first rotates the first phalanx joint while the relative motion of the other phalanges with respect to this first one is prevented. When a contact occurs, the closing force/torque acting on the finger is able to overcome the spring keeping the second phalanx open and thus, this phalanx begins its closing motion. This procedure is then repeated until all the phalanges are closed on the object. The design of an underactuated finger with a shape adaptive and stable behavior is however quite complex: an impressive amount of possibilities can be looked upon and the solutions cannot usually be found intuitively. Nevertheless, the objective of this paper is to present the difficulties and the lessons learned in the physical realization and the early tests of a novel underactuated finger. The design of this finger will be introduced here, but the exact way on how the lengths and the shapes of the links are obtained will be discussed in an upcoming paper. 1 A SOFTWARE ASSISTED APPROACH As discussed in [6], there are 3,024 two or three degree-offreedom (DOF) possible architectures of underactuated fingers with seven or less links. To select the best architecture for a specified task, a topological optimization software has been developped by the authors [7]. Details of the software are presented in the latter reference, which currently handles only architectures with revolute joints. The input parameters of the software are: weighting values for the optimization criteria, typical objects to be seized, and lengths of the phalanges. Each architecture is then optimized separately with a genetic algorithm. The mechanical design computations handled by the software allow to establish the forces in the finger, the generated contact forces, the actuator position of a specified grasp, and the (non-straight) shape of the links of the finger if required to eliminate interferences. The criteria evaluated are the stability of the finger, compactness,