Titre : Title : From flapping wings to underactuated fingers and beyond : a broad look to self-adaptive mechanisms Auteurs :

In this paper, the author firs reviews the different terminologies used in underactuated grasping and illustrates the current increase of activity on this topic. Then, the (probably) oldest known self-adaptive mechanism is presented and its performance as an underactuated finge is discussed. Its original application, namely a fl pping wing, is also shown. Finally, it is proposed that the mechanisms currently used in underactuated grasping have actually other applications similarly to the previously discussed architecture could be used for both an underactuated finge and a flappin wing. This paper was presented at the IFToMM/ASME International Workshop on Underactuated Grasping (UG2010), 19 August 2010, Montréal, Canada. 1 Self-adaptive, adaptive, intelligent, underactuated, differential, or compliant? Self-adaptive mechanisms as used in underactuated fin gers (Birglen et al., 2008) are often confused with other classes of mechanisms because of the lack of a clear defi nition. According to Gosselin (2006), adaptive mechanical systems are define as systems “in which the ability to adapt to new external situations relies strictly on mechanical properties” (quoted). Although the denomination of “adaptive mechanisms” proposed in the latter reference is absolutely correct, the author of this paper often prefers referring to them as “self-adaptive mechanisms” to avoid confusion. Indeed, the latter expression emphasizes that the adaptation capability refers to the mechanical system itself and does not describe an algorithmic procedure. As noted in Gosselin (2006), adaptive systems are usually associated with computer systems and this has lead to some confusion in underactuated grasping. It should also be noted that “self-adaptive” has already been used by other authors to describe these systems (Rubinger et al., 2001; Carrozza et al., 2004). They are also sometimes referred to as “intelligent” systems (Ulrich, 1988; Gosselin, 2005), which is more vague and therefore, should probably be avoided. Correspondence to: L. Birglen (lionel.birglen@polymtl.ca) The objective of self-adaptive mechanisms is to delegate part of the control tasks from the electronic board to the mechanical layout of the system itself. Hence, with selfadaptive robotic hands, parts of the possible motions of the finger have to be uncontrolled electronically. However, these motions must be carefully predicted and studied in order to achieve the desired closing sequence, or else, they can lead to degenerate behaviours (Birglen and Gosselin, 2006b). Since some degrees of freedom (DOF) of these hands are not controlled, they are often referred to as “underactuated” (Laliberté and Gosselin, 1998). Again, this adjective is technically correct but has also lead to some confusion. Underactuation in robotic hands is different from the concept of underactuation usually presented in robotic systems and the differences between both notions should be made clear. An underatactuated serial robot is define as a manipulator with one or more unactuated joints. On the other hand, “underactuated” or self-adaptive finger use passive elements (the most common of which are springs) in the design of their unactuated joints. Thus, one should rather think of these joints as uncontrollable or passively driven instead of unactuated. With self-adaptive robotic systems, and conversely to usual underactuated manipulators, the actuation torque (or force) is distributed to each joint of the system. This distribution property is essential and can be related to the Transmission matrix of the linkage (Birglen, 2009) which is characteristic of the type of transmission mechanism required to achieve this distribution. In principle, this distribution Published by Copernicus Publications. 6 L. Birglen: From flappin wings to underactuated finger and beyond