A variable compliance, soft gripper M. E. Giannaccini · I. Georgilas · I. Horsfield · B. H. P. M. Peiris · A. Lenz · A. G. Pipe · S. Dogramadzi Abstract Autonomous grasping is an important but chal- lenging task and has therefore been intensively addressed by the robotics community. One of the important issues is the ability of the grasping device to accommodate varying object shapes in order to form a stable, multi-point grasp. Particu- larly in the human environment, where robots are faced with a vast set of objects varying in shape and size, a versatile grasping device is highly desirable. Solutions to this prob- lem have often involved discrete continuum structures that typically comprise of compliant sections interconnected with mechanically rigid parts. Such devices require a more com- plex control and planning of the grasping action than intrin- sically compliant structures which passively adapt to com- plex shapes objects. In this paper, we present a low-cost, soft cable-driven gripper, featuring no stiff sections, which is able to adapt to a wide range of objects due to its entirely soft struc- ture. Its versatility is demonstrated in several experiments. In addition, we also show how its compliance can be passively varied to ensure a compliant but also stable and safe grasp. Keywords Grasping · Soft robotics · Continuum robot · Variable compliance · Shape invariant grasping 1 Introduction In the rather large research area of autonomous grasping we are focussing on the design of the physical manipulator. A Electronic supplementary material The online version of this article (doi:10.1007/s10514-013-9374-8) contains supplementary material, which is available to authorized users. M. E. Giannaccini (B )· I. Georgilas · I. Horsfield · B. H. P. M. Peiris · A. Lenz · A. G. Pipe · S. Dogramadzi Bristol Robotics Laboratory, Bristol BS16 1QY, UK e-mail: maria.elena.giannaccini@brl.ac.uk novel design of a highly compliant gripper with variable stiff- ness was formulated. Its initial high compliance is exploited to passively adapt to the shape of the objects. For robotic systems to grasp in human environments, the ability to address the uncertainty of the setting and the objects in it is of an utmost importance. For this to be achieved, a sta- ble grasp must be realised under the ambiguity of the particu- lar geometry of the object. Thus, it is important for the robotic end-effector to conform to the shape of the object, increas- ing the contact area and creating multiple contact points. This results in better stability and smaller forces between the end-effector and the object during manipulation. For this rea- son, initial end-effector compliance when forming the grasp is desirable. Compliance permits the gripper to conform its surfaces to those of the object without needing explicit con- trol and sensing, a process called shape match by Deimel and Brock (2013) and Eppner et al. (2012). However, once the grasp is established, an end-effector must be stiff enough to impress sufficient force on the object surface to make the holding and lifting of the object possible. Due to the very high initial compliance of our gripper, this must be increased to a level which allows force transmission to other objects. Thus, in the final phase of the grasping, the gripper needs to be stiffer than it is in its completely compliant initial stage but it is always relatively compliant compared to stiff structures such as, for example, metal ones. One approach to achieve stable grasping is to utilise robotic end-effectors that have some degree of anthropomor- phic structure, composed of two or more digits with as many as 20 degrees of freedom (DOF) (SHADOW 2013) in total, the control of which could be rather complicated. A good example of this type of end-effector is the highly anthropomorphic DLR hand arm system (Grebenstein et al. 2011) with 19 DOF in the hand and kinematics similar to the human on a functional basis. Another example of a state-of-