Exploiting Proximal F/T Measurements for the iCub Active Compliance Matteo Fumagalli, Marco Randazzo, Francesco Nori, Lorenzo Natale, Giorgio Metta and Giulio Sandini Abstract— During the last decades, interaction (with humans and with the environment) has become an increasingly inter- esting topic of research within the field of robotics. At the basis of interaction, a fundamental role is played by the ability to actively regulate the interaction forces. In this paper we propose a technique for controlling the interaction forces exploiting a proximal six axes force/torque sensor. The major assumption is the knowledge of the point where external forces are applied. The proposed approach is tested and validated on the four limbs of the iCub, a humanoid robot designed for research in embodied cognition. Remarkably, the proposed approach can be used to implement active compliance in other non passively back-drivable manipulators by simply inserting one or more force/torque sensor anywhere along the kinematic chain. I. I NTRODUCTION Robot interaction has been hypothesized to be at the basis of cognitive processes [1]. Through interaction, cognitive systems create their own knowledge (their epistemology) of the surrounding environment. These concepts are at the basis of a new research direction that mixes robotics and research in cognitive systems, where robots and humans share the same physical space [2]. In this sense, safety becomes necessary in unstructured environments. In this context, autonomous behavior and learning capabilities are fundamental, and the role of perception and sensor inte- gration becomes of primary importance [3]. Within this framework, force information covers an important role for the exploration process during the interaction of the robot with the environment and humans. Force control here can be considered as a tool of the learning phase, which contributes to preserve the robot and its surrounding environment. The ability to detect and counteract forces helps in preserving the robot own safety and that of the environment (including people). Compliance plays a central role for the interactive and explorative behavior of the robot. Robot safety has been considered in [2] and [4]. All aspects of robot design should be considered in order to increase safety [5]: mechanics design but also software, and electronics. From the mechan- ical point of view, passive compliance has the peculiarity of decoupling the link and rotor inertias, thus resulting in an intrinsically safe actuation system [5], [6]. Light-weight designs have also been investigated in [7] where it is shown that impact forces can be reduced, resulting in a safer robot. Similarly, the macro-mini actuation design proposed in [8] relies on relocating the major source of actuation at M. Fumagalli, M. Randazzo, F. Nori, L. Natale, G. Metta and G. Sandini are with the Robotics, Brain and Cognitive Sciences Department, Italian Institute of Technology, Genoa, Italy. Corresponding author email: matteo.fumagalli@iit.it G. Metta is also with the Department of Communication, Computer and System Sciences, Faculty of Engineering, University of Genoa, Italy Fig. 1. The iCub humanoid robot. the base of the manipulator while employing light-weight and small motors which maintain high-frequency torque capability without increasing the overall system size. An alternative to these mechanical solutions is the im- plementation of active force control on non back-drivable manipulators. Even though this approach is far from being intrinsically safe, it does not require to increase the complex- ity of the mechanical system. Standard industrial approaches employ a single 6-axis Force/Torque (F/T) sensor located in a distal configuration, e.g. at the end effector of the manipulator [9]. The obvious assumption in this case is that the robot interaction with the environment only occur distally at the tool level. Joint torque sensing might be an alternative solution to this issue [10], [11], but it again requires a specific joint design for torque sensing. The solution proposed in this paper consists in exploiting one (or more) F/T sensor placed in a proximal configura- tion 1 . In our interpretation, this solution can be seen as an intermediate alternative which merges some of the benefits of localized (distal) F/T sensing together with some advantages of distributed joint torque sensing. As clearly described later in the paper, a proximal sensor allows measuring forces applied at different levels (not only at the end-effector) and gives information about joint torques. In this paper we first present the iCub, the humanoid 1 Within this framework, proximal, as opposed to distal, refers to the natural order of joints in an open kinematic chain. Therefore, proximal refers to objects close the base of the chain. Distal refers to objects close to the end-effector.