Sensory subtraction with cutaneous feedback to ensure safety in bimanual robot-assisted surgery Leonardo Meli, Claudio Pacchierotti, and Domenico Prattichizzo Abstract—In this paper we present a novel approach to force feedback in robot-assisted surgery. Haptic stimuli, consisting of both kinesthetic and cutaneous components, are substituted with cutaneous feedback only. This new approach to sensory substitution in haptics is called sensory subtraction, since it can be thought as the subtraction between the complete haptic interaction and its kinesthetic component. In order to evaluate the feasibility of the proposed technique, we carried out a bimanual teleoperation experiment, wherein we compared the performance of our sensory subtraction approach with that of (1) complete haptic feedback, (2) visual feedback, and (3) auditory feedback. Results assess the proposed method as a viable solution to substitute haptic feedback in complex teleoperation scenarios. Moreover, as any other sensory substitution technique, this approach allows to overcome any stability issue affecting the haptic loop. I. I NTRODUCTION Haptic force is widely considered to be a valuable navigation tool during teleoperated surgical procedures [1], [2]. However, the kinesthetic part of the haptic interaction can lead to undesired oscillations of the system, which may be unsafe for both the clinician and the patient being operated. Stability of haptic systems can be in fact significantly affected by communication latency in the teleoperation loop, hard contacts, stiff control settings, and many other destabilizing factors that dramatically reduce the effectiveness of haptic force feedback in teleoperation [3]. For this reason, feedback approaches that disregard kinesthetic feedback are lately gaining great interest. A popular non-kinesthetic approach to provide information about forces exerted at the slave side is sensory substitution. It consists of substituting kinesthetic force with alternative forms of feedback, such as vibrotactile [4], auditory, and/or visual feedback [5]. In this case, since no kinesthetic force is fed back to the operator, the haptic loop becomes intrinsically stable and no bilateral controller is thus needed [3]. Similarly to sensory substitution, Prattichizzo et al. [3] presented a feedback approach that substituted haptic force feedback with cutaneous feedback only. Results showed higher transparency levels than that obtained compared to other conventional sen- sory substitution techniques. The authors named this technique sensory subtraction, since the force provided (i.e., cutaneous stimuli only) can be thought as a subtraction between the com- plete haptic interaction, consisting of cutaneous and kinesthetic components [6], and the kinesthetic part of it. The research leading to these results has received funding from the European Union Seventh Framework Programme FP7/2007-2013 under grant agreement n ◦ 601165 of the project “WEARHAP - WEARable HAPtics for humans and robots”. L. Meli, C. Pacchierotti and D. Prattichizzo are affiliated with the Dept. of Information Engineering and Mathematics, University of Siena, and with the Dept. of Advanced Robotics, Istituto Italiano di Tecnologia, Italy. meli,pacchierotti,prattichizzo@dii.unisi.it Fig. 1. The da Vinci Skills Simulator (Intuitive Surgical, USA, and Mimic Technologies, USA) contains a variety of scenarios (right) designed to give surgeons the opportunity to improve their proficiency with the da Vinci console controls (left). In this paper we exploited the idea of sensory subtraction in a challenging medical scenario: a bimanual 7 degrees-of- freedom (DoF) teleoperation task, very similar to the Peg Board module of the da Vinci Skills Simulator (see Fig. I). Performance were compared while providing (1) complete haptic force feedback through a couple of haptic interfaces, (2) cutaneous force feedback through four cutaneous devices, i.e. the sensory subtraction approach, (3) visual and (4) audio feedback in substitution of force feedback, which are two popular sensory substitution techniques. II. FINGERTIP SKIN DEFORMATION DEVICES Cutaneous stimuli, sensed by skin pressure receptors, are useful to perceive local properties of the objects such as shape, edges, embossing and recessed features. This is pos- sible, mainly, thanks to a direct measure of intensity and direction of contact forces, and to the encoding of the force spatial distribution over the fingertip. Conversely, kinesthesia provides the user with information about the relative position of neighbouring parts of the body, principally by means of sensory organs in muscles and joints. In this work we employ a wearable fingertip deformation device, which consists of two platforms: one fixed to the back of the finger and one in contact with the fingertip, connected by three cables. Three small electrical motors, equipped with position encoders, control the length of the cables, thus being able to tilt and move the platform towards or away from the fingertip. As a consequence, a 3-D force can be displayed to the users fingertip. The force to be provided was estimated according to a mathematical model of the fingertip [7], which considers a linear relationship between resultant wrench at the fingertip and devices platform displacement. More information about these devices can be found in [8]. III. EXPERIMENTAL EVALUATION The sensory subtraction technique has been evaluated in two bimanual peg board experiments, with and without delay in the haptic loop.