Autocalibrated Gravity Compensation for 3DoF Impedance Haptic Devices Alessandro Formaglio, Marco Fei, Sara Mulatto, Maurizio de Pascale, and Domenico Prattichizzo Department of Information Engineering University of Siena, via Roma 56, 53100 Siena, Italy {formaglio,fei,mulatto,mdepascale, prattichizzo}@dii.unisi.it http://www.dii.unisi.it Abstract. The apparent mass of haptic device end-effector depends on its po- sition inside the workspace. This paper presents a recursive algorithm to detect effective direction of gravity force, and to automatically estimate the apparent mass of the end-effector when placed at the vertices of a cubic grid contained into the device workspace. Then an on-line technique is proposed to actively compensate gravity, exploiting trilinear interpolation to compute an estimate of end-effector apparent mass in any position of the workspace. Experiments have been performed with three different haptic devices, and results shown that the apparent mass of the end-effector is compensated almost homogeneously with respect to its position in the workspace. 1 Introduction In the last decade impedance force-feedback devices have been strongly improved thanks to both mechatronic and software developments. As a consequence, improved performance devices at lower prices are available today, as for example the Phantom Omni or the Falcon. Thanks to technological development, using haptic devices has be- come a common practice in many disciplines such as medicine, where these devices are used as training systems for laparoscopy and microsurgery [1]. Besides, haptic devices represent also an important tool to study cognitive and physiological mechanisms in- volved in manipulation tasks, as for example in Psychophysics or Neurophysiology [2], where the interest of experimenters is often focused on measuring finger motion, tra- jectories or exerted forces. In all these applications friction, inertia, and apparent mass characterizing the haptic manipulator represent a disturbance which can strongly affect experimental results. This paper deals with the problem of gravity compensation for haptic systems. Sev- eral works can be found in the literature where different techniques are employed in order to actively cancel effects of gravity on haptic manipulators [3, 4, 5, 6]. Although the large variety of approaches, they share some common traits. Generally, gravity com- pensation is based on the knowledge of device kinematics, or otherwise force sensors are used in order to achieve device mechanical transparency. M. Ferre (Ed.): EuroHaptics 2008, LNCS 5024, pp. 43–52, 2008. c  Springer-Verlag Berlin Heidelberg 2008