39 Copyright © 2013, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 3 DOI: 10.4018/978-1-4666-3634-7.ch003 INTRODUCTION Parallel kinematics machines, PKMs, are now used in many different application fields, both in industrial environments and in advanced robot- ics developments. Such machines are given by a moving platform that is actuated in-parallel by several limbs, each one driven by actuators that are fixed with the ground. On the other hand, due to the complex kinemat- ics of 6-dof full mobility PKMs, research com- munity is addressing the studies towards simpler Luca Carbonari Polytechnic University of Marche, Italy Luca Bruzzone University of Genova, Italy Massimo Callegari Polytechnic University of Marche, Italy Impedance Control of a Spherical Parallel Platform ABSTRACT This article describes the impedance control of an in-parallel actuated orientation platform. The algo- rithm is based on a representation of platform orientation which exploits the equivalent axis of rotation: this approach is more intuitive and easier to visualize than conventional methods based on Cardan or Euler angles. Moreover, since for small angular displacements the Mozzi’s axis lies very close to angular velocity, impedance control algorithms based on such representation provides better performances and smoother motions. Results of numerical simulations and experimental tests are shown and commented with reference to the spherical parallel machine.