Abstract— This work discusses the design and control approach of a recently developed 3 DOF waist joint for the “iCub” humanoid robot. “iCub” is child like crawling robot that resembles a 2 and a half year child. The ultimate goal of this project is to provide the cognition research community with an open human like platform for understanding of cognitive systems through the study of cognitive development. The design of the mechanisms adopted for the waist joint is discussed. This is accompanied by discussion on the control scheme design and presentation of experimental results showing the performance of the mechanism. I. INTRODUCTION nthropomorphic type robots combine many desirable features such as natural human like locomotion and human friendly design and behavior. As a result of this multi degree of freedom human like robots have become more and more common and many humanoid robots have been recently designed and fabricated. The first biped humanoid robot was WABOT developed at Waseda University back in 1973. This biped which was able to perform simple static walking was followed by the development of WABIAN I and II. WABIN-RII[1]. Following these first prototypes a number of other human like robots were developed including the H6, H7 at the University of Tokyo [2], the impressive humanoid robots P2, P3 and ASIMO developed by HONDA [3-5] and some more recent prototypes as the JOHNNIE anthropomorphic autonomous biped robot [6] developed at University of Munich and the HRP, HRP-2 developed by METI in Japan [7-10]. Other less know medium and small size humanoids include SAIKA [11] and KENTA [12], the MK.5 a Manuscript received March 9, 2006. This work was supported by the European Commission through Unit E5 “Cognition”. W. M. Hinojosa is with the Centre for Robotics & Automation (IRIS), School of Computing Science and Engineering, The University of Salford, Salford, Greater Manchester M5 4WT, United Kingdom (e-mail: W.M.Hinojosa@pgt.salford.ac.uk). N. G. Tsagarakis is with the Centre for Robotics & Automation (IRIS), School of Computing Science and Engineering, The University of Salford, Salford, Greater Manchester M5 4WT, United Kingdom (e-mail: N.Tsagarakis@salford.ac.uk). D. G. Caldwell is with the School of Computing Science and Engineering, The University of Salford, Salford, Greater Manchester M5 4WT, United Kingdom (e-mail: D.G.Caldwell@salford.ac.uk). compact size humanoid robot constructed by Aoyma Gakuin University [13], the PINO platform constructed by ERATO [14] and the SDR-3X (Sony Dream Robot-3X) and SDR-4X developed mainly for entertainment [15, 16]. In the above humanoids examples the waist joint is usually implemented using a simple serial mechanism with 2 DOF. This waist configuration is adequate for these robots since the tasks that are usually perform are still limited in entertainment and amusement applications and in demonstrating walking capabilities. The concept behind the development of iCub is to provide the cognition research community with an open human like hardware/software platform for understanding of cognitive systems through the study of cognitive development. The iCub will replicate a human-like robot with the size of a 2 and a half year old baby acting in a cognitive scenario, performing the tasks useful to learn, interacting with the environment and humans. In the early definition of the project two main tasks were considered from which the design requirements for the waist mechanism were derived. These are: crawling and manipulation [17]. Based on the requirements implied by these two tasks the design of the waist mechanism of the iCub was realized. A 3 DOF differential based mechanism was employed to provide not only increased stiffness but also increased range and flexibility of motion for the upper body. This paper presents the design and control of this differential based waist mechanism of the iCub. The paper is organized as follows: Section II gives the general specifications of the waist joint in terms of DOF, range of motions and torque requirements. The following section describes the mechanical design adopted for the waist mechanism and highlights the advantages of this approach. Section IV introduces the system model used for the design of the control scheme while section V presents the control system design. Estimated performance measures of the control scheme are presented in sections VI and VII, by means of simulation and experimental result. Finally, section VIII introduces the conclusion of this work II. WAIST SPECIFICATIONS The kinematics specifications of the waist joint of the Performance Assessment of a 3 DOF Differential Based Waist joint for the “iCub” Baby Humanoid Robot W. M. Hinojosa, N. G. Tsagarakis, Giorgio Metta, Francesco Becchi, Julio Sandini and Darwin. G. Caldwell A