1534-4320 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNSRE.2014.2365697, IEEE Transactions on Neural Systems and Rehabilitation Engineering XXX 1 Abstract—Powered exoskeletons can empower paraplegics to stand and walk. Actively controlled hip ab/adduction (HAA) is needed for weight shift and for lateral foot placement to support dynamic balance control and to counteract disturbances in the frontal plane. Here, we describe the design, control, and preliminary evaluation of a novel exoskeleton, MINDWALKER. Besides powered hip flexion/ extension and knee flexion/extension, it also has powered HAA. Each of the powered joints has a series elastic actuator, which can deliver 100Nm torque and 1kW power. A finite-state machine based controller provides gait assistance in both the sagittal and frontal planes. State transitions, such as stepping, can be triggered by the displacement of the Center of Mass (CoM). A novel step-width adaptation algorithm was Manuscript received November 05, 2013; revised August 21, 2014; accepted October 19, 2014. Date of Publication November XX, 2014; date of current version November XX 2014. This work was supported by EU FP7 Programme under contract #247959. MINDWALKER was the project title and the name of the exoskeleton. In short, the goal of the project was to create a mind-controlled orthosis to help paraplegics walk. * Authors have equal contribution to this paper. S. Wang and C. Meijneke are with Biomechanical Engineering., Delft University of Technology, 2628CD Delft, the Netherlands (phone: +31 152788634; e-mail: shiqian.wang@tudelft.nl; C.Meijneke@tudelft.nl). L. Wang and E. van Asseldonk are with Biomechanical Engineering, University of Twente, 7522NB Enschede, the Netherlands (e-mail: letian.wang@utwente.nl; E.H.F.vanAsseldonk@utwente.nl). T. Hoellinger and G. Cheron are with Laboratory of Neurophysiology & Movement Biomechanics, Université Libre de Bruxelles, 1070 Brussels, Belgium (e-mail: hoellint@gmail.com; gcheron@ulb.ac.be). Y. Ivanenko, V. La Scaleia, and F. Sylos-Labini are with Dept. of Neuromotor Physiology, IRCCS Fondazione Santa Lucia, 00179 Rome, Italy. V. La Scaleia and F. Sylos-Labini are also with Dept. of Systems Medicine, University of Rome Tor Vergata, 00133 Rome, Italy. (e-mail: y.ivanenko@hsantalucia.it; v.lascaleia@hsantalucia.it; f.syloslabini@hsantalu cia.it). M. Molinari, F. Tamburella, and I. Pisotta are with Neurorehab. Clinical Unit A and Experimental Neurorehab. Lab, IRCCS Fondazione Santa Lucia, 00179 Rome, Italy (e-mail: m.molinari@hsantalucia.it; f.tamburella@ hsantalucia.it; i.pisotta@hsantalucia.it). F. Thorsteinsson is with OSSUR, 110 Reykjavík, Iceland (e-mail: fthorsteinsson@ossur.com). M. Ilzkovitz, J. Gancent and Y. Nevatia are with Space Applications Services N.V./S.A., 1932 Zaventem, Belgium (e-mail: michel.ilzkovitz@gmail.com; jeremi.gancet@spaceapplications.com; yashodhan.nevatia@spaceapplications.com). R. Hauffe is with eemagine Medical Imaging Solutions GmbH, 10243 Berlin, Germany (e-mail: Ralf.Hauffe@eemagine.com) F. Zanow is with ANT Neuro, 7521PT Enschede, the Netherlands (e-mail: frank.zanow@eemagine.com). H. van der Kooij is with Biomechanical Engineering, University of Twente, 7522NB Enschede, The Netherlands. He is also with Biomechanical Engineering, Delft University of Technology, 2628CD Delft, the Netherlands (e-mail: H.vanderKooij@utwente.nl). proposed to stabilize lateral balance. We tested this exoskeleton on both healthy subjects and paraplegics. Experimental results showed that all users could successfully trigger steps by CoM displacement. The step-width adaptation algorithm could actively counteract disturbances, such as pushes. With the current implementations, stable walking without crutches has been achieved for healthy subjects but not yet for SCI paraplegics. More research and development is needed to improve the gait stability. Index Terms—MINDWALKER, exoskeleton, SEA, XCoM, gait assistance, balance control I. INTRODUCTION pinal cord injury (SCI) has high personal impacts and socio-economic consequences. Patients with SCI place a heavy burden on the health-care system [1]. The prevalence of SCI in Northern America, Australia, and Europe is estimated to be between 223 and 755 per million populations; and the incidence of SCI lies between 10.4 and 83 per million inhabitants per year worldwide [2]. Two thirds of SCI patients are estimated to be paraplegic; most patients with SCI are young men in their thirties, who need to work to support their families [1]. They have to rely on help from the health-care system and social security system. In a survey [3], 59 percent of paraplegics rated the restoration of walking as their first or second priority for improvement in quality of life. Orthotic devices have been developed to provide paraplegics with some degree of locomotion capability and to reduce the occurrences of secondary complications. Passive (unpowered) orthoses are often prescribed. However, due to the passive nature of these devices, the metabolic energy expenditure in gait causes frequent abandonment or low utilization [4]-[6]. Seeing the limitations of passive orthoses, researchers started developing active exoskeletons as early as the 1960s [7]. However, rapid developments have only been achieved in recent years, resulting in several wearable exoskeletons [8]-[19]. Several of these devices are specifically designed to restore walking for SCI subjects [10]-[19]. Despite the impressive progress and promising results, there is a need to improve the technology. Although SCI patients are enabled to walk again, for stability they rely on crutches, and their walking pattern is less fluent and slower than natural human gait. Design and Control of the MINDWALKER Exoskeleton Shiqian Wang*, Letian Wang*, Cory Meijneke, Edwin van Asseldonk, Thomas Hoellinger, Guy Cheron, Yuri Ivanenko, Valentina La Scaleia, Francesca Sylos-Labini, Marco Molinari, Federica Tamburella, Iolanda Pisotta, Freygardur Thorsteinsson, Michel Ilzkovitz, Jeremi Gancet, Yashodhan Nevatia, Ralf Hauffe, Frank Zanow, and Herman van der Kooij S