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Due to the complexity of the human musculoskeletal system and intra/intersubjects variability, powered exoskeletons are prone to human-robot misalignments. These induce undesired interaction forces that may jeopardize safe operation. Uncompensated inertia of the robotic links also generates spurious interaction forces. Current design approaches to compensate for misalignments rely on the use of auxiliary passive degrees of freedom that unavoidably increase robot inertia, which potentially affects their effectiveness in reducing undesired interaction forces. Assessing the relative impact of misalignment and robot inertia on the wearer can, therefore, provide useful insights on how to improve the effectiveness of such approaches, especially in those situations where the dynamics of the movement are quasi-periodic and, therefore, predictable such as in gait. In this paper, we studied the effects of knee joint misalignments on the wearer's gait, by using a treadmillbased exoskeleton developed by our group, the ALEX II. Knee joint misalignments were purposely introduced by adjusting the mismatch between the length of the robot thigh and that of the human thigh. The amount of robot inertia reflected to the user was adjusted through control. Results evidenced that knee misalignment significantly changes human-robot interaction forces, especially at the thigh interface, and this effect can be attenuated by actively compensating for robot inertia. Misalignments caused by an excessively long robot thigh are less critical than misalignments of equal magnitude deriving from an excessively short robot thigh.Index Terms-Active leg exoskeleton (ALEX II), force control, human-robot misalignment, rehabilitation robotics.

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We report the first observation of high-order harmonic waves from rare-gas-like ions excited by a subpicosecond KrF excimer laser. At an intensity of 10 15 W/cm 2 , the highest orders of observed harmonics are the ninth (27.6 nm) in Li + , the eleventh (22.6 nm) in Na + , and the thirteenth (19.1 nm) in K + . The advantage of such ions as nonlinear media for extreme ultraviolet generation is discussed in relation to ionization potentials.PACS numbers: 42.65.Ky, 32.80.RmThe recent development of ultrashort, high-power lasers allows the generation of high-order harmonic waves in the extreme ultraviolet (XUV) and soft x-ray regions [1][2][3][4][5][6]. Using a Ndiglass laser, the high-order harmonics up to the 53rd (19.9 nm) were observed in Ne by Ferray and co-workers [2][3][4]. Miyazaki and Sakai [5] observed the harmonics up to the 41st (15.0 nm) with the use of a visible dye laser (616 nm). In these experiments, the appearance of a plateau in the intensity distribution among harmonic orders was found. On the contrary, such a plateau did not appear with the KrF-excimer-laser excitation (248 nm) by Sarukura et al. [6], which attained the shortest wavelength of harmonic generation, 9.9 nm at the 25th order.Generally speaking, the use of shorter-wavelength lasers in experiments on harmonic generation has an advantage for XUV generation because of the reduction of harmonic orders [1,6]. For pumping with KrF or XeCl lasers, however, multiphoton ionization of the medium becomes serious and limits the maximum input power. According to the experiments using a Nd:glass laser by Lompre, L'Huillier, and Mainfray [4], there was a saturation in the highest order observed for increasing pumping power. They also found that the saturated order of the harmonics depended strongly on the ionization potential, so that the highest order of harmonics increased linearly with increasing ionization energy of rare gases. Therefore, much-higher-order harmonics would be expected if a nonlinear medium with a higher ionization potential were available. From this point of view, singleionized alkali-metal ions such as Li + , Na + , and K + are chosen because they are isoelectric to neutral rare-gas atoms, and they have higher ionization potentials than those of rare-gas atoms. The ionization potentials of Li + , Na + , and K + are 75.6, 47.3, and 31.6 eV, respectively, while those of rare gas atoms are 24.6, 21.6, and 15.8 eV for He, Ne, and Ar, respectively. The increase of ionization potential, however, results in a decrease of the nonlinear response of the medium [7]. Therefore, high laser intensities and high number densities of the medium are necessary for media with high ionization potentials. In this Letter, we demonstrated harmonic generation with the rare-gas-like ions of Li + , Na + , and K + , using a sub-picosecond KrF excimer laser. Such ions are produced and utilized in laser plasmas. The effect of coexisting free electrons on phase matching is also discussed. Figure 1 shows the experimental arrangement. Two KrF excimer lasers were...

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