A tracking problem for a mechanical system is considered. We start with a feedback controller that is designed without attention to disturbances, which are assumed to be adequately described by a dynamic LuGre friction model. We are interested in deriving a superimposed observer-based compensator to annihilate or reduce the influence of such a disturbance. We exploit a recently suggested approach for observer design for LuGre-friction-model-based compensation. In order to apply this technique, it is necessary to know the Lyapunov function for the unperturbed system, as well as the parameters of the dynamic friction model, and to verify that a certain structural property satisfied. The case when the system is passive with respect to the matching disturbance related to the given Lyapunov function is illustrated in this brief with a DC-motor example. The main contribution is some new insights into the numerical real-time implementation of a compensator for disturbances describable by one of various LuGre-type models. The other contribution, which is built upon the main one, is experimental verification of the suggested model-based observer design procedure.

The importance to postural control of the mechanoreceptors of the soles was investigated in thirteen healthy subjects. Body-sway velocity was evaluated before and after exposing the subject's feet to hypothermia, and when calf muscles were exposed to vibration at frequencies between 20 and 100 Hz. Subjects were tested both with eyes open and closed. Body-sway velocity was found to increase significantly during hypothermia of the feet. The difference in body-sway between hypothermal and normothermal conditions was less prominent when the subject's eyes were open though the difference was significant in both cases. The present results indicate the importance of the mechanoreceptors of the soles to postural control and elucidate their interaction with compensatory visual input in maintaining postural control. These findings also suggest, that factors affecting pressor input should be taken into consideration when assessing patients with complaints of dysequilibrium.

Algorithms for continuous-time quadratic optimization of motion control are presented. Explicit solutions to the Hamilton-Jacobi equation for optimal control of rigid-body motion are found by solving an algebraic matrix equation. The system stability is investigated according to Lyapunov function theory and it is shown that global asymptotic stability holds. How optimal control and adaptive control may act in concert in the case of unknown or uncertain system parameters is shown. The solution results in natural design parameters in the form of square weighting matrices, as known from linear quadratic optimal control. The proposed optimal control is useful both for motion control, trajectory planning, and motion analysis.

The significance to human postural control of pressor information from the feet was investigated during vestibular disturbance in seven normal subjects who were exposed to bipolar biaural galvanic stimulation of the vestibular nerves before and after their feet were anaesthetized with hypothermia. The increase in body sway in the lateral plane induced by the galvanic stimulus was enhanced when the feet were anaesthetized, and adaptation of postural control to the galvanic stimulus was delayed. It is concluded that pressor information from the feet contributes significantly to postural control in humans and is important in compensating for vestibular disturbance.