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This paper analyses the problem of modelling joint friction in robotic manipulators with gear transmissions in the sliding regime, i.e. at joint velocities varying from close to zero until their maximum appearing values. It is shown that commonly used friction models that incorporate Coulomb, (linear) viscous and Stribeck components are inadequate to describe the friction behaviour for the full velocity range. A new friction model is proposed that relies on insights from tribological models. The basic friction model of two lubricated discs in rolling-sliding contact is used to analyse viscous friction and friction caused by asperity contacts inside gears and roller bearings of robot joint transmissions. The analysis shows different viscous friction behaviour for gears and pre-stressed bearings. The sub-models describing the viscous friction and the friction due to the asperity contacts are combined into two friction models; one for gears and one for the pre-stressed roller bearings. In this way, a new friction model is developed that accurately describes the friction behaviour in the sliding regime with a minimal and physically sound parametrisation. The model is linear in the parameters that are temperature dependent, which allows to estimate these parameters during the inertia parameter identification experiments. The model, in which the Coulomb friction effect has disappeared, has the same number of parameters as the commonly used Stribeck model. The model parameters are identified experimentally on a Stäubli Rx90 industrial robot.

The human motion analysis, for gait or for most of other activities, relies mostly on the use of multibody formulations applied as kinematic or dynamic tools. In many biomechanical applications to gait analysis the choice between using direct or inverse dynamics to obtain the solution of the problem, even in pure kinematics, only depends on the personal preference of the user and not in any particular form of the data available or structure of the equations to be solved. In this work the structure of the equations of a multibody system are reviewed for direct and inverse dynamic analysis. It is shown that if the time dependencies of all degrees-of-freedom of the system are known the inverse dynamics is equivalent to a direct dynamics problem. This equivalence is particularly useful when the problem of the biomechanical analysis consists in finding the muscle forces in an over-actuated biomechanical model that leads to a prescribed motion, which is obtained by using video data acquisition or simply by designing such motion. The problem can then be solved by using optimization procedures in which the objective functions are physiological criteria and, eventually, a measure of matching the prescribed motion. If not used as part of the objective function the prescribed motion is introduced in the optimization problem as nonlinear constraints. The variables of the optimization problem are, for all type of analysis, the muscle forces, directly, or their corresponding muscle activations. It is shown that the natural choice for design variables of the optimal problem is the muscle activations. Two representations of the time history of the muscle actuation are tested in this work: the input sampling where the activations are found in a finite number of time instants and then linearly interpolated in between; the smooth exponential function approach where the actuation is described by a sum of exponential functions being the width and the size of the bumps of each of the functions the unknown quantities. Then the muscle forces are simply obtained by using a Hill type muscle model where the state of force-velocity and the forcelength relations are obtained directly from the kinematics of the biomechanical model. The methods presented in this work are demonstrated and discussed in the framework of two problems associated to the human locomotion apparatus.

With the increasing integration of mechanical, electrical and hydraulical components in advanced engineering systems, the integrated analysis of coupled physical phenomena and coupled technical systems gets more and more important. The methods and software tools of multibody dynamics are used successfully as integration platform for these multidisciplinary investigations. The present paper summarizes some multidisciplinary applications in the context of multibody dynamics and considers common problems and solution strategies. A novel modal multifield approach for coupled field effects like thermoelasticity is discussed in more detail.