Applying the models of magneto-rheological substances in the study of exoskeleton variable-length link with adjustable stiffness

The article considers the existing mathematical models of magneto-rheological substances and describes some of their properties. As a result of the open sources analysis, it was found that there are no exoskeleton models with variable-length links with adjustable stiffness, based on the application of magneto-rheological fluids. Therefore, the application of these fluids in other technical systems is considered. A mathematical model of an exoskeleton variable-length link with adjustable stiffness is proposed. This link can be used for supporting and strengthening the lower limbs of the human musculoskeletal system. The difference between the proposed mathematical model of the link and the existing ones lies in the fact that the section that changes its length is considered weighty. Therefore, the mathematical model of the link with a variable inertial characteristic, the moment of inertia relative to the axis perpendicular to the longitudinal axis of the link symmetry and passing through its beginning – the point where the link is fixed to the stationary mount with a cylindrical hinge, is considered. A method of motion control based on the assignment of differentiable functions is applied. The trajectory of the link movement is found, linear and angular velocities and accelerations are calculated. To showcase the link motion, the computer-animated visualization of the link motion control problem solution is presented. The control actions required for the implementation of the given motion have been calculated in the numerical experiment. The drag coefficient range of the magneto-rheological substance has been identified during the implementation of the proposed link motion. The software implementation of the proposed mathematical model of the exoskeleton variable-length link with adjustable stiffness has been done in the Wolfram Mathematica 11.3 universal computer math environment. The software package including the unit for deriving the differential equations of motion in analytical form, the kinematic trajectory synthesis unit, the computational experiment unit, and the unit for animated visualization of the model motion and its export in the wide-spread 'gif' video format has been developed.