A fairly detailed exoskeleton variable-length link model with adjustable stiffness is considered in the article. The refinement of the previously created models in terms of discretization is made. The geometrical dimensions of the top and bottom rods, the cylinder, the cylinder covers, the piston that moves inside the cylinder with magneto-rheological fluid, as well as their masses and moments of inertia relative to coordinate axes are refined. The payload mass at the end of the link, resulting under the effect of other links or the attached actuator, is simulated by a homogeneous ball. The change of the mass in the bottom and top parts of the cylinder, and the change of the moment of inertia of the link as the magneto-rheological fluid flows inside the cylinder from one part to another through the piston with microchannels are made more precise. All these made it possible to bring the model closer to the real technical device. The mathematical model of a more accurate link model is compiled. The possible model of actuator, functioning based on the proposed refined model of the exoskeleton link with magnetorheological fluid, is described. The possibility of its functioning and the options to control it are studied. The accuracy estimation for the refined model in comparison to the previously created models is made. All the results presented were obtained in a program created in the universal software environment “Wolfram Mathematica”. The links of the proposed model can be used in designing next-generation exoskeletons, which are more comfortable comparing to the existing models.
A 2-D exoskeleton model with five variable-length links is proposed in the article. The model features not only the active control in the hinges joining separate links, but also the controlled link length alteration. The controlled alteration of the angles between the links in this model is implemented by the drives with reduction gears. The electric drives with rack and pinion gears control the change of the link lengths. Thus, all the degrees of freedom describing the 2-D exoskeleton model are controlled. The electric drives can implement various control algorithms. The method of programmed motion control by specifying information about alteration of kinematic properties describing it is used in this article. Based on this information and the system of differential equations of motion, which is solved algebraically for the control torques and lengthwise forces, their analytical expressions are derived. Thus, the inverse dynamics problem is solved. On the basis of the found control actions, the electric drives have been selected. Electric motors, reduction gears, and rack and pinion gears have been selected from the list of the currently manufactured standard products.
The article discusses three spatial models of exoskeletons: with four, five and n links. In accordance with the biomechanics of the human musculoskeletal system, the models use cylindrical, combinations of two cylindrical hinges with mutually perpendicular axes of rotation, and spherical hinges. The change in angles between the links is controlled using electric drives located in the hinge area. Unlike those considered earlier, in this model, a section of variable length is assumed to be significant, and the drive that controls the change in the length of the link is located in this section. The models provide for the presence of a battery attached to the hip joint, simulated by a concentrated mass. In addition, the five-link model includes auxiliary elements that support the user's head in the form of a point mass. As a simplifying assumption, it is assumed that the entire mass of a section of variable length is concentrated in its middle. A drive that changes the length of a link can be implemented in the form of a rack or pinion gear with an electric motor. Using the method of software motion control, for the considered models with 4 and 5 links, the inverse problem of dynamics was solved and the influence of the body on the forces developed in the lower extremities of the exoskeleton was analyzed. The significant impact of adding a body on exoskeleton control has been established. Based on the analysis of the obtained systems of differential equations of motion, a generalization of the exoskeleton model to the case of an arbitrary finite number of links is proposed.
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