A simulation study to improve fixation mechanisms between exoskeletons and limbs

Exoskeletons are rigid mechanical structures designed to assist human beings in activities of daily living, to enhance natural abilities or to provide interaction with virtual reality, among other uses. Nowadays, one of the most common uses for exoskeletons is in the rehabilitation field, assisting patients in physiotherapeutic exercises during the recovering process after neuromotor diseases, such as strokes, or even in the treatment of pathological deformities –eventually correcting them. Exoskeletons are connected to the body parallel to the limbs and joints. Since the beginning of the 20th century exoskeletons are being developed, but studies about the mechanical interaction between the robotic structure and the human limbs have been performed in an unsystematic way. The goal of this work is to propose and assess different types of fixation between the exoskeleton and the forearm and evaluate the system’s behavior through computational simulation, using the software MSC ADAMS. The final objective is to identify the best method of fixation in terms of how the arm follows the exoskeleton’s movements – movement transmission –, security and comfort. A study of the mechanical interaction among the parts was carried out, so the forces involved could be better understood. Then, in order to reduce computational costs, simplified forearm models have been proposed, using different approaches. Three different kinds of fixation were proposed: Ring, Thick Band and Two Bands, then compared used during ADAMS simulations, so the best one, according to the obtained results, could be chosen. The first criterion, Movement Transmission, was analyzed through the use of the calculation of the root mean square error. The other two criteria, Security and Comfort, were analyzed in a qualitative way, being related to velocities and accelerations reached, since at the moment proper literature discussing these topics was not found. A discussion was made regarding the damping factor of the body. Since this is another factor that does not have enough literature available, an arbitrary value was chosen to the first tests. At the end, once the best fixation was chosen, simulations were run considering other damping coefficients. An arbitrary frequency was set to run the first simulations. Again, once the best fixation was chosen, new simulations were made, considering different frequencies, so the system behavior could be analyzed. Considering the presented criteria and running the simulations following the mentioned steps, the best fixation was found to be the Thick Band.