Analysis, modelling, and design of a multi-joint under-actuated leg prosthesis based on EHA

Leg prosthesis in the market and in literature that actuate both knee and ankle joint are usually heavy and discarded by the patients for lighter semi-active ones. The purpose of this thesis is to study the possibility to realize a multi-joint under-actuated leg prosthesis based on Electro-Hydrostatic Actuation (EHA), as to have only one power unit for both joints, reducing the bulkiness and overall weight. Hydraulics motion transmission can serve an optimum role in such application, as power transfer between the components is an easier task as the motor does not have to be placed close to the actuated joint. The hydraulic circuit actuating the prosthesis is made up of a EHA unit composed of a fixed-displacement pump and an electric motor; two hydraulics actuators (double-acting hydraulic cylinders) are connected to the EHA and are in direct configuration (upper chamber connected to upper chamber). These actuators, positioned behind knee and ankle joint respectively, allow the joints to rotate. Finally, a bi-directional valve is linked in series to the upper chamber of each cylinder to allow stroke modulation. The theoretical analysis of the available physiological dataset and the dimensioning of the components was performed in the MATLAB environment, while simulation and control of the prosthesis was carried out in Simulink via Simscape and Multibody modelling tools. The simulation allowed us to confirm the theoretical results and to verify that the prosthesis works with the predefined components and follows the physiological trajectories of the gait cycle with a low RMSE in a open loop control. Moreover, this prosthesis, by under-actuated design, has the advantage of being able to power both joints with one motor and decouple joint movement for different tasks thanks to the implemented valves. These design traits make such prosthesis possibly able to actuate both knee and ankle for different ambulatory tasks while maintaining a lighter weight (~4 Kg) with respect to its fully active counterparts.