Optimized Design for the Knee Structure of a Humanoid Robot

The objective of this work is to design and to make a part of a humanoid robot, named HYDROÏD. The keynote is a development of a self-sufficient robot by minimizing energy inputs required for its activity. Currently humanoid robots have a power/weight ratio lower than human, as a consequence a limited autonomy. In this work we propose an innovative knee structure in order to reduce friction, and as a result, increase energy efficiency. In classic knee architectures, the rolling elements are balls in bearings with relatively small curvature radii. Here, the idea is to increase this curvature radius to minimize rolling friction. This new joint is realized by rolling between two pieces (femur and tibia) linked by ligaments, and thus get an architecture similar to that of a human knee. As such, the contact is made by rolling movement without sliding between two cylindrical surfaces with circular section, and for which we need find an innovative actuation mechanism. To take advantage of energy savings achieved, we must optimize the mass distribution so as to achieve the smallest global inertia of the mechanical system. In this work we propose various technological solutions for actuation mechanisms. A comparative study is performed between the different technological choices for actuator (cylinder or rotary actuator) and for transmission (connecting crank arm, belt, gearing, etc.). Of course, this new structure must be in accordance with specifications for the knee about size and weight, as well as amplitude and speed rotation of joint. In this work, our choice is to use electric actuators. These different solutions are evaluated according several criteria such as inertial characteristic (mass and inertia matrix), overall size, energy efficiency and the complexity of the system (number of used pieces). Initially, solutions with pulley and belt or rotary actuators and cables seem to have best performance those other systems with connecting crank arm or gearing. Results should be confirmed from a more accurate determination of transmission efficiency. For prospect, the future works will be about optimization of pieces geometry, and in particular as study the gain due to using curvilinear surfaces with elliptic section. Calculation of stresses in the materials by finite elements will provide more information about optimization of dimensions and shapes. Ultimately, energetic gains obtained with this architecture should be confirm through experimental tests.