Principle performance requirements, safety analysis, and control for elastic joint robots

Kurzfassung

Intrinsically elastic joints have become increasingly popular over the last years. Commonly, they are considered to outperform rigid actuation in terms of robustness, energy efficiency, and peak dynamics. However, despite being a popular argument in favor of elastic actuation, it was not shown yet that a potential speed gain is truly inherent to the physical properties of the mechanism. In order to argue that ``elasticity is superior to input torque'', i.e. size and weight, it still needs to be derived that this new feature does not come at the cost of increasing weight for a given actuation technology. Therefore, it is analyzed in this thesis, under which circumstances ``extracting'' a certain amount of mass from a rigid joint and ``investing'' this into an elastic transmission leads to a performance increase. For this, the general scaling behavior of a rigid joint is derived and analyzed. Then, its capabilities are compared to the performance of an elastic joint in terms of maximum velocity. In this context, also a closed-form solution is given for reaching the maximum link side velocity in minimum time with an intrinsically elastic joint having a spring with limited deflection. While using joint elasticity is beneficial in terms of performance, high link velocities pose a potential threat to humans in case of collision. In this thesis, a representation of injury severity for blunt impacts is provided that can be accessed in real time, given the reflected inertia and velocity of a robot. Since collisions can generally not be avoided under all circumstances or contact might even be part of the task, one must take appropriate countermeasures in control and/or mechanical design of the robot to guarantee a safe human-robot interaction. In terms of control, collision detection and reaction have extensively been addressed for rigid and ``classical'' flexible joint robots (such as the DLR LWR-III) in robotics research. However, intrinsically compliant robots were only marginally taken into account. In this thesis, collision detection schemes are applied to an elastic robot, and a controller for collision reaction is proposed, which extracts the kinetic energy from a robot with intrinsic joint elasticity as fast as possible.