Comparison of Control Strategies to Excite Intrinsic Oscillations in a SEA-Driven Robotic Joint

During the execution of periodic motions, such as locomotion, animals exploit the elasticity in their body to increase efficiency. Adding elasticity in robotic systems, e.g., through a Series Elastic Actuator (SEA), enables to mimic this biological solution by storing energy in the spring. The standard strategy to efficiently drive such systems in periodic motions is to assume the motor static such that the SEA behaves like a single-mass-spring system, excited at its natural frequency. However, when regarding the SEA as a two-mass- spring system, we can derive another control strategy to excite periodic oscillations, where the motor and link inertia exhibit anti-phasic oscillations. This paper compares these two control strategies on a hardware SEA test bed regarding performance metrics such as maximal input torque and electrical power consumption. The control objective for this comparison is to excite a link oscillation with a desired amplitude, as could be needed for a pick-and-place task. We find that less current is needed for the given task and hardware for the first control strategy. The second strategy causes more friction that needs compensation but also increases stored system energy for the desired amplitude. When adding motor inertia shaping to this second strategy, we find a flexible controller that can shift the system to either behave like a single- or two-mass-spring system. Thus, we propose a promising control approach that can adapt system behavior to best suit a given oscillatory task.