Design, modeling, and control of a Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS)

The recent application of the principle of discrete adjustment of stiffness has substantially enhanced the performance of Clutched Elastic Actuators (CEA), especially in terms of their capability for rapid stiffness switching. This key improvement plays a vital role in safety considerations, which are of particular relevance to physical Human-Robot Interaction (pHRI) applications. However, currently the only proposed CEA for this purpose suffers from bulkiness, which limits its potential application in multi-degree-of-freedom manipulators. In this work, we introduce a Series Elastic Actuator with Discretely Adjustable Stiffness (SEADAS) as a novel CEA, in which the stiffness can be modulated by adding/subtracting the appropriate number of elastic elements via clutches. The novelty lies in the unique design topology, where three concentric shafts are connected in series through clutches. Each shaft is further attached to a pair of elastic elements. The number of involved spring pairs, which depends on the engagement/disengagement of the clutches, allows for the attainment of three different levels of stiffness. The realized design is significantly compact relative to the previously proposed CEA that was based on this principle. This paper details the working principle, design and development of the actuator, stiffness and dynamic modeling, as well as a gain-scheduled PID controller used for accurate, stiffness-adaptive joint position control.