Mechanical Influences on the Design of Actuators with Variable Stiffness

In the 1990s compliant joint actuation was introduced to robotics with the Series Elastic Actuator [1] and the Mechanical Impedance Adjuster [2]. Such concepts provide safer human-robot interaction, can store energy and decrease force control effort [1]. In the following decades, energy efficient actuation with such concepts had high impact in mobile applications such as bipedal robots, since the stiffness adjustment can be utilized to match the natural frequency of the drive train to the frequency of the desired trajectory [1, 3]. A review on designs of actuators with variable elasticity between drive and link is given in [4] and followed by an energetic examination of specific concepts in [3]. Early solutions for actuators with variable stiffness are designed focusing on the drive side and hence only the input inertia of the drive train is considered as in [1]. Although the input and output inertia of the drive train were included in models of the complete robotic application according to [5], the dynamic interaction of those inertias is not considered for drive train design sufficiently [2]. In contrast to this, input inertia is not considered appropriatly in the dimensioning of recent approaches due to the assumption that drives represent ideal torques or position generators [6, 7, 8, 9]. Further, the analysis of power consumption in [3] is based on the same assumptions and hence does not regard the inertia interaction during dynamic operation. So far, the influence of more than one inertia is mainly considered in complex antagonistically-controlled concepts as AMASC [10] and VSA [11]. While the influence of interaction does not show significant influence on the characteristics of AMASC, its impact is not examined in the case of VSA. In the structurecontrolled and comparable complex VSJ [12], the interaction influence is rather low as in AMASC. Yet, this is not the case in general applications and thus both inertias and their interaction should be considered, as this has strong impact on the drive train dynamics and hence its dimensioning as well as an energy-efficient strategy for stiffness variation. This paper investigates those influences on a linearized model of a generic actuator with variable stiffness based on the parameters of VTS [13].