A Novel Compact Torsional Spring for Series Elastic Actuators for Assistive Wearable Robots

Abstract

The introduction of intrinsic compliance in the actuation system of assistive robots improves safety and dynamical adaptability. Furthermore, in the case of wearable robots for gait assistance, the exploitation of conservative compliant elements as energy buffers can mimic the intrinsic dynamical properties of legs during locomotion. However, com-mercially available compliant components do not generally allow to meet the desired requirements in terms of admissible peak load, as typically required by gait assistance, while guaranteeing low stiffness and a compact and lightweight design. This paper presents a novel compact monolithic torsional spring to be used as the basic component of a modular compliant system for series elastic actuators. The spring, whose design was refined through an iterative FEA-based optimization process, has an external diameter of 85mm, a thickness of 3mm and a weight of 61.5 g. The spring, characterized using a cus-tom dynamometric test bed, shows a linear torque versus angle characteristic. The com-pliant element has a stiffness of 98 N m=rad and it is capable of withstanding a maximum torque of 7:68 N m. A good agreement between simulated and experimental data were observed, with a maximum resultant error of 6%. By arranging a number of identical springs in series or in parallel, it is possible to render different torque versus angle characteristics, in order to match the specific applications requirements. [DOI: 10.1115/1.4007695

Design and Characterization of a Novel High-Power Series Elastic Actuator for a Lower Limb Robotic Orthosis

“…This shape implies that the transfer of the torque between the outer and the inner parts of the spring occurs through flexible elements. The shape and dimensions of such flexible elements are defined through an iterative FEM simulation-based design and optimization process, as described in [22]. Considering the stiffness and the torque requirements in safe conditions (safety factor ≥ 2), the target storable elastic energy is 7.5 J, above the maximum value of the spring described in [22].…”

“…Based on the authors' previous work [22], a compact, monolithic, torsion spring was designed for the rotary SEA. The desirable physical stiffness of SEAs for locomotion assistance, as retrieved from a literature analysis, may range from 100 to 300 N·m·rad −1 [20,21,[29][30][31].…”

“…The shape and dimensions of such flexible elements are defined through an iterative FEM simulation-based design and optimization process, as described in [22]. Considering the stiffness and the torque requirements in safe conditions (safety factor ≥ 2), the target storable elastic energy is 7.5 J, above the maximum value of the spring described in [22]. To increase the level of storable elastic energy, a novel topology was investigated, which includes a symmetric structure with a radial replication of four elastic elements (worm-shaped lamellae).…”

“…A similar approach is proposed in [18], with the HD used in differential mode: in this device the motor and the spring are functionally connected in series but topologically they are connected to the two input shafts of the differential mechanism (Differential Elastic Actuator, DEA). In [19], a prototype intended for a humanoid robot is presented; its compliant element is composed of a three-spoke shaft with six linear springs.In [21], a compact SEA with a flat DC motor, a HD and a custom lamellar spring, designed according to the method proposed in [22], is used to actuate an active knee orthosis. In the lower limb rehabilitation robot LOPES [23], SEAs are implemented using brushless DC motors, connected to the actuated joints through Bowden cables.…”

Design, Development and Scaling Analysis of a Variable Stiffness Magnetic Torsion Spring

“…Examples include robotic hands, where the elastic elements are used to perform a stable grasp [1][2][3], flying robots, in order to estimate the pitch torques generated by wings during flapping [4] and robotic snakes, for estimating ground contact forces [5]. Furthermore elastic elements are key components of Series Elastic Actuators (SEAs) [6][7][8][9], Variable Stiffness Actuators (VSAs) [10][11][12][13][14][15][16][17][18] and, more generally, of Variable Impedance Actuators (VIAs) [19], where damping can also be adjusted by properly controlling the device. In this field, a seminal work is that of Fasse et al [20], where an electromagnetic variable impedance actuator is demonstrated, capable of exerting a torque much higher than that of the prototype described in this paper.…”