Design of a Compact Actuated Compliant Elbow Joint

Abstract

Compactness is a valuable property in designs of assistive devices and exoskeletons. Current devices are large and stigmatizing in the eyes of the users. The cosmetic appearance will increase by reducing the size. The users want a device that is small enough to be worn underneath the clothes, so it becomes unnoticeable. The goals of this paper are (1) to provide an overview of the shape-changing-material-actuated large-deflection compliant rotational joints, (2) provide new introduced performance indicators that evaluate the designs on performance with respect to volume or weight and (3) design a compact active assistive elbow device as a case study. In order to reach these goals, two evolving fields of study are brought together that have great potential to reduce the size of exoskeletons: smart materials and compliant rotational joints. Smart materials have the ability to change their shape, which make them suitable as actuators. Compliant joints can be compact, since they are made out of one piece of material. An overview of shape-changing-material-actuated large-deflection compliant rotational joints is presented. Performance indicators are proposed to evaluate the existing designs and the prototype. As a case study a compact actuated rotational elbow joint is presented. An antagonistic actuator made from shape memory alloy wires is able to carry an external load and to actuate to move the arm to different positions. The compliant joint is optimized to balance the weight of the arm and to auto-align with the rotational axis of the human elbow joint. A prototype is able to generate a volume specific stall torque of 5.77 ⋅ 103 Nm/m3, produces a work density of 7.27 ⋅ 103 J/m3 based on volumes including isolation covers and the half-cycle efficiency of the device is 3.6%. The prototype is able to balance and actuate a torque of 1.1 Nm.

Soft Elbow Exoskeleton for Upper Limb Assistance Incorporating Dual Motor-Tendon Actuator

“…In the figures, the simulated signal is represented by a blue line and a red graph illustrates the experimental signal. The performance for steady-state response in the experimental work has a higher steady-state error than the steady-state error in the simulation based on the transfer function model in Equation 3through (6). This can occur because of the un-modeled dynamic of the friction force between lead screw and nut, as well as Bowden cable and the exoskeleton.…”

“…Researchers have developed a hard exoskeleton elbow for assisting people with the upper limb [2][3][4][5][6][7][8]. They succeeded in building a hard elbow exoskeleton robot to provide mechanical assistance for a user/wearer mostly for flexion and extension motion.…”

Misalignment Compensation for Full Human-Exoskeleton Kinematic Compatibility: State of the Art and Evaluation

“…Compliant elements can also be implemented at the joint level. The joint axis is then replaced by a compliant coupling, which gains its degrees-offreedom from an elastic deformation [86].…”

“…A compliant joint element based on leaf springs has been implemented in an elbow exoskeleton by Kleinjan et al [86]. It allows for a compact design and auto-aligns itself with the human axis of rotation.…”

“…On the other hand, a non-zero torque can be used to counteract the mass of the device, as noted in Ref. [86].…”