Session: SYMP 4-4: Human Integrated Smart Systems

Paper Number: 140337

140337 - New Concept of a Bidirectional Sma-Actuated Stepper Drive for a Hand Exoskeleton 

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In the context of rapid demographic change innovative technologies that contribute to the maintenance of health, rehabilitation, and restoration of mobility after impairments are becoming increasingly important – e. g. exoskeletons. Despite intensive research into exoskeletons, there is still a lack of evidence and technical suitability for everyday use. To enable the access to these technologies, it is necessary to develop cost-effective and reliable drive technologies that actively support the movement of the affected extremities with low energy consumption.

Rehabilitation of the hand is a crucial area of research, given that limitations in fine motor skills or grip strength resulting from illness or injury can significantly impair daily functioning. The development of an exoskeleton for the hand poses a particular challenge due to the limited installation space and complicated movement sequences. In Literature various design principles for hand exoskeletons are introduced and analyzed. Tendon-driven mechanisms have proven to be a promising approach due to their low weight and small installation space requirements. The antagonistic arrangement of two tendon systems enables a relatively natural flexion and extension of the finger with actuators. For this purpose, electromagnetic, pneumatic, or hydraulic principles, and smart materials are conventionally used. Nevertheless, these concepts are characterized by increased mass and a high spatial requirement, as well as high energy consumption in some cases. Stepper drives are a promising alternative to conventional actuators, as they are characterized by both high functional density and low energy consumption.

In the following, the kinematics of a novel shape memory alloy (SMA)-actuated stepper driver for use in exoskeletons is described and analyzed. The kinematic concept is based on a bidirectional ratchet mechanism that can be used to wind and unwind the tendons on a pully for active flexion or extension of the affected finger. With two antagonistically arranged SMA actuators, the mechanism can be driven bidirectionally or kept in a stable state of equilibrium without using energy. Upon activation of the actuator, the lever undergoes deflection, causing the ratchet to rotate by one tooth in the direction of deflection. After the actuator has cooled down, the system returns to its original equilibrium state, with the adjusting wheel maintaining its position. This process can be repeated as required, with the cable being wound and unwound accordingly. Activation of the antagonistic actuator leads to the reversal of the direction of rotation. Analyzing the mechanism shows that the kinematics enables overcoming a finger force of around 40 N.

Presenting Author: Kenny Pagel Fraunhofer-Institute for Machine Tools and Forming Technology

Presenting Author Biography: K. Pagel received his Dipl.-Ing. degree by the Technical University Chemnitz in 2006. In 2007 he joined the Smart Materials Department at Fraunhofer IWU as a researcher and project manager. He received his doctoral degree in 2018 with a thesis about the development of shape memory actuators with inherent guidance function. Since 2022 he is head of the Shape-Memory-Alloy Department. His work is focused on the design of shape-memory-alloy applications and especially SMA micro actuators.

Authors:

Alina Carabello
Kenny Pagel
Welf-Guntram Drossel