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

Abstract The development of an exoskeleton for the hand poses a particular challenge due to the limited installation space and complicated movement sequences. 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 distinguished by their augmented mass and extensive spatial requirements, alongside occasional high energy consumption. 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 kinematic 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 wound and unwound the tendons on a pully for active flexion or extension of the affected finger. The mechanism can be driven bidirectionally or kept in a stable state of equilibrium without energy using two antagonistically arranged SMA actuators. Upon activation of the actuator, the lever undergoes deflection, causing the ratchet to rotate by one tooth in the direction of deflection. Upon cooling of the actuator, the system reverts to its initial equilibrium state, with the control wheel maintaining its position. This process can be iterated as necessary, with the cable being wound up or unwound accordingly. Activation of the antagonistic actuator results in a reversal of the rotation direction. The gear analysis indicates that the kinematics enable the overcoming of a finger force of approximately 25 N.