Shape Memory Alloy-based Stepper Drive for a Hand Exoskeleton

Due to the increasing importance of hand exoskeletons, recent literature has discussed various solutions for actuating finger movements. Development efforts aim to further reduce the size, weight, comfort, and energy consumption of the exoskeleton to improve patient compliance and evidence. Today, soft and hybrid exoskeletons are common solutions to provide movement and strength assistance via pneumatic muscles or cable-driven concepts. While pneumatic muscles provide high forces and physiological movement, they require large ancillary components compared to cable-driven concepts. These concepts are usually based on electric motors and cables for force transmission. Other approaches investigate the chances and limitations of smart materials, such as shape memory alloys (SMA), for actuating finger movement. While these approaches have shown promise, they suffer from high energy consumption, especially during endurance tasks, e.g. holding finger positions. Therefore, suitable mechanics and design approaches are needed to enable energy-efficient assistance for endurance tasks. This work presents an SMA-based stepper drive as an alternative actuation approach for hand assistance, especially for movement and endurance tasks. For this purpose, a bidirectional stepper mechanism driven by two SMA actuators is described and analyzed. The results show that the mechanism can realize the stroke of an artificial tendon and can remain in its position without further power supply. Furthermore, a mass of 100g can be lifted and lowered. It could therefore support fine motor skills and slow passive movement of injured fingers.Clinical relevance - This investigation can lead to improvements in rehabilitation, e.g. for slow movement of the fingers or training fine motor skills in therapeutic training via a hand exoskeleton.