Development and performance comparison of an assistive hand based on shaped memory alloy and linear actuator

Abstract

Number of people with hand disability is increasing recently due to diseases and accidents. Rehabilitation and assistive robotics is an emerging field of research where researchers are trying to develop tailored made robotic devices to address the challenge of disability. However choice of proper actuator for designing assistive device is important to make it safe and compact. Moreover, feedback integrated system of structure-less wearable hand for partially disabled people is not available yet so this dissertation contributes to make it done. This work presents a study on development of two wearable assistive robotic hands for grasping based on Shape Memory Alloy (SMA) and Linear Actuator. The proposed designs are compact and sufficiently light to be used as an assistive hand. Each design is tendon driven and joint-less structure that has the potential to be used as an assistive device for stroke patients. The proposed designs have been implemented for index and thumb fingers as a first prototype to enable grasping. SMA actuator and bias force mechanism were used together in the first design for the purpose of hand's flexion and extension where a linear actuator was used in the second design. This work provides a review on the previous work done in the field of hand rehabilitations and describes the mechatronic design of the wearable hand, simulation, modeling, and development of the actuation force mechanism and sensory system. Experiments of open loop controller were conducted to understand the hand characterization and grip force provided by index finger. A feedback controller (proportional controller) has been implemented for this prototype with gripping force as the feedback parameter. In the first design, it was observed that approximately 2.25 A current caused 4 cm displacement for SMA actuator. The maximum temperature of the SMA actuator was achieved to be 100 Celsius. The attainable gripping force was around 2N for a load free finger. The conducted experiments showed promising results that encourages further development on this. The same procedures were implemented in the case of Linear Actuator. However Linear Actuator consumed much less power in comparison with SMA. Performance comparison and evaluation between the assistive hands based on SMA and Linear Actuator were conducted. Various experiments were carried out to explore the performance behavior of these two actuators.