Frequency Response Analysis of Actively Cooled Nylon Twisted Coiled Actuators for Use in Wrist Rehabilitation Devices

Recovery from an upper limb musculoskeletal injury can take months or years, and often requires continuous visits to a therapist for specialized training and evaluation. Research has found that active assistance using mechatronic devices can significantly improve the quality and speed of recovery, especially when used for periods beyond a typical clinical appointment. However, current devices commonly use conventional actuation methods that are too heavy and rigid to be worn as a portable system outside of clinical settings. A recently discovered actuator made from twisting and coiling nylon thread (TCA) has the potential to improve wearable mechatronic designs due to its high power density, large strain, and inherent compliance. TCAs require heat to contract, which limits the actuation bandwidth due to the slow cooling rate, making it difficult to implement in wearable devices that must respond to voluntary motion. This study presents an active cooling method for TCAs to improve their frequency response and evaluates their feasibility in a wrist orthosis design based on real anatomical constraints. The frequency response was evaluated using a square wave input of varying frequency to provide actuation cycles of resistive heating and forced convective cooling, while measuring displacement at constant load. The results show that the TCA was able to respond to frequencies of up to 6.5 Hz, and also achieve full wrist range of motion at 0.55 Hz for loads equivalent to the weight of an average adult hand. This indicates that the proposed wrist orthosis could effectively assist the wrist throughout its entire range of motion and at the speeds required for voluntary motion using parallel TCAs.