Training techniques and enhanced kinematic modeling for extended range twisted string actuators

As a linear actuator, the twisted string actuator (TSA) offers ease of miniaturization, flexibility, and the capability to exert a strong actuation force, due to its large gear ratio. Owing to these characteristics, TSA has recently gained attention as a suitable small actuator for wearable devices, and surgical robots. However, the operating range relative to its size has been reported as limited. This study aims to overcome this limitation by extending the operating range of TSA using the coiling phase, following the twisting phase. To achieve this, we first analyze the training process to effectively mitigate the irregular overlapping phenomena in the coiling stage of TSA, considering the hysteresis and the training effect of TSA under different loads. Second, we address the limitations of conventional TSA kinematic models and propose an improved kinematic model for the extended operating range of TSA. Specifically, this study presents the precise transition points between the twisting and coiling phases of TSA and proposes enhanced analytical models for each phase. Finally, through various experiments, we validate the proposed training process and kinematic model for extended TSA operation. It is expected that the proposed training process and kinematic model for TSA will enable precise actuation within the extended operating range, facilitating a wider array of applications.