Modeling, Design and Verification of Twisted String Mechanism

Abstract

Low back pain (LBP) imposes significant economic losses, particularly in the industrial field. The causes of LBP are diverse and excessive force being is one of them. One method to prevent excessive force is exoskeletons. Exoskeletons can reduce maximum voluntary contraction (MVC) by up to 47.4%, effectively minimizing the risk of LBP. Among the methods of actuating exoskeletons, rigid and soft actuation exist. Soft actuation, known for its compact, and lightweight, is particularly attractive. Among soft actuation mechanisms, the Twisted String Actuator (TSA) stands out due to its compact, lightweight, and cost-effective, making it suitable for exoskeleton applications. However, TSA has a limitation of its fixed contraction range. To address this limitation, we propose a TSA device that can adjust the contraction range by adjusting the offset between strings. This device comprises a rotation plate, rotation axis, gear, curved link, motor, and motor housing. It has an expanding mechanism and planetary gear to enable offset adjustments while maintaining twisting operations. Additionally, we present an analytical modeling appropriate for the proposed TSA device. The proposed modeling is based on the geometrical shape of TSA according to the Number of Rotation (NoR). We evaluate the proposed modeling by Normalized Root Mean Square (NRMSE) and Nominal Range Sensitivity Analysis (NRSA). The proposed device exhibited the capability to widen the offset, resulting in a 17% increase in the contraction range at the same NoR, and to narrow the offset, leading to a 42% increase in maximum force under the same input (PWM=0.565%). NRMSE of the proposed model was 3.243%. As a result of NRSA, it was confirmed that the existing modeling did not respond to changes in load and offset at the same time or was not within the standard deviation range of actual changes, whereas the proposed modeling was within the standard deviation range of actual changes for changes in load and offset, so it is suitable for the TSA device proposed in this paper. In conclusion, our proposed TSA device, with its adjustable offset feature, is expected to excel in adapting to external environmental changes, when equipped with motors that have limited specifications in exoskeleton.