Demonstrator of a low-cost active knee orthosis with twisted string actuation

This paper presents a demonstrative active knee orthosis to support sit-to-stand (STS) transfer movement, which is silent, simply constructed, small, lightweight, and low-cost. The demonstrative active knee orthosis is driven with a twisted string actuator (TSA) fixed to the lateral side of the upper rail. The TSA consists of two polyethylene strands which are twisted along its main axis by a DC motor. Linear guidance rails are used to counteract the motor torque on the load side of the TSA. The resulting helix structure formed in the twisted strings leads to a tensile force along the length of the TSA. At the knee joint, a cam disc converts this force into the desired support moment. Low-cost parts and materials were used when putting the demonstrator together to ensure the overall costs were kept to a minimum. An Arduino microcontroller and a H-bridge circuit provide the motor control, with a user controlled potentiometer specifying the desired speed. The demonstrator is 3D-printed via fused deposition modelling, with scaled down dimensions (scaling factor 0.7) compared to a normal knee orthosis. The costs, size, and noise of the demonstrator are kept low via the use of a simple TSA, replacing the need for heavy and noisy mechanical gear. Due to the simplicity of the system, the total costs are low (106 EUR), and thus are also expected to be low when creating a full scale version. The demonstrator shows a successful implementation of a TSA driven knee orthosis able to achieve STS-movement while being silent, lightweight and low-cost. Future work will look into improvements of the cam disc's geometry and motor selection to achieve the desired knee torque profile in relation to the transfer movement. In addition, a full-scale version of the TSA knee orthosis will be constructed.