Swantje Janzen

Abstract

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.

Abstract

The twisted-string actuation (TSA) principle provides simple, lightweight, silent yet powerful actuators well suited for human-machine interaction. This comprises of not only the use in orthotic and prosthetic systems for lower extremities, arms, and wrists but also exo-skeletons. A TSA consists of a bundle of at least two fiber components and an actuator to twist it along its main axis. This forms a helical structure, which shortens the axial length - given non-elastic behaviour of the material. In practice, an axial bearing compensates for the load force and a linear guide counters the motor torque. In cases where a bi-directional force is required, a passive spring return mechanism can be included. Our work focuses on the integration of the described TSA components into a single elastic tube. Three TSAs are arranged in series and six in parallel such that they form an array that can bend in three dimensions with muscle-like behaviour and elastic properties. By switching motor units on and off, the length and force of the array can be varied between zero and maximum force without the need of an internal feedback loop. A first experiment is carried out to validate the force control. A possible application of this technology is soft medical robots for diagnostic and therapeutic purposes. The first demonstrator consists of three motor units in series. Six of these serial arrangements are combined in parallel forming an array of 18 motors. Each TSA unit consists of a DC motor, a string coupling including the axial bearing, the string made of high-density polyethylene, a cylindrical housing that protects the motor against axial forces, houses the DC motor, and provides the support for the previous unit. The entire TSA unit is encapsulated inside a thermoplastic polyurethane (TPU) tube which takes over the counter-torque, acts as linear bearing, is the return spring, and provides the elastic base for the TSA array. Thus, by using these modular units an infinite chain of TSA can be built and arranged as desired. The TSA units can be current-controlled and independently activated. Experiments showed that a no-load stroke of 18 mm and a maximum pulling force of 11 N can be achieved by a single TSA module. The spatial arrangement of the tubes allows muscle-like use in larger and anthropomorphic systems and also 2-dimensional bending and torsion of the array for soft robotic systems. Further work will comprise the improvement of the module, their miniaturization, simplified axial bearings, and integrated control electronics.

Abstract

In this study a basic principle for the gas supply of a pneumatic actuation for a mobile active knee joint orthosis is described. Instead of using a compressor or a bulky pressure reservoir, the system is supplied with pressurized gas directly from a thermodynamic process to reduce size and weight of the device. To achieve this goal a literature search was performed identifying chemical processes. These options were then analysed and evaluated considering the aspects energy density, safety, eco-friendliness, and technical feasibility of the construction. The expansion of liquified carbon dioxide with phase change achieves the best result due to its high level of safety and the simple technical feasibility of the system. However, CO2cools down considerably during the expansion, so heating up again to room temperature is necessary. Therefore, the technical construction includes a passive and an active heat exchanger.