Series and parallel actuation array of elastic micro-twisted string actuators

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.