String Lifetime Evaluation of Twisted String Actuators-Applied to a Soft-Wearable Robot

Twisted String Actuators (TSAs) are used in many areas of robotics. Some studies have used strings, such as ultra-high molecular weight polyethylene strings, commonly called Dyneema strings. The twist tensile tester was specially designed to evaluate the strings in TSAs. In addition, the lifetimes of 14 string types and diameters (from 1.0 to 2.0mm) manufactured by four companies (Armare, Gottifredi, Liros, and Alpha Ropes) were evaluated using an experimental device. The experimental conditions were a motor rotation speed of 2500 rpm, stroke of 10%(25mm), 20% (50 mm), and 30%(75mm), and loads of 9, 15,20, and 25kg. The rest of the times at the upper lower strokes were 1s. This study recorded the longest lifetime for Liros (1.5, and 2.0mm) when the weight was 9 kg. Gottifredi's Evofly had the second-longest cycle when the diameter of the string was 1- to 1.6mm (47,423 cycles), and SK78, (also from Gottifredi), had the second longest when the diameter was 2.0mm (121,942 cycles) at 9 kg. Both Gottifredi's Evofly78 and SK78 had the longest lifetimes when the weight was 15, 20, and 25kg. When applying this Dyneema material to a wearable robot, it is generally heavier than 9kg, therefore, Gottifredi (Evofly 78 at 1.5mm and, SK78 at 2.0mm) is the most suitable material.