  In this paper, we propose a compact twisted string actuation system that
achieves a high contraction percentage (81%) on two phases: multi string twist
and overtwist. This type of system can be used in many robotic applications,
such as robotic hands and exoskeletons. The overtwist phase enables the
development of more compact actuators based on the twisted string systems.
Furthermore, by analyzing the previously developed mathematical models, we
found out that a constant radius model should be applied for the overtwisting
phase. Moreover, we propose an improvement of an existing model for prediction
of the radius of the multi string system after they twist around each other.
This model helps to better estimate the bundle diameter which results in a more
precise mathematical model for multi string systems. The model was validated by
performing experiments with 2, 4, 6 and 8 string systems. Finally, we performed
extensive life cycle tests with different loads and contractions to find out
the expected life of the system.

Robotics