Design and Modeling of Soft Pneumatic Helical Actuator with High Contraction Ratio.

Soft contraction actuators are becoming important elements particularly for human-friendly robotic applications. However, it is challenging to achieve both a large operating distance while generating practical force. Hence, we present a new soft contraction actuator capable of realizing a high ratio contraction by pneumatic power. It can be easily fabricated using soft materials, including rubber tubes, one-way extensible cloth, and inextensible wire. Its initial shape is tubular but it can curve and coil to a helix shape owing to its different extensibilities on two sides when pressurized. A maximum contraction ratio of 78% and a 23 N contraction force can be achieved with an 11.6 mm initial outer diameter tube under 0.3 MPa. The effect of the tilt angle of a one-way extensible cloth on the helical shape is investigated, and a mathematical model illustrating the relationship between the contraction ratio and force is derived. Our experimental results suggest that this helical actuator has a much higher contraction ratio than a McKibben actuator under the same conditions. Finally, we discuss the potential application of the proposed actuator to a wearable device, i.e., for assisting the dorsiflexion of an ankle joint requiring a wide range of motion.