  Soft pneumatic actuators are used to steer soft growing "vine" robots while
being flexible enough to undergo the tip eversion required for growth. In this
study, we compared the performance of three types of pneumatic actuators in
terms of their ability to perform eversion, quasi-static bending, dynamic
motion, and force output: the pouch motor, the cylindrical pneumatic artificial
muscle (cPAM), and the fabric pneumatic artificial muscle (fPAM). The pouch
motor is advantageous for prototyping due to its simple manufacturing process.
The cPAM exhibits superior bending behavior and produces the highest forces,
while the fPAM actuates fastest and everts at the lowest pressure. We evaluated
a range of dimensions for each actuator type. Larger actuators can produce more
significant deformations and forces, but smaller actuators inflate faster and
can evert at a lower pressure. Because vine robots are lightweight, the effect
of gravity on the functionality of different actuators is minimal. We developed
a new analytical model that predicts the pressure-to-bending behavior of vine
robot actuators. Using the actuator results, we designed and demonstrated a 4.8
m long vine robot equipped with highly maneuverable 60x60 mm cPAMs in a
three-dimensional obstacle course. The vine robot was able to move around sharp
turns, travel through a passage smaller than its diameter, and lift itself
against gravity.
