  Inspired by the dexterous handling ability of an elephant's trunk, we propose
a pneumatic actuator that generates diverse bending and extension motions in a
flexible arm. The actuator consists of two flexible tubes. Each flexible tube
is restrained by a single string with variable length and tilt angle. Even if a
single tube can perform only three simple types of motions (bending, extension,
and helical), a variety of complex bending patterns can be created by arranging
a pair of tubes in parallel and making the restraint variable. This performance
takes advantage of the effect of the superposition of forces by arranging two
tubes to constructively interfere with each other. This paper described six
resulting pose patterns. First, the configuration and operating principle are
described, and the fabrication method is explained. Next, two mathematical
models and four finite element method-based analyses are introduced to predict
the tip position changes in five motion patterns. All the models were validated
through experiments. Finally, we experimentally demonstrated that the prototype
SEMI-TRUNK can realize the action of grabbing a bottle and pouring water,
verifying the effectiveness of the proposed method.
