  Origami structures have been widely explored in robotics due to their many
potential advantages. Origami robots can be very compact, as well as cheap and
efficient to produce. In particular, they can be constructed in a flat format
using modern manufacturing techniques. Rotational motion is essential for
robotics, and a variety of origami rotational joints have been proposed in the
literature. However, few of these are even approximately flat-foldable. One
potential enabler of flat origami rotational joints is the inclusion of
lightweight pneumatic pouches which actuate the origami's folds; however, pouch
actuators only enable a relatively small amount of rotational displacement. The
previously proposed Four-Vertex Origami is a flat-foldable structure which
provides an angular multiplier for a pouch actuator, but suffers from a
degenerate state. This paper presents a novel rigid origami, the Self-Lock
Origami, which eliminates this degeneracy by slightly relaxing the assumption
of flat-foldability. This joint is analysed in terms of a trade-off between the
angular multiplier and the mechanical advantage. Furthermore, the Self-Lock
Origami is a modular joint which can be connected to similar or different
joints to produce complex movements for various applications; three different
manipulator designs are introduced as a proof of concept.
