Origami mechanisms

Overview

An origami mechanism is a body folded from a flat sheet along set creases, so its shape and motion come from the pattern rather than from separate parts. The one paper on this slug uses a Kresling cylinder, a hollow, triangulated tube folded from a flat pattern 2203.10122 p. 3. The authors contrast this with existing millimeter-scale origami devices, which usually need separate geometric components for locomotion and for each function [arxiv:2203.10122 p. 2].

Three properties of the fold do the work. Its high geometric symmetry, with a height-to-diameter ratio close to 1, lets it roll like a sphere [arxiv:2203.10122 p. 2]. Its tilted triangular panels act like propeller blades when it spins in water 2203.10122 p. 2-3. Its hollow cavity shrinks when the body folds, which pushes out liquid stored inside 2203.10122 p. 4, p. 7-8. The body is monostable: once the field that folds it is removed, it recovers the unfolded shape by itself 2203.10122 p. 4-5.

The fold does not supply power. The shape only responds to an external actuator; in this paper that actuator is a magnetic field, covered in Magnetic actuation 2203.10122 p. 4.

Variants

Single-plate Kresling for locomotion

One thin magnetic plate is attached to one hexagonal end. A rotating field makes the body roll on the ground, flip over obstacles or spin to swim, and the rotation axis of the field sets the mode 2203.10122 p. 3-4, fig. 1E-G. In this variant the fold supplies symmetry and panels rather than a shape change. The paper reports that the robot moves over a wall, spaced stairs, a 30 degree incline, a column array and a cylinder of about its own size, switching between rolling and flipping without a change to the field 2203.10122 p. 6-7, fig. 2A-C. Card: Spinning-enabled Wireless Amphibious Origami Millirobot.

A frontal hole and six radial cuts in the end caps are reported to raise swimming speed from 66.0 to 81.2 mm/s at 10 mT and 30 Hz. The authors attribute this to the hole drawing in water that is pushed out through the cuts, which reduces the high-pressure region in front of the robot [arxiv:2203.10122 p. 10-11, fig. 3C-E]. The same paper uses the spinning end as a suction grip: the low pressure at the hole draws cargo in, and gravity releases it when the hole faces down 2203.10122 p. 14, fig. 4B.

Two-plate Kresling for fold-and-release

Two plates sit on the two hexagonal ends, with magnetizations of equal size set 90 degrees apart. Their vector sum gives a net magnetization. A field applied along that net direction produces two opposite torques, which fold the body. The cavity shrinks and pushes stored liquid out through the radial cuts, and a needle at one end punctures a dye container at the other [arxiv:2203.10122 p. 7-8, fig. 2G-H]. Repeated fold and unfold cycles release liquid in what the authors call controlled doses 2203.10122 p. 8. The paper gives no volume per cycle, dose accuracy or residual volume, so the dose claim has no number behind it 2203.10122 p. 8.

When to use which

Implementation sketch

The pumping variant is the only one in the paper that changes the body's shape on purpose, so it is the clearest case of the fold as a mechanism. The sketch below is the cycle as the paper describes it. Fluid volume per cycle is not stated in the paper, so the code does not compute one.

# Fold-and-release cycle of a two-plate Kresling body  [p. 7-8] [fig. 2G-H]
# Input:  reservoir (liquid in the inner cavity), n_cycles (int)
# Output: list of release events; volume per event is not stated

def pump_cycles(reservoir, n_cycles):
    events = []
    # Repeat the fold and unfold cycle  [p. 8]
    for _ in range(n_cycles):
        # Field along the net magnetization folds the body  [p. 7]
        apply_field(direction="M_net")
        # Cavity shrinks and pushes liquid out through the radial cuts  [p. 8]
        events.append(eject(reservoir))
        # Remove the field; the body recovers its unfolded shape  [p. 4-5]
        remove_field()
    return events

Contradictions

None known. Only one card covers this slug so far.

Papers

• Spinning-enabled Wireless Amphibious Origami Millirobot: a magnetically driven Kresling cylinder that rolls, flips, swims, pumps liquid by folding and grips cargo by spinning. All results are author-reported.