Sunday, July 20, 2014

Windbot Muscles

Me with a newly-built windbot muscle.

A simple, muscle-like actuator for wind animated sculptures can be made by supporting a Savonius wind rotor on a taut thread. The thread needs to be very fine and strong. I have used Berkley Fireline of 1-lb test strength. A small Savonius rotor can be made by splitting two disposable cups and reassembling them with hot melt adhesive.

The wind rotor twists up the thread causing it to contract. When the wind dies down, the contraction slowly relaxes as the rotor spins in reverse.

The relaxation can be made very fast and controllable by using high-performance ball bearing swivels. Lazer Eagle Claw 75-lb test swivels work well enough with the windbot muscle shown here.

A Savonius wind rotor made from two disposable cups.

Detail of the wrap-around wire fitting that confines twist to the outboard sections of the doubled length of FireLine.

Suitable ball-bearing swivels for the muscle shown.

Saturday, July 12, 2014

Wind-powered plectoneme actuators

A plectoneme. Image quoted from B.C. Daniels et al., "Discontinuities at the DNA Supercoiling Transition."

I am working on a proposal for an artwork at World Maker Fair New York that uses wind-powered plectoneme actuators. A plectoneme is the twist instability that forms a hairpin side branch on a highly twisted fiber. As has been discovered in DNA biomechanics, when a twisted fiber forms a plectoneme it becomes the mechanical equivalent of a threaded screw: additional twisting at constant tension shortens the length of the fiber. The shortening continues to nearly full contraction of the fiber as demonstrated in my video on "writhing thread" actuators. That demo used a high-current pulse to drive a cellphone vibrator motor to lift a weight weighing about 30 times the motor's weight to almost 100% contraction of the fiber.

After plectonemes form, further twisting of a fiber under constant tension is converted into linear motion. Image quoted from S. Forth et al., "Abrupt Buckling Transition Observed during the Plectoneme Formation of Individual DNA Molecules."

Tuesday, October 1, 2013

Kids make completely foldable objects

Scene at the "Make a Completely Foldable Object" booth.

Folks of all ages made completely foldable objects at the Silver Spring Mini Maker Faire on September 29, 2013. The example I demonstrated was an octahedron that folds down to a single stack of triangles. Participants as young as six years-old made their own. Two participants went further, creating their own freeform completely foldable objects, and succeeded in folding them down to a single stack of triangles.

An ambitious, freeform CFO...

and, a little later, all folded down to one stack.

A freeform completely foldable object.

After some puzzle solving, it's all folded down to a single stack.


Me with Crumple at Nuit Blanche.

Crumple is an animated sculpture composed of 128 identical folded pieces of aluminum sheet metal. It is animated by inflation of an internal plastic bag. It was exhibited at Washington DC's popular Nuit Blanche on September 28-29, 2013. This album shows photos of Crumple under construction. This videos shows Crumple in action.

Friday, September 6, 2013

Maquette for a completely foldable sculpture

Maquette for Crumple, a kinetic sculpture proposed for DC's Nuit Blanche.

Crumple maquette in a different state of inflation.

Left and right patterns.

Cutting from aluminum flashing with strong scissors.

Assembling with Bassetti-style rubber band hinges (#19 bands doubled.)

Saturday, August 31, 2013

Improved completely-foldable octahedron

Completely foldable octahedron with Bassetti hinges and box closures on the outside.

View of the rubber band hinges and the unfolded shape of the triangle units.

Wednesday, August 28, 2013

Triplets of Dyck words code the physical configurations completely folded surfaces

In phantom configurations of a completely folded surface, the triangles and hinges can intersect and coincide. In semi-physical configurations of a completely folded surface, triangles do not coincide (they have a physical, stacked order,) but hinges may cross through each other. In physical configurations of a completely folded surface, triangles do not coincide and hinges do not cross. Since a surface in a physical configuration has no self-intersection, such can never be a non-orientable surface without boundary.

A completely folded surface in either a physical or semi-physical configuration has a stack of triangles—and consequently three stacks of edges—hinged together in a particular way. We assume in the following that the surface has no boundary: therefore every triangle edge is connected to one other triangle edge in the same edge stack by a hinge.

If hinges are permitted to cross each other (the semi-physical condition), then the hinge connections in a given edge stack are a perfect matching of those edges.

If hinges are not permitted to cross (the physical condition), then our boundary-less surface must be orientable, and the hinge connections in a given edge stack are a non-crossing partition of those edges. A natural way to code a non-crossing partition is as a parenthesis word, or Dyck word. When written out as a Dyck word, it is easy to imagine the non-crossing arrangement of the hinges in the edge stack by simply connecting the paired parentheses with arcs above the Dyck word, e.g.:

( ) ( ) ( ( ) ).

Since there are three edge stacks, a triplet of Dyck words of the same length can encode a completely folded physical configuration.

The number of Dyck words of length 2n are counted by the nth Catalan number:

1, 1, 2, 5, 14, 42, 132, 429, 1430, ....

and triplets of Dyck words are counted by the cube of the nth Catalan number:

1, 1, 8, 125, 2744, 74088, 2299968, 78953589, 2924207000, ....

For triangle-faced maps that are, as usual, without boundary, but possibly composed of multiple disjoint components, the same daunting integer sequence counts their completely folded configurations when 2n is the number of triangles in the stack. In using the cube of Catalan numbers we are counting as different every possible difference in the completely folded configuration: e.g., simply rotating the stack, flipping it over, enantiomorph-ing the surface or the folded configuration, etc. More significantly, since the map is permitted to have multiple disjoint components, we also count every possible way the multiple components can be arranged in the stack, including the different ways they can enclose each other.

Though it is appealing to think that we can now dial up the phone number of any completely folded configuration that will ever be designed, the combinatorial explosion frustrates "cold-calling" as a technique to discover new designs. Nonetheless, dialing from even the shortest phonebook puts us in touch with shapes that we humans have largely avoided thinking about. A phonebook with n<=2 has 1 + 1 + 8 = 10 entries in it. The configurations that pick up the phone are the empty set; the triangular pillow; a stack of two triangular pillows; a triangular pillow enclosing another triangular pillow; two triangular "ears" that have seamed along a double edge and folded along the seam—in three rotations; and the same "double ear" with one ear tucked inside the other—also in three rotations.