Section of Final Report: NASA ACRP NCC7-7, Bush Robots, Hans Moravec, Jesse Easudes

5: Fabrication of Structural Models

We cannot yet build an actuated mechanical model, but there are techniques that allow us to make static "sculptures" of bush robots. The most convenient avenue seems to be a ten-year-old prototyping method called stereolithography. Stereolithography is a three-dimensional printing process that produces a solid plastic model. A computer-controlled ultraviolet laser draws cross sections of the model onto the surface of photo-curable liquid plastic, hardening it. A vertical elevator system lowers the newly formed layer, while a leveling system establishes the thickness of the next layer. Successive cross sections, each of which adheres to the one below it, are built one layer on top of another to form the part from the bottom up.

We have two stereolithography machines located near us, and old, neglected one in a Carnegie-Mellon lab, and a better-maintained newer model at a local Alcoa company facility. The working volume of the first is a cube 10cm on a side, the second is a 20cm cube, both have a resolution of 0.25mm. The costs of fabricating a 10cm object can range from a few hundred to a few thousand dollars, primarily determined by the fabrication time. A large, detailed model can consume several weeks of stereolithography machine time. The process has various limitations, for instance partially fabricated objects must be self-supporting at all stages. The software controlling the machine automatically inserts struts as needed for temporary scaffolding, which must be manually removed on completion. The myriad floating fingers of an awkwardly posed, partially fabricated bush robot could require an unwieldy number of struts, so we had to plan the poses carefully. Following is a picture of the SLA-350 20cm stereolithography machine.

We prepared files for 9 and 10 level B=3 bush robots, having 19,683 and 59,049 end fingers, and made of 196,828 and 590,488 traingular facets, respectively, using the triangular tesselation scheme described in Section 3. The machine was able to poduce the degree 9 part, but repeatedly failed in attempts to make the degree 10 bush, with different errors, probably because of memory limitations. Interestingly, our 3D graphics systems also broke down at degree 10, or just beyond, surely reflecting the capacity of typical computers in 1998. Given the steady rise in computer power and memory, we would expect the number of elements that upgraded graphics and SLA systems can handle to approximately double each year.

The following four pages show two graphical representations of the files used to make the degree 9 model (which itself strains our graphics computers), followed by two images of an actual stereolithographed model.

3D graphics views of B=3 N=9 bush robot

Views of stereolithographed B=3 L=9 bush robot model, 4.25" tall

Mechanical Model Design

We tried to choose the simplest and most economical approach to making a passively articulated three-way-branching bush robot model. Standard parts do not come in the scale steps required, so we loosened the requirements, and allowed up to 20% deviation from the norm sizes in structural members and joints. We designed a double-cone socket piece to contain three ball joints connecting a branch its three smaller twigs. We require 5 different scales of this part for our degree 6 bush:

Each such part is capped by a three-petaled sheet metal cover to hold in the ball joints. The upper portion of each ball and the twig it supports protrude through a hole in each petal sized to allow a +/- 45° swivel from the 45° normal of the cone orientation, allowing the twig to swing from parallel to orthogonal relative to the orientation of its parent branch. Here is a graphic of a fully filled level 6 mechanical bush model:

Photographs of Mechanical Model