18 SERVO 01.2016
DISNEY DOES SOFTWARE
For most hobbyists, building a robot mostly involves buying a robot, assembling it, and then (if you still
like robots) programming it to do stuff. Designing your
own robot from scratch is much more difficult —
especially if it’s a robot that has legs that are supposed
to do something practical.
Fortuntely, ETH Zurich, in collaboration with
Disney Research and CMU, has developed “an
interactive design system that allows casual users to
quickly create 3D-printable robotic creatures.” From
walking bipeds to salamanders with actuated spines to
quintapeds, the software does all of the hard work for
you. With just a 3D printer and some servos, you can
design robots that are exactly as bizarre as you want
them to be.
This approach is based on how easy it is to design
digital characters by pointing, clicking, and dragging.
Making physical robots this way isn’t nearly as easy, of
course, because it involves dealing with minor
annoyances like whether motions are physically possible.
There also needs to be some consideration of cost and
complexity — especially if the fabrication process is
based on a consumer 3D printer.
Disney’s method involves a combination of
fabrication-oriented design, physical character design,
motion planning, and, of course, robotics.
From the look of things, using the software is
alarmingly easy. Each robot starts with an initial skeletal
structure with bones connected by virtual motors
placed at each joint position. This is freely editable, and
you can click to add or subtract motors or alter their
orientation to change the structure of the robot.
To get the robot walking, you can adjust which legs
are on the ground when, while keeping your robot from
falling over simply by making sure that the green ball
representing the center of mass of the robot stays within
the red box representing the stability polygon created by
whichever legs are in contact with the ground. (Whew!)
More detailed gait customization can be applied as well,
including directionality, speed, turning rates, and individual
feet trajectories. Behind the scenes, the software deals with
all of the complicated stuff, optimizing the motor values to
yield dynamically stable motions that can then be previewed
in a physics-based simulation.
Once you’re happy with how things look, the final step is
to generate 3D geometry for all of your robot’s body parts,
including motor connectors. The software takes into account
what kind of printer you’re using and what materials you’ve
chosen. For example, if you’re using a MakerBot or
something else with a filament material, the software will use
infill for strength. However, if you have a laser sintering setup,
the software will change to a much more efficient truss
structure.
To test out their software, the researchers built
themselves a few different robots from scratch (including a
five-legged robot called Predator) using 3D printed parts,
Dynamixel MX-28s, a servo controller board, and a battery.
Predictably, the real world performance of the robots varied
somewhat from the physical simulation. Things like friction,
slightly bendy 3D printed body parts, and actuators that
don’t respond like an ideal actuator should all lead to small
but cumulative amounts of uncertainty. However, the
researchers “observed good agreement between the overall
motions of our physical prototypes and the behavior
Image: Disney Research
Snapshot of the design interface. On the left is the design viewport with the
footfall pattern graphic. On the right is the preview window showing the center of
pressure of the robot (green) and the support polygon (red).
