the ankle joint next. A deceptively tricky
joint, the ankle’s design challenge comes
in allowing for a large range of motion
while keeping a lower pivot. The ankle
from our design has two DOF. Both
axes of rotation intersect to more closely
imitate a human’s ankle and make
the kinematic and dynamic equations
simpler for control of the robot. Figure 9
shows how we made our ankle and
feet. Notice how it can be challenging to
figure out how to support your links.
We used ball bearings in most cases
to reduce the bending moment where
∨ FIGURE 10. Exploded view of the hip.
42 SERVO 01.2007
< FIGURE 9. Exploded view of the feet.
the servo motor attaches to the link.
The Hard Part — Hips
Even if you can handle the design of
the rest of the body, the hips can stop
you in your tracks. Creating kinematically
spherical joints with human proportions
(optional), good range of motion, and
good structural stability is very difficult —
especially if you have a waist motor in the
area. Figure 10 shows how we made our
hip joints. Two of the motors are joined
together in the hip to reduce the space
requirements of each motor. If you have
trouble fitting all of the motors together
in the hips, moving the waist motor into
the chest can free up some space.
Links
Once you design the joints, you’re
about halfway done. Next, decide how
you want to join each link together.
There is a careful design balance
between structural rigidity and range of
∨ FIGURE 11. Exploded view of the legs.
motion. You’ll notice that as you start
connecting joints together, your links
start to interfere with each other and
limit your range of motion. Figure 11
shows how we designed the link going
from the knee to the ankle. We put a
bend in the link to add structural rigidity and to increase the range of motion.
You may also notice that the rendered
drawings are a little different from our
final design. In our final design, we
removed material from the knees,
shins, and forearms to decrease weight.
This is also a good point in time to
consider how you want to mount the
motors to your joints and links. What
kind of screws and nuts do you need?
What kind of holes (threaded or not)
and hole placement do you need?
Deciding on a standard screw pitch
and size greatly helps in assembly.
Link design is also an appropriate
time to start considering wiring issues.
How are you going to get power/
communications to your actuators?
How are you going to connect your
sensors, CPU, and power source?
We created many of our own wiring
connections since we wanted to reduce
any excess weight or volume. Robotis provides male and female connectors so you
can create wires to your desired length
for wiring up your servo motors (Figure
12). Figure 13 shows a picture of one of
our custom-made wiring connections.
Force Sensors
Obviously an optional feature for
your robot — but important enough for
consideration during physical design —
force sensors can be used to give information to your CPU to help stabilize
the walking motion of the robot. The
number and placement of the force
sensors can be tricky. We recommend
using four for each foot: one placed at
each corner. Once you place the force
Thank Yous
Thanks to the 2005-2006 senior
design team members that built DARwIn
1.0 (Patrick Cox, Joo Gil, Chris Greenway,
Jeff Kanetzky, Karl Muecke, Patrick Mulliken,
Raghav Sampath, Daniel Zokaites) and to
our advisor Prof. Dennis Hong.
