Select Your Motor
One of the biggest decisions you
have to make when building your bipedal
robot is what kind of actuators to use.
We used Robtis’ Dynamixel servo motors,
model DX-117; and will upgrade to the
RX- 64 for the next version of DARwIn
(Figures 2 and 3). The DX-117 has a maximum torque of 39 kg-cm and the RX- 64
∨ TABLE 2. Table of specifications for
Robotis’ Dynamixel DX-117 servo motor.
Mass 66 g
Angle Range 300°
Torque 29 kg-cm — 39 kg-cm
CPU PC/104 (Planned)
Height 600 mm
Mass 4 kg
Degree of Freedom 21
Power 2 Li-poly @ 7.4V
Sensors Rate gyro, force sensors
< FIGURE 1/TABLE 1. Here is a photo and
statistics table for DARwIn.
has a maximum torque of 64 kg-cm
(Table 2). Both have built-in position and
speed controllers. All user control is done
with RS-485 serial communication.
If you are on a tight budget, regular
R/C servo motors are fairly inexpensive,
but since you get what you pay for, the
performance may be unacceptable. You
can also use miniature DC motors, but
you need to design your own position
feedback controller or purchase one
such as the “allmotion” boards, which
can be expensive. Different actuators
have different torque properties and
mass properties; so if keeping your
robot lightweight is a must, you may
want to put the stronger, heavier motors
in the joints that see the greatest load.
Once you pick your actuators, you
need to decide on your kinematic structure — this is one of the most important things to do in the design process!
You can make your own or use ours
(Figure 4). Your kinematic model will
determine how you design your joints
and how you attach the actuators.
Where to Put Stuff
legs. It is possible to house hardware in
other cavities in the robot, some such
cavities may be in the leg or foot
(depending on the design). Though the
batteries are in the chest for the current
design, next year, we plan on placing the
batteries in the feet of the robot. This
placement will make room for our new
PC/104 board computer, IEEE 1394 PC
board camera, rate gyro, DC-DC converter, and protection circuits.
Let’s Be Honest:
Finally, before you start designing
the links and joints, determine what
materials you can afford to buy and
what machining tools you have
access to (milling machines, welding
equipment, etc.). This will dictate how
ornate (or how simple) your robot’s
design can be. If you have access to a
four-axis CNC mill and large amounts
of bar stock aluminum, you have much
more freedom in your design than if
you only use sheet aluminum.
For DARwIn, time constraints and
availability led us to use sheet aluminum for almost the entire structure.
With some experience under our belt,
we’re designing and building DARwIn
2.0 in half the time and using a four-axis CNC mill to create most of our
parts. Figure 6 shows a CAD drawing
of our new hip design. We recommend
using aluminum for the structure of the
robot because of its price, weight,
strength, and the ease of machining it.
If you have the funds, using rapid prototyping would be an excellent option.
∨ FIGURE 2. RX- 64 motor.
40 SERVO 01.2007
∨ FIGURE 3. DX-117 motor.
Next, decide on
a location for your
extra hardware: CPU,
etc. We decided to
except for the force
sensors in the chest
for simplicity (Figure
5). One issue with
this high is it creates
a larger torque on
the motors in the
Now that you’ve nailed down
the essentials, you’re ready to start
designing the links and joints of your
robot. You can get as creative as your
imagination lets you, but if you need a
starting point, here are some tips and
examples from DARwIn.
Start Simple — The Elbow
A good place to start design is
coming up with a simple elbow/knee