FIGURE 1.
A screenshot of
RVIZ showing the
robot's arm, Kinect
point cloud data,
and colored
results of object
segmentation.
Dynamixel Family of Servos
Most people involved in robotics are aware of the
Robotis Dynamixel AX- 12 servo. This 'smart' servo
incorporates a small microcontroller which gives much finer
control over the servo than a traditional hobby servo.
Dynamixel servos have long been known for their half-duplex TTL serial bus, precise joint
position measurement, over-torque
and over-temperature shutdown
protections, and the ability to tune the
torque output.
For years, many have drooled over
the higher end RX- and EX-series
servos which boast even more torque.
Until recently, those higher end servos
required an RS-485 bus to
communicate, meaning you had to
either build your robot entirely out of
high end and more expensive servos
or you had to have two buses.
However, Robotis has recently begun
to release their new MX-series of
servos (shown in Figure 2), starting
with the MX- 28. The MX series uses
the original half-duplex TTL serial bus,
making it easier than ever to upgrade
a servo when you need more torque.
The new brackets released over the
past year also have a number of
mounting patterns, meaning that
most brackets can be connected. You
can finally build an arm with a variety
of mixed-size servos using only off-the-shelf components.
The MX-series servos improve
things in a number of other areas which are essential for
building robot arms. The original AX- and RX-series servos
had 10-bit resolution over a 300 degree range of motion.
This meant that you had 1,024 positions, or a resolution of
0.3 degrees. While 0.3 degrees sounds nice, that can cause
a huge displacement at the end of a longer robot arm. The
new MX-series servos have 12-bit resolution encoders giving
FIGURE 2. New Dynamixel MX-series servos.
SERVO 04.2012 41
