FIGURE 6. A nice little package of 18
Dynamixel MX- 28 motors from Robotis.
It is possible to make
the motor operate stronger
in one direction which is useful for
a hexapod leg where it is mostly applying
a force in a single direction. If set right, a person can
physically move part of the robot without causing much
strain on the motors. However, the simple linear or
proportional feedback ( P ) nature of the onboard
controller results in position errors or overshooting, and
does not follow any standard control system strategy. If a
derivative feedback term ( D ) were to be added, then the
adjustable slope could be increased to help tighten up the
error and increase the speed of the position
polish things off, an integral
feedback term ( I ) could be
implemented to get rid of the
The motors use a high quality
potentiometer that enables
continuous rotation. The
potentiometers work by having a
resistive material in a ring with a metal
brushed wiper attached to the shaft. Long
days of repeated motion in the same place
causes the resistive material to wear. This is
highly unusual, and I am inclined to believe that
other factors such as hot temperatures helped
induce wear. The resulting wear causes a huge
misreading for feedback. The drastically changed
value results in a violent shaking behavior of the motor.
The only way to remedy the problem is to replace the
potentiometer, which is not too bad using a screwdriver
and soldering iron.
The more I write code, the better I become.
Considering that the code currently operating the
hexapod was written about two years ago, my skills have
vastly improved. The current user interface for the
demonstration mode with the Sixaxis
controller is very limited. It is difficult to
see what is going on when the fit-PC2 is
cranking away at processing face
tracking and kinematics algorithms.
Also, the use of the Sixaxis controller is
seamless but limited.
The face tracking algorithm was
only able to effectively work at 11
frames per second at a down sampled
image resolution of 160x120. This
was the most effective I was able to
have the hexapod track a face. The
operation is not very robust as a
person must be facing right at the
camera with no tilt of their head in
any direction. The face tracking
mode is very difficult for most
people to interact with.
FIGURE 7. All the motors neatly laid out
for the next-generation hexapod.
Having listed out the known problems, I can proceed
with an upgrade plan.
60 SERVO 12.2011