the opposite end. The more weight at the
end of that limb times the length creates
uncontrollable oscillations in the system.
Two things help. Tighten up the slop and
lighten the load.
The tolerances of the servos are fixed. I
was doing my best as a hobbyist to build
tight linkage but in the end, the most bang
for my buck was the weight at the end of
the limb. So, I decided that the servos at
the foot had to go.
I mounted the two foot servos at the
knee. One controls the first metatarsal of
the foot, the other controls the fifth
metatarsal of the foot. The heel joint just
rotates. The linkage pulls and pushes the
other two points allowing for foot leveling.
Small movements are key as it does not
take much movement in the servo to
change the CG. The servos are fast and the
LPB has hardly anything to do except work
at keeping the weight inside the triangle. I
will go into more details of this in next
Limb Processing Board
When I started this project a few years
back, there were not as many controller
boards as there are today. I could not find
one that had the interfaces I needed to
read the sensors, control the servos, and
work in a fast distributed system. I had
played with ExpressPCB a few times before
and found it very easy to build a board with
everything just the way I wanted it. I
designed a board around the Microchip
dsPIC30F4012. It has plenty of processing speed,
comes with a free compiler and lots of libraries to
do PI calculations in real time. Microchip has a
plethora of new devices every year and at some
point I will probably redesign the board with
something faster than the I2C bus I use to talk
between boards. For now, it does the job.
I use the same program for each LPB. One is
designated as the master and the others as slaves.
There is a command line interface that can
communicate wirelessly with a PC's terminal
window via a Digi XBee board. These boards have
a simple-to-use serial interface. I only talk to the
FIGURE 2. My foot prototype.
FIGURE 3. Standing
FIGURE 4. The LPB.
In the next installment, I will move away from
the prototype to show the mechanical design of the
full size biped. I will detail the weight distribution,
foot linkage, belt driven gear box, and the power
SERVO 02.2011 49