Typical hexapod and octopod robots display incredible agility. When each leg has four DOFs (degrees of freedom) and four servos, remarkably lifelike motions can be achieved. Of course, there is great mechanical, electrical, and software
complexity to control 24 to 32 servos. For a less daunting
first walker, I chose to hack a simple $8 "Spider Robot" kit
from eBay.
This motorized, eight-legged mechanical spider walks in
a straight line. A Klann linkage converts the motor's rotary
motion into each leg's D-shaped path. There’s a nice
kinematic presentation at www.mechanicalspider
.com/ comparison.html if you’d like to check it out.
The spider robot consists of identical left and right
gear-driven leg assemblies, powered by a central gearmotor
through a common steel driveshaft. A single alkaline AA
battery drives the spider, but it's a little underpowered. It
was "peppier" after I replaced the battery with a four volt
14500 Li-Ion cell. I suggest building the stock kit before
hacking it. There's a helpful video at www.youtube.com/
watch?v=aKIbu4pf2zw.
A similar $40 IR RC spider robot kit I found on eBay is
fine for basic remote control, but for programmability,
expandability, onboard sensors, and autonomous operation,
we must get our hands dirty. You'll build a better bot with
that $40 than with a kit.
You'll need to order some parts, so plan ahead. I enjoy
finding parts bargains and stocking up to build stuff, even if
some waiting is involved. Good things come to those who
wait ... and hack! Design Considerations
The simplest way to steer the spider chassis is to use
differential steering. That means two motors drive the left
and right leg assemblies independently — much like tank
treads. Servos modified for continuous rotation are easy to
use. The plastic spider legs are skinny and somewhat
flexible, and there is some slop in the linkage pivots. This
eases assembly and improves durability, but it limits the
load capacity of the bot. We want to minimize total weight
and keep the CG centered to avoid overloading the legs to
make the spider walk efficiently. Servo Selection and Modification
Not all nine gram servos are suitable for the well-known continuous rotation hack. Some don't have a
complete gear on the output shaft; some have unstable low
speed performance. I chose Hobby King HXT-900 nine gram
servos. They are inexpensive,
strong, and reliable. They must
be modified mechanically and
electrically.
The feedback potentiometer
provides support for the output
shaft, so even though we will
eliminate its electrical function
we must preserve its mechanical
integrity. (See my videos at
www.youtube.com/
watch?v=wfmM3cuihJI and
www.youtube.com/
watch?v=7pxRhxBPSUM).
Working on a clean surface,
remove the four screws and the
top of the case where the gears
are. Pull the large output gear
straight off and remove the two
small molded mechanical stops
from the back of the gear. Use
cutters and a hobby knife to
remove them completely. Next,
modify the potentiometer
internals so the shaft can rotate
360 degrees freely. Two fixed
black mechanical stops and some
thin rotating metal contacts
inside need to be cut back.
Post comments on this article and find any associated files and/or downloads at
www.servomagazine.com/index.php/magazine/article/march2014_Ostendorf.
Assembly.
68 SERVO 03.2014