The BEAM Bestiary is a BEAM
2 motor walkers
BEAM walkers with 2 motors and 2
or more legs
There are a surprising variety of walking 'bots you can
build that are driven by just two motors. Probably the
biggest thing differentiating these designs is the number of
legs the walker has -- 4,
6, or 8.
2 motors walkers with 4
By far, the most common BEAM walker is some variation or
another of a 2-motor / 4-legged robot layout:
This kind of 'bot walks via careful positioning
of walker CG, good timing of motor motion. I've go
into more detail on this in a subsequent page, but
basically, you want the walker CG to shift between
the front to back as the walker motors move.
Meanwhile, the walker is being supported by 3 of 4
legs almost all of the time. This means you need to
build this kind of walker such that its CG is in a
selected region (in particular, one that will allow
the walker's weight to "shift" correctly).
I've tried to show these mechanics in a
/ quadrangle shows where the walker's weight is
being supported at a given step, the green diamond
shows the walker's allowable CG range.
A wide variety of frame designs, leg designs, and motor
geometries have been developed for 2-motor / 4-legged
walkers -- but I've got more on that in my page on 2-motor /
4-legged walker design. Note that I also have subsequent
pages detailing control schemes, plans, and circuits for
2-motor / 4-legged walkers.
There are also some designs with more-complex mechanical
linkages that use two motors to drive more than 4 legs.
Here, the legs are attached to a gear, and pivot around a
"fixed" pivot point some distance away from the gear. As a
result, the "feet" attached to the legs move in sort of a
In both 6- and 8-legged configurations, groups of these
legs are "coordinated" via small "secondary" gears.
Normally, the drive motor(s) is/are coupled to secondary
gears. What really differentiates 6- and 8-legged 'bot
configurations is how 'bot turning is accomplished.
2 motors walkers with 6
A 2-motor / 6-legged walker has a single motor to provide
all drive force (i.e., the two rows of 3 main gears are
directly coupled), and so turns by moving its legs' hinge
points -- this makes one side's legs "row" in a smaller
circle than the other.
Pivot point close to
Pivot point further from
The configuration of this kind of walker then looks a bit
like this (note the very sketchy diagram of a "frame" to
move the leg pivot points in concert):
I've had one of these walkers on the "drawing board" for
some months now; watch this space for pix when I get it
built (as you may have guessed, designing a mechanism to
reliably move the leg pivot points left and right in concert
is not a trivial exercise)... Meanwhile, here's my first
2 motors walkers with 8
A 2-motor / 8-legged walker has two motors to provide
all drive force (one motor driving each side's legs), and so
turns by differential steering (moving one side's legs
faster than the other). Note that it is able to accomplish
this because 2 legs are always touching the ground on each
side of the walker (i.e., each leg on a side is 180 degrees
out-of-phase with the next).
8-legged walker motion
Steve Bolt's "Spider" is the best (well, actually,
only) example I've found of this design -- go
out! Steve's page has lots of pix, and explanation of
all the parts he used in building his 'bot.
For more information...
Andrew Miller used to have a site up with a walker
tutorial; his tutorial was adopted by Bram Van
when Andrew shut down his site. Andrew's tutorial
is educational, but pretty font-intensive; a less
"busy" version is hosted here.
Meanwhile, Bram has a 2 motors tutorial of his own
(for the Z-Walker Mark III).
Chiu-Yuan Fang's "Miller" walker tutorial is
he also has another 2-motor geometry here.