Build Your Own
Planetary Rover Bogey
Both the Mars Pathfinder and
MER rovers used a rocker
bogey suspension system of six
wheels. The rocker bogey is an amazingly simple design that has proven
its effectiveness. It is passive — no
computer is required to control it. The
goal of the system is to keep as many
wheels in contact with the ground as
possible over uneven terrain. I
wanted an easy-to-build, inexpensive
platform for students to use to
simulate planetary rover operations.
My requirements were the platform
had to be able to carry a payload of
up to 10 kg, the entire robot could be
lifted by one person, and that it could
fit in the average car trunk. My target
price for the platform would be $200-
500, depending on how good a
scrounger or recycler I turned out to be.
I’ve seen lots of student projects
where someone has put a camera on
a RC car, and called it a rover, but I
don’t feel that teaches kids anything
about robotics. Most commercial kits
are only suitable for use on smooth
level surfaces indoors. Now, where’s
the fun in that? If I am going to build a
rover, I want one that can get its
wheels dirty! I call my robot the
PMMP = Pneumatically Mediated
Mobile Platform. I am going to show
you how I built it.
by Alan Federman
Why Go Bogey?
Just think about it. Suppose you had
a four wheel vehicle with no suspension.
If one wheel hit a rock or a curb and
tried to climb it, you’d soon have two
wheels up in the air, and only 50% of
your traction. If your suspension could
automatically adjust to the terrain, you’d
always have at least three wheels on the
ground and 75% or more of your traction. A rule of thumb is a round wheel
can’t climb a curb generally greater than
1/3 of its diameter. A four wheel system
with a bogey suspension can usually get
by at a 1/2 to 2/3. Add a rocker to the
bogey, and you can usually climb an
obstacle 1 to 1.5 diameters. Another
advantage of a bogey suspension is the
platform tends to stay level with the
ground. So a flush mounted camera is
always aligned with the local horizon.
There are several ways to make a
bogey. NASA’s rovers use differential
gears. You could also use simple tie rods,
like the control rods used by RC modelers. For my bogey, I chose pneumatic
cylinders, because I could get away with
being a sloppy machinist, and I happened to have a couple of extra ones
lying about. New cylinders are kind-of
expensive, but used ones aren’t. If you
were really strapped, you probably could
make your own by using a pair of screen
door closers or bicycle pumps.
■ This photo shows the end of the box beam,
made of three 1.5 inch wide pieces of 1/2
inch birch plywood, glued and screwed to
one piece of six inch wide plywood.
The motors are bolted to the wider piece of
plywood. Two assemblies — one for each
side — are made.
I am a big fan of using plywood box beams for structural elements. Plywood is
cheap, easy to work with,
and almost everyone can
get access to a table saw. I
■ Complete view of the rover
showing the freedom of motion
and the legs at opposing angles.
started by making two 30 inch box
beams to hold the motors. I used four
12 volt seat positioning motors. These
use a two stage worm gear reduction
for a top speed of about 75 RPM. I
used lawnmower wheels mounted
directly to the output shaft by a pin
and endcap tapped into the shaft end.
The Central chassis is just an open
plywood box. A few pieces of lumber are
used to stiffen the box and act as support
struts for the battery and warning light.
Both wheel assemblies are attached
to the central chassis box by ball bearing
races and 8” aluminum shafts. The
shafts are either drilled through and
bolted, or drilled and tapped to attach
additional structural elements.
Before attaching the pneumatic
cylinders, it is important to check the
balance and freedom of motion of the
wheel assemblies. The cylinder attachment brackets are made from some L
channel, some C channel, and some
1/2: aluminum rod. The assemblies are
either bolted directly to the plywood box
or drilled, tapped and bolted together. I
used a small piece of plumber’s steel