years ago, Team Found Objects' Brandon Davis sat down
and put together a "How To" guide to help all new
builders learn the basics of combat robot design.
Hopefully, this will get you moving forward on the right
What This Is
a thrown together set of rules of thumb, suggestions,
hints and general officiousness, with the intent of
assisting in the construction of a CHEAP 12 pound robot
from more or less easily found bits & pieces. This was
created to supplement informal robot construction
seminars conducted after a robot fight. The idea goes
like this; have a robot fight and while watching folks
are drunk on mechanical destruction & carnage, show them
how easy it is to build their first bot. Right there,
right then (recognize the economic model? First taste is
free kiddo!). Give the folk a hand-out to take home for
reference. They come to the next fight with their
creations and a new group of ravenous mechanical
destroyers is born (BWAH_HAH_HAH_HA!... Sorry). Repeat.
And since this is being re-printed on the Robot Battles
a link to a PDF version to print
out for yourself.
Who This Is For
targeted at folks who have never built a combat robot
before. It is not a technical document. Folks who have
built a combat robot before will be bored silly by this.
This will not help folk who have access to mills,
lathes, CNC machines and degrees in electronics (you
don't need my help-go build something cool).
Why Listen To Me?
are asking yourself the above question, the answer is
that you probably shouldn't. I am not a heavyweight in
the robot combat world (or any other world). What I am
is an intermediate level builder who really enjoys a
hobby and would like to get others involved. I don't
have lathes, mills, or CNC machines (OK, there is a
degree in electronics but that isn't going to come into
play here - promise).
What Is A Combat Robot?
purposes of this exercise, a combat robot is an R/C car
in a very bad mood with the desire to find other R/C
cars and kick them into scrap. Purists will tell you
that to qualify as a robot, a machine must be
autonomous. Fine. Good. Go read something else.
one if the few times in this little article that I am
not in any way kidding. Combat robots are inherently
dangerous. You do this at your own risk - I accept NO
liability for ANYTHING. This hobby, in both its
construction and execution, has the potential for fire,
explosion, trauma and other injury as well as death. Be
smart, be safe. Read manuals. Read warning labels. Ask
The Basic Parts
To get a
combat robot to do anything (other than be an vaguely
amusing sculpture) you need six fundamental elements
(this is the order we'll be discussing things):
Speed Controller - accepts the commands from your
receiver, and translates those commands between the
batteries and motors.
Transmitter - issues your commands to your robot.
Receiver - receives the commands at your robot.
Motors - makes the robot go (and hopefully kill the
Battery / Charger - provide the power for your
Chassis - holds everything but the Transmitter in
one snug package.
transmitter, receiver and the speed controller are the
items that most folks just can't do for themselves. (If
you are one of the folks that can build your own
electronics hooray for you! Now go bask in your own
superiority-QUIETLY, while the rest of us get on with
our pitiful ignorant lives.) The bad news is that these
items are relatively expensive and absolutely
indispensable. The good news is that in recent years
these items have become essentially commodities. You are
encouraged to spend your money here, buying for the
future. Investing in good quality radio equipment and
high quality speed controllers is not only less
frustration in the long haul, but much cheaper. With
care, these items will fight on in multiple different
For this project, we're taking a short-cut - the
selection of our source of motors, specifically the
cheap & nasty battery operated drill. This is the source
of the motors, the batteries & the charger. Outside of
this project, how to try and save money on your robot is
the subject of a fair bit of debate.
My personal spending priority is:
Transmitter / Receiver (a.k.a. Radio system or R/C)
Battery / Charger
Especially the battery vs. motor priority portion can be
the subject of disputes no less vicious than Mac vs.
Wintel or any of the innumerable schisms that make
religious studies such a side splitting (or slitting,
these folk get SERIOUS) hoot. Well, I'm the one writing
this, those are my priorities, and the rest of you can
go schism among yourselves (again QUIETLY).
Speed Controller (ESC) is really the beating heart of a
combat robot. It will probably be the most expensive
item on your shopping list. Do not skimp here. Many of
the items that can blow up or be damaged during a fight
can be often be borrowed from other competitors in the
pits (No really, one of the strange things about a robot
fight is that your opponents are almost as interested in
your bot being able to fight as you are. Otherwise, how
else can they beat you to scrap? Oh yeah, the pits are
where to find the bot builders who aren't fighting at
the moment. Look for tables off to one side that look
like a hardware store mated with an electronics shop &
then exploded.). There are frequently enough spares
floating around the pits to replace most of the items
that die during a fight-but the ESC is expensive &
consequently rare. Make protection of the ESC a basic
criterion of your design.
The basic "buy, don't buy" elements for selection of an
Type of Motor - there are two basic motor types out
there to be controlled; Brushed & Brushless. Most
ESCs will only work with one type or the other. Make
your system match. For the cheap 12 lb bot, get a
Voltage Capacity - all ESCs will specify the maximum
voltage that they can be supplied with. This
specification is not a suggestion. It is an absolute
maximum. Don't exceed the voltage; you will not be
pleased with the results.
Current Capacity - all ESCs will specify the maximum
continuous current that they may supply to a motor.
This specification is not a suggestion. It is an
absolute maximum. For the cheap 12 lb bot, anything
over 15 amps should be fine. But, like I said before
- try to buy for the future.
Reversibility - many of the ESCs designed for R/C
cars will not reverse. In combat robots, this is
bad. Unless you are blessed with supernatural
driving ability (you are not), if you cannot backup,
you will lose.
Braking - there are quite a few ESCs out there,
mostly for R/C boats & cars, which do not have
electronic braking. This is a bit of jiggery-pokery
in the circuit that stops the motor when your
transmitter stick is in the middle (neutral)
position. This can be the difference between
stopping before you hit a spinning saw of death and
coasting obliviously into it. There are some drivers
out there (disgusting youngsters with the reflexes
of a coked up snake, you know who you are) to whom
this feature is a matter of indifference. For the
rest of us, this should be a buy/ don't buy decision
- Battery Eliminator Circuit is a great feature to
look for on an ESC. This will supply the power for a
Receiver from the ESC, eliminating the requirement
to make alternative arrangements. Keep in mind that
if you have multiple speed controllers in a bot (say
a dual channel brushed for drive motors & a single
channel brushed for a weapon), that you should only
use the BEC from one of them.
important consideration in the ESC of your dreams could
be number of channels. There are many ESCs available
that will control 2 motors (Left & Right Channels) from
the single board. This can simplify your layout
considerably. Some ESCs allow for control of 3 channels,
Left Right & Aux. This is very handy if you are
interested (now or in the future) in having an active
weapon on your robot.
is a feature found on many ESCs. There are two steering
techniques available to the combat robot; Tank & Mixed.
In Tank steering, the left hand stick controls the left
side motor(s), and the right hand stick controls the
right side motor(s). Both sticks forward= forward, move
one stick back and the other forward= a very fast turn.
In Mixed steering, the ESC places Forward/Reverse on one
stick axis (Y axis, up & down) & R/L on the X axis of
the same stick. This results in a one handed drive,
leaving the other hand and stick available for weapon
control (BWAH_HAH_HAH_HA!... sorry).
have an ESC- READ THE DOCUMENTATION! Keep it around.
Download a copy & store it in your hard drive (you know
you'll lose it one day).
Radio Systems (R/C)
a Transmitter (TX) & a Receiver (RX) that match each
other. A handy guide to concepts in R/C basics
can be found here while
a guide to R/C terminology
can be found here.
Best - 2.4 GHz Spread Spectrum: This tech will
be the most glitch free and avoids channel
Better - FM 75 MHz (channels 61-90): Good
range & resistance to glitches.
Allowed - AM 75 MHz (channels 61-90): Cheap
Avoid - 27 MHz: Not allowed in most rule
decide on an FM or AM system, you will have to also
purchase channel crystals (XTAL). Crystals cost anywhere
from $7-15 each. You need matching XTALs for both TX &
RX (example: a channel 65 for your TX AND a channel 65
for your RX). Plan on having at least 3 channels (more
is better) that work with your system to go to a fight.
The reason for this is that you want to control only
your bot-not other folks'. Of course the reverse of this
is true-you REALLY don't want somebody else to have
control of your bot! Having multiple channels to select
from is frequently a requirement of the rule sets. Be
prepared for this; it is really disappointing to not be
allowed to fight because of channel conflicts. It is
also a very good idea to test each channel pair in your
completed bot BEFORE you show up at the fight. Like
anything else, XTALs break.
completely avoid channel conflicts, consider a 2.4 GHz
spread spectrum system. While the initial investment is
higher, these radios will bond with their receivers,
creating a unique pair that that does not conflict with
other systems. The result is fewer giblets to cart
around and less scrambling in the pits to resolve radio
consideration is the NUMBER of channels a TX/RX pair can
control. For example, the GWS system cited for a FM
system will control 4 separate things; say Left Motor,
Right Motor, and a Weapon motor, with a channel left
over. The example cited for an AM Radio only has 2
Channels of control which will be generally used up by
Left Motor/ Right Motor. Try to not only consider what
you build today, but what you may wish to build
ESC you select does not have BEC, you will need to
provide a 5v source to the Receiver.
at least two motors for your bot, one for Left side
drive & one for Right side. The motors by themselves are
not particularly useful; they spin too fast and with
little real power. Some sort of transmission is also
called for. For your first combat bot, allow me to
recommend the quick "one-stop shopping" technique of the
battery operated drill. This is a great inexpensive way
to get both motor & transmission in a single fell swoop
(there is an additional benefit to this that we'll
discuss in the next section-multitasking baby!). The
motors in the drill are the brushed type.
eye open at the hardware or "home improvement" store for
sales. Perhaps the easiest way to get hold of very
inexpensive motors is to purchase on-line from a site
Harbor Freight. After I
wrote that sentence, I found an 18v drill motor for $27
and a 4.8v for $15. An 18v drill motor would be
delightfully adequate for a 12 lb to 30lb fighting robot
(4.8v might be a bit anemic for a 12lb). Anything in the
9v and up range is a nice place to start. These drills
cost less than most of the "specialty" motors alone.
have the drill, there is a bit of surgery required (we
call it hacking). I am lazy, so this will not be covered
in this document. Instead, I refer you to
a nice web article by a
builder of almost mythic stature in the Robot Battles
family of fighter/ builders, Dale Heatherington (spend
some time on this site; there is LOTS of good stuff for
the geek here. There are lots of other drill motor hacks
out there, as usual, Google is your friend). Pay careful
attention to the July 2006 Update, those set screws are
have after the hack (and say $60, including shipping) is
a pair of motors (transmission included) with a shaft
already attached (the threads on that shaft are 3/8" 24,
you can mount wheels directly to this), batteries to run
them, and chargers for the batteries. This combo of
materials, purchased separately, can easily cost (no
kidding) hundreds of dollars.
something to power your bot. Many fights don't allow
Internal Combustion Engines (a.k.a. IC or ICE) because
they have an unreasoning fear of fire (silly, yes, but
the times we live in require tolerance of many strange
opinions). We won't be talking about IC. For most of us,
this leaves the electric option; for the vast majority
of this group it means a battery (you mad scientists out
there can work out your own alternatives, again
is your battery & charger for this project? You already
have it- it came with the drill guns you acquired and
hacked for their motors. The batteries are low-end
NiCads. You need a (possibly) new tool for this next
bit-the Multi-meter (a.k.a. DMM or just meter). This
will allow you to test the battery and all kinds of
electrical thingummies in a number of ways. You don't
need anything fancy (Though they are nice). Radio Shack,
Home Depot, any hardware store is a good place to look
for this item. Talk to the nice person at the store &
tell them what you are doing (you're gonna blow their
mind!). There are instructions in the box with your new
toy (and best diagnostic friend).
A mile-high overview of the battery harvesting process
goes like this:
Check how the battery fits into its charger before
doing any surgery. This will ensure that the correct
terminals touch the charger (+ goes to +; - goes to
-. This is called polarity- Use that meter!).
Make notes as you go, marking polarity on both the
batteries and the charger (Use that METER).
Carefully remove the cells from their plastic
may be able to solder wire (length is up to your
requirements, at least 6") right onto the connector
that was on the plastic housing (at least to the
wires that connect to them). Alternatively, you can
solder right onto the ends of the battery itself
(especially if you were not careful in the above
step). It is a good idea to use Red wire on the
positive & Black for the negative (at the very least
try to use different colors for this. Use that
Once you have the battery exposed & soldered, make
sure to wrap it up nice & snug in electric tape.
Repeat this process for the battery from the other
test jumpers to (wire with a clip on each side-make
your own or go to Radio Shack) connect your battery
to the charger. Be VERY careful to use correct
polarity (USE THAT METER!).
some basic guidelines to the care & feeding of your
battery and charger. Please note that items that include
words in all capital letters (like NEVER & READ) are not
suggestions; these have the potential of explosions,
fire, injury and (I kid you not) death.
Only combine identical cells into batteries.
Only combine identical Batteries.
NEVER short circuit your batteries or cells (a short
circuit is when you connect a battery's (or a
cell's) own + to its own -).
REALLY NEVER short circuit Li type batteries or
MATCH your charger to your battery type (NiCad
chargers will not work with Li)
MATCH your charger setting to your battery
NOT over charge your battery
READ the manuals & documentation for your batteries
& and your chargers (take them to your first couple
of fights so you can refer to them if needed).
THE FLIPPING METER!
where you get to use as much imagination as you may
possess. I have built robots that, aside from the
electronics, were constructed entirely out of materials
acquired from (multiple trips to) Home Depot. There are
bots made from R/C Cars. There are bots made from wood.
There are bots made from closed cell foam. There are
bots made from exotic materials like Carbon Fiber &
Titanium. The choice (and the skills & tools to
implement that choice) is yours.
warning - odds are good that your first robot will… and
there is no nice way to say this… suck (this is true for
almost all of us. If you aren't one of US, then you are
one of THEM. And you should know what happens to one of
THEM that get mouthy. Remember, QUIETLY). Don't sweat
it. Your next one will be better, and so on.
guidelines to ease you into the process:
Figure out how much you are willing to spend.
Target a first event (might I recommend Robot
Battles @ DragonCon in Atlanta GA?). Read the event
rules CAREFULLY. Keep these rules firmly in mind
while designing & building. It is really
disappointing to be disqualified from fighting
because your ignored a rule. (PS-the safety rules
are NOT dumb).
Spend some time looking at other robots (Google is
what you know (example-if you can't weld, don't
count on welding your frame together).
Draw it first. You don't need to be Leonardo, a
sketch is fine. Chances are good that if you can't
draw it, you won't be successful building it. Put
the drawing away for a bit and look at it later,
critically. Revise in the cold light of day. (see 1
Make a materials list based on your drawing. I find
it helps to order everything at once (I live on the
outside of the galaxy and therefore order most
Take the components you have collected and lay them
out in the approximate way you have drawn. Does it
all fit? Revise & repeat. (see 1 & 2)
Budget your time as well as your money. Finish
construction BEFORE you show up to the event.
Practice your driving BEFORE you arrive at the
Don't be afraid to change things.
all I have for you.
Good Luck, fight well.
- Brandon Davis, Team Found Objects
Sample List-12lb robot
(I'm picking inexpensive value here; make up your own
mind. These are OPINIONS):
Transmitter - GWS GWT-4A 75MHz 4-channel Radio
2) Receiver - GWS Pico 4channel 75 MHz 4 Channel
3) ESC - IBC Dual Speed Controller (~$299)
4) Motors, battery, charger - 2 x Harbor Freight 12-
18v Cordless Drills (~$60)
5) Chassis - Use materials you are familiar with