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Wiring your
robot
By Brett "Buzz"
Dawson
Team
DaVinci Robotics
One of the most overlooked parts of robot building
that I have found is proper wiring of your robot. Electrical failure
is the number one cause of robot failure (non-combat induced).
There are several reasons for this. The builder chose a wire that
was too small to handle the current. The connector (crimp) was
not secured to the wire correctly. Builder did not tighten down
all connectors (screw type) correctly.
Let's tackle the first problem, choosing a wire
that is too small. When electricity goes through a wire the movement
of electrons creates friction and the friction creates heat. This
can be best demonstrated by looking at the burners on an electric
range in your kitchen. The more current that you feed through
the coil the hotter it gets until the heat starts vibrating at
the same frequency as the low end of the visible sprectrum (ie.
it starts glowing red). Now imagine what that would do to a wire
with plastic sheilding around it. It would start to melt. Now
imagine that you fed enough current to go from cool to red hot,
instantly. You run the risk of vaporizing the wire (I've done
this a few times on accidental shorts). This is the same reason
why you hear all of the horror stories about dropping a wrench
across the terminals of your car battery. There is enough current
in a sealed lead acid battery that if you were to short it out
that the current would heat the battery up so much that it could
make the Sulfuric gasses trapped inside the battery ignite (rather
violently I might add). I have seen the hood of a truck blown
off and into the bed of the truck when it was closed and a misplaced
wrench shorted the battery.
So, what size of wire should you use? Well, there
are some really nasty formulas for determining that (none of which
i can recall at the moment) so I'll just say, use the biggest
that you can get. Now, I don't mean use 0 gauge wire in a lightweight
robot. It depends upon how much current you will be pulling and
for how long you will be pulling those currents. I use 8 gauge
wire for my weapon motor that pulls 300 Amps for 3 minutes from
my SLAs and they barely get warm. But, I have batteries that can
source that kind of current. If you are running NiCads or MiMH
batteries then you probably will melt your batteries before you'll
hurt anything heavier than 10 gauge wire. The way I see it, 12-10
Ga. is good for most Lightweights, 10-8 Ga is good for most Middleweights,
8-4 Ga. is good for most Heavyweights, and 4-0 Ga. is good for
most Super Heavyweights. There are exceptions to this but in general
is seems to fit.
What type of wire? Do you have some old speaker
wire laying around? Forget it! That stuff will vaporize in a brilliant
flash. You need high strand count wire with a high temp silicone
jacket. Solid core isn't very good and it isn't very flexible
either. Something like Dean's Wet Noodle that can be found at
Team Delta's site is good. Another good source are the high power
cables used for high end car stereos. You can get these at most
car stereo shops and even at electronics surplus shops and flea
market vendors. Make sure to get something that is pretty flexible,
you'll thank yourself later when trying to run the wires in your
bot.
Measure the lengths that you will need and cut your
wire about an inch longer (to account for a bad crimp, need to
cut back a little more, etc.)
Okay, you have the wire, now what? Now you need
a good crimp to put on the wire to act as a termination point
of the wire so that it can be connected to a distribution block
or a motor or a variety of other things. Note: There are power
distribution blocks that you insert the wire into a hole in the
block and pinch it down with a set screw. These work but I would
be afraid of the wire getting pulled out under the high shock
loads that your bot will endure. Back to the crimps. Make sure
to get a crimp that is made for the size of wire that you are
using. Get a crimp that is too big and it won't grab the wire
securely. To small and you'll have to open it up a bit and can't
get it around all of the wire.
You can get crimps at your local auto parts store.
They are relatively inexpensive. Most of these are fine but I
have found that the metal that these crimps are made from can
be too flexible for my tastes. I prefer to use the gold plated
ones that I get from my local electronics surplus store (you can
also get them from the car stereo shops). They cost a little more
(about 35 to 50 cents each versus 99 cents for a bag of ten) but
the peace of mind of knowing that I have a quality part far outweighs
the extra $20 that I spend in these over using the cheapo ones
from the auto parts store.
Once you have the crimps you will need to trim the
insulation back by about 1/4"~3/8". If you trimmed the
wire correctly it should just poke completely through the opening
in the crimp. Insert the exposed wire into the crimp and crimp
it down with a good crimping tool. You should be able to tug on
it pretty well without it coming out. If it does come out then
it's not because you pulled too hard, it because you had a bad
crimp job. Scrap the crimp and start over (don't try to reuse
the crimp because the metal will fatigue if you try to open it
back up and then clamp it back down).
Now, the next part would be much easier if you have
this little thing sometimes called "helping hands".
It is basically a couple of alligator clips on movable arms with
a weighted base. You can get one at Radio Shack for less than
ten bucks. Trust me, they are really "hand"-y to have
(sorry for the pun :-p). This part calls for some soldering. This
is easier than soldering on a circuit board because you don't
necessarily need to be neat or precise. Clamp the wire in one
of the alligator clips so that the crimp is being held out in
front of you with the ring/spade/fork on the bottom because you
are going to add solder to the wire where you crimped it. A good
soldering gun or 40 Watt soldering pencil will do (I use the latter).
Heat the area where the wire and the crimp meet. then add a bit
of solder. Heat more area and add more solder. Molten solder aids
in heating the surface quickly. When the entire area that we are
soldering gets hot enough the solder will flow like water. This
was what we want because it will flow into the minute crevasses.
I personally am not satisfied until all of the wire that pokes
through the crimp is tinned (covered in solder) and that it flows
down onto the crimp and tins it as well. After you are done let
it cool and you will have a high power electrical connection that
will not fail.
Okay, we're almost done with the wire. The last
thing that we need to do is cover the area with a good high quality
shrink wrap. You will need to get some shrink wrap that has a
high temp capability. The stuff you get from Radio Shack is worthless
for this. Try your local electronics surplus shop or car stereo
shop. The good stuff is usually very rubbery feeling (not stretchy)
and flexible and will have temp ratings on it. The stuff that
I use is rated to 125 degree Celsius. You can get it in a variety
of colors. Make sure that you get the right size for your wire
as it can only shrink so far. Cut off a piece a big bigger than
the area that you want to cover as it shrinks in three dimensions
(gets shorter as well as constricting in size). Get enough to
completely cover the connection from the wire jacket to over the
crimped area leaving only the connector exposed. This will help
prevent shorts.
Well, now that we have prepped the wire we have
one last thing to do and that is to connect it to the appropriate
components. Make sure that you tighten the connection down well
if you use screw type connectors like those found on Vantecs and
Victors. And, after a battle, tighten them down again because
no matter how tightly you did it the first time some will come
loose. Sometimes you will have to make a connection to something
that has no screw terminal but you want to make sure that the
wire doesn't come loose, like on a SLA that has blade connectors.
I tried soldering to the blade connector directly but found out
that the heat from the soldering damaged the battery a little
internally and when I started pulling major amps from it the heat
generated aggrivated it and the battery shorted internally costing
me a match. So, Adam from Team LOGICOM gave me a great idea. Use
auto repair epoxy to hold it down. It is the type that you can
buy in the auto parts store as a two part mix of clay like materials
that you 'squish' together with your fingers. Then just pack the
area around the terminal and let it set for an hour and that wire
won't go anywhere. And, the stuff is non-conductive to boot! Check
out the pictures below.
And there you have it! It may sound like a lot of
trouble to go through but you gotta ask yourself, how much are
you willing to do to make sure you win and not get taken out like
a chump because of poor cabling?
Automotive epoxy covering the terminals. Also notice the
clear shrink wrap. |
Here's a finished wire connection. This connector will not
ever come free of the cable on its own. You can also see
some of the solder tinning the connector under the shrink
wrap. |
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