April 8, 2004
Yes folks, the apocolypse draws near. I am using wheels. After nearly suffering a mental breakdown constructing my last walking robot (there's no build page yet on that one sorry), I felt that it was time to design something a little less draining. It was not easy to accept these strange round things everyone else seems to use, so to soften the blow I decided to use "Omni" wheels. I first saw these cool devices on Tony Hall's "Y Chromosome". For months I stared at them, fascinated by the ability to move in every direction. To that end I designed "Skittlebraü" an Omni wheeled 12lb robot. Of course for a weapon it has something spinny, and as it's invertable as well.
|To begin with I needed to choose the correct
omni wheels. For those unfamiliar with this type of wheel it consists of
a driven (or in some situations undriven) hub with rollers on the periphery
that rotate at 90 degrees to the hubs axis of rotation. While being driven
in one direction the omni can roll in the perpendicular direction. With
a minimum of three omni wheels you can make a robot base translate in any
direction, forward, backwards, and sideways without turning first. It is
my hope that this increased freedom of motion will be both entertaining
as well as effective in combat.
I had originally planned to make my own wheels, as it is certainly within my capability to do so, however I figured it might be worth the time to purchase ready made options, at least to begin with. On the left is an all acetal plastic omni with 8 rollers. While it rolls great, the hard plastic rollers lack traction, making it hard to drive. The rear green rollered wheel is huge, ~3.5" in diameter, and rather heavy. It does, however, sport a urethane roller which provides superior traction. The black rollered wheels in the foreground are just right. They also have urethane rollers, but are a much more manageable ~2.375". Available from www.acroname.com they aren't the cheapest, but are just what I needed.
|The motors I selected for drive are these, the so called "Masek Mystery Motor". I happened to have a couple lying around, and they love high voltages, perfect for this project. They also have 4 brushes and a relatively high torque/low rpm, making them well suited for minimalist reduction techniques.|
|Here are the parts which make up an individual drive "pod". There are three in total, spaced 120 degrees from each other. Some of these will be pocketed a bit further for increased weight savings. A simple single stage belt reduction with 3mm pitch HTD belts (www.sdp-si.com) reduces the RPM to an acceptable range. Missing from this shot are the belt and keyed shafting.|
|A shot of the gearbox assembled. The angled side blocks allow the frame rails to be machined flat, instead of making them at an angle.|
|Multiply the above times 3 and add side rails. One of these side rails will be replaced later with the one that will mount the weapon, but I wanted to drive test before I had completed the weapon. Plus it never hurts to have a spare frame rail.|
April 15, 2004
A bunch of stuff came in this week, and a good thing too as the comp is a few short weeks away. Although my mill is quite useful, it's steel handling capabilities leave a little to be desired. I could have done them myself, but after considering having to source the appropriate alloy, machine it, and then properly heat treat it, I decided to just have terry from Team Whyachi make these for me. He did an excellent job, exactly as per my specs, and a very reasonable price. Can't wait to get the rest of the weapon done to try them out!
|I spent a morning this week working on these as well. They are mounts to hold the ball casters (from perpetual fav McMaster). Sadly there are no small stem mount ball casters within a reasonable price range. These plates allow me to mount up the $3 cheap versions to the weapon standoff plates.|
April 16, 2004
|Up until now everything for this bot has been fairly uncomplicated, totally against everything we stand for at Team Nar. Thankfully the weapon provides ample opportunity to overcomplicate things. Pictured here is the worlds most machined ashtray....No, not really. This is one of two weapon hubs. The other one does not contain the series of 12 notches around it's perimeter. This is the interior of the weapon hub.|
|I didn't spend the hour precisely cutting those notches for no reason. For those unfamiliar with it, the part to be machined is mounted in a rotary table, which allows the part to be rotated a specific ammount. With it you can make all sorts of interesting things, from precise hole patterns, gears. to little castle like notches like in this part. The hub above was made in the same way. This particular rotary table has an attachment so the lathe chuck screws directly to it, making it easy to transfer round work back and forth from the lathe to the mill without rechucking. The part shown here is the magnet ring. Made of magnetically soft iron, it cannot be magnetized like a piece of steel can. It's purpose is to direct the magnetic flux back into the motor. The reason for the notches will become clear as the entire weapon is shown. I wanted to make the weapon as easy to disassemble as possible with a minimum of gluing. The notches aid in this.|
April 18, 2004
|The last time motor I built, I relied on making thin plastic spacers to hold the magnets apart as they are glued into the ring. Adjacent magnets have opposite poles, therefore they snap together, making the chore of gluing them in and maintaing spacing very dicey to say the least. My plan this time is to make space rings, to hold the magnets at an even spacing, and then glue the ring itself. A piece of thin plate is clamped to the rotary and the fun begins. The first series of cuts opens the center of the ring. A spreadsheet was done beforehand to make a table of the angle to cut each "tooth" in the ring at. The last cut around the outside frees the ring from the plate.|
|Here are the rings in their intended use. Not all the magnets are shown here, these are some old ones I had from my last order, the new ones should get here later this week. 14 will go in the ring in total. I got these from Amazing Magnets. They are composed of Neodymium, some of the strongest magnets that you can buy.|
|I couldn't resist taking this shot. The motor axle is a piece of 3/4" DOM tube, which although really strong, wasn't precisely the right size. Taking cuts on the lathe with a tool bit is kind of difficult for the tiny ammount of material I had to remove, so strips of emery cloth were used to sand off a couple of thou's and polish things up in the process. Since I'd played this game before, I knew full well that inhaling nostrils full of steel dust is no fun at all. My solution was to put one of the Neodymium magents on the steel body of the chuck, what you see here is the metal "fuzz" caused by the magnet attracting all the tiny bits of steel that came off of the shaft.|
April 22, 2004 : The motor IS the weapon...
|Ok, time for a bit of discussion here. I've kinda
kept quiet about the weapon concept for this bot, although the general idea
has been out there (brushless, spinning disc, etc.) the full scope has not
quite been revealed. For this project I wanted to do something a little
different than the standard "motor through a reduction to a spinning
mass" deal. It's not the most efficient design for a number of reasons,
notably losses in the reduction, and the typically essentially useless mass
in the center of the disc, which contributes little to MOI for it's weight.
The solution I found eliminates both the reduction, and the center useless mass. The weapon motor IS the spinning disc. The disc itself is dounut shaped (see, you are never too far away from the Simpsons, mmmm donut). The magnet ring fits into the hole in the donut and is keyed into one of the disc hubs, preventing it from moving relative to the disc. The other hub provides further support, as well as preventing the ring from slipping out. The motor armature sits on the shaft between the two hubs. But wait, how do I get wires to the armature with hubs on either side? Simple, just use a hollow shaft, and run the wires through it and out the other side.
Now, with all that out of the way, lets get on to the construction...
|I wanted to make the disc out of 1" thick
stock, which is not the easiest thing to cut, and I don't have a metal cutting
bandsaw (or a bandsaw of any kind for that matter). The local metal shop
was happy to chop me off a chunk of 1" bar stock, but wasn't about
to cut any circles, and a piece of round stock would be impossible to work
with, as I have no chuck that can grip something that big in order to get
the faces parallel. Using bar stock makes the faces parallel, and then I
used this technique, known as "Gang Drilling" to begin the cutting
process. Using CAD, I layed out a series of holes, that came very close
to intesecting with each other. Taking a printout of these, I punched the
center locations and drilled them on the drill press. By the time this was
done, the miniscule ammount of metal that was left was cut easily by my
scroll saw!! While this method certainly takes some time, I'd much rather
let the drill press do the work, than me with a hacksaw, plus the accuracy
of this method is much better.
The four holes in the center allow me to fasten the piece to the rotary table. A couple of passes round the outside cleaned up the rough edges, and brought the disc to the correct size.
|This has to be my favorite setup of the whole project. This is to cut the pockets that hold the teeth. To begin with the inside of the pocket needs to be a right angle, thus making it impossible to use the milling cutter from the top, as this would leave a radiused inner corner. Also the holes that hold the teeth on must be drilled into the surface of the pocket. To do both of these things, the head of the mill is tipped 90 degrees. To make up for the height lost in this fashion the whole rotary table must be raised ~1.5", you can see the big piece of Al round that I keep around for this purpose. If I didn't do this, the cutter could barely reach the top surface of the disc, let alone cut across it.|
April 26, 2004
|The weapon will be held off the ground using these weapon standoff pieces. The 90 degree stuff was easy to mill with the part clamped square to the table, but how to do the 45 stuff without all sorts of hastle and aligning the rotary? Since the pivot axle and weapon axle holes line up on the 45 deg axis, plugs were cut to fit tightly, and holes tapped into a scrap bar. The vise at 90 provides all the hold I needed.|
|Here is the completed weapon armature. I used an armature core from another motor and took all the wire off. It was wound stock in a completely different manner than I wished to use in this project. The 3/4" DOM tube shaft was sanded ever so slightly in the lathe to fit, and holes were drilled to let the wires pass through the center of the shaft. The aluminum collar seen on the left prevents lateral and radial motion on the shaft, and 4 screws secure the pole plates to this collar. The holes at the ends secure the shaft between the two standoff plates.|
|A little bit of pocketing to save some weight, and the frame is assembled. Note the coffee cup for scale and santity. All it needs now are the top and bottom covers, and to be wired. Not shown here, but worthy of note is the manner I used to cut those covers. They are made from garolite, which is very hard on standard HSS tooling, and I have nothing big enough to cut them with anyway. I printed a sheet of paper with the bolt hole pattern on it and drilled them. I then bolted the sheet to the frame, and used a laminate trim bit (a straight flute bit with a ball bearing on it to follow the contour of the aluminum) in a router, to cut the plates exactly the size of the frame. The carbide router bit did a good job.|
April 30, 2004
|No time to talk, I finished on time, fought tooth and nail for the last couple oz. First pic is some of the weapon parts, when the bot is dead, I'll fasten them togther like that and wear them around like Mrs. T. Second is all done!|
May 1, 2004
Skittlebraü did great at it's first competiton. Following is a brief
First round was against Tyrant. In an unforseen design oversight, the
weapon motor had so much torque that it would kick downward into the floor,
stalling the blade on startup, and putting the controller into a protective
reset. After the reset I would try again, but to no avail. Al's blade
actually caused some damage, and when he sliced my weapon wires, I tapped
Skittlebraü took 4th place overall, a very good showing for one
of my robots, and certainly respectable for a robot on it's first time
out. Many things were learned, and many improvements are planned
May 4, 2004
Videos Are UP! Thanks to Rob and Pound of Pain for hosting these, as my meager web site does not posses the file size to do so. Right click on the link and select "Save As" to get them onto your hard drive. Enjoy.