Servo - August 2008

veterans of my small robot classes, some of them also having experience on my Destination Imagination teams.

The first session I tossed out a bunch of stuff from my workshop: cordless drills, cordless screwdrivers, Barbie Jeep gearboxes with and without removal of the final gear for a “speed hack.” I had the kids weigh them, and test the amp draw running free and at stall. They liked the standard Barbie gearboxes, which was fortunate as I had a box full of them.

Next, I had them draw up some materials stats, such as weighing and measuring various thicknesses of plywood, aluminum, Lexan, and foam; and calculating how heavy a robot would be if it were made of each material and had some plausible dimensions for this project. I suggested a simple pushy/wedge bot for a first build and at the end of session 1, we had a shopping list.

It helps a great deal to have a workshop full of junk. The expensive components such as speed controllers and batteries were all lying around from years of previous projects. In fact, I had built something very much like the kids’ design a few weeks earlier when the high school drama department needed a robotic goat on rather short notice.

Speed controllers are kind of the soul of a robot. That being the case, it appears that some Eastern religions are correct about reincarnation, as electronics for this project had lived previous lives as everything from the 340 pound “Newton’s Claw” down to the candy delivering “Pumpkin Bot” that scared my trick-or-treaters a few years back.

Our basic drive system was a pair of Barbie Jeep gearboxes, hubbed to eight inch rubber wheels that were about a buck at the local surplus shop. We added a pair of 7.2 volt R/C car batteries wired in series for 14. 4 volts; a mild over-volt for the motors but no problem for

SUMO’s drivetrain.

the Victor 883 speed controllers that I pulled out of RoboGoat.

One of the kids brought a big slab of half-inch plywood that made up the basic frame and base armor. I tossed in a broken snow shovel blade for the pusher.

So far, not much ground breaking technology, but a serviceable machine. We did get a bit more creative with the secondary armor, which was a tricky composite of dense foam, 1/32” Lexan in two layers, and plenty of Gorilla Tape. In fact, two rolls of this stuff at about 5 bucks each were the single biggest expense of the project.

The new machine — dubbed SUMO for its final pudgy look — was controlled with a 75 MHz JR receiver and an IMX mixer for simpler handling.

With time to spare, we actually started a second 30 pounder, with a working name of NSP (No Spare Parts). This was to have a similar drive system and an active weapon.

But alas, with two sessions to go a problem arose. Spring arrived. Middle school boys are not the most focused primates on their best days, and warm weather, track practice, and the attire of middle school girls all became major distractions. So, we scrapped NSP at the half-built stage and upgraded SUMO a bit.

Basically, we ended up swapping in 24 volts’ worth of NiCd batteries, which made SUMO a very effective pusher.

In actual combat, lessons were — as usual — quickly learned. The “close enough” fit between the axle

The build team pauses for a photo-op.

and shaft collar proved to be not quite close enough, and we lost one match when one hub slipped off the output gear of the gearbox. The design was a bit tight, making emergency repairs difficult. This could have been avoided, as we were four pounds underweight. (Note to self: Next time, bring an accurate scale. The one from the school nurse’s office must have been jumped on too often.)

The composite armor proved more than sufficient against flail and blade, and with some additional refinements will make more appearances in future projects.

The kids all had a fun time, and the relative success of the project is largely a tribute to their outstanding driving skills.

Total out-of-pocket costs came in under $40, which at just over a dollar a pound must be some kind of record. True, you could claim that we cheated by raiding my robot graveyard/workshop. But I suspect that with a bit of eBay trolling and dumpster diving, it would be possible to do the entire project including electronics and radio equipment for somewhere around $150.