Totally Offensive Build Report (Robots)

After Anorexia mutated into a lifterbot, the kids started bugging me for a robot of their own, so I started thinking about how to build them a seriously nasty 30lb robot. They clearly wanted something with an active weapon and a simple strategy.

My design constraints were that I wanted to make something that would be easy to construct, would reuse a lot of parts I already had (like intercooled battlepacks, victors, etc), and would use standard parts for the tricky bits to save time and make the build easier (my time was limited and I only had a few months to be ready for the Southwest Divisional Championships).

What I came up with was a design that's “totally offensive”. Everything is sacrificed in favor of the weapons system, a magmotor-powered lawnmower blade. I tried to pare off every ounce I could so that the blade could be as hefty as possible, and it ended up being a 6.9lb 22“ hunk of tool steel.

By using a lot of standard parts, I figured that if the design wasn't successful, I'd have lots of bits left over to reconfigure into a different layout. As it happened, this turned out to be a fateful decision

The first iteration was a simple squat “T” of aluminum C-channel, aggressively hole-sawed to reduce the weight. The foot of the T mounts the magmotor and Whyachi gearbox underneath, and an intercooled battlepack on top. Terry of Team Whyachi was nice enough to put a nice radius bevel on the bottom of the gearbox shaft so it could glide around more easily.

The arms of the T contained the Victor speed controllers, radio, gyro, power switch, and weapons relay (both from Whyachi).

Underneath the T were the translational drive wheels, based on Whyachi T-Boxes. I love these things, especially with the inexpensive Small Johnson motors.

The drive wheels were set so that they were tangent to the arc of the weapons blade. This means that the bot could easily turn left and right without moving the blade at all, at the cost of having to scrub like nuts to drive forward or backwards. A pushybot this wasn't, but given the lack of armor on the bot, being able to always put the “nasty” part of the bot towards the enemy was a big plus, I thought.

The plastic drainpipe at the back of the robot was the first part of a passive self-righting mechanism. And the black netting was there to basically give some minor protection against flying debris.

Two weeks before SDC, however, I started having some serious second thoughts. The problem was that because the hub of the gearbox scraped on the ground, the bot wasn't very manuverable. The askew orientation of the wheels let it turn on a dime, but it often had trouble moving forwards and backwards.

So I went back to the drawing board and came up with a tilted spinner design, somewhat reminiscient of the lightweight 2EZ. Worst case, I figured that I'd get some extra practice welding.

With one day left before I had to ship the bots to SDC, I got everything together and gently fired it up in the garage. It drove pretty well. Then I fired up the spinner… and the bot promptly turtled itself!

Whups! Because of good old “equal and opposite reaction”, when the blade started spinning, the robot wanted to turn in the opposite direction, and because the blade was at an angle, this caused the bot to lift up on a ball caster and flip over.

Several chunks of garage floor later, some emergency wheelbase widening resulted in a bot that wouldn't immediately knock itself out, and into the crate it went.

But at SDC, I had problem after problem. The most vexing one was some sort of bizarre interference problem that was locking up my IMX-1 mixers. This actually affected both of my robots! It appears that under certain circumstances the small johnson motors can create a spark whose RFI nukes the IMX-1. So after losing fights because my robots were locking up, I had to reconfigure both of them to use in-radio mixing.

But more serious was that Totally Offensive should have been named Totally Suicidal, because every time it hit something, it would flip upside down and lie there helplessly. After one memorable hit it danced its way on its side around the arena.

So it was back to the drawing board – again!

This time I came up with a simplified version of the original undercutter design. The drive wheels are exactly in line with the spinner hub, so I can rotate around that axis while still having full forward and backward drive without scrubbing the wheels. The frame is made out of T-angle with some minor lightening holes.

Underneath the curved UHMW bonnet are all the electronics and the battery. The radio is mounted under the battery in a compartment formed by some UHMW and the bottom of the T-angle members. The Victor speed controllers are located between the battery and the magmotor. At the rear of the robot is a single ball-caster, so the frame is tripod-supported on the caster and the two drive wheels. This means that the blade is suspended off the arena floor and the blade tip height can be adjusted a bit by adjusting the length of the ball-caster support.

There are a several cute design features that I came up with. First, I noticed that the motor mount screw access holes in the T-box were just the right size to be tapped for a 9/16-18 bolt. So I did that and used it to make the motor mountings more sturdy.

Next, I mounted the C1 contactor that powers the Magmotor on top of the gearbox! Yeah, it sounds stupid, but it solved a space problem, reduced the frame size, the two bolts I installed to support the UHMW bonnet protect it from incidental impacts (as does the bonnet), and it's far enough out of the way that I'm not too worried about direct damage, only about impacts caused when the bot gets flipped or tossed. If I have the time and the weight I'll add front and back UHMW covers for some added protection.

Finally, I dispensed with the power switch entirely, and now activate main power by clicking together some powerpoles and making them secure with a ziptie. Saved me a few precious ounces.

At which point, it was back into the garage and… it doesn't kill itself! Yeah! It drives pretty well, spinup only causes a 10-15 degree turn, and it seems to be stable. Spinup to about 80% of max speed takes about a second!

Who knows, perhaps third time's a charm!

Update: Totally Offensive went 5-0 with 4 KOs in its first tournament, qualifying for Nationals. In the second tournament, it went 3-2, and we discovered some flaws in the design, so we've redesigned it yet again.