TB4.5MCESP1 Update 16: Barrel Roll edition

With about a third of the bot totally nonfunctional, I thought I was going to have to drop the event. However, quite a few things have changed and I’m able to at least make the bot run at a slightly reduced power level, so I will go ahead and attend Motorama ’08.

I have been a bit overwhelmed by classwork and nonclass-work over the past three days and so haven’t been able to finish the bot as completely as I like. However, the major work is all but complete, and the only machining-related things left to do revolve around trying to deter the arm from jamming on every throw.

I managed to get more time on the waterjet and cut out the front armor plates. They are all 1/4″ thick 2024-T851. I haven’t even seen that stuff before (most 2024 is T3 or T4), but it seems slightly stronger but more brittle than the usual. Not by much, so I’m not concerned about things breaking. Due to the blessing that is EBay, I snagged a 12″ x 40″ plate for $40.

Too bad the 100th buildpic can’t be a finished bot. SP1 is actually the first build which I have taken over 100 pictures of. Almost every detail is covered. Hey, documentation is good.

This is the “new” 3S 2250mAh lithium battery. I essentially just desoldered the dead cell, reversed the one next to it (since the polarities have to keep matching) and called it a day. Since my charge port is adaptable, there was no change to the charge plug except to cut one of the pins off to avoid disaster. The rest of these cells are healthy, and the bot is plenty maneuverable on them.

However, I think the Victors might suffer from low-voltage dropout syndrome again. We’ll see.

For this bot, I decided to exploit my new 2D shape-cutting techniques and explore the world of uber-nuts and nutstrips. They are threaded bits of metal that match the hole pattern of whatever part you’re trying to retain, and do two important things:

1. To spread the fastening force over a larger area, which cuts down on focused stresses, and

2. So you don’t have to jiggle a bunch of nuts every time. Isn’t that lovely?

These nutstrips are 1/4″ 2024, made of the same sheet of aluminum as the wedges. They go on the fastening side of each hinge where I would have used some locknuts before.

Wedge plates drilled and countersunk. Notice the redundant holes on the arm plate. These were the result of numbers mutating inside my head, causing strange things to happen IRL. What were going to be 7/8″ hole spacings suddenly became 3/4″ hole spacings on the arm beams themselves, so each hole in the pattern was progressively an eighth inch off. The new holes were drilled on the mill so I could control the hole placement (I actually used an endmill since a drill bit’s pointy tip would cause it to explode drilling a half-hole). They were countersunk in the same operation, since the mill is infinitely stiffer than me and more adept at keeping the countersink where it needs to be.

There’s some chatter marks in the countersinking, but BFD, amirite?

Here’s MCE’s old 12 volt NiCd pack of about 1800mAh. It managed to work with two deficient cells all through BBIQ 2007 (not well, but it did), so I’m taking the opportunity of finally possessing a smart charger to condition it to see if it can still function in the bot. Results are so far promising.

The Media Lab had some snazzy clear shrinkwrap to put the cells in. The lithium battery is also coated in a few layers of the stuff.

It’s ALMOST there! Everything on the front is test mounted. No, they’re not upside-down. The corners will actually be bent back 45 degrees such that they wrap around the front of the bot, forming an additional angled deflection surface for opponents using less material and weight than MCE’s separate side skirts.

The plates are a perfect fit with zero clearance, just like the CAD. Of course, it’s so perfect that friction from flexing and mounting inconsistency will become an issue, so I’ll sand the ends of the plates that touch such that they… well, don’t touch.

Nutstrip structures in place. Along with the tack-welded hinges and the internal nutstrips, the whole front of the bot has to come off before anything comes off.

This could very well happen, by the way. I’m not discounting it.

When I put everything together,something just didn’t LOOK RIGHT. I wasn’t sure what until I tried making the wedges flush against eachother and found that they were misaligned, but coplanar, by about 1/8″. D’oh.

The only way for the wedges to do this and still be slightly off is me cocking up one of the hinge hole spacings. I will probably try to move those holes up 1/8″ as well (Or just grind them.)

One side effect of turning holes into slots is ADJUSTABLE WEDGES!

Less than 24 hours left now before departure (I head out Thursday morning to join a traveling party)! What can happen?





Problems remaining:

  • Arm jamming due to inadequate pinion support
  • Victor-Spektrum interface difficulty
  • Current-limiting too restrictive on arm ESC
  • Wedges not yet machined or bent

New problems:

  • 0 volt cell in lithium polymer battery pack. Like, zero. Zip. Nada. No recovery.
  • Dead arm ESC. That’s why current limiting has been solved. Mysterious non-current cause of failure. Update: Ghettobooster seems to have blown its 5v regulator early (could have been a bad one to start with) and was putting out over 8 volts on the 5v line. I’m surprised the Spektrum and Victors are still alive. Test custom electronics before putting them in the bot, kids.

At this point, it’s almost frivolous to continue. At least the bot moves, and I was able to drive it around for a few minutes.

I have only a few options – drop the lithium pack and go to the MCE 12v NiCd pack, or drop one cell out of the lithium pack and run 11.1v. I’m not sure which one is more effective at this point. I will have to borrow, steal, or beg an arm ESC at the event.

Otherwise, I might as well concentrate on solving the arm jamming issue. There’s a few solutions to that, too.

The final solution (Plan Z!) is to just forget about the arm completely, lock it down, span the front wedge with a durable material, and brick it! That’s right, run as a pushybot.

However, it’s unlikely I’ll activate plan Z. There’s absolutely no point in doing that.

Bot on. I hope the rest of you have better luck.