Category: dumb robots

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Null Hypothesis Grand Update: From Zero to Something, I’m Not Sure What

chorl2 Comments

What do you mean Robot Battles at Dragon*Con is in 3.5 weeks?!

Again? I swear they move the thing up one week every year just to surprise me. Actually, it’s more like 2 and a half weeks before I get to Epic Road Trip once again from Boston to Atlanta (ideally not through The Sprawl) and temporarily set up base camp at the Invention Studio. So maybe it’s time to start on a robot or something.

Luckily, I’ve actually been working on Null Hypothesis on and off for about a week. As explained in its introductory post, I was needing to separate a large 8 x 48″ panel of 1″ thick UHMW into slightly funny-shaped bars with the waterjet. I test cut one of the less important frame pieces to check for quality of finish, and determined that only cutting on the highest quality was going to even have a hope of getting through the material. UHMW is notoriously hard to do anything abrasive to since it resists abrasion really well.

One mistake I made but only realized later is that I was testing that piece on a setting meant for polypropylene plastic, which machines much easier – one of the most machinable plastics.. Every material is assigned a “machinability” value in the OMAX software, and this is one factor in controlling how fast the jet moves – and cut quality is pretty much a function of slow. Even “generic” plastic and acrylic-specific settings have machining speeds around half of polypropylene. Therefore, I elected to just throw the panel on the machine and hope for the best using the highest quality finish.

 

By the way, cutting a mushy plastic like UHMW – or any plastic, really – causes a disgusting film of plastic goo to cover the surface of the pool, because the particles both float and stick together. I was skimming handfuls of plastic snot out of the tank as the job progressed.

The job took an hour and a half, which I can’t really say is much faster than me cutting off a bunch of barstock like I would have done Back In The Day. I had bought enough UHMW barstock in the correct size (1″ thick, 2.5″ wide) to make most of the frame anyway because I was uncertain that the huge panel would have been useful. But, with the combination of very high quality cut (slooooow) and actually selecting the correct material properties, the frame pieces came out very smooth and square. I was otherwise prepared to redesign the frame a little to get rid of those hard internal square corners and slots to relieve me of the extra machining.

Also on deck in the same session was the front and rear steel parts, the top and bottom polycarbonate covers, and the internal electronics mounting structure. Above are the front wedge and eventual rear bumper thing. Both are made from annealed 4130 plate steel purchased from McMaster.

While I don’t really have facilities big enough to heat treat these plates short of throwing it on top of a gas fired barbeque grill for an hour, buying a plate of generic mild steel was actually more expensive – a 6 x 36″ plate of 4130 in 1/8″ thickness costs $36, whereas generic 1018 steel in the same dimensions is more like $50. Strange, given how much more versatile 4130 can be –  not only is it stronger even in annealed form than mild steel, but I could harden it to the high Rockwell C40s.

I attribute it to McMaster knowing what people buy most frequently and giving those product some… extra markup.

Here’s the pile of structural parts. The top and bottom plates are 1/8″ polycarb, and the supplementary electronics armor is 3/8″.

I’ve pre-bent the rear bumper because it took me a few tries to get it right. First, the extra-strength 4130 defeated my attempts to bend it using the largest metal brake I could find – a 10 gauge rated, 50″ box-and-pan brake. The cold 4130 plate, a foot wide and at the limit of the machine’s capacity, would not bend.

Next, I tried heating it up with a torch, but I don’t think I got the part nearly hot enough (that or my torch was too small, another possibility). While the hot plate did bend, I didn’t tighten the finger clamps enough or something similarly bad happened, so my bend line slipped about 1/4″ back on one side. Whoops, asymmetric part…

I gave up and recut the plate out of some scavenged 1018 mild steel plate – this bent just fine. Honestly, for Robot Battles I could well make this plate from jello and it would still be enough.

UHMW is almost always manufactured oversize and to rather loose tolerances because its primary industrial application seems to be skid pads and wear plates. For that, precision is not necessary. My 1″ UHMW plate was more like 1.03″ (not 0.003, 0.03), so some of the rectangular slotwork in the top plate didn’t fit. I plowed off the extra thickness in the region of the slots using tinymilll, our super adorable LittleMachineShop mini mill.

The frame standoffs are stock from McMaster. Now, these things come threaded, but I actually just need spacers (unthreaded, pass-through), so I drilled them out.

It turns out that large standoffs only cost about $1 in quantity. McMaster must have found a cheap supplier for standoffs recently, because I definitely remember almost all of the small ones like 4-40s and 6-32s also costing $1 each or so. But now they seem to hover around 30-40 cents.

This is great, because it means it’s actually far cheaper for me to buy a bag of them as “stock” than to machine them myself. Previously, if I needed a bunch of odd standoffs, I’d machine all of them, which takes alot of time, and I’m sure an industrial manufacturing process can produce more consistent parts than I ever care to.

Because I was able to keep the slots and cutouts, the frame acted as its own machining fixture very well. I clamped the parts to the table and drilled where needed. It was pretty much assembled in situ. All of the fasteners are 3/8″-16 button head cap screws. On the top and bottom, I’m going to have 1/4″-20s because larger screws have impractically large heads to countersink into 1/8″ thick material.

The back plate was clamped in place on the bot first, then the mounting holes drilled and tapped in-place. The steel was very slightly too acute of an angle, but the screw pressure was enough to keep it fully flush with the beveled surfaces. I like how this back plate came out.

One advantage of keeping with the strict waterjetting diet is that I was able to easily make these ‘form fitting’ mounts for the drills. Granted, 3 holes and a big center bored hole would have gotten me the same.

Because the material is so thick, the entirety of the gearbox is well supported. The motor is actually fixtured from movement on the outside of the frame rails…

…like so. I’m really hoping these 3D printed ABS hub-things stand up to physical abuse. They’re 100% solid, but ABS is still a pretty soft plastic, especially when laid down in noodle form using the Lab Replicator (because we live in the future now). This let me build the double-D shape of the drill output exactly.

I didn’t actually get to mount the motors permanently – the frame wasn’t done with hole drilling, and I was also missing the correct screws to secure the flange to the frame and those giant set screw things for the motors.

Most of the frame is now completed. The top and bottom plates were also drilled and tapped in-place using the holes in the polycarbonate plates as a template. Afterwards, they were clearance-drilled and countersunk appropriately.

Frame fully assembled now, but without the front wedge holes drilled yet. I was waiting on the correct length 1/4″-20 screws to show up before I mount everything.

In the mean time, I started dissecting drills. I have, in total, 8 dril….err, drive motors for this thing, and hopefully I don’t go through all of them. These are all Harbor Freight p/n 68239, the latest model of shady 18v drill HF offers. They have a verified 24:1 gearbox, though with a plastic first stage (not necessarily bad, in my opinion).

Even 24:1 is a little high ratiowise for the speeds I want to get to with this thing. The bot will hit about 15mph on 26 volts (8S A123s, 3.2 volts each), which is a little on the slow side for my tastes despite Clocker never hitting over 10-11mph. It is, after all, a 100% drivetrain bot. I’m also uncertain that the drill motors will like such drastic overvolting. That question will be answered concretely once I put this thing on the ground and push some things around.

The drills’ torque clutches were hacked in the classical fashion. I used to know many guides on to hack these things, but they were from years ago and many have gone offline as builders retired. Basically, the hole is the perfect size to tap for #10-24 or #10-32. Short set screws are used to entrap the dog clutch element that is the main component of the adjustable torque limiter – without these, the ring gear would just spin freely and no torque can be transmitted to the wheel. Dale has another method of locking the torque clutch which is a bit more compliant.

I decided later to proceed with drilling the wedge mounting holes, since I had a bad idea coming up for the plate itself. The wedge was clamped directly to the frame, aligned horizontally, and the holes drilled and tapped in situ.

The process I’ve been using is first drilling a dimple using a pilot drill of the template hole diameter (in this case, using an F size drill because I designed those holes to clear 1/4″-20 scrws), such that the dimple is reasonably well centered. Next, I switched to the tap drill diameter and let it settle in the dimple, then just freehanded it. Not all the holes are dead square, but that’s just a consequence of free drilling and only practice and experience can get them close.

Alright, so this is the bad idea. Not shown in the robot’s 3D model, but in the works all along, is turning the wedge holes into slots. This would have been a trivial operation if I were actually just slotting, but because bolts protuding from the surface of your wedge is a glaring sign of n00b, I had to make countersunk slots.

It involved a 6-flute metalcutting countersink (no, do not try this with a cheap hardware store 1-flute wood & drywall countersink), lots of cutting oil, and going very slowly. The whole machine would occasionally rumble and generate a small local earthquake every time I began cutting. In the end, countersunk slots!

And they fit!

Well, most of them – there’s at least 2 which I felt were “oversunk” and wouldn’t be as strong as one of proper depth. That’s why there are 16 screws up front, right?

it’s also being screwed into uhmw.

The ride height of the wedge is adjustable from essentially floor-scraping to about 5/16″ up. The D*C arena/stage/combat surface is not guaranteed to be flat anyhow, so this lets me calibrate the height to the stage as needed.

The last operation of the night is coring out the wheels. These are McMaster’s p/n 2243T41, or “McMasterbots” wheels as I call them, 40A durometer donuts of rubber overmolded onto polypropylene hubs.

As shown in the design previously, these are supposed to partially envelop the motor mounts and use them as a rudimentary secondary bearing. Hopefully, this will make the drivetrain nearly indestructable (from axial and radial impacts like being dropped or flipped) because the majority of shock loads will not be transmitted through the gearbox nose.

The machine of choice for this job is hueglathe, the 19″ LeBlond Regal, because its chuck can effortlessly grab the entire wheel.

Four wheels cored out to the appropriate depth. Can you guess for which one I forgot to tighten down the chuck before plunging the boring bar into it?

This is how it will go together.

Still to come: Making the wheel hubs, permanently mounting the motor and wheels, and then…

Hmm.

That’s about it. This is a pushybot, after all.

A New Pushybot

chorl8 Comments

IT’S ROBOT SEASON!

My annual robot party, Robot Battles @ Dragon*Con, is right around the corner. Again.

This is a competition I’ve been attending since 2002 and competing in since 2003 – it’s been my “end of summer festivity” for every year since then, except 2007 when I was retained by the mits for freshman orientation.

For most of my RB history prior to the MITs, I’ve had a primarily drivetrain-centric entry, Test Bot, that worked all the time and was undefeatable so long as I didn’t mess up. After that, beginning with Test Bot 4 in 2006 and really kicking off with Uberclocker version 1 in 2008, I began focusing more on cool actuators and weaponry.  That probably explains why I haven’t won anything since, or really have a robot that can put on a performance without being unreliable. Cool weapons and whirly things are funny until you can’t get it to move straight across the arena to beat the other guy senseless with it.

I’m kind of sick of that. Lack of any real motivation to fix the drivetrain issue with Überclocker Remix thoroughly has led to its losses in 2009, 2010, and 2011. That’s 3 straight years of fail, and I can even name the reasons why. In 2009, the robot lost drive functionality early on because I dared to think that using $2.50 drive motors was a good idea (and brought no spares, also a good idea). I upgraded the gearbox and motor rig to a custom DeWalt solution for 2010, but it slipped a poorly-made output shaft and I didn’t care enough to try and ninja-fix it during the actual event. And finally, last year, with the drive motors finally reliable, the robot ditched chains left and right. Literally. Both of them. Poor tensioning results in the chains catching on… something – I couldn’t tell what was actually going on, but it cost me the tournament. Again, what just totally sucks about it is that it was no fault of the design itself, but just my inattentiveness or ambivalence.

Sigh

So I’m starting a new 30lb build which will be 100% drivetrain. The goal of this build, really, is to just come up with something that’s fun to drive and run into things with again. Piloting the (comparatively well-handling) Cold Arbor test rig around a few weeks ago reminded me of how important it is to keep your robot short, dense, and 4WD with the center of gravity smack in the middle. Soft wheels helps too, as does an immense amount of horsepower in the motors. This 30lber will act as a baseline to compare all my future designs with. The drive will be modular, with one motor per wheel, quickly replaceable and with easy to find spares (something Clocker has never been very good at). The frame will be pretty damn near indestructable. If I can’t beat the all-drivetrain pushy-brick with a new design, then anything else is pretty much hopeless. It will function as the null hypothesis test of robots – to be a proven design, it has to reject (/defeat) the null hypothesis.

Incidentally, that’s exactly what I’m calling it. Introducing Null Hypothesis:

Well gee, it looks like I’m almost done designing it already. Usually my CAD posts start off with a rectangular extrusion or something equally simple, but I’ve been slowly digesting this design for a little while.

Short rundown of the design: Four 18v Harbor Freight 900rpm type drills (now on sale again as item 68239) direct-driving four 40A durometer “McMasterbots” wheels. 1″ thick UHMW bar frame, and polycarb top and bottom. True 0.75″ ground clearance each side. Classic indestructable pushbot! The outside of the bot measures 18″ x 18″, which is a reasoanble 30lber size.

The design above is the “first pass” appraisal of the concept. It has a solid UHMW front wedge that was intended to be carved from a 3 x 4″ block. That turned out to be $150 of UHMW, which seems to have gotten way pricier since my last UHMW brick in 2006, because UHMW is just condensed natural gas or something.

Note how it gets a little weird and irregular out back – I wanted this bot to have no rear traction gaps. A classic “box” frame leave you a finite, usually less than 45 degree, tip angle before the frame lifts the wheels off the ground (for Test Bot 4.5 this was only about 25 degrees because it’s so damned low). One solution was to cut up a pipe and round off the end of the frame, but this left me little space to mount the motors. Hence, I settled for 45 degree ‘facets’ that can get pushed further back to let me add mounting features for the drill gearboxes.

The ground clearance is called “true” 0.75″ because the top and bottom covers are inset into the frame. This leaves me with a 2.5″ frame that has 2.25″ of internal space given the choice of 1/8″ polycarbonate top and bottom, which actually turns out to be not that roomy.

Here’s a “second pass” of the design. When I do a “first pass”, it’s generally just to puke ideas and generate shapes. Some times I find that the shapes end up being nonsensical or impossible.

There wasn’t such an impasse with this design, but I decided to get rid of the solid UHMW brick front. As funny as it might be, I really can’t justify spending $150 on UHMW just to use maybe $70 of it. The wedge ‘sections’ are made of the same barstock now as the frame. A 1/8″ 4130 steel plate forms the business end – I got a plate of the alloy for reasonably cheap, but there might not be an oven big enough for this thing with immediate reach to fully heat treat and take advantage of its strength.

The battery and its retaining mechanism is now shown too. It’s actually really hard to make an all-UHMW 30lb bot – UHMW is just not dense enough, really, so the weight needs to be made up. In this case, it’s with theoretically enough  batteries to run 4 or 5 matches back-to-back. The intended voltage of the system is going to be 25.6 nominal, 8S A123 cells, with 4 effective cells in parallel, physically arranged as two separate 8S2P packs. This should yield a decent top speed of 15mph: not too quick, but also not sluggish. Middle of the road pushybot.

The battery packs will be made with shock mounting padding, then captured inside a 3/8″ or 1/4″ polycarbonate ‘cage’ inside (hard to see, with polycarb being modeled as transparent). The cage is part of the top and bottom plates.

Electronics-wise, I’m trying to decide between the Botbitz controllers that I got to test for Arbor, or my full-custom board (yes, again):

The hell is that thing? I christened it RAGEBRIDGE because, somehow, I have never managed to produce a reliable small H-bridge controller. Landbearshark’s custom controllers were a miserable failure for several now-obvious reasons. I’m also really in need of a synchronous-rectified current-control-capable H-bridge for an outside consulting project – which is really what this board was developed for.

So I took another stab at it. I took every precaution to not loop my grounds and cross my logic with power, so I hope this can become a reliable hardware base.

That is, if i ever receive it. It’s been nearly a month, MyroPCB. The excuse I got was that my order was ‘damaged in fabrication’ and had to be redone. Well hhmm, I hope it wasn’t all those vias….

The reason I’m still holding out hope for these things is because the BB controllers can’t do “true 24v” systems – the Hobbyking ESCs they’re based off of are rated to only 6S lithium polymer cells, or around 26v peak. There are 35v parts all over the place, including the main power capacitors, and the FETs are 30v parts. In other words, a freshly charged 8S A123 battery at 28.8v is most likely too much for them. And I’m not going to back down from that – NH will be too slow if I only run 6S.

Worse come to worst, I have all of those Victor 883s (classic ones, mind you… The FET layout above is clearly borrowed from the Victor883107-7.).

Here’s the front end of the bot. I’m most likely going to just stencil-paint (or laser-etch?) the H_0 onto the front, instead of making it a cut-through, because that weakens the steel too much. Else, I might weld a backup plate behind it.

The top and bottom (and wedge) hardware on this bot is all 1/4″-20 countersunk cap screws, with the side/frame screws being 3/8″-16. Big, meaty threads. I considered using giant lag bolts for maximum grip strength, but they were not available in anything other than “giant oversize decking and timber-clamping Hex head”.

Random little electrical details will be added soon, when I can decide on the choice of electronics.

If you’re wondering where the hell the drill motors are disappearing into…

Hey, first real picture of the build!

Here’s what’s going on. The McMasterbots wheel will use the nose portion of the black flanged piece as a secondary ‘bearing’. This is a scheme to avoid hanging the entire wheel solely off the 3/8″ shady drill steel axle (as is common in drillboxes). The D-shaped nose of the drill gearbox is additionally supported by the mounting flange, giving it more plastic meat. The motor is prevented from torquing and moving axially using fine-threaded set screws (the cross holes in the picture). The flange itself is screwed into the UHMW side rail.

The black plastic piece is made of ABS, rapid-prototyped on the Lab Replicator. The final version will also be made of ABS, and incidentally also to be rapid-prototyped on the Lab Replicator. On a side note, most of my complaints in that post seem to have been wrapped up in the latest firmware for the Replicator – they were probably common enough that an official fix was distributed.

I’ve ordered pretty much everything I’m going to need to make the mechanical bits of this bot in the coming two weeks or so. I have eight drills now, and the buffer pile on my desk is becoming insufferable.

I have to make a decision about how to build the frame. While I was initially going to buy and mill slots in UHMW barstock, I snagged a large plate (48 x 8″) of UHMW on eBay for very cheap. It has all of the square footage needed to build the frame, but separating it into chunks will be a bitch.

While my normal tactic when faced with a giant plate and needing bars and shapes would be to Just Waterjet It, UHMW is well known to be so soft and abrasive resistant that it cuts poorly.

I laid out some test parts (these are the side bumper things) using different qualities to see if any were remotely tolerable. Anything short of high quality (or in Omax terms, “Quality 5”) just doesn’t even make it through all the way, and I can’t get the part out of the material! Even the above piece, which was on Q5, needs all the holes drilled out. The outline is reasonably acceptable, though, and I may make the parts which don’t need to be critically square using the cheap plate. This include those side bumpers as well as the little wedge triangles.

I bought a 4 foot stick of UHMW for the parts that can’t be waterjet-cut, but I might have to get friendly with this plate and a slitting saw in the near future.

This is not it. I have several other robot updates on deck as August rolls in, including:

  1. My kinda-open-secret project of creating a new arena hazard for the new Atlanta Bot Arena
  2. The last round of updates to Überclocker before it will be retired after this D*C event
  3. Maybe a new beetleweight design that I’m tossing around.