Archive for June, 2009

 

The Neverending Tragedy of the LOLrioKart

Jun 28, 2009 in LOLrio Kart, Project Build Reports

It has been an eventful few weeks.

I started at iRobot as a summer engineering intern, tasked with building Terminators.  I was invited along, and subsequently went to, the Buckminster Fuller Challenge award ceremony in Chicago with Smart Cities.

And I finished, broke, finished, then broke, then finished LOLrioKart. I’m waiting on the next “broke” cycle.

Let’s start with a batterygasm.

From the deepest dredges of “Well, they technically never asked for it back”, here is a pile of A123 lithium nanophosphate cells. These are the most ballin’ shit in terms of batteries available today.

The backstory is that A123Systems, through its ancient ties with MIT, donated Over 9000™ cells which failed quality control to the Electric Vehicle Team. The Media Lab also received some for sampling, and being Smart Cities, we quickly snapped them up because… well.

Anyway, I was tasked with the fantastic task of determining what “failed quality control” means. For the most part, it means “smudge” or “wrinkle in the cardboard”, but a few cells were genuinely low voltage or had high internal impedance. Through a series of trials and strictly controlled processes involving a giant power resistor, car battery, and multimeter, I determined that essentially 95% of the cells in each case of 100 were most likely good for our purposes.

Great news for us, better news for me, because A123 probably has more.

Anyway, to equal the 54 volt, 30AH nickel batteries I already have (nominally – these cells are totally fucked and probably return less than 20AH as a pack), I would need about 225 cells – 15 cells in series for a nominal 48 volts, and 15 cells in parallel for about 33AH. The reason I calculated the numbers for 48 volts is because I do have access to 15S chargers for the chemistry, from the car. That’s something like $3500 in batteries if I were to drop some cake for it.

Or two boxes of cells. Come on EVT, you know you want to donate some to The Cause.

In order to resolve the “dude, what the hell is 42 volts doing on my frame?” issue, I stripped the entire electrical system down and pretty much rebuilt it. At the same time, I took apart the back end for cleaning. If you’ve never seen the kart’s running gear, here it is.

While rebuilding the electronics, I decided to try to minimize the footprint of the ginormoFET controller. Here’s the dismantling in progress.

And here’s the completion.  By stacking the busbars, I gained a few square inches more… board space? Deck space? Scrap-of-plywood space? Additionally, it was easier to arrange the wiring to suit the rest of the electricals.

I decided to forego fan attachment until heat was determined to be a major problem.

Contactors donated to The Cause by another MITERer. They are rated to switch 100 amps. In theory, they should never be switching current in my electrical system, merely passing it after closing. Switches tend to conduct far more current in a circuit than they can reliably close or open, so I’m not concerned about melting the contactor.

Also included in the deal is a Hugeasspacitor™. 33,000 microfarads of love at 75 volts.

Most everything in place. I mounted all components with short wood screws this time, which saved alot of drilling and… yes, threading of wood. That was such a dumb idea that I only could have done it at 4am.

The DC key switch is a battery cutoff switch, and isn’t intended to actually turn the kart on and off.

The power system has two stages. First is the closing of the battery switch with the contactor still open. A resistor bypasses this contactor and goes straight to the controller, which allows the Hugeasspacitor™ to charge at a reasonable rate.

The reason for this precharge resistor is, without diving too much into Course 6 theory, that capacitors appear as an instantenous zero-resistance  to a sudden step in voltage. You know, like closing a switch really fast. What that means is if I just threw the capacitor onto the battery, there would be a Big Spark as infinity amps tries to flow into the cap at once. The problem is that if the cap is now charged, there is still infinity amps trying to cram into it. Not good for the capacitor, and especially not good for whatever poor switch gets caught in the middle. It’s easy to weld contactors like this.

So the second stage of the power electronics is powering the contactor, which opens a very low resistance path. I can now drive off into the nearest oncoming semi.

A 12 volt DC-DC converter provides the contactor current. This is placed before the resistor but after the switch so I can run accessories without the main controller being powered.

Alright, a furious night of building and debugging without pictures later, and here’s the system ready for a test run. The night was mostly consumed with debugging my dumb hardware PWM generator that I pledged never to make. Because it’s on a breadboard, there were Over 9000™ things that could have been wrong with it.

It ended up that my comparator was dead. After tearing the whole thing apart to discover that, I threw on a new comparator, coated the board in hot glue, threw it on the kart, and called it a night.

The safest way to perform a test run is of course to strap a 36 volt 10AH lithium polymer pack right under your ass.

A beauty shot, if you stretch the definition of “beauty”. Notice the emergency stop button. This is the contactor controller.

Yeah, yeah, testing… It was late and nobody wanted to grab the camera. So LOLrioKart sails down the hallway successfully. In this configuration, I drove it to Swapfest the next day for some lulzy in-field testing.

So I couldn’t resist. I had to put the voltage-leaking Nicads pack on the kart, because there wasn’t another way to get greater than 36 volts. My DC-DC converter, rated for 48 volts, shuts off exactly at 36. What’s the nominal voltage of the lithium test pack? 36. If I floored the throttle, the kart would turn off.

Not exciting.

Before throwing the packs back in, I thoroughly coated the bottom of each pack in rubber sheeting, just in case I missed a spot and metal touched metal while in the basket.

Despite my efforts, there was STILL live voltage present at the frame. How’s about them electrical gremlins?

However, it was a high impedance leak, so I wasn’t worried about shorting something through the frame. I decided to go ahead with this installation.

Scoping out a(nother) PWM generator problem. I went through FET driver chips like crazy, for reasons totally unexplainable; not even the resident EEs could figure out what I was doing wrong.

The voltage leak was ruled out as a cause because the DC-DC converter provides voltage isolation and no component is frame-grounded. That I know of, anyway.

Out-of-range operation was also ruled out, since I’m running the chips at less than half their maximum voltages. The FET gate had a bleed resistor and the throttle input has an RC filter inline.

Transients were the main suspect, so I arranged some more low-value caps around the important parts.

It seemed to be stable. Time to figure out which way the motor is supposed to be hooked up!

This is what we call “Nope, it hooks up the other way”. I like where this is going.

Alright, so maybe not this picture, because “where this is going” was straight backwards into a shelf.

Giving the nicads a wakeup charge. By virtue of sitting for two weeks, some of the cells have fallen back to zero volts.

That’s how totally fucked they are.

Using the HOLYCRAPWHATISTHAT, I dumped 25 amps into the cells, which is pulling something like 1600 watts.    After an hour and so, they were nice and warm.

Totally the most legitimate throttle pedal ever. That’s a spare bettery switch key zip-tied to a bike hand throttle zip-tied to the frame.

Video time! I decided this was legit enough to take on the streets and have some fun, so I drove to campus and found some testing grounds. It had been raining for two weeks at this point, and I couldn’t really hold it in any more.

Afterwards, I decided this pedal was just not legit enough to keep, and that my scooter needed the throttle back really badly, so I fabbed up a pseudopedal. It’s not really any more legitimate.

I had a “resistive throttle box” already, so I just scrounged some parts from it and assembled them onto a mount I cut out long ago. While it’s functional, the positioning is the least ergonomic thing in existence. I kind of have to side-roll my right foot onto it.

Alright, so it’s good for now. Time to start working on the priorities, like…

…instead of, you know, functioning brakes.

Seriously, the little band brakes have deteriorated to the point that stopping doesn’t really take less distance if I completely step on the brake pedal. They really were not made for the task of stopping a 350+ pound vehicle at n miles per hour, where n is between 20 and 30.

And so this is how the kart was set up for its first ever road trip, from MITERS to the extreme western tip of campus, a distance of roughly one mile each way. No video is available due to it being completely spontaneous

I’m clearly still alive. More work to come, like BRAKES.

It exists.

Jun 12, 2009 in Project Build Reports, Project RazEr

Happy now?

Video below, build reports, project page.

The Summer Build Season 2009

Jun 03, 2009 in LOLrio Kart, Nuclear Kitten 5, Pop Quiz 2, Project Build Reports, Project RazEr, Stuff, Überclocker Remix

It has begun.

While I seem to be in “build season” mode year-round, it is during long breaks with little in the way of academic or life obligations that I get the most done. Last summer, I began work on LOLrioKart and built Überclocker, Pop Quiz 2, and Nuclear Kitten for Dragon*Con.

… which sort of sucked horribly for everything. Except NK, but only by about *this* much.

So what’s coming down the projectubes this summer?

Mostly the same thing. D*C is my biggest bot-celebration of the year, so once again the combat robot fleet takes high priority. Since there’s really just one robot that needs rebuilding, I also have the usual pile of small electric vehicle projects, of which only one is actually urgent.

Übercløcker RЭmiχ

I started redesigning Uberclocker some time in the fall of last year, hoping to get it done by Motorama 2009. Of course, due to scheduling concerns and logistics, this didn’t happen. But what that presented me with was the chance to put it away and not look at it for several months.

This is pivotal. The basic design has already been hashed out, but now I get to return to it after not thinking about it for a while. I am now in the process of analyzing the 3d model for any “impossible objects” that I might have included, or Really Bad Ideas. Such design flaws plagued the real life Uberclocker 1.0 at D*C last year.

Planned upgrades from 1.0? Well, besides EVERYTHING, the primary focus is on drivetrain reliability, center of gravity, and the upper clamp arm.

As a member of the pushybot school of combat robotic thought, I value maneuverability and driving above jawesometacular weaponry. Uberclocker 1.0 had a strange serpentine timing belt setup that seemed like a really awesome idea at 5 in the morning, but… wasn’t.

The robot also suffered from “centrally located center of gravity” syndrome at the event. While a CoG near the geometric centroid of the robot is good in practically every other case, the fact that the bot’s sole purpose was to grab another opponent and lift it off the ground meant that it just sort of faceplanted every time I attempted a lift. Not a very impressive show. The redesign lengthened the wheelbase of the bot, and selective weight reduction moved the CoG back about 4 inches, without additional ballast.

Oh, that’s right, Uberclocker 1.0 weighed in at an incredible 22.5 pounds out of 30 at the event. I’ll fix that too.

What I didn’t really get to (properly, anyway) in the redesign was the upper clamp arm. The previous arm was both weak and structurally unsound. While I think I took care of the “unsound”, I still have my doubts as to the clamp mechanism’s effectiveness.  In the past, clampbots have used pneumatics to actuate the upper half of the clamp. This is advantageous because a pneumatic actuator requires no “holding power”, unlike an electric motor, which has to be continually powered to produce torque. Pneumatics also have a certain amount of spring-back ability that a solidly coupled electric actuator doesn’t.

But robot-heaven forbid that I make Überclocker even more complicated by incorporating a pneumatics system for the one actuator that might need it. Thus, I’m still partial to a (spring-coupled) leadscrew-type mechanism, over the current design candidate’s motor-on-a-weird-gear. Except this time it won’t be driven by a beetleweight motor.

I intend to keep the “Chinese puzzle” frame, and will be refining it for ease of assembly. I devoted a few weeks to just fabricating the frame parts last time – no, never again. That’s what computer-controlled machine tools are for.

Pop Quiz 2√2

Incidentally, 2√2 is about 3. Not quite there, which also describes this planned rebuild of Pop Quiz 2. It’s not quite a complete conceptual revision, but there will be significant upgrades all around.

PQ2 is one of the (if not the) flattest 1lb class robots around that has an active weapon. It hits lower than some undercutters. The problem is that going the extra 1/8″ down in this current design meant that I had to ditch practically all the well-known, battle-proven parts – Sanyo gearmotors, SPEKTRUM 2.4ghz receivers, etc.

It was a fun thought experiment come to life, but the robot had a horrific reliability record, almost no reception due to the FM ground-band receiver, and a 5 minute chopped hack of a master power switch that ended up disintegrating after exactly 1 hit at D*C 2008. Pop Quiz had about 15 seconds in the arena.

Not cool. For ’09, I am INCREASING the height of the bot. Me, making a robot taller. How many times does THAT happen?

The robot height will be increased to about .400″, enough to cram in a set of real Sanyo micro gearmotors. The rest of the robot’s electrical system is sound, and so is the weapon motor. I’ll most likely end up reusing the electronics anyway, minus the cheesy little FM park flyer receiver. Instead, it will be swapped out with the latest Spektrum DSM offering, and I will run one transmitter between all the robots.

There’s no current virtual model for PQ2.8284171, but just imagine the current bot 0.025″ thicker.

Nuclear Kitten 5.1 Digital Surround Sound Edition

I’m actually satisfied with the performance of one of my combat ‘bots for once. NK needs very minor rework to take another run at D*C. The weapon motor needs some magnet reglued, and the weapon pod pivot axle is slightly bent and needs to be made better anyway. Past that, I have a spare blade to replace the faceplant-into-steel-bumper bent blade.

The only point of concern with NK is the drivetrain. Despite having a mechanically isolated weapon, I’m still blowing drive gearboxes, just because the bot is that much more powerful. I might switch to something like the 50:1 Copal motors || redesign the motor mount || use softer wheels.

No frame changes are necessary, since the bot escaped D*C rather unscathed.

LOLrioKart

Since I discovered that the main battery pack was leaking voltage all over the place (somehow, through an eighth inch of rubber?), I stripped down the entire electrical system and tested all the batteries. It turns out that the steel casings of the cells are live, something which I’m fairly certain should never be the case. While it’s fairly common for the battery negative terminal to also be the casing, the errant voltages are always somewhere between 0 and 1 volts.

This case voltage doesn’t seem to have negatively affected the cells, but I’m fairly certain it’s the culprit behind stray frame voltages. Somehow.

The focus for LOLrioKart work will be the electrical system. I intend to complete and test the ginormoFET controller and possibly implement dynamic (or regenerative!) braking using the upper leg of the half-bridge. Mechanically, the kart is fine.

Well, except for the brakes, but they’ve always been undersized and insufficient.

Ultimately the goal is to run it for longer than 1 minute on all 54 volts, or the full pack voltage of whatever eventual power system I might come into. I’m heavily considering crating up LOLrioKart and shipping it down when Dragon*Con comes around, so I can drive it in the parade. This could possibly be the worst idea I have ever thought of.

Project RazEr

It’s been hanging on a utility hook since the last controller fire. Everything works and the batteries are still charged, so all I need is a BLDC motor controller. Since everything still technically “works”, I don’t intend to touch the scooter that much, if at all. Any work on it will be replacing the shell of the wheelmotor with something more substantial (and better engineered, and more reversably built).

Time to get crackin’.