The Ongoing Tragedy of the LOLrioKart

chorl

The next “broke” cycle has arrived. Once again, it’s (you guessed it) the motor controller, under mysterious and  nonintuitive circumstances.

But first, I am proud to announce that the kart is now able to stop.

That is, in under half a block’s distance.

Here’s the reason why. I bought a set of 140mm disc brakes and cable-actuated brake calipers from electricscooterparts.com, which incidentally sells all kinds of electric scooter parts. Now that I know that things like this exist, I wonder why the hell I didn’t spec them out for the kart originally.

Oh, right, because I didn’t know they existed. The quality of components seems to be about par for Orient-imported small vehicle parts; by which I mean the brake disc vent holes had burrs around the edges, the bolt circles were not quite concentric, and the left and right brake calipers were, while sharing the same mounting dimensions, different parts physically beyond being simple mirror images.

So it was out with the old and in with the new. I dismantled the front wheel assembly on each side and cleaned everything off.

The inside of the wheel rims were thoroughly caked in small brake band particles. The fact that there was little in terms of brake left over on the bands themselves probably contributed to the kart’s dismal stopping ability (read: none).

So extremely bald front tires.

The tread was not exactly deep on them to begin with, but all the rough handling, skidding, and serendipitous toe angle of the kart has essentially vaporized the tread off the tires. The rubber thickness is still adequate, but I just shouldn’t be driving in the rain.

Then again, there are bigger problems to expect if I try to drive in the rain.

Integrated wheel-o-brake hub, to be made from hugeluminum round stock. It carries the bolt pattern for the cheap wagon wheels on one side and the brake disc on the other, and is bored for a .5″ bore R8 type ball bearing. I tried my best to perform an interpolation of the intended bolt circle diameter using the three bolt holes on the brake disc.

Here’s the hugeluminum billet in question, set up in a position ripe for disaster. Real machinists and South Bend lathe lovers avert thine eyes.

I needed to turn this billet into two smaller billets, but our horizontal bandsaw was broken, the N51 auto shop’s was optimized for steel cutting, and I was not going to wrestle this through a conventional bandsaw. The last option was chucking it in the lathe and parting down the middle, which filled my imagination with vivid images of tooling setup explosions and broken back gear teeth.

However, with judicious use of centers and power crossfeed, disaster was averted, and I had two equally sized not-hugeluminum billets.

A little while later, a hub emerges. The bolt circle was drilled using my handy dandy indexing fixture.

Test mounting everything. Surprisingly, taking the average of the three bolt radii resulted in a disc that was centered with minimal wobble. It almost makes me think they did it on purpose or something…

Nah.

It’s time for Pretend-O-Brake. Here is a setup testing prospective brake caliper mount positions. The brake caliper was designed by real engineers, so there’s not a single straight line on it to reference dimensions from. Mounting it would be a nontrivial matter, so I decided to resort to some cheating in the form of the abrasive waterjet.

I designed a caliper mount based off existing part dimensions and alot of caliper-balling (eyeballing the dimension measured from an imaginary line projected off the end of your caliper tips, directed towards the feature in question).

Here’s the designed part. Just for kicks, I threw it into Inventor’s built-in ANSYS stress analysis add-on to see theoretically what might happen if I brake too hard. The verdict is that I could make this part out of jello and still have it be able to lock up the front wheels and skid.

Alright, so not jello, but at least birch plywood.

The mounting points for the calipers are slotted such that I have an ability to adjust them a small amount if I found that caliper-balling wasn’t enough.

A few hours later, parts cut out of some leftover half-inch aluminum. Abrasive waterjets are beautiful things.

You know that extra hole next to one of the caliper mounting slots? That was originally for a design-on-the-fly widget to connect the caliper mount to the steering pivot block. However, when I performed a test fit, I realized that I could just cut out a step in the caliper mount and have it slide over the block in question. Square objects cannot rotate over eachother by nature, and the Nut of Wheel-Retaining will hold everything in place.

Well then. Cutting down a portion of thickness is certainly easier than making two more whole parts.

The end result is a quasi-floating brake caliper. Better than a fixed one, IMO, in taking up for the lack of alignment inherent in stuff I build.

All mounted up. The spacer length between the wheel-o-brake and the steering pivot block required a bit of trial and error to get right, but once everything was cranked down, the assembly was solid.

Another view of the assembly, all cabled up.

And now duplicate for the other side, accounting for chirality.

It turns out that real brakes stop moving objects substantially better. I was able to lock up the front wheels and skid during runs in the hallway – sort of the opposite extreme of not being to stop at all, but at least I have the choice of locking up or not. The tires have substantially more traction outside on concrete, and I was not able to lock up (without stomping excessively hard), but that’s a good thing. Stopping distance from top speed was reduced to “under the length of the N52 parking lot”, scientific tests be damned.

Overall, I consider adding brakes to the kart a great success.

Guess what? It’s SWAPFEST time! LOLrioKart has been the unofficial promotional vehicle for Swapfest since May. This time, I could show up and have a fighting chance at not extensively damaging property or causing wanton personal injury.

Then I

Well, I sure as hell didn’t, but something did. I put the kart in a Prominent Advertising Position™, then went around to gather cruft. When I returned to start putzing it around, this happened.

As soon as the battery switch was engaged, the precharge resistor set on fire. This tells me that the ESC is stuck wide open, and so the Etek tries to drink all few hundred amps of its stall current through a 50 ohm, 1 watt straw. It, in turn, does not last long.

Because the ESC was damaged in the ON position, I elected to not hit the contactor button. At that exact moment, it would have taken off and landed in a pile of server parts. Servers are fundamentally more expensive than anything on this vehicle.

And thus I kept the kart off until I grabbed a friend and rolled it back into MITERS. I have not yet opened up the electrics to see what went wrong (besides it existing in the first place), but my suspicion is on the gate driver again.

Anyone have a real, 60 volt commercial DC motor controller of over 300 amps capacity they want to donate to the cause?

Swapfest finds

Swapfest is always an interesting adventure because of the variety of people it brings. By variety, I mean old ham radio enthusiasts. However, the distribution of cruft and oddities is quite Gaussian in nature. There’s tons of the usual – electronics supplies, small discrete components, computer parts. A steady amount of the esoteric but not out of the ordinary, such as vacuum tubes, antique radio equipment, and random shit from someone’s attic/basement/garage/hole-in-the-ground. But every once in a while, you stumble upon something that is so weird or awesome that “Holy Iridium Jesus” is the only proper response.

This is one of those finds. I was told that it is a military aircraft radio component of some sort, an early form of spread spectrum radio called a data translator. I prefer to call it AWESOMSESAUCE.

I didn’t have a real camera available, so the multi-kilopixel cell phone camera has to do. But I think the astonishing engineering detail is visible even from here. The thing is packed solid with conductors, tubes, motorized digital-to-analog converters, and crazy components that I don’t even know the function of. Seriously. A motorized DAC. It even has NUVISTORS. I don’t even know what the hell NUVISTOR is, but it sounds badass.

This is from an era when mechanical engineering and electrical engineering were truly intertwined and engineers had to have a deep understanding of eachothers’ practice. This is not like “mechanical engineers design a structure and the electrical guys slide a board in”. This is “your product is so incredibly part-dense that your fucking components are structural members.

I stand by my position that old people, no matter how weird they smell, are more hardcore than my current generation will ever be.

The only specs that my 3G-enabled friend could dig up was this milspec, which doesn’t really say anything besides “register for our website”. Anyone know what on earth it is?

At the $60 quote price, I was tempted to buy it just so I could put it in a display case. But a display case hardcore enough for this thing would have to be made from cast magnesium with solid hand-refined fused quartz windows and lit by radioactive phosphorescent compounds.

Anyway, to return to Earth, I got a few things that I could, you know, actually use. Past the supply refills for MITERS, I got this box of giant transistors.

A scan of the datasheet when I returned ousted them not as FETs, but IGBTs. Because the ginormoFETs that I bought at Swapfest the last few times were dubbed Swapfets, these are now Swapbutts, because IGBT is only properly pronounced “igbutt”.

The total count is

3x 1000v, 200A

1x 1200V, 150A

and a single 2000 volt, 300 amp unit. H00t.

I also got a Hot Wheels RADAR gun. This is a real RADAR gun, but with reduced power and sensitivity so small children can bite it.

Maybe now I can actually find out how fast I go? It’s also fairly hackable.

The Neverending Tragedy of the LOLrioKart

chorl

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.