Make-a-Segbearshark: Random Updates

With the holiday and end-of-year business shutdowns finally ending, the steady trickle of parts shipments for Landmelonsharkpigbeartankboard is flowing once again. I’ve finally gotten my trippy PCBs in for Make-a-Bot too, but haven’t gotten the chance to make the heat spreading plate and test it yet. Otherwise, I got Segfault running once again, now with its own enormously overkill battery.

Overall not much to say, so let’s just start with the grocery list.


LBS is still a pile of parts that has been steadily increasing in size. The materials needed to start the entire project off are the aluminum plates, which have yet to arrive. Otherwise, I have essentially everything – motors, motor-side sprockets, chains and links, the four shock bodies, a whole mess of stainless steel hardware, most other drive components, and this cute little contactor.

Here’s what the whole mess looks like right now…

Still on the way for whatever reason are three 24″ x 24″ x 1/4″ aluminum slabs. Other frame materials are used in small enough quantities to just be scrounged.

The design has been filled out with the requisite t-nuts needed to hold the panels together.

Some minor touches are missing, including places to mount the rider-sensing switches. The contactor and other major electrical components are also homeless at the moment. I also need to make the “second deck” of electronics which will handle tasks other than motor control. I did collect a model of the Giant Red Key Switch, and it hangs out in the back.

If I’m lucky, the metal will arrive tomorrow and the frame can be cut out by the weekend. That’s really the only hard part.

(Okay, minus the electronics…)


At last, the trippy PCB heaters!


I made one change from the version I keep linking to – there’s a center hole in the board so I can wedge a thermistor between it and the aluminum heat spreader. You know, so I can actually find out the temperature of the working surface. Otherwise, the trace resistance checks out (the squiggles on the bottom side in the design were put there in case they did not…) I’ll need to cut a single square of aluminum for the heat spreader. The aluminum will then be thermal-epoxied (not bolted or sandwiched) to the top of the board.

I got two boards, but will only prepare one of them for now.


Poor Segfault.

No, I haven’t completely trashed it yet again. It’s been working, but always became weak after 30 minutes or so because all I had in it were two of Überclocker’s packs. It would usually just fall over after an hour. So after the term ended, I swore I would make a new battery just for it such that I can reliably bring it out for demos. Naturally, with the ennui of the break, I felt unmotivated to do anything. Additionally, during an unfortunate scooter-organizing incident, the cable leading to the control knobs was sheared off, so it was just one more impediment and grunt-work repair job I had to tackle before it could even work again.

So maybe I did completely trash it. Either way, I guess it counts as New Year’s resolution to repair Segfault? I’m not sure.

Here’s where it starts.

Ah, another brick of A123 26650 lithium nanophosphate cells; here, being prepared and tinned.

Segfault’s completely empty right side was just begging for a brutally large battery. I measured everything out and found that I could easily fit a 5 x 7 cell array. Since Segfault already demonstrated operation on 7S (about 23 volts), I’d have to make a pack that had 5 cells in parallel. This is more or less a 146% \m/etalpaKkK. With 5 2.2 amp-hour cells in parallel, the total watthours count of this pack comes out to be around 250. It ought to be enough to keep Segfault running for two hours or more.

The enormous \m/etalbraid makes a return on this pack. Grounding braid is now my staple “battery bar”, as shown by the \m/etalpaxXx themselves and RazEr rEVolution’s pack (and the Clockerpacks, and the monstrosity I made for Cold Arbor). Segfault will never draw enough current to overload these busbars, but hey – maybe one day this thing will be repurposed. Have to plan ahead, you know.

Soldering is discouraged on cells like this because of the risk of melting the polymer separator close to the terminal, which results in bad. If you’re very fast and have a soldering iron with a large tip (high thermal mass, effective thermal bath), it’s definitely possible. I stuck to my 3-second rule here – once the joint starts melting, I count 3 seconds to smash it down and add more solder. Once that time is up, I immediately move on to another cell, and don’t return to that one until I’ve visited the rest of the pack.

It’s probably not very legit, but I haven’t overheated a cell yet…

After the whole pack got busbraided, it was time to add the wires. There’s three heavy-gauge wire pairs coming out of the pack this time. The two off to the right interface with the existing double battery connector in Segfault. There’s no reason to have a double connector in the thing, but it’s the way I originally made it to accept the robot batteries.

The single cable to the left is only used for charging. But I guess it could be a third discharge port if needed.

I ordered JST-XH connectors in several different sizes from Digi-key, so I was actually able to make a legitimate balancing harness.

Now the fun part begins: Packaging the whole thing.


So there’s no soda bottle big enough in this world (please prove me wrong) to swallow up 5 cell wide rows. The \m/etalpaxXx required a 3-liter soda bottle, and they were only 4-parallel groups. And while I could have planned ahead and ordered wide heatshrink tubing, that just doesn’t work with how I like to build these things – i.e. right now.

I did, however, buy a six inch wide roll of Kapton (Crapton, since it was from a Chinese ebay seller, and doubtlessly not real DuPont polyimide film) for Make-A-Bot’s future build surface. So I decided to just give it a try with finishing the packs up. With some cut up sheets of adhesive-backed foam rubber fitted on the cells for shock isolation, I wrapped the Giant Crapton around the whole pack several times in two separate loops. I think it came out great. The tape doesn’t really stretch, so it doesn’t look as “heat-shrinky”, and I wouldn’t say it’s waterproof. But it got the job done.

After a brief interlude to reconnect six little wires, Segfault is now once again attempting to kill innocent riders.

Segfault Re/BOOTED Part 3: How Seggfault is Formed

The first part of this Segfault update, and the second, are linked herein respectively. Whoa… can I actually get back to building things now? The last state that Segfault was left in mechanically consisted of two stripped drive motors and a few banged up bits of sheet metal (well, and the loss of two “frame standoffs” on the right side).  Before I could use it again, I would have to replace those drive motors with something else.

The answer was found hiding in a closet at the MIT Edgerton Center. These are 27:1 Banebots P80 gearmotors, similar to ones formerly provided with FIRST kits of parts. The new P80s are pretty legitimate in terms of construction and materials used, and I rather like them. Only problem is, they don’t share the same mounting pattern as the KOOOOLmorgen pancake gearmotors. That was solved rather quickly with a waterjet-cut adapter plate made from frame scrap.

The hole that passed the Kollmorgen motors didn’t fit around the square diagonal of the Banebots. I got out the killing-head and opened up the circle another quarter inch or so. (That’s actually a boring head, but with that giant slab of tool steel mounted in there, it can only kill things and ruin livelihoods.)

A few selective spacers later and the motors once again drop into place in the subframe. Since the wheel hubs and shaft diameters match, I didn’t have to process the motors any further. And thus Segfault was pitched back together.

back to course VI

Two weeks ago (or thereabouts), I sent out the locked-antiphase/class-D motor driver H bridges to Advanced We’re So Awesome We Send You Free Food Circuits on the quick-and-dirty turnaround option.

They really are quite cute. I did these up all through-hole this time around because of component availability (no guarantees on finding all the SMT values and packages I needed in the rush schedule). I had a few IR21844 DIP chips left over, and they found homes on these boards.

Of course, every rush-sleep-deprivation-designed short-turnaround board is bound to have some kind of boneheaded mistake in it. In my case, it was accidentally switching the high side source (/half bridge midpoint) and the high side gate drive supply connections and not noticing until I wondered why my 15v gate drive supply was directly accessible through said phase midpoint. A few chops with a knife and then some Little Blue(and yellow, and purple) Wire followup fixed that.

I began putting together the new filter board alongside the motor drivers. This time, instead of using my custom 555 negative-5-voltificalator, I purchased some cool 24-to-15 volt DC/DC converter modules that gave isolated +/- 15 volt rails. Unfortunately, I found out the hard way that the LMC6484 quad op amps couldn’t handle a whole 30 volts across the power supply pins, so I ended up having to down-regulate both sides to +/- 5 volts. There is also a 3.3v LDO regulator just to feed the sensors.

Motor drivers temporarily hooked up for testing. Consensus: Yeah, I guess they work. Actually, it wasn’t quite that easy. Because of my isolated power supply on the front end, the gate drive chips couldn’t get a common ground between the input and output sides. I had to jump the “VSS” and “COM” pins on the 21844 to make them actually turn on. But wait… what on earth are those wires on the op amps?

I had to wing-jump +/- 5 volts to the LMC6484 on the motor drivers. This chip serves as the 21kHz triangle wave oscillator for the PWM outputs. Unfortunately, moving the supply rails way closer and changing the “divider” value together reduced the frequency to a paltry 6.2kHz. That’s the constant ear-piercing noise in the video.

Concurrent to the electronics assembly process was the repair of the control panel. Previously, this had been wired up to give voltages out at each of the potentiometers. Since the new controller needed the resistance of the pots to control the PI & K gains, I had to perform some air-wiring on the 10 pin cable.

After bench-testing the motor drivers, it was time to start hooking up the Serious Power wiring. I just reused most of what was already in Segfault for this one. The controllers themselves have already been super-professionally mounted using chunks of velcro.

The 100th build report of Segfault is a picture of me taking everything back out after checking for wire length and bench-testing the controller with all the gain knobs hooked up, on live power.  A small test motor was used initially since the power supplies could only push about 5 amps max, and the Banebots motors don’t get up in the morning for anything less than like 40. You can really see the difference between the AFFC and the old controller. Overall, the layout is much cleaner. Granted I did move alot of the H-bridge control circuitry onto their own PCBs…

With temporary Clockerpaxxx dropped into place. I’ll be making a more permanent 7 or 8S pack array for Segfault later. For now, the two robot packs run it just fine for about half an hourt of net run time.

And we’re ready to roll. We’ve already seen what happened after this. By the end of the day of demoing and test riding, I had taken out 2.1 out of the nominal 2.3 amp-hours in each battery. These things were toast – it was hitting the 18 volt cutoff of the DC/DC converter, so near the end of the day, Segfault would occasionally just fall over dead.

A1234 cells being amazing as always, they charged right back up to take in 2.1 amp hours…. at a rate of 9 amps


end of segfault

Well, not truly the end. But now that the project is demonstrably working, I’m ready to move on. I’ll be making the bigger battery pack and possibly making the control mounting more robust, but otherwise, I’m going to consider Segfault a (finally) finished tale after a year of head-scratching.