Pan-Project Update: Little things here and there

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Not much has happened in the last week or so with regards to anything reaching completion. Sadly enough, not even the melontank has escaped the wrath of final semester. I’ve been filling in some minor details on the vehicles, though, in part preparation for the Energy Night Showcase, which I’m attending with some cohorts mostly as an excuse to ride scooters around to annoy the staff, and partly because said scooters is actually a hardware display at the event.

razEr rEVolution

Ever since getting back from Singapore, RazEr has just been kind of hanging out around MITERS giving curious onlookers test rides. One day, however, it just stopped. I don’t recall if it was on acceleration or deceleration, but the entire controller simply shut off. Not in flames, to my utter surprise. Further investigation revealed that the ATMEGA328 chip that is the core of the controller just straight up died. I don’t know what the cause could have been to completely kill a microcontroller besides voltage transients above its maximum voltage – after all, the logic voltage is fed by the gate drive voltage, which is one step closer to the very noisy and high current battery rail. And I don’t use a hardcore switching voltage regulator like some other people do… rather, a completely rigged linear voltage regulator fed by a resistor.

So instead of, you know, fixing the thing to use a real voltage regulator, I decided to fix the symptom for now:

Dropped (almost literally) on the Arduino carrier board is a 5v transient voltage suppressing diode, and right next to it is a 100uF 1206 capacitor. Seriously – 100uF in 1206? What is wrong with the world?

The addition of massive buscap and a TVS should absorb any transients on the logic rail in the future. I’m saving the real parts for the full v2 redesign, for which I have additional upgrades in mind.

Another issue that Razer had faced before its untimely demise was some kind of strange, no doubt current induced behavior where the motor operation would become very unstable if full throttle (or really anything above mid-throttle) was applied. The symptom manifested itself as a sudden loss of torque, almost like the motor was spinning off a clutch somewhere and little torque was making it to the wheel. Now, I know I slammed that motor can together on a 20 ton arbor press, so there’s no way it could have been mechanical. Further investigations into the gate drive voltage revealed no significant “early shutoff” of the high side bootstrapped FETs, which could cause such behavior past a certain PWM duty cycle. I was out of ideas, so on a wild suggestion, I made a common-mode choke out of a nut and a few turns of 14 gauge wire.

It solved everything.

I’m not even going to ask. The Common Mode Nut will, for now, become a permanent feature on RazEr. It allows me to floor it with reckless abandon down the hallway (shortly before discovering the hard 90A urethane wheels, covered in floor dust, have zero traction in the waxed hallway corners).

landmelonbearsharktank

Not much news here, but I finished mounting the sprockets!

I made the spacers in the Pappalardo Laboratory, during my 2.007 lab assistant session. I decided to tow the entire thing to campus in order to do this, and the other students taking the class found it amusing that such a thing could possibly exist. Mission accomplished.

make-a-bot

Unrelated to the showcase, but still worth an update, is the death and revival of MaB. I had run out of PLA plastic (which sticks to everything) by printing like 8 versions of Chuckranoplan, and needed to revert back to my stock of ABS plastic. My surface heater had shed its thermistor a while back for whatever reason, meaning it couldn’t be software-controlled, and getting ABS to stick to cold plastic was a difficult affair. So I did what any reasonably intelligent person would do – hotwire the heater to 12 volts.

Backwards.

Luckily, I realized how backwards it was after a split second, so there was no cascading destruction of the entire electrical system, but it was long enough to bake the entire bank of ADC inputs on the extruder controller.

It took a while and alot of help from people with more patience for SMT soldering, but with the AWESOME MITERS HOT AIR REWORK STATION!!!! the ATMEGA168 on the extruder controller was replaced, and MaB was operational again.

While it was away, I got a reel of white ABS instead:

I like white ABS alot better. Everything doesn’t come off as a featureless black blob, and it changes color slightly dependent on temperature and how long it’s been cooking in the nozzle, so it’s one way to gauge if my temps are wrong within a wide band of errors (No, they’re not.) For some reason, it doesn’t smell as death-filled as black ABS. However, it does seem to lose a little more volume in the extrusion process for whatever reason.

That’s all for now. If anyone else is going to the Showcase, be mindful that I might broadside you at full speed on RazEr.

i’ll just leave this here.

Land-Bear-Shark: Beasting Everything Ever

chorl

At the start of today, LBS was a small pile on the ground whose satellite components were slowly being diffused into the MITERscape.

At the end of today, I’m proud to say that LBS has reached criticality (in this case defined as when a project can support its own weight and leftover parts). It’s not rideable yet, and it’s still missing a control system, but mechanically speaking it’s pretty much there. I would like to thank… uhhh, the waterjet, t-nuts, my parents, and… wait, what?

Anyways.

First order of business was to get the missing parts cut. Or, in the case of the front and rear skid plate, recut. I really have no clue where they ended up. But, I found a piece of plate aluminum that had their exact profiles missing, so I must have made them once before.

I’ll probably discover them in a few weeks and use it as an excuse to commission a second LBS.

The biscuit plates are the axle mounts that turned up short last time because I forgot what symmetry was.

Assembly now begins for real. I bolted in the shafts (which are stationary spindle pins) into their respective side plates, then mounted the resprung shocks. The large center 20mm axle was trimmed to length on an abrasive cutoff saw (the temperature decoloration is visible at the bottom there). The shocks themselves are secured using shoulder screws.

Slipping the tracks on was a matter of pulling things the right way. I had to loosen the front tensioner axle in order to slide the whole assembly on. Axial alignment of the plates was done by tightening down all the axle screws, which forced the plates to be 6 inches exactly apart and planarly aligned with eachother, then applying the shaft collars to the center pivot axle to keep them in place.

It turned out that “Bottomed out” was a good tension setting for the axles, so I left them there and secured the pins with 262 flavor Loctite.

Here’s a view from the other side, showing the sprocket spacers and the shaft collar arrangement. Most of the spacerwork was Dynamically Generated on the lathe using 1″ delrin stock.

After rinsing and repeating for the other side, accounting for chirality (oooops… let’s try that again):

Hey, this could totally be a kickass 60lb robot.

At this point, I took it for a leg-powered joyride down the hallway, almost running down some MIT Museum tourists in the process. For having tank treads, the thing glides pretty far. The treads were, of course, “set” into a favorite position from being parked in someone’s back yard for half a decade.

The rubber was dry and cracked in many places, so I began an emergency WD-40 marination treatment. The solvents in WD-40 penetrate the cracks and the rubber surface, softening it and infusing it with the oil component. A more legit method is making a tank tread and kerosene soup, but I’m not that hardcore (and had a can of WD).

While the treads were exfoliating, I started stripping down the skateboard. The very rusted truck attachment hardware took alot of effort and careful force application to remove. I also decided to try and repair the splintering front end of the board by filling the gaping holes with wood glue and epoxy. That’s what those clamps are holding together.

The new deck hangers (planters?)  are now attached. There’s a 0.400″ thick UHMW plastic block separating the bottom of the board from the hangers, since I needed to slip a bolted joint in that region.

Here’s what the whole system looks like from underneath. The rubber blocks let the board tilt a few degrees to emulate the action of a normal skateboard. It’s not perfect, but I didn’t particularly feel like engineering a linkage to use the existing trucks.

Note that the pivot pin attachment point is slotted. This is to enable the board to travel up and both about 1/8″ or so when the pivot bolt is installed. I decided that this interface would be a good one to put a “rider is still on vehicle” switch  on, since if this thing is going to be wireless, I absolutely do not want runaway conditions. A set of microswitches (which were not thought out well enough to have mounting holes designed) will detect the depression of the pivot in the slot, and interlock actions will be handled in the controller software. It’s as simple as cutting the throttle command or logic power to the motor drivers while opening up the main battery contactor.

As simple.

Watch that come back to haunt me.

Alright, with the board mounted and still drying, I turned my attention back to the motors. Previous rewound but never finished, these things needed wire pigtails and Hall sensors added. Both were easy enough affairs. The hall sensors are current attached using hot glue, which admittedly is not the best of materials for holding things in a motor.

But it was right there.

After more cutting, soldering, gluing, and some hammering too, the genetically modified melon motors get reassembled. I’m not sure how two motors in the same order came with totally different screws, but one was also way harder to put back together than the other. Quarity!

I Dynamically Generated these motor mounting standoffs using 1/2″ aluminum rod stock. The outermost holes in the cruciform motor mounting flange were threaded for 1/4-20 threads. This assembly is less meaty than the enormous billet mount that’s on melon-scooter, but the shorter overhang distance makes up for it.

One of the legs of the cruciform had to be cut off because it interfered with the rotation of the track’s drive sprocket. The nubby things protrude further, it seems, than what I designed for. That and the fact that the holes in the cruciform mount seemed to be hand-punched and were all over the place. It was fine in the CAD model.

And here it is. No sprocket on the motors yet, because I ran out of Beast It points for the day.

Overall, the rocker suspension worked great. The re-sprung shocks are just at the “right” spring rate; perhaps just a little soft, but definitely in the range where jumping up and down causes deformation and springing action, but just standing doesn’t. I wouldn’t mind taking a curbhop with this thing.

The skateboard hangers worked about as well as I expected, which was in fact not at all. Those rubber shock mounts are just too soft, and they deform almost immediately to flat when someone gets on. Maybe if I get ambitious I’ll engineer a torsion spring or leaf spring system, but for now, leaning action (roll axis) is poor but pitching and heaving (forward and backward tilt; up and down motion) are acceptable.

Left to do:

  • Sprockets?
  • Design the double wireless hand controller!
  • Make the double wireless hand controller!
  • Salvage the deprecated Kelly KBS motor drivers from the defunct Citycar model
  • Melontank?