Category: MIT & Boston

Anything that happens in MIT and beyond.

A Preview of 2.00Gokart and Finishing BurnoutChibi

chorl

With the semester winding down (or, perhaps, finally ramping up!), many of the 2.00gokarts are in the process of being wired up and tested. The final product is due next week, and our competition (last year’s video)  is on May 5th!

Some of the students have been industrious and scheduled their checkoffs and inspections early. Here’s a preview of the action that will unfold in a much larger space next week:

Because conventional controls and riding postures are for wussies, apparently. I’m both amused and somewhat terrified at the prospect of there being three (out of eight) karts in which you ride head first. As it was my stated mission to not interfere much with the design and construction of the karts to let students experience as much of the design process, I might have to start padding BurnoutChibi and run interception for wayward karts.

Speaking of which…

Here’s a picture of the aftermath of BurnoutChibi’s motor detonation. As I would later find out, the sparks seen in the video were not the magnets grinding on the can, but rather them cutting up the phase wires.

Here’s a better picture of the ownage. The red wire, in particular, was cut almost all the way through. The annoying thing about this is that the wires were so close to the stator. If they were further out, patching would be a simpler job. I’d have to loosen the epoxy holding the wire stubs in place and also trim the heat shrink selectively.

While I await better motors, i decided to try and repair these. First step was to pop them open. There is a front retaining ring that comes out, then 2 set screws loosen up to free the shaft from the rotor. Then it’s a matter of pushing the shaft out to the right in the picture – this step was done on an arbor press.

Ouch. In total, five magnets broke loose. I figure this must have been a chain reaction where one magnet ditched first, and the resultant imbalance caused can deformations which broke the rest loose.

This is why I recommend motors that have “rotor bearings” or “skirt bearings” to everyone who asks me about them for vehicle apps. Even though it adds a little drag, the distal end of the can is properly supported on its own bearing. The only exception is if the motor is very short, like a more “pancake” style design.

I mixed up a generous dose of long-cure epoxy with glass microsphere (microballoon) filler, to slightly under nutella-like consistency. The offending magnets were pried out, the mating surfaces cleaned, then this epoxy smeared into the new joint. I replaced the magnets and used as much of the remaining epoxy as possible to completely fill in the gaps between them.

Evidently, I didn’t add enough microballoons, as the mixture did sag a little. To keep the cure symmetric, I actually chucked this thing into Tinylathe and ran it on a very low speed for several hours.

After the mixture was firm (but not cured), I set it on a radiator to cure with heat. Luckily for me, the radiators in the building were still on; they were switched off successively as recently as 2 days ago!

I didn’t get a good picture of the wiring repair before, but it basically involved exactly what I described before – carefully scraping away the heat shrink tubing to expose as much wire as possible. The wire was actually all magnet wire, so it would have been difficult to solder. To combat this, I “frayed” each lead as much as possible to expose the maximum amount of magnet wire surface area. Then I cranked the 80W soldering iron up all the way to 850 fahrenheit and literally burned away the enamel by embedding the frayed ends in a big ball of solder for heat transfer.

I think I managed to get back 75% of the red lead. The rest were patched similarly, but did not need as drastic soldering measures.

After the real epoxy fully cured, I reassembled the motor and crammed it back into the left side transmission.

I have yet to ditch a single magnet. Though I figure it’s only a matter of time before the right side lets go…

And with that, BurnoutChibi is ready to lasso its rogue… brethren? Bastard children? Offspring conceived via assistive reproduction technologies? Something. The only thing it does not do very well, sadly, is burnouts! Because the rider weight is basically square in the frame, and is up so high, it really just like to drag the front wheels along even if I’m holding the brakes. The same reason contributes to its severe power understeer (and associated lift-off oversteer!) behavior. Oh well…

What’s Happening in 2.00gokart?

chorl

It’s been a month or so since my last post about my personal undergraduate victory garden, “2.00gokart”. At that point, nobody’s really assembled anything or completed their designs yet. That’s all changed. Here’s what’s going on now, and what will happen in the next few weeks!

Chaos and half-assembled karts is the law of the land as the “Milestone 7” checkoff and inspection draws near. This is a full “rolling frame” demonstration – brakes must work and steering must be hooked up and functional. The vehicle mechanicals do not have to be final, but to get to this point, it’s sort of implied. Hacking systems together to pass the inspection was discouraged, and nobody really tried to push anything sketchy. After MS7, the only tasks remaining should be to finish up electrical assembly.

At this point, things were being raised onto wheels and the true creativity of the students began to show.

This thing, for instance, had a custom wooden coachwork/centerpiece which was partially CNC router machined and also featured living hinges laser-cut into thin plywood.  On top of all that, the steering isn’t a normal wheel or handle, but it’s tilt based. The builders, Nelson and Carolyn, both have blogs and I swear they add content more often than I do. This creation has been officially named Zoran, which I briefly confused with Zorak.

Some of these things are conventional, others just a little off the wall, and still others… well, they have interesting operating postures. I’m kind of glad to see that nobody is really building a normal 4 wheel kart – there’s really only one. But hey, that’s not bad either – last year, the most normal and innocuous vehicle was done on time, on budget, exactly as described, and performed reliably.

Other vehicles are defined by a central feature, such as Dat Wheel.  That’s a 18″ lawn tractor tire that the team specified off Surplus Center. You’d  basically be sitting directly over it.

So that’s a sampler of the oddball creations coming out of this crew of MechE sophomores. What’s next for everyone is putting electrical systems together. I’ve been giving short “mini lectures” about places to get electrical parts such as switches and contactors, and also good wiring practices and other safety-related device (such as in the background of that picture – no male-side connectors allowed on batteries!)

In about 3.5 weeks time, everyone will once again (hopefully) fly down the service road and up the garage. This year is going to be tremendous.

On my end of things, I’ve been recently tasked with creating a relevant homework assignment for the EV students. One of the homework assignments for 2.007 proper relies significantly on you having built a competition robot. Since none of the alternate lab section students have, everyone was a little distressed. I therefore had to invent an alternative.

My alternative is a miniature (1-3 amps) current mode controller for a DC motor. That’s right, your homework is to torque control. I created the hardware hookup most of the way, requiring students to read a datasheet or two to discern what else needs to be hooked up, and also how to read the DC current sensor. The half bridge is standard fare for me – an IRS21844 gate driver hammering on some obsolete but sufficient IRF2807 N-channel FETs. The whole rig is synchronous rectified by nature. Really, this can be scaled up 100 times and be fully legitimate plus or minus some power supply changes.

Once the students finish said homework, I’ll release the working code & “class solution”, which is the most documented and commented thing I’ve ever written, as a general resource.

Here’s a picture of testing the whole setup, with a DC motor plant, ammeter to verify output, and a battery which can take regeneration current. Power supplies may be safer to hand off to a fuzzy duckling, but they cannot handle regeneration current, so a 10 amp fuse in the test battery will have to prevent things from going too awry. The controller is based off a simple integral-only (i.e. ramping) loop, which is more than suitable for driving highly inertial, dynamic loads like a vehicle.

So that’s explanation for the video last week! What really happened there was that I was pushing 15 amps (at 20 volts or so) into the field winding of the large blue motor (it’s a “separately excited” DC motor, so the mini half bridge board could crank 5 amps (briefly – I raised the limit just for the video) into it and have it just barely spin up. No, it is not creating perpetual energy. Without the torque control loop, it would have grenaded instantly.

From here out, I’m only bumping the students in the right direction to finish, and also making sure the appropriate paperwork and signoffs are in order for the final contest. In other words, “Wait, you want to do what with the garage?” “Yeah, we did it last year.”