The 2.007 class is structured with 12 weekly “milestones” which students must use their class lab notebooks for and write down their progress, thoughts, calculations, sketches, etc. Some students are detailed or previously experienced in using notebooks / journals (such as from an internship at a company which requires it for engineers), others write down pretty much exactly what the milestone requires and that’s it.
I was definitely part of the latter crowd. It was difficult for a professor to actually squeeze out of me a competent lab notebook of any sort.
To encourage more diversity and accessibility in design documentation, this semester I encouraged people to write about their builds on their personal websites or blogs. Now, the Department™ still requires the paper notebook as part of the grade. But, for the last milestone, a reflection and summary type writeup, I decided to break from that and give the students some flexibility. You now had the option of submitting the final MS as a site entry or blog post.
Here’s who took me up on the offer, and those who’ve had a running log of everything they’ve been doing too!
Almost exactly one year after my post concluding “2.00EV”, which laid out the foundations of what would become “2.00gokart”, I’m glad to say the semester is over and done with, and that it basically went down exactly like that. Sixteen undergraduates in Mechanical Engineering, together with some clowns from EECS, built eight kart-like devices over the course of twelve weeks (so that’s 0.04166 kart-undergrads/week?) , and at the end, like last year, there were games. For the full story of this year’s class, see the three previous relevant posts: Intro, progress update 1, and progress update 2. The bottom line is: $500 budget, some free materials, and practically no design constraints. The class goal is to teach you how to shop for parts and use rapid-prototyping resources that we have to build nice, functional things very quickly. So basically, 2.00-my-Instructable.
The crowd this year ranged from those who had some experience with the “design” part of EV Design from taking Mechanical Engineering’s introductory freshman course, to those who had experience building larger mechanical systems through FIRST, or who were basically pros already. I was afraid a little of going 2-person teams this year, since inevitably you might come to hate your partner, but the group was small enough, and enough of the class knew each other already, to avoid unwanted team dynamics for the most part.
Designwise, it’s particularly worth pointing out that nobody built a “normal” go-kart. By that, I mean 4 wheels, single rear axle, sit-down. There were many “tadpole trike” style designs which tried to save cost and complexity by using one rear wheel, so as to afford a better powertrain. That’s as ‘normal’ as the crowd got, and it only became more exciting from there. “Bobcat” style lever steering was infectious – of the three designs which steered with “rudders” as we called them, nobody used tank steering, but the linkages were hooked up to the front wheels to pivot them. And on top of all that, there were not one but two designs which placed the driver face first, lying down. That’s what I get for setting very few rules about the actual design, I suppose.
This post is just going to be a competition recap – after the previous update, there was only wiring and drive testing involved, most of which was in the parking lot so was not particularly exciting to watch. Skip to the bottom for the highlights video!
The most pressing task on my end in the last few weeks was preparing the defense strategy for the garage events. Last year, in response to safety office concerns about people nosing head first into concrete walls (a real and legitimate problem this year!), we set up those big orange fishing nets construction site debris barriers strung up on steel cables. While they could have been okay for scooters and other situations where the rider is roughly net height, the go-karts would all be too low for them to work (in the worst case they’d just strangle you on the way out). The nets were also a pain to set up, and once they were un-deployed, we really had no hope of ever saving them again since the steel cables became tangled up.
Last year, I bought a few bricks of blow-in natural fiber insulation (made of shredded cloth and paper, more or less) to pad the structural columns of the garage. My goal this year was to convince the safety office that a row of these (soft, fluffy) bricks was all you needed for go-karts. After all, everyone will hit things more or less at the same level this year, and the combined mass of the bricks plus their deformability will do a better job than a slingshot cable-hung net. Fortunately, convincing them was not particularly an issue – I basically got back the professional version of “lol k”.
So up they went. Myself and 3 students, the evening before the competition, set up the garage’s narrow wall end with a discontiguous wall of fluffy bricks. Notice that there’s “panel gaps’ on the order of 5+ feet. It would bad if you approached right at it, but on most floors, people came into this turn very wide, so it was highly unlikely for someone to slip through the gap. Plus, you’d have to be completely ballistic and motionless to not at least try to aim for the soft parts. This solution is much more scalable – setup and teardown took around 30 minutes each.
Besides the narrow end-turn region, a few of them were scattered around the columns as usual. 78 little fluff bricks were used in total, and after seeing how little of the average 14 foot box truck they filled, I’m completely fine with filling in the big gaps on the next runaround.
The morning of the competition, the same box truck was used to haul everyone’s karts over to the contest site.
Well, those who arrived by 9am anyway. The stragglers were forced to push their karts over.
First event of the day was the drag race. As the event organizer, I couldn’t get many pictures of the setup myself, and we were short-staffed enough in the morning that nobody else really got any either. At least there was video.
The two top placers in the drag race were a hundredth of a second apart yet represented opposite ends of the traditional EV design spectrum. One was huge, had giant balloon tires, and two massive DC motors. The other was small, lightweight, and had a single brushless motor.
After the drag races were done, we broke for lunch and then began the garage hoonage that is now a hallmark of silly vehicle activity at MIT. This is a Dramatic Photograph of the drivers’ meeting, basically me telling everyone where the start and finish are, in a protracted fashion.
Recall what the “garage challenge” was. Your total time climbing the four levels *and* your energy consumed (in watt-hours) is recorded. The product of the two is your score (with units Joule*seconds, though we used Wh*s for shorthand). Here’s what the results from 2012 looked like:
It’s basically a measure of how efficient you are at doing things. This actually has a mathematical basis in the concept of action. The closer you are to the original (0,0) of the graph, the less energy you used AND the faster you did it. So, there are actually myriad ways to score in something seemingly as simple as driving in a long upward spiral (for the same driver – the score is, of course, affected strongly by mass). You could run wide open all the time and get the best time, but you might use a ton of battery energy in the process – and hence could lose to someone more middle-of-the-road and who took it easy.
Here’s some photos from the garage, since by this time, more people were awake and ran over to take pictures:
“Supermankart” (which I whimshically termed “Suicidekart #2″) on the last leg of the climb, with the finish line around the corner.
The only thing to resemble a normal 4-wheel kart if it were not for its “reverse lawn tractor” steering patched on during the middle of term, the affectionately named “Beschleunigen Warenkorb” – as far as I understand it means “accelerating shopping basket“.
Suicidekart #1 “RoachKart”, so called because it’s extremely flat and quick, and on top of that seems to like going in random unintended directions.
“MiloKart”, named after a dorm cat with allegedly a very disproportionate rear end. The wheel is an 18″ lawn tractor wheel.
Zoran, the Norse God of Marshmellow Cream Puffs speed, and not to be confused with Zorak. The twobuilders of this machine blog more often than I do.
Bull Kart, nicknamed “lugekart” during the semester, so called because of its two long vertical footrest bars. It was basically a motorized street luge built using longboard components. Here, one of the drivers pulls a tighter-than-usual turn.
“Derpscooter”, one of the two contenders for first place along with RoachKart. The power system of this contraption was a C-8085-180 “short melon” motor with a 150A Kelly controller. It’s the first usage of a melon-class motor with a controller that can actually feed it well – often, people try to use these large motors with small controllers that inevitably detonate or are severely underpowered. Derpscooter edged out Roachkart in both competitions by single significant figures to take the overall win.
Camera car service was provided by shewu operating the DPRC…and an iPhone 5.
What’s left to do after the competition is over? Dispense with the clean, orderly arrangement of loading up and just start piling. This beautiful traditional go-kart-and-fluff arrangement was promptly shat out unceremoniously into our shop building’s loading zone.
This year, unfortunately, due to “No, I do not want to deal with the paperwork of you driving under a bus” reasons, students will not be allowed to keep their vehicles. So the last hurrah for everyone was at the 2.007 main competition:
During the intermission between the elimination and playoff rounds, there was a parade in front of everyone.
I was presented with this set of “nonfuzzy dice”, laser etched with everyone’s creation logos or CAD images, in walnut wood. My students had all heard of mikuvan by now, so I guess some of them banded up to create something for it. These nonfuzzy dice will be mounted proudly in wherever you’re supposed to hang dice that aren’t fuzzy in a car.
And now for the tl;dr: the highlights video from this year!
And here’s the compiled graph of results from this year:
Compared to some historical vehicles:
But wait! There’s more.
the summer session
I’ve been specially requested by the SUTD Collaboration and Mechanical Engineering to run a summer session of this class! 28 Singaporean sophomores and juniors will be coming over to MIT this summer, and they needed something to do. Well, do I have the adventure in stock for them…
The plan for this crew is *eight* teams of three and two additional teams of two for those who are more skilled. The students are less mechanically inclined than my class this semester, and they will have to learn to deal with our curious units system. So, there will be some changes to the course material. The class is only 8 weeks instead of 13, so I’ll have to plan which “milestones” to condense accordingly. And, more materials will be provided, because the impact of having to wait a whole week for a part will be unacceptably great in this timeframe. People will still have the ability to specify their own parts if they want, but otherwise, I’ll have a giant bin of things to pick from.
Stay tuned for the third great race-off at the end of summer.
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…
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!)
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.”