Sunday, March 10, 2013

The Long Overdue Go-kart Post: The Birth of SmartKart

So, I'm probably going to have to give a little bit of background on this one. 2.007 is one of the primary design/build courses for MIT mechanical engineering undergrads, and also served to inspire tons of other robotic competitions across the globe (eg FIRST). Incidentally, the class is also one of the reasons that high-school me was so enraptured by the idea of attending MIT. And now, like so many funny twists that life often brings, I can safely say that I will never take 2.007. I probably wouldn't have believed you if you'd told me that, even just a year ago. But when I found myself in a position to actually sign up for the class, I realized that there was no longer much reason for me to do so. Through clever (not really, this was actually totally accidental) selection of my requirements for my 2-A degree, I'd inadvertently rendered 2.007 no longer necessary for me to graduate. Additionally, by the time I got to the point where I'd be taking 2.007, I felt I'd gathered enough design experience to render the class largely redundant for me. Sorry to disappoint you, high-school me, but it turns out the alternative is even more fun.

And what is this magical alternative, you ask? Well, to be honest, it's not really even a real thing. More just a chain of serendipity leading to me racing around in a go-kart and teaching other MechE's how to build their own. It all pretty much started when I decided that I really wanted to build a hub motor-powered scooter, and solicited advice from MIT's resident silly-hub-motor-powered-thingsCharles. Basically, this initiated a new major interest in electric vehicles for me, and after going to the 2012 Maker Faire last October and witnessing all the crazy go-kart racing, I knew what I had to build next. Fortunately, Charles had run an electric vehicles special section of 2.007 last year, and had plans to bring it back as a more specific go-kart section this spring. I didn't want to wait and take the section for a number of reasons; such as not wanting to wait for a whole semester before starting, as well as not wanting one of my projects to be given all the additional constraints that come along with being part of an academic class. This was about when a rather clever idea came to mind: Charles was a grad student at the time, and an interesting property of grad students is that they're able to acquire UROPs. So, naturally, I decided that I wanted to UROP for Charles and build a go-kart. Conveniently, this actually lined up exactly with what he was looking for at the time: someone to run a sample, 'pilot' of the 2.007 go-kart section (affectionately dubbed '2.00gokart') in order to iron out kinks in the intended curriculum, and give a better idea of what was expected for the course.

Administrative details handled, I pretty much dove right in and started designing. Initially the proposed budget for the course was $300 for the kart (not including batteries, three sticks of 80/20 aluminum extrusion, and 24" x 24" plates of both 1/4" and 1/8" aluminum), so I went with just about the cheapest option for motors I could find: two CIMs. If you did FIRST, you almost certainly remember these. Since the rated 337W max output of the motors sounded a bit low to me, I opted to run them on 24V batteries instead of their rated 12. For reasons that I'm going to try not to go into too much detail about, this effectively bumps their maximum power by a factor of 4, up to 1348W. I whipped up a couple of MATLAB plots to show both conditions:

 As you can see, the motor's power output is the product of its speed*torque, which has a maximum at the halfway point of each. However, owing to the stall current of each motor (133A each at 12V), I'll never actually be able to hit anywhere near their max power output. At 24V, each motor would draw 266A at stall, or 133A at max power output, for a total of 266A for both motors operating at max power. What's the problem with that? Well, that's 1348W*2 = 2696W of power output at the motors, but unfortunately both the batteries I was given and the controller I selected are limited to 100A. 100A*24V = 2400W maximum going into the controller, so I'll never be able to hit 2696W output (not to mention the ~15-20% loss due to resistance in the motor).

Why am I okay with this? Well, the controller is able to limit its output to 100A so nothing in it will explode, meaning I'll be capped at 2400W when I'm getting going. However, as soon as my motors are spinning faster than 556 rad/sec (5310 rpm), I'll be to the right of the max power point, meaning my motors will be outputting less power but operating in a more efficient region (less torque generally means more efficiency, because torque is related to current, power loss is I2*R, so minimizing torque/current means less power loss). How fast do I need to be going to make this happen? Well, I chose a relatively high (fast) gear ratio of 13:70 coming off of my motor, and I'm using 8" diameter wheels. At 556 rad/sec, I'll be going 556 rad/sec * 13/70 * 4 in = 23.5 mph, so any faster than that and my efficiency starts improving.

Okay, enough math, on to CAD:

I started with a frame mock-up, which consisted of spending somewhat inordinate amounts of time slicing up 80/20 into chunks and then mashing it together into a shape I more or less liked (funnily enough, that's a decent summary of the physical build process as well). I knew I wanted to keep the frame fairly short and compact, because I wanted it to be (relatively) portable and lightweight.

Next I threw in motor mounts and brackets:

I used 1/4" plate for all the primary structural frame brackets, because I was worried about 1/8" plates bending. At this point I started building whenever I had parts in and then CADding in parallel. This was probably a poor methodology for design, but I had a fairly complete structure in my head of how the cart would turn out, and certain elements like brake calipers and the seat were impossible to CAD until they were physically in front of me to measure for mounts.

Here's how the final CAD turned out. This assembly doesn't contain a few things, so I'll just go ahead and describe them/mention why they aren't present:
  • Battery/electronics mounts, because I quickly whipped them up in 2D and didn't update the CAD with them (mostly laziness here)
  • Steering wheel, same as above
  • Seat mounts, because the seat was rather organically shaped which forced me to produce a mount by hand, as such it was more something that I just threw together rather than something meticulously planned.
So, on to building! I started out by cutting some of the 80/20 frame elements on the MITER saw.

I then got my CIM motors in, which ship with a keyed shaft:

In order to interface this with the small, smooth-bore drive sprocket I bought from McMaster, I had to mill a flat on the shaft to convert it into a D-shaft.

Afterwards, it looked like this:

Next was similar treatment for the sprockets themselves - I drilled out the 1/4" bore to 8mm to match the 8mm shaft of the CIM motors, and I drilled and tapped a set screw hole.

And here it is fixed on the motor shaft:

Next, I waterjet out the preliminary mounting brackets, cut some more 80/20 lengths, and just started throwing screws into things.

Because I had designed (and actually cut out, which was a mistake) the aluminum sprockets prior to getting my Harbor Freight wheels shipped in, I estimated the hole pattern as a 2.5" diameter when in fact it's actually 2.6". Accordingly, there was a bit of this involved:

As the waterjet can only cut parts in 2D, I also had to chamfer the edges of the sprocket to allow the chain edges to slide over it. I just eyeballed this one on the MITERS lathe.

Next, I had to part and tap a bunch of little spacer doohickeys to attach the sprockets to the wheels.

I also made some delrin spacers to axially support my wheel on one side, they have a chunk milled out of them so they can slide under the bar of 80/20 that the axle mounts are attached to.

And a shot of the part in action:

The chain was pretty sticky on my waterjet sprockets, so I went ahead and tensioned them and ran them in for 15-20 minutes each, just to get the chain to grind down the interfering aluminum on the sprocket.

Somewhere along the line, boredom and lack of a front half of the vehicle led to a rather monstrous contraption, with the combat robot Null Hypothesis serving solely as a temporary motor controller:

Then I waited a couple weeks and focused on Blizcopter while I waited for parts to ship in. I also made another giant waterjetting run, resulting in a pretty massive pile of parts.

I had to make some real tiny Delrin bore adapters for my steering knuckles, which honestly scared me a bit to turn because of how thin/fragile they were.

My waterjet holes on the front of the steering knuckles also didn't quite fit the 5/8" shaft I was using because it was about .005" over diameter, coupled with the fact that waterjet holes end up a little smaller than designed. So, naturally, the solution is to find a pointy 5/8" thing and just beast the hole wider (the shop I was in at the time definitely doesn't carry 5/8" drill bits).

And here's what the steering knuckles look like when mounted on the frame:

I then hack-sawed some threaded rod to put in the tie rods driving the steering linkage, and mounted front wheels.

Unfortunately the rest of my 5/8" stock didn't quite fit inside the bearings I'd bought for my steering linkage, so I had to move to the GIANT lathe downstairs and turn down the whole thing by just a few thousandths.

Seriously, this thing is massive. That shaft is easily 2.5' long.
Now the basic frame was close to complete! Obligatory celebration dance:

As you can see, the wheelbase is pretty miniscule.
Next, I needed a lot of T-plates to set up the attachment for the seat to the frame, and I didn't have enough patience to go waterjet these. So, I enlisted the help of a friend, and we started beasting out mount plates by hand. I started by cutting out some squares, clamping them together, tracing out the outline of the part, marking holes, and then drilling them out:

Next I bandsawed off the excess.

I went ahead and drilled some holes through the bottom of the seat and attached them to the tops of some 80/20 mounted to the brackets I'd just made. I also threw the steering 'wheel' (lol) on top of the steering rod, but it wasn't fully mounted at this point.

The seat was super wobbly in left/right bending, so I just attached a giant plate of 1/4" polycarbonate (yeah, that bullet-proof glass stuff) to the back to support it better. I also lasercut some quick acrylic battery mounts, and got started on wiring up the electronics.

Shh, don't tell the shop guys I did this...
I fit a laser-cut acrylic electronics mount into the center of the kart, where it holds the motor controller and ignition key.

I went ahead and installed some requisite glow-lighting, finished up the wiring, and was left with a reasonably functional go-kart:

Done? Not quite, there are still brakes to worry about. I cut out a cute little brake-pedal-mount-box, accidentally failed the clearance on the sides a little bit, and milled them out a touch.

I turned some little spacers to go between the brake discs and my sprockets, drilled more holes in the sprockets (same process as last time) and attached them together:

I still had some slight clearance problems with the brake calipers, so I had to dremel the face down just a little bit.

And a close-up showing the brake caliper mounted, as well as exactly where it was hitting the chain:

I also finished the brake pedal and added that to the front.

So, the state the kart is currently in:

And of course, obligatory underglow picture.

As you can see, the original front bumper I designed hasn't been fabricated yet because it's not essential to the function of the kart, and I haven't gotten a chance to go waterjet the mounts for it yet.

I decided to dub this kart the SmartKart, owing to its short wheelbase and passing resemblance to the similarly named car. The top speed is a calculated ~33mph, unfortunately the range is rather lacking as it only has two batteries, so it's only able to run for a continuous 5-10 minutes before running out of battery. Overall I'm pretty happy with how the project came out, and I'm looking forward to 'back-modding' it a bit now that my original design constraints (pretending to be in 2.00gokart) have been alleviated. It's not the most practical project, but it's a lot of fun to joyride in, and the tight turning radius makes drifting pretty easy. Now that the weather's picking up, I'll try to take it garaging sometime soon and update this post with a bit of video.

I've been pretty busy with classes this semester, which is why SmartKart took me so long to write up (it was effectively done a couple of months ago), but hopefully I'll be able to write another post soon (BE3P is way overdue for an update).

With SmartKart finished, the real 2.00gokart section started this semester, and I'm tagging along as a TA for the section and helping other students go through this process on their own. I'm hoping for some pretty crazy ideas/execution from them, and if I can manage to talk any of them into writing up their kart builds on blogs, I'll be sure to put up links here.