Tuesday, October 4, 2011

I'm Alive I Promise

Well it's been a while. I've been busy with uhh...being poor as a result of buying one of these. In case you don't know anything about R/C rock crawlers, or if the name isn't self-explanatory enough, these trucks are freaking beasts and are designed with super low gear ratios (this one is 38:1), super awesome solid axle suspension, and super powers, allowing them to conquer just about any terrain. Exhibit A:

But of course, as with pretty much anything R/C, there are competitions that require rock crawlers to actually crawl...over rocks...ok that was a no-brainer but you know what I mean. They actually have a purpose that matches up with what their name implies, as opposed to just crushing people and making them beg for mercy. I don't know...I think it's pretty neat. Check it out.

Anyways, with that out of the way, I have a new project I've begun work on that was actually conceived right around the time I blew all my money on an Axial XR10, but the new toy actually helped my new project become more of a tangible idea, since I didn't really know if the project was...possible. This needs explaining.
Ok so I always found it fascinating that it's easier to balance something tall if it's...taller, and is heavier at the top, like a yardstick or wooden dowel, as opposed to a pencil or something. It seems like it would be harder to balance a taller object, and it looks more impressive, but those guys in the circus balancing ten-foot poles on their foreheads would have a hell of a time doing the same with a ruler.
After doing no research whatsoever and thinking about it for like 10 seconds, I figured out why taller things are easier to balance. I thought it was kind of cool, so I made a little presentation explaining the phenomenon.

Figure 1

As you can see in Figure 1, we have an object that is 300mm tall, and if it falls all the way to the ground, the very top of that object will have to travel about 471mm.
Figure 2
Now in Figure 2, we can see that whatever lazy asshole drew these pictures simply scaled Figure 1 up to get Figure 2. The object is 1200mm tall, and the very top of that object has to travel about 1885mm before it hits the ground.

Even though the object in Figure 2 is scaled up and looks just like Figure 1, gravity is constant. Therefore, since the object in Figure 2 has a longer arc to fall across, it will take more time to fall to the ground. Since a taller object takes more time to fall, it is easier for a human to balance that object on their hand or head or finger or whatever, because there is more time for our slow reflexes to react to whatever that object is doing.
By the way, I promise I'm going somewhere with this. Stay with me.

My little illustration explains why taller things fall slower, but ignores weight altogether, which is very important to this whole...idea. What actually matters is not just how tall an object is, but where the center of gravity is. My illustration works if both of those objects, let's call them "poles", are of uniform mass everywhere. In that case, both poles have a center of gravity right in the middle of them, which still allows the taller pole to take longer to fall.

Now, if you were to put a lot of weight at the top of the shorter pole in Figure 1, bringing the center of gravity very close to the top, you can get it to take longer to fall to the ground. In addition, if you put a lot of weight close to the bottom of the taller pole, lowering the center of gravity, you can get it to fall much quicker.
So...height and center of gravity are both of equal importance when talking about how easy a tall object is to balance. Optimally, it should be as tall as possible and have as high of a center of gravity as possible.

Anyways, now that we're all good and learned, the project idea that I had in mind that exercised this...concept was a two-wheeled vehicle with a long pole extending out the top, with a weight on top, and the operator must keep it balanced to drive it around. At about the same time I had this idea, I was screwing around with my newly acquired R/C rock crawler. One thing led to another and eventually it ended up having two wooden dowels zip tied together, with a heavy piece of steel zip tied on the end, all zip tied onto the axles so that this big long top-heavy pole stuck straight out the front. I wish I had a picture. This setup allowed the truck to be balanced on end for upwards of 20 seconds and made for a pretty fun game, and ultimately proved that it was possible for a human to balance a vehicle like that.

Ok, so we've made it through physics and history class. Here's what I'm currently working on building:

That carbon fiber rod coming out the top extends out of the frame about an extra 940mm or so, and will have a heavy weight at the very top. The design has actually changed just a little bit, but it wasn't necessary to update in the CAD file...so I didn't. All I changed before I sent the drawings to Ponoko was the orientation of the motor, which will be mounted facing the opposite direction so that the spur gear is closer to one of the bearings, minimizing stress on the drive-shaft, and I added some fancy cutouts to the chassis pieces to make it look awesome reduce weight. So here's exactly what I'll be receiving from Ponoko, cut out of 6.6mm Delrin:

Seriously, Ponoko is the best. I highly recommend their services. I only had to pay $17 for them to cut all of this stuff out. On top of that was $30 for the 6.6mm Delrin but...that's because it's 6.6mm Delrin.

I put an extra gear on there because why the hell not/just in case/I like gears and I wanted another one to show off and play around with. I should have just put like six of them on there. The word "balance" will actually be laser-etched. The laser-etching, along with laser-cut gears and working with delrin at all, are all things I've never done before, so this should be a huge learning experience for me, which is always fun.

You can probably notice that the whole assembly is actually clamped together by four huge standoffs running along the width of the chassis. I'm lucky enough to have an awesome friend that will make those kinds of things for me in exchange for gifts such as chocolate chip cookies.


Moving right along. I got all the Pololu stuff I needed. Wheels, motor, hubs, all that. Even the little Team Losi Micro Rock Crawler ESC I'll be using for this thing. It doubles as a DSM receiver which is why I chose it. It's super compact.

I purchased 60mm and 70mm wheels just because I wasn't completely sure on the gear ratio. This thing needs to be geared down low enough to allow precise adjustments, but it can't be incredibly slow to the point where it can't catch up if it tilts more than 1 degree either. I'll have to mess around with it once I get it all put together. I'm hoping to have my Ponoko order within the next week and a half.

No comments:

Post a Comment