Tuesday, November 23, 2010

TOBL: Slowbot

So there are a couple new roadblocks in the way of TOBL, but I guess by definition engineering is solving problems. No, that's not right. "Engineering: the ultimate pursuit of laziness through hard work -Shane." Much better. After all I did throw a ton of time at a watch winder that will never do more work for me than I did to construct it.

Not a bad shot for a phone.

Anyways, the new servos are in and tested. They work very well, a substantial upgrade in torque from the HS-81s. Infact, they're better in every way EXCEPT for one kinda-super-critical detail: the spline gear does not have 360 degrees of teeth! Not sure what Hitec was thinking in that design meeting but then again I'm not their average customer. After being taken advantage of one night on GChat (and being stubborn) I think the best way to proceed is to make my own replacement gear. I don't even want to know how small pitch those gears are, but I'm going for it. Also I already opened them, and told The Internet about it so eBay probably won't take them back.

Current Gear (left), Continuous Motion Friendly Gear (right).

Unfortunately the digital readout on the mill at the shop I work out of is broken so, the aluminum plates will not be machined for a while. That gives me time to think about the rest of the frame though. There are a couple of things to consider if TOBL is going to work as planned, namely weight and rotational inertia.

Okay, nothing on this robot is going to weigh that much, but consider the following. The output torque of the servos are 38.8 oz/in, which means that 38.8/16 = 2.425lb force can be applied with a 1" moment arm. Take this times two, since there are two servos and no planned gear reduction to get a total of 4.85lb. This is about right too, taking into consideration the tipping condition where the front wheels are wedged and the entire frame must rotate around the front axle, the moment arm would be extended to the center of mass, hopefully somewhere in the vicinity of the centroid or ~1" radially.

Therefore I need to be careful about adding weight and where. The brainchild of this realization is to extend the rods supporting the gears throughout the frame, make them out of aluminum, and thin them out a bit. Then, just two small brackets to attach the electronics, and some good old zip-ties for the battery. Something like this:

Board shown in green and battery shown in blue.

Also, the heart of the software is almost complete. I started experimenting with both of the new servos at once and have so far managed to control one and both on the same signal, through the touchOSC > Processing > XBee > Arduino routing process. However, I would like the robot to turn too so right now I'm working on processing the signal to the two different servos. My lack of programming experience has definitely caught up to me but I'll figure it out. Once that's together I'll try to add in some cool buttons to the "control surface" for say spinning or trying to wheelie.

That's all for now. I'm posting mainly out of guilt knowing that the next few weeks may be busy with the holidays and finals on the horizon. The winter months should be far more productive. Happy Thanksgiving!

Monday, November 8, 2010

TOBL: Intro

The project name is TOBL and a prize goes to the first person to figure out why. You'll be hard-pressed though, it's very obscure! When I do a project, in addition to giving it a senseless name, I like to start with what I'm least comfortable with and once I'm reasonably comfortable with that, have a field day with what I do know. In the last few posts I proved to myself (maybe you too?) that I can tap my finger on a little wireless black box and touchOSC, Processing, and an Xbee/Arduino medley will know what that means. Now that I have [any] confidence in the software, it is time to have fun designing a cool robot. Here's a recap of some wired speed controlling:

So, TOBL. It'll be a 6WD robot driven by either two or four wheels at a time. This means it's pretty inefficient. However, the goal is to make this little guy freaking unstoppable! The triangular six-wheel configuration therefore is to allow it to roll over and then carry on its way (see MS Paint diagram below). For torque, I've bought a pair of Hitec HS-82MG servos, as promised. I explored the option of upgrading the gearset in the one working HS-81 and buying a new HS-81MG, but for $5 more ($45 total with shipping) I got two brand new servos with upgraded motors, controllers, metal gears, and ball bearings. Anyways, they put out 28kg/cm (whatever that means) each at 4.8V.

The drivetrain will consist of a splined pinion gear coming straight off of each drive servo to spur gears on each of the three wheels. From what I could find online it made most sense to use 48 pitch gears, which means I will have to be extremely careful when machining the plates that align the them. In the interest of cutting a little weight and getting practice with small-scale precision machining, I've made these plates as ridiculously intricate as possible. I should have a plate of 1/8" 6061 aluminum tomorrow so look for a fabrication post sometime soon. I'll do my best to stick to these designs no matter how much I'll regret it while on the mill.



There will be two of these "drive units," with the servo-sides facing each other and the electronics and battery sandwiched in between. Speaking of the electronics I've decided to go all out and print a board for this little robot. It'll basically be an Arduino Nano 3.0 shield, so I think it's definitely worth while to design an accomodating platform that could be of use in further projects, and allow the expansion of this one. Perhaps once I get TOBL to move I'll start adding some sensors to the leftover pins. For now, It features a socket for an Xbee and an off-board linear 5V regulator. This may seem excessive since the Nano has an onboard regulator built-in. However, I'm a bit of a servo nut and traced four of the PWM pins to 3-pin headers specifically for servos. While I'm only using two micro servos in this project, I can imagine using more on future iterations and Nanos are not cheap to replace.

Click the schematic for bigger version.

First Schematic! The boards may not be printed for a while, kinda hoping Santa's elves will pick up the tab on this one because this project is getting quite pricey. I've already put some work into this so it shouldn't change much. I'll post a shot of the board layout and more details once I finalize and send that out. With the dimensions of the board I'll have a better idea of what the chassis will look like. I will be working hard and machining as quickly (and carefully) as possible over the next couple weeks before Thanksgiving Break!