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!

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