"Waffle" East Angelbots 1339 Offseason Project

[End-Game]

Another update on our progress. We are in the midst of wiring and plumbing the Robot, and should have the structure, with the exception of a few parts, done by the end of tomorrow.

http://imgur.com/a/634HB

[jkelleyrtp]

Originally Posted by BBray_T1296

tl;dr Churro tube has an exceptionally poor torsional stiffness per unit weight, for weight savings use a hex axle with a hole drilled in it instead

Does that mean ThunderHex by Vex is the best COTS option for weight savings and torque distribution?
By your numbers:

ThunderHex is 1/2" in size
.2" central bore
15% less weight by cross sectional area. ((.5^2)pi-(.2^2)pi))/((.5^2)pi)
The central .2" carries less than 2% of the total load?

[s_forbes]

Originally Posted by jkelleyrtp

Does that mean ThunderHex by Vex is the best COTS option for weight savings and torque distribution?

I would assume that to be the case, but it would be interesting to see what torque is required to make a ThunderHex shaft fail vs a standard Vex Hex shaft... the rounded corners on the ThunderHex and bored out center might let it round off inside of a hub with enough torque. Or not... it sounds like a neat test.

Keep in mind the weight savings of the hole in the shaft are pretty minuscule, something you might be able to offset with several strategically placed helium balloons.

[End-Game]

An image album of the completed structure, with most of the wiring and pneumatics finished.

This also shows our strategy for acquiring the flag.

http://imgur.com/a/4Uoq6

[Sl3dman]

Waffle bot update

[BBray_T1296]

Originally Posted by jkelleyrtp

Does that mean ThunderHex by Vex is the best COTS option for weight savings and torque distribution?
By your numbers:

ThunderHex is 1/2" in size
.2" central bore
15% less weight by cross sectional area. ((.5^2)pi-(.2^2)pi))/((.5^2)pi)
The central .2" carries less than 2% of the total load?

Those should be powers of 4, but the result is correct. A hollow round shaft with those parameters is just 2.56% less torsionally rigid than a solid shaft.
The existence of the hexagon-generating surface features complicates things if you care for precise numbers, but from this simplified situation, ThunderHex is the most efficient choice.

What is happening on a molecular level is, the atomic bonds on the outside edges of the shaft are being stretched in plane shear more than the atoms on the inside of the shaft for the same rotational displacement. In addition, there are more atoms on the outside edge than the inside near the center that are being stretched, and therefore resisting the displacement. Coring out the shaft removes the lightly-stretched central atoms while leaving the atoms actually seeing the most stress. Also, consider the stretched bonds on the edge are further from the axis of rotation and thus generate a higher moment.


Out of curiosity I wanted to find the internal radius of a hollow shaft that would maximize rigidity/weight. I calculated the moment of inertia ratio of the hollow shaft compared to a solid one, then calculated the mass ratio of the hollow shaft compared to a solid one. I then calculated inertia ratio/mass ratio. By setting the second derivative with respect to the inner radius equal to zero (maximizing), I found that the internal hole should have a radius equal to sqrt(3)/3 of the shaft radius (or 57.7%), which for a shaft of 0.5" diameter is 0.29" This shaft would have 66.6% the weight of a solid shaft, but maintain 88.9% the rigidity.

[mrnoble]

http://youtu.be/ymrUYVZW844

Waffle's first run.

[asid61]

Originally Posted by mrnoble

http://youtu.be/ymrUYVZW844

Waffle's first run.

I love that quick snap on the PVC, and the flawless lifting and lowering of it.

[Bluman56]

Originally Posted by mrnoble

http://youtu.be/ymrUYVZW844

Waffle's first run.

This is very cool. Reminds me of 179s can manipulator that creatively utilized mecanum wheels.

I noticed it takes a bit of adjustment to get the grabber to snap onto the PVC tube properly. In a game where there may be many of these tubes, I would try to aim for a quick lock on the tube after your robot aligns with the edge of the box. I can imagine putting an encoder or (probably even better) a potentiometer to the extending mechanism to program it to stop going forward when the grabber is aligned with the pipe.

I'm curious what the team is doing (if anything) to address that.

[Sl3dman]

Originally Posted by Bluman56

I can imagine putting an encoder or (probably even better) a potentiometer to the extending mechanism to program it to stop going forward when the grabber is aligned with the pipe.

I'm curious what the team is doing (if anything) to address that.

Actually, we are indeed using an encoder (which is reset by some limit switches that the linkage hits when in its upright position) and a PID loop to track how far forward the linkage is from the robot. The only problem is that the pipe isn't always at the same position in the box which proves to be a challenge for automation. So we ended up deciding that it would be better to overshoot the game piece by a little bit rather than coming up short to it.

[mrnoble]

Gotta drop this here. The team is now competing to make the best off-season build video, and since this is the first one, I figured it should be shared.

https://m.youtube.com/watch?v=OSH0sN4_U5k

We had our "competition" with 2240 last Saturday. It was fun, and gratifying to see how the team's work paid off. The robot was as good as any practice bot during build season, in spite of limited hours and resources. It would have been nice to see additional sensors used, including more limit switches and a gyro, and of course drive practice is something we will always need more of. Nevertheless, I'm very proud of the team.

[End-Game]

We are releasing the CAD for Waffle, Available on our GrabCAD account.

Link to this here
https://workbench.grabcad.com/workbe...H0_vBO4C75WvaU

There are two new Reveal Videos on our Youtube account.

https://youtu.be/3aahoGNRPow
https://youtu.be/E5dKcjiZz3s

I hope everybody enjoys them.
Can't wait till Kickoff, good luck to everyone!