pic: Off Season CAD Practice

[Whippet]

Thank you all for the comments! I will upload some side view screenshots soon that show the full range of motion that this arm currently has.

Originally Posted by asid61

Looks very lightweight. I do have to say though, I would expect that claw to wobble significantly due to the lack of stiffness in the structure.
Have you thought about adding intake rollers?

My main issue with a roller claw would be counterbalancing the extra weight at the end of the arm. My team that year had a similar arm with a heavier claw, and the motor we used for the arm was constantly stalled out due to the forces involved. It seems like there is a lot of support for a roller claw in this thread, though, so I'll see if I can incorporate one into this design.

Originally Posted by BBray_T1296

Seconded
Note how the claw will be on the ground outside the bumper perimeter, and is liable to be ran into by other robots. The moment on the joint from such a collision is massive and could bring the whole tower down

The viewing angle in the picture is a bit deceiving, but the claw actually doesn't contact the ground, and is fully contained within the frame perimeter during starting configuration. It sacrificed a floor pickup for it, but I decided that this design is pretty close to what my team can realistically manufacture in six weeks and would allow us to have some practice time before ship/bag.

Originally Posted by LCJ

The geometry of the current design is set so that once you pick it up, the tube is always going to have a certain angle relative to the arm. This means the robot can only score when the arm is at a certain angle (which makes it difficult/impossible to place tubes once that row is filled).
I would suggest something like what 148 did, where a piston changed the angle of the tube to be parallel to the scoring rack, making it easier to place on a peg. To do this, you will have to change the arm into a 4-bar linkage, or have some other method of keeping the grabbers at a same angle relative to the chassis as the robot lifts its arm.

I was envisioning the human player feeding the tube so that it would extend straight out from the arm, theoretically allowing the robot to score on all three levels without too much trouble. I do see how the tube could rotate because of a loose grip, so perhaps a 4 bar could be useful.

Originally Posted by BrendanB

Nice work for your first large scale project in Inventor!

Learning that will be very helpful come January.

Thank you! I hope that having a detailed CAD model beforehand will enable my team to build more complex robots than our past years!

Originally Posted by Mike Marandola

Also, some teams with two independently powered rollers were able to spin the tube in the claw, by rotating the rollers in the same direction.

I have seen that, and will definitely try to incorporate such a controlled roller claw in the next iteration.

Originally Posted by Lil' Lavery

Perhaps that could create a large enough moment to cause a tower failure, but I suspect there would be other failure modes that would occur well before the tower failed.

Could you point out elements of the design that would be likely to fail in this way?

Originally Posted by D.Allred

Interesting critiques, but seems like most missed the key point.

"I taught myself how to use Inventor over the summer."

Nice job Whippet.

How did you approach designing for proper reach to hang tubes on the top peg? Did you work out the basic geometry in 2D sketches or iterate your design through 3D modeling?

David

Thank you! I built the model to be parametric, so I was able to easily change the lengths of the key different frame members to optimize the arm through the 3D model, and then built the support structure around it.

Originally Posted by GeeTwo

I was wondering a bit that you were a rookie and 2011, but are still a student. I guess by that you do not mean an FRC rookie, or are you on a team where you can play as a student six years?

I won't add any more to the "roller claw" discussion, as you claimed "basic tools and COTS parts" in the OP, and most if not all of what I would have said has been covered, anyway.

Most importantly, as far as "modern 2013+ rules" are concerned: 2015 had no lateral "sprawl" rules beyond fitting on your alliance's half of the field, but In 2012-2014, the robot was required to start the game completely within the "frame perimeter", with a maximum height restriction. This arm does not seem to meet this requirement; you can't get it to fit both requirements at the same time. The obvious answer is to have another articulated joint or two. I think there is an alternative, though I haven't run the numbers: make the mobile segment of arm a bit shorter, and mount the claw on the opposite side of that arm. Give it a mobility of about 350 degrees rather than 170 degrees (doesn't seem to be a problem from what I can see). Then, you could start with the arm stowed on what is currently the "front" side, pick up logo pieces off the floor from what is currently the "back" side, and score them high on the "back" side with less rotation issue than you would have with what you designed. If I recall the 2011 game correctly (I did not play it, but skimmed the rules a few months ago), this would probably also reduce the number of times you would have to turn the robot around.

Finally, I find that motor sticking out the side of the "wrist" to be cringe-inducing. This would be a great place to have the "thumb" actuator respond to a "muscle" pull along the arm. This could be done via pneumatics, a lead screw (or even threaded rod and a coupling nut), or a light chain or belt. Especially if you use pneumatics, consider actuating both jaws of the claw; this would allow you to "rotate" pieces for more solid placement.

My old team, 3043, operates out of a combined middle-high school, and allows middle school students to be on the team under certain circumstances. I started hanging around the team in the 2010 season while I was in 6th grade, and was officially recognized as a member beginning in the 2011 season, when I was assigned to help build our (admittedly terrible) minibot. I also worked on the school's FLL and FTC teams at the time, but FRC was my main thing.

By "modern 2013+ rules," I was specifically targeting the new and smaller frame perimeter and the recent motor additions. The game-specific ones, such as the unlimited height during match play, the maximum size cylinder, and the starting configuration rules, were all left intact. Due to being a single-jointed arm and having to start within the frame perimeter, this arm did not include a floor pickup.

I hadn't previously considered increasing the arm's range of motion like that. It's currently not possible due to the gas shock counterbalancing the arm, but if I moved the claw motor towards the shoulder as you suggested, and added a physical weight behind the shoulder as a counterweight, then the torque needed to rotate the arm might be small enough for the motor to handle on its own.