254 Robot

Hey

I’m new to FRC, so sorry if this sounds like an obvious question. Houston Worlds was my very first FRC tournament I attended and got a feel on how FRC tournaments were (Hint: they were fun).

However, I couldn’t help but notice the design of 254’s robot with its independent rotating multi-purposed arm. How does the arm rotate independently of the drive base? What’s stopping the wires connecting to the intake from bundling up when you rotate the whole way? Is there a CAD I can download with 254’s robot?

Thanks

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254 has not released their CAD ever as far as I know. They have released a technical write-up on their robot the last few years on their website.

The arm is on a turret, probably custom designed. It probably can only rotate a limited number of times so the wires don’t get tangled.

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Looks like a lot of well designed magic.

it might be a good idea in future to visit teams with robots you like, ask them how they do things. They generally are willing to tell you.

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The best thing to focus on for the 254 design for 2019 is the ball & hatch intake/placement. Just doing that thing on a well-driven basic robot with a basic lift would have won most events.

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I would imagine that it would be something like this turret which I believe was based on 254’s 2016 turret. I’ve heard that some teams have used it and have found success but I haven’t used it so I can’t comment on any specifics about it.

As for the wires not bundling up I would assume that would be done in software with a limit to how much the turret can rotate so it would just rotate back the other way to get to the same position.

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Looks like they custom 3D printed a cable management system. That is fine if you have the skills and equipment. But just getting some Igus energy chain will get you most the same benefits. Just takes a bit more space. Learning what works and what does not, takes some study and or experience. Our Igus rep helped us out a lot with what to look out for. Surprisingly the most magical part of of our setup is the cable tray. We made a polycarbonate pie pan arrangement and could have stopped there. We could make over 720 degrees of motion.

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I’m on team 1305 and we went with a rotating tower like 254, we actually couldn’t spin a whole 360 to avoid wires tangling, I don’t think 254 cant either because of they way they demoed their robot on behind the bumpers:

we also went through various iterations of how the turret rotated such as a large bearing and a CNCed bearing made out of lexan for the tower to sit on. And a 3D printed gear to rotate the tower using a belt to rotate the tower. So i could imagine 254 using a system similar to ours.

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They use a 72T Vexpro sprocket that has been machined out to drive the arm.

The bearing setup they did wasn’t the same as their 2016 turret. The booklets they had in their pit had a description of what they did this year, if they publish those digitally.

3476 also did an elevator on a turret and just posted some of their design docs over in their robot reveal thread.

Another alternative is a slip ring. 971 had one on their 2017 turret. With one of these you can rotate endlessly assuming all devices on the turret are powered through the slip ring.

A good place to accumulate this info is from watching seminars which are held post season by teams such as 1678, 254, 971, 973 etc. Lots of info is up for grabs (especially from prior years).

I am glad to hear that you had fun at your first competition!

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I know it’s too late but you should have just asked the students. Any top tier team if you ask them, could explain so far in depth every little detail that you couldn’t retain all the information if you wanted to. I could probably talk about my teams robot for over 2 hours describing different ideas and strategy’s used to create each mechanism. Just come and ask us anything, even just say “what’s your favorite part on the robot”… That will get any student going.

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The turret has two bearings actually.

The inner “core” of the turret is the rotating portion, and has a very large thin section bearing (like 8" +). The outer race is held stationary and attached to the robot.

In the middle of the “core” is a heavy duty thrust bearing, which keeps the turret pinned down. If you can imagine, the turret assembly can only be pulled out (if you were taking it apart), due to the way the OD and ID of the thin section bearing are held by flanges. This thrust bearing holds things “down” so that the thin section bearing doesn’t have any “bad” forces on it, like trying to wrench out the ID from the OD. All the forces are kept strictly radial and sideways.

It’s a pretty ingenious setup. There’s not really another kind of equivalent COTS bearing that would allow for such a large member to hang off one side like their lift mech.

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We had an arm on a turret this year and we used the armabot turret. I was worried about it when we were first building it and contacted them. They said they were more worried about the bolts than the bearings. I knew the bolts could take it so I went with that design.
This proved to be true. As we drove off of hab level 2 in a sandstorm at championships our climber dug in and stopped the bot cold. The arm broke a #25 chain on a 72 tooth sprocket. That force also went through the turret bearing with no ill effects. I’m not saying that 254’s design is not better or stronger, but there is a COTS part that can do this.

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They could. Go 41 seconds into this video and you will see a clip of their turret spinning multiple times.

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I think what Vinny was saying was that while the actual turret itself could rotate multiple times, the actual turret, with the wires, cannot. The example in the video is before 254 has added in the wiring for their arm. Perhaps with the bidirectional energy chain and the wires, they can’t rotate multiple times because the chain ends up wrapping around the turret.

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This. We have a similar clip of when we first put our spinning arm together and had it spinning around and around.

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Foreward:
We happy to answer questions about our robot (our kids especially love talking to anyone in the pits). We do not upload our CAD because we feel that incentivizes people to merely blindly copy and develop their own design skills or understand why we did things the way we did.

Another note about our designs, just because we are 254 doesn’t mean it’s perfect, we have only a 6 week season like everyone and sometimes that means “just sending it”. Do we overbuild things and rely on custom billet parts a lot? Yes. Do we overvalue aesthetics? Yes. Should we have built a turret in the first place and made our season feel like a dumpster fire? Perhaps not, though it turned out well in the end. This was by far (a solid order of magnitude) the team’s most ambitious, stressful, and challenging build/competition season in recent memory and I’m not sure we’d make all the same decisions again. That said, here’s some details that you won’t find in our technical binder (which will likely be uploaded sometime in the summer).

Wire Management:
Due to the limitations of the bi-directional IGUS (“Bigus”), the turret could only rotate +135deg and -310deg, the code told the turret which way to spin to point to the target, and sometimes if the drivetrain was quickly rotated the turret would have to whip around to keep pointing whilst not going past the hardstops. The Bigus was constrained above and below by a shelf and cover made of bent 1/16" polycarb that attached with velcro onto the bumper rails and could easily be removed to replace batteries/bumpers and service wiring.

Turret Design:
The bearing design for the turret was done to maximize stiffness and strength. Making a turret that was strong enough to withstand the moment generated from our arm running into a rocket or other robot with the elevator at max travel and drivebase at full speed was the driving spec.

The turret shaft (highlighted in blue) consists of a giant billet part which holds an upper, 6.5" ID, 7.25" OD, 0.375" WD Kaydon X-contact bearing we purchased from Ebay. This bearing outer race was held from below by a counterbore in the fixed 3/8" plate that was bolted to the 1x1x1/8" crossrails that were welded to the drivetrain. The outer race is clamped from above by clamp ring via screws into tapped holes in fixed 3/8" plate. The inner race of the bearing rotates with the turret and is held from below by a clamp ring via screw into tapped holes in the turret shaft, and above by a shoulder on the turret shaft.

Lower on the turret shaft a shoulder exists where a #25 72T vex sprocket with a milled-out center is attached (#10-32 screws clamp and transmit torque). This sprocket is driven by #25H chain from a 22T 1/2" hex sprocket that is the output of the turret gearbox. An SRX Mag Encoder counts rotations 1 gear reduction back from this output due to packaging issues. The gearbox is driven by 1 775pro.

On the bottom of the turret shaft another 1" ID, 2" OD, 0.5625" WD high load radial ball bearing is held from above and below by internal and external retaining clips, respectively. It is held radially in another billet piece that attaches down to the drivetrain weldment’s bellypan via standoffs (this billet piece just barely sits above the breakers on the PDP). This extra bearing greatly increases the moment capacity of the turret, giving it a second bearing point 1.75" away that can transmit load into the bellypan, which is very strong in inline tension.


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Claw Design

The claw (V7 in CAD, V3 of what was actually built) is designed to intake both discs and balls from all sides of the robot. The claw in the robot photo at bag’n’tag was bad, it popped balls and couldn’t pick-up discs, not even from the ground. We abandoned floor discs and focused on making a versatile intake that would be great at funneling balls and could still do discs. Thus, we took inspiration from others and redesigned the intake in time for SFR to feature 2 rollers to hold the ball and 4 3" flex wheels on arms that could actuate between a ball funneling state and a disc intaking state.

At SFR and SVR, the intake was massive and game object would sweep outside the bumper perimeter when turreting, requiring the drivers to carefully drive around the rocket and cargo ship. The new intake for champs was much shorter but required a longer (16.5" at SFR/SVR --> 21" at Champs). It was also lighter (15.6 --> 10.2 lbs) by removing the bottom roller (we determined the bottom roller was unnecessary after seeing others), one of the 2x2 and both 1x1 crossbars. These combined to result in a ~50% reduction in torque, allowing it to be counterbalanced by a lighter gas spring (90 --> 40 lbs), accelerate faster, and not cook our wrist 775pro.

Neos were used to power the top roller (2:1 reduction) and side wheels (10.125:1 reduction). 3D printed pulleys via the Markforged were used all over to make deadaxle combined pulleys, 16T pulleys with built in spacer, etc. The Neos were certainly excessive in their power output, but allowed for less gearing than a 775pro would require and the brake mode allowed for the disc to be held with little current draw.

A retro-reflective banner sensor was used to detect when the ball was all the way in and automatically stow the arm and intake.

The arms of the side wheels were actuated by a single pneumatic cylinder. Routed 1/4" aluminum 10DP “gear plates” were attached to the polycarbonate plates to keep the arm from parallelogramming. 1/4" polycarbonate plates were used here and to hold the top roller due to their ability to absorb shock loads without permanently deforming.

The top roller is our standard 1" OD, 1/16" wall roller with surgical tubing stretched over it we’ve used for lots of intakes in the past. We made custom aluminum endplugs (one side counterbored for a 3/8" shoulder bolt, the other with a 3/8" thunderhex CNCd to drive the roller). This roller setup is lighter than using a 1/2" thunderhex with numerous flex or mecanum wheels.



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Is the purpose of the initial belt run from the 775 to the first reduction just to reduce backlash?

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We try to use a belt for the first reduction of 775’s wherever possible because they are quieter and don’t wear down like the 32DP gears spinning at thousands of RPM do. It’s also nice to be able to package the motor in a weird spot out of the way if needed (see our carriage gearbox). This belt run still has some backlash (there’s no tensioner in place).

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