An answer about FRC arms

With this year’s game being a pick and place with a large horizontal extension, we have seen a lot of arms. Specifically, we’re seeing a lot of telescoping arms. To most people, these are just called “pink arms”. But defining a pink arm as “a rotating telescoping tube that is extended by a motor” means that 1678’s climber from 2022 is a pink arm. I don’t know about you but I can’t really call that a pink arm.

In this post, I will be outlining definitions for different types of named arm archetypes and going into detail about why these classifications are important. I am not going to include arm types that do not have a dedicated nicknames. My hope with this is to help clear up some of the confusion around terms like pink arms, 330 arms, an sushi arms. It’s also partly because I’m under the weather and have nothing better to do today. This is not meant to be a comprehensive classification of all arm types. There will be arms that don’t fit into the categories bellow (like 971 in 2018 and 2910 in 2019).

A lot of the content in this post was made by me with some contributions from @Boomie, I will credit him whenever I use any of his definitions or content. For all the definitions, this excludes any possible end-effector designs unless said otherwise.

Key terms:

Near the center of the robot: If you split the robot into equal thirds lengthwise and widthwise to for 9 equal boxes and the axis is in the middle section, the arm can be considered to be in the center of the robot. (provided by @Boomie).

Mid pivot: If you split the arm tube into equal thirds lengthwise and the axis is in the middle section, the arm can be considered to be a mid pivot.

End pivot: If you split the arm tube into equal thirds lengthwise and the axis is in either of the outer sections, the arm can be considered to be an end pivot.

High pivot: a high pivot is where the axis of rotation is placed as high on a robot as feasibly allowed by that year’s rules.

Telescoping tube: a series of hollow tubes (regardless of construction or shape) that nest inside one another and are able to extend out

A commercial off-the-shelf (COTS) telescoping tube system.

Mid pivots

The most famous mid-pivot arm is arguably 233’s 2011 telescoping arm. But not all mid pivots are telescoping. The main benefit of a mid-pivot arm is the ability to counterbalance the arm, and put any motors for a second Degree of freedom (DOF) on the arm.

Pink arm

Definition: A pink arm is a type of Mid pivot telescoping tube arm where the axis of rotation is near the center of the robot. using motor power for both extension, retraction, and rotation of the arm. Pink arms generally have multiple stages of extension.

examples:

FRC 3847 in 2018

This is a really classic example of a pink arm. Instead of using extruded box tubes, they use bent sheet metal that is fastened into a rectangle. and they put the two motors powering the telescoping extension in the very back of the first stage tube.

FRC 233 in 2004

This is the first pink arm that team 233 built. They used it again in 2005 and most famously in 2011. This robot helped define what the pink team is.

Not a pink arm:

FRC 7492 in 2023

Cavbots design from this year is a mid pivot and it does in fact telescope but it does not have the nesting telescoping tubes that are signature of a pink arm.

FRC 125 in 2023

While this arm does feature telescoping tubes what keeps it from being called a pink arm is the fact that the pivot is an end pivot and the pivot is not near the center of the robot.

Image credit: @Boomie

End pivots

End pivot arms are the most common type of arm in FRC. End pivot arms offer more reach and are not commonly counterweighted but can be counter-sprung.

330 arm

Definition: A 330 arm is the main type single degree of freedom end pivot arm where the axis of rotation is high up in the robot.

examples:

FRC 330 in 2007

There is not much to say here. Obviously, 330 is the team that this arm is named after and this robot is a classic example of their arm style. This robot made it all the way to einstein in 2007.

FRC 5414 in 2018

Partially biased here but this is my team’s 2018 robot. A really light arm allows it to rotate fast.

Not a 330 arm

FRC 233 in 2004

While this is an end pivot it has 2 degrees of freedom and the pivot is relatively low in the robot.

FRC 330 in 2016

While built by team 330 this arm has a pivot in the bottom of the robot. Despite not being a 330 arm it still managed to win the world championships that year.

Sushi arm

definition: sushi arms are a type of single-stage telescoping arm that has a pivot high up at the end of a tube. one of its other key features is that the arm in its starting/ stowed configuration points straight or nearly straight down between the two uprights that support the pivot. some additional characteristics include the use of a COTS telescoping tube system and a max planetary dead axle. Sushi arms are considered by many people to be a subcategory of a 330 arm.

examples:

FRC 7461 in 2023

This is the robot that defined the term sushi arm. It uses the AndyMark “climber in a box” design with very long pistons to telescope the arm.

FRC 4481 in 2023

While this arm does not rest in line with the uprights it does still stow in a vertical position.

Not a sushi arm


FRC 7461 alpha bot in 2023

Many people consider this a Sushi arm but there is no meaningful diffrence between this and a 330 arm so I choose to make the distinction between the alpha and competition versions of the sushi squad robot. This is a 330 arm, not a sushi arm.

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FRC 5414 in 2023

I couldn’t find a really good photo of my team’s arm for this year so I just pulled a CAD screenshot. While this arm does stow vertically it does not telescope so it does not meet the definition of a sushi arm.

The End

I would love for people to add to this. If you know of any other notable arm archetypes please feel free to post an example. Again this isn’t meant to define every arm so you will be able to find arms that don’t fit into my categories.

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Would you say 233’s 2008 robot does not have a Pink arm?
https://i.imgur.com/IB3mDwXh.jpg

What about their 2010 robot?
https://i.imgur.com/EzTEEjOh.jpg

Pink’s 2008 robot had an end pivot. Pink’s 2010 climber was spring-loaded and an end pivot. Both of these break the definition you’ve provided, and yet both Pink and everyone else would call them pink arms in a heartbeat.

Rather than try and impose a definition that you have intuited on everyone else, when everyone else is using something different, maybe let the consensus win this one. Especially when there is some logic to the consensus. It happens all of the time in the course of human language, even in other cases when the common definition is clearly “wrong” in origin! (Don’t get me started on “meta”)

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Let people call things what they want… if it works well it works well.

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Its called a BOBCAT arm.

My intention was never to impose my definition of something on people. FRC has a long history with lots of robots and designs, more then I could possibly ever hope to know. I think that there can be exceptions to anything that I’ve said, take 330 from 2016 in my post I say that it’s not a 330 arm but if you had shown me a picture of it before I sat down and wrote this I would have said, “yea that is absolutely a 330 arm”.
I didn’t write this post alone, I had many people proofread it before I posted to make sure that it aligned with the consensus of people’s opinions that I trust and value. If you have a better definition I would love to hear it.
The same thing can be said for sushi squads alpha bot. People are still going to call that a sushi arm even when I disagree and that’s perfectly ok.
I can certainly see your argument about why those two robots are pink arms and I didn’t know of their existence. If I had I certainly would have taken it into consideration but I choose to make this post because people would see a robot that had a telescoping arm and automatically assume it’s a pink arm.
The point of my post was always to inform and never to impose so if it came off that way please tell me how to fix it. At the end of the day, I’m an idiot on the internet and you are more than welcome to disagree and ignore me.

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You take a team who built something similar to what they normally build, and say it’s not one of “that team’s” arms. I suggest that that sort of thing is Not Done.

Oh, and by the way, I take issue with your statement that 330 arms are typically box tube, 1". You didn’t go back to 2005, which is unquestionably a 330 arm even by your definition. It’s also PVC. 2004 is a little less clear… but it counts, IMO, and is C-channel and some tubing. And it’s kind of hard to fit many pneumatic cylinders in a 1x1…

Signed, a 330 alum.

Also, @troy_dietz to finish any roasting required.

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awesome, I went ahead and amended it.

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Yeah but everybody else is wrong and that can’t stand on the internet!

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When my limbs lose circulation while holding the robot waiting to queue onto the field because some other team’s robot cart is in the way, do they not then become pink arms?

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No pressure 7461 but the two other robots that had arm archetypes named after them won 3 regionals, a division and a technical award on Einstein.

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Me watching Sushi get another 4 finalist medals:

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We can probably settle with “XYZ Mechanism is typically characterized by X, Y, and/or Z. See 20XX robot by Team 123 or 4567 as examples”

The defining feature of a pink arm is the telescope, in an era where that was very tricky to pull off, followed by the center pivot, which is also tad unusual and influenced driving strategy.

I (and most FRCers) would call a 330 arm “a chain-and-sprocket driven single-jointed arm with a top (rear) pivot”, and that was notable because of how many problems it managed to solve, especially in years where people thought it wouldn’t be viable or make sense (see 2012 especially, and 548 that year). Obviously, almost as many 330 robots follow that definition, broke it. 2010 is sort of a bottom pivot (and telescopes), 2013 is a bottom pivot, 2015 has like 3 joints depending on how you’d like to count, 2016 is a bottom pivot, 2018 is two joints, etc.
Even construction materials are inconsistent…like that one time it was 1.5" 6063.

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:sweat_smile:

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