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.
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.
