[FTC]: Caster?

[pilum40]

I'm just a teacher, not an engineer and wanted to see if we could use a caster (360 degree mounting). Some of my sophomores had an aha moment and wanted to build a triangular shaped robot with a caster in the back to facilitate turning. I won't crap in any student's hat when they bring an idea to the table. Just wanted to know if this caster assembly would violate COTS rules of one degree of freedom. I'm thinking probably because it can rotate 360 degrees. If so, it should pass because it only goes back in forth in any direction except vertically. If not, I'll get them to prototype the design in wood for viability. If we don't use that were thinking a couple of Mechanium wheels in the back for turning. Our front wheels are 8", 8 pointed stars that were 3D printed and covered with rubber edging. They navigate the churros without incident, don't mar the floor pads and were fun to build. Any pearls of wisdom would be appreciated. If not a 3 wheeler, we'll just add 8 inch FRC wheels with an FTC hub adapter on the back to handle the churros on the ramp.

[GeeTwo]

I don't do much with FTC, so I'm not even going to make a tentative rule interpretation, but speaking as a physicist, a caster wheel with an integrated swivel definitely has two degrees of freedom.

As a workaround, you may be able to combine a non-swiveling caster wheel with a small lazy susan bearing to achieve the same result yourself, using two COTS items each with one degree of freedom.

Thinking more about DoF, do the rules state that actuators must only have one DoF, or COTS mechanisms in general? A caster wheel with a swivel has the same number of DoF as a ball joint, but neither of them are actuated (controlled). A wire or a pneumatic hose has several non-actuated degrees of freedom.

Oh - another easy way to achieve 2 DoF with separate COTS items: an idle omni wheel with bearings that are a separate COTS item. We used these on our 2013 FRC robot as functional equivalents to swivel casters.

[dradel]

Not familiar with ftc rules, but would this work?
http://www.mcmaster.com/#ball-casters/=10ll5k3

[pilum40]

Yes those would be cool. We'll try a larger Omni wheel too and see. I'm going to see if I can get a response from the Q&A. Thanks

[dradel]

Again not knowing the rules at all, but the way I see it the caster ball only has one moving part

[GeeTwo]

Originally Posted by dradel

Not familiar with ftc rules, but would this work?
http://www.mcmaster.com/#ball-casters/=10ll5k3

Originally Posted by dradel

Again not knowing the rules at all, but the way I see it the caster ball only has one moving part

A ball and socket joint, while only having one moving part, has two degrees of freedom: altitude and azimuth, or latitude and longitude.

[IndySam]

For competition robotics purposes, casters are evil and should be avoided.

[pilum40]

Indy:
sorry to be obtuse. My team is asking me why casters are evil? Again, I'm just a teacher, not an engineer. Not trying to be funny but legitimately would like to give them a reason why. It's a teachable moment for all. thanks for your patience.

[IndySam]

They are never in the position you want them to be in and can cause irregular movement.

They also violate one of the rules of competition robotics: If it touches the floor it must be powered.

[RRLedford]

Originally Posted by IndySam

They are never in the position you want them to be in and can cause irregular movement.

They also violate one of the rules of competition robotics: If it touches the floor it must be powered.

You are conflating the term "rules" With "rules of thumb" that good designers follow.
The official game rules of the Res-Q competition do not exclude the use of casters, unless perhaps they have both a swivel and a shock absorber too (2 degrees of movement freedom → rotation & elevation).

Three wheeled drives are rather tippy and unstable. Casters can typically get turned away from desired direction too often and too easily, especially if you will ever be trying to back down the mountain.
If your star wheels work well, I assume that going for the 40 points that reaching the high mountain zone would give you in autonomous would be possible, but getting back down without a rear caster becoming stuck on churros would be near impossible and risk a rollover backwards.

I am glad to hear someone has done a pair of front "star wheels" that successfully navigate the mountain & churros.

Mecanum wheels will not work very well as only a pair at rear, since they are meant to be used as a set of four located at the corners of a square.

A driven pair of of at least 4" diameter omni wheels at rear would likely work best, and give good results both on the mountain and in the floor.

-Dick Ledford

[DavisDad]

From FTC forum: "Thread: Robot Parts and Materials - Answer Thread"

Casters

Originally Posted by FTC5950 View Post
We were wondering if rubber caster wheel (http://www.lowes.com/ProductDisplay?...llow&cId=PDIO1) was legal. We were not sure if it would fall under wheels (legal) or COTS (illegal due to 2 DOF).

Thanks!
5950 - The Bio-Bots

A: This caster wheel is a COTS and has more than one degree of freedom. Therefore it is illegal.

We used a castor to make a gimbal. We removed the wheel and attached a linear slide. Maybe if you use a different wheel and only use the castor as a "lazy susan" the 3 DOF rule would be satisfied. You could post this question to the FTC forum to be sure.

A fixed Omni wheel would achieve the same performance without the issues from castor pivoting.

[RRLedford]

A castor with just a fixed vertical pin and no pancake bearing being integrated with it should be legal.
If you can then fit the pin into another component that provides a slip fitting swivel socket in which the pin can rotate you can still get castor functionality from combining two separate, single DOF items.

-Dick Ledford

[howellroy]

I'm with Team 3490 and we used two casters on our bot during the Ultimate Ascent. it made the bot really quick and responsive but a lot of other teams said it was a mistake because we could get pushed around a lot. we made the finals but a team member fell off the tower and we never recovered in points.