In almost every robot that uses a hood shooter, I see that it is driven by rack (if that is the correct term here, maybe sector gear?) and pinion.
Was wondering, what are the advantages of this over driving it with chain/belt? I can see that it would require a larger torque, but that could be solved with a greater gear reduction (though speed is lost)
Thanks!
There a ton of teams that didn’t use rack and pinions. In fact I would say most teams didn’t use one in 2020-2021.
With a chain or belt, you need something on the far side to hold a pulley or sprocket. And pulleys/chains tend to do better with straight-line movement while hoods tend to move on curves.
[for our robot for this particular game] Our calculations (and later testing) showed us we really only needed two positions, so we just used a simple servo.
In general, my opinion is a rack-n-pinion is easier to package than a belt/chain driven. YMMV
Sector gear would be the correct term. You’ll hear it called a “curved rack” by a lot of teams but that’s just because “sector gear” isn’t a term a lot of people are familiar with (a curved rack isn’t a thing, by definition).
In terms of why teams use sector gears to drive launcher hoods, it’s mostly because of slop. Driving the hood with a chain or belt near the pivot would end up with more slop than using the sector gear. Plus you get the save the weight of that extra gear reduction you would have to add.
There is also a packaging consideration. Generally the hood pivots around the same axis as the launcher wheel spins. It’s easier to just let that be a free pivot and move all of the actuation hardware out to the outside of the hood.
Couldn’t you technically just stick a max planetary for a chain driven hood? (I’ve only build rack and pinions)
I think there’s a couple answers to this:
That being said, I don’t think there’s inherently anything wrong with other motor-driven setups. Team 20 used a geared hood in 2016 and won a regional/got picked at champs with it:
I also think the vast majority of teams should consider static hood/pneumatic hood before motorized hood. We (3467) used a pneumatic hood in 2022 and had every shooting position you could possibly want and more.
I would say “packaging”.
A chain/belt requires extra space for the motor (can’t be as close, so needs to be farther away), PLUS it has to not have any flex particularly under load (like when a ball goes past it under compression). This leaves a few options, not necessarily just rack-and-pinion.
Pneumatic cylinder: Great if you only need 2 positions (close/far), lousy if you need more positions. Can’t be deflected by shots.
Servo: Good packaging, not-great strength, good at being in the right position.
Linear actuator: Similar to a pneumatic cylinder, but more position options. Holds position pretty well.
Rack-and-pinion/sector gear: See above posts.
Direct-drive off the motor: Not enough torque unless you’ve got a pretty large reduction in your gearbox. Still an option, though.
You still need to mount it somewhere, route the chain/belt, and have a motor. That’s a “packaging” problem. It’s not that it won’t work.
Oh, and one other consideration: There’s an awful lot of shooters on turrets. Whatever option you use needs to fit on/inside/etc the rotating part of the turret, which could be anything from just the hood and wheels to the entire loading stack.
Reminder that this means two physical positions of the mechanism, not necessarily two positions on the field - you always have the ability to vary the speed of your shooter as well to change how far you shoot.
The 2022 robot I mentioned above had near-infinite shooting distances. The “long shot” shooter position was used for everything ~10ft and out, where the “short shot” shooter position was used for everything inside that. There was technically a tiny dead spot in there we couldn’t shoot from, but the robot would simply back up slightly to a position we could shoot from.
We were not at all limited by having a two position hood in pretty much the only game where shooting from everywhere was really valuable.
Low backlash is the main reason. Using an extremely large gear on the hood means that for a given linear distance of backlash, the angular distance of the hood moves less. This linear distance for backlash acts upon the large circumference which results in only a small angle change. In contract, belt and chain setups act upon a much smaller circumference of the sprocket/belt, meaning that there will be a much larger angle change on the hood.
Eliminating backlash on hoods is an extremely high priority because they are extremely sensitive to backlash. The hood being even a single degree off from where you expect it to be can cause shots to miss.
The curved rack setup also has the advantage of taking care of most of the gear reductions for the hood. The large gear on the hood and the pinion could easily form gear reductions in excess of 20:1 which is very space and weight efficient and means you need less gear reduction before the hood mechanism, meaning yet even lower backlash.
“secret” technique: put two cylinders of different stroke lengths “back-to-back” and you can have 4 positions.
“super secret” technique: put two cylinders of different stroke lengths “side by side” and you can have 3 positions.
seems like you should be able to get 4 there as well.
I completely agree. Not only is this simpler mechanically (and thus both more robust and less likely to fail) but it can be easily as effective. We used a fixed dual flywheel shooter for our 2022 robot and it worked very well from pretty much any distance (and 2022 had some highly variable shooting distances.) Once our programmers worked out the tables, the rpmRevolutions Per Minute of each (independently powered) flywheel set was tied to our Limelight’s distance calculations and that was that. It was also a season that showed the limitations of hood shooters. Unlike in 2020, where backspin on the balls was advantageous, that was a serious problem in 2022 and only the teams that solved this problem by either using dual flywheel systems or hoods with wheel/roller components to defeat backspin did very well at shooting.
Short |Long
Retract|Retract
Retract|Extend (same as Extend|Extend)
Extend|Retract
The double extend position is the same as just the longer one extend.
The OP wasn’t asking about how they’re driving wheels/rollers on the hood, they were asking about how they were actuating.
The image you posted looks like that hood is actuated by an air cylinder.
6328 ran a hood actuated with a belt and pulley. I will grab a picture of their cad when i am at my computer again
Just looked at the CAD, is belt actuated there:
That being said, the images on TBA (including the one you sent) are pneumatically actuated:
https://imgur.com/rLKwIC7
Was the belt drive version later in the season or earlier?
