Gear ratios for launcher

[swwrobotics]

Originally Posted by artdutra04

How much compression were you putting on the ball?

Other than the fact that the tread pattern on the 2012 KOP wheels chews up the foam basketballs, any single-wheeled shooter with six inch wheels (when directly driven from a CIM) should easily be able to shoot the balls a more than adequate distance.

Simply changing the gear ratio won't solve this problem; the real problem is probably related to ball compression, shooter hood design, and/or misalignment/binding/friction in the moving parts of the shooter.


FYI: The 2.7:1 gear ratio is what is inside the CIMulator from Banebots. There will likely be a fair amount of teams using this gearbox this year, as this gearbox makes a 775 or 500-series motor "emulate" the output characteristics of a CIM motor. This allows teams without a lot of machining resources to keep their CIM motors in their drive train and use "emulated" CIM motors elsewhere on their robot.

Our mentor said the we should have as little amount of contact with the ball as possible to pervent too much deformation and loss of accuracy. So you're right, that definitely could be the problem. But just out of curiosity, would it be worth more to have a high gear ratio or a larger wheel diameter?

[JamesCH95]

Originally Posted by swwrobotics

Our mentor said the we should have as little amount of contact with the ball as possible to pervent too much deformation and loss of accuracy. So you're right, that definitely could be the problem. But just out of curiosity, would it be worth more to have a high gear ratio or a larger wheel diameter?

It's true that you should have as little contact with the ball as possible, but you still need some minimum amount of contact, otherwise you won't impart much energy to it. Many teams seem to have found that 1-2" of pinch on the ball will work well.

With a 6" wheel you could be looking for 2,000-5,000rpm axle speed, depending on what range you want. the actual ratio will depend on what motor(s) you're using.

[Ether]

Has anybody posted test data anywhere that includes wheel rpm, wheel diameter, squeeze, launch angle, and distance?

[edit] and wheel moment of inertia, if available? (yeah, I know: dream on) [/edit]

I've been keeping an eye out, but haven't seen anything yet. It would be most useful as a sanity check for some of the calculations.

[Aren Siekmeier]

Originally Posted by Ether

Has anybody posted test data anywhere that includes wheel rpm, wheel diameter, squeeze, launch angle, and distance?

I've been keeping an eye out, but haven't seen anything yet. It would be most useful as a sanity check for some of the calculations.

We took a decent amount of data with ball compression, I'll see if I can't dig that up and compile for public posting.

We're gearing for a top speed at the wheel edges of about 80 ft/s (54 mph!), since this is about double the speed of 40 ft/s that worked well for basically any shot in the near half of the field in our testing. So for a 4" wheel that's a 5:1 off of FPs (0673) and we have nearly the equivelant of 4 of them (2 FPs, 2 550s), so we're making up for lost torque (see here for more of the reasoning behind that). We're using the same 8" plaction wheels we tested with because that tread seemed to work better than others we tried briefly. I pushed for wheels smaller than the ridiculous 8 inches, but nobody ever tested anything smaller (we have screwed up ways of setting priorities, working on fixing that...) and people were worried about differences in traction with smaller wheels, we stuck with 8". The reasoning for smaller wheels was mostly weight, with a marginal benefit in how the frame could be layed out (more freedom), as well as just not having ridiculously huge wheels. But no one listens to me anyway... In any case, with our 8" wheels we have 10:1 total reduction off of 2 FPs and 2 550s in the works.

[Garrett.d.w]

Two weeks ago I made a pitcher using two rollers made out of 2.5" PVC covered in wedge top tread. This was powered by two 775s on banebot 46-1 gearboxes with no chain reduction. The compression of the balls was about 2 inches.

I would be comfortable shooting from half court with this thing. It is also good for rapid fire, the 775s have so much torque that the rollers will spin at a steady speed no matter what.

[wireties]

FIRST Team 1296 is trying 2 FP 801 motors at 6:1 and 2 AM motors at 4.64:1 driving 2 8" wheels (both under the ball) - have not got it running yet, a proof of concept with a CIM at 3:2 worked nicely compressing the ball 1-1/2"

HTH

[klmx30302]

Team 166 is direct driving 2 axels with 2 8" wheels each. Each axel has a cimulator with 2 rs-550 motors each. We have not clocked its top rpm yet but the wheels spin quite fast as you can see in this video: http://www.youtube.com/watch?v=YtiK87pE_9U
We don't know the range of the final shooter yet, but the prototype could shoot 46-50 feet, the prototype was direct drive cims instead of cimulators.

[pfreivald]

We're making very consistent shots from key range with AM motors and planetary gearboxes, though the spin-up time between shots is too long. We're working on that problem now!

[DampRobot]

Originally Posted by pfreivald

We're making very consistent shots from key range with AM motors and planetary gearboxes, though the spin-up time between shots is too long. We're working on that problem now!

My team is using a similar set up. Were getting about 16ft using belts driven by 1:1 sprockets off a AndyMark gearbox (the one for the Fisher Price). Accuracy is OK, but we've really been underwhelmed by its performance.

[h1n1is4pigs]

using RS-775s with 4:1 reduction direct driving to 8" wheels with about 3" of compression we are getting very consistently balls going well over 30 feet

[Ether]

Originally Posted by pfreivald

the spin-up time between shots is too long.

What voltage are you running the motor(s) at? i.e. what is the motor voltage when the wheel has come up to speed and no balls are being fed to it?

[pfreivald]

Originally Posted by DampRobot

My team is using a similar set up. Were getting about 16ft using belts driven by 1:1 sprockets off a AndyMark gearbox (the one for the Fisher Price). Accuracy is OK, but we've really been underwhelmed by its performance.

Accuracy at key range will be more important than being able to chuck a ball 30+ feet, IMO.

Originally Posted by Ether

What voltage are you running the motor(s) at? i.e. what is the motor voltage when the wheel has come up to speed and no balls are being fed to it?

They were using home-built speed controllers with a knob control -- I'm not sure what the duty cycle voltage was. (I wasn't there... had a funeral this weekend, so Saturday was the first FRC day ever without me there. Quite weird, I must admit!)

[Ether]

Originally Posted by pfreivald

Saturday was the first FRC day ever without me there. Quite weird, I must admit!)

Not weird, but impressive. I hope your students recognize and appreciate your dedication!

[pfreivald]

Originally Posted by Ether

Not weird, but impressive. I hope your students recognize and appreciate your dedication!

I believe they do!

[artdutra04]

Originally Posted by swwrobotics

Our mentor said the we should have as little amount of contact with the ball as possible to pervent too much deformation and loss of accuracy. So you're right, that definitely could be the problem. But just out of curiosity, would it be worth more to have a high gear ratio or a larger wheel diameter?

From what it sounds like, your shooter barely compresses the ball. If so, the best thing to try out first would be to increase the compression between the shooter wheel and the ball; otherwise every motor and wheel combination you can try will likely have the same disappointing results.

The trick is to find a balance between not enough compression (not enough energy from the wheel gets transferred to the ball, leading to wimpy shooting range) and too much compression (too much energy is taken from the shooter wheel without any shooter range improvements). If you do some experimenting, you'll find an a good balance between the two.

Once you find the ideal level of compression, then you can look into substituting other motors, gear ratios, and wheel sizes.