Side vs. Top Shooter

[farmersvilleRob]

Originally Posted by squirrel

It's pretty easy to get close numbers....find the no load rpm of the motor, find the wheel diameter, decide if you want a wheel on each side of the ball or just one side, and figure out the gear ratio needed to make the wheel(s) spin that fast, or just a bit faster.

If the motor speed drops too much when shooting the ball, add another motor, or add mass to the wheel and wait longer for it to spin up

But isn't it different with no load rpm? Is no load rpm affected by the load of the wheel? Even if the wheel is 90 degrees vertical, it should still give fight back to the motor thus lowering the no load RPM. Also, the torque would have to pick up the weight of the ball and at that moment in time, the torque is also having to speed the ball up "through" the launch plane and that reduction also slows down the RPM.

If calculating proximities though, like squirrel is suggesting, I would like to add that taking the max efficiency RPM and torque would leave wiggle room for all the complicated math if you don't want to run a long algorithm for precise numbers even if preciseness is probably the best if you don't have the financial or physical means of prototyping.

[Alex.q]

I know it will change based on design, but how will having wheels on the bottom vs wheels on the top and bottom affect the range/accuracy? Will two sets of wheels provide more power/range as one would believe, or would power be entirely dependent on the motors themselves?

Sorry if this is hard to answer based on little actual design.

[farmersvilleRob]

Originally Posted by Alex.q

I know it will change based on design, but how will having wheels on the bottom vs wheels on the top and bottom affect the range/accuracy? Will two sets of wheels provide more power/range as one would believe, or would power be entirely dependent on the motors themselves?

Sorry if this is hard to answer based on little actual design.

Well to my knowledge, it would give more range. Think of it this way: If you have a pitching machine on, to throw a fastball with the appropriate slope through the strike zone, the top wheel is set at 40 mph and the bottom wheel at 65 mph. That gives you a 60 mph fastball and that is because of the bottom wheel still having to put spin on the ball otherwise you have a frozen rope (linedrive) at the same level it came out as.

It would, however, make the top motor run at about 30% efficiency compared to the bottom one because it is programmed to run slower. You could throw a frozen rope at the goal, but if that worked, then basketball players would do it... and they don't.

So, you need the curve. The bottom only motor gives you the curve, but with a plate on the top using friction to create the slope, it gives a lot more rotational translation of power than linear. This slope actually works against you. It's like programming for a 45 degree shot from a cannon and getting a 60 degree angle anyways with less power and that huge slope. So, you need one for shots further than 5 feet, if you can make it. And they would have to be run by a CIM motor, otherwise there isn't enough torque. But, you could actually make a super light bot that runs on 2 CIM motors no problem. It's all up to the rest of the bot, but torque and RPM is your focus for the whole competition. Torque for driving. RPM/torque for shooting. RPM for ball collecting/grabbing. Torque for bridge pushdown. Torque for arm/shooter raising/lowering. Every team needs to balance their motor layouts to correspond with every other part of the bot, otherwise it won't doing any good if you have an amazing 4 CIM shooter with a slow body and vice verse.

[MrForbes]

Originally Posted by farmersvilleRob

But isn't it different with no load rpm? Is no load rpm affected by the load of the wheel?

The wheel itself offers little load when it's up to speed...design it to run at 90% of no load RPM (or max power, which isn't far from there), and you can always reduce the speed with the control system.

My point is that you shouldn't worry too much about it, just put something together that's estimated to maybe work, and see if it does work.

We only have 5 weeks left

[dtengineering]

I'm not surprised at the 50% mass increase in the ball... amazing what a little change in radius does!

The shooter video I linked to, however, was driven by only one FP, geared down about 4:1 using a banebots planetary, if I recall. The design was originally set up so that a second gearmotor could be placed on the other end of the axle shaft, but we didn't need it... in fact it would have broken the shooter speed limit that was in place that year so we left it off.

I wish we had known more about programming and PID loops when we built that robot. We could have added the second motor and used a control loop to maintain a constant speed on the shaft. That way the second motor would have brought us (back) up to shooting speed more rapidly, but then would have dialled back the power to control range. The "new" controller (okay, I guess to most students doing FRC, the cRio isn't exactly new...) makes closed loop motor control MUCH easier... most experienced teams will have had some experience with closed-loop feedback, but for newer teams looking for a good excuse to learn more about closed-loop speed control, the shooter would be a great place to start.

So while I fortunately/unfortunately don't have a robot/team to experiment with, I'm reasonably confident that two FP's, properly arranged and geared, should have good range and repeatability when configured as a vertical shooter.

And I do have to say the backspin was incredible... occasionally the balls would land in the goal, and backspin themselves right out. This year I can only see backspin as being a good thing!

Jason

[Thermal]

Originally Posted by dtengineering

So while I fortunately/unfortunately don't have a robot/team to experiment with, I'm reasonably confident that two FP's, properly arranged and geared, should have good range and repeatability when configured as a vertical shooter.

So far with our testing, your hunch is pretty reasonable. Right now we're hitting 15ft with a 3" radius grouping over 10 balls with 2 fisher prices. The FP's are mounted to a 3:1 planetary, and are both attached to a CIMple box. We can reach out and hit 20ft max. I'm confident in that range with the hood we're using on our competition robot, but I doubt we'll ever attempt to score from that range. It's pretty much a guarantee to hit from the fender, so there is not much reason to not use it.

Right now our plywood and polycarb hood with a shooter wheel thats on some very poor bushings is too rough of a mockup to give entirely concrete results to say that a single wheel hooded shooter can hit from half court reliably, but if I was to give a guess, i'd say you'll see a few robots at each regional that are entirely capable of hitting from half court on a consistent basis.

[Racer26]

@farmersvilleRob: Have you been involved with FRC in the past?

You've made some pretty wild suggestions that are not only flying in the face of what some long-standing, well-respected FRC mentors have said based on math, but flying in the face of things they've DONE, as in, actually built, and witnessed in operation.

I personally watched a prototype shooter on Saturday shoot the 2012 game piece approximately 30ft. It was a very crude setup, and the final product would shoot much better, and more accurately. Torque, in my mind, is not at all the issue. Your suggestion that a wheeled shooter wouldn't be able to make a bigger shot than 5ft is flat out wrong. Period.

[JamesCH95]

A picture of our prototype shooter. Over-under rollers, the small roller diameter means less gearing (4:1 from a 775 direct-driving the rollers) and that it can be very close to max height. Having two separate rollers lets us adjust spin. Backspin for making shots, and top spin for throwing balls across the field with a bounce or two.

http://i.imgur.com/b018x.jpg

Very similar to our 2006 shooter that we really liked. We have since added grippy material to the rollers, and made it adjustable to pinch the ball a bit more to get more grip.

Just a tidbit: backspin will dramatically increase shot range, just ask anyone who plays airsoft and had to adjust their kick-up.

[gpetilli]

Originally Posted by JamesCH95

A picture of our prototype shooter. Over-under rollers, the small roller diameter means less gearing (4:1 from a 775 direct-driving the rollers) and that it can be very close to max height. Having two separate rollers lets us adjust spin. Backspin for making shots, and top spin for throwing balls across the field with a bounce or two.

http://i.imgur.com/b018x.jpg

Very similar to our 2006 shooter that we really liked. We have since added grippy material to the rollers, and made it adjustable to pinch the ball a bit more to get more grip.

Just a tidbit: backspin will dramatically increase shot range, just ask anyone who plays airsoft and had to adjust their kick-up.

I really like your concept and I am trying to convince my team to move away from the default pitching machine to something similar to yours. Can you please share a video and/or some data on current shooting range? Can you adjust angle or only velocity? Why rollers instead of wheels?

[MrForbes]

We did a little more shooter testing today, the single wheel shooter is looking good so far. We set up an older fisher price motor that has similar specs to the AM non-geared 2012 motor, with gear/chain reduction to give the 8" 2010 kit wheel a max theoretical speed of 4000 rpm. At full power, the ball goes most of the way across the wood shop...nice, flat trajectory after about 8 feet of travel, with plenty of backspin that makes it bounce off the backboard at a nice angle down towards the hoop. We're still setting up the backboard, etc, but it looks like it works as I hoped it would.

[JamesCH95]

Originally Posted by gpetilli

I really like your concept and I am trying to convince my team to move away from the default pitching machine to something similar to yours. Can you please share a video and/or some data on current shooting range? Can you adjust angle or only velocity? Why rollers instead of wheels?

No really good video yet, we're still working out a number of finer details including how much 'pinch' to put on the ball to get good range and what sort of roller materials will work well. In this video we're getting tons of slip between the rollers and the ball, so very little energy is making it to the ball.

http://www.youtube.com/watch?v=CSHx0...lcp&playnext=1

So far we can only adjust velocity, no angle.

I like rollers over wheels because you can get the same tangential velocity with a lower gear reduction and therefore less drive train loss. Though a big wheel probably has more inertia than a roller for a given amount of mass, which may be beneficial. Rollers are also a substantially smaller in profile, so they can be packaged in a smaller volume.

[MrForbes]

Originally Posted by JamesCH95

http://www.youtube.com/watch?v=CSHx0...lcp&playnext=1

An autocross video?

[JamesCH95]

Originally Posted by squirrel

An autocross video?

Ah! I'm sorry! That's the next video in my channel.

Here is the direct link.

http://www.youtube.com/watch?v=g8lR4-BlvOA

Unimpressive range, but it was a very first prototype and showed a great ability to control spin magnitude and direction.

[mikegrundvig]

This is the design we are planning on using:
http://www.youtube.com/watch?v=l8U9u2iqMEs

The plan is to use two 550 motors to spin the four 12" wheels we are machining from aluminum. The wheels have beveled edges to provide a lot more surface area with the balls when shooting. We are aligning them vertically so we can control backspin and minimize drift from side to side. We will be using a pair of PID control loops to get the RPM on the wheels as accurate as possible. The RPM itself will be tracked via hall effect sensor and magnets.

The students have done a lot of math and prototyping and everything seems to work out as planned. I'm actually very hopeful for an accurate long-range shooter to be honest.

-Mike