Shooters: What have you learned?
 

I doubt that next year will be a shooter game, but I can guarantee that some years down the road, there will be another year where a shooter is the de facto method of scoring. I hope this thread will be useful to teams when that year does roll around!

So what less than (or more than) obvious facts have you learned about shooter design this year? What problems did you run into that you weren't expecting, and how did you fix them?

I'll start off: Make a single axle shooter if you want backspin. And if you make one with two axles, make an easy way to adjust the squish and make it easy to take apart. Because you'll be doing a lot of that.

Re: Shooters: What have you learned?
 

Backspin is nice, try to have it.

Don't put used wheels on your shooter, it's a dumb move.

Prototype your idea thoroughly, don't just assume it will work because you see that every team has built a shooter prototype that works.

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Ummmmm......maybe not so dumb.

Re: Shooters: What have you learned?
 

1712 had no issue getting backspin using a dual-axle shooter. We had each axle controlled independently so we adjust the backspin, but you could also find the ideal ratio and gear them to it. We never really adjust our backspin, so that capability is not the most useful in the world.

Re: Shooters: What have you learned?
 

One thing to keep in mind when prototyping is the amount balls squish change the distance they go (at least for this year/this type of ball), so to build a prototype which can quickly go from one distance to another can help see what you want to do. For us we had a top shield and bottom shield, with a space between the bottom shield and the shooter wheel to make contact with the ball and different amounts of spacers between the top and bottom shield to adjust the amount of space between the bottom shield and the wheel.

Not sure if that makes sense but hopefully it does.

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Darn, I guess we'll scrap ours as well.:p

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The theory I learned in Feedback System Analysis and Design actually works. :D

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Consistent feed and positioning of the ball as it enters the shooter is absolutely critical. Our initial design allowed the ball to move left and right in the lift ~1/2" each way, after adding guides (and inducing a slight amount of lateral compression in the ball) we noticed a large increase in our accuracy.

For reference our shooter is a hooded shooter, 2 6"x1.5" colson wheels ~1.5" apart, and between 1"-1.5" in compression.

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This depends on the wheel, I guess.

Many teams using 2008/2010 KOP wheels on their shooters found it necessary to remove the crowns left on the wheels from the molding process, essentially breaking in the wheel before use.

Though, I know of a team using 2006 KOP Skyway wheels that ran into Issues with them wearing over the course of the season and losing grip.

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We're using this year's kit wheels, right out of the box, with no problems with wear after 2 events. I guess it all depends on your set up. Prototype, prototype, prototype!

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^Truest statement I've ever heard.

Some balls lose their squish...and other balls just don't like to be squished all that much.

If the game requires precision distance shooting, do us all a favor and get the design done so you can tune said design. Just ramping up the power does NOT equate to making a shot.

Oh, and if you're using heavy, custom fab wheels, make sure the motors you pick have enough power to make them go...

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Consistency can be gained through LOTS of compression. We ran our competition robot with 4-5 inches of it in order to ensure precision.

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Don't solid-mount your gyro to your turret or other vibrating chassis components. Use a soft material that will damp vibrations. Note: we only learned this yesterday, when we took the time to do proper analysis on our gyro's heading over time. Makes it hard to auto-aim straight if your gyro starts drifting about 1deg/sec when the shooter turns on.

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Since I'm looking to mentor new teams next season, these are the things I'll be bringing if a shooter game comes back up:

• Lighter wheels - With lighter wheels, the wheel will be back up to speed faster. This is especially necessary if you're using a motor with less torque and power to run the shooter wheel.
• More time on programming - Shooters are almost definitely the best way to score this year, but a lot of shooters aren't used to their full potential because the programming wasn't up to par. Spending more time on the target tracking algorithm can allow a shooter to be very succesful.
• Watch the weight distribution - Our robot this year is a bit top heavy. That's being worked on and resolved for champs, but it hurt us at FLR. When designing a shooter in the future, I'd like to make sure the shooter doesn't have an uneven weight distribution. This prevents disadvantages in game situations such as the bridge, and prevents the robot from rocking and potentially tipping as it moves.
• Responsive, consistent turret - Part of the key to succesful shooting is having a quick, responsive turret to shoot at the proper angle to meet your target. With a turret that has the proper hardware and coding, shooting can be made greatly effective.
• Make sure the ball enters the shooter quickly and consistently. We have a nasty habit of coming up with some unnecessarily complex storage-to-shooter designs. With some thought, I think there could be a quicker, more consistent way of getting the balls into the shooter.
• Don't let the balls enter the shooter at an angle - Without making sure of this, balls may enter (and come out) at an angle, or not get into the shooter at all.

These things aren't NEEDED for a good/great shooting robot, but they are things I'm going to greatly consider next time there is a shooting game.

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Interesting... If our shop wasn't getting knocked down next week (and rebuilt hopefully by September) we were going to add a bunch of weight flywheel-style to our shooter to see if that increases accuracy!

Also, this is just an anecdotal observation, but single-axle hood shooters seem to be much more reliable than double-axle shooters.

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Yeah. We used a pretty heavy custom wheel for our shooter (modeled off our 2006 shooter). I noticed we really need to be careful of the timing we shoot at, or else the balls don't go nearly far enough. Again, we're working on it for later in the season, but it kind of hurt us at FLR. Especially before we got our shooting to be somewhat accurate.

Re: Shooters: What have you learned?
 

1.) The amount of contact arc between a single-wheeled shooter and the ball doesn't appear to matter much with respect to shot consistency, though this also makes the assumption that entry speed/position/angle is identical from shot to shot.

2.) Re-calibration of shooter distances is a non-stop thing to do. Expect to test calibration every few matches.

3.) There were two teams in DC that used back-EMF in unpowered motors to detect how fast the shooter was spinning. This seems like a much more elegant solution than an encoder or tachometer/light sensor since the Voltage across the analog sensor port is directly proportional to the SPEED of the shaft the motor is on. Both teams used KOP-legal motors (one was a window motor without its gearbox, the other was a RS-395).

4.) Flywheel vs no Flywheel, that was the question. I'll post back after I talk to a few teams at championships about their shooter controls.

5.) A single 8"x1.5" IFI wheel with roughtop tread was a very good choice for a shooter wheel this year, and it is a choice we will revisit in the future.

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Our wheels have scuffs on them in odd places from years of prototyping. They are beyond broken in.

Re: Shooters: What have you learned?
 

Our shooter was designed in such a way that rotation was handled by the drivetrain, and distance by shooter rpm.

If using a CANjaguar, extremely repeatable results can be obtained by measuring the voltage instead of the rpm.

Shooting at a very high angle can be a lifesaver, because you can avoid shooting for the backboard, which makes shoots somewhat unreliable. Not to mention shots that are all net look pretty cool when you make them every time!

Make sure that you tighten bolts/other fasteners on your shooter whenever possible, because the vibration + excessive movement = inaccurate shots.

Recalibration is a must. During our regional, the balls would keep getting squishier and squishier at a rate which we did not expect. So between matches (when our robot wasn't being repaired :) ), we would head down to the practice field and change constants in our program.

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Our team made 2 diffrent shooters this year one was a wide belt design using really heavy pulleys and the other was a single axle shooter. Interia will only help you so much, you need the power to bring your wheel up to speed faster. Compression of the ball or object your shooting was a great variable as well but once again there is a limit on that as well. We ended up going with a custom wheel that was 3 seperate pieces and effectly built our fly wheel into the middle of the wheel powered by 3 rs 550's. I think we determined that for us, at least for this game, a single wheel rather then double wheel is much more effective.

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This is what we learned last night.

When you think the shooter is tuned just right. Hold on to camera very tightly.

http://www.youtube.com/watch?v=OiAo1Oiw5X8

:ahh:

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Control the shooter wheel speed with a bang-bang controller rather than PID.

Tip of the hat to Martin for reminding me of this.

Re: Shooters: What have you learned?
 

You sir just made my day.

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I wish I saw this earlier! I don't follow the LabVIEW forum on the portal because we don't use LabVIEW :(

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Nice, quick and easy. I like it!
Mind if I attempt to port it over to LabView?
It can be placed in a periodic task with the interval time of your choice. That way it is one less thing for Teleop to process and it can be used during Autonomous without having to build it in two places.

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By all means, please do.

This post explains in a bit more detail why it works for the shooter wheel speed application:

Re: Shooters: What have you learned?
 

Really? We did about the same thing with our original shooter, and we found DRASTIC variation between shots because of its explosive exit as it uncompressed...

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Conveniently knowing both systems well allow me to answer.

Our shooter does not shoot the ball with a very high velocity (compared to most). The ball is compressed between a roller and a flat piece of poly carbonate. This allows the ball to be decompressed over a fairly large distance (~5 inches). So, it's not exactly an "explosive" exit like it is from a shooter with dual wheels spinning much faster than ours.

Morale of the story:
Allow the ball to compress and decompress over a distance rather than "instantly".

Re: Shooters: What have you learned?
 

Er, yeah? I don't recall comparing the exits from the two shooters...

Re: Shooters: What have you learned?
 

In designing a shooter, you must design around the ball you will be shooting and what you will be shooting it into. In 2006, I think the Poof balls were much more consistent in their squishy-ness or it was just a lot easier to shoot at a 30inch diameter near vertical goal. Either way, shooters that year did not require as much work due to a more consistent game piece and/or a large goal to shoot at.

This year however, we are playing with a game piece that changes quite a bit throughout the regional and is not consistent in squishy-ness to start with, along with a much more difficult goal to shoot at (18 inches and horizontal). With these factors, creating a shooter that would make every shot without adjustment took a little more time and effort.

From what I have found, the top three options for having a consistent shot are:

1. Catapult (16 is the best that I've seen: http://www.chiefdelphi.com/forums/sh...d.php?t=103876)
This approach removes ball squishy-ness from the picture. While it's harder to get significant backspin, you can land the ball in the same spot every time if you do it right. The trick is the getting the thing lined up and the right distance away from the hoop.

2. Extended acceleration period (2949 using belts: http://www.chiefdelphi.com/forums/sh...highlight=2949)
With lower compression and extended acceleration, each ball will exit the shooter at a consistent speed regardless of squish. Backspin is tweakable based on your setup. 2949 had a good setup with their shooter on a turret and their laser targeting system.

3. Shoot it hard with a lot of backspin (341 and I believe many others: http://www.chiefdelphi.com/forums/sh...d.php?t=103579)
For this setup, you need a decent amount of compression for this years ball and I think it helps a significant amount to be a tall robot. I will definitely be inspecting 341's shooter at champs to see just how they did it. As long as you can hit the backboard in this case, the backspin should put the ball right into the hoop as long as you are lined up correctly.


As to what we (525) did this season, we had to find another way to do it. We didn't wind up having enough height (we are about 3 ft tall) or enough compression on our shooter to easily do what 341 did. We thought about speeding up the last stage of our feeding mechanism to do more of what 2949 did. And obviously a catapult would have been quite a drastic change from what we had to work with. Our shooter used a 6inch Colson with one FP 0801-0673 with just short of 2 inches of compression on the ball and an adjustable hood. We decided we had to adjust our shooter speed/hood angle on a per ball basis in order to have a consistent shot. To do this we added a compression load cell (http://search.digikey.com/us/en/prod...6948-ND/809394) to measure each balls squish factor. You can find more information about our setup for the load cell here: http://www.chiefdelphi.com/forums/sh...d.php?t=105394. This setup got us to the point where we hit 9/10 balls in tele-op for one of our finals matches at 10,000 Lakes, hopefully something we can carry on to champs.

I'm sure there are other options for having a consistent shot, bot these are the three designs that have stuck out to me.

I hope this is helpful and I hope I remember all of this for the next game involving shooting a ball.

Kellen Hill

Re: Shooters: What have you learned?
 

Based on the above linked paper, I threw together the LabView "equivelent".
I chose to set it up with a counter instead of an encoder, but you can still use an encoder as the input to the counter. With this configuration, you can also use a simple Banner sensor and a piece of Retroreflective tape on the wheel if you would like. I also chose to use the 10ms. periodic loop.
With this version, you should be able to modify the loop delay without changing anything else. It will compensate for the change in interval time.

Please test this before you use it. This is built purely in code only and has not been debugged on actual hardware......yet.

[EDIT] I realized I had not made a compensation in the slew rate limiter to take into account different loop execution times. So, in the next version I'll have it added. [/EDIT]

[EDIT#2] See my next post, post #36, for the updated version of the Periodic tasks.vi. [/EDIT#2]

Re: Shooters: What have you learned?
 

I would say shooting for the backboard increased our shot consistency significantly.

Also, if you use a flexible back plate, make sure its braced to handle the force from the ball. Ours would lose a lot of energy as it was only held in place at a single contact point by a winch. You could see the plate buckle somewhat as the ball forced it back.

1218 had a similar back plate if i remember correctly, but it was braced by a solid assembly with multiple contact bearings so they had greater range and accuracy.

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That is probably a more complete and better analysis than just "moar compression" I think that using relatively smaller wheels (3.875inches) also made the compression behave differently than most other shooters.

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What have we learned? Cannons are awesome :rolleyes:

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I really want to feel your robot up at Champs... :D

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The attached Periodic Tasks.vi include compensation for different loop intervals. This allows you to set the priority/frequency that you want to update the shooter motor drive. It also includes Slew Rate limiting that compensates for the interval time. Lastly, I cleaned up some data types conflicts.

[EDIT 4/13/12]Ether has made some refinements to his document on the Bang-Bang control. This latest version for LabView reflects those changes. [/EDIT 4/13/12]

Re: Shooters: What have you learned?
 

You forgot #4 Fling-A-Pult originated by 3940 and used by 292, 829 and 217

Re: Shooters: What have you learned?
 

What exactly differentiates a Fling-A-Pult from a catapult? It's just something I don't have experience with and haven't personally seen this season. After just watching a couple of 829's matches, it seemed like a pretty consistent shot. More details/pictures of your Fling-A-Pult would be helpful.

Re: Shooters: What have you learned?
 

PID in the Jaguar works well. It would consistently recover in about 0.5 seconds after a shot. See attached graph for details.

-Hugh

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We have a single axis shooter that doesn't have a consistent compression. Basically the ball enters the shooter at the 0 degree mark with no compression. At 90 degrees it has 1" and at 180 degrees it puts 2" on the ball. There is an extra strait (tangent) 2" after the half circle that holds the ball on track while it is decompressing and it definitely helps accuracy. We've never had any ball go left or right (I'm not saying anything about a vertical accuracy because we had some sensor issues). Anyway, the ball auto centers in the rails before being grabbed by the wheels. The wheels are basically polished and that really helped us not tear up balls (at 5200 rpm having a grippy wheel will tear up any ball) and they worked because of the compression.

All this being said, the amount of back spin we achieved was amazing and we could hit the very top of the backboard and still make it. During prototyping we tested from beyond the co-op bridge and made it, with about a 25% accuracy :p In the end it comes down to having had experience with designs you make during prototyping.

We wanted to keep our center of gravity as low as possible. So we ended up with it about 10" of the ground. Very helpful when balancing. I've seen too many teams flip their bot in the last seconds trying to balance. With that low of ground clearance you'll never flip. To show off we tested ours to about 70 degrees before tippage :D

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Our shooter hood has a gradual arc on it that allows the ball to decompress gradually, which seems to help consistency. Also, we always shoot from the closest to the hoop as we can get while touching the key, with the lowest rpm possible that will still allow the ball to hit the backboard. This combination of factors allowed us to have a very consistent bot this year.

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Our team was one of the very few that made a shooter with one flywheel on top of the ball causing it to have front spin. This made it super hard to score shots unless they were a perfect swoosh, because otherwise the ball would just jump right off the backboard. So definitely give yourself time to test out your ideas because they might not always work they way you think they will.

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Re: Shooters: What have you learned?
 

The condition of the balls really takes its toll on shooting accuracy.

All I want next year are durable game pieces.

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Looks like another great way to deal with ball inconsistency. Consider it added to my list!

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Agreed.

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In terms of shooter this year, I think a big factor that seemed less important when compared the rpm & torque of your motors, the angle at which you shoot, or the horizontal distance from bot to basket was shooter contact time (the amount of time that the wheels, lexan or any other material used to guide or propel the ball is in contact with the ball). During the LA regional, we practically tripled the size of the lexan guide on our shooter and drastically changed the angle so that we could get an easy, consistent shot on the middle basket versus a high arching shot that was mildly less reliable for middle and high baskets.

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I think 33 did quite a few things correctly on our shooter this year due to experience with 2006 and 2009.

Absolutely 100% #1 most important thing is feeding consistancy. You can have the most consistant, perfect shooter in the world, but if it has 1 inch of tolarance side to side its not actually going through the shooter the same way each time. We took a lot of care to get this right. We made our shooter turret at the bottom. We found from 2006 that with the lazy susan mounted higher it was basically impossible to put the ball in the shooter the same way every time. We also went with a three point contact within the tower, two pieces being adjustable plastic skids one being a gates belt. This ensured that the balls are entering the gun at the same place as there is no room for lateral movement in the tower. We also have "kicker" wheels which basically push the ball up into the gun eliminating any potential dead spot between the gun and gates belt. Finally, our programming sequence is such that each ball stops just before going into the gun so every ball enters the gun at exactly the same speed from the same place within the tower.

Shooter design-wise we have almost 150 degrees of wrap around the gun before the ball exits. This assists considerably with ensuring that each ball accelerates to gun speed before exiting. We have done an enormous amount of calculated and dynamic shooter work to ensure that we have the least amount of variation in shooting as possible.

Of course, I can't share too much before worlds. Perhaps after.
Regards, Bryan

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Assuming we're talking one-wheeled-hooded shooters:

#1 - Make sure your shooter is sturdy. For us, this was probably the most important factor in ensuring repeatability. A janky shooter will Wobble and warp, affecting the ball's trajectory significantly.

#2 - Use as LITTLE compression as possible. We found that high compression caused big variations depending on balls' squishiness and how new/old they were. To make up for the force you lose to create the necessary friction, it's good to use a surface like grip tape on your shooter hood and wheels. this increases friction force with the friction coefficient rather than the normal force, so to speak. (Grip tape is great on both the wheel and the hood!)

#3 - Make sure the ball enters the shooter the exact same way every time. We found that changing the speed at which the balls are fed also can significantly impact their trajectory.

#4 (This goes with #3) - Surfaces in the shooter must be flat in the direction perpendicular to the balls' motion. This means that your hood should be one smooth "plate" rather than a set of rails. (It can still be curved around the wheel, so long as it curves with the wheel and not across it). It might also be worth looking into to make the shooter wheel a "drum". The reason for this is that you want the shooter to be tolerant to any side-to side error the ball might have upon entering.

With all that and a good control loop for wheel speed, you have a shooter that will fire almost exactly the same shot every single time.

Re: Shooters: What have you learned?
 

What I learned from shooter designs:

Back spin shooters are great due to the fact that they cause the ball to fly straight down after hitting the backboard.

.5 inch compression is the most decent amount of compression for foam balls. any tighter, the ball would rip apart. looser would cause the ball to shoot out with low power.

curved backplate for a backspin shooter helps get more traction on the ball.

rapid fire causes each ball after the first one to lose a small fraction of power.

the angle of fire can easily be adjusted by the motor for the wheel on the launcher without the backplate having to move if it ends at a 45 degree angle.

the best wheels are the ones with smooth rubber on it.

Re: Shooters: What have you learned?
 

One thing i noticed this year is that when you have multiple balls being fired, if they are fired in quick sequence it may not give the motors time to spin back up to the required speed thus reducing range/accuracy.
From what we saw with out robot this year that can be solved two ways. The first is to ramp up the shooters power to a point where a shot doesn't slow the wheel speed a noticeable amount. The problem with this is that it really prevents you from putting serious compression on the ball which may or may not be a problem depending on the shooter style you choose. Also lots of power may require transmissions or multiple motors which adds weight. If it is a fixed shooter this may not be a problem but on turreted shooters it may.
The other option is a mechanical feature designed around the idea to not feed multiple balls through in a line but only let them go through when the wheel was back up to speed. This was the option our team chose in the form of a pneumatic powered lever arm that pushed the balls into the shooter only when the trigger was pulled. This enabled us to shoot only when the speed was where it was needed. Hope this helps!

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We went 10 straight matches (starting with our 3rd match and continuing through SF's) and hit 18/20 balls in auton. Here's how we did it:

1.) Pick a point on the key that can be utilized every time as a reference. There are 4 such points: 2 corners and 2 edges where the middle line meets.
2.) Make a shooter than has insane backspin, such that a ball can be shot up at 60 degrees under full speed, hit a wall a few feet in front of the robot while still ascending, and still come back down to the hit robot.
3.) Make a sighting mechanism consisting of a hole and a narrow slit. Calibrate the slit to where the ball hits the wall.
4.) Make an adjustable autonomous where the driver's station can delay both balls such that they fire anywhere between 3 and 12 seconds.
5.) Make an adjustable autonomous where the driver's station has a "hard ball" and "soft ball" setting for both balls that fire. Calibrate. This is the hard part.
6.) We set the bot down with the front right wheel on the left front corner of the key. Then I sighted the direction of the bot to either the left or right bolt on the back of the rim: right for hard balls, left for soft balls. The shots would be off a degree or so depending on which we had.
7.) If the balls are ROCK HARD, I had to make a manual adjustment, moving the bot around the back of the key by 6-12". This is why we missed the 2 that we did, but they were in different matches.
8.) Pick 2 balls that are similar to the balls that were calibrated with, if at all possible.
9.) Profit.

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Pay close attention to the varriations between other balls that really killed us this year, one second we are shooting perfect when we get on the field and use there balls boom all is diffrent.

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When you have a 2lb aluminum pseudoflywheel that works going 3000 rpm, don't use a roll of duct tape as a makeshift brake pad. Things just go down hill from there...the roll itself got shredded, then the adhesive residue drastically changed our shooter grip. From needing to fire at 110% power, we had to drop it all the way down to <30% power.

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When coaching on the Curie field I remember picking through the available balls on the coop bridge. After seeing the stark differences between them we started "racing" the other alliance every time we got onto the field to procure the best balls for hybrid.

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I've written a quasi-successful swerve program in LabVIEW. In that the robot drives, but doesn't turn. Our team's analyses, however, state that the turning problem is mechanical (too heavy, swerve drive treads on carpet produce high coefficient of friction). pm me for source code.

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? Was this accidentally posted here?