Catapult?
 

I haven't seen any discussion on anybody using a catapult design - is anybody going this route? Has anybody tested this? If so, is it accurate and effective? I imagine this design would be very difficult to defend (aside from breaking the arm off). Just curious....

Re: Catapult?
 

I can't speak for everybody, but team 1318 dismissed the catapult idea early in our first brainstorming meeting as hard to reload, hard to make accurate, hard to shield, and hard to make as powerful as necessary. I will be very impressed with any team that pulls this off successfully.

And I'm not sure what you mean by "hard to defend". I don't see how it would be harder to defend against a catapult design than any other type. And of course, breaking the arm off would be against the rules, so that isn't a legitimate strategy.

Re: Catapult?
 

I think a well-constructed catapult is likely to be more consistent in its aim than a wheel-driven shooter. I also think it's pretty obvious that a catapult will be slower than a wheel-driven shooter.

Any teams designing a catapult will have to be careful not to run afoul of the very first rule.

Re: Catapult?
 

A trebuchet would potentialy work (especially a Floating Arm Trebuchet) and would still be considered safe by <S01> regarding the large mass being stopped abrubptly, however, a large arm, and a large mass would be required for a good trebuchet, both of which would be difficult to acheive with the game limitations...

-Leav

Re: Catapult?
 

I don't think a catapult is a good idea. If any team can build a successful one, then by all means, go ahead and try...but it will indeed be difficult to work with..Unless it can scoop and shoot.

Re: Catapult?
 

I dont know, a catapult for this application would not have to be very big - maybe a 10 or 12" throw

esp when it can be actuated with a pnuematic cylinder that can provide up to 180 lbs of force (2" diameter cylinder charged with 60psi)

Re: Catapult?
 

Actually, my spring tensioning cam was stolen from a catapult of sorts. I don't know the name and thus can't google it, but it was designed to throw basketballs. It's a little hard to explain the exact mechanism, but effectively, it was a lever with springs on one side of the fulcrum and a ball holder on the other. The thrower has super heavy duty springs (possibly garage door springs) so that it has a short throwing arm.

With the basketballs, it was able to throw about 30' and hit the same spot within a 1' radius. Personally, I think a similar device would be much more repeatable than most other ball throwers that are being discussed in the forums. For one thing, nothing is moving just before firing. This ensures that you have the same conditions every time.

Re: Catapult?
 

i also dont think that a catapult is a good idea. it may be accurate but it takes even more to get 1 ball to fall into the catapult at once, unless you want to shoot more than one at a time

Re: Catapult?
 

Not to toot my own horn, but two or three years ago, when we had the last non-autonomous period, we had a game based on soccerballs. We built a catapult powered by constant force springs (the springs made of coiled sheet metal) using 6 5lb springs, we got our robot to shoot soccer balls 99% effectively. The good thing about this was that our robot did not shoot at different velocities because of the change in our battery level.

Re: Catapult?
 

Our first design was a modified trebuchet.
But it was 1500 kg and was over 8 meters tall.

Next we built a giant rabbit...and we crawled inside it waiting for the right moment after autonomous mode...

Re: Catapult?
 

Now I'd *love* to see how someone designs a Trebuchet "repeater".
Resetting it to make the SECOND shot (esp autonomously) is
NOT a trivial robotic design problem.

But even if you *could* solve that one, it'd have to be a TINY
Trebuchet to comply with this year's build shooter sizing rules.
IMO, your biggest problem is getting it into the original volume,
to comply with shooter sizing rule <S03>:

> <S03> Shooter Mechanism must remain inside the ROBOT
> - Any mechanism used to throw balls must be contained within
> the original 28” x 38” x 60”starting envelope of the ROBOT [...]

Hmmm... How would a catapult shooter deal with this rule?

- Keith

Re: Catapult?
 

how much 'throw' do you think a catapult will need to launch the nerfball at 25mph?

we are not talking about launching a car 2,000 yards through the air here :^)

Re: Catapult?
 

Well, let's take a crack at that one...

Furthest case: At the end of the stroke, the center of the ball needs
to be moving at 12 m/s max. [edit] IOW, a point on the circle made by
the ball cradle's sweep that intersects the center of the ball must
be traveling at 12 m/s. That means the final rotational speed will be
related to the Throw Radius, which I'll define as distance between
the pivot center and the ball's center. [/edit]

We've analyzed 48 Poof-Slinky balls. Amazingly, their sample
weights are fairly evenly distributed between 175 and 186 grams,
which places the median at about 180-181g. Therefore, we then
calculated a ball will contain roughly 13.7-13.8 J of energy at 12 m/s.
(Can someone else here, please check our math? Are we right?
Anyone have more weight samples, to improve our data set?
We suspect our sample set is too small, because we didn't
get the expected 'Bell Curve distribution'...)

[edit] To calculate the ENERGY required, you now require: the
arm's parameters (specifically the arc range of motion, the
distance between the pivot and the center of the ball, and
the rotational inertia of the arm). [/edit] From those, it should be
possible to calculate the torque required to accelerate the ball
over that arc to impart the required energy, and guarantee the
system still meets the "12 m/s max muzzle velocity" FIRST spec.

You then check to see if you have a motor still available capable
of imparting the required torque, and design an arm torque system
that'll properly clutch itself to prevent self-destruction from
repeated "stop shocks"... :)

BTW, a big question for all: Is there an easy way to hand
calculate the rotational inertial of a wheel and/or arm design,
to evaluate them before construction? Weighing it is insufficient,
as you need the radial mass distribution.

Professionally, the only way I know of is to Solid Model it with
mass properties on, and run it through a Mechanical Simulation
System (MSS) software suite like ADAMS. But we don't have
that software here now. It'd be VERY expensive (and take awhile
for approvals) to buy it for the school, and we don't have training
time during the build to learn it, even if we could get ahold of
a seat or two of it... (Darn...)

We also don't have the time to build a bunch for testing. Otherwise,
the other "simple" way would be to "make one, apply a known torque,
and measure its acceleration" with some improvised measurement
widgetry (like the time it takes it to traverse a known arc between
two retroreflectors). Then tweak, and repeat... <sigh>

How do you simply measure (or calculate) the rotational
inertia of an arm system, or of a sample object (like a wheel
you have on hand), before building up a system using it?

Back to catapults...

Now a problem with catapults in general is that (when compared
to continuous feed methods like belts or flywheels) unless you
"complete the arc full circle", you are wasting a LOT of energy
per throw. With a traditional arm catapult, you have to put in
the energy to accelerate the ARM as well as the ball, and the
remainder is all lost when it hits its stop. Since the arm is
normally a lot heavier than the ball, the relative energy cost
is huge to launch a bunch of balls. And, you still have the whole
"recocking cycle time" issue to contend with.

OTOH, if you think a set of "Jai Alai" style shooters (Cestas) arranged
like spokes around a wheel, you may have something there! :)
You KEEP the energy you spent revving up the wheel, and now
only have to replace the momentum lost by throwing each ball.
You either "drip in" balls, or catch them off a dispenser to throw.

Now getting this all synced up to have the Pelota (Spanish
for a Jai Alai "game ball") properly intersect the Cesta
(Spanish for a "basket", the Jai Alai "shooter") at the right
point in each cycle to make it repeatable would IMO be an
interesting technical challenge.

Controlling exit angle is another problem. A combination of where
it first intersects the Cesta and its rotational speed determine
how long it remains on the Cesta accelerating. So, exit angle
COULD be controlled by adjusting the intersect or radial drip point,
(or by changing the wheel's speed), but all this could require a LOT
of experimentation and tweaking!

Of course, assuming that all worked, how do you then "shield"
the whole assembly to match FIRST's safety rules?

I think other rotary designs may be easier to package,
aim, and be more repeatable, which is your true desire
here. (You need repeatability first, before you can possibly
calibrate your shooter.)

But man oh man, it'd be REALLY COOL to see a two to
four armed "Cesta Wheel" (Cesta Rueda? Basket Wheel?)
in action, whipping out balls! :D

Comments?

For more info on Jai Alai, some terminology, and to see
a real Cesta (a traditionally made wicker basket shooter),
Google it, or go to: http://www.dania-jai-alai.com/page6.htm

- Keith

Re: Catapult?
 

I think you have made some assumptions that are not necessarily true.

1. whatever your catapult 'arm' is made of, once a ball is fired you must stop it before you can fire again, so the inertia of the arm is always lost (unless you are going to have something more like a bat that wacks balls that are dropped in front of it.

2. an arm that rotates only part of a circle, 90° for example, could hit a spring at the end of its motion that bounces it back into its starting position. If fact, when it gets back to its starting position there could be a second spring that absorbs (recovers) the momentum of the arm?

3. a motor would be my last choice for transferring energy into a catapult. First choice would be a pneumatic cylinder.

Re: Catapult?
 

Actually, I stated that as a disadvantage. (Sorry if I wasn't clear,
but I mentioned about the energy lost with a "traditional arm"
design.)

BTW, in FIRST contests (IMHO) "continuous operation widgets"
are always superior than any oscillating "batch" device,
The total average cycle time per item handled is much shorter
(especially if the batch device only holds ONE field object, and
there are many you need to process in a round to win)...

That's why I mentioned a "Cesta Wheel" ([TM] ;) ) as a possible
"catapult variant".

Hmmm.. It sounds cool, but I love to see that work in practice,
without risking tipping over the machine. I believe the energies
and lever arms involved are high enough compared to the total
machine mass (and its distibution) to be of concern. In a
traditional catapult, the base to arm & rock mass ratio is pretty
high. Now it may not be a problem, but without actually crunching
some numbers on a proposed design to make me feel better, I'd
worry a lot about the possibility of the machine being kicked over,
or at least having its aim ruined for the next shot. It wouldn't be
good to have to "set up" every shot with a move.

Well, in this application, I'm not so sure...

Let's consider cylinders for a moment. You can get some pretty
strong forces (up to 188 lbs directly), and you can stall it forever.
That's cool.

But a big cylinder is SLOW. I have no clue how you'd fire a lot
of balls with it in a short period of time.

Also, you don't have much total work available in a FIRST
pneumatics system, when compared to the motor/battery
system. The air storage is pretty small. The pump delivers
only a small amount of air in 2 minutes. And, you're limited
by the small hoses, causing the cylinder's slew rate limit
to be of concern when looking at cycle time.

This is a fast access, high input "total energy / round" application.
To be considered a real shooting machine, you'll want balls
to <wham> <wham> <wham> out of this thing! For Auto
mode, it should cycle faster than 1 ball/sec, and that's
doesn't even include any drivetrain positioning time to
set up the shot.

So if you're serious about shooting a lot of balls (vs just one
or two during the round for show), I believe the sheer number
of balls you want to process in 2:15 probably requires a lot
more total energy than the pneumatic charging system can
possibly provide to you, both totally and incrementally.
The cylinder slew rate limit alone suggests you should charge
a fast release energy storage device (e.g."a spring").

To ME, given that (and FIRST build rules limiting what
actuators you're allowed), it implies our choices are between:

- A BIG coil spring and an incremental release system
(kinda dangerous to work with);
- A direct motor drive; or
- Some sort of spring & motor, or spring & cylinder
combination, where the spring is recharged by either
device (and I'm still not convinced a cylinder can keep up).

However, you've raised a VERY valid point about energy recovery.
Whether you're using a cylinder, a spring, or a motor, recovering
least some of the energy is a good idea.

How about using a spring and a ratchet to fire the arm?
You "<recharge> a spring" with it, via a ratchet. (Hmmm...
CDF won't let me use the "c" word... :)) That way, IF
you can recover any energy with a "kickback spring" as
you suggest, it's stored with a <click> <click> back on
the ratchet when the arm slams home. The remaining required
Potential Energy is then added in via a cylinder stroke.
or a motor/gear combination (no solenoids allowed yet...).


Now, let's consider MOTORS. Motors are by definition a
torque device. Near stall, they put out a LOT. And, you
have quite a bit of total work energy available in the battery,
which can be tapped fairly quickly.

With a motor, one option would be "one way drive": Use
a motor/gear combination and a ratchet (or a non-backdrivable
gearbox) to <set> a BIG spring. With this system, you'll need
some kind of a triggerable clutch to unload the motor/gearbox
and release the arm. That catch could be run by either a
small cylinder, a small motor, or a servo. Once shot, re-engage
the motor/gearbox, and "wind 'er up again". (Your release is
a potential safety hazard and wearout item, so design it WELL,
with safety "lockout pins" for pit work, etc...)

The other way is to DRIVE the arm BOTH WAYS, with a
high torque, continuously connected motor/gear system.
Use mechanical or air springs on the "shot end" to help
decelerate the arm, and recover a bit of energy for
starting the return motion. Having some opto switches
or a pot to monitor the arm allows you to auto sequence
the shot with software, and eliminates "end of travel"
stall currents. It would also allow software to program
the entire cycle as a "semi-automatic or full-automatic"
shooter. But with this one, you're going to want some
kind of over-torque clutch or shock coupler, to help
manage stresses on the motor/gear system.

[edit]
Actually, I just thought about one more option: A direct
motor/gearbox, and a spring acceleration startup assist.
Now that spring could be recharged with a cylinder!
But, now you're talking about both a pneumatics package
and a big motor system. Weight might become a problem.
[/edit]


I hope this helps!

- Keith

Re: Catapult?
 

We rejected a catapult early on because of the inherent limitations of rate of fire. Spinning wheels or belts have such an obvious advantage in fire rate that we didn't seriously consider a catapult for long, although some students did some interesting math on one.

So, this is just FIRSTnoodling, and not a serious design proposal. I'm also not going to do the math, so forgive me for that, please. :)

I agree that one of the problems with catapults is the waste of energy involved. A lot of energy is used accelerating the arm, which is then usually wasted at the end of the stroke. How does this calculus change when the arm weighs essentially nothing? You could build a very stiff launching arm three feet long of hand-laid carbon fiber and fiberglass on a foam form that would weigh about about six ounces. If you molded a CF/glass dish on the end for the ball it might add another two or three ounces. I wonder how the math would work out if a pneumatic cylinder pushed on a 6-inch lever arm driving a 30-inch throwing arm on the other side of its ball bearing pivot? I have to get back to work, but perhaps some enthusiastic physics student wants to work out the numbers.

Re: Catapult?
 

Hey Rick! Email me privately and tell me how your build is going
(and what you did with the data I sent you... )

Yea even if you COULD do one, it's obviously not going to win anything
(but the crowd) this year, so don't take my comments too seriously.

But as you can see, if someone asks me to "noodle", I'll take a
real crack at it... :)

Now if a catapult was a serious payload widget contender, you'd
still have to power the thing, so my comments about pneumatics
vs motors (et al) stand.

Another one of "Keith's Rules":

"Stall can be managed. You must first look at the 'total energy
per round' and the 'energy transfer rate' requirements of
each device, before attempting to allocate actuators to
widgets on your machine. Right off the bat, those two criteria
may point you to the best actuator for the job."

I'm hoping at least that part of this discussion may be useful to
the next student trying to the classic "motor vs spring vs
cylinder" choice.

But yea, though it wouldn't win, I still feel a catapult, a "Cesta Wheel"
or some other "Rube Goldberg" throwing widget would offer a LOT
of "ohh, ahh" crowd entertainment value! (Especially if it whips
the ball wildly off course!)

Well, please realize that a catapult arm is not simply a stick,
whose strength you're trying to maximize, while at the same time
minimizing weight. It's a distributed spring. Just like a bow, the
arm itself acts as an energy storage device. The material local
to the pivot rotates much faster than the load itself. In essence,
the arm "<sets>" itself (there's that banned CDF "c" word again!)
by transferring potential energy from the spring into its length.
It then re-releases that energy, converting it into Kinetic Energy
in the load mass as it accelerates. IOW, the compliance of the arm
is a huge part of the total system.

- Keith

Re: Catapult?
 

I don't want to get into the fine details of actually making one work but the power is certainly there. A 2" bore pneumatic cylinder pushes with about 180+ pounds of force. Since work is force x distance you can multiply that out. (180 lbs of force * 9 inches of travel) inch pounds in joules is according the Google calculator 183 joules. Thats a ton of energy! I'm not sure if thats exactly how a pneumatic cylinder would translate into energy but its got to be considerable none the less. As was stated above a pouf ball needs about 13.7 joules to to go at 12 m/s(instead of the 18 joules for a single flywheel design due to the spin). That means that a 2" bore pneumatic would in theory only have to travel about an inch or two to put enough energy into the ball assuming a pretty thorough transfer of energy and a very light throwing arm. Just some numbers to think about. I'm still sticking to a fly wheel. :)

Re: Catapult?
 

Consistency is very beneficial. But the same consistency can be accomplished with the right sensors. Of course this is only in theory for my team, seeing as how this is the first time we've incorporated a sensor into regular game play.

Re: Catapult?
 

understatement of the day! 183 joules could fire 13 balls all at once!

what was all that talk about cycle times and being able to fire fast enough? :ahh:

Re: Catapult?
 

The problems I see with a catapult is that you can only set it to shoot at 1 angle I cant think of any way to change the angle in the middle of a match meaning you'll have to be the same distance away from the goal at all time. You cant set up to shoot 2 feet away and than be like 4 feet away and hope to make a goal. Also you'll only be able to shoot once, reload, and shoot again. There are much better designs that allow you to fire more than 1 in one load. ^_^

But thats just what I think.

Re: Catapult?
 

and thats assuming that the compressor keeps the tanks at a consistent pressure throughout the stroke of the cylinder... unlikely.
-edit- i guess the tank pressure doesnt quite directly matter in all cases... it just has to stay above 60psi right?

Re: Catapult?
 

<SIGH> Hmmm... How do I put this?

What you're talking about is the theoretical total energy contained
in that much compressed air. That's a lot different than what a
typical cylinder can provide in output work, in a short time.

And YES, you are still ignoring cycle time of the system
(IOW, how fast it can get ready for the next shot).
Think about it... Well SURE, it sounds great! Yes, if you could
release all of that theoretical energy in an instant, you'd fire
a whole bunch of balls, and that would be pretty cool!
(You've then just "shot your wad". It'll take time to get
it charged up again.)

But that's not how cylinders work. You can't just use the
raw air (that's "making your own cylinder", a no-no), and
you're STILL limited to whatever that darn thin tubing can
supply you.

Ya can't get all of the work out, either. (Well, not in any
reasonable amount of time, anyway... :) ) Cylinder motion
is quite a bit like a capacitor charging in electronics.
The difference between the load's back force and the
cylinder's force is very important. If you actually tried to
get that full energy out of the cylinder by backloading it
heavily, it would take a very long time for the cylinder to
move to its final position (which in reality it wouldn't...
it'd stop from static friction first.)

The point is that cylinders work best when you can vastly
overpower the load's back force, or else they slooooooow
dooooooown to a craaaaaawwwwwllllll. Not very useful
in contest.

OOC, Have you guys ever actually tried to USE a 2" cylinder
in a contest before? We have. It's SLOOOOOOW, and you
can cycle it fully only a couple of times in a round. You just
simply don't have the air, nor the slew rate to drive it any
more than that!

Now I've used the 2", full length cylinders in several contests
with a couple of my teams. In fact, 1502 used one last year with
a multipositioning shoulder joint for our tetra arm. BUT, we
had to be very careful in our design to not ask it to move
around too much, or we'd risk running out of air. That's why
we used a motor for our arm's elbow joint. If need be, we could
use the "last gasps of air" to move the shoulder into a good
position, and leave it there. That happened a couple of times
in contest. No sweat. We could still keep lifting tetras from
the HP to the top of a goal via just the elbow joint, while the
air system slowly regenerated..

But what you're suggesting is to move it back and forth
repeatedly. Sorry to bust your bubble, but you just can't
do that more than a few cycles over several minutes with
a 2" cylinder, or it stops. (I'm getting the impression some
people here have NEVER worked with the FIRST pneumatics
before...)

But don't take my word for it. Try it out for yourself when
you get the time! You don't have to build the catapult,
but you really need to know how the kit pneumatics behave,
or one of these days you may get yourselves caught by
expecting too much out of them.

Try this: Put together the basic kit pneumatics package.
Nothing fancy. Add ANY 2" dia cylinder. Point it downward
and attach a good sized weight to the clevis. (Let's say about
100 lbs.) Now try to run it continuously up and down full strokes,
and see what happens. Does it do it quickly? Does it slow down?
How many total inches were you able to lift that 100 lb weight
up in 2:10?

Do that, and you'll gain some good insights into how much you
can get out of the kit's pneumatics. Good luck!

- Keith

Re: Catapult?
 

[smileing] Whenever someone comes out and says that something 'cant be done' at this point in the design cycle, sure enough when you get to your 1st regional you will find someone who found a way.

If a pneumatic shooter can launch ten balls at once, how fast does it need to cycle? how many times per match do you think you will be launching 10 balls at once?

The balls dont weigh 10 lbs each, so you are not pushing or lifting a 100 lb mass.

How do you get all that air in the cylinder 'all at once' ? You dont: you block the cylinders motion halfway down it length, precharge the cylinder up to 60 psi, pull the blocking pin out of the way and POW! its like a garage door spring with 180 lbs of force being released.

How much will ten balls weigh? with 180 lbs of force behind them what does F=MA tell us?

Re: Catapult?
 

im not to sure wat team or if this is even the exact design but i think a pneumatic cylinder pulled back a spring or surgical tubing so a ball could be loaded from the topand then it somehow relesed it and went back to shoot another one. :confused:

Re: Catapult?
 

The only good catauplt would be a tribuchet, but if you used that, good luck aiming/firing at the goal consistently.

But seriously, if someone's team could pull off a catapult with... say 75% accuracy, I would be amazed.

Re: Catapult?
 

Actually, a trebuchet would be an accurate and consistent, but really impractical device for a robot. Trebuchets rely on a counterweight to pull the arm down. In a robot, where every ounce is critical, anything relying on a counterweight is a Bad Idea. Likewise, I cannot think of any practical way for a robot to reset the sling of a trebuchet after each shot. A rotating wheel, or the energy of a wheel imparted to a belt, is a much more appropriate mechanism for a robot. Even a mangonel or oranger would be easier to build than a treb, even if you have to design for the end-of-stroke shock.

Re: Catapult?
 

I suppose, but a trebuchet would be frickin awesome.

Play Age of Empires?

Re: Catapult?
 

Im am not completely sure but I dont think that catapult is legal because you can not use anything that uses a sudden stop. But I do think you could use a trebuchea.

Re: Catapult?
 

-a catapult design does not have to come to a sudden stop
-the energy stored in the arm's rotational motion doesn't have to be lost (it can be restored into its original form)
-an effective & accurate catapult can be made that is not a trebuchet (ie doesn't use weight @ height to store energy or an arm/sling)
-trajectory could be changed in-match, but it becomes complicated. With an intelligently selected single trajectory it is possible to score at a wide range of distances from the center goal.
-a catapult can be implemented with a rate of fire of > 1 ball/sec
-a clutch or similar device is not necessary to "k"ock the mechanism when driven by a motor
-a working prototype using a whammo-type jai-alai cesta achieved much greater than 12m/s and has many variables for adjustability.

Re: Catapult?
 

Check our catapult-like device out in prototype: http://marsbot.org/files/arm_demo.zip

We finally got our hardened shaft and everything is coming together now. I will try to post a video of it throwing tommorow (assuming it can do that yet :rolleyes: )...

Re: Catapult?
 

well even tho u might get some distance with the catapult idea its not very acuarte and plus u will eed a lot of tention to get it right speed and distance our team was the long island champions of


pumpkin chuckin contest

Re: Catapult?
 

Boy was that impressive!! I remember that one.

Re: Catapult?
 

Our shooter was just rapid fired for the first time yesterday and I am proud to present a video of our bot firing. Please let us know what you think (leave a comment here)!


Shooting.wmv (19.6 MB)