Catapult?

[kmcclary]

Originally Posted by KenWittlief

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.

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".

Originally Posted by KenWittlief

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?

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.

Originally Posted by KenWittlief

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

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