A friend of mine is designing and building a web browser controlled system that includes a 2-axis camera and, of all things, a ping pong ball launcher (don't ask).
He has the camera and servos working and is in the conceptual stage of the ping pong ball launcher. Neither of us have any experience launching ping pong balls, so I thought I'd ask the group for ideas on the best simple way to do this.
He doesn't want to use compressed air or anything else that requires a complex support system - just an electro-mechanical system.
Go buy or borrow one of those little electric indoor auto-return putting cups. No, not pudding, putting. When someone actually manages to put a golf ball into one, the ball rolls into a little cavity where a switch is activated. The switch, in turn, applies power to a decent solenoid. The golf ball is propelled back where it came from, usually at a pretty good clip.
I'd use a similar concept, if not the actual solenoid, depending on how far you're wanting to shoot these balls.
Simply make a ping-pong ball cannon like this:
\OOOOOOO/ \OOOOO/ direction of projectile
^ ^ | | barrel | | solenoid
Crude, but you get the idea (maybe). The balls funnel down to a PVC pipe big enough for a single-file of ping-pong balls. The bottom ball happens to line up directly in front of the solenoid. Perhaps a couple of very light flat springs or plastic tabs could be glued into the beginning of the horizontal barrel, to keep the balls from rolling out when the barrel is at a negative angle, and to keep the ball near the solenoid. When power is applied to the solenoid, it will kick the ball past any restraining tabs, down through the barrel, and hopefully some distance in the direction the barrel is pointing.
If you just need to ping the balls out there a ways, this should work. If you want these things to leave welts, then compressed air is probably the only simple option. Mechanics are the same, except your solenoid controls an air valve.
Lego Mindstorms has a disc-launcher project that might give you some useful insight. There are two wheels spinning in opposite directions and a little conveyor that feeds the discs into the wheels. Since the wheels are spaced juuuuust a little smaller than the disk and have a little give, they grab the disk and send it flying. I seem to remember the setup actually had 1 wheel slight faster so it would impart a spin on the disc, not sure what effect that has on the aerodynamics of a sphere.
Rotate the axis of rotation 90 degrees for a floater or a sinker, depending on wether the top of the ball is rotating into or out of the direction of motion..
How about using a simple solenoid? Voltage gets applied to the coil, which pulls the core in. The core can be made long enough to strike the ping pong ball. The core gets returned to it's resting position by a spring after power is removed from the coil.
Thanks for the ideas. I like the golf ball putting return device. We were thinking about solenoids, but most solenoids are way to wimpy. Those putting returners do have some powerful solenoids, though.
You could use compressed air to drive a pin that strikes the ball...something like a pinball machine pull. BTW, the solenoids in pinball machines can launch a large heavy steel ball pretty fast. You might want to do Google search on pinball machine parts.
Ok here is a new, and clearly superior, solution - it just took me a couple of days to think of it. Cut a strip of metal at least one foot wide, 3/8" thick, and as wide as a lane of traffic. Put the strip of metal on the road outside the location of the webcam. The metal strip should have one long edge raised an inch or two on the side opposite the flow of traffic, and the short edge allowed to pivot as cars drive by and compress the device. Some suitable torque transmission element that minimises frictional losses should be used to connect the metal strip to the apparatus indoors. I would recommend 1 inch steel shafting fixtured with sealed bearings and pillow blocks, with universal joints where absolutely necessary. The output end of the shafting is used to rotate the spring about the axis which it is wound. For a spring a surplus coil-over from a small car strut might. The spring is released via soleniod, and a ratched arm that holds the spring tension until triggered is used to throw the ball. Perhaps a caddy could be loaded with balls, that allows a new ball to be gravity loaded each time the spring firing assembly is released. Heavy traffic is a must, as the ball can not be fired until after the spring is tensioned. For more rural locations a windmill might be used to pump water into a 100+ gallon container that uses a pulley system to tension the spring. I'm suprised nobody else came up with this first.
How far can one throw a ping-pong ball? The mass is so small; wouldn't wind resistance dictate increasingly smaller gains for larger launch velocities?
On the robix thrower, it says that 3 servos can result in 90% in tossing balls into a cup. Why would 3 be more repeatable than 1 servo?
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