Tennis Ball Thrower

[dan]

Hi,
I'm thinking about trying to build an iPhone-controlled ball-thrower for my dog.  Reading this thread made me think about the torque involved in throwing a tennis ball.  What is the limiting factor in creating torque: the servo, or the smart board?  When the reset you described above occurs, does that mean the smart board just can't handle that load?
Forgive me if my questions are naive... I'm new to all this stuff.  I imagine one of the downsides to making a product that is so user friendly is you are going to get a lot of questions from newbs.

[jason]

The torque limit of a servo comes from the size of the motor inside the servo.  But there is a limit to how much current the servo can draw from the module.  I suspect the torque needed to throw a tennis ball far enough to give your dog any degree of exercise is beyond what the modules power supply can handle.

But don't give up yet.  My wife made an ioBridge controlled ping-pong ball thrower out of Legos and servos.  She went through a few iterations but the design that seemed to work the best used two spinning drums to shoot the ball out.  The spinning drums were connected to DC motors controlled by a relay board.  She used a servo to drop the ping pong balls down a chute that led in between the drums.  The drums were spaced just wide enough to grab the sides of the ball and shoot it out.   

You could probably use her approach on your application and avoid having to use huge expensive high-torque servos.  You could still use a servo to point you tennis shooter though.

[thecapacity]

Dan,
I'm sure you saw this video;

http://gadgets.boingboing.net/2008/03/17/video-tennis-ball-la.html

and probably got inspired, and I thought it looks like he uses a servo to retract the slingshot but relies on the tubing for power.

I also saw a similar technique in this lego crossbow;

http://www.youtube.com/watch?v=4rXeO2rPgmY

He relies on the servo for retracting but uses a mechanical system to store the potential energy.

So there are potentially a few ways to work around the issue.
Hope some of those help inspire!

[Fodder1]

You could use a bottle of compressed Air and some valves.
Put the balls in a hopper and feed them one at a time into the chamber using a servo. then use the switch to fire off a jet of compressed air to shoot the ball.

Distance could be controlled via amount of compressed gas released buy the solioid.

[Fodder1]

Hi Everyone,

I have been thinking about this and there is an even better way to do it.
Still using compressed air but using a an actuator conected to the thowing arm.
These actuators really have a lot of power and with a bit of stuffing around the throwing arm could be reloaded and even the direction changed randomly so the dog does not get board with it.

using the solioids this should be really easy.

CH