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  #16   Spotlight this post!  
Unread 26-01-2010, 09:06
mrcolling mrcolling is offline
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Re: kicker idea input

Quote:
Originally Posted by AndrewJS View Post
That is close but not quite correct. Momentum is conserved, and energy is conserved.

Therefore:

Conservation of Momentum
M(kicker)*V(kicker[before]) = M(Ball)*V(Ball) + M(Kicker)*V'(kicker)

Where V(kicker[after]) is the speed of the kicker after hitting the ball (do not assume this is 0).

Conservation of Energy
KE(kicker[before]) = KE(ball) + KE(Kicker[after]) + Eint

(Where KE = 1/2mv^2 and Where Eint is the internal energy lost to deformation of the ball, I suppose you could include others like heat as well, but I don't see too much heat being created here). I ran an experiment in my garage (bouncing a soccer ball on my cement floor, recording with a camera, and analyzing the height of each bounce) and saw that roughly 60% of the kinetic energy of the soccer ball was lost on each bounce. I have not run equations taking this loss into effect, but my guess is that Eint = .4*KE )

Collisions can be elastic, inelastic or somewhere in between. Elastic means that kinetic energy is completely conserved (Eint in this case would be 0). Inelastic means kinetic energy is not conserved.

For sake of equations, lets assume the collision between a kicker and a soccer ball is completely elastic, so Eint = 0. You can then solve the two above conservation equations (took me a full page) but you get in the end:

V(ball) = 2 * V(kicker) * M(kicker) / ( M(ball) + M(kicker) )

Lets take the case where M(kicker) is much greater than the mass of the ball. Taking a limit as M(kicker) -> inf, Then:
V(ball) = 2 * V(kicker)

This is the maximum velocity you can ever hope to achieve with the ball (twice that of the kicker). As Walter Lewin (MIT) would say, "very non-intuitive." Nevertheless, don't concentrate on making your kicker too massive. The real key is balancing having enough mass and speed, as mentioned before using F = ma where your kicking force will be constant (if you are using a spring/elastics and pull it back to the same spot each time).

Yet this doesn't take into account the loss of energy due to Eint. You'd have to re-solve all the equations for this. Or, just plan on designing something about twice as powerful as it is in theory.

This is the point where I stopped doing equations (trying to calculate everything was getting quite ridiculous) and just built a kicker prototype, which actually worked quite well at throwing balls across the room and breaking ceiling tiles. We are going with a spring loaded pullback (our prototype used bungee cords) Still working out details for a fast and strong winch to pull it back. Our wish is to have the release be adjustable, this will give use the advantage of selecting how hard we want to hit it. If all else fails, we can use a (probably simpler) system to release it at the same spot every time.
Greetings

The ball when compressed is like a spring and stores some energy which is then released as the ball regains its shape. When a human kicks a ball the follow through past the point of impact is important. The mass at the end of the kicker should stay moving in the direction of the target during this compression of the ball otherwise energy will just be sent back to the robot. I would think some forward motion of the robot combimed with the kick would give positive results for maximum force.
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Unread 26-01-2010, 12:42
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Re: kicker idea input

I tried to post this before but since I am new to the site I don't think it allowed me to make a post with a link.

RoboCup has been around for a long time now and they have really perfected the kicking mechanism. Here is a link to a white paper that may help you out. Keep in mind the solenoid is illegal.


http://roboturk.googlecode.com/files..._mechanism.pdf
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Unread 26-01-2010, 12:43
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riptide riptide is offline
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Re: kicker idea input

I tried to post this before but since I am new to the site I don't think it allowed me to make a post with a link.

RoboCup has been around for a long time now and they have really perfected the kicking mechanism. Here is a link to a white paper that may help you out. Keep in mind the solenoid is illegal.


roboturk.googlecode.com/files/robocup_shooting_mechanism.pdf
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Unread 26-01-2010, 13:17
tomgee4me tomgee4me is offline
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Re: kicker idea input

Has anyone considered the bumper overhang? I'm having a tough time with all the bumper restrictions severely limiting design choices. 10" high over a 9" diameter ball. If the ball rolls further than 4.5" under the bumper, a 45 degree kick will not clear it!
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Unread 26-01-2010, 16:06
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Re: kicker idea input

Quote:
Originally Posted by tomgee4me View Post
Has anyone considered the bumper overhang? I'm having a tough time with all the bumper restrictions severely limiting design choices. 10" high over a 9" diameter ball. If the ball rolls further than 4.5" under the bumper, a 45 degree kick will not clear it!
Raise your bumpers up to the maximum height of the bumper zone which is 16". This should position the bottom of your bumpers at about 10.75" - 11" from the floor. (assuming bumper height of ~5")

This should free up a bit more room for your kicker.
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