Science Olympiad Grabber Arm Robot

[BMX670]

http://www.vexforum.com/gallery/showimage.php?i=1004&c=8
this is the robot I built for my high school Science Olympiad competition. It fits inside the required 30x30x30cm box and can pick up anything from a CD or penny to a golf ball. The only problem with it is that the lowest joint on the arm doesn't actually work. I have a 12-84 gear ratio on it, but that doesn't help if the teeth on the gears skip. (note that the two gears on the arm other than said ones are only there for the purpose of slowing down the arm when it does this.) I incorporated it into the robot's design so that that section of the arm will hang in the position shown and not fall any further forward, but I would really like to fix it. Any ideas?

close-up of the gears on the lowest joint
http://www.vexforum.com/gallery/showimage.php?i=1005&postid=732#post732

[robofreak]

Thats really cool, you have a lot of joints on that arm allowing it to reach behind something.


If i were you i would add a wireless spy camera where the motor is on the gripper that way you could grab things behind an obstacle without having to see it.

You can get wireless spy cams off ebay with no sound for $9.99 and $20.00 to ship it from china lol!

Anyways nice robot.


Also you could elevate the tank treads a bit with the rollers that are included in the kit that way the robot would be able to pass over low obstacles.

[robofreak]

Sry for the double post but i did not answer your question!


The 12 to 84 tooth ratio might not pack enough bang to hold the arm up in some areas.

To fix this get the worm gear mount and the advanced gear kit, the worm gear is like a 4 to 84 tooth gear ratio and it prevents the gears from slipping backward.

[jgraber]

The usual problem with the first arm joint is that it has to lift the whole dead weight of the arm, in addition to the weight of whatever the arm is holding.
You can somewhat 'cancel' the torque of the extended arm by using a round-ish pulley (like a 4" wheel) on the arm shaft, with a string wrapped around it going to a spring (rubber band). Put the arm straight up (no torque) when the string is straight out, then it will wrap around the wheel in either direction to provide counter-torque as the arm moves.
As with any engineering effort, a few iterations (try rubber bands in series or parallel) will greatly improve the results.
To provide the greatest lift capacity, set the springs so that the motor has to work nearly to its gear slip limit just to push the arm down. Then the motor and the spring can work together to lift nearly twice the weight that the motor could lift on its own.
-- jgraber

[GGCO]

Maybe if you creatively used gears it would solve the problem. Just think about it a sec.

[BMX670]

the wrist got stuck pointing straight up during the competition (although I fixed that problem,) and it still took silver and is going to state level.

[bikehikeclimb]

To fix this get the worm gear mount and the advanced gear kit, the worm gear is like a 4 to 84 tooth gear ratio and it prevents the gears from slipping backward.

I agree it is just too much weight for that to work.

Anyway, great placement of your joints!

Kyle