Three joint arm

[akoscielski3]

Originally Posted by Nemo

Team 2056's 2011 robot is another example of a successful 3 jointed arm.

2056's 2011 robot was only a 2 joint arm. But it had a "jaw" like tube manipulator.

EDIT: BrendanB beat me to it

[pfreivald]

Our too-slow, wonky-clawed two joint arm could pick up from the floor and place objects up to 12 feet off the ground. Why would you need a third?

http://www.youtube.com/watch?v=m3TE2FCp9A8

[rachelholladay]

For the 2011 year we built a three jointed arm: shoulder, elbow and wrist. The picture below shows them fairly well -
http://team1912.com/photos/mediapage...ach11/_h21.png

From that year we learned 3-jointed arms are probably not the best idea. (although i dont doubt that effective ones can be beautifully made). We could have built something with less degrees of freedom that would have weighed less, been easier to control, and operated in faster and safer mode. It was a hard learned lesson and I hope we don't make similar mistake any time soon.

[PAR_WIG1350]

My suggestion would be to do everything you can to make the segments farthest from the attachment point of the arm as light as possible. and those closest to the attachment point as strong as possible. Flexing and torque are your enemy. I suggest very thin wall high-strength aluminum round tubing for the last section or two, 1/16th for the second segment, and 1/8th for the first. I also recommend that all segments except for the last one be constructed from two parallel sections. this makes the load distribution more symmetrical and results in less twisting. Your first joint will need either a strong motor, or large bore pneumatic cylinders. to reduce the torque caused by the large cantilever, place the motors at the beginning of the segment before the one that it moves. Also, the first segment should be balanced with surgical tubing so that it is stable with the arm folded. To get an idea of the kind of load the arm will be under, bolt three pieces of aluminum together and bolt one end down to something sturdy so that it pivots at that end. Then try to lift it from the pivot end.

[Mark McLeod]

Here's a short clip of our 3-jointed arm from 2007. The controls were fairly simple, because for that game most of the joints could be slaved to the position of a master joint. So the arm driver just specified where the tip needed to go and the rest of the arm followed.
There was a tightly constraining 80" cylinder written into the rules that year, so the software managed all the joints to keep the whole arm within that imaginary cylinder.

The arm was modeled and controlled in Vex first to test the control logic.

[Lil' Lavery]

My personal design philosophy is single-joint or bust. Simpler to build, easier to control, easier to program, fewer points of failure, and require fewer motors (and speed controllers). There are plenty of other options to increase an arms "reach" other than adding more joints.

I do appreciate the desire and lessons that can be learned from taking this on as an off-season project, especially in regards to the control perspectives.

[JosephC]

68's robot from 2011 had 3 joints (well 2 and 1/2; we had a shoulder, a elbow, and could flick our "wrist".)


Skip to 2:57 for the arm, sorry I couldn't find any better pictures. They're all at school.
http://www.youtube.com/watch?v=QWKMNXKR7Ns

[AdamHeard]

Originally Posted by PAR_WIG1350

My suggestion would be to do everything you can to make the segments farthest from the attachment point of the arm as light as possible. and those closest to the attachment point as strong as possible. Flexing and torque are your enemy. I suggest very thin wall high-strength aluminum round tubing for the last section or two, 1/16th for the second segment, and 1/8th for the first. I also recommend that all segments except for the last one be constructed from two parallel sections. this makes the load distribution more symmetrical and results in less twisting. Your first joint will need either a strong motor, or large bore pneumatic cylinders. to reduce the torque caused by the large cantilever, place the motors at the beginning of the segment before the one that it moves. Also, the first segment should be balanced with surgical tubing so that it is stable with the arm folded. To get an idea of the kind of load the arm will be under, bolt three pieces of aluminum together and bolt one end down to something sturdy so that it pivots at that end. Then try to lift it from the pivot end.

The entire structure could easily be 1/16". 1/8" is very inefficient in comparison.

[Kyler Hagler]

Here's a look at out 2 jointed arm. The Shoulder is chain driven by the cim on the top half of the robot and the second stage of the arm is driven by a window motor. The claw was controlled by a pneumatic piston that opens and closes. The metal on the arm was .060 i believe, and had aluminum circle tubing that spanned to each side that we riveted into it. This would be something to look at. Message me if you want some more pictures. You could use this for some ideas.