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Unread 05-13-2012, 10:39 AM
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Three joint arm

Hey, I'm the new build team leader of our team (#2212 The Spikes) and I decided to have the crew bulid soething we've neve tried before. We decided on building a multiple-joint arm, preferrably more than two, just to make things a little more interesting.

Did anyone ever see (or build) such a thing and direct me to someone who can explain it to me, or atleast a team? Pictures would be very helpful too.

Thanks in advance,
Bar, #2212
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Unread 05-13-2012, 12:48 PM
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Re: Three joint arm

Wildstang's 2007 robot comes to mind (3rd joint pivoted along arm's axis, allowing them to reach to the side). If you do this, you should also devise a way to control such an arm reliably, there is no point in having a fancy arm if the driver can't use it effectively.

http://www.chiefdelphi.com/media/photos/27788
http://www.chiefdelphi.com/media/photos/27192
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Unread 05-13-2012, 01:09 PM
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Re: Three joint arm

Quote:
Originally Posted by PAR_WIG1350 View Post
Wildstang's 2007 robot comes to mind (3rd joint pivoted along arm's axis, allowing them to reach to the side). If you do this, you should also devise a way to control such an arm reliably, there is no point in having a fancy arm if the driver can't use it effectively.
I actually meant arms with all joints on the same plane ( --O--O--O-- ). And you're right, for compatition purposes it's pointless to have a fancy and uncontrollable arm, but in this case the goal is to have a big fancy arm that has no purpose (maybe Isould have mentioned that earlier...).

But thank you very much
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Unread 05-13-2012, 01:26 PM
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Re: Three joint arm

Team 33 2005 was a double jointed arm, and 2007 was a triple (shoulder, elbow, and rotating wrist). There is a lot of good documentation on the 2007 arm in the Behind the Design book from 2007.

Most FRC games I have seen can effectively be played with the 2 joint arm. The third joint adds more complexity than it would likel be worth for many games, but it would be a good learning exercise.

I think it is wise to protype an arm as the likelihood for a pick and place game is pretty high this next year. Also important to any arm design is the object manipulator. Instead of doing a 3rd joint, I would focus some effort on trying to pick up an odd shaped piece like a cone, cylinder, or some other non-ball shaped piece (inner-tube, tetra, box...).
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Unread 05-13-2012, 01:32 PM
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Re: Three joint arm

Our 2001 and 2007 robots have several joints.

Another picture of the '07

The CAD will be posted to FRC-Designs.com soon.
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Unread 05-13-2012, 01:35 PM
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Re: Three joint arm

Team 2056's 2011 robot is another example of a successful 3 jointed arm.
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Unread 05-13-2012, 01:45 PM
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Re: Three joint arm

Quote:
Originally Posted by Nemo View Post
Team 2056's 2011 robot is another example of a successful 3 jointed arm.
2056 was only a two joint. Shoulder and wrist.

Team 1519's 2007 robot was a triple joint.
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Last edited by BrendanB : 05-13-2012 at 01:49 PM.
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Unread 05-13-2012, 01:51 PM
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Re: Three joint arm

How about 25's 2004 robot's arm?

http://www.chiefdelphi.com/media/photos/17115
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Unread 05-13-2012, 01:55 PM
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Re: Three joint arm

Quote:
Originally Posted by Nemo View Post
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
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Last edited by akoscielski3 : 05-13-2012 at 01:57 PM.
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Unread 05-13-2012, 02:21 PM
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Re: Three joint arm

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
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Unread 05-13-2012, 04:07 PM
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Re: Three joint arm

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.
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Unread 05-13-2012, 04:10 PM
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Re: Three joint arm

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.
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Unread 05-13-2012, 04:40 PM
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Re: Three joint arm

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.
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Unread 05-13-2012, 07:22 PM
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Re: Three joint arm

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.
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Unread 05-13-2012, 08:45 PM
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Re: Three joint arm

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
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