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Originally Posted by M. Hicken
Since i have been told that arms with 2+ joints to them usually fail, we are looking into making a telescoping arm. All of our attempts in the past have failed miserably.
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There are many industrial robots that have more than two joints. However they were designed by people who have been working on such things for years and they do without a lot of consious thought things that make the job much easier. Starting from scratch is what makes it hard. The really hard part about a mobile robot with a multi-joint arm is keeping the CG within limits. Another key problem is making the arm go where you want and getting it to stay there. Some of the new "canned" code can help with that if you install some form of position sensor.
There is no inherent reason why a 2 joint arm should always fail, but there are many non-obvious reasons why a particular design might.
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Originally Posted by M. Hicken
Does anyone have any ideas on how to make a telescoping arm ~10' tall. I am also trying to aviod using a lot of cables and pulleys to extend it.
Thanks
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Cables and pulleys are very light weight ways to transmit power and adjust the force requirements of a system to something a motor can handle. The keys to getting cables to work in any application are to:
1) keep the proper tension
2) make sure that cables only have to move in the plane of the pulley
3) minimize slack
4) have a means of adjusting the cable length to make it easier to do 1 and 3
5) have cable to pull both ways, the cables should be driven off of the same drum if possible
6) keep the proper tension
7) be nice to your motor (this goes for arms too) never design it to require more than 25% of stall torque in normal operation then you will have plenty of margin for those exceptional occurrences.
8) keep the proper tension
We have had telescoping lifts that have gone to 12' and more and have never had an issue with binding or had a major failure in competition. We have broken cables but nothing more serious than that and even those have been rare and due to operator mistakes. Done properly a cable system can be very low maintenance, but ignore the above and you risk having a kinked rat's nest.
There are other ways of telescoping, but compared to a cable set up the range is limited and they are heavy. Ball screws are linear actuators thatwill work if you don't want to go too far. I'd say if you only want to extend a foot or so they might be worth a look. But if you want to go to 10' then they will probably be pretty heavy.
If you have questions or need help with a specific design feel free to PM me.