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. 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.
Also, im thinging of using a cabinet clip. You know, the little metal nipple with the 2 rollers attached by a spring to keep the cabinet door closed, as the mech to hold the tetra, with some base to hold them lateraly still
and Yes i did a search and the other 2 arm threads do not address this
we did a teloscoping arm-ladder last year to hang with, and it was very reliable on hanging. (getting up the stairs was a different story, but that’s not the point). last year was our rookie year, and we used a cable that did NOT change length to extend it. the geometry was so that all the segments were parallel. its hard to explain, and i dont have schematics on the computer… or anywhere really… (i SAID we were rookies…)
i was in charge of the mechanism last year, so feel free to PM me if you want help. (but you -should- have decided your design by now, but im not one to judge… based on my team…)
because the cable is always the same length, we used a winch drum and 2 (fisher price ones i think) motors to extend the ladder (it even had rungs!) and retract it. (same motors! go cable geometry!)
hope this helps, if i find a schematic of our cable somewhere i’ll be sure to post it,
because my team is a class. Not a team, we have not chosen a design yet, that is what our “goal” for tuesday. We are so sick of our teacher, we are going to start taking it home to my friends machine shop. But, if anyone has detailed pics or drawings of an arm is greatly appreciated.
Last year i think our team (#269) had the best telescoping arm… I wasnt involved with the design much but i welded for some of it… it was a triangular tubing shape and it went out ten feet with a hook on the end… then to pull our selves up we used a winch and the rope went to the the top of the arm on the hook… this hear we have a control system with two pistons to turn quicker that require pneumatics so our arm extends with the pneumatics compressor also… very heavy but works great… a telescoping arm isnt hard to build. search for good motor ideas and just build prototypes for it and see what works best…
The mechanism was pretty simple: three lengths of 80/20 extruded aluminum linked together with nylon sliders. Pulleys guided spectra cable (from Small Parts) which raised and lowered the arm (one set of cables for up and another for down). We used 50lb test cable for up and 300 lb for down, since that had to lift the weight of the robot.
Our only mistake was using two different motors for the two directions. We thought we could pick motor speeds that would keep the cables in synch, with tensioning loops to give us some slack to play with. That didn’t work well at all and resulted in broken cables, including one disaster that broke the 300 lb cable in the middle of a match. It made quite a load SNAP that we clearly heard from the nose-bleed seats.
We didn’t produce engineer documents, but I’m happy to answer any questions and/or take more pictures to show the mechanism.
Last year, our telescoping arm used no pulleys and was pretty easy to fabricate. We used three different sizes of square aluminum tubing that folded in on eachother to fit within the dimensions allowed. We used a van door motor directly connected (as can be seen in the photo below) to rotate out to full length. I don’t think the van door motor will cut it this year lifting those 9 pound tetras, but it could be possible to do what we did with the arm setup of the new transmissions (outlined in “Guidelines, Tips & Good Practices” available in the manual section of the FIRST website) and lift the tetras. I haven’t done the calculation yet.
well we are doing the same thing and it looks like we almost have to do the pullys we have tryed many ways also. we actually had a couple ppl on that team go to a hardware store and asked them to see a fork lift and we analized how that worked. once u know how a forklift works then make it a little simpler and go with it!!! good luck
Hope I helped
In addition, we did some experiments and found that the robot can go into this year’s competition nearly unmodified (not that we would, or could per the rules). We’d only need to change the manipulator on the end (which, in theory, could be just a stick).
Here’s some pictures from the Silicon Valley regional SVR
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.
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:
keep the proper tension
make sure that cables only have to move in the plane of the pulley
have a means of adjusting the cable length to make it easier to do 1 and 3
have cable to pull both ways, the cables should be driven off of the same drum if possible
keep the proper tension
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.
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.