telescoping arm

[CharlieWilken]

I am wondering if anyone has ideas for actuating a telescoping arm made of say square tubing. I am assuming that filling it full of air pressure would be illegal. How could three or more pieces reach 10 foot to hang a hook on the horizontal bar?http://www.chiefdelphi.com/forums/ne...ewthread&f=22#
http://www.chiefdelphi.com/forums/ne...ewthread&f=22#

[ahmed]

My team built a telescopic arm for last year's Stack Attack, also made out of square material..

http://www.chiefdelphi.com/forums/sh...&highlight=843

[Jedi Padawan]

My team has built them in the past (shudder) and they are "extremely complex to build" not that I would say that but I trust the 4-5 engineers who almost collapsed into fits when we said moving arm to know what they are doing. plus if you work it right you might not need a telescoping arm to begin with....

[_NiKki_]

you could make a pulley system with cables running through the telescoping components with a few motors. my team was contemplating on doing that.

[KenWittlief]

ahmed: Nice photo of your bot from last year. How did the arm extend?

did it use cables and pulleys? how many stages did it have?

[ahmed]

Originally Posted by KenWittlief

ahmed: Nice photo of your bot from last year. How did the arm extend?

did it use cables and pulleys? how many stages did it have?

Actually, quite sadly, it's my first year on the team. I *almost* certain though it used cables and pulleys

I'll try and check with my teacher

[KenWittlief]

that works pretty well as long as there is NO WAY for the cable to jump off the pulley - it has to be covered or the cable must have no where else to go.

this is how fork lifts raise way up BTW. same idea

[Mike Norton]

http://www.valleytech.k12.ma.us/robo...1Robot2001.jpg

this has five stages half of the 1/8" cable is inside of the machine. This was a push pull system. 2 different cables. one to pull out and one to pull in. This is not hard to do at all. the hardest thing was to make the squares out of thin alum. we went 14' with this machine. we went 9' the year before.


we were only 17"when the machine was completely closed. this kept our center of gravity pretty low.

[kevin.li.rit]

We had a non functioning telescoping arm for our 2002 robot. It would extend to almost half the field but it would always jam. We did it just by stringing it and then using the motor to wind up the wire.

[Yan Wang]

For the Canadian teams, see if you guys can get the engineers who worked on the arm for the Space Shuttles to help you guys.

I'm sure they'll provide the best input.

In fact, how can I get ahold of them?!

[KenWittlief]

That arm was designed over 25 years ago (Space Shuttle)

those guys are all walking around in florida now wearing white shoes and polyester sports jackets :c)

[Craig]

Hey Guys I'll shed a little light on our arm from 2003

It took 4 solid weeks to work out all the bugs in the telescoping mechanism. No pulleys or cables in that arm either. We used "fish tape" which is commonly used for running electrical wires thru conduit. it's like a 15' long piece of spring steel about 1/4" wide that we moved with two rollers. When retracted the tape was wound up inside the big brass ring that you see on the side of the mechanism. To tell the truth that thing was a pain in the $@#$@#$@#, it worked really well but only 60% of the time. That is until the bot fell on the mech in the first practice heat, from that point on it worked so well we could shoot the smallest extension like a spear, Almost lost my head the first time it happened!

[bheller]

Originally Posted by CharlieWilken

I am wondering if anyone has ideas for actuating a telescoping arm made of say square tubing. I am assuming that filling it full of air pressure would be illegal. How could three or more pieces reach 10 foot to hang a hook on the horizontal bar?http://www.chiefdelphi.com/forums/ne...ewthread&f=22#
http://www.chiefdelphi.com/forums/ne...ewthread&f=22#

I was curious about this too, and saw many teams doing it successfully in 2000. There is a pretty good white paper powerpoint at ChiefDelphi by ChrisH on different ball lifts that shows how wires/chain could be used for a telescoping lift. So I built one out of Lego in a few hours to learn more about them.

Motorized Lego Telescoping Lift

Photo Contracted

Photo Extended

If you exclude the hook at the top, the structure doubles it length with three segments, going from 6.5" to 13".

The design certainly taught me some lessons about these lifts. Here they are:

1 ) There is a lot of tension in the plastic lego chain, so much that the chain pulls apart before the structure can actually lift itself up the bar, though the motor/gearing has enough torque for that. I would not use chain for a mechanism like this. Instead, I would use flexible steel cable and pulley. At the base, instead of a driving gear from the motor, the motor would turn a spool. The spool would have enough extra wire to move in both directions without running out of wire. Don't forget a tensioner... with cable or a non-continous chain (it can be fixed in place, like the locked gear, on the top segment) tensioning may be pretty easy.

2 ) Side loads. Just like overhanging a wheel can be bad, overhanging the shafts with the chain in the model (the ones you see moving, closest to the camer) is bad. It caused a side torque that jerked twist the assembly toward the side, almost breaking it, when the model reached the top of its travel. To fix this, I would route the chain/wire in the middle of the extension segments, via a hollow extension segment or extension segments on each side of the chain/wire.

3 ) Related to #2: The distance between the bottom-most two gears (the ones on the base) causes a FORWARD torque on the assembly. The longer this distance is, the more twisting force on the linear bearings, and the higher the chance of binding. The solution to this one is keeping that distance tiny, or doing the same solution to #2.

4 ) Adjustable speed. The first time I set this up, it was way too fast. Adjust the gearing from 1:1 to 5:1 helped quite a bit! Two sprockets and some chain could do the same thing. Make sure your motor is running at about half the no-load speed when it's lifting the robot up! This is where DC motors make the most power.

5 ) Overtravel. It breaks when you keep tellling the motors to pull up. The easy solution: limit switches that prevent the motors from moving when the thing's just about to hit a physical stop.

6 ) Your linear bearings MUST be super-smooth. The lift would bind when I had a bunch of 2-stud lego plates against 8-long lego slides. Bad! I replaced the 2-stud plate with 8-stud plates and the binding was less frequent, but still happened. If the plates weren't pressed ALL the way in, a tiny gap would catch.

7 ) Your linear bearings must have sufficient overlap (In my case, 6 out of 16 studs overlap). The greater this overlap, the less each segment can rotate with respect to another. This is an engineering tradeoff between reduced chance for binding and extra extension length.

8 ) I would consider using stretched rubber tubing between segments to reduce the torque the motor is required to produce.

9 ) Exposed chain when the lift is fully extended; may be vulnerable. Left as an exercise to the reader ;-)

The designs that used 80/20 or other aluminum extrusion in 2000 for hanging seemed by far the easiest lifts to construct. It can be done with entirely off-the shelf Al extrusion, linear bearings, pulleys and cable. A preliminary pricing from 80/20 showed a price on the order of $500 for the assembly, which would weigh about 20 lbs.

If anyone listening can chime in about their teams' experience with a lift like this, it'd be appreciated!

Thanks,
Brandon Heller
449 Alum / 931 Mentor

[Stephen Kowski]

Awesome video and comments....

[Trinora]

This is a helpful little tidbit, maybe:

The less telescoping pieces, the better. Two would be enough to reach the bar. As my team discussed; more than 2 would be impossibly difficult. Rmeember; it gets stronger and weaker as each telecoping piece size decreases.