telescoping arm

[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.

[Gabe Salas Jr.]

Roccobot 2K

Roccobot 2K2

We used a two piece telescoping tower that is able to tilt all the way down. The good thing about Roccobot 2K was that it had a nice base, and it was able to evenly distribute weight even when extending to retrieve the big round balls. But it was not very strong, but did its job well. On Roccobot 2K2, we improved the arm design, but we were unable to maintain stability and balance when using the arm. Note that it had three wheels. But we made sure that if Roccobot 2K3 tipped over (which it has during competition) that it was able to right itself up easily using the arm.

[nuggetsyl]

WOW no one mentioned us team 25 in this artical i though we built one of the best telescoping arms ever made for first. Well if you need to see what we did get a hold of wayne.

[Meredith Rice]

Our telescoping arm made use of a pully system with extruded aluminum attatched by plastic sliders. We used pistons to extend. Very effective.

[Goober65]

thats right, if i remember correctly, we (team 25) built a telescoping arm that reached about 12 feet out in order to steal the black balls from the other teams bin and then put into ours (2000 game), im suprised as well that no one has mentioned us....

[Jeremy_Mc]

Originally Posted by Goober65

thats right, if i remember correctly, we (team 25) built a telescoping arm that reached about 12 feet out in order to steal the black balls from the other teams bin and then put into ours (2000 game), im suprised as well that no one has mentioned us....

I don't mean to be rude, but why not mention yourselves? If you'll notice, most teams are mentioning mechanisms they built themselves. I'm not sure why you see it as everyone else's duty to talk about your team (and I'm not just referencing this thread. Another memeber of your team has been posting "Well we did this, why didn't anyone say anything about that?" on other threads, too)? I know you're a very old and respected team (25 is a very low number haha), but honestly, not everyone is going to know you built some (apparently) amazingly fancy mechanism. If you want it to be known, then say so.

I've located some pics and videos of 442's arm from 2002 and I'll be posting those soon (hopefully).

[Natchez]

Our team built two 4 staged telescoping arms in 2001 which could put the balls on top of the goals while the goals were on the ramp. Here are some pics

Practicing capping goals

Don't really need 4 stages

Oops ... the red one ended up on the floor

Kinda see how they work … best pic I could find

Under the bar

In the Easter spirit

Here are some of my memories of the arms that might help

-Globe motor was used to extend arms
-Surgical hose was used to "assist" the globe motor
-Window motor was used to lift arm (about 6" moment arm with 1" hub) … arm had to fit under bar
-Seat motor was used for turret
-Spectra string was used to "pull" things
-An Eagle Scout tied all of our knots … pretty frustrating when a knot lets go.
-First stage was pinned to turret
-Globe motor "pulled" second stage into place (up or down … did not depend on gravity)
-Third stage strings were attached to second stage ... not directly to motor
-Likewise, fourth stage was attached to third stage
-The cross section was a triangle with flat corners
-Delrin blocks (Teflon was cost prohibitive) were used as anti-friction blocks and "pulleys"
-The aluminum used for the arms was 0.032" sheet
-Bending the triangular sections is definitely an art but can be mastered with a good sheet metal break

Although the structure of the arm was very strong, the actuators were all fairly weak (using all of the strong ones on the base and balancing mechanism). This was okay because it was a 4 team alliance with no defense. This year, you need to have more powerful positioning actuators; thus, this year we are thinking about simply using our base to point the arm or go to a very strong turret.

All in all these arms performed very well even though our main strategy that year was to balance both of the goals. Telescoping is a good way to go and if you go this way, try to leave yourself a solid week to get out all of the kinks & binds.

Good luck,
Lucien

[Stephen P]

All these telescoping arms are SWEET. I think I have the general idea of how they work, but does anyone have any specifics? A design sketch would be great.

[ngreen]

Originally Posted by Goober65

thats right, if i remember correctly, we (team 25) built a telescoping arm that reached about 12 feet out in order to steal the black balls from the other teams bin and then put into ours (2000 game), im suprised as well that no one has mentioned us....

Acutally I saw 25's arm on another thread. It was very impressive. I don't remember which thread. You would have to search.

[TGreen]

Originally Posted by CrazyBear

Roccobot 2K

Roccobot 2K2

We used a two piece telescoping tower that is able to tilt all the way down. The good thing about Roccobot 2K was that it had a nice base, and it was able to evenly distribute weight even when extending to retrieve the big round balls. But it was not very strong, but did its job well. On Roccobot 2K2, we improved the arm design, but we were unable to maintain stability and balance when using the arm. Note that it had three wheels. But we made sure that if Roccobot 2K3 tipped over (which it has during competition) that it was able to right itself up easily using the arm.

By any chance do you remember how much the 2 pieces weighed?

[Gabe Salas Jr.]

By any chance do you remember how much the 2 pieces weighed?

I'm afraid I do not remember how much the two pieces weigh, but I could ask my teammates and find out. But before I do, which robot you would like to know the weight of those two pieces?

[TGreen]

The 2000 robot arm

[Gabe Salas Jr.]

Originally Posted by TGreen

By any chance do you remember how much the 2 pieces weighed?

For Roccobot 2K, the entire arm design (minus the battery) was about 67 pounds (more or less). Towards the bottom part of the arm, is where we kept the battery. So overall, it would weigh about 75 to a maximum of 80 pounds (which includes 6 big 30" balls). These are rough estimates from what the team could remember.