Steel Tape Drive (for telescoping arm)

FIRST General Forum
#1

Inspired by 118’s reveal video, I tried to design a winch for a telescoping arm based on a spring steel strip. The neat thing about their mechanism is it appears to be able to push the telescoping arm up and pull it down to lift the robot. Usually extending a telescoping arm requires an external spring and cable rigging to extend each of the arm sections. I’m pretty sure they did this with a steel tape on a spool, similar to the tape measure climbers we have seen in the past, but more reliable.

As a starting point, I measured the steel on my fish tape, which is 0.125" wide and 0.045" thick. It wraps around a 6" spool. This seemed a bit too flexible to extend straight up 50" or so, but confined to the inside of a telescoping arm, I bet it would work. For my design, I used a 0.25" wide strip, something like this: https://www.mcmaster.com/9075k413

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The spool for the steel tape is 6" , and the feed rollers are 2". They are driven 3:1 relative to the spool to match speeds.
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The feed rollers aren’t really necessary when retracting the tape, but extending it by simply reversing the spool would cause problems with all of the slack.

Anyway, this was an interesting design and helped curb some of the isolation boredom.

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Team 118 2020 Seson Climb Mechanism
#2

So what you’re saying is the steel strip is attached to something in the outer tube and since it’s steel, it pushes the tubes out. Then when it’s time to climb, the spool winches down the climber similar to how a “normal” climber works

#3

If you wanna go el cheapo, I’m pretty sure a normal steel fish tape is the same thing as the McM part, so I’d just buy that regardless. Seeing that diameter spool in their robot reveal immediately reminded me of messing with those as a kid.

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#4

Cool concept. I actually thought it used some sort of bendable plastic rack and pinion, which is what a few FTC teams have done to do the same thing. https://shop.sdp-si.com/catalog/?cid=p764

Also, what is your ratio in the planetary gearbox? just curious if its possible to build a planetary gearbox into the larger 6" spool, with the planets fixed and the ring is the spool. I know its not practical, but this is summer CD

#5

I built a measuring tape based arm as a backup plan. I used a Bosch seat motor with a dual pulley on it. A long section of tape was attached at the middle to the base of the moving arm (made from a painters pole). Each end was terminated on one groove of the pulley. Pulleys on each end of the static tube turns the tape around. As the motor turns one tape is pulled in and the exact same amount is put out by the other pulley groove. I added limit switches after I tore the tape out twice… I was able to fiddle things where it would swing up on an air cylinder, then a single stage telescope would take it to the highest possible hook point. In hindsight, I should have made it a LOT shorter… Its tricky to hook when the bar is level.
For some dumb reason I don’t have a photo of it, and its locked up in the school :frowning:

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#6

The end of the steel tape would be attached to the innermost tube of the telescoping arm.

You could, but I like how many options mcmaster carr has for different widths and thicknesses. I bet some experimentation would be required to find the right balance between stiffness and flexibility.

I did look into the flexible rack, but I doubt you could hang a 150lb robot from it. They do have several sizes, but the larger the teeth, the larger the bend radius.

The planetary gearbox is 36:1. There certainly is a lot of space in a 6" diameter spool, but designing a custom planetary gearset is not something I want to pursue. To your point though, I think this mechanism could be made a fair bit smaller with some effort.

#7

Love the climber, Rob. Hopefully not a dumb question, but why did you pick a 6" spool over another size? Is this driven by a bend radius constraint of the spring steel, or something else?

#8

Pretty sure it’s just driven by the normal size these come in:
https://www.homedepot.com/p/Klein-Tools-25-ft-Steel-Depthfinder-Fish-Tape-56005/100660167

Nothing better than taking a COTS part and slapping it directly on the robot when possible. (Though I’m sure 118 found some way to complicate it)

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#9

More or less, yes. I based this size off of my fish tape spool, which is just a thin coil of spring steel. You probably could go smaller, but at some point you run the risk of permanently bending the spring steel. Obviously this principle only works if the tape springs back into a straight line once it is uncoiled. I bet with some experimentation you could find a coil diameter and steel thickness that would be more compact.

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#10

Did you see any close up views of 118’s while working on this? At least I assumed they are doing the same. I’d thought about doing something like this with one of our climber re-designs, but instead we end up with a ~6 ft compression spring inside, mostly to avoid the powered spool design like this. But I definitely like the idea of having the fish tape extending and retracting cleanly for this.

#11

Is steel tape basically tape measure material

I remember a 2016 video where a bot climbed with tape measures. Not sure who.

#12

1986 did

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#13

Ah, yes

#14

Sadly I will never again get to look at one of 118’s robots up close. I think what I designed is similar to theirs, but can’t say for sure.
Your 6ft compression spring sounds really interesting. Do you have any pictures? Where did you source the spring?

#15

The steel tape I refer to here is much thicker than a measuring tape, and rigid enough to extend a telescoping arm. A measuring tape relies on it’s curved profile for rigidity, but once it buckles, it wouldn’t work. I know there have been several successful measuring tape climbers built in the past, but I think they all had a fairly small hook on the end, so didn’t have to lift much weight. My impression is they can work, but you have to be very careful not to buckle the tape when you are extending it.

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#16

Late to the party.

We used a similar system in 2016, but with a single hook.

Note each side actually have two tape measure strips doubled up (back-to-back, on the same spool) for extra rigidity:

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It works, but at least for us it was finicky. Getting the double strip reset perfectly before each match was a Herculean effort that only two students really mastered. Watching it extend to place the hook was always nerve-wracking, and it was single shot. If we jerked the robot too much while aligning, the extension would crumble and we’d be stuck with a “park”.

I’m not opposed to using similar things in the future, but I’d definitely add extra stuff around it to keep it rigid, and not rely on the tape itself to stay straight.

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