What would be the main advantage of running a custom PINK style telescoping arm on a central pivot vs. a COTS telescoping arm on a central pivot not including cost or manufacturing? A telescoping arm seems to be what our team is leaning towards, but the rigging seems a lot more complex for something that does not give any noticeable advantage. We have a sheet metal sponsor who can very easily manufacture a design like Spectrum 3847’s 2018 robot, but the design itself is still quite complex.
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A cots telescoping arm is not powered on the extension. This means that if you have an intake on the end of it and push on it, it will freely move back. Depending on how you look at it, this could be good or bad. If you want a simpler approach than spectrum’s chain driven telescoping arm, check out Bear Metal’s(2046) belt driven telescoping climber from this year. The main complexity comes from wire management of an intake and possibly wrist at the end of the telescoping tube, not the actual extension.
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Makes sense, can’t believe I didn’t think of that
Is there a good link for understanding more details on how that arm works?
Thanks!
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Not the same arm that was referenced, but here’s a really detailed look at a telescoping arm from 2019.
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As someone who believes in using COTS to elevate your team’s capability the only telescoping tube systems that I am currently able to think of off hand are the ones that WCP and Thriftybot make. Both of which are in base design form a spring extend and string retract based design… which is good for climbers but I’m not sure how well it would work for an arm with some more weight on it that you want to more precisely control.
I think you basically have to investigate a telescoping system that has the flexibility to be rigged up internally to function similar to how a cascade elevator works. It’s tricky to do but definitely possible.
None of the documents I’ve read on the COTS options offer a clear step by step method to set them up to work this way so it would require your own inventiveness to modify them to work that way. This is assuming they don’t offer a modified version soon that does exactly what I’m describing…
@Karthik , Thank you!
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Hello! I think I can talk a little bit about custom telescoping arms since I’ve had a few tries on making my own.
Building a single-stage (i.e. 1 moving part) telescoping arm is relatively easy. In fact, I would go so far as to claim that it’s no harder than building a single-stage elevator, especially if you have access to a decent 3D printer.
However, building a telescoping arm with more stages grows in complexity much more quickly. The main reason for this is that you have to fit all the bells and whistles of a normal elevator into a very small package. If it’s not compact enough, the arm will balloon in weight to the point that it is nigh impossible to use. My first telescope design weighed roughly 30 pounds. My most recent design would likely weigh less than 15lb when assembled. The more the arm weighs, the harder everything else gets.
You will have to be very conscious of DFM (design for machining) and DFA (design for assembly) when designing your arm. In a normal elevator, having your moving parts occluded is the exception, but in a telescoping system, it’s the rule. Be extremely conscious of how you will access screw heads and tie down points - in my experience, if a hole isn’t at least 1.5" wide, there’s no way that anyone can get their hands in there to do work. The bearing assemblies are often the hardest to fabricate and assemble, but a good 3D printer can go a long way to easing your suffering there. My most recent design was oriented around keeping the number of CNC setups to a minimum.
It’s most common to classify telescoping arms using the outer width of the tubes. For instance, I believe the telescoping arms that Team 233 made in 2011 were 5-3-1, meaning the outermost stage was built with a 5" wide tube, the intermediate stage was made with a 3" tube, and the carriage stage was a 1" tube. These days, the gold standard is 4-3-2 and 3-2-1 arms.
You will also need to actively design in routing for your wiring harness. You cannot, under any circumstances, skip this part. I’ve found it’s best to treat the energy chain like its own bulky belt, which must always travel supported by pulleys. In a 2-stage arm, the energy chain must travel twice the length of the first stage, so you typically need a clever constant-force-spring setup to keep it stowed properly.
In short: a telescoping arm is a very complex assembly. However, when designed well, it can be extremely effective (and I think that we will see at least one telescoping arm robot on Einstein), especially in games like this where you need to extend diagonally. If you don’t have a lot of design experience, or you don’t want to iterate much, don’t try this at home - the WCP telescoping setup is pretty good.
When it comes to what you need to make a COTS telescope work for you, the big elephant in the room is electrical routing. Unfortunately, this is the no-compromises part of a telescoping arm. If you don’t think about the routing in your arm, there will be many tears in your future. What I would really love to see (RC, are you reading this?) would be an additional kit for the WCP telescope which handles routing some kind of energy chain through the bottom.
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Fixed your question to give credit to the team that actually created the design everyone copies.
The original versions of the extending are were sheet metal monocoque made from .032" Al sheet that was solid riveted using aircraft assembly techniques. We went to square tube for ease of manufacture in 2001 and stuck with that.
The extension motor is typically mounted near the base on the extension, and for the early iterations used window tape to drive the extension because it came in the kit every year. Window tape is kind of like a cross between a timing belt and a rack and pinion. We went to a cable drive when the window tape was harder to source.
The trickiest part is designing the sliding bearings for the telescoping mechanism. We machined HDPE or Delrin blocks to fit between the tubes, affixing one to the top of the lower tube and to the top of the upper tube.
For a wrist mechanism this is simplified by making the smaller tube the “Stationary” tube on the inside and mounting them to the outside of the sliding tube on top.
I feel like it’d be easy enough to install a screw drive in the thrifty telescoping kit. (ignore the crude drawing)
(basically one of these in the center of it)
It’s real easy with 1 stage…2 stage completely doable but it’s more annoying. Also, idk how fast it would be.
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Screw drives always have some disadvantage. Not only do you have to custom source one, but if you have fine control, it’s slow. If you have a fast screw drive, you don’t have control. It could work with the right pitch balance, but it’d be very heavy
Leadscrews tend to be heavy, slow, inefficient, and a pain to use. Under nearly all circumstances I would advise against them in an FRC setting, unless the non-backdrivable part is a feature for you (such as in some climbers).
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One good question to ask - how many stages do you need in the telescoping arm? If it’s just a single stage, then you could modify a COTS one pretty easily. Remove the springs, and replace them with pulleys. Run a string* from your winch up to the pulley and down the inside of the tube to the bottom of the stage. If you wrap it around your pulley opposite of how the string* going into the bottom of the tube is wrapped, then the two strings will give you a pretty solid in and out motion. Note that some design work is needed on the pulley to get everything lined up right, and you may have to run it on both sides, similar to the springs being on both sides.
*something durable and strong enough, of course!
I could be wrong here, but if the CF springs were strong enough could this still be a viable option of it not being a powered extension?
We were thinking of going with something thicker for the sheet metal like 0.090" or even 0.125". Is that overkill?
Btw I didn’t know about the Bobcats, sorry about that.
How would a return on this mechanism work? In my mind it goes up well, but would still require gravity to return the arm to its original state.
Crude sketch:
In this example, by rotating the spool counter clockwise, the red string pulls in, pulling the tube out, while the blue string is released, allowing the tube to go out. Reverse the direction, and the red string is released, while the blue string pulls in, resulting in the tube retracting.
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hey sometimes a crude sketch is all you need, and in this case it worked lol. I understand what you mean now, thank you!
What are people’s thoughts on just rotating a COTS elevator for the same effect? Seems easy enough to make the TTB elevator and rotate it on a pivot.
Ri3D Cranberry alarm is doing this. Seems like a center pivot may have a little less torque on it/be easier to deal with. You could also make the elevator narrower, and probably get this to look more like a Pink arm, without all of the difficult rigging.
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Yeah we are going to rig it like this for 2 stages and with timing belts
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