On what types of challenges would an elevator do better than a telescoping arm and vice versa?
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It depends! Admittedly the situation in the prompt is fairly vague, so the first part of my answer will be similarly vague.
They’re both linear motion, so they solve a very similar problem space. I’d say the bigger deciding factor becomes packaging - which one fits better given the other constraints of the robot? Which one is easier to make robust enough to handle side loads? Which one is easier to make light enough to be moved by another mechanism?
Elevators tend to be attached straight to the drivetrain. Arms tend to be attached to the end effector that reaches outside the robot. But that’s not a hard rule.
Given some of that framework for how to think about the problem… Why do you ask? Is there some specific scenario you’re looking for advice on?
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telescopic elevator 
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In our Behind the Bumpers we explain it well, but in summary, our team was considering a telescoping arm, but we had a lot more experience with elevators in general, the only new thing we had to learn was putting it on a slant.
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I’ve seen a lot of teams have high success with telescopic arms. But I likely won’t be recommending it to my students again. It wasn’t that it was overly complex or difficult. It’s mainly that if something breaks or goes wrong it’s significantly more difficult to fix than an elevator that has most of the components exposed and not inside other tubes
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Telescoping arms were really popular in the early years of FRC because prior to the first time we debuted one in 96 or 97, 2 and 3 jointed arms were only way to reach the tall structures on the field or reach the long distance needed. We came up with the telescoping arm to reduce the degrees of freedom and make the arm easier to control in the days of very limited controls, motor power, and sensors. These worked great for a lot of games during the unlimited extension era and gave great functionality.
In 2007 we had a big rule shift. It was the first year we had a scoring object requiring a high reach and were extremely limited on extension. Again our processor power and controls were more limited than they are now so having believable extension limits controlled in software was something that didn’t really prevent you from getting called for over extending in a match. It was better to have a vertical style telescope (what is now called and elevator) so you could prove it was impossible to break the rule.
This was when two different philosophies started to develop. One was to use an elevator for less degrees of freedom and easier to control. This tends to work well for teams with limited programing and that want to make things simple and straight forward to control. The other was using a telescoping arm that requires a lot of practice or hard software limits that are well programed and defined.
With the sensors and processors we have now its much easier to define all your setpoints in software than it used to be but it doesn’t always make sense and you can evaluate the same criteria for a game and come up with to opposite views on how to do accomplish the same task.
Historically elevators have been very dominant in games with “climbing” endgames like 2018/2019 since you can use the existing superstructure for support.
Elevators usually have a permanent high center of mass, while a telescoping or jointed arm can usually collapse lower.
High Tide/4414 had a 3 stage elevator that retracted to a very compact bot, but there were numerous other examples of bots with a high elevator towers that were wobbling all over the place.
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There’s also examples of tippy telescoping arms - our robot, for example, is very tippy, because we didn’t have the CAD experience to make a multi stage telescoping arm, so we just have a singe extension for extension and rotation, meaning that we are at our highest in a “stow” position.
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This isn’t always the case if the elevator is properly designed. When stowed, all the mass of the carriage systems will sit nice and low in the robot leaving only a few critical drive components and structure up high, which altogether weigh next to nothing in the grand scheme of things. This means that a properly designed elevator bot will be very stable while traversing the field. Deployed, the story can be much different, as we proved when scoring game pieces this year.
ThunderChickens arm for 2018 or 2019 was a multi stage Carbon Fiber arm made of square tubing. They came to our shop to borrow the CNC routers/Bridgeport to cut the larger pieces out. I don’t recall the exact details of how it was assembled but I remember it used the constant force springs on rollers to force the arm out unless it was pulled back by a winch motor.
There were smaller inner tubes that the eventual “Claw, grabber thingy” would attach to. I think it was 3 tubes in total and it pivoted from a dead axle in the middle of the chassis. Many robots this year had a similar style of arm. Truck Town Thunder at Kettering #2 I believe was one that comes to mind. They were definitely a top contender!
There are many advantages and disadvantages to both elevators and telescoping arms, but I believe that an elevator is better in most situations.
Advantages of using an elevator:
- Stability - An elevator is typically far more stable than a telescoping arm, with much less side to side oscillation
- Load capacity - Generally elevators can carry more weight
- Simple programmatically - You can only go two ways, you may not even need set points depending on the scenario
Disadvantages of using an elevator:
- Mechanically complex - A telescoping arm can be made to be as complex as an elevator, but usually an elevator is more complicated, especially when it comes to multiple stages
- Weight - An elevator is usually heavier than a telescoping arm, and considering its vertical nature, it can result in a higher center of gravity
- Limited horizontal reach - An elevator itself doesn’t have horizontal reach. Even if the elevator is slanted it would need another mechanism (pivot, 4 bar, elevator, etc.) to reach the highest cone nodes this year
Advantages of a telescoping arm:
- More simple mechanically - As I said, a telescoping arm can be made to be as complex as an elevator, but a telescoping arm can be made to be very simple.
- Faster extension - A telescoping arm is moving far less mass, which can make its extension quicker.
- Light weight - Telescoping arms are usually far lighter than elevators, which can give you extra weight for ballast for a lower center of gravity.
Disadvantages of using a telescoping arm:
- Stability - Side to side oscillation can make a telescoping arm harder to control
- Programmatically complex - I have seen a few teams this year struggle heavily with programming a telescoping arm, the extension + pivot requires complicated control algorithms and can take up tons of time.
- Limited load capacity
In one of spectrums videos, they said that a telescoping arm is better for reaching over things, while an elevator is good at lifting vertically. Virtually every FRC game requires primarily vertical extension, with minimal horizontal. The obvious exception is this year, which is why there were so many telescoping arms. I doubt FRC will do a horizontal extension game again soon, so if you are wondering what to build in the offseason, I’d say build an elevator. Elevators are versatile and could have been used in every recent game except for 2017 (as a primary mechanism or for climb).
Otherwise, telescoping arms are perfect for climbing in a shooter game, as their lightweight and compact nature is very useful as a standalone mechanism (though, you may need to use two). If possible, I would try to use an elevator for your primary mechanism and climb in a pick-and-place game.
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There is what 1351 did this year, which as far as I could tell, was an elevator with no distance between the rails of the carriage, so it was basically the size of a telescoping tube.
(cc @Andrew_L)
Ah yes, the televator. Born out of the desire for the characteristics of a telescope without the packaging requirements of one. Basically cascade elevator packaging with telescopic motion. We loved it. I believe citrus did something similar.
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Usually I’ve seen it be the other way around. Telescoping tubes are an elevator but smaller, which means the same functionality has to be packed into a much smaller space. Packing everything into such a small space results in telescopes usually being more complicated than elevators due to the packaging difficulties.
Citrus did continuous instead of cascade
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Yeah, cascade rigging a telescoping arm, designing bearing blocks, and wiring a telescoping arm can get really complicated. But i have seen incredibly simple telescoping arms. We used a lead screw and just ran energy chain on top of it, we also just used slides that worked basically first try. Sushi squad ran just a few pistons outside of their arm.
Comparing a multi-stage elevator to a single-stage telescope is a bad comparison. Of course the one with fewer stages is going to look simpler. If you compare a single stage elevator to a single stage telescope or a multi-stage elevator to a multi-stage telescope, most cases, the telescope is the same thing as the elevator but put into a much smaller space. You can make a single-stage elevator just as simple and they can are used in the same ways single-stage telescopes are used.
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Both alternatives are examples.
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Maybe a single-stage elevator and a single-stage telescoping arm are roughly comparable complexity-wise. Still, a single-stage telescoping arm is far more versatile than a single-stage elevator. This is mostly because the size of an elevator makes it more difficult to mount and pivot. A telescoping arm’s light, square, and compact structure is far easier to pivot than an elevator. Also, wiring a single stage on a pivot would give you far less space to work with than a telescoping arm, as the moving stage is fully concealed in a telescoping arm, giving you the entire top of the static stage to route wires.
While I agree overall both telescoping arms and elevators are very similar, they have different uses. Which (in some instances) allows you to have a simpler design with a telescoping arm. That is why I said that telescoping arms are simpler; in the cases where you would want to use a telescoping arm over an elevator, the telescoping arm allows for a simpler design.