Using a continuous or cascade elevator, is it possible to “backdrive” it so that it actuates not only up, but down, so that the elevator extends past the bottom of the robot?
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That largely depends on your bearing/slide setup. The bearings are usually not optimized to support travel below the main frame. Check the bearing interaction between the frame and the 2nd stage on the Greyt Universal Cascade Elevator for reference.
Edit: after thinking about it for a bit this might be impossible on a cascade elevator, since the carriage (innermost stage) moves at the same time as the 2nd stage, they are already level with one another at the bottom. Of you intended for your carriage to move down you’d also need your 2nd stage to move down which idk if that’s possible. Additionally you couldn’t drive it with sprockets and chain
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I can see why a Cascade elevator wouldn’t work, is there anything stopping a continuous elevator if setup correctly?
Continuous? Depends.
Cascading? No.
What would the continuous elevator be dependent on?
Bearing clearances for one.
Additionally most continous (and cascade for that matter) elevators have thier power and pnuematic lines plummed such that they hang from the top of elevator and attach to the carriage at the bottom and then raise up as the elevator raises. You would need to add some extra length to both to ensure they don’t snap.
You also need extra cable winds on your drum to allow for more travel.
Finally many teams use the rigid frame at the bottom as a hard stop for limit switches to zero the elevator encoder. You couldn’t do this if the elevator moved below the frame. Infact, nothing besides the cable rigging is preventing your elevator from sliding out of the frame on either end.
I just think the extra complexity added to the elevator isn’t worth it. If you are trying to to reuse a component to save weight, PTO out of the elevator gearbox to power another mechanism is always possible.
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How you build it. I’m not entirely sure how you would use a continuous elevator, but i know that cascading would not work.
My old team 1741 designed a completely new lift for IRI, and we turned our lift from a single stage into a double stage and just cascaded it all together. We ran our roping the way you normally would when trying to lift multiple stages. Roping tied down to First stage, then ran through the second stage, and then tied down again to the stage that is being cascaded. However we just added another cable that worked the opposite way. Where instead of of running it through the top of the second stage we ran it through the bottom so when the first stage would go down it would also pull the other stage down.
For the record, the term “backdrive” usually refers to the ability of the mechanism to be moved by hand (or other external force) without power from the motor. So a typical arm or drive gearbox is backdrive-able, whereas a mechanism driven by worm gear or leadscrew would not be.