Understanding Continuous Elevator Rigging

Hello!
I’m having an issue understanding continuous elevators in FRC. I have worked with and designed cascade elevators in the past, so I’m mainly having issues understanding the continuous rigging itself. I have looked at pictures of robots with continuous elevators but the rigging is often blocked by other mechanisms on the robot. (I would imagine seeing one in real life would help).

If anyone can point to any resources that can help, it would be appreciated!

Thanks!

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Here is an example that our team found useful when designing our continuous elevator for Power Up:

It is important to pay attention to which sprockets/pulleys are fixed to which stage, and which runs of belt/chain/cable have to be perfectly parallel, along the axis of motion to ensure smooth operation without loss of tension.

For a FRC scale design, we have our CAD from last year published on GrabCAD as an example:
https://grabcad.com/library/team-20-2018-frc-robot-zenith-1

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Basically you have 2 runs of “cable” on your elevator. One run is the lift run, the other is the lower run and the two are terminated to the same pulley(at least our was last season). When you run the pulley it will spool in cable on one of the runs causing the distance between that run’s idler pulleys to become shorter and the other run of cable plays out, allowing the idler pulleys on that run to gain distance. When you reverse the spool it reverses those actions. You anchor each cable run to the carriage, so when your lift run is getting spooled in it lifts the carriage from a pulley at the top of your first stage(the carriage is the second stage). When the carriage can move no further the next pulley in the sequence at the bottom of the first stage is now effectively the anchor point and gets pulled to the pulley at the top of your fixed elevator frame which is being pulled by your cable spool. The continuous doesn’t care about what gets shorter as long as it get shorter somewhere, that is why you see a lot of constant force springs to make the carriage “weigh” less so it always goes up first.

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Keep in mind that for a continuous lift system like this, the net force on any intermediate stage from the cable is zero until the carriage (or any other nested stage) reaches the end of its extension. We used a continuous system last year with no springs, and the 15lb carriage/manipulator always started ascending before the 5 lb frame of the intermediate stage. A free body diagram of the forces acting on an intermediate stage shows why this is the case.

If there is a lot of friction in the pulley system, an intermediate stage might go up first. Usually gravity keeps them down.

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This animation shows a cascade type chain routing: https://youtu.be/diXEm9aw1Dc

All segments rise together. It’s faster than a continuous-routing elevator but requires more torque. Of course speed depends on your load, motor & gear ratio.

Last year 340 did an elevator like this except that the elevator was powered down by gravity. The video in the post that this post is replying to is basically what we did except take out the chain leading down to the bottom of the elevator to bring it down did not exist as it wasn’t needed. We powered our elevator with 2 mini cims. It worked well with few to no problems.

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Yup, that’s certainly another way to do it! Team 20 had a much heavier payload since we had 4 pneumatic cylinders on the carriage (3 for wrist actuation, 1 for gripper open/close), so in order to have fast enough motion, we used constant force springs to counteract gravity. This meant that the motors had less work to do to lift the heavier load (we were able to accomplish this with a single MiniCIM, despite the heavier carriage), but it also meant that the motors had to power the downward motion of the elevator since gravity was “cancelled out” by the constant force springs.

Both 20 and 340 had fast, effective elevators, despite the different style of power transmission. I’m sure you could find plenty of successful examples of either style, as well as other possibilities not described here.

@s_forbes suggestion of using free body diagrams on the various stages can definitely provide insight to the motion that would occur. I’m sure technical judges would love to see that presented as well!

I always found this image to be illuminating.

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Thanks for all the help so far! I think I understand how the elevator goes up, but I don’t get the rigging downwards. The images seem to have one cable powering up and down, but the elevators I have seen have two different cable runs on the same spool. If the cable powering the elevator downwards is connected to the carriage, why does the first stage lower first? Also, how is the cable powering the elevator downwards routed? Is it just a cable anchored at the bottom of the carriage going down to a fixed pulley near the bottom of the elevator and then connected to the spool?

Sorry for all the questions!

Thanks

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In an ideal world, I believe the carriage should lower first, but this may change due to fraction. My team built a continuous elevator last year and the first stage always went up before the carriage when going up, and carriage went down first when going down. This changed sometimes due to load, tension of the line, and between our comp bot and practice bot.

This depends on what setup you want to run. You certainly can run the cable directly downward, through a pulley and on to the spool, much like the picture I linked. You can also route the return line opposite to the lift line. This is what my team did last year. Note that we had 3 cables on our spool because we had 2 return lines as we used our elevator for double climb last year. This was to spread the ~300 lbs of 2 robots across two cables instead of one.

Don’t be sorry for wanting to learn my dude.

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How did you guys make that spool?

A mentor who is basically a master machinist made it on a lathe. Realistically, the grooves cut into the spool did not add much, and are largely unnecessary. So long as you mount your pulleys such that the line makes a ~15° angle from the pulley to the spool it will wrap and unwrap nicely without overlapping. Look it up, this angle is called a “fleet angle”.

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Thank you Nick!

Why 15° ? How did you determine that was the optimal angle?

The main thing is that the angle is enough that each wrap is separated from the previous one. Smaller spools and larger line would require a more aggressive angle.