Hi everyone, my team has unfortunately never manufactured or tested an elevator and I’m trying to design an optimal elevator and am stuck on if I need to design a continuous elevator or a cascading because I saw a lot of teams using cascading this year but would like a technical reason for why its better (I’m not a programmer so if it makes programming easier then great).
I’ve been designing around 111, 1678 for cascading and been designing around 3476, 254 for continuous.
If anyone can give me some tips for rigging that would be great as well!
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Most of the available COTS elevators – which you should use one of if it’s your first time doing this – default to cascade rigging. Thrifty Elevator Stage Kit and GreyT Cascade Elevator – WestCoast Products are two examples of kits with rigging.
That said, just like many other things, there are tradeoffs.
In 2018, there were often two tasks the elevator was used for: lifting game pieces (light, fast) and climbing (heavy, slower). In a cascading elevator, each subsequent stage moves at double the speed of the previous one, and at half the torque. If your climbing mechanism was attached to the top of your first stage (the slowest-moving), it meant you had 2-4x the lifting force available at that point (depending on number of stages), compared to your carriage – so, hopefully no need to resort to more complex methods (e.g., shifting gearboxes) to get enough torque to lift the robot, maybe just overpower the elevator slightly. It takes far fewer parts to counterbalance a cascade elevator with a constant-force spring; only one spring is required, versus one or two springs per stage for a continuous elevator.
In games where you score at multiple heights (e.g., 2019, 2023), a continuous elevator provides a slight center-of-mass advantage when scoring at lower levels. Unlike a cascading elevator, where all stages move at the same time, in a continuous elevator, the end stage moves first, gets to the top of its range, and lifts the second-to-end. Once that combo gets to its end, it lifts the next stage, if there is one. This means that the mass of the elevator stays lower when the robot is only extended to lower scoring levels, reducing tippiness. A drawback of this is that you must have hard stops for each stage, or the elevator doesn’t work. You don’t necessarily need hard stops for every stage of a cascade elevator, just the last one, which is often built-in (the “box” that forms the second-to-last stage frame).
With respect to programming, feedforward is slightly simpler for a cascade elevator (kG is constant), but it’s negligible in most elevators.
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There are a few things to consider:
- Power requirements. If you do the math, you’ll see that the force needed is different for continuous vs cascade, due to the way the tension on your rope works relative to the moving stages. The amount of rope pulled in is different as well, with continuous requiring twice as much rope to be pulled in by the motor. To give a good example, in 2008 we built a continuous elevator, in a large part because the payload (a large trackball) was fairly heavy. By going continuous, we reduced the overall power requirement. Today, we have better, more powerful motors, with no limits on quantity, so that is less of a concern. Game pieces, particularly this past year, weighed a lot less as well. Looking at 2011, where the inner tube game pieces were fairly light, we used a cascade elevator. You can see a rough layout of the math below.
- Position control can be easier with cascade. When you have a cascade elevator, you know exactly how it’s going to move, as everything moves together. With a continuous elevator, you can’t be sure of that. The final stage might move first, with the middle stage moving only once the final stage gets to the top, or the middle stage might move first until it gets to the top, then the final stage. With that difference in motion, you might also see a difference in speed depending on what part is moving. You might see that it behaves differently if you have a game piece versus if you don’t. That uncertainty, particularly these days with our powerful motors, often leads teams towards cascade over continuous. This is largely due to the different masses involved combined with friction in the system.
- Complexity. Cascade is typically a little easier for routing the cables and wires, in my experience. The known motion profile makes designing around wire management a little easier.
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Thanks this helps a lot, I have been looking at the COTS ones but learning how to rig up a rope elevator would be helpful for future years I believe. Just a question but is there a difference in performance for cascading vs continuous in this years game since i’ve seen more continuous in higher up teams.
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I think the improved position control ability of cascade helps with this year’s game, given the multiple scoring levels. The game pieces are fairly light, so that’s not a huge deal either. If I was making the choice, I’d probably go with cascade.
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Not exactly; the torque requirements are different, but the work is much more similar. The diagram shown doesn’t show that, while F2 is almost double F1, it requires half the displacement of the [string]* to achieve the same displacement of the end stage.
If I were building the cascade-rigged version, I would gear the drive for the elevator quite differently – approximately double the gear ratio, and because the end stage moves twice as far in a cascade for the same input [string] displacement, it results in nearly the same end stage speed.
It’s also important to note the relative magnitudes of m1 and m2 in the examples. In a typical FRCFIRST Robotics Competition-style elevator, m1 might be 3 lbs. and m2 might be 15 lbs.
*string, belt, chain – depends on implementation
Ideally, yes, but not always. It’s a question of the mass of the different stages and the friction in the system (both amount of friction and location). It’s best not to count on this behavior!
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If we make the assumption that the end speed of the two elevators should be identical, then that means continuous has half the gear reduction that cascade does, and cascade twice the torque available at the motor as continuous does. But because of those equations, you end up needing more than twice the torque in cascade, all else being equal. When planning things out for motion, you’llLimelight, an integrated vision coprocessor look at the motor curves and realize that moving your mechanism at a constant speed, when you increase the torque requirements (even by a little) requires more power to the motor.
One more important note for cascade vs continuous that’s getting a bit lost in all the discussions: direction of force. In cascade rigging the force from the motors is only being applied to lift (i.e., extend) the elevator in most cases. It is gravity or (maybe) springs that are retracting the elevator. In continuous rigging the force from the motors is applied in both directions. Thus it can be used to climb and other applications that require force to retract the elevator. A proper continuous rig is a complete loop, which is why it works this way. Some of the current COTS offerings are actually hybrid continuous systems, like the Thrifty Elevator, which features a fully powered loop from the base to the first stage and then a non-powered (separate) loop for the first to the second stage. It’s actually a great compromise, since it pulls in both directions but is much simpler than traditional continuous rigging.
This might be the safe way, but some teams do count on this behavior. Most modern FRCFIRST Robotics Competition elevators use ball bearings running on aluminum tube for both axes, resulting in very low friction. If the elevator is more upright than lateral (e.g., not a 254 or 1323 “laterator”), gravity will tend to keep the outer stages low while raising. This is especially true if the elevator is not counterbalanced in any way. Or, stages can be biased with pull-down springs to reinforce this behavior.
Agreed – adding another stage of gearing results in a 10-20% penalty, either as additional torque required of the motor, or as lost speed if geared for the same torque.
Either cascade or continuous can be open or loop – it’s just that continuous has one or more great loops in parallel for the entire elevator, whereas cascade has one or more parallel loops per stage. In diagrams, they are often shown as open, though.
Cascade-with-loops is what enables it to be counterbalanced so easily. (e.g., The Thrifty Elevator with constant-force spring) The CF spring spool lifts the top of the carriage and its spool is mounted to the top of the second stage. Yet, through the “return”/“lowering” rigging of the elevator, it can actually lift the second stage with respect to the stationary frame (provided the spring applies enough force).
I would suggest continuous rigging unless you have a good reason to run cascade.
The main advantage continuous has over cascade is simplicity with an additional slight weight and speed benefit. Cascade elevators typically require many parts to do the rigging correctly because there is one or two rope loops per stage. Each rope loop requires one tensioner and one clamp point per stage. When you are potentially looking at 2-6 rope loops, that becomes a lot of parts which makes maintenance and packaging fairly annoying. It is also difficult to have cascade elevators fully retract due to all the ropes needing to be the perfect length while under tension. When ropes due their thing and lose tension, they won’t always extend or retract fully due to one stage’s position relative to its bottom shifting relative to another stage. This is a large hassle solve and will take quite a lot of time to align the strings. In contrast, continuous elevators only have to worry about a single rope loop ever. A single tensioner with a single clamp point can tension a continuous elevator of any number of stages, making it really easy to tension and maintain. Continuous elevators also typically don’t need very large reductions, allowing you single-stage gearbox in many circumstances which saves weight, friction, and backlash.
There’s a couple reasons to run cascade over continuous however. If you want to use your elevator to climb, you can climb off the first stage of a cascade while using the higher stages to score. This means that you are effectively climbing using a much higher ratio than you are scoring with to get good enough performance for both tasks with a single mechanism with an additional benefit of the weight of the higher stages helping you climb because they need to move down to pull the robot up.
Additionally, if you want to counterbalacne the elevator, you only need a single spring to counterbalance a cascade elevator. However, in the brushless era, counterbalncing is often unnecessary and just leads more more maintainace and the safety risk of powerful constant force springs.
Attention to detail, and lots of it. Regardless of running cascade or continuous, your goal is for the spool to always be feeding and receiving the exact same amount of rope at all times and to never lose tension. Seemingly-small details can cause this to not be the case so you will want to pay attention to them.
Make sure that your spool is correctly designed. An ideal spool spools only a single layer of rope with no overlap and should never allow the rope to jump off it. You want to minimize the fleet angle of the spool by using larger diameter spools whenever possible and placing the spools far away from the first pulleys it interacts with. Even though large diameter spools are large and need higher reductions, I recommend using them because they significantly reduce the fleet angle leading to more reliability. Placing the spool at the very bottom of the robot typically is far enough because there’s several feet of distance between the bottom and top of the elevator in which is where the extension spring first redirects. However, you will want to keep an eye on the retraction string as in some setups, the carriage can come quite close to the spool which will cause a high fleet angle when the elevator is almost retracted.
If you have the manufacturing capabilities, I would suggest putting grooves in the spool. A grooved spool forces the rope to spool correctly and will generally be more reliable than a non-grooved spool and is more tolerant to higher fleet angles. However, grooved spools will not compensate for extremely large fleet angles and won’t fix the problem.
Additionally, make sure any pulley runs in which the amount of rope between two pulleys “changes length” are perfectly straight. If the ropes are not straight, spooling in X inches of rope would cause the elevator to move slightly less than X inches while the spool is still outputting X inches of rope, meaning you are losing tension while this happens. Small amounts of unstraighness can be compensated for by a good spring tensioner, but large deviations will cause the system to completely lose tension and can allow the ropes to jump off the pulleys.
If for a given game, you have determined it is nessesary to use two ropes on both sides of the elevator for an even pulling force instead of one rope in the center (such as needing an even force for climbing), you can combine both ropes using some clever rigging. Shown here is a 1-stage elevator using this rigging, but the concept is easily expandable for both continuous and cascade rigging
Running the rigging like this uses a single rope looped around twice to create a block and tackle setup which drives both sides of the elevator, uses only a single rope loop, and provides an extra 2:1 reduction meaning you can run fewer gearbox stages. This comes with the cost of needing to spool up twice the amount of rope on a single spool, however, this challenge is solved exactly the same way as continuous rigging spooling is solved meaning it isn’t much of a challenge.
Cascade retraction can be done by forming closed loops for each stage which avoids the unreliability of gravity/spring retraction, but comes with the complexity of yet even more parts per stage.
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I think you switched up cascade and continuous a few times.
I’d say the only real downsides to cascade are, you have to tension every stage rather just having 1 tensioner (or 2 for rigging each side separately), and you need a higher reduction on your winch. I think the main advantages to a cascade are, you can run the first stage with chain eliminating the need for a bulky spool system while still using a lightweight rope for subsequent stages to reduce mass, when driving both sides of an elevator you can’t get miss matched extensions, and you can put different mechanisms on different stages to get different “gear” ratios. Also for anyone using a rope with a lot of creep (stretch for non materials people), several tensioners in a cascade elevator divide the creep so each one only needs to take up a fraction of the slack. This can save a lot of retieing of knots if you use dynema and turnbuckles like we do.
I noticed at least one post got the “gear” ratios of a cascade wrong though. The ratio doesn’t double or multiply by anything with each stage. The ratio of the directly driven stage to the final stage is 1:N, where N is the number of stages being used. This means that if you attach a climb to the second stage of 3 stage elevator with a gripper, the gripper will move 3 times faster than the first driven rope is pulled, the climb will move twice as fast as the first rope, thus the climb will move 2/3 the speed of the gripper with ~50% more force.
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