Gearing a Rotating Arm

Our current robot design includes a dual-flywheel shooter on an angle-adjustable arm which we are currently trying to figure out how to rotate. For reference, the arm can be approximated by a mass of 3.5kg at .3m from the pivot. Our question is what would be a good way of gearing the arm such that it is responsive. To make another approximation, let’s say “responsive” means traveling from horizontal (0 degree) to 45 degrees in .5 seconds, which should really be taken as a guideline over a restriction for the design.

The current model involves a versaplanetary (motor and stages undecided) followed by a sprocket stage which is necessary for the placement of the gearbox but can be made a reduction anywhere from 1:1 to around 4:1. Is a versaplanetary a good choice for this situation? If so, where to go from there and if not, what other options have teams used?

JVN calculator is my go-to for rotary stuff. Just plug in your ratios and it spits out speed.
For your application, I get a speed of 0.27 seconds to go 90* if I used 2 miniCIMs on 100:1 versaplanetaries (no extra reduction).

Just a heads up, that speed sounds like it could be quite fast if you are looking to control the arm accurately, and you might want to play around with it a bit. I don’t know the specifics of your mechanism, but you could probably mess with the chain reduction part of it to get it to a quick controllable speed in any case, and that would probably only require the minimal maintenance of changing sprockets. This is by no means a must-do, but it might be something you’d want to keep in mind.

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No arms please :ahh:

Out of curiousity why would you suggest against that? Our team is looking on doing something similar

The Second Amendment begs to disagree :wink:

Two teams wanting to work with severed arms? Wow… Where are you sourcing these spare body parts?

Their 2015 Robot could have something to do with it…

I think the teams in question are dicussing a very different kind of arm though. If I’m picturing it correctly, it’s just a pivoting intake/shooter on a lever arm. A very simple concept as there is just 1 range of motion to worry about.

Back in 2011 when we had a long arm, we used gas springs on moment arms to balance the weight of the arm. If you do it right, the arm will stay at any position you put it without having to stall the motor to hold it there. A sprocket or gear bolted directly to the arm, driven by a miniCIM in a 100:1 planetary with a decent secondary gear/sprocket reduction should get the job done.

You may want to tone down the speed if you are rotating the shooter up and down at those angles unless the “boulder” is firmly secured you could lose it when rotating.

If you can swing that heavy an arm that fast, why not just remove the dual-flywheel shooter and its motors and throw the boulder a bit faster?:stuck_out_tongue:

Arms can be tricky!

I would avoid driving a long lever arm with a live 1/2" hex axle, and I would avoid driving one directly off a VP, especially 100:1. The 10:1 VP stages are the least strong, and arms exert a lot of shock loading that can really do nasty things to the thinner gears of the VP.

How many positions does the arm need to be in? 2, 3, 4? If at all possible, using pneumatics to rotate the arm between a few positions will simplify control a great deal and is generally more durable.

If you must motorize a long lever arm, it’s probably best to do the final reduction for the arm using chain, with the sprocket bolted to the arm to transmit torque. 973 RAMP has some good videos on how to design a simple durable arm power transmission system. The chain can deal with some of the shock loading the arm will face a bit better than a final gear stage could, as there is some flex in the chain and the sprockets are engaged at multiple teeth versus 1 or 2 teeth for a geared system.

Your gearbox will almost definitely backdrive, though you may be able to feed-forward the motor at a small voltage to prevent this. Only do this at low voltages or your motor will heat up quite fast.

To answer a few questions and raise some more…

The arm is relatively short, and only rotates about one axis. The ball is contained by the arm as it moves, so the ball falling out isn’t a concern.

Now, according to the chart here: http://content.vexrobotics.com/vexpro/pdf/VersaPlanetary-User-Guide-20151221.pdf (page 25, miniCIM simple load ratings) a miniCIM is not recommended for a versaplanetary at a reduction of 100:1. How should I interpret this?

Thanks for the help so far, everyone.

  1. JVN Design Calc is very good for this sort of thing

  2. Read and understand your chosen transmission’s applications manual ::rtm:: there are a number of motor/gearing combinations that quickly exceed VexPro’s recommendations (or bane-bots, or whatever). For example: running a mini-cim with a 2x10:1 vesaplanetary is counter-indicated, but running a mini-cim with a 9:1+4:1+3:1 gives you 108:1 reduction and is approved. I recall that when BB planetaries were first used widely in FRC there were a number of complaints about the transmissions failing, and it was because they simply weren’t rated to deal with an RS775 geared down 256:1 (for example).
    [moving on from addressing the above post]

  3. Arms can usually stall to hold position, but you must make sure this current is acceptable (see JVN calc for this). I’ve designed arms that stalled RS775s to hold position with no issues, use your judgement here

  4. If you counter-balance, and I think most arms should be counter-balanced, I suggest working out the spring geometry to counteract gravity. I.e. you want the most counter-balancing torque when the arm is horizontal, and the least amount (or zero) when it is vertical (meaning its CG is over the pivot point).

  5. Some amount of damping on your counter-balance is fantastic for smooth control. We really like using air cylinders to counter-balance because they provide a constant restoring force, damping, and mechanical hard-stops all in one.

  6. Back to actual arm gearing… my general practice is to pick the power I want the arms to have, which drives my motor decision. I gear as much as possible with a planetary transmission, staying inside the MFGs load guides, then I do I final sprocket reduction to the arm itself. Sprockets let me keep my drive motors down low in the robot, and generally help with arm packaging (many small details that I won’t go into here). Chain is also a great place to couple on an encoder for arm control.

Examples of 95 robots’ arm gearing:

2011: 2xRS775 + 64:1 bane bots + 14:72 sprocket reduction (IIRC), motors were mounted under the ‘deck’ of the robot, shoulder joint was near the height limit. Motors stalled to hold position. Some minor pneumatic counter-balance

2015: 2xCIM + 50:1 versa planetary + 16:64 sprocket reduction, pneumatic counter-balance kept arm neutral without game pieces, motors stalled to hold position. This robot needed over-balance protection in code since it could pick the arm+game piece up fast enough to flip itself over.

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Woah woah woah! You guys were the masters of arms last year!

A team here in Houston twisted some 1/2" hex so that it looked like part of a wrought iron railing last year by transmitting the torque from the sprocket to the arm through the 1/2" hex.

The RS550 and RS775’s rely on fan cooling. If the stall current is high enough, the windings will start smoking pretty quickly, possibly causing permanent damage to the motor.

We made a wooden cam last year to raise an arm, I think the rate and load would be similar to what you’re trying to do. The students come up with some neat ideas!

We plan on running a small arm on a cim geared by a versa planetary 100:1. The arm should be done this week and will definitely be tested next week. We may tone the gearing down and if it seems like a situation of durability we may substitute our P80 in. But, It looks like it will be fine. One thing I will see is do not directly drive the axle with the motor. Use chains, belts, or gears. We plan on using gears.

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I’ve learned the hard way that it’s better to be overly cautious with your ratios when designing an arm, and especially so when you need precision control. A more aggressive gearing may get you to the position you need to be faster, but it’s also going to apply more torque to the motor which means it’s going to draw more current. As a result, you’re going to get an arm that “spring to life.” Fine tune movements will be difficult, since by the time you apply enough power to get the motor moving with an aggressive gearing, it’s going to be moving fast. Further still, the more you gear back, the less you have to stall your motor in order for your arm to hold position (if you have to stall it at all).

careful with the versa planetaries. if you look at the published load specifications it says that a CIM on a 100:1 is well outside of the load limits for those gear boxes. Also your speed seems high. My team is doing a similar thing to you however our arm mass and length is larger. We plan on using two mini CIM’s we will have a total of a 150:1 gear ratio. 50 of that will come from a 3 stage gem planetary then another 3:1 reduction by chain and sprockets. also be carefull with what you use as the axle. we are using a 1 inch aluminum pipe.