Balancing an Arm

[Brandon_L]

After my teams horrendous mess of an arm in '11, I've been doing quite a bit of research on arms in FRC. The more I find, the more questions I have about them. I've picked up that you'd want to balance your arm with either surgical tubing or gas springs, so that your motor driving the arm doesn't have to deal with gravity as much. But what is the best way to balance an arm? I'm specifically interested in gas springs and have the following questions (I'm sure I'll have more later):

1) Where do you want your arm to sit when neutral?

-I've read that you'd want it to sit horizontal to the ground. But wouldn't it make more sense to be more neutral where you'll be reaching? (example, closer to the "up" position in 2011 to ease reaching the upper racks). I'm familiar with formulas to get the arm to sit horizontal (weight * distance on one side = weight * distance on the other, or in the case of a gas pring pushing up, weight * distance = distance from center of rotation to where the spring mounts * force of the spring) but I'm not quite sure how to apply that to get the arm to sit more in the 1st quadrant then directly horizontal.


2) Once you have your arm balanced, wherever balanced may be, how can you start a match with the arm down?

-My thought process here: The gas spring or surgical tubing are supplying a constant force to keep your arm up, and I'm using a motor that backdrives. So whats keeping it down with no power to the arm motor? Do some team do quick releases for surgical tubing arms?

The main question I'm currently fighting with: is there some property of gas springs I'm missing?

-I've been told you could manually push a gas spring arm down into your starting config and leave it there, and it won't spring up. So this tells me that the spring isn't just putting out a constant force upwards onto the arm. If its not pushing up, how does it help at all? What am I missing here?

[Joseph Smith]

In '11, our team along with 1718 used a pair of gas shocks to balance our arm, mounted on either side of our tower. The arm was balanced in the stowed position, about 30 degrees from our vertical tower. Since the mounting point of the gas shocks moved with the arm, the force that they apply on the arm increases when it reaches horizontal and decreases when it is above or below horizontal, so that the arm is balanced at all points of rotation. I'm not going into the physics behind this, but from my personal experience it works.

[BJT]

If you are driving the arm with a motor, the motor will usually provide some resistance to movement especially when using high reductions. a gas spring just needs to cancel out gravity and the arm will stay put wherever you stop it. In theory

[rcmolloy]

To just introduce a brief comment, we know that there isn't really a "best" way to balance an arm. If we did, every robot in 2011 would have used that mechanism or whatever applied on their robot somewhere that year

Not know a whole lot of information on the best use of either, I really can't give too much help on this but I can elaborate a bit on both of them and one or two other things to aid in balancing arms or systems.

With surgical tubing, gas springs, and constant force springs (very applicable for elevators), they are ultimately going to have some constant force pulling/pushing the arm up or down (however you orient them). In this case, I wouldn't see a non-backdriveable motor an option unless you have an equalizing force colinear with that one being applied to your arm by your mechanism. (Someone please correct me if I'm wrong )

I know that some teams had great success in 2011 with just gearing right and using control loops to establish a great balance of weight without too much load.

To end, I do believe it does depend on how you are designing your arm and system and then from there a determination, from design to actual performance, you can asses your situation to move to springs, surgical tubing, ect.

[EricH]

Why do you need gas springs or other means of balancing an arm other than its power source?

This is not an idle or ignorant question. I have seen multiple arms with no balancing (other than the power source) that did quite well. Single-joint, multi-joint, even with a large-ish mass at the far end like a game object.

As for your second question: Your motor does not backdrive? So why are you worrying about keeping the arm in a specific place? For that matter, why are you looking at things other than the motor to keep the arm up? Motors that backdrive (may) need these things, but motors that do not backdrive almost certainly do not.

For the first question... You're actually asking two. Where you want "neutral" to be is game dependent--ask again sometime in January. But the math behind sizing/placement at neutral...

You're actually pretty close. You would use an equation something like, (weight of arm side 1)*(horizontal distance from pivot point to arm CG)=(weight of arm side 2)*(horizontal distance from pivot point to arm CG)+(vertical force component from balancer)*(horizontal distance from pivot point to balancer attach point).

This would actually fall into the Statics category of problems, which many engineers study early in their college careers--the basic method of balancing out forces is to set up an X-equation, X-unknown system of equations where the sum of all the forces and moments in a given direction or about a given axis of rotation is zero. Then you can solve for your unknowns.


Just to bring something up... I have yet to see one of 330's single-joint arms use any sort of balancer that wasn't the drive motors. Neutral for some was all the way down; for others it was wherever the arm was left--one at least needed a little bit of power to maintain a position. Typical drive, 1-2 FP motors, stock gearbox, heavy reduction afterwards (exception: 2x Globe motors mounted on the arm itself--2004). No braking capability.

Oh, and pneumatics are also an option, if you don't mind having rather limited numbers of postitions.

[AlexH]

gas springs are the things that make the rear door of a hatchback or minivan stay up.

on a first bot the trick with them is to have them apply just slightly less force than it takes to lift up whatever they are supporting.

[BrendanB]

In the fall of 2011 our team went from our low row rookie machine to a top row scorer. We balanced our arm using surgical tubing and we kept it down using some small magnets. It was just enough to keep the arm down with a slight tap to release it from the stowed position. We loved how light, simple, and fool proof the system was with no latches or pins to get stuck, jammed, etc.

Another solution would be to use a latch to hold your arm down. At the beginning of the match you lower the arm allowing the latch to fall/release and your arm is free to move as it pleases.

[Brandon_L]

Originally Posted by EricH

Why do you need gas springs or other means of balancing an arm other than its power source?

I'm not quite sure what you mean here, but I was trying to stay away from the "strap the biggest motor you can and a bit of reduction" method. See here.

As for your second question: Your motor does not backdrive? So why are you worrying about keeping the arm in a specific place? For that matter, why are you looking at things other than the motor to keep the arm up? Motors that backdrive (may) need these things, but motors that do not backdrive almost certainly do not.

My mistake, It was supposed to say a motor that does backdrive. Fixed.

For the first question... You're actually asking two. Where you want "neutral" to be is game dependent--ask again sometime in January. But the math behind sizing/placement at neutral...

That itself answered the question. I was asking if it should just be balanced at 90 degrees or something game relevant (like higher then 90 in logomotion)

You're actually pretty close. You would use an equation something like, (weight of arm side 1)*(horizontal distance from pivot point to arm CG)=(weight of arm side 2)*(horizontal distance from pivot point to arm CG)+(vertical force component from balancer)*(horizontal distance from pivot point to balancer attach point).

Pesky CG, I knew I left something out. This formula would give you an arm that balances at 90 degrees though, no? What about something angular?

This would actually fall into the Statics category of problems, which many engineers study early in their college careers--the basic method of balancing out forces is to set up an X-equation, X-unknown system of equations where the sum of all the forces and moments in a given direction or about a given axis of rotation is zero. Then you can solve for your unknowns.

I'll have to look more into that

Just to bring something up... I have yet to see one of 330's single-joint arms use any sort of balancer that wasn't the drive motors. Neutral for some was all the way down; for others it was wherever the arm was left--one at least needed a little bit of power to maintain a position. Typical drive, 1-2 FP motors, stock gearbox, heavy reduction afterwards (exception: 2x Globe motors mounted on the arm itself--2004). No braking capability.

The bit I bolded there is what I'm specifically interested in, would that not be a gas spring? or did they just use two window motors together? Our arm in '11 used two window motors, and well, I can just say it wasn't fun. Then again we were probably doing everything wrong. Thanks for the help!

[Jared Russell]

Balancing an arm is a bit of an art. There are tons of options for supplying the balancing force (coil springs, constant force springs, gas springs, surgical tubing, even counter weights), tons of different sizes/strengths, and in most rotary arm cases you are dealing with changing the angle of the balancing force during travel.

Without getting into specific designs (others here are far more adept than I at arm design), here are two meta-level thoughts to keep in mind:

1. Does an arm ever really need to be "balanced"? You are using material deformation to supply a force to counteract the effect of gravity in order to reduce the torque/force requirement on some actuator. Even if you aren't really balanced, every bit of torque/force you are relieving is aiding the actuator. Don't lose sleep over making it perfect (that is, F(gravity) = F(counterbalance) for all angles of the arm)

2. All motors stop back driving at some point due to friction. The bigger the gear reduction, the more friction is present in the system. Our 2011 robot's arm was powered by a Fisher Price motor through a multi-stage planetary transmission. There was enough friction in this system to keep our arm stowed at match start even though the arm was balanced in the horizontal position - we had to be careful when doing maintenance on the chains to our arm because once you remove them, the arm springs up.

[BJC]

Originally Posted by EricH

Why do you need gas springs or other means of balancing an arm other than its power source?

This is not an idle or ignorant question. I have seen multiple arms with no balancing (other than the power source) that did quite well. Single-joint, multi-joint, even with a large-ish mass at the far end like a game object.
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Just to bring something up... I have yet to see one of 330's single-joint arms use any sort of balancer that wasn't the drive motors. Neutral for some was all the way down; for others it was wherever the arm was left--one at least needed a little bit of power to maintain a position. Typical drive, 1-2 FP motors, stock gearbox, heavy reduction afterwards (exception: 2x Globe motors mounted on the arm itself--2004). No braking capability.

I don't think there is any reason not to "balance" your arm in FRC. Why would you make your motors work harder then they have to? By making an arm close to neutral your improving the arm's loaded speed and consequently allowing it to take on a greater loads without slowing down. In theory, the only weight the motors should actually be lifting is from the game piece. While 330's single joint "stick with claw" arms weight much less than many others it doesn't weight nothing to the motors like it would with balancing.

[MrForbes]

We've made two arm robots, each with a small motor and a "spring" to support most of the weight. Neither arm was fully balanced, but they did work ok.

Gas springs do not really have a constant force, the force is higher when the spring is compressed all the way. You can play with the geometry of the mounting points, to try to get a more linear spring force out of it, but it will never be perfect.

And surgical tubing also has this problem, even more than the gas spring. The fun part is trying to get the geometry right so that it works the way you want, ie. naturally holds the arm retracted in the "stowed" position, then provides more help as the arm raises up. We haven't been able to do it yet, but one of these years we'll actually design this part of the robot.

The 330 approach of using a lot more motor power, seems to be quite successful. I'd listen to Eric.

[Chris is me]

Originally Posted by MrForbes

And surgical tubing also has this problem, even more than the gas spring. The fun part is trying to get the geometry right so that it works the way you want, ie. naturally holds the arm retracted in the "stowed" position, then provides more help as the arm raises up. We haven't been able to do it yet, but one of these years we'll actually design this part of the robot.

The way we've found some success using surgical tubing is to pre-stretch it a little so that even when its in its most relaxed position, it's still providing a bit of resistance. It seems a lot of the nonlinearity is in the first few inches of elongation, so this seemed to work for us. Then again, arm balancing is the kind of thing you can just get "good enough" at.

The 330 approach of using a lot more motor power, seems to be quite successful. I'd listen to Eric.

There's a lot to be said for just throwing a gear reduction at the problem. It naturally slows your arm down a lot more so you are less reliant on software to control it. Plus depending on your configuration, you can probably self-right your robot. I mean, adding a spring can only help an arm, but if you gear like 330 it's not mandatory.

[BJC]

I can't find the original spot on the internet where I got them but JVN's design calculators will allow you to figure out exactly how an arm/intake/elevator/drivetrain will perform with specific motors/gearing. I highly suggest anyone who hasn't seen them before saves these somewhere safe for the build season.

Balancing an arm can do nothing but improve its performance. Team 33 always springs our arms/elevators up without exception because it is *better* then it otherwise would be.

*Goes faster, lifts more, pulls less current, makes it easier to maintain a selected position, etc.



Edit: Answering the OP's actual question.

Brandon: Gas Springs are awesome in certain situations. Team 67 is the most noctorious for having some crazy powerful gas spring "thing" on their robot. 2012 they used gas springs to hold up their "arm" in the collecting position. That said, big gas springs are also pretty heavy. Surgical tubing can also work really well if you do it right and weighs very little. The biggest factor with surgical tubing is:

1. The lever arm. Depending on where the tubing is attached relative to the pivot point it will change how much force is being exerted on the arm. This will determine how much surgical tubing you need. Thankfully, because surgical tubing is easy to work with and change the amount can be experimentally found in testing.

2. The stretch distance of the surgical tubing when the arm is fully up vs. fully down. By minimizing the difference in length you can get the force on the arm almost constant throughout the arm's travel. The longer the sugical tubing and the shorter the lever arm the less change in length there is.

If you're running a linkage arm, you can also put your surgical tubing between the pivoting points on the linkage.

I don't know much about using gas springs as counterforce so I'll let someone else take that one.

[Akash Rastogi]

Brandon, were you looking for specifics about how to select the right gas shock?

[daniel_dsouza]

I don't think you're missing anything about gas springs.

The last time we had an arm (2011), our goal was to be able to score on both sides of our robot (our arm was designed to rotate more than 270 degrees).

We had two gas springs that we bought (maybe ordered) from our local hardware store. We had attempted to connect the arms to the springs in such a way that the springs would perfectly cancel out the force of gravity. We then used a (not very powerful) window motor, which in theory should have been able to effortlessly lift our arms.

Theory only works when the math is done correctly. If anyone looks at our match statistics that year, they will find low scores. Because our arms were not properly balanced, our window motor would overheat, and we'd spend some matches without a working arm, and some with our our arm straight up!

Used correctly, gas springs can offset the force of gravity on many mechanisms (like arms)