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

[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.

[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.

[EricH]

Originally Posted by Brandon_L

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?

It would work for angular. HOWEVER, the distances will change in that case. They'll become closer to the pivot point. There will also be a change in the force vector of the balancer relative to the arm--could be in terms of magnitude; almost certainly a direction will change.

It's kind of hard to explain without going through a diagram. I've attached one with the main equations you'd need for two positions--note that they're the same equations for both positions, but the numbers WILL be different. You use a lot of components when you're working at angles (the component of force in the Y direction, for example).

[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.

[ChrisH]

BeachBot design is driven by requirements developed during the design/concept phase. Some of these requirements are driven by the game or rules and others by experience. We call the later "BeachBot requirements". One of those requirements is that the robot will be self-righting to the extent possible. Extreme examples of this were our 2008 and 2010 robots who could right themselves from any stable position, given enough time.

Because we mount our arm motors low and do the reduction in chain between the motor and the pivot point, the arm system is quite weight efficient. Self-righting may not be possible with a "balanced"arm.

Due to our design philosophy of keeping everything as low as possible on the robot we rarely have to use our self-righting ability. Twice in a season during competition would be alot, but it sure is handy (and a crowd pleaser) when you need it.

Every design has its trade-offs. We like using brute force in the arm because it helps with secondary issues. But there might be a good reason for using a balanced approach. Just be aware of what you might be loosing in going with a particular approach

Originally Posted by Chris is me

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.

[DampRobot]

I'm more than a little surprised that there is some debate on this thread whether or not arms/elevators should be balanced. In my mind, this is sort of a no-brainer.

As affirmed by JVN and other designers from teams like 148, 111, and 254, balancing an arm or elevator increases it's speed while decreasing current draw. The less weight that must be overcome by the arm/elevator, the faster it can be geared. While it's not too difficult to slow an arm down, it's very hard to speed it up. As the drivers (and software) adapt to the higher speed, performance benefits will become apparent.

I hate to pull out the old "you can always slow it down in software," but I feel like I need to. Most beneficial effects of large reductions (non-backdrivability, high resolution) are the kind that can be essentially duplicated with good software. The only benefit mentioned in this thread that cannot be duplicated with a balanced arm , self righting, is really not a huge advantage 99.5% of the time. Sure, it's nice, but I'd much rather have a quick arm.

[MrForbes]

Originally Posted by DampRobot

I hate to pull out the old "you can always slow it down in software," but I feel like I need to. Most beneficial effects of large reductions (non-backdrivability, high resolution) are the kind that can be essentially duplicated with good software.

We have enough trouble getting an arm balanced....and you expect us to be able to develop good software too?

[Andrew Schreiber]

Originally Posted by DampRobot

I hate to pull out the old "you can always slow it down in software," but I feel like I need to. Most beneficial effects of large reductions (non-backdrivability, high resolution) are the kind that can be essentially duplicated with good software. The only benefit mentioned in this thread that cannot be duplicated with a balanced arm , self righting, is really not a huge advantage 99.5% of the time. Sure, it's nice, but I'd much rather have a quick arm.

Yes, you can "always slow it down in software" but personally I would prefer a mechanical reduction. Reducing speed in software is going to make your motor run harder. Maybe not an issue if you are using something like a CIM but if you are running the FP/BB style motors which are actively cooled by a fan when running you may be asking for trouble running them slower. Additionally, as much as I hate to admit it, software isn't perfect. Sometimes we get weird edge cases that are inadequately tested and an arm that moves physically slower means we have more of a chance to kill the power before it breaks something.

[Jon Stratis]

One point I haven't seen on here... try to avoid a situation where you're driving around with the motor stalled most of the time! The way we had our elevator set up 2 seasons ago, we had to be driving it down in order to pick up tubes. As a result, most of the match it was either stalled going down as we went after tubes, or stalled going up as we attempted to hang. We ended up burning out a lot of FP motors that year! There is some debate between myself and another mentor as to the reason for the burn out... he thinks it was due to friction in the elevator, making the motor work more to raise/lower, while I think it was due to the motor being almost constantly stalled!

All that said, counterbalancing any moving part, whether linear or rotary, isn't necessary in our applications... but it is a simple tool to use to get increased speed at a decreased effort!