Balancing an Arm
 

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?

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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:)

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

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

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)

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?

I'll have to look more into that

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!

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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.

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.

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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

Re: Balancing an Arm
 

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) :P

Re: Balancing an Arm
 

As Joe said regarding our and their arm in 2011, we counterbalanced. Our gear ratio was 573 (go mechwarriors!) To 1.

We do not have facilities to make a light arm. If you look at chickens arm from the same year, they were able to fab it light enough that they used s 600:1 reduction with no counter balance.

Holding an arm in position using motors requires stalling them. If you have too much weight and draw too much current, you will fry them. You can always gear down, but then you are trading off on speed

Re: Balancing an Arm
 

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).
Attachment 13266

Re: Balancing an Arm
 

How do you balance an elevator? Do you use a torsion spring to reel in cable? Somehow you need to get around the problem of needing to stretch the spring/tubing to 5 times its starting length if the overlap between elevator sections is only 20% (or whatever) of the elevator's stage height, right?

Re: Balancing an Arm
 

Constant force springs

Re: Balancing an Arm
 

My garage doors are each balanced with a torsion spring reeling in a cable.

Re: Balancing an Arm
 

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

Re: Balancing an Arm
 

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.

Re: Balancing an Arm
 

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

Re: Balancing an 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.

Re: Balancing an Arm
 

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!

Re: Balancing an Arm
 

^This.

Here's a rather contrived example, but it illustrates the point.

Suppose you are using a BB550 motor to raise an arm, and you want to drive the arm at 15 rpm (90 degrees per second) at a torque of 500 oz-in.

If the total mechanical speed reduction from motor output shaft to arm rotation is 10:1 you'll be drawing ~60 amps

If the total mechanical speed reduction from motor output shaft to arm rotation is 50:1 you'll be drawing ~12 amps

Re: Balancing an Arm
 

The bigger advantage of course is that if you reduce the load your motor needs to lift, you can use a less powerful motor to accomplish the task at the same speed. 148 (and 217) have used a single globe motor as our shoulder joint in years where many other teams are using a CIM or 1 or 2 FP motors...

The weight of the arm is (reasonably) balanced, so the motor just needs to lift the weight of the game object.

-John

Re: Balancing an Arm
 

And Team 1296 dutifully read the awesome JVN blog and did exactly the same thing! We used surgical tubing to save costs. Our arms worked beautifully - now if we can only make more elegant grippers...

I saw a couple of references to "slowing things down with software" in this thread. In general it is good practice to never ask more (with your software) out of a motor than it can give. Don't design software that asks the motor and the appendage to violate any laws of physics. This is often as simple as adding a trapezoidal velocity profile (or let the PID classes do it for you) or filtering the linear input from the driver station with a cubic function.

HTH

Re: Balancing an Arm
 

Im quite interested in how 148 decided on where to have your arm balanced. Just straight out horizontal? Or something game specific like near the top rack in logomotion?

I didn't mean for this to turn into a "balancing an arm vs gearing" thread, I was more interested in methods of balancing an arm. Not that I'm ruling out any of the pro-gearing points, just looking into balancing at the moment. It seems like the easiest way would just be surgical tubing. I'm just concerned with how you decided where you want it balanced now.

Re: Balancing an Arm
 

The TechnoKats "Overdrive" robot had a perfectly counterbalanced windmilling trackball manipulator mechanism. It used a pneumatic cylinder which could be selected to two different pressures in order to balance it both empty and when holding a trackball. The design feature that made it perfectly balanced in any orientation was that the cylinder was connected to a sprocket that turned with the "arm", so that it applied maximum upward force when the arm was horizontal and zero force when it was vertical. The arm would stay exactly where it was placed with no motor power applied at all.

The main benefit I saw of the perfect counterbalance was in the control software for setting the arm position. Gravity simply was not an issue, and a single set of PID constants worked for the entire circle of arm travel. As a programmer, I consider the fact that we didn't burn out Fisher-Price motors by stalling them against the weight of the arm to be of secondary importance.

Re: Balancing an Arm
 

In 2011 we disconnected the motor from the gearbox and balanced at about horizontal as baseline value, then tweaked until the drivers & programmers were happy. In some years, we've had to reduce this amount due to the arm lifting up at the starting position. In 2011 we had a very simple "drive until position, then cut power" control scheme. Combining the Victor brake-mode with surgical tubing balance and this control allowed for easy pre-positions.

We tuned in surgical tubing & position code through iteration -- test, tweak, repeat. (Iteration is always the answer, of course.)

In my experience, it is better not to overthink it, just add some surgical tubing and tune it to taste...

-John

Re: Balancing an Arm
 

How do you counter balance an elevator??

Re: Balancing an Arm
 

I've said this earlier in the thread but most teams I've heard of counterbalance with a constant force spring.

Re: Balancing an Arm
 

Constant force coil spring.

http://www.mcmaster.com/#constant-force-springs/=kg1vlp

Our 2011 elevator will naturally float up when the robot is turned on its side, and will also walk upwards if the return cable is disconnected. Since it's slightly over-balanced, the weight of any game piece evens it out, and the mechanical resistance in the motor gearboxes holds it in place fairly well in almost any position. We initially built the system and estimated a certain portion of the load would be balanced, and geared accordingly. After building, we changed the gear ratio for more speed due to the relatively light load the system was under (for 2x 550 motors).

Re: Balancing an Arm
 

We also have used constant force springs with success.

Our 2011 elevator was 2 stages, with a main raising section and a "trolley" that had the roller claw mounted to it. To offset the entire weight of the elevator/roller claw, we needed to counterbalance both stages of the elevator.

To do that, we used one spring connected to the main raising section with the other end of it connected to the robot frame. We then used another spring that was mated to the frame of the main raising section with the other end connected to the trolley on the 2nd stage.

Because the weights of each stage were different, we used two different force springs. The springs together offset about 75% of the weight of the elevator. Like 33's, our elevator also shot out sideways if you turned it on its side.

Even without the weight being totally offset, the springs enabled the elevator to be faster, easier to position with PID and overall just more effective.

-Brando

Re: Balancing an Arm
 

This is important for many things that invovle tuning. I am a huge proponent of "doing the math", but often in development cycles, you plan to get close, and then tune for optimum performance.
When doing any sprung element development on a car (springs, shocks, bushings, engine mounts...), you typically will do analysis and design to pick a nominal, and then have a tuning set of +/-10% to +/-50% to see how changing the rate might help performance. The reason for this is usually, your analysis is based off of some assumptions. Without tuning, the quality of the output is reliant on the quality of the assumptions (which sometimes are poor). Planning for some tuning can vastly improve this.

With things like balancing an arm, you can use the:
"JVN says balance at horizontal" as a rule of thumb, but then try a little extra and a little less and see what works best. If you vex robot works best with it tuned to balance when feeding in the trough... then that is the ri9ght answer for you and your robot. If your robot works best with just a little bit of assistance to keep the motors in the friendly half of the power curve... so be it.

You can get pretty fancy with the way you do the counterbalance as well. Using "over center" principles, you can get some nice variation in forces. The gas shocks on a minivan liftgate are a great example of this. At almost closed, the forces in the shock are at their highest, yet they offer very little lifting force due to the hinge and the push point and the reaction point nearly being in line. This makes it easy for the gate to stay at or near closed. Yet, when fully extended, the shock is at its weakest from a force in the shock, but it is able to hold the liftgate up all by itself. Pretty neat when you think about it.

You can also get this behaviour using bungee and "cams" to change the lever arm length that the bungee has. This can have some very neat and dramatic effects.

Re: Balancing an Arm
 

Thanks for posting this here, Brandon! Our team had the same problem in 2011. Arm was pretty bad and required really heavy springs to balance, which just killed our weight budget.
So from what I've picked up so far, you can either use a super-powerful motor or use surgical tubing to balance said arm, correct? If using surgical tubing, would the tubing be attached to the arm both below and above the arm?

Re: Balancing an Arm
 

This is the most common way to attach surgical tubing.

Hope this answers your question.
Regards, Bryan

Re: Balancing an Arm
 

You can also use the proper gear ratio so your motor doesn't have to work so hard. But be careful with the super-high-reduction gearboxes: they have an output torque limit that frequently is less than the torque the motor will generate with them.

Re: Balancing an Arm
 

Our 2011 robot used a gas shock from McMaster on the same side of the arm as the tube manipulator. It was sized in a way to provide a slight assist in lifting the arm, but not so much that it required a lot of motor power to lower it. We did a lot of match to determine the length and pressure and final location.

The other advantage of the gas shock was that it provided some damping and gave stability to the arm as it rotated up or down.

Re: Balancing an Arm
 

Our 2012 arm drew around 4 amps when stalled, holding the arm horizontal. This was around 1 volt. We never had a problem dissipating 4 watts.

Re: Balancing an Arm
 

Thanks! It does

Re: Balancing an Arm
 

From what I've picked up, basically yes.

If I were able to go back and play logomotion again, I think I'd balance my arm with surgical tubing (and a lighter manipulator wouldn't hurt) and use a window motor (possibly with torque limiter) to avoid the motor stalling issue. You'd have to gear down its speed a bit though.

If it were a heavier game piece ('08) I'd probably use gas springs instead.

This thread turned out to be very informative, thanks to everyone who threw their $0.02 in

Re: Balancing an Arm
 

With the high-speed FP motors, it's pretty hard to provide too much speed reduction when used for an arm.

Consider the following rough calculation for an extreme example: an FP 9013 motor with 1000:1 speed reduction

At 0.8 volts, 9013 provides 4.5 oz-in torque @ 4watts & 5 amps when stalled.

That translates to 23.4 ft-lb holding torque (stall) at the arm.

That's equivalent to approx 6 lbs of unbalanced weight at the end of a 4 ft arm, or 12 lbs in the middle.

As Joe mentioned previously, FP should be able to sustain a 4 watt stall condition without overheating.

At 12 volts, the 9013 can drive at that load (4.5 oz-in) at 15580 rpm.

That's equivalent to 94 degrees per second rotation at the arm.


As I mentioned in a previous post, the real concern with these super-high gear ratios is not the speed reduction, but rather the output torque rating of the final gears. This can be mitigated by making the final stage rugged.