M11 and counterbalancing mechanisms...

Rule M11 (limiting stored energy in a spring at the start of a match to 20 ft-lbs) seems to dissallow the (historically) common practice of counterbalancing arms, baskets, etc.

For example, suppose a robot has a 10 lb gripper. If the way the arm stores at the beginning of the match puts it at the bottom of the box, the springs on the robot could only lift this gripper 2 ft before exceeding the 20 ft-lb limit.

Another example, the smallest gas spring sold by Small Parts Inc. puts out 60 lbs of force and has a stroke of 3.5 inches. Compressing this single spring would store almost 90% of the legal limit.

This does not seem reasonable.

This is a very significant limitation to a very important aspect of proper robot design (specifically, using springs to compensate for gravity effects of robot arms & mechanisms).

Counterbalance of robot arms is a “best practice” that is effectively forbidden by this rule.

Safety is an important aspect of robot design and springs can be dangerous if not designed properly, but lack of counterbalance can be dangerous as well.

Do others read the rule the same way as I do?


Joe J.


I wholeheartedly agree. It’s been three years since the Bobcat required a counterbalance spring but, when we did, it was at least two orders of magnitude larger than that at the beginning of a match…

I wonder what FIRST is thinking? We are limited to the latex tubing (and to a specific maximum length) which has always defined the upper bound of stored energy. This has worked rather well for the nine years I have been doing this…

Will they also limit any precharge in the pneumatics?

I liked it better when the engineers designed the game and rules…


great points
(Actually thanks for reminding me about gas springs…now to find out if they are legal)

But just to make sure I am understanding you. The counter balanced arm of 10 lbs you use as illustration is counter balanced by some spring and not a 10 lb weight on a lever arm. FIRST has always been cautious about springs. I don’t recall being able to use any really big springs for the 5 years I’ve done this (with the gas spring being the exception as being from Small Parts)

And just to bring up safety (if that is the intent of the 20 lbs stored rule), well I imagine a few 130 pound robots in auto mode causing all sorts of safety problems. Will they have 4 people on kill switches just to watch the 'bots?

I may send off a question about the gas springs…they could solve a few problems at the moment.

If possible, we always try to make our arm “neutral balanced” in some orientation (say horizontal) so that gravity and our counterbalance just equal eachother out at that point.

If you do this, your robot’s motor does not have to do any work to keep your arm in that position. Basically, you turn off the motors and, low and behold, the arm stays put.

It is not impossible to make a well behaved robot arm without an effective counterbalance, but it is MUCH more difficult (for instance, you can add a “gravity” term to your motor output to try to have the motors automatically add more output when gravity is putting more torque into your robot arm).

But… …the bottom line is that clever software has a much more difficult time dealing with gravity than a few yards of latex tubing judiciously wrapped around your robot arm.

We have done this for years and have never had any safety issues.

I hope that many of you will join me in seeking to have counterbalance return to the legal side of the rules.

Joe J.

The 20 ft-lb limit came as a surprise to me. We have in the past used quite a bit more than this to help counterbalance arms. It makes for better machines.

I definitely think the rule limits the capability of robots with big arms. It would be nice to know how FIRST arrived at the 20 ft-lb figure (maybe they used a slingshot… now THERE 20 ft-lbs will do some serious work…)


Simple solution. The rule states that you cannot have more than 20 ft-lbs of energy stored in the spring at the START of the match. It never says anything about a maximum limit to stored energy during the game.

Design your system so that at the start of the match, it is in a gravity neutral position (i.e. arm at an elevation angle of 90 degrees or so. As you move the arm away from that point (and toward horizontal, when a gravity offset is most necessary), the springs/tubing are stretched further, adding energy. That energy can be recovered as necessary by the springs as the arm moves back toward the original position.


Y = AX^2 + B… ehhh, whatever.


For some arm joints, the initial size limitations (2.5ft by 3 ft by 5 ft) allows for the arm to start with its GC in the highest position so that the spring energy can be at its minimum, but for the most part, this is not possible.

I am not saying that we cannot get a system to work without counterbalance or even that effective counterbalance is impossible within the current rule structure. I am only saying that it is much more difficult.

For my part, I am not willing to wait to see if we can turn FIRST around on this ruling, so I have already designed in a way to make our machine work without relief.

I believe that it is a much bigger problem for teams without clever sheep like myself :wink:

The MAIN reason I bring this up is that I believe that, to the extent that fewer teams use effective counterbalance or that teams just ignore the rule, the FIRST community is moving in the wrong direction.

Comments welcome…

Joe J.

I also wonder why they set such a low limit for potential energy at the start of the match? Was it for safety reasons or just to make it challenging?

BTW - If you have ever use a compound bow for shooting arrows, you will know that there is a way to get variable and non-linear force over a specific range of motion. IF you are a clever sheep (as Joe would call it) you could design a counter balance that produces the required 20ft-lbs potential energy but get a greater force at the point you need it.

Another option is to use constant force springs - like those used on tape measures.

As to why the limit was set where it was, I think that the limit was made up by folks without real team experience.

As to storing enery using a constant force spring, this too is limited to 20 ft-lbs (per mechanism if you read the FIRST forums, not per spring).

To understand just how low this amount of enery is, think of this.

The Chiaphua motor has a peak power output of about 300 Watts. This is 300 Joules per Second. A Joule is a N-m. The conversion from ft-lbs to N-m is 1.4 (approx). So… 20 ft-lbs is approx 30 N-m or (a.k.a. Joules).

So… the amount of energy that is allowed to be stored in a spring is the amount of work that at Chiaphua motor can do is 1/10th of a second!

Think about it this way. In short bursts, the battery can provide 200 Amps at approx. 10 Volts. This works out to an astounding 2000Watts. So…

The amount of energy that can be legally stored in a spring can be provided by the battery in 15 milliseconds (1/65th of a second)!

This is obsurdly low in my opinion.

But… …I can live with it if you can…

Joe J.

The only reason I can think of is Safety.

Any chance of failure of mechanisms using springs with lots of force, and the spring might snap, or the parts holding it, and parts start flying toward the audience. That will be a really bad thing at FIRST events, and people might get hurt, not to mention they might sue FIRST because of it.

It’s just so much easier making a rule like this, and not worry about the trouble of not having the rule.

Maybe when FIRST field have the shield around it like battle bots, then we will get the springs back into the robot. Before then… I am afraid we will have to deal without it. Which is ok, consider everyone is under the same rule…

Safety of the participants and spectators is indeed very important, but I still don’t get rule M11.

It does nothing to limit the amount energy sorted in a spring, it only limits the amount of energy stored at the beginning of the match and that to a level that is so low as to be silly.

Another example of how little energy they are allowing: 20 ft-lbs of energy will allow you to lift your robot 1/23rd of the way up the ramp (i.e. about 4 inches up the ramp or raising your robot’s CG a little more than an inch off the carpeted surface).

There are an infinite number of ways to build a unsafe robot. They are all illegal. Limited spring energy to 20 ft-lbs at the beginning of the match does very little to increase the safety of FIRST robots (imho).

In fact, I believe that a case can be made that increasing the number of badly behaved robot arms will result in a higher chance of someone getting hurt at a robot competition (i.e. a decrease in safety).

The rule is a puzzlement…

Joe J.