Balancing an Arm

[JVN]

Originally Posted by Brandon_L

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?

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

[akoscielski3]

Originally Posted by BJC

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.

How do you counter balance an elevator??

[MichaelBick]

Originally Posted by akoscielski3

How do you counter balance an elevator??

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

[apalrd]

Originally Posted by akoscielski3

How do you counter balance an elevator??

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).

[Brandon Holley]

Originally Posted by akoscielski3

How do you counter balance an elevator??

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

[IKE]

Originally Posted by JVN

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

-John

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.

[gabrielau23]

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?

[BJC]

Originally Posted by gabrielau23

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?

This is the most common way to attach surgical tubing.

Hope this answers your question.
Regards, Bryan

[Ether]

Originally Posted by gabrielau23

\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?

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.

[Chris Fultz]

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.

[Joe Ross]

Originally Posted by Tom Line

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

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.

[gabrielau23]

Originally Posted by BJC

This is the most common way to attach surgical tubing.

Hope this answers your question.
Regards, Bryan

Thanks! It does

[Brandon_L]

Originally Posted by gabrielau23

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?

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

[Ether]

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.