Springs, Bungees, Counterweights

I’ve been watching this board for a couple of weeks and haven’t seen a proposed design for an arm that uses either a counterweight or a spring system on the arm itself.

The problem with a counterweight, of course, is that if it is mounted on the arm it raises the CG and does not contribute to the other functions of the 'bot. I suppose the counterweight could have been mounted on a 1/2-height tower using a 2-1 pulley-and-cable system, but that would be clumsy, still add weight up high, and only apply half the downward force as a weight connected through a 1-1 purchase. Of course, if there were a backwards-extending arm just as long as the main arm, you could hang a fairly small weight on the back (see construction cranes). You would still have the high CG problem.

If you committed to a backwards counterbalance arm as long (or almost as long) as the main arm, you could use bungee cord material or a spring to store energy when the arm is descending that could be used to help lift the arm-and-tetra. There are some issues to be solved regarding the effective length of a spring and whether it could be used over the effective length of the arm’s motion, but there are ways to work around this. A spring-over-shaft design, sort of like a roll-up garage door spring, might work pretty well.

If you think about it for a few minutes, it would be trivial to design a counterbalance that could be extended automatically after measurement.

Anyway, our team rejected the concept of counterbalancing forces (grumble, grumble), and went with a straight lifting arm system. I am curious if anyone is planning to use this sort of stored energy (smart teams), or considered it and rejected it. Talk amongst yourselves.

We tried putting on a large spring to assist in lifting tetras yesterday. We were hoping that it would stop us from destroying worm gears, as it took off 1/6th of the downward force on the loaded arm. Unfortunatly, we then preceded to destory another worm gear. :mad:

We use a spring on our arm with great success. However, i am unable to find any pictures of the mechanism just yet. I would say that springs are the way to go, they are light and provide the force needed.

We use a pair of springs made of surgical tubing to cancel the
force on an arm that tilts at the base of the robot by means of a
screw. Because sin(x) is approximately x for small angles, we
get quite good force cancellation and the tilt motor runs at free
speed.

We also use surgical tubing to help lift the heavier section of
a telescopting tube setup, but in this case we tune the spring to
cancel the force at a spot we think we will likely spend a lot of time
at. We would need a constant force spring, lifting 16 pounds,
to do better than that.

Worm gear life is a problem, we have to do everything we can
to extend gear life.

Here’s our counterweight for the shoulder of our arm:

It only helps us once the arm is past vertical, but there is bungee on the other side to balance it out.

We use a spring to help counter balance. It gives a maximum counter-balancing torque of 42 lb-ft with the arm at 110 degree from vertical, about 38 lb-ft at 90 degrees.

counterspring.jpg


counterspring.jpg

Our team decided that counterbalancing was an absolute must. Our arm has a constant-force gas spring to help counteract the weight. Unlike most other springs that exert a force proportional to the distance compressed/extended, our gas spring exerts a constant 225 lbs in every position. A simple sum of moments calculation is done to figure out the distance to mount the spring.

The arm is designed so that one tetra completely balances the arm and unloads the motor (van door). An empty arm means the motor works a little to keep it down and two or more tetras means the motor works to keep it up. We have found this to be highly effective as the motor runs a lot cooler than arm designs in previous years.

You can see it here.

We are using springs. Without the springs we risk buring out our van door motor trying to hold a tetra up even with our arm not extended. So we added springs to the back of the arm and now we risk burning the motor trying to hold the arm down… Actually the braking on the victor holds it in place nicely as long as the robot is on, but we have to turn the robot on to make the arm stay down. Makes storage more interesting.

We on team 343 are using a heavy duty bungie as a counter balance on our lifting mechanism. You can see pictures on our site, www.metalinmotion.com. The bungies are tucked into the diagonal supports (they’re white with blue stripes). They keep the current draw on the fisher price motors down around 10 Amps. For a closer look come by our pit at Florida, Palmetto or the Championship.

Have a great Season!!