Robot Arm C

[chalker7]

Most teams struggle to complete as many tasks as they can in the allotted time. One problem I've run into practicing is simply knocking the ping pong ball out of play rendering it unusable.
This means that a lot of teams will not even get to the height task. And for teams that use up all 3 minutes, this means there will still be plenty of ties. However the situation is much better than last year and I doubt we'll be seeing many ties between teams who would've ended up tied at 94 points last year.

I'm not sure I follow. Not trying to be leading or anything, but what makes you say that a lot of teams won't get to the height task? Do you mean they won't be able to pick up a ping pong ball at all?

[iwonder]

Adding the counterweight will increase the moment of inertia, and require more torque from the motor to move the segment, but generally it simply results in slower movements, unless you have a really underpowered servo. The advantage is that it can require absolutely no power to keep to the arm in any position(in fact, I can remove power from my arm almost regardless of position and it stays still). The other downside is the weight(mine weighs in at about 35lbs).

Woah. How much does the counterweight itself weigh?

Also, since my counterweight is set at a fixed distance from the shoulder joint of my arm, I find that when I move the elbow, such that the claw is raised up, the arm isn't balanced the same as it was when I first set the counterweight. Of course this situation doesn't arise much during the run, but just one thing that could be improved upon.

The counterweights are 2.5 lbs on the elbow, and something around 10 lbs on the shoulder, then there's a heavy plywood base and a couple large lead acid batteries to to keep it from tipping over.

Not quite sure I follow your logic about the balance changing... I think I know what you mean, but since the full weight of each segment is fully supported at the pivot point, no matter how you move(for example) an elbow joint, the counterbalance on the shoulder would remain the same, is that the kind of thing you were getting at?

[OldSpice]

Most teams struggle to complete as many tasks as they can in the allotted time. One problem I've run into practicing is simply knocking the ping pong ball out of play rendering it unusable.
This means that a lot of teams will not even get to the height task. And for teams that use up all 3 minutes, this means there will still be plenty of ties. However the situation is much better than last year and I doubt we'll be seeing many ties between teams who would've ended up tied at 94 points last year.

I'm not sure I follow. Not trying to be leading or anything, but what makes you say that a lot of teams won't get to the height task? Do you mean they won't be able to pick up a ping pong ball at all?

I was trying to convey that most teams did not finish all of the tasks they had to complete last year, ie: They would use up all three minutes and only have moved 5 pencils, 4 PVC, and 1 nail or something.
Only a small fraction of teams I competed with last year finished the tasks with time to spare, therefore, teams that run out of time would not get to the height task. If a team did not get to the height task then they would not have fractions of points to differentiate themselves from other teams. Therefore there will still be ties. However, as I stated before, there most likely won't be ties between teams who would've tied at 80+ points last year as they will have time to do the height task.

[chalker7]

I was trying to convey that most teams did not finish all of the tasks they had to complete last year, ie: They would use up all three minutes and only have moved 5 pencils, 4 PVC, and 1 nail or something.
Only a small fraction of teams I competed with last year finished the tasks with time to spare, therefore, teams that run out of time would not get to the height task. If a team did not get to the height task then they would not have fractions of points to differentiate themselves from other teams. Therefore there will still be ties. However, as I stated before, there most likely won't be ties between teams who would've tied at 80+ points last year as they will have time to do the height task.

EDIT: Okay I just read over my rules sheet and just figured out why what I said does not make sense. Sorry about that. I'm leaving this post here so you know what I was thinking. However, everything I said in this post is negated by the fact that the height task comes after times stops.

I understand. I just didn't want people to think that they can't attempt the final task if they haven't finished everything else (already scored a perfect score.) I agree that many teams might not try it because of the low relative value, but still think enough teams will try that it will prevent a lot of ties.

[jander14indoor]

For first comment, usual statement about this being an opinion, not official clarification, blah, blah.

Height task is the last scorable thing you do, but you don't have to get all other points FIRST. It does require a scorable ping pong ball of course.

Strategy comments.
Since the first tie breaker is time, you may want to make sure you ALWAYS call time before the three minutes is up. Especially if you are either unlikely to get any more points in the remaining time, or it will be small at best. You may be part of the ties, but you are at least not at the lowest possible point for the first tie breaker.
Similarly, consider the height task. You may want to make it a priority to accomplish something. Again, if otherwise tied, this MAY make the difference since it is likely to keep you out of the tiebreakers.

Comments on load balancing. With proper design, you can get a tension and cam system to work as well as actual counterweights, and it does have the potentional advantage of vastly lower mass than iwonder mentions.

At the Ford assembly plants where I spend a lot of time, all heavy tools are on devices known as load balancers (you might try googling "mechanical load balancers" or "tool balancers", don't bother with just "load balancer", that talks almost exclusively about balancing workload on computers). They look like one of those things you put keys on with a string that zips in and out of a holder, but much larger. They suspend heavy tools for the operators who only need to move them into position to use them, they don't really pick them up or put them down. They allow a wide range of heights while maintaining a constant force to support the tool. The secret is inside the box that zips the cable in and out. It has a clock spring to apply force. Now normally as you wind the spring by stretching out the cable the force would go up overcompensating for the weight of the tool. But the cable itself is wound on a spiral pulley (see http://www.ergonomicpartners.com/produc ... 3-lbs.aspx and http://www.flexibleassembly.com/Product ... ASB-Series for real ones that show this spiral pulley). When the cable is pulled it leaves the pulley at a narrow diameter. As it pulls out, it moves to a larger diameter, increasing its leverage/torque. Designed correctly, the increasing lever exactly offsets the spring increase as it tightens giving a constant force at the end of the cable.

Now, you don't need that range of motion on your robot. You won't go more than 360 degrees. Unfortunately, while I see these in use all the time, I don't design them. I understand conceptually how they work, but haven't ever worked out the equations for a particular design myself. So I can only point the way, I don't have simple plug and chug answers on this.
- You'll have to design your own using spring equations and torque equations vs load.
- When the balance force needed is low, the cam diameter should be low (less leverage for the spring). When its high, the cam diameter should be high (more leverage for the spring).
- And of course you have to compensate for the changing spring force as it lengthens and shortens.
- If I were setting this up myself, I'd:
-- Measure the force needed at a couple of specific angles from highest to lowest.
-- That will give you the needed torque from the system at various points.
-- Then figure out the spring constant for your tension spring.
-- From that calculate the needed cam diameter at each angle.
-- Draw out those points radially on graph paper to match your chose angles. Connect the with a smooth curve.
-- Go back and check the math at points in between. You'll probably need to refine your first cut as you adjust for actual diameter/string stretch on the cam.
-- Once you are happy with the conceptual idea, cut out a cam and try.

Note, I don't know that there aren't plug and play calculators for such things on the web, given everything else that you can find I wouldn't be surprised. But I've got a cold and will leave that to you computer savvy youngsters tonight.

Jeff Anderson
Livonia, MI

[OldSpice]

For first comment, usual statement about this being an opinion, not official clarification, blah, blah.

Height task is the last scorable thing you do, but you don't have to get all other points FIRST. It does require a scorable ping pong ball of course.

Strategy comments.
Since the first tie breaker is time, you may want to make sure you ALWAYS call time before the three minutes is up. Especially if you are either unlikely to get any more points in the remaining time, or it will be small at best. You may be part of the ties, but you are at least not at the lowest possible point for the first tie breaker.
Similarly, consider the height task. You may want to make it a priority to accomplish something. Again, if otherwise tied, this MAY make the difference since it is likely to keep you out of the tiebreakers.

While calling time before 3 minutes up in order to win ties is a good idea in theory, I disagree that you should do it every time. If you think you have enough time to move a scoreable object into a bin then most definitely go for it. The risk of negating the first tiebreaker is worth the chance of adding an extra 2-4 points to your score. Those points could potentially be the difference of 2 or 3 places.

As for the height task because the rules dictate that it must be performed last and most teams who run out of time probably won't be able to raise it more than a meter or so, it's worth it to go for those 3 points you get from PVC, pencils, or nails instead of getting 1.XX points from the height task. While you might not be avoiding a tie, you'll be getting more points and thus be higher in the rankings regardless.

Because of this I have come to the conclusion that working on the height task before you hit 94 points is a bad idea. (unless you have some crazy idea that will get you like 10 points, and once again I doubt many teams that didn't have the ability to get 94 points last year will have that)

[FawnOnyx]

Has anyone thought of using a sticky, gum-like adhesive as an end effector? I was thinking of a release method using a solenoid, but I'm not sure practically how well it would work as there could be several problems (too sticky, recoil after release).

[harryk]

I thought about, but I think there would be too many issues with it. Mostly with making sure it will always have enough "stick" and able to pick up the objects. Though this year with the allowance of removable passive end effectors, you may be able to have several tools with adhesive that you could grab and use to pick up the objects.

[chalker7]

Has anyone thought of using a sticky, gum-like adhesive as an end effector? I was thinking of a release method using a solenoid, but I'm not sure practically how well it would work as there could be several problems (too sticky, recoil after release).

If you do go with an adhesive of some type, I'd strongly suggest making sure it is either not strong enough to damage the Event Supervisors' setup or very easily removable. You don't want to risk a DQ for damaging things.

[FawnOnyx]

If you do go with an adhesive of some type, I'd strongly suggest making sure it is either not strong enough to damage the Event Supervisors' setup or very easily removable. You don't want to risk a DQ for damaging things.

Hmm yeah that's a good point. So the supervisor can still kick you out even though a rule against leaving residue or something on the items isn't explicitly mentioned in the event rules? I don't see anything in the general rules either that would forbid it. But all that aside I'm thinking something like this museum putty would satisfy both of the conditions you mentioned and be pretty safe.