Robot Arm C

[Ron_Swanson]

So this might be a tacky question but I was wondering if someone could address it:

Rule 6. c. ii. states that the run must end if the team says "stop." One the surface this appears to be a pretty benign rule but I could see this causing some headaches. For example, last year I supervised this event at regionals. One of the teams used a tandem control method where one teammate controlled the shoulder and elbow joints and the other teammate would control the gripper. When the one teammate would guide the arm over an object the other teammate would say "stop" to get him to stop moving the arm. Obviously it was not his intent to stop the run but if we go by the letter of the rule, they did say "stop" so the run should stop. Luckily, no one noticed or pointed this out, and to be honest I didn't even think of it until the run was over and the team had left, but if someone had filed a protest I'm not sure what I would have done. Does anyone have any insight here? I feel like that rule should be worded better, maybe including something about intent.

[jander14indoor]

OK, the following response REALLY needs to be sensitive to the not place for clarifications, statements are only opinion of the author, etc...

Unfortunately, we can't cover ALL possible cases in one or two pages of rules for the event. When I supervise SO events I try to keep in mind what the overall purpose is (encouraging lifelong enthusiasm about science and technology learning) and make judgements accordingly. As part of this, I scale how 'rigid' or 'hard ass' I am depending on the level of the competition.

Invitationals are meant to be learning opportunities, run them fair as awards are given, but recognize there will be many mistakes due to true ignorance. Try to help the students understand their mistakes and learn/improve for the tournaments they could advance from. Note, I don't help the team perform at the event or violate basic rules, but I try to give a little slack on things like time, or setup. And after a team has performed, I offer suggestions on how to improve, or discuss violations in depth. If time allows immediately after, if not, I offer to discuss with everyone after the event. I typically run the tech events so this is an especially good time to offer a quickie clinic to all teams attending.

Regionals I'm more formal, but still try to recognize that for many teams this is their ONLY time to perform. I'd probably react to your example as you did and not worry about it. Next time MAYBE make a note of the first stop time and what had scored by then, but not count it unless it became clear the students really meant the stop for the judges. As for protest I'd make my case and let the chips fall where they may. If you have a note of time of the first stop and score you can at least make a correction. Note, at most tournaments protests are not allowed of how other teams are scored, only how you are. Another action might be to add this to your list of things you counsel the students on when you start the event. I'm not really into 'gotchas'.

States and Nationals, things become more rigid, trying to follow the rules exactly. But even there I've made a practice to review scoring with the students at the end of their run. I've seen a few cases where they catch my mistakes, and to many more where the student thought they'd done well but didn't and worse, DIDN'T KNOW WHY. A true tragedy in my mind.

At all levels I try to make it clear how I'm going to run things. I spend at least a few minutes with the students beforehand telling what I expect them to do, and what I'll be doing. This would be a good place to review a rule like this.

Just one person's thoughts,

Jeff Anderson
Livonia, MI

[chalker7]

So this might be a tacky question but I was wondering if someone could address it:

Rule 6. c. ii. states that the run must end if the team says "stop." One the surface this appears to be a pretty benign rule but I could see this causing some headaches. For example, last year I supervised this event at regionals. One of the teams used a tandem control method where one teammate controlled the shoulder and elbow joints and the other teammate would control the gripper. When the one teammate would guide the arm over an object the other teammate would say "stop" to get him to stop moving the arm. Obviously it was not his intent to stop the run but if we go by the letter of the rule, they did say "stop" so the run should stop. Luckily, no one noticed or pointed this out, and to be honest I didn't even think of it until the run was over and the team had left, but if someone had filed a protest I'm not sure what I would have done. Does anyone have any insight here? I feel like that rule should be worded better, maybe including something about intent.

Please submit an official clarification if you want this answered for how it will be run at nationals. Also, please understand that if we sat down and attempted to write rules for every conceivable scenario, the manual would grow to be hundreds if not thousands of pages long. If I were coaching the team in the above scenario I'd instruct them to use a different word (and practice with the other word) to err on the side of caution.

[mrsteven]

Hello all,
I'm starting to test my robot arm and finding that my main 'shoulder' servo is bouncing around alot while it moves. I have it completely counter weighted (I've also tried 70% counter weighted) with a 1 kg mass on the end secured. The bouncing is only a major issue when I'm moving fairly quickly. Has anyone found a way to fix this?
The difference between the bouncing around from .7 kg to 1 kg isn't much if any, so I'm not sure the issue

[iwonder]

The issue is really your servo control, I'd start by just slowing down, it'll cost you time, but it's the easiest solution. Other than that, you could increase the ramp up/down times of the servo during the move, or just get a bigger servo.

[harryk]

Is the bouncing being caused by the inertia of the arm+counterweight? If so you you could try what iwonder said and move slowly or use a more powerful servo, or you cold try using a motion dampener or static brake

[illusionist]

How do you calculate how much mass is required for the counterweight? I don't think this question has been brought up before... Should I weigh the arm and add that to the other end?

[mrsteven]

How do you calculate how much mass is required for the counterweight? I don't think this question has been brought up before... Should I weigh the arm and add that to the other end?

I used some AP Physics stuff we learned on center of masses and moments of interia with calculus. What it boils down to mostly is that you want your pivot point to be the center of mass of the system, so your counter weight needs to allow for that. The equation (XM)/Mt = Cm where X is the position from the pivot, M is mass of that part and Mt is mass total. So you would set...
(X1M1)/Mt = (x2M2)/Mt
So the mass total of the system drops out.
X1M1=x2M2
So the center of mass will be in the pivot if the position of center of mass of your arm section times the mass of the arm is equal to the mass of the counter weight times the position of it
This is where things get a little hairy. This is treating each weight as a point mass, which works remarkably well for the arm since you know the implicit masses and such involved. But your counter weight is not including the implicit weight of the bar or w/e used to hold the counter weight X2 far. Thus id tell you to talk to a calc teacher about teaching you to integrate to find the true position to use for your counter weight including the bar used to extend it, but youll have to know the density of the material your using, which is simple.

Or just test and add/subtract weight as needed i prefer the math but im a little weird

[illusionist]

How do you calculate how much mass is required for the counterweight? I don't think this question has been brought up before... Should I weigh the arm and add that to the other end?

I used some AP Physics stuff we learned on center of masses and moments of interia with calculus. What it boils down to mostly is that you want your pivot point to be the center of mass of the system, so your counter weight needs to allow for that. The equation (XM)/Mt = Cm where X is the position from the pivot, M is mass of that part and Mt is mass total. So you would set...
(X1M1)/Mt = (x2M2)/Mt
So the mass total of the system drops out.
X1M1=x2M2
So the center of mass will be in the pivot if the position of center of mass of your arm section times the mass of the arm is equal to the mass of the counter weight times the position of it
This is where things get a little hairy. This is treating each weight as a point mass, which works remarkably well for the arm since you know the implicit masses and such involved. But your counter weight is not including the implicit weight of the bar or w/e used to hold the counter weight X2 far. Thus id tell you to talk to a calc teacher about teaching you to integrate to find the true position to use for your counter weight including the bar used to extend it, but youll have to know the density of the material your using, which is simple.

Or just test and add/subtract weight as needed i prefer the math but im a little weird

I'm currently in calc, so I understand integration and such (unless it's something specific thing I haven't learned yet). My counterweight is held ~20cm away from the pivot point (shoulder) of the arm using light material. If I weigh the whole arm, and treat the arm as a lever with the fulcrum at the shoulder and use Distance1*Mass1 = Distance2*Mass2, that should also get me pretty close, right?

[mrsteven]

Decently close. Besides, you dont want it to be entirely counter weighted. Make the servo do SOME work.
Its more of an AP physics topic than a calc one but we learned in both.

But if you were to inclined to make it more exact it requires a more detailed explanation than i think anyone really wants on here... You could probably google 'center of mass integration' learn, and apply.