Centrifugal Arm Claw

[farmersvilleRob]

Our claw needs to be super lightweight and hold in 78 kg of force (multiply by 9.8 m/s^2 and you get the Newton force). But we were thinking of using 3, 553 oz/in servos for three claw pieces made of welded bike chain pivoting on small ball bearing mounts. Does that sound reasonable? I'll have CAD ready by tonight but for now, does that sound reasonable? The claw pieces will probably be the arc of the basketball, but only 4.5 or 5 inches in horizontal length.

[the man]

I am having a very difficult time picturing what you are describing. If you put up some cad drawings it will be much easier to see.

[farmersvilleRob]

Originally Posted by the man

I am having a very difficult time picturing what you are describing. If you put up some cad drawings it will be much easier to see.

Here is our arm in CAD. Too bad CAD is unsupported, but I took plenty of pictures and I have one more to attach as a drawing of the claw itself

[farmersvilleRob]

Here you go

[artdutra04]

I wouldn't use servos for this application. If you really want to make a motorized pincher claw, using a Window motor and a potentiometer for angle feedback would be much more preferable. Or if there are only two states needed (such as retracted in the robot OR fully extended), a pneumatic cylinder would be even easier to use if you already committed to having pneumatics on your robot.

As for the claw, is there any compelling reason why you are looking to weld dozens of links of roller chain together? It would be much easier, faster, and significantly lighter just to cut a claw arm out of 1/4" polycarbonate (Lexan) sheet.

[MCahoon]

Same point Art pointed out - bike chain is probably made from steel. Three pieces of chain the length you describe would probably weigh more than the ball itself.

I would also suggest using polycarbonate (lexan) cut into the same shape as a lighter alternative.

For the servos to hold the ball as the arm whips around to throw, then release the ball, the centripetal force they would have to hold against just for the ball would be in the neighborhood of 35 N (weight of an object of about 4 kg - about 8.8 lb) to throw the ball at 20 foot per second. If you are - the force of the holding fingers being whipped around in the arc would add to that.

My gut feel is there isn't enough holding force of the 3 servos to hold the fingers and the ball. At 5 inch length, they are pressing inwards at about 300 oz for all 3 - about 19 lb. but only a small component of that inwards squeeze is actually holding the ball up - the rest is just squeezing the ball and holding center of mass of the fingers from flying straight out from the bearings.

If you covered the fingers with a really grippy material such as urethane rubber, then the squeezing would provide a normal force such that friction would keep the ball from flying out.

Hard to tell much from an analysis because of the complex shapes and the curved fingers - it might work - my gut feel says it probably won't. Suggest you prototype it.

[farmersvilleRob]

Originally Posted by MCahoon

Same point Art pointed out - bike chain is probably made from steel. Three pieces of chain the length you describe would probably weigh more than the ball itself.

I would also suggest using polycarbonate (lexan) cut into the same shape as a lighter alternative.

For the servos to hold the ball as the arm whips around to throw, then release the ball, the centripetal force they would have to hold against just for the ball would be in the neighborhood of 35 N (weight of an object of about 4 kg - about 8.8 lb) to throw the ball at 20 foot per second. If you are - the force of the holding fingers being whipped around in the arc would add to that.

My gut feel is there isn't enough holding force of the 3 servos to hold the fingers and the ball. At 5 inch length, they are pressing inwards at about 300 oz for all 3 - about 19 lb. but only a small component of that inwards squeeze is actually holding the ball up - the rest is just squeezing the ball and holding center of mass of the fingers from flying straight out from the bearings.

If you covered the fingers with a really grippy material such as urethane rubber, then the squeezing would provide a the normal force such that friction would keep the ball from flying out.

Hard to tell much from an analysis because of the complex shapes and the curved fingers - it might work - my gut feel says it probably won't. Suggest you prototype it.

We don't have the funds to prototype and the pneumatic or window motor or regular motor is too heavy. If I use the polycarbonate it might only be slightly lighter, but I think it'll work better than the chain. Anybody know how much urethane friction would help against flying out?

[artdutra04]

Originally Posted by farmersvilleRob

We don't have the funds to prototype and the pneumatic or window motor or regular motor is too heavy. If I use the polycarbonate it might only be slightly lighter, but I think it'll work better than the chain. Anybody know how much urethane friction would help against flying out?

I just ran the numbers* and the 553 oz-in servo you want to use is likely illegal due to rule R48-L (depending on what its maximum rotational speed is, as it will likely have a peak power output over the 4W at 6V limit).

* Assuming max torque is 553 oz-in and the max rotational speed is .21 sec/60 degree (which seems to be the average speed I found when looking up high torque servos in the 400-600 oz-in range at 6V), this results in a peak power output of 4.86 Watts.

[the man]

It is defiantly a very unique idea.

[farmersvilleRob]

Originally Posted by the man

It is defiantly a very unique idea.

Well we're solving it by using a 1/4 cup to go un the underneath of the ball then using the servos (new ones cuz we figured out we cant use a servo the is rated right at 6V and just use the 12V option ) and 1 cm urethane fingers to hold it in tightly and hopefully if we ever get our checks, we'll have a prototype up next week? lol

[MrBasse]

What if you ran the motor remotely? Use a cable like a bike brake system and have the motor off the arm. That way you can transfer force to the claw system and have it spring loaded to open or close while the motor and cable provide the opposite action. We used this for 2011 to grip tubes and it was incredibly flexible and allowed us to pinch tubes as tight as we could have wanted. Plus parts are cheap, you can get cable and clamps at Lowes, Home Depot, or any local bike shop.

[EricH]

There is one thing I'd be worried about, if my interpretation of this setup is correct (which it may not be).

Am I right in assuming that the claw(s) in question will be mounted on an arm, the entirety of which will be spinning at some speed sufficient to launch a ball if the claw holding said ball is opened, at some point during a match?

If I'm not right in that assumption, then you can ignore what I'm about to say. If I'm right, then your servos are going to be in a lot of trouble, whether they're servos or some other motor. You have them out at the end of a rapidly rotating arm, so they have to be mounted securely. However, I am confident that that is doable.

What concerns me, if the assumption is correct, is: I think you will need either a metric ton of extra wire, a slipring, or some other linkage to get the servo power out to the servo. You'll be winding up wire and possibly pulling and damaging connections, or having to reverse, otherwise. I would strongly suggest looking into sliprings for PWM (or whatever wire you end up using) to avoid this problem. Sliprings of appropriate gauge are legal per [R44] (this for those who were around when they weren't). Or an alternative linkage for claw release that doesn't involve the servo going around and around.

Again, this is only if I'm right about the intent of this arm design. If I'm wrong about that, then mea culpa for assuming something that wasn't clear.


The other thing I could say, but at this point I'm willing to bet that it could be done: I think the entire system is too complicated for any team, let alone a rookie team. However, that is for the team to decide, and they obviously have.

[farmersvilleRob]

Originally Posted by EricH

There is one thing I'd be worried about, if my interpretation of this setup is correct (which it may not be).

Am I right in assuming that the claw(s) in question will be mounted on an arm, the entirety of which will be spinning at some speed sufficient to launch a ball if the claw holding said ball is opened, at some point during a match?

If I'm not right in that assumption, then you can ignore what I'm about to say. If I'm right, then your servos are going to be in a lot of trouble, whether they're servos or some other motor. You have them out at the end of a rapidly rotating arm, so they have to be mounted securely. However, I am confident that that is doable.

What concerns me, if the assumption is correct, is: I think you will need either a metric ton of extra wire, a slipring, or some other linkage to get the servo power out to the servo. You'll be winding up wire and possibly pulling and damaging connections, or having to reverse, otherwise. I would strongly suggest looking into sliprings for PWM (or whatever wire you end up using) to avoid this problem. Sliprings of appropriate gauge are legal per [R44] (this for those who were around when they weren't). Or an alternative linkage for claw release that doesn't involve the servo going around and around.

Again, this is only if I'm right about the intent of this arm design. If I'm wrong about that, then mea culpa for assuming something that wasn't clear.


The other thing I could say, but at this point I'm willing to bet that it could be done: I think the entire system is too complicated for any team, let alone a rookie team. However, that is for the team to decide, and they obviously have.

Well we were thinking of leaving a lot of slack in the wire and either having the robot pre -wind backwards or after autonomous (our team is heavy programming) we would let it reverse while it it moving which isn't that big a problem. As for damaging wires, we were going to at least hook the servo cable extenders at the weak points just in case it somehow gets out of hand then the break happens where it is designed to break.

We looked through the rules and it says that the actuators can't be changed under x, x and x, but we might pose a question to the GDC because it says in the blue box about it being a rule so teams can't get more power, but ours is just for functionality.

[farmersvilleRob]

Originally Posted by MrBasse

What if you ran the motor remotely? Use a cable like a bike brake system and have the motor off the arm. That way you can transfer force to the claw system and have it spring loaded to open or close while the motor and cable provide the opposite action. We used this for 2011 to grip tubes and it was incredibly flexible and allowed us to pinch tubes as tight as we could have wanted. Plus parts are cheap, you can get cable and clamps at Lowes, Home Depot, or any local bike shop.

I'll bring this up next meeting! I like it, and eventually we'll have some prototypes to share at the end of week three, or even a full robot depending if we get money in time :/

[EricH]

Originally Posted by farmersvilleRob

Well we were thinking of leaving a lot of slack in the wire and either having the robot pre -wind backwards or after autonomous (our team is heavy programming) we would let it reverse while it it moving which isn't that big a problem. As for damaging wires, we were going to at least hook the servo cable extenders at the weak points just in case it somehow gets out of hand then the break happens where it is designed to break.

We looked through the rules and it says that the actuators can't be changed under x, x and x, but we might pose a question to the GDC because it says in the blue box about it being a rule so teams can't get more power, but ours is just for functionality.

So it's just a back and forth launch? If it isn't, see slipring. It's more durable than a lot of slack, and eliminates any need for slack beyond what's need to plug in.

What I meant by linkage change was something like a helicopter uses for control--I believe it's called a flybar or something like that. You can do whatever you need to to the output of any non-integral gearbox. (See [R49-A])