Centrifugal Arm Claw

[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.

[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])

[couvillion]

Your post so far suggest that the timing of the release would be computer controlled. That seems really hard, and difficult to debug unless you have a high speed camera around. I would suggest that you make your release mechanical based on rotation of the arm. And the use a motor to adjust that release point forward or back. It is mechanically complex, but that can be understood without a high speed camera.

[farmersvilleRob]

Originally Posted by couvillion

Your post so far suggest that the timing of the release would be computer controlled. That seems really hard, and difficult to debug unless you have a high speed camera around. I would suggest that you make your release mechanical based on rotation of the arm. And the use a motor to adjust that release point forward or back. It is mechanically complex, but that can be understood without a high speed camera.

Well we havbelieve with an encoder (might need a better one than the FIRST choice ones) we can track the exact RPM the arm is moving at then have it correct tself before launch. It all is set to happen <2.5 sec, but we have an amazing programming team. We think the only hard part is finding how the motor corrects itself because we don't want an oscillating graph where RPM is oscillating on the y axis over the x axis of time because it's over correcting. So that's an experimental stage. We are actually hoping the cup design would be able to release the ball on a tangent to the rotation without touching the ball. If the cup is too deep, we have a shallow one and we mount the servo arms (1 cm long urethane fingertips) parallel to the x axis (perpendicular to the rotation) to also decrease chance of even bein involved in the ball's release other than the release of force.

[farmersvilleRob]

Originally Posted by EricH

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])

We we're sure if sliprings were in compliance with rules. Is there anything to back it up that they are? Because that was our number one choice but we needed a backup which was the slack in the wires. Also, could you explain the flybar? I found what it basically is, but why would we need it again?

[EricH]

Originally Posted by farmersvilleRob

We we're sure if sliprings were in compliance with rules. Is there anything to back it up that they are? Because that was our number one choice but we needed a backup which was the slack in the wires. Also, could you explain the flybar? I found what it basically is, but why would we need it again?

Is there anything to back it up? You betcha. [R44] includes the following:

The branch circuit may include intermediate elements such as COTS connectors, splices, COTS flexible/rolling/sliding contacts, and COTS slip rings, as long as the entire electrical pathway is via appropriately gauged conductors.

If you wanted to avoid a motor or servo on the rotating arm (and thereby the electrical hassle of extra wire or sliprings), you could run a setup similar to a helicopter's flybar. The flybar allows the rotor on a chopper to maintain a commanded orientation, regardless of where in the rotation the rotor is. It also translates changes up to the rotors. Something like that provides a mechanical solution to the problem (expense of weight). A slipring is probably the best option.

[farmersvilleRob]

Originally Posted by EricH

Is there anything to back it up? You betcha. [R44] includes the following:

If you wanted to avoid a motor or servo on the rotating arm (and thereby the electrical hassle of extra wire or sliprings), you could run a setup similar to a helicopter's flybar. The flybar allows the rotor on a chopper to maintain a commanded orientation, regardless of where in the rotation the rotor is. It also translates changes up to the rotors. Something like that provides a mechanical solution to the problem (expense of weight). A slipring is probably the best option.

Ah I see! So basically since We HAVE to mount the servos directly on, it would be easier to do a slipring? And we're actually mounting the encoder to the outside of the shaft so that wiring is independent then taking the shaft and in the middle of the arm we're creating a "keyed" (really just filed) section the then move with the arm. No extra anything needed it just let's the encoder sit on the outside. So then we only have 3 servos on the claw. Sorry about the grammar, but I am on my iPhone getting posts in before sleep lol

[couvillion]

I believe that swashplate would be another term for the flybar mechanism, with wiki blacked out my IQ has dropped.

In terms of timing with the release, it might be possible, but your programmer may not realize how much difficulty of real-time programming has been covered up by the WPIlib and cRIO gate arrays. I would suggest working on a backup plan at the same time.

[farmersvilleRob]

Originally Posted by couvillion

I believe that swashplate would be another term for the flybar mechanism, with wiki blacked out my IQ has dropped.

In terms of timing with the release, it might be possible, but your programmer may not realize how much difficulty of real-time programming has been covered up by the WPIlib and cRIO gate arrays. I would suggest working on a backup plan at the same time.

Our mentor that wi be teaching us his ee skills is fairly familiar with it all so he kinda said it's possible like your saying. The only thing is we probably will get a high speed camera if their under $2000 to time the releases. It's factoring out the release time with the angle of release (which will be a set ideal angle) that's the tricky part. And hopefully it'll get to a point where the robot can either stick with an angle or adjust using our algorithm to find it's own ideal angle based on shots to correct itself.

[ToddF]

Rob,
I'm going to make a friendly suggestion. From reading your previous posts, I know you are pretty much set on a catapult rather than a wheeled shooter. You have also said a couple times that you don't have the money for prototyping. But, if you have $2000 to buy a high speed camera, you have money to prototype.

My suggestion is that you spend a couple hours putting together a single wheeled shooter prototype. It will take relatively little time and money, and may provide you a viable backup plan in case your catapult doesn't work out.

You have nearly everything you would need to prototype a single wheeled shooter in the kit of parts. To power it, use a CIM motor directly driving one of the kit of parts wheels. You'll probably need to buy some stuff (1/2" keyed shafting, 1/8" keys, a motor to shaft coupler) from McMaster Carr. With overnight shipping, this stuff will probably come to less than $100.

Just so you know, our team also is hoping to use a catapult style shooter. But, we're working on it knowing that if there are unexpected difficulties, we have a workable fall back plan.