pic: Off-Season Kicker Design

[rahilm]

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[Hawiian Cadder]

what motor are you using? our kicker this year was a CIM in a 144-1 bane-bots planetary that could generate 200ftlbs of torque and pulled back 275 LBS of surgical tubing. if using a cim, that gear reduction might not be necessary.

[rahilm]

Yeah, we're using 1 CIM. The reason for the huge reduction is in case we want to add more spring force if necessary. The other part of it is that this is also the backstop for our roller, so we can't afford to have the roller + ball drive the kicker back and have the ball encroach past 3".

[Ether]

When you kick, does it back-drive the motor?

~

[apalrd]

1. where is the sensor?

2. Are you releasing it with an AM dog clutch? I guarantee it takes too much force to release - more then a small piston can provide.

[rahilm]

Originally Posted by apalrd

1. where is the sensor?

We're planning on putting a potentiometer on the shaft for the kicker

2. Are you releasing it with an AM dog clutch? I guarantee it takes too much force to release - more then a small piston can provide.

Yes, we are. Shouldn't the ratchet-pawl make it easier to release? And also, we could replace the standard piston used on shifters with a larger piston of the same bore, couldn't we?

[R.C.]

Originally Posted by rahilm

Yes, we are. Shouldn't the ratchet-pawl make it easier to release? And also, we could replace the standard piston used on shifters with a larger piston of the same bore, couldn't we?

Yes but your talking a lot of force, I would personally move up to a 1.5" bore or 2" bore. If you have the weight and air why not use a 2" bore actuator?

Or you could ask someone how to do the math and get the correct size?

-RC

[nitbaj]

Originally Posted by R.C.

Yes but your talking a lot of force, I would personally move up to a 1.5" bore or 2" bore. If you have the weight and air why not use a 2" bore actuator?
-RC

Yeah, you really need a huge amount of force to release the dog gear. We had trouble this year with our dog gear, even with a 2" bore pneumatic. Actually, we had to significantly angle the faces of our dog gear to reduce the force required to fire it. There might be a better way to address the problem, like using a larger diameter dog gear. I also saw some teams this year use a motor with a lead screw/cam to push out the dog gear, which you could also try.

[apalrd]

We used an anti-backdrivable window motor, and attached it to parts of a toughbox (1 stage). We made an assembly that, using a 2" bore piston, separated the gears in the direction of force, so the piston held the two gears together when not kicking. This was after we were unable to release a dog gear, even after greasing it and pulling it by hand. I don't know if there are any good pictures of it, but if you're at IRI, you can come to our pit and ask us about it.

In a nutshell: We un-mesh the gearbox by attaching the motor side to a plate that moves to pull the two gears apart.

[Ether]

you really need a huge amount of force to release the dog gear

If you design the spring attachment points so that the spring is almost parallel to the kicking lever when in the armed position, then the holding torque will be very low, even with large kicking energy stored in the spring. Then it is easy to actuate the release mechanism.

I drew a sketch but this particular thread has attachments disabled. Think of an over-center latch.

The disadvantage to this approach is that you cannot vary the kicking energy in software.

~

[rahilm]

Originally Posted by apalrd

We used an anti-backdrivable window motor, and attached it to parts of a toughbox (1 stage). We made an assembly that, using a 2" bore piston, separated the gears in the direction of force, so the piston held the two gears together when not kicking. This was after we were unable to release a dog gear, even after greasing it and pulling it by hand. I don't know if there are any good pictures of it, but if you're at IRI, you can come to our pit and ask us about it.

In a nutshell: We un-mesh the gearbox by attaching the motor side to a plate that moves to pull the two gears apart.

I was thinking about this, and the way you guys did it inspired another thought for me:

Make a slot in the gearbox where that final gear is so that the gear can slide, and then have a pneumatic cylinder attached on either side of the shaft. When the pneumatic is at one end, the gear meshes. At the other end it disengages and you get a kick. That shouldn't take nearly as much force.

(The slot shouldn't be too hard to figure out. Just something following a line at a smaller angle than that of the line tangent to the pitch circle of both gears, right?)

[apalrd]

We had a sorta C-shaped bracket, with the shaft supported on both flanges and the motor on one, a pivot point on one end and a pneumatic piston on the other. It was a really big piston, 2" bore.

One other thing to keep in mind is the slight rotation of the output gear that is generated when you unmesh the gears. Since you are not pulling the gears straight out, you are pulling them on an angle, that action will rotate the output gear slightly. Make sure this rotation causes the kicker to go out rather than in. This way, the gears will be easier to unmesh and require a smaller piston (it is not fighting the springs to unmesh).

One final thing to keep in mind is the default state of the pneumatics. We used a double-action cylinder (air pushes both directions) but a single coil valve. We made sure that the default state (when no power is applied to the valve) left the gearbox meshed, so it kept the kicker in the box and avoid accidental firing. We left the pneumatics system full all the time to prevent the gearbox from firing (actually, we shipped the robot to Atlanta with 120psi of air in it's one air tank, and it still had 80psi in that one tank when we took it out of the crate)

[MCahoon]

[quote=rahilm;968133]We're planning on putting a potentiometer on the shaft for the kicker

I would recommend not using a potentiometer. We tried using a potentiometer for sensing the position of our surgical tubing powered kicker. It lasted for about 20 kicks before the acceleration of the shaft twisting the potentiometer wiper caused it to fail. We replaced the potentiometer with an encoder which has lasted two regionals (well into the eliminations of both), and 3 or 4 demonstration/exhibition showings.

[apalrd]

I would actually recommend a pot over an encoder because it's position is absolute instead of relative. Your programmers will thank you.

The key is to isolate the pot from the shaft. We did this with a short length of 25 chain and a plastic sprocket on the kicker shaft and pot.

[MCahoon]

[quote=apalrd;968294]I would actually recommend a pot over an encoder because it's position is absolute instead of relative. Your programmers will thank you.

I completely agree. That is the same reasoning that caused us to use the pot in the first place. The pot we used was driven from a shaft on the winch we used to pull back the kicker. Because it needed to make multiple revolutions, we used a 10-turn pot. If your kicker mechanism allowed use of a standard 270 degree pot, the mechanism might not have been subject to the same stresses ours saw (smaller amount of rotation during the very short time interval of the kick), or may be more robust. I wouldn't think there is enough give in the plastic chain/sprocket to reduce the angular acceleration appreciably.