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A few details:
-Overall 326:1 gear ratio
-Ratchet pawl system to prevent backdrive
-~200lbs of spring force
-Load and kick time is a combined ~1s
Discussion
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03-07-2010 15:03
Hawiian Cadder
Re: pic: Off-Season Kicker Design
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.
03-07-2010 17:51
rahilm
Re: pic: Off-Season Kicker Design
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".
03-07-2010 19:09
Ether
Re: pic: Off-Season Kicker Design
When you kick, does it back-drive the motor?
~
03-07-2010 19:12
apalrd
Re: pic: Off-Season Kicker Design
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.
03-07-2010 19:16
rahilm
Re: pic: Off-Season Kicker Design
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Originally Posted by apalrd
1. where is the sensor?
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| 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. |
03-07-2010 19:34
R.C.
Re: pic: Off-Season Kicker Design
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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?
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Or you could ask someone how to do the math and get the correct size?
-RC
03-07-2010 20:01
nitbaj
Re: pic: Off-Season Kicker Design
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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 |
03-07-2010 20:29
apalrd
Re: pic: Off-Season Kicker Design
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.
03-07-2010 20:46
Ether
Re: pic: Off-Season Kicker Design
| you really need a huge amount of force to release the dog gear |
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.
~
03-07-2010 22:33
rahilm
Re: pic: Off-Season Kicker Design
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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. |
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?)
03-07-2010 23:24
apalrd
Re: pic: Off-Season Kicker Design
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)
06-07-2010 13:01
MCahoon
Re: pic: Off-Season Kicker Design
[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.
06-07-2010 13:19
apalrd
Re: pic: Off-Season Kicker Design
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.
07-07-2010 02:39
MCahoon
Re: pic: Off-Season Kicker Design
[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.
07-07-2010 08:08
Ether
Re: pic: Off-Season Kicker Design
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Originally Posted by apalrd
I would actually recommend a pot over an encoder because it's position is absolute instead of relative.
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http://www.austriamicrosystems.com/e...5030-Downloads
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Originally Posted by apalrd
The key is to isolate the pot from the shaft.
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~
07-07-2010 09:10
apalrd
Re: pic: Off-Season Kicker Design
The key to not damaging the pot.
We anticipated that the stresses on the shaft would break a potentiometer if it was connected directly to the shaft. We isolated it with a chain so that the rotary motion was transferred, but any other forces were not. We used a standard 270-degree pot.
Although a magnetic encoder would remove any mechanical connection between the sensor and the shaft, so that might be a better solution then a potentiometer.
07-07-2010 09:50
Ether
Re: pic: Off-Season Kicker Design
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Originally Posted by apalrd
The key to not damaging the pot.
We anticipated that the stresses on the shaft would break a potentiometer if it was connected directly to the shaft. We isolated it with a chain so that the rotary motion was transferred, but any other forces were not. |
If you use the pot to replace one of the shaft bearings then of course that's going to put unacceptable loads on the pot. But if you mount the sensor directly to the shaft (through a simple coupling) on the outboard side of one of the end bearings it will not bear the shaft loads.
~
07-07-2010 12:17
MCahoon
Re: pic: Off-Season Kicker Design
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Originally Posted by Ether
Isolating the pot from the shaft (via a chain or belt etc) is certainly a solution, but not the only way.
If you use the pot to replace one of the shaft bearings then of course that's going to put unacceptable loads on the pot. But if you mount the sensor directly to the shaft (through a simple coupling) on the outboard side of one of the end bearings it will not bear the shaft loads. ~ |
07-07-2010 12:48
Ether
Re: pic: Off-Season Kicker Design
We had a gear on the end of the winch shaft. That gear drove an idler shaft that had the pot attached to the end of it.
Sure, that's a good solution. But the point was it's also possible to mount the pot (or encoder) directly to the shaft through a simple in-line coupler on the end of the shaft. With that approach there would be no undue loads on the pot. It's not the direct connection to the shaft per se that causes the loading problem, it's if you try to use the pot in place of a load bearing.
~
08-07-2010 00:42
EricH
Re: pic: Off-Season Kicker Design
In-line connection, 330 style: make the shaft extra-long by an inch or so, and turn that inch or so down to about the same diameter as the pot shaft. Put the ends of the shafts together in line, then put surgical tubing around the small portion and ziptie it down solidly. That way, even if something goes really wrong, the pot (or encoder) isn't going to be too banged up (it has its own mounting point off the axle).
08-07-2010 08:03
Jared Russell
Re: pic: Off-Season Kicker Design
Our team used a US Digital MA3 absolute magnetic encoder directly coupled to a small bore in the end of our kicker shaft and it held up great. We used the sleeve bushing model, rated up to 10,000 rad/secē acceleration and 100 RPM continuous top speed, but you can get the ball bearing model (250,000 rad/secē; 15,000 RPM) if you want even more of a safety margin. No detents or hard stops, no gap in sensing, and no wiper wear over time - I would heartily recommend using them in this application (or wherever a pot would be appropriate).
Link: http://www.usdigital.com/products/en...ary/shaft/ma3/
08-07-2010 19:23
Garret
Re: pic: Off-Season Kicker Design
| In-line connection, 330 style: make the shaft extra-long by an inch or so, and turn that inch or so down to about the same diameter as the pot shaft. Put the ends of the shafts together in line, then put surgical tubing around the small portion and ziptie it down solidly. That way, even if something goes really wrong, the pot (or encoder) isn't going to be too banged up (it has its own mounting point off the axle). |
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