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  #31   Spotlight this post!  
Unread 27-09-2003, 16:35
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Quote:
Guys, this debate is basically just about terminology. Everyone has their own.
Omni-directional steering's official name is holonomic steering.
Swerve drive steering's official name is synchro-drive.
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Unread 27-09-2003, 21:15
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Quote:
Originally posted by patrickrd
We used to use wheels made by this company on our RoboCup robots in 2001-2002. They also had the hex bore. This is how we (or my past teammates) did it: You can purchase hex stock from McMaster (among other places), and since it is an .874" bore (which seems huge to me), you can drill out something like a 3/8" hole centered in the hex stock. Then, you can connect a standard circular shaft, and use a keyway or pin or dutch pin to secure the hex shaft to the circular shaft.

- Patrick
I understand the idea, but what kind of machinery would one need to insure that the hole would be dead-center? Would a standard drill press be OK for this?

I think we will buy .874" hex stock, and use it as a hub for the wheels, . . as in it just fits onto a standard smaller circular shaft, and is secured using cotter pins or something of that nature.

BTW: that airport picture is awesome! Did you try to ride on it (I know I would have)?
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Unread 28-09-2003, 08:02
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You might need to order .875 hex stock, which is slightly oversized, but will make a good press fit into the wheel. As for centering the hole, the best option is actually to put it in a lathe and drill out the hole that way. The second best option is to mount the piece vertically and use a milling machine. Then use a zeroing tool to locate the center of the hex stock. Then you can drill it out quite easily.

But it sounds like you don't have either... So my recommendation is to mark very carefully the center of the hex and use a centerpunch to ensure the drill bit will not drift. Then mount the hex in a drilling press, and drill through. You'll probably need to start with a small drill (1/8)" and then work up to the final size (1/4" and then 3/8"). If you have a reamer, it would be a good idea to drill just under 3/8" and then use the 3/8" reamer to do the final cut. The key to making this work is mounting the hex perfectly horizontal. Use a right angle of some sort to do this.

You'll probably get some no-good pieces using this method. But some will be good. If the hole is centered on both sides of the hex, it should be good. If it is only off by 0.005" or less, then I do not think a slight wobble will affect performance too much.

- Patrick
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Unread 28-09-2003, 14:16
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tough to decide what's what

we never got caught up in definitions when we built our drivetrain............

since there is so much talk about omni-drives, i thought i'd let you know that we're still waiting to use our next generation Kiwi Drive............stay tuned, its been two years in the works.

for now, check out our old video from the pre-season of 2002, this was a video we unveiled before competing with our reworked chassis. it was more fun to drive than to watch.

http://stuweb.ee.mtu.edu/~alkrajew/FIRST/kiwi.mpg
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Unread 28-09-2003, 19:07
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Unread 29-09-2003, 10:45
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Quote:
Originally posted by patrickrd
Actually, upon further consideration I will make a deal to any teams out there interesting in constructing an omni-drive. Here's the deal (of course, let me know someone if this breaks a FIRST rule, I will not do this if it does):
  • I will provide your team with quantity six of a 5" version of the omni-wheel above... I've started the design for this, and I plan on making about 12 of them at the end of the semester. I will make CAD and engineering drawings available to ALL TEAMS asap so any team with sufficient machining capability can make their own.
  • I will ask your team to pay for the material cost of these wheels. The cost should be somewhere between $50 and $100 for the set of six.
  • The catch: I will only do this for at most TWO teams -- those who demonstrate feasiblility of implementing an omni-drive for their 2004 robot. (through a written report -- I'm leaving this open ended. The most convincing cases get the wheels.)

That's the deal. The deadline's is Dec. 15. I will send them out at latest the first week of competition (you will have all the dimensions well before to do designs).

- Patrick
Patrick -

Depending on the exact schedule you plan for construction of the wheels, as I interpret the rules, this may not be allowed. Your message is not clear about when you will be constructing the wheels - right after your December 15 deadline, or during the first week of the competition.

Under the current FIRST rules (emphasized to point out that we don't yet know what will be in the final version of the rules for 2004), it would be illegal if you start manufacturing the wheels prior to the kick-off, and any team using your wheels would be in violation of the rules. All materials and components on the robot must be manufactured ONLY during the competition period (other than those bought as straight off-the-shelf commercial commodity items, e.g. you can purchase and use a Banner sensor that was actually manufactured by the factory earlier). Wheels that you manufacture prior to the start of the competition cannot legally be used on your robot, or by any other team. You can share your designs all you want to. But you cannot, and other teams cannot, start to actually build the wheels prior to kick-off.

-dave
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Unread 29-09-2003, 13:39
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Hi Dave,

Thanks for the reply. It turns out my design couldn't be used anyway, since O-Rings are not allowed :-/ Maybe I'll be able to come up with an alternative.

thanks
Patrick
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  #38   Spotlight this post!  
Unread 29-09-2003, 14:48
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Why wouldn't o-rings be allowed? They are listed in small parts?

Our rollers were initially delrin with rubber o-rings.
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  #39   Spotlight this post!  
Unread 29-09-2003, 16:32
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<<<ajlapp wrote...>>>
for now, check out our old video from the pre-season of 2002, this was a video we unveiled before competing with our reworked chassis. it was more fun to drive than to watch.

http://stuweb.ee.mtu.edu/~alkrajew/FIRST/kiwi.mpg
<<<>>>


I disagree!!!

Very cool to watch.

I have toyed with the idea and always stop when I look at power loses. But in terms of being able to manuver it would be pretty cool

Thanks for reminding us about it
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Unread 29-09-2003, 17:25
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no problem

power loss sucks, but we designed around the power loss.........

the robot we used that year had no power what so ever, because we designed strictly to pick up balls. last year we didn't use it, because of power loss, instead we built a tried and true swerve.

don't give up yet, there are ways to use the kiwi and keep power on the turf, trust me
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  #41   Spotlight this post!  
Unread 29-09-2003, 19:51
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Re: no problem

Quote:
Originally posted by ajlapp
power loss sucks, but we designed around the power loss.........

the robot we used that year had no power what so ever, because we designed strictly to pick up balls. last year we didn't use it, because of power loss, instead we built a tried and true swerve.

don't give up yet, there are ways to use the kiwi and keep power on the turf, trust me
like polyurethane rollers in a four-wheel configuration! nearly as efficeint as tank drive!

You see, with three wheel killough(kiwi), if you spin all three wheels you get a robot that is turning . . and you won't get all three motors adding forward power. With four wheel, you can have all four motors putting all their power into forward push. Whether or not you are powerful at that point depends on your traction, and polyurethane is a great sticky thing, and your gearratios, which are entirely up to the team.

In motion, some energy is lost to the omniwheels inefficiency, but if you buy 'real' omni wheels, as linked above by myself, you get nearly the same efficiency as say, a tank drive going forwards.


So you see. You get to make the same choice between power and speed you do when designing a tank drive, but you get super maneuverability thrown it! No extra charge! Well, a little for the wheels, but not too much!

Last edited by Frank(Aflak) : 29-09-2003 at 20:06.
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Unread 29-09-2003, 23:12
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Re: Re: no problem

Quote:
Originally posted by Frank(Aflak)
You see, with three wheel killough(kiwi), if you spin all three wheels you get a robot that is turning . . and you won't get all three motors adding forward power. With four wheel, you can have all four motors putting all their power into forward push. Whether or not you are powerful at that point depends on your traction, and polyurethane is a great sticky thing, and your gearratios, which are entirely up to the team.

So you see. You get to make the same choice between power and speed you do when designing a tank drive, but you get super maneuverability thrown it! No extra charge! Well, a little for the wheels, but not too much!
I direct you to my earlier post here. Omniwheels allow lateral motion but prohibit perpendicular motion (to the drive direction). Therefore, only a limited amount of torque would be exerted. It's simple trigonometry. Only anywhere from 50% to 70% of the total torque of all 4 motors can be exerted in any one direction. If you look at the numbers, with a killough platform you get 57% of the total torque (all 3 motors). Of course, with the four wheel variant you have more motors contributing to torque, but it's not more efficient by any significant margins.
Quote:
Originally posted by Frank(Aflak)
In motion, some energy is lost to the omniwheels inefficiency, but if you buy 'real' omni wheels, as linked above by myself, you get nearly the same efficiency as say, a tank drive going forwards.
I don't see where you're coming from. How would you lose efficiency? You might experience some lateral friction since the rollers don't have individual bearings, but I don't see how you would experience any significant power loss. Other than that, omniwheels are omniwheels, as long as they permit lateral motion. I don't see what makes those omniwheels 'real'.


As for which, crab drive or an omniwheel design, is better, I don't know. It depends on your design strategy. If your strategy (or the game) requires torque over mobility, then a tank/crab drive is better, because all 4 motors contribute 100% of the available torque to the direction of motion. If mobility is considered to be advantageous over torque, then an omni-wheel design might be better. However, an omni-wheel design does present a programming challenge, or opportunity, depending how you look at it. One idea I had for last year was a saucer-shaped wedge robot and use a digital compass and programming to create relativistic motion (from the drivers POV). However, even though local magnetic interference can be compensated for (your own motors), other robots would be a challenge. You could do the same thing with an accelerometer, but the error would add up, and toward the end the robot may not go in the desired direction.
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Last edited by Jnadke : 29-09-2003 at 23:41.
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Unread 30-09-2003, 00:36
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see if anyone remembers our solutions

Quote:
However, an omni-wheel design does present a programming challenge, or opportunity, depending how you look at it.
see if anyone can remember our solution way back in 2002.......

no programming needed, no calibration tricky of potentiometers
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Unread 30-09-2003, 00:51
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Patrick,

O-rings are perfectly fine, provided that they are accounted for per rule K3.

The rule that I would be worried about is rule m17 (traction devices).

As far as off-the-shelf components go, the term off-the-shelf isn't used in k3 but is used in the parts use flowchart. It is also used in m1, but only in reference to previous years components. Given that FIRST always said refer to the flow chart if there are any questions, I would say that your wheels are not allowed if they are manufactured before kickoff, but I wish that FIRST would use consistant terminology throughout the rules.

Edit: ignore the rules hyperlinks and just look them up in your manual or on hyperrules

Last edited by Joe Ross : 30-09-2003 at 01:47.
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Unread 30-09-2003, 10:20
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By 'real' i meant professionally made with either low friction non-bearing rollers or bearings for the rollers. Also, professionally produced omniwheels are more likely to withstand a beating.

And, i believe it is possible to get 100% motor torque using four wheels.

look at it this way,

._
{_}
.
where the '{' and '_' are the wheels.

say you power the {} wheels in the same direction. you get full power/torque out of them.

Say you power the '_' wheels in the same dircetion, same deal.

Now lets say we geared the motors so that when only one set of wheels are powered, you get 6 feet/second and whatever push that translates to. If you power BOTH sets you get the robot traveling in a diagonal, at the new speed 6rt(2) feet per second, and a new power that is sgrt(2) times the power of an individual set of wheels. so, you get 2x the power of one set of wheels, and one set of wheels gives you the same power from he motors that a tank drive would, so with twices the motors ou get twice the power of a four-motor tank drive, which gives you 100% power except for gearing and wheel inefficiencies. I don't see where your 70% is coming from . . perhaps I can get a link to an explanation . . . ?

at the 45' diagonals you get 100% power out of the motors. The only way it isn't as powerful as a tankdrive at same geared to he same speed is that you are loosing power to make the rollers turn, which is a near-negligable loss.

I don't understand how it could be different . . I accept that you only get full power at the 45' diagonals, but that is still another axis of freedom on which full power is available over a tank-drive.

edit: your idea about using a compass/angular acceleration sensor: We have been planning that since last season. It will be in place using one or two solid-state gyros. Error will add up, but we may build a control that allows for the secondary driver to manually adjust the robot's percieved heading so the error can be corrected as the match progresses.

edit: I looked over your post a second time, and it is true that at the diagonals you get sgrt(2) times the power of each individual set of wheels, but I think you failed to consider that you ALSO get sqrt(2) times the speed, so twice the power. You gotta look at both. And you need a driver who is aware of all of this.

Last edited by Frank(Aflak) : 30-09-2003 at 10:25.
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