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Jnadke 25-09-2003 13:07

Quote:

Originally posted by WakeZero
I still think this is an unnecessary requirement in the definition of an omni-directional drive :rolleyes:
om·ni·di·rec·tion·al ( P ) Pronunciation Key (ahm-nee-die-rek-shu-nal)
adj : not directional; being in or involving all directions

Let's apply this to radio waves. Sure, with a rotator a directional antenna can signal in a certain direction well, and has the ability to change directions, but can it signal in multiple directions at once? No.

As the definition says, the crab/swerve/synchro drive is directional. It cannot move along other axises without first changing the direction of the wheels. True holonomic drives have this instantaneous ability to move in other axises than oriented. Some more reading. A suitable analogy would be if I cold move really fast, say 1/4 the speed of light. I could say I teleported myself. To the average person, they'd believe it, but the educated person knows better. This analogy is suitable because the fast and powerful motors we have allow us to turn the swerve wheels very fast. If we did not have such powerful motors, the limitations would become aparent.

Everyone is entitled to their own opinion, I'm just telling you the dictionary and accepted definition. Nobody said yes couldn't mean no.

Matt Reiland 25-09-2003 15:29

Quote:

Originally posted by patrickrd
Hi Frank,

The best options is to gear the motors such that the torque-speed curve lines are parallel for both motors. Then, for the motor with more power (ie the one further away on the torque-speed graph), you want to multiply the voltage applied by a constant less than one to bring the curve inward to the weaker motor. For example, the weaker motors would be used fully (PWM range -127 to 127) while the stronger motors would not be used fully (something like -108 to 108). This does waste power, but believe me, with four powered motors geared to an appropriate gear ratio and with high-traction wheels (see below), you will not have a power problem. Ensuring the same torque-speed curve using my method above will also help a LOT with control problems. There will still be some variance from motor to motor and over time and with varying heat conditions: If I were to do it, I would actually place encoders on each wheel, and use feedback control to ensure the robot is going in the commanded direction... But I don't know if FIRST rules allow purchasing of encoders. I know there are teams that make their own with the optical sensors. But to start with, ignore the encoders idea, and if things don't work well enough, you might consider it.

Now, about wheels, I have a lot of experience designing omniwheels, Here is a design I worked on last year (I've also attached a picture of final product):
Exploded view of wheel
Engineering Drawing
Engineering Drawing of Lexan "Hub"
Engineering Drawing of Roller (small rollers around the wheel)
CAD View of Robot w/ Wheels

The wheels have lexan hubs and delrin or aluminum rollers with rubber Buta-N o-rings wrapped around the outside of the rollers. The coefficient of friction between wheel and ground of this particular design is 1.3 for a felt-like material. I suspect it is at least 1.5 (probably closer to 2.0) on a FIRST carpet.

You can machine very similar wheels with manual mill & lathe, but you will need a rotary table to do the lexan pieces. It is better if you have access to CNC. You will also need to make larger wheels, probably about double the size of these wheels (which are 2" diameter). You can order O-Rings from McMaster or try and find them at a hardware store.

If you are really interested, you can actually read our mechanical design documentation at:
Cornell Robocup 2003 Mechanical Design Documentation

There may be some things of use in the documentation that I did not include here.

Good luck! And let me know if you have any questions.

- Patrick


We used a similar design to those (More like Wildstang 2000) and we couldn't get the o-rings to last longer than about 10 minutes once you start spinning the wheels on the carpet. We ended up machining the rollers and putting a knurl on them which lasted a lot longer

Frank(Aflak) 25-09-2003 17:19

Quote:

Originally posted by Tytus Gerrish


A Crab drive seems Great because you can point all your power in any direction, But omni wheels only a fraction at the most half in any given direction

not quite.

say you have the wheels like so:

. . _
. .[_]

you only get .5 of possible max power if you are moving directly verticle or horizontal. however, you go diagonally (45 deg. from horizontal) you will get full power.

To PatrickRd: Thanks for the offer of help with the omniwheels, however we don't have access to a machine shop and I think the best way to go is to buy our wheels. I found this source (kornylak) that looks perfect, about 5 inch diameter, polyurethane rollers (wooo!), made of aluminum. Only downside is the wheels we would need would total to 516 USD. We can probably do that . . . . if we raise 6large in time to apply for the NASA grant again.

pics:


The wheels pictured have 3 rollers, you need two wheels back-back to provide for 360 degrees of rotation.

Here are schematics:


linkies:
omniwheel


I think that this is the best way to go for long-term durability and for overall funcionality (any rollers we make probably will not be as efficient as these.

downside, is again, 64 dollers per wheel. But we will be able to swing it. I asked Kornylak is they would like to sponsor us, so maybe we can swing it for cheaper. Who knows. These are what I'm drawing around as of now.


edit: These wheels are also ideal because they are nearly circular, so there will be smooth weight transfer from one roller to another and it will make for a smoother ride than, say, the lexan/disc wheel you pictured above. It is also probably lots easier on the wheel and the gearbox.


I'm reading that documentation, its pretty cool. Especially your nod to strongbad: "Eating one battery . . . . Eating Five Batteris"

It was so serious up until then, too. I guess you college students just didn't have it in you to stay that way, huh?

WakeZero 25-09-2003 20:18

Quote:

Originally posted by Jnadke
om·ni·di·rec·tion·al ( P ) Pronunciation Key (ahm-nee-die-rek-shu-nal)
adj : not directional; being in or involving all directions

Meriam-Webster
Main Entry: om·ni·di·rec·tion·al
Pronunciation: "äm-ni-d&-'rek-shn&l, -"nI-, -(")dI-, -sh&-n&l
Function: adjective
Date: 1927
: being in or involving all directions

Same as your definition above.

So, if you put boxes on both a killough and a swerve drive, both boxes could move in ALL directions without changing the orientation of the box. Since this fits the definitions above, both killough and swerve are omnidirectional.

Yes, you can be more technical and argue that swerve can't do them all at once... but like I said before, what is the point? You can't physically be going in two directions at the same time, so why make that a criteria? :yikes:

Again, these are my thoughts. I don't however want this turning into a flame war; we should all have the ability to reason and debate without getting angry or start calling each other names.

Oh, one more thing.

Quote:

Originally posted by Jnadke
Nobody said yes couldn't mean no.
George Bool kind of did. The above statement is what I blame for the continued digression of moral law in the world. It's the kind of thinking that says, "In his mind, there is nothing wrong with killing someone; therefore, it's ok and we should be more understanding" :ahh:

Joe Ross 25-09-2003 22:39

None of the proposed drive systems are omnidirectional because they don't involve the z-axis.

:p

Cory 25-09-2003 23:51

what would the z axis even be? Logic would say it would involve up and down motion, but common sense would tell you that would never be useful in a drivetrain.

Cory

Solace 26-09-2003 00:09

the three axis as far as FIRST is concerned are x, y, and theta (the angle)

Jnadke 26-09-2003 12:03

Quote:

Originally posted by Joe Ross
None of the proposed drive systems are omnidirectional because they don't involve the z-axis.

:p

Due to the curvature of the earth, all robots involve the z-axis.

:D :p :cool:

indieFan 26-09-2003 12:14

Thanks John,

That's exactly the information I was looking for.

indieFan

patrickrd 26-09-2003 16:52

Quote:

Originally posted by Frank(Aflak)
I'm reading that documentation, its pretty cool. Especially your nod to strongbad: "Eating one battery . . . . Eating Five Batteris"

It was so serious up until then, too. I guess you college students just didn't have it in you to stay that way, huh?

LOL, I'm glad somebody commented on that... I don't think our advisor ever saw that, because he never commented on the documentation at all. We had been working about 3 days straight and it was the last day, we were doing proofreading and decided to add that in.

BTW, two of our robot names our "Burninator" and "The Cheat"

Some comments on the wheels you found: The design concerns me because a wheel roller is not always in contact with the ground. Usually with this type of wheel there are staggered rollers, such that there always is a roller in contact. So the wheel will be rolling without friction one instant, and then a split second later be scraping against the ground. Please correct me if I'm misinterpreting the design of the wheel.

Good luck, I really hope to see more omni-drives around this year :) Seriously, if you run into any roadblocks at all you can't figure out please contact me at prd8@cornell.edu. I'd even make the wheels for your team if I had more time, just to see a four-wheel omni FIRST robot.

- Patrick

Elgin Clock 26-09-2003 17:12

Quote:

Originally posted by RogerR
the first FIRST killough platform (i believe) was built by team 857 in 2002, and was called the 'kiwi-drive'.
Gotta love that "Kiwi Drive" video that the team released to showcase their drive system!!! :D

patrickrd 26-09-2003 18:10

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

Frank(Aflak) 27-09-2003 00:38

Patrick:

you did miss a detail: Those wheels are designed to be put back to back two on an axel,180 degrees apart, so you have six rollers per axle, with either one or two always being in contact, and a nearly circular shap for smoothness.

I contacted Kornylak, and they said that the wheels have a .874" hex bore. Is this type of axle readily available? Can I get bearings and standard size gears that will go on this axel? If not I may have to take you up on that offer.

I began chassis design the other day (this is all pending on whether this years game will allow for this sort of thing . . ) and I came up with a way to build a solid chassis with four long rectangles of aluminum plate and some angle peices. Light, should be strong, and damaged peices could be easily switched out. as of now, its just a sketch, but tommorow I will calculate sizes and dimensions and whatnot. Maybe I will even begin the gearboxes. Who knows?

er, about the omnidirectional debate.

This may clarify things.

Group A says that killough/similair are the only true FIRST omni because they can move in any x,y direction in an instant. They say that swerve/crab drive is not omnidirectional because the crab modules must turn, so that all a crab module is a quickly reconfigureable chassis, but it can't zoom north and then east with only a few instants in between to do all that accelration stuff, but it can go north, rotate its wheels to the E-W line, thus reconfiguring the chassis, and go east . . .


Personally, I think that omniwheel-based systems are cheaper and easier (built like a tank drive, only put the wheels at 90 degrees to each other) to construct, but quality omniwheels are hard to build or obtain. I think that the kornylaks are winners, and I hope that our bot this year reflects that. Essentially, all our robot design is is four wheel drive, one motor per wheel independant of the others, turn the wheels perpendicular to each other, and then put omniwheels on the drive shaft. Same gearboxes. No steering mechanism that uses extra stuff, just standard gearboxes. I just hope it works. But, I'd say the killough platform is as KISS-style as you can go with all these 'omnidirectional' drives described in this thread.

patrickrd 27-09-2003 16:20

Kind of like these wheels I spotted at the airport in Zurich, Switzerland :D


patrickrd 27-09-2003 16:25

Quote:

Originally posted by Frank(Aflak)
Patrick:
I contacted Kornylak, and they said that the wheels have a .874" hex bore. Is this type of axle readily available? Can I get bearings and standard size gears that will go on this axel? If not I may have to take you up on that offer.

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


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