Re: Square Wheels?
 

well offtopic but square wheels would keep the axle movement linear if the ground it were rolling upon was half circles... theres plenty of articles onthat all over the place if you're interested.. like square wheeled bicycles.

waaaaaaaay offtopic but with all the ways people have done and attempted multispeed robots, anyone try a variable wheel diameter? not in the transmission but like the wheel(or w/ tire) that touches the ground... it expands, and itll basically be like a higher gear. and also give more ground clearance... so little junk wont be in the way when its going fast but when it pushes, the center of gravity goes lower andmore torque is applied via smaller diameter wheels.
if you're tricky, you can probably pull that off automatically with some sort of centrifugal system.

Re: Square Wheels?
 

wow, I think you just blew my mind!

a sort of cone shaped wheel with a bit of a curve to it.

when one end is contacting the ground the 'wheel' diameter is very small, the other end large, and inbetween, infinitely variable!

all you need is pnuematics to tip the cone towards one end or the other.

Re: Square Wheels?
 

Holy monkey! so if you could put them flatish, you'd be almost un-pushable! But I think i'd try to run them with a motor setup Ken. It'd be smoother, and more variable, unless you rig up a special exhaust valve.

Re: Square Wheels?
 

like this!

Re: Square Wheels?
 

Okay, I think I'm correct now -- wouldn't this end up being a hemisphere?

If you started with the driving shaft parallel to the ground, the wheel circumference would equal that of the hemispherical equator. As you pitch the shaft toward vertical, the effective wheel circumference approaches zero as the hemispherical pole comes into contact with the floor.

Does that make sense?

I have examined other methods of using centripetal force to engage a shifting mechanism. I would like, someday, to build such a thing to better get an appreciation for its challenges.

Re: Square Wheels?
 

whoa. i was thinking a MUCH more complicated idea. but that is nice! hm.... if you'rw going to have variable angles on the axles, there has to be a way to incorporate that to do a multidirectional... something different from an omniwheel or a transwheel, or a rotating thing... w/e they were called. cant rememebr it right now tho.... hmmm.... darnit.. shoulda thought about it myself and blown everyone's mind at the competition lol. except i didnt have the variable andgle idea.... darn... woulda been cool for rookies to come out with a totally new concept like that and had it work tho. oh well' ill think of sumthing else :rolleyes:

Re: Square Wheels?
 

yup. makes sense. although i doubt anyone'll really need it to turn 90 degrees tho.. so a hemisphere will work fine but by having a "custom" curved cone shape, you can vary the surface contact patch are too. well i guess diameter and tire material play an effect into it, the skinnier it is the easier it is to turn so maybe you can have a small contact patch in "high" and a bigger contact patch in "low". and well you can make the transition between "highest" and "lowest" at different rates too. oh. you can even use different materials so high can be slipperier than low. WOW. this idea is incredible. you can have the grippiest tires possible in the "lowest".. you dont even need to be able to turn.... and have an omniwheel at the "highest" so you can manuever as fast as you want..... again. WOW... the possiblities of such a system...

wanna share?

Re: Square Wheels?
 

oh... sorry..... this is getting WAAAAAAAY off topic.... anyone want to move this into a new thread that would be more appropriate?

Re: Square Wheels?
 

I'm liking the idea of hemispheres as wheels. You could actuate their tilt with pneumatic clyinders if only high and low were desired, or you could do it with a lead screw for much more variation. I'm just wondering what to make the hemispheres out of (and how) and what tread material to cover them with and how).

Re: Square Wheels?
 

What's further interesting about using hemispheres as wheels is that you can maintain constant driving power to them. You can drive all four "wheels" with a single gearbox. You do not even need to vary the power output to vary its speed.

This, to me, is far more interesting the square wheels. Who wants to file for a patent? :)

Re: Square Wheels?
 

you could also have one of these somewhere in your drivetrain, and have a variable speed transmission

instead of having the cone/hemisphere in contact with the ground, it drives an internal wheel that is part of the drivetrain.

Re: Square Wheels?
 

but again, with frictiontype gearboxes, more can go wrong. i think its more worth lookinginto having the cone/himisphere as the final part rather than within somewhere.
i have a feeling something of the like has been patented tho. i have seen tilt wheel rc toys.... not sure if variable speed/torque was the point of those tho..
but if not, twould be sweet.
if you drive all 4 wheels with the same gearbox how can you turn? or are you saying about the angleing?
a leadscrew...hmm... or wormgear....or....

Re: Square Wheels?
 

By varying the diameter of the effective wheel on one side or another, you can vary the speed at which it travels a given distance. Make the wheels on the left larger than those on the right and your robot will turn right, for example. :)

Re: Square Wheels?
 

one of the benefits of a variable transmission like this: it can be in the high rpm part of the drivetrain, right on the motor for example

where the energy is hi rpm and low torque, so you would not need as much friction between the two surfaces, and the parts could be fairly small.

Re: Variable Diameter Wheels
 

This is really what a toriodal CVT is. Just with one wheel.



http://auto.howstuffworks.com/cvt6.htm

Re: Variable Diameter Wheels
 

oh MAN! they stole our idea! darn time travellers! :^)

Re: Variable Diameter Wheels
 

In 2005 we had a long talk about "Dude, think about how cool a CVT would be", and then we had a longer talk about "Dude, think about how cool a 300 lb robot would be"

Re: Variable Diameter Wheels
 

Ken, this is me >:) smiling.

Re: Variable Diameter Wheels
 

Easiest way to make your hemispherical wheels tilt is to have the motor and wheel all tilt together. 10 lb tilting assembly, with the weight of the robot on it. Sorry, but I'll pass.

But actually... it can be done. If you think about it, as the hemi-wheel tilts, it does not change the height of the robot. All you need is strong bearings on the tilting axis, and thrust bearings on the hemi-wheel. All it takes is the will to do it, and soon enough you'll see that it isnt that hard. You know, I bet you swerve drive is harder to do.

And as for me, I would control it with pneumatics. 4 cylinders, all at the same height, digitally set. When they reach the correct height, use a double and single solenoid to lock the cylinder where it is. fun fun. I will bow to the team that pulls it off.

Re: Variable Diameter Wheels
 

I can't say i'll bow, but I will spend every spare minute ogling their drivetrain!

Re: Variable Diameter Wheels
 

Dang, you just had to ruin it for us with that one little line! :D You are 100% correct though. I had totally forgotten about the thrust load issue. Nuts.

Like you said, it can be done, but now it doesn't seem as easy or fun anymore.

Re: Square Wheels?
 

oohhhhh i see.... well thats kinda akward tho. no more skidsteer...
hmm.. true. but well for the sake of this thread, lets not talk about those cvts. lets just talk about variable diameter wheels. <nice title btw.

you mean balls? :D
[/quote]Like you said, it can be done, but now it doesn't seem as easy or fun anymore.[/quote]
well i saw a robot that had omniwheels on an extreme angle. people are doing it already. just not on a swivel...

and as for not changing robot height, then the center (of the sphere) of the face of the hemisphere must be within the angling axis. so dpendin on how bulky that wheel+gearbox+motor tilting assembly is, it may not be that practical.
but other than load issues for the cylinder or w/e angles the assembly, why not let the robot height vary? it might come useful. high off the ground for high speed movement so you cant get snagged on each carpet crease. and low down to push. and its kinda easy to lift the robot on a bar again if the game calls for it now too. retractable...

Re: Square Wheels?
 

Save for making a zero-radius turn, it'd be almost exactly like skid-steering. In fact, since you could keep the motor output at a full 12V throughout the turn, you'd be putting out more torque at the wheels during a turn than a traditional skid-steer arrangement would.

Re: Square Wheels?
 

true. and you worded that very nicely.. i was only imagining zero radius turns and one side put. one side on turns. i guess depending on how much angle you can turn... like if you can turn the hemispheres a full 180 then you have a conventional skidsteerable robot.
but i just noticed... even if you do use only one motor for the entire drive, youd need 2 motors(or cylinders) to move the angle of each side independently. while if you do a rightside motor, left side motor, you can do the angling with a single motor or cylinder.

Re: Variable Diameter Wheels
 

For tilting the wheels it could be done quite easily:

have two sets of motors like an average robot this year, then have it drive a shaft that goes to the wheels, make a u-joint and an extending shaft that allows the angles of rotation you need,

the wheels would be on a folding down assembly that would pivot with the u-joint and extend the shaft as it needs, also i would use pneumatic pistons to raise and lower it setting the pressure desired for a certain height and the robot will push the pistons back in, so if you get hit in the front your front wheels begin getting more torque and the rear more speed to allow you to level yourself out becuase of these conflicting drives and then you lift the pistons up all the way and BOOM high torque

Gonna try it,
Cuog

Re: Variable Diameter Wheels
 

waaait! i dont understand! where does the ubolt go? im so confused with your description... can you draw a picture for me?

Re: Variable Diameter Wheels
 

why use a Ubolt (universal joint) on each side? each set of wheels (left and right) can have one motor, and the motor tips with the wheel assembly as a unit.

Im wondering, would you want the high torque on the outside of the wheels or the inside? when you are leaning on the outside edges of the wheels your bot is more stable, which would be ideal in a shoving match, having more power to steer under heavy resistance

but having the outside edges down at high speed would also be good, if you are going to make high speed turns, to keep your bot from tipping

I could go either way on this one, depending on the game itself.

Re: Variable Diameter Wheels
 

yeah im bad at explaining, if the person on my computer leaves soon i can finish inventoring it, otherwise it may have to be hand drawn and scanned oh well

heres try II:

each wheel is mounted on a bearing setup that rotates on the base, the wheel has a shaft that runs into it, the shaft goes up to the pivot point and has a u-joint at that spot, the u-joint connects to a telescoping rod inside the robot which is then connected to the motor however you like.

Re: Variable Diameter Wheels
 

im with ken on this. no need for a universal. a "conventional motor>gearbox>wheels" can all rotate. no problems there..

hmm. well it does depend on traction and speed but a high speed robot will be faster at zero radius turning (as in angular acceleration) if the track width is smaller. but also harder to control that way. if you do the high speed on inside, high torque on the outside, and have all four wheels on a sort of parallel 4 bar link, and at "neutral", the robot is on about the "center" of the cone, you lean all 4 wheel to the left and the robot turns left in a gentle swoop. so itll kinda handle like a motorcycle. kinda...
but thats offtopic. sorry.
um. personally i think i would go with torquey on the inside. faster to turn with even massive "low gear" and more stability at high speeds.

Re: Variable Diameter Wheels
 

another application for these cone shaped wheels would be railroads and subways.

as you enter a train station the tracks would be wider, the train would slow down and have more torque (stopping and starting power) (no need to tip the wheels)

then out in the great far-betweens the tracks would be placed closer together, and the train would take off like a bat out of you-know-where

Re: Variable Diameter Wheels
 

If I'm not mistaken, wouldn't the track need to be changed in two axes? It needs to move laterally to be positioned under a different radius, and it also needs to be raised/lowered to make smooth contact.

This reminds me a little of a roller coaster called 'X' at Magic Mountain. The cars move along a 2-railed track like a normal coaster, but there is a third rail between them that changes height. The wheel that rides on this track is attached to a rack and pinion system that causes the car to rotate forward and backward. It's a pretty wicked ride...

Anyways, I would imagine that the same could be done in the case of your wheels. You could have a third track that rotates the wheel assembly. I don't think this would be the greatest of ideas, but it would be an option nonetheless.

Re: Variable Diameter Wheels
 

X has two cam rails rather than one, but essentially, you've got it right. It also has been a technical nightmare to operate and maintain.

I don't believe that the rail height would need to change as Ken's described things. Instead, the height of the trainset above the ground would vary with the track guage.

Railroad trucks have flanges on the inside of each rail that keep the train going in the correct direction around curves and through turnouts, however. Without a flange immediately abutting the rail, it seems that the trainset Ken's describing might tend to be thrown from a curve by its centripetal force. I suppose you could always super-elevate the turns so there is no lateral force at all, but that may be a bit extreme for riders.

Re: Variable Diameter Wheels
 

Our team (1002) has actually been looking into the hemispherical wheels thing since we figured it out over the summer, but we haven't figured out how to get the wheels to have enough friction to move the robot, but little enough that the ratio can change.

To make sure your ground clearance stays constant, you pivot the "wheels" around a diameter.

I think we decided on some sort of rubber-coated aluminum spheres.

Don't know if this helps anyone with figuring this out, but it will be a really neat innovation for whoever can get it working.

Re: Variable Diameter Wheels
 

Given that the ratio change -- or pivot -- of the hemisphere is going to happen under power, isn't that motion effectively the same as rotating another wheel? If so, I don't see that generating sufficient friction is any more a concern here than it is to any conventional drivetrain.

Re: Variable Diameter Wheels
 

For a rigid wheel (non deformable tread surface) a wheel contacing the ground plane will give you a line whereas a hemisphere contacing a plane will give you a point. This may be cause of concern in the traction department.

Re: Variable Diameter Wheels
 

For as long as we're running on carpet, nevermind what tread or material the wheel is made of, there will be significant deformation at the interface between the hemisphere and ground.

Re: Variable Diameter Wheels
 

Or it might make things better. It depends on the surfaces involved. If the playing field has a clean metal surface, metal wheels might actually be a good bet. Metal on metal actually results in a very high coefficient of friction and a small contact area enhances this. That was a key principle in the CVT that 190 did a few years back. We also used in on our slide drive in 2004.

However, if we are back to the standard carpet then the line contact would probably be better. Because of the carpet, not the wheel.

Re: Variable Diameter Wheels
 

within a clean gearbox, metal/metal friction may be high but on a field. its hard to keep it clean. THAT clean. and even a small scratch may affect it.well for the highest gear its easy to have a "Conventional flat faced tire.... its just the low gear... which i guess is also possible but it can also generate a torque not just longitudally but laterally? or... well. a sideways torque... which may actually work too. kinda like toe-in (on the rear axle of a car would be a good example). aggressive tread would work for that. basically clawing into the carpet. kind of.

Re: Variable Diameter Wheels
 

Ignore for a moment the horrid material attached to this rendering as Solidworks does a poor job with plastic.



Green represents the robot chassis and how each wheel assembly may be mounted.

Red represents the variable pivot mechanism. This doesn't presume to explain how the pivot happens -- but one could use sprocket and chain, gears, rack and pinion, or a cam to achieve the required roll.

Blue represents the drivetrain. The small blue shaft is the input to the bevel gear driving the semi-spherical "wheel".

The wheel, as it were, is not a full hemisphere. The pivot point passes through the center of the sphere and thus, a full half sphere would have interfered. It represents at its largest a wheel of 8" diameter and, at its smallest, a point that has no diameter.

Re: Variable Diameter Wheels
 

I believe team 116 did something similar. Im not talhing drive train wise but in the mounting of the wheels and the point contact. Maybe someone might mention how good the traction was? Actually wouldn't you get better ratios with concave disks rather then hemispheres? less rotation would be needed to make a large gear shift.

Re: Variable Diameter Wheels
 

do you mean convex? because if its concave then the low or high will be the only points that can touch the ground with nothing inbetween.

Re: Variable Diameter Wheels
 

:D Yeah after I posted that I was kicking myself because I knew that I had gotten them mixed up. Another nice side effect which Epsilon Delta capitalized on is that this drive set up allows omni directional movement. Anyways, yeah I believe a convex dish would give better ratios with a minimum amount of movement.

Re: Variable Diameter Wheels
 

Could you better explain what would be different about a "dish" as compared to a hemisphere?

I think you're suggesting that the wheel be made with an elliptical cross-section rather than spherical, but am not certain. An elliptical cross-section would still require a 90* rotation to move through its maximum ratio, but would reduce the movement needed to pass through the most practical ratios. It may be harder to manufacture, however.

Re: Variable Diameter Wheels
 

Wouldn't it also suffer from a variable ellipsoidal radius, meaning that the whole trunnion assembly would have to pivot to counteract the vertical motion of the centre shaft? It could be done with some sort of bar linkage, I think, but keeping the spherical radius constant is far easier.

Re: Variable Diameter Wheels
 

Believe it or not the wheels on a train are cone shaped even though you can't tell by staring at them. Don't ask me how I know this because it's from watching too much television. I think they are shaped like that because otherwise the train would derail easily.

Re: Variable Diameter Wheels
 

Physics--There's a reason for it (to keep the train on the track, in this case). I forget the name of the principle. Here's a quick demo of how it works: attach the bottoms of two styrofoam cups (you know, those little white ones that come in huge packages) together, set up two rulers/metersticks (or something similar) parallel to each other and roll the cups down from one end to the other. That's what keeps the trains on the track (the flanges are backup).

Re: Variable Diameter Wheels
 

its more that that. The train wheels are welded to the same axle, so as the train goes around a bend the centripetal force causes the outside wheel to ride up on its inside (larger diameter) and the wheel on the inside of the curve is on the smaller part of its diameter. This allows the wheels to stay in contact with the rails without slipping, because the outside rail is longer than the inside rail on a curve.

Re: Variable Diameter Wheels
 

Just to finish off this train of thought (ha!), the AAR (Amercian Association of Railroads) standards established a wheel profile with a 3 degree slope across the width of the wheel tread. These standards were later adopted by the Federal Railway Administration. The fillet between the wheel tread and the flange used to be a simple circular radius. But in the past thirty years, as metallurgy has advanced and resulted in harder railhead materials, this simple profile caused uneven wheel wear and hollowing. In response, the fillet proifle is now a convoluted shape that minimizes wheel wear on the new, harder rails.

...and now back to our originally scheduled topic of discussion.

-dave

Re: Variable Diameter Wheels
 

Quit railroading everyone. We need to keep this thread on track.

Re: Variable Diameter Wheels
 

Har Har Har... Quite punny.

So anywhoo, The only real problem that I see with this from a FIRSt bot perspective is weight. The wheels would would have to be considerably heavier than normal, as well as adding a whole mechanism for tilting the wheels controllably, while retaining some structural stablity. Anyone got any ideas on how to keep the weight down (besides cheeseholing) ?

Re: Variable Diameter Wheels
 

I was thinking instead of making it infinately variable, you could use the same concept and have maybe 3 wheels (three speeds).

This would also allow you to make the smaller diameter wheels have wider treads, for more traction, and have the taller wheels wearing skinny tires.

and you could angle each of the three wheels to keep the tread parallel to the floor when it is rotated into positionl

Re: Variable Diameter Wheels
 

but then again you dont have a couple gears that would be needed to gain the gear shifts. and well other than a cvt, its probably one of the smoothest gear transitions possible. and infinite.

Re: Variable Diameter Wheels
 

Out of curiosity, how would you fabricate such a machine?

Re: Variable Diameter Wheels
 

i woulda say lathe. not too hard to shape flat surfaces on an angle. just need a protractor and a chunk of wod and a ruler. well or a CNC

Re: Variable Diameter Wheels
 

The 3 wheels idea sounds scary to fabricate since you cant take a rod and stick 3 wheels on it the rod would have to constantly bend so that only 1 wheel would contact the ground at a time or the relative wheel diameters would have to be carfuly selected and placed with very exact positioning

Re: Variable Diameter Wheels
 

Well, if you are talking about variable diameter wheels, but now continuously variable diameter wheels, 190's 2001 robot did use two sets of different sized wheels to shift on the fly (one set could be raised or lowered). There has also been a clever design for a continuously variable wheelplan we've been kicking around for as long as I've been on the team, but I don't think any of the recent games have really made it necessary to use.

Re: Variable Diameter Wheels
 

Believe it or not, i used to have an RC truck that worked just like you're describing. I actually only realized how it worked after reading this (given I havn't played with it in years - I knew it went slower when stood up, but i never had thought about why).

It had the motors built into the rotating part with the wheels, as you've discussed. It was skid steer, and it would stand up or squat down - giving it the adjustable speed.

I've found one picture online. I'll see if i can find mine in the garage tomorrow to get better pictures. We moved last summer though, so I dunno how lucky i'll be.




- Jeff

Re: Variable Diameter Wheels
 

yup. thats what i was talking about.. such an ugly rc truck tho lol.
obvously that toy's rideheight changes a considerable amount. have we figured olut if thats good or not in our robots?

Re: Variable Diameter Wheels
 

I'd say it's almost certainly not a good thing, for several reasons; it's easy for the car to lift itself up, but a 120 lb. robot will have a harder time supporting its own weight, I should think. Beyond that it can depend on the game and the robot, but changing the height of the robot certainly makes it harder to be certain of where anything is for autonomous or even just operation, although I doubt this would be as much of an issue as the weight thing.

Re: Variable Diameter Wheels
 

OK i will finally come clean that i have been working on this system for quite a long time now, you will see it sometime soon, once i get it completely finished. As for ride height i personally wouldn't want my CG to change (to be at it's highest) when i am getting maximum torque.

Re: Variable Diameter Wheels
 

depending on where the axle pivots, the robot can be designed to be higher on the larger wheels, or higher on the smaller wheels, or it can be exactly the same height throughout its CVT range

it all depends on the shape of the wheel, and where the pivot point is.

Re: Variable Diameter Wheels
 

Let's think about this for a second:

Let's make a sphere. Mount a motor, gearbox, and whatever else inside of this sphere. Drive a shaft throw the center of the sphere, perpendicular to the motor shaft. Mount the assembly to the robot using this new shaft. Remove any excess portion of the sphere that will not contact the ground during normal operation - the sphere has become a wheel. Now, you need some way of pivoting the assembly about the mounting shaft.

This configuration will make it so that the height of the chassis never changes. However, you can change this by moving the mounting shaft closer or farther away from the robot (relative to the assembly).

Re: Variable Diameter Wheels
 

yes, relative to the center of the sphere

or if you enlongate the sphere the height of the robot can also be different, depending on where the pivot point is.

Re: Variable Diameter Wheels
 

For what it's worth, the sketch I posted earlier in this thread is, more or less, exactly as Phil has described. The notable difference is that I presumed that the wheels would not be powered independently.

They pivot, however, such that the axis of rotation passes through the center of the sphere, thus only the effective diameter of the wheel changes and never its position relative to the chassis.

Re: Variable Diameter Wheels
 

Actually looking at your drawing it apears to me that since the pivot point is slightly behind the center it would then lift and lower the robot however i am estimating this difference is about an inch and not a large problem in most competitions(unless they force us to fit under something)

Re: Variable Diameter Wheels
 

The pivot is not slightly behind the center; it passes through the center. The 'wheel' is not a full hemisphere.

Re: Variable Diameter Wheels
 

As you move the wheel from "fast" mode to "torque" mode, do you believe the frictional load caused by sliding the sphere sideways on the carpet will cause issue?

My goal would be to make these as high traction as possible, to get the use out of my torque in the low end. I'm worried that this high traction would seriously impede the movement from high-low and vice versa.

We're talking about a not insignificant amount of force.
Just something to consider. :)

This is definitely an interesting concept. I wish I had more time to build a proof-of-concept out of Vex. I hope this gets further developed, and possibly put into practical application.


-JV

Re: Variable Diameter Wheels
 

The force required to rotate the sphere would be about the same as that required to move the robot since the rotation is, in effect, acting as little more than another wheel. While moving the robot certainly requires a decent amount of force, it is often not the most difficult task to accomplish on a robot and, given that this rotation need only be ~90*, it is likely possible to accomplish it using some method that, while powerful, is not suited to drivetrains.

I don't think anyone's really done any considerable calculations regarding this; at least not here. I certainly have not. I just tossed the model together as a visual representation of the concept. I have my hands full with something else just now, so I won't be trying to build something like this for a long time to come.

Edit: This reads as if I'm being preachy -- I'm not, since I know you this stuff. I was just being expository.

Re: Variable Diameter Wheels
 

Nobody said the sideways rotation has to be symmetric around the center of the robot. If the hemispheres are all turned in the same direction, the whole robot will shift a bit to one side, and there won't be any carpet friction problem.

Re: Variable Diameter Wheels
 

Well my team has a meeting today after school so I'll probably bring it up and see if we can build a test component to see how well it works

Re: Variable Diameter Wheels
 

well if we have stairs again it might be useful. two birds with one stone.
the changing rideheight i mean.

Re: Variable Diameter Wheels
 

[quote=JVN]As you move the wheel from "fast" mode to "torque" mode, do you believe the frictional load caused by sliding the sphere sideways on the carpet will cause issue?

John,

I think the bigger challenge to overcome would be the losses from scuffing while going straight. The contact patch where the tire meets the carpet would approximate a section of a cone. This conic section will want to turn about a point at the apex of this cone. Just think of how a styrofoam cup rolls in a circle. Car tires will do the same thing and can cause your vehicle to pull yo one side if they have excessive conicity, which is one side larger than the other.

Assuming the conicity of the tires would all point in (or out), the forces would cancel each other out. There would still be high rolling resistance to overcome.
Sounds like an interesting idea.

Jay