Problems turning

[Bochek]

Well we shipped the robot with 2 main problems, we are 5 lbs over weight, and the robot doesn't turn.

im working on a sollution to the weight problem, but i have a question about the turning problem.

if you watch this video:
http://video.google.ca/videoplay?doc...15467259458065

you can see that we have issues with turning, we have to take a run and then quickly reverse one side to get it to turn.

i was thinking as a simple and easy sollution would be to replace all 4 wheels with the omni wheels available from andymark. as well as premounting the wheels to aluminum sprockets from ifi robotics so that its a simple swap at the competition.

my questions are as follows:

will this help us turn?
what are the disadvantages?
how much weight can we expect to loose by swaping to the aluminum sprockets?

Thanks alot.

- Bochek

[Cody Carey]

Originally Posted by Bochek

Well we shipped the robot with 2 main problems, we are 5 lbs over weight, and the robot doesn't turn.

im working on a sollution to the weight problem, but i have a question about the turning problem.

if you watch this video:
http://video.google.ca/videoplay?doc...15467259458065

you can see that we have issues with turning, we have to take a run and then quickly reverse one side to get it to turn.

i was thinking as a simple and easy sollution would be to replace all 4 wheels with the omni wheels available from andymark. as well as premounting the wheels to aluminum sprockets from ifi robotics so that its a simple swap at the competition.

my questions are as follows:

will this help us turn?
what are the disadvantages?
how much weight can we expect to loose by swaping to the aluminum sprockets?

Thanks alot.

- Bochek

Yes, it will help you turn, but you will get pushed sideways. A lot. A more cost-effective/easy solution would be to just switch out two wheels for omnis. The only thig is that you would pivot around the two wheels that still have sideways traction, but if you replace your back wheels, it will allow you to pivot around the front of your robot... which could be a huge advantage while positioning around a target such as a tube.You can reasonably expect to lose between a pound and two pounds by switching out the sprockets.


-Cody C

EDIT: Your drivers have learned to cope with the decreased ability to turn very well. Good job.

[Jizvonius]

Omni wheels will make turning much easier. The major disadvantage to that system is that while you can turn yourself with less effort, so can everyone else.

I would suggest only changing out either the two front or back wheels with omnis. This way you can still turn easily, but you can resist a little pushing from another robot. Note that your turning center will shift towards the non-omni wheels in this case.

The sprocket weight thing can be calculated based upon the size of your sprocket and the densities of the metals. If your current sprockets have large hubs, the weight savings could be significant

Edit: so a lot of this repeats Cody's post, but the sprocket calculations still stand.

[Lil' Lavery]

Originally Posted by Jizvonius

I would suggest only changing out either the two front or back wheels with omnis. This way you can still turn easily, but you can resist a little pushing from another robot. Note that your turning center will shift towards the non-omni wheels in this case.

The omni-wheel placement depends on where they want the center of rotation to be. If they wish to pivot around the front, place the omnis in the back. Vice versa for pivoting around the back. If they wish to pivot around the center (as well as reduce opposing robots ability to pivot them compared to other omni placements), the best solution would to be place the omni wheels on opposite corners of the drive.

[Cody Carey]

Originally Posted by Lil' Lavery

The omni-wheel placement depends on where they want the center of rotation to be. If they wish to pivot around the front, place the omnis in the back. Vice versa for pivoting around the back. If they wish to pivot around the center (as well as reduce opposing robots ability to pivot them compared to other omni placements), the best solution would to be place the omni wheels on opposite corners of the drive.

Since surface area has no bearing on friction, I would assume that this would be a bad Idea. By putting the two "sticky" wheels beside each other, you are reducing the amount that the wheel has to be pulled sideways when turning, and increasing the amount that it rolls along the turn. This decreases over all turning friction.

If you were to place them on opposite corners, it would do nothing to the amount that the sticky wheels have to be pulled sideways to follow the circle of your turning radius, and since One wheel has the same amount of friction as two, I wouldn't expect this to solve your problem at all.

[Goldeye]

One of the best solutions to turning difficulties is very often overlooked.
Lower you gear ratio, significantly
It gives your robot the force to turn against friction, and saves energy while doing it.
Also, you will probably find it makes the robot much more controllable anyway.
This is just a suggestion, and seeing as it doesn't help you with weight either, might not be the best solution.

[Lil' Lavery]

Originally Posted by Cody Carey

Since surface area has no bearing on friction, I would assume that this would be a bad Idea. By putting the two "sticky" wheels beside each other, you are reducing the amount that the wheel has to be pulled sideways when turning, and increasing the amount that it rolls along the turn. This decreases over all turning friction.

If you were to place them on opposite corners, it would do nothing to the amount that the sticky wheels have to be pulled sideways to follow the circle of your turning radius, and since One wheel has the same amount of friction as two, I wouldn't expect this to solve your problem at all.

From experience with vex bots, it definitely improves turning. Also look at 494/70 last year.
Omni placement all depends on how you want your robot to drive and how you want your robot to react to defense.

[meatmanek]

Originally Posted by Cody Carey

Since surface area has no bearing on friction, I would assume that this would be a bad Idea. By putting the two "sticky" wheels beside each other, you are reducing the amount that the wheel has to be pulled sideways when turning, and increasing the amount that it rolls along the turn. This decreases over all turning friction.

If you were to place them on opposite corners, it would do nothing to the amount that the sticky wheels have to be pulled sideways to follow the circle of your turning radius, and since One wheel has the same amount of friction as two, I wouldn't expect this to solve your problem at all.

Not true. One wheel with 60 pounds on it has the same amount of friction as 2 wheels with 30 pounds each. However, one can assume that the weight of the robot will be approximately equally distributed across the four wheels, or at least the weight on the omnis will be the same as the weight on the traction wheels. Therefore, the traction wheels have only half as much turning friction as they used to.

Another solution would be to move your wheels closer to the middle of your robot. This increases turning capacity without decreasing traction. However, it causes balance problems, as your wheel base is shorter. Add a few trick wheels or casters that hover just above the ground to fix this problem.

[Cody Carey]

Can someone please explain this to me mathematically? I still don't get it.

It takes less force to rotate a wheel along the diameter of the turning circle than to scrub it sideways along the same circle. With the omni wheels in the front (right picture),you are basically applying force tangentially to the circle which you turn on, and with the omni wheels on opposing corners, You are waisting a lot of your force going outward from the turning circle, and scrubbing both traction wheels sideways.

Is it just that having the omnis on opposite corners woud decrease the force required to turn the robot, but not as much as the omnis on front of back would do? A little help would be greatly appreciated.

[Tom Bottiglieri]

Originally Posted by Cody Carey

Is it just that having the omnis on opposite corners woud decrease the force required to turn the robot, but not as much as the omnis on front of back would do? A little help would be greatly appreciated.

Yes.

While the second scenario you illustrated will get you closer to a "zero friction" turn, it also changes your turning radius from dead center in the robot to inside of the axis between your back two wheels.

Putting the omni's in opposite corners still gives you the problem of turning scrub, but it will significantly reduce the problem as half of your total weight is being distrbuted along wheels which experience little to no axial frictional force. For most situations, this should be enough to kill the bouncing robot problem. This also allows you to keep your 0 degree turn radius, and keep a solid wheel on the ground on each side of the bot at all times.

So really, its all up to driver experience and preference. I have been told that drivers who like to use one stick drive will be much better on a "power slide" bot (omnis on same side) and drivers who prefer two stick will be much better with a 0 degree turn radius.

[Bochek]

well today we ordered 3 andy mark 6" omni wheels,

the reson for 3 was incase one breaks.

we will be replacing the 2 rear wheels with the onmi wheels.

does anyone know the difference in weight between the ifi aluminum sprokets and the steel KOP wheels?

- Bochek

[Arefin Bari]

Originally Posted by Bochek

well today we ordered 3 andy mark 6" omni wheels,

the reson for 3 was incase one breaks.

we will be replacing the 2 rear wheels with the onmi wheels.

does anyone know the difference in weight between the ifi aluminum sprokets and the steel KOP wheels?

- Bochek

The KOP wheels weighs .8 lbs. I believe you were trying to ask about the sprocket that was provided in the kit. If you are wondering if it would help you lose weight when you switch those steel sprockets to aluminum ones, yes you are on the right track.

Also, you are already going to lose .6 lbs by planning on swtiching those kop wheels to AM omni wheels. Those AM omni wheels weigh 5 pounds.

If you need help figuring out how to lose more weight and if you have detailed/close up pictures of the robot, feel free to contact me. I will be more than happy to help you guys and give some suggestions.

[Ben Piecuch]

Bochek,

You didn't tell us what your drivetrain consists of, and your video doesn't give us much insight into it either. However, based on a previous picture you posted of your kitbot, I'm going to assume that you're using the standard kitbot setup with 4 kit wheels, a single motor Banebot 56mm tranny, and 2wd.

Your problem comes with the fact that your robot requires more power to turn than the single motor setup provides. The kit wheels provide a lot of traction in both the lateral (front/back) and transverse (side) directions. You need to either:
a) increase the supplied turning force.
b) reduce the required turning force.

You can solve (a) by adding more power with more motors, or by reducing the gear ratio to reduce your speed and increase your torque. With your existing weight problem, adding more motors doesn't help your cause.

You can solve (b) by reducing the transverse friction in your wheels. Most teams accomplish this by adding one set of "omniwheels" to their drivetrain. For 2wd setups, you would add the omniwheels to the non-driven wheels. Andy-Mark makes a nice omni-wheel, but they are not the only ones on the market. There are cheaping and lighter alternatives, such as ones found at McMaster.com or Omniwheel.com You can also reduce your required turning force by reducing your wheelbase, or powering all 4 wheels.

There are several whitepapers, as well as numerous threads on this site that explain this problem in more detail. Some simple changes should be able to be made at your competition to have you up and running in no time. Best of luck,

BEN

[Bochek]

Originally Posted by Ben Piecuch

Bochek,

You didn't tell us what your drivetrain consists of, and your video doesn't give us much insight into it either. However, based on a previous picture you posted of your kitbot, I'm going to assume that you're using the standard kitbot setup with 4 kit wheels, a single motor Banebot 56mm tranny, and 2wd.

sorry, the drivetrain is the banebots 56mm gearbox with a CIM and 4wd.

basically the standard kitbot chassis but with added sprokets and chain to make it 4wd.

so replacing the rear wheels with omni's could prove a good solution then?
and i would still keep it 4wd correct?

- Bochek

[Jimmy Nichols]

Yes, that would keep it four wheel drive. My question is, how are you operating your bot. If you are running a single motor and gearbox, think about adding another and running it tank style, this makes turning much easier without purchasing the omni's. The only drawback would be your wheelbase, the larger the wheelbase on your 4WD system, then the harder it is to turn if with a tank drive system.