pic: GBX-114 swerve with PTO

[Mike Marandola]

Originally Posted by asid61

With swerve drives, if we get into a pushing match, we have three
options:
1. Translate away. Good for offensive tactics, especially with this blistering speed.
2. Push back. Useful only against lightweight robots or if we are stuck somehow. Or to slow other's robots down.
3. Cross the wheels. If the wheels are in an X-formation, then we can't be pushed unless they lift us (cue opponent's breaker blowing). Esentially, the driver has a trigger to lock the wheels, and when the opponent tries to go another direction our driver releases the trigger and moves. This is an extremely effective defense (in theory) as long as our driver can move to block them when they try to get past. I'm confident our speed is high enough for this.

How did you come to the conclusion that you can't be moved when the wheels are in an X?

[asid61]

Originally Posted by Mike Marandola

How did you come to the conclusion that you can't be moved when the wheels are in an X?

In order to move the robot, the pushing robot would be acting against the traction wheels. Any direction that they are pushing from they would need to scrub the tires.
You could turn the wheels 90* from their push too.

[asid61]

D'oh. The low gear is actually 9.8fps in the photo. I accidentally marked it down as 36 tooth instead of 44 tooth in the calculator.
Then the robot is definitely traction limited. It draws 240 amps at maximum (lifting) torque. I was worried about increasing the pulley to get the free speed under 12fps, but it turns out that won't be an issue.

So the speeds are 24.0fps/9.8fps.

[nathannfm]

Originally Posted by asid61

In order to move the robot, the pushing robot would be acting against the traction wheels. Any direction that they are pushing from they would need to scrub the tires.
You could turn the wheels 90* from their push too.

Assuming your contact patch is square an X with 45° wheels is the most effective (I know that a 1" wide, 4" dia wheel with Blue Nitrile is approximately square) because you are applying force along the longest line you can draw in that shape. If you measure your contact patch and find it is a rectangle you can calculate the most effective angle to put your wheels at when you make this maneuver. (Not sure how much difference that would make tho...)

[asid61]

Originally Posted by nathannfm

Assuming your contact patch is square an X with 45° wheels is the most effective (I know that a 1" wide, 4" dia wheel with Blue Nitrile is approximately square) because you are applying force along the longest line you can draw in that shape. If you measure your contact patch and find it is a rectangle you can calculate the most effective angle to put your wheels at when you make this maneuver. (Not sure how much difference that would make tho...)

It would increase contact area, but I want to be able to lock the wheels without having to spend power on the motors. I guess putting them in brake mode would do the trick.

[Mike Marandola]

Originally Posted by nathannfm

Assuming your contact patch is square an X with 45° wheels is the most effective (I know that a 1" wide, 4" dia wheel with Blue Nitrile is approximately square) because you are applying force along the longest line you can draw in that shape. If you measure your contact patch and find it is a rectangle you can calculate the most effective angle to put your wheels at when you make this maneuver. (Not sure how much difference that would make tho...)

Since the contact area and amount of force would be the same regardless of the wheel angle, there might not be any difference. This is not taking the tread pattern into consideration though. Which is why the X formation might be beneficial. I think the X pattern will both reduce the wheel's ability to rotate when pushed and utilize the effects of the tread "fingers".

Originally Posted by asid61

It would increase contact area

The contact area would remain the same.

Nathan, I know you said that 365 did testing on diagonal vs. normal tread. Did you guys happen to test the tread sliding sideways as well?

[asid61]

Originally Posted by Mike Marandola

Since the contact area and amount of force would be the same regardless of the wheel angle, there might not be any difference. This is not taking the tread pattern into consideration though. Which is why the X formation might be beneficial. I think the X pattern will both reduce the wheel's ability to rotate when pushed and utilize the effects of the tread "fingers".



The contact area would remain the same.

Nathan, I know you said that 365 did testing on diagonal vs. normal tread. Did you guys happen to test the tread sliding sideways as well?

Oh wait, you're right. I was thinking of another tread with diagonal tread in it...
Regardless, it shouldn't matter. An x-configuration would allow the motors to just sit in brake mode I think.

[nathannfm]

Originally Posted by asid61

I guess putting them in brake mode would do the trick.

Yea, I figured whenever you sent them to an X configuration you would want to be breaking.

Originally Posted by Mike Marandola

Nathan, I know you said that 365 did testing on diagonal vs. normal tread. Did you guys happen to test the tread sliding sideways as well?

Yes, I wrote up a procedure in that thread. Thats actually what started us doing the testing, we were getting pushed sideways more than we liked.

[Mike Marandola]

Originally Posted by nathannfm

Yes, I wrote up a procedure in that thread. Thats actually what started us doing the testing, we were getting pushed sideways more than we liked.

Awesome. I will check that out.

[asid61]

Originally Posted by nathannfm

Yea, I figured whenever you sent them to an X configuration you would want to be breaking.



Yes, I wrote up a procedure in that thread. Thats actually what started us doing the testing, we were getting pushed sideways more than we liked.

Pushed sideways while in an X? That woule be very hard to do if you set the turning motors in brake as well; they are geared down 1:26 with a secondary of 30:84.

[nathannfm]

Originally Posted by asid61

Pushed sideways while in an X? That woule be very hard to do if you set the turning motors in brake as well; they are geared down 1:26 with a secondary of 30:84.

Sorry, we did the tests last year on our 8 wheel WCD. With a swerve that has straight cut tread it should be just as hard to do as being pushed backwards. (I will have to think about how best to "break" with diagonal tread)

[asid61]

Originally Posted by nathannfm

Sorry, we did the tests last year on our 8 wheel WCD

Thank goodness. I was thinking, "Oh god, did my numbers come out totally wrong!?"
They did, as a matter of fact. This is only geared for 20fps, sadly. I messed up the numbers in the calculator (again). Fortunately, the speed is easy to increase, but hard to decrease.
EDIT: I added a 3-position cylinder to the CAD, but for some reason these cylinders from Bimba are around 6" tall. So you can technically put on one here, but I would probably learn to live with this version.

[nathannfm]

Originally Posted by asid61

Thank goodness. I was thinking, "Oh god, did my numbers come out totally wrong!?"
They did, as a matter of fact. This is only geared for 20fps, sadly. I messed up the numbers in the calculator (again). Fortunately, the speed is easy to increase, but hard to decrease.
EDIT: I added a 3-position cylinder to the CAD, but for some reason these cylinders from Bimba are around 6" tall. So you can technically put on one here, but I would probably learn to live with this version.

I'm confused, what's that have to do with being pushed while in X mode?

[asid61]

Originally Posted by nathannfm

I'm confused, what's that have to do with being pushed while in X mode?

If you get pushed from the side while in x mode, then because the contact patch with the floor (ideally) is a line or thin rectangle, the pusher will torque the wheel sideways a bit like a caster. It seems like it would take a ton of force to shove those wheels back into a straight position, but it is doable. I'm sure that the robot will lift before that though.

[nathannfm]

Originally Posted by asid61

If you get pushed from the side while in x mode, then because the contact patch with the floor (ideally) is a line or thin rectangle, the pusher will torque the wheel sideways a bit like a caster. It seems like it would take a ton of force to shove those wheels back into a straight position, but it is doable. I'm sure that the robot will lift before that though.

Theoretically it's a line, if your wheels and the floor were both hard and solid, ideally it's as large a rectangle as you can make it because with tread on carpet the physics class friction model goes out the window and surface area is quite important. But 45° wheels pushing from the side should be identical to pushing from the front. You are right though, it's quite hard to turn a (for us) 100:1 braked planetary gearbox. As for lifting, unless your bumpers are higher than your opponents there is no force that is going to lift your robot. Infact, if your bumpers are lower they will be pushing you into the floor a small bit, increasing your normal force and your friction.