Drive train questions

[Ether]

Originally Posted by Bjenks548

Thanks for the help, I thought mecanum wheels worked by pulling at a 45 angle from the rotation

That is how they work. And it's why you need 4 to strafe. You need the back wheels to cancel out the fore-aft force vector components.


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[Bjenks548]

Originally Posted by Ether

That is how they work. And it's why you need 4 to strafe. You need the back wheels to cancel out the fore-aft force vector components.


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This is why I'm confused. Let's say that you do have the design I mentioned, if you drive both forward you drive forward because the right wheel is pulling forward and left, and the left wheel is pulling forward and right. Therefor left and right cancel out (assuming same speed). Same can be said for driving both backwards. Now if you drive the right wheel forward it pulls forward and left, and the left wheel backwards, backwards and left. Shouldn't you go left? And running the right one back and the left one forward shouldn't you go right?

As I see it the only reason i would need 4 mecanum wheels is to turn, not to strafe.

Someone explain to me how I'm wrong considering no one else sees this happening.

[Hawiian Cadder]

that would work if the two mechanum wheels were occupying the same point in space, however this method would cause the robot to twist when you tried to strafe due to the fact that they wheels being apart from each other would cause a rotational force to be applied about the center of gravity. in a perfect world this would work, but not in the real world.

[Clem1640]

These days, I live in the real world.

[Ether]

Originally Posted by Bjenks548

Now if you drive the right wheel forward it pulls forward and left, and the left wheel backwards, backwards and left. Shouldn't you go left?

Here's where your upcoming physics course will be helpful. Since the forward force from the right wheel and the backwards force from the left wheel are not colinear, they create a torque moment which causes the vehicle to turn left as it is strafing left.

As I see it the only reason i would need 4 mecanum wheels is to turn, not to strafe.

If you want to strafe without turning, the rear wheels are necessary. To strafe to the right, the front left and rear right are driven forward, and the front right and rear left are driven backward. The front left and rear right generate forward and right force components; the front right and rear left generate backward and right force components. The backward force component of the front right wheel is colinear with and cancels the forward force component of the right rear wheel; the forward force component of the left front wheel is colinear with and cancels the backward force component of the left rear wheel. What remains is the right force component of each of the four wheels. So the vehicle strafes to the right, with no torque to cause turning.

[Jon Stratis]

Originally Posted by Bjenks548

This is why I'm confused. Let's say that you do have the design I mentioned, if you drive both forward you drive forward because the right wheel is pulling forward and left, and the left wheel is pulling forward and right. Therefor left and right cancel out (assuming same speed). Same can be said for driving both backwards. Now if you drive the right wheel forward it pulls forward and left, and the left wheel backwards, backwards and left. Shouldn't you go left? And running the right one back and the left one forward shouldn't you go right?

As I see it the only reason i would need 4 mecanum wheels is to turn, not to strafe.

Someone explain to me how I'm wrong considering no one else sees this happening.

There's a good way to visualize how this would happen too. Imagine a merry go round you might have had in a playground growing up. Now, put a person on the east (right) and west (left) side of it. If both people push north, it doesn't spin (ie the robot would go straight forward). However, if the guy on the east pushes towards the North West, and the guy on the West pushes towards the South West, they can spin the merry go round (Because one is pushing towards the north and the other towards the south). It's not a perfect spin, however, because they aren't pushing tangentially to the merry go round - some of the force goes into translation.

This analogy isn't perfect, as it's hard to visualize that translation with it. But it does a great job with rotation. For a normal Mecanum drive train (with 4 Mecanum wheels in the corners), you can stick people on the North East, North West, South East and South West corners and have them push in different combination's. You'll find that when they're all pushing tangent to the merry go round, in the same direction (clockwise or counter clockwise), the merry go round will turn without translation. And when you have them working in opposing pairs there won't be any rotation, but the force has to go somewhere - you should be able to figure out what the translation would be.


In physics, this is called a free body diagram. You imagine your forces acting on a pivot arm coming from the center of mass of your object (arguably the center of your robot, for all practical purposes... although your mileage may vary based on specific robot designs). If the forces line up properly, they'll cause the object to rotate or translate (or both!). It's one of the more important concepts in physics, and one that students seem to have the hardest time grasping.

[Ether]

And when you have them working in opposing pairs there won't be any rotation, but the force has to go somewhere - you should be able to figure out what the translation would be.

One minor quibble with this explanation. The phrase "the force has to go somewhere" could be misleading to a new student. For example, if the front wheels are being torqued forward and the rear wheels being torqued backward, the "force goes nowhere". There is no motion. No translation, no turning.


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[Jon Stratis]

Good catch... When we worked on implementing Mecanum, we only looked at the productive scenarios (since the one you point out would only serve to waste battery power), so any unproductive ones like that just didn't come to mind when i made my post. Thanks for helping to clarify that