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Originally Posted by dougwilliams
My team typically uses mecanum, and we are starting an off-season drivetrain. We understand most but can't see / make sense of some of the omni based drivetrains.
http://www.vexrobotics.com/vexpro/examples-guides
What are the pros and cons of this 2+2 drivetrain? Why only go two omnis vs 4? I could imagine some scrub reduction, but why not all 4 wheels? And if you are using 4 independent gearboxes, why couple each side with belts/chains? Are they trying to mechanically couple and equalize wheel speed per side? |
There are a few pros to having two traction wheels in a setup like this. The biggest thing is that you retain some amount of tractive force for pushing matches - while omni wheels have some forward traction, most traction wheels have more, so you'll still be able to push with a 2+2 setup. The second is that your turning center is usually directly between the two traction wheels. There is, theoretically, *no* scrub in a 2+2 wheel drivetrain, because the omni wheels are the only wheels sliding laterally at all. The traction wheels become the point of rotation for the system.
The pros to belting the two gearboxes together on each side have less to do with speed and more to do with motor load. It's most obviously useful in a pushing match, but any time that your CG isn't perfectly centered it provides some benefit. Consider the extreme case where you're trying to push a robot, but your omni wheels leave the ground. This means your traction wheels can only push with 2 CIMs of mechanical power. If the wheels are belted together, you get the force of all 4 CIMs on those wheels. This same principle applies when all four wheels are touching the ground, but the force on each wheel is not even, such as when the CG of the robot is closer to one set of wheels than the other set. Essentially it lets the mechanical power of each motor go to where its most needed. It also gives the drivetrain a little redundancy, but that's not really why it's done in most cases.
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Then we were looking at 1114's 2014 robot, and it looks like 4 wheel traction, but two wheels have butterfly like modules where you can drop omnis on one side (not sure if it was the front or back). Similar to the above, is temporarily reducing scrub important enough to add that weight?
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It's a six wheel traction drive, with dropped center wheels. The drive behaves as a 4WD effectively, as the dropped center wheel raises one end of the robot slightly, but with a short enough wheelbase that the robot can still turn on a dime.
The reason for the drop down omni wheels (which I believe are unpowered) is not quite obvious, but it has to do with the mechanics of a high traction drive in a pushing match between two robots with compressible bumpers and rough fabric. This may sound like an extremely specific edge case, and it is very specific to FRC, but it's a real issue. You can T-bone pin a 6WD robot by pushing it hard on the center of its long side. The combination of the direction of pushing force, the two robot bumpers compressing into each other, and the friction caused by the compressed bumpers will result in the defended robot being virtually unable to escape. The drop down omni wheels change the center of rotation for the robot from the center to an extreme end, as the robot temporarily becomes 2 traction 2 omni, and the robot is able to spin free without pushing into the compressed bumper. Or, more likely, the T-bone robot continues to push as the module goes down, spinning 1114's robot out of the pin for them.