Quote:
Originally Posted by JohnFogarty
I do understand that the mobility is the advantage, but I have suspicions that people on this forum seriously overstate the difference in torque in between the two drive systems. I don't dismiss the experience people have here off hand, I just want some real world data specific to this argument.
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http://www.chiefdelphi.com/media/papers/download/2739
TL;DR, given the same coefficients of friction for a standard and ideal mecanum wheel, the mecanum wheel can exert 30% less force on the floor before breaking static friction in the forward direction. (Non-ideal mecanum wheels - with friction in the rollers - suffer less in the forward/backward direction, but more when strafing). Moreover, in practice you can find standard wheels that are far more tractive than you can get in a mecanum roller due to the size and geometry, which amplifies the difference, potentially to 50% or more.
Static friction provides an upper bound on your ability to accelerate, change direction, push, and resist pushing. Even on an open floor without obstacles and defenders, the traction-limited tank drive matches or exceeds the maneuverability of a traction-limited mecanum drive in many cases. For example, if you want to move 10 feet to the left, a mecanum drive can strafe for 10 feet, but a tank drive can reorient, drive, and reorient again (or drive any number of smoother maneuvers that accomplishes the same thing) and be able to execute each segment of the motion more quickly because of its superior ability to exert force against the floor.
Which one gets you to a given arbitrary goal location first? There are going to be cases where a traction-limited mecanum clearly wins (e.g. move 1 inch to the left) and cases where a traction-limited tank drive clearly wins (e.g. turning in place or moving purely forward). For hybrid motions, it depends on the details of the respective robot dynamics. The further away the goal, the more likely it is that the tank drive can drive a path that gets it there more quickly.
(The actual study of the optimal path/time to get from one spot to another for different types of drivetrains is fascinating and complex...see
http://planning.cs.uiuc.edu/ch15.pdf for a good [graduate level] introduction).
So it may make sense to use a mecanum drivetrain if you are optimizing around short, controlled sideways movements in tight spaces. But most FRC games (2015 being an exception) are not dominated by these types of maneuvers. Once you add in the presence of "adversarial dynamic obstacles" (defense), things skew in the tank drive robot's favor in most cases.