Quote:
Originally posted by Tytus Gerrish
A Crab drive seems Great because you can point all your power in any direction, But omni wheels only a fraction at the most half in any given direction
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Not entirely true... with a 4 wheel omniwheel design you can focus at minimum 1/2 your total power in a direction. This is considering that 2 motors power the X direction and 2 motors power the Y direction. Simple trig tells us the maximum we can focus (in a direction of 45 degrees) is 0.70 of all 4 motors. For the 3 wheel omnidirectional design, due to symmetry, you will always exert 0.57 times the total power in any direction. It is possible to increase this to 0.66 times the three motors if you orient the wheels parallel to the Y instead of perpendicular, however this is at the trade-off of losing the ability to control your heading (making this option not really feasible, unless you didn't care what your heading was such as with a saucer-shaped wedge robot).
Keep in mind that with the 4-wheel omnidrectional platform you can have your Y-axis geared for high-speed and your X-axis geared for high-torque without motor fighting.
As said above, with robots drive systems there are three variables to control. Heading, Y-axis velocity, and X-axis velocity.
Tank drive gives control over heading and y-axis velocity, but you can only manipulate one of those at any given moment.
2-wheel / caster drive gives a little bit more control over the two, and you can even do both at the same time for turns up to 45 degrees. However, you cannot make a 90 degree or more turn while maintaining Y-axis velocity.
Swerve/crab drive gives the illusion of omnidirectionibility due to the ability to change the robot heading without changing robot orientation. Still, you only have control over robot heading and Y-axis velocity. Although, you have the ability to manipulate both at the same time.
3-wheel/4-wheel holonomic drive systems have true capability to manipulate all 3 at once, as Jeff said above. It's possible to change robot heading/orientation while maintaining a constant velocity in a certain direction.