Freewheeling with Multi-Wheels
For those uninitiated in the world of omnidirectional wheeling, it can be a mind-bending experience to watch. Let's watch, shall we?
To be clear, multi-wheeled systems like this are not that new. The first such system was created by Swedish inventor Bengt Ilon in 1973, and eventually found its way onto the cramped decks of ships. The chief advantages of wheeling systems like this are true maneuverability in all directions, including rotation, and low friction while doing so. They truly exhibit true 3-degrees of freedom.
A vehicle with tank treads, by comparison, really only has two: rotation and one direction movement. Additionally, treadded vehicles use the 'skid-steer' technique, which involves quite a bit of friction and will tear up your lawn.
A Honda Civic (or an Ferrari for that matter) typically only have only 1 1/2 degrees of freedom. They have directional movement going for them, but the ability to rotate actually depends on forward or backward motion happening. The rotation dimension is kinda' rolled right into the directional dimension.
How in the world??
How do these things work? How do I get a grip on how they move? Fortunately, like many things in the universe, it is not actually too complicated if you think about it the right way... using physics. In this case, vectors.
If you consider an ordinary rubber wheel, the wheel can be seen to move by pushing against the ground. This push in terms of the physics involved can be called a mechanical force, which has a magnitude (how hard of a push?) and a direction. The main difference between ordinary rubber wheels and multi-directional wheels is this: If you park an ordinary wheel, friction holds it on the spot, while multi-directional wheels are free to slide along some direction. Like ordinary rubber wheels, multi-directional wheels exert force against the ground by rotating. Unlike ordinary wheels, however, they are free to slide in the directions perpendicular to the forces they exert.
There are two main types of multi-directional wheels I have seen:
Here is a little diagram that shows how these wheels extert force against the ground, and how they are free to slide when parked (or rolling for that matter!):
These two systems operate slightly differently, but they use the same concept of vector-addition to determine how the vehicle moves. When vectors get added together, you add not only the magnitude but also the direction. These wheeled systems essentially add the forces of all of the wheels, and the sum of the forces determine what is happening to the vehicle's body.
Mecanum wheels are the first kind, the ones invented by Bengt Ilon in 1973. A mecanum wheel is a big wheel made of smaller wheels that have all their axis of rotation set at 45 degrees off of parallel from the axis of the big wheel at large. Mecanum vehicles usually have four such wheels and have them oriented as such:
This video (which bears the company logo) does a great job of demonstrating all of the modes of movement for a mechanum system:
The Omni-wheels are big wheels made of smaller wheels that have all their axis of rotation perpendicular to the axis of the big wheel at large. Vehicles that use them usually have three or four of them, and they orient them like this:
This video does a good job demonstrating its modes of movement.
It's important to note that, though each wheel in a multi-directional system is free to slide in some direction when parked, a parked multi-directional vehicle is anchored quite firmly. That is to say that the sum of the friction forces of the parked wheels will not let the total vehicle budge. This makes it great in terms of control. These systems are great for maneuvering and parking things!
I hope you've enjoyed my little ad-hoc tour of multi-directional wheeling. If this isn't enough, here's another type of omnidirectional drive for you to ponder: