I’ve seen those special drive systems some teams have and i dont seem to understand how they work.
Im talking about those robots that can go back and forth and right left. withou turning, they can just drive to 4 different directions, using those special wheels…
How does it work?
its called holonomic drive. they use a set of four indipendently driven - angularly oposing wheels that have roleres on them that allow the wheels to slip sideways. or in the case of mecanum wheels diagonally.
there is also swerve AKA Crab Drive it uses wheels mounted on independntly steered modules that can be steered in 360 degres.
see http://www.chiefdelphi.com/media/photos/tags/mecanum
But how do u ge those rollers to slid?
I meat, the wheel can only go back and forth, so what PWMs do u give those motors in order to make the wheels slide on those rollers?
here’s a good explanation by ian
The rollers are a passive feature of the wheels. So, if you get pushed, you will simply slide that direction. Mecanum drives and mechanum wheels deal more with diagonal forces ( imagine the linear motion of a screw, it helped me). Holonomic drives are far easier to push because omni wheels slide so much. Mecanum is better in that aspect, but i’d certainly advise talking to a team who has done it before pursueing such options. We did a holonomic, which was sweet, but it definitely posed a lot of challenges.
There are 3 popular styles of holonomic drivetrains:
Omni-wheel/Trick wheel drive:
This drive type uses trick wheels that are typically aligned perpendicular to each other. Each trick wheel allows unrestricted motion in one direction, and can be powered in the other direction. Most teams have 2 wheels driving the robot forward and two driving it sideways, though some three wheel drives, with 2 driving forward and one driving sideways are commonly seen too. Though I haven’t seen a robot in FIRST with triangular omnis, they can be found outside of FIRST. (Each wheel is mounted at the point of a triangle, and each is aimed 120 degrees to the others)
Mecanum wheels:
Mecanum wheels are similar to omni/trick wheels, because they have rollers mounted around the edges. On mecanum wheels, however, the rollers are angled, rather than perpendicular to the wheel. A mecanum wheel is driven in one direction, and allows free movement at 45 degrees to the driven direction.
Mecanum wheels work on the same principle that roller skates work on. In a pair of roller skates, to drive yourself forward, you turn the skates at an angle and push them sideways. Similarly, in mecanum wheels, you turn the rollers at an angle, and push them forward or backward. Each wheel on a side of a mecanum drivetrain has oppositely angled rollers - one will have the rollers pointing in, one will have them pointing out. By driving your front wheel forward, and your back wheel backward, you force your robot to go sideways. Otherwise, the driving is very much like standard tank drive.
Swerve or Crab:
This is a more difficult holonomic drivetrain to build. In Swerve or Crab drive, each wheel is powered on its own, and each wheel can be steered independently. The difficulty of this drivetrain lies in the choices you have to make: How many wheels you want to drive (swerve/crab can be done with 2 driven wheels and a few stabilizing casters), the range of motion of each wheel (should each wheel be able to spin freely, or will there be a limit?), and how you will steer each wheel.
I believe The HOT team 67 did it in 2005, it wasnt actually having each wheel aimed at 120degrees but was arranged this way.
The HOT team used a three wheeled crab drive in 2005 on their road to a championship. Note that this was likeyl one of the most competitive championship series that i have ever seen.
And what do u guys think about this kind of system?
Ever seen it? (sorry for the lame illustration but i think youll get it right) =]
http://img407.imageshack.us/img407/9923/untitledbh3.jpg
That’s a basic holonomic drive with wheels mounted at 90º. A number of FIRST teams have used it, including 116 (2005), 1541 (2006), and 422 (2007).
Also, swerves do not HAVE to have each wheel powered individually. A “co-axial swerve” has all 4 wheels powered together, then steered in pairs.
As Lil’ Lavery said, it is possible to steer pairs of wheels, but to me it seems like this would be similar to keeping the 2 front wheels on your car parallel. It works, just not as well as it could, and you’re going to lose some energy and traction.
I’d like to see a mechanical solution to this problem, but it’d be pretty tricky (When you’re turning your front wheels to the left, sometimes you _do_want them parallel)
in 2002, our team (343) had a similar base, but with 6 omni’s. two on each side, and one on the front and back. i would have to say that it worked very well.