All this really means is that if you are trying to execute any omnidirectional drive other than swerve, you will have to use omni wheels or the likes of them. However with the swerve drive you can use any wheel you want: roughtop, wedgetop, rubber, plastic, etc. This is how you can have better traction than other holonomic drives.
It kinda refers back to the point of “the wheels have to slip for the system to move” under the holonomic drive column on that same page.
I did not contribute to this presentation but I was involved in the brainstorming, choosing, and testing of this project. This point came up a lot when we were discussing what type of holonomic drive to test.
When you say “the wheels have to slip for the system to move” what do you mean by that? Are you referring to the rollers spinning, or are you referring to slipping between the floor and wheel contact surfaces?
If the latter, where did your team get that understanding?
sorry i didnt notice this was already responded to, but this explains the mechanum and holonomic drive situation in a little more depth if needed.
Its both actually. This understanding comes from any holonomic drive that uses omni wheels. Each wheel has 2 directions it wants to go. The way the system goes one direction specifically is by each direction canceling out with an opposite direction from the adjacent wheel except the direction it will travel. You can only achieve this type of drive that uses this implementation with wheels that slide easily or omni wheels.
A mechanum or a holonomic drive where the wheels are angled in the four corners in a diamond pattern for example:
the front right wheel wants to go forward and right
the front left wants to go forward and left
the back right wheel wants to go forward and right
the back left wheel wants to go forward and left
This results in the system moving forward because the lefts and rights canceled out. To be able to cancel out directions like this, you have to use a very low friction wheel or omni wheel. In theory, if you have wheels that slide very easily, they will slip on the floor and move the robot. I have never actually seen this done, as all the holonomic drives I’ve seen use omni wheels.
A mecanum vehicle does not require low traction tread surface on the rollers in order to move sideways. The sideways motion of the vehicle is accomplished by the combined action of the wheels turning and the rollers rolling. It does not depend on “sliding” at the roller-to-floor contact interface.
If you have access to a good university library, an excellent discussion of this is provided in chapter 3 of Introduction to Autonomous Mobile Robots by Siegwart and Nourbakhsh. You can also probably get the book via inter-library loan through your local community library.
… which derives the inverse kinematic equations which show how to properly program a 4-wheel mecanum vehicle’s individual wheel speeds in order to accomplish any desired combination of simultaneous vehicle motions (fore/aft; sideways; rotation) without scrubbing the wheels against the floor.
An explanation why a mecanum vehicle has less traction than a standard-wheel vehicle (even if both vehicles have the same tread material) is given here:
Thank you for the response and questions. Matt and Chris have pretty well captured our team’s thoughts going into the decision making process. Our largest reason for not selecting a holonomic or mecanum drive for the fall project was the loss of pushing force commonly experienced by robots with those drive systems.
I haven’t been able to read the links you posted (dang job…), but will put them on my to-do list.
Thanks again for the comments. I’ll get back to you once I am able to read your links.