The Vexmen of Brandywine Robotics

Blog Post

Exploring the New Vex Mecanum Wheels

Have you seen the new Mecanum wheels from Vex? They are a bit different from the normal Vex Omniwheels in that they are on 45 degree offset angles. So this way you can scrap that four wheel base and switch to a holonomic drive with a change of the wheels (as long as you are in four wheel drive that is).

Supposedly you get a bit better getting pushed-around protection with these wheels than normal omni wheels but i am not entirely convinced without seeing them in action myself.
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Think of the mecanum wheels as high performance tires like you’d find on a Corvette or a Ferrari. If you were to look at some high performance tires like this Goodyear Eagle F1 tire. Notice the tread pattern is not the same from left to right. You need to mount these with the big tread part on the outside and the tight part on the inside. This helps with cornering mainly as well as being highly effective at removing more money from your wallet.

If you don’t mount these tires right, you will get the opposite effect and either wobble a bit or drift one direction. These different treats are just a relatively small set of forces left to right. The mecanum wheels take it to a whole new level. The 45 degree angles of the rollers can have the robot drift in a 45 degree angle if pushed from the side. So that’s why the mecanum wheels need to be different orientations.

As we showed in the choosing your robot’s drive post, we looked at the holonomic drive. The wheels drive the robot in the X or Y directions based upon the power you send to the wheels. The rollers are at 90 degree offsets in the Omniwheel configuration.

Say you want to go froward. As you spin the wheels, some force is going to be pushing in the X direction and some is in the Y direction. Combining different spins directions of the wheels offsets these forces in the X direction to drive you forward along the Y axis. The free spin of the omni wheel allows the robot to not grind to a halt.
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So let’s take a closer look in this document at the new mecanum wheels. This overview written by FRC team 357 talks about the mecanum wheels in quite a bit of detail Mecanum Wheel Overview. It shows how the forces are in the X and Y axis just like the regular omnis in the holonomic 45 degree offset. If you need an overview of vectors and how to draw forces as vectors please come ask, we can show you how engineers draw forces using vectors and then apply some math using algebra and trigonometry to figure out what’s going to happen. Maybe we’ll show the math in detail in another post because we’ll definitely need that before showing you all how to use the new Vex accelerometer.
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Now programming these wheels is pretty much just like the 45 degree offset holonomic drive we showed above. It seems weird but the wheels are planted at 45 degrees on the floor where it touches. This link at robotc.net has some holonomic code used for the mecanum wheels as well as some more pictures. Check them out here: http://www.robotc.net/blog/2011/11/22/vex-mecanum-drive-using-robotc/..