My team and I were wondering about the Holonomic Drive Train with Omni Wheels…
Do we need 2 Omni wheels per edge? That would be 8 wheels per robot, wouldn’t it be?
Is it better to go with mecanum wheels instead?
8 Omni Wheels are like 240 dlls, and a full set of 4 mecanums are just 100 dlls…
You can use as few as three omni wheels in total for a holonomic drivetrain, in a style called kiwi drive. This can be complicated for a number of reasons, namely that it works best with a triangular frame. You should only need four omni wheels in total for a more traditional holonomic drive, each one mounted at a 45 degree angle relative to a square or rectangular frame. I would recommend looking at pictures of holonomic drivetrains posted in various threads from past years.
The reason you see what looks like two sets of omni wheels is to ensure each corner is for stability; with a set of single omni wheels, the robot would have a very bumpy drive that may not always slide consistently. Teams used to solve this issue by using two wheels per corner, mounted so that the rollers were offset from each other and would therefore maintain fairly even pressure on the ground.
However, AndyMark and VEXpro both offer ‘double’ omni wheels (in fact, I believe VEXpro only sells doubles), which is a single wheel that has two sets of rollers to achieve the same effect. For FTC use, I’d recommend VEXpro’s omni wheels or AndyMark’s DuraOmnis.
@Aaron_Li, I think the OP was specifically thinking of a holonomic X drive (4 omnis at 45 degrees on each corner) rather than 118’s H drive or any of the other holonomic drive systems.
@Mau, one of our FTC teams (FTC 731) used X drive for Velocity Vortex with great success. The wheels were direct driven off of AM Never-rest 20s and it was very zippy and maneuverable. I would not recommend the Never-rest 20s for this application as the gearbox is not strong enough for a drive system. But the newer Orbital 20s are plenty strong. X drive is a pretty good choice for FTC (in some ways, better than mechanum).
I would not recommend it for FRC as it is really easy to get pushed around by defenders.
As others have mentioned, the dual wheels on each corner are really to smooth out the ride so that you always have one roller solidly touching the mat. With the little bit of compressoin that the field has, using a single omni wheel on each corner will cause the non-roller part of the wheel to come in contact with the ground and it will make the drive sticky. Also, the ride gets pretty bumpy as you roll.
If you are looking for FTC omni-wheel options for an X drive, the 4" duraomni wheels have the dual rows of rollers already built in. Vex has 4" dual roller omnis as well but you will need to get hubs to adapt them to FTC motors/shafts. GoBilda offers single row 96mm omnis where you would need to mount 2 on each corner, but they are pretty cheap, so the cost is not really all that bad.