Encoders on mecanum drive?

[Tom Line]

Since doing this is very difficult, you have a couple other options.

You can put a couple unpowered casters with encoders are them to measure location. Or you could use distance sensors on the robot to determine positions from the walls.

Each has it's own issue. The casters may bounce on the bumps and be inaccurate, and different sensors have their own issues with different wall materials and surface finishes.