Has the IMU sensor to be located in the center of the wheels?

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There's nothing set in stone anywhere where you need to place any particular sensor. You can place it where ever you like. If you're trying to integrate some other stuff into your IMU (i.e. fuse orientation) then creating a IMU link in TF is probably necessary so another node can transform that data with respect to your base.

However you'd need to do that either way to set things up correctly so there's no additional overhead to placing your IMU in any given location. If you use an IMU that produces an orientation vector out, rigid body transformations tell you the orientation of a part is the orientation of the whole, so it actually doesn't matter at all where it is. If you're integrating angular velocities it may depending on the type of algorithm you're using.

tl;dr: no, you can put it anywhere. Just define the appropriate frame.