How to setup and control the dynamixels for the three omni-wheels?

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I will suggest you to checkout Dynamixel WorkBench to familiarize yourself. After that checkout Dynamixel SDK that will show you how to command your motors. There are very good examples given already. I think they are more than enough atleast to move a robot and an example(Python and CPP) how to use dynamixel motors and SDK.

Calculate robots velocity commands. A nice tutorial can be found here and its corrresponding ros example.

Now just connect step 1 and 2 and your robot should move.