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Every robot is going to have a unique way of handling the conversion from Twist message into motor commands. It depends on the geometry of the robot, the motor controllers, what feedback is available, what type of motor is involved, the existence of APIs, etc. So it is difficult to give a general answer for how to do this.

For wheeled robots, likely the biggest thing that is needed is an understanding of the kinematics of the robot. For a differential drive robot (like a TurtleBot), if we have a desired forward and angular body velocity (the data contained in the Twist), there is a unique mapping that tells us what translational velocities each of our wheels needs (assuming we know how far apart the wheels are). Then if we also know the diameter of the wheels, we can uniquely calculate a desired angular velocity for the motor. This information can then be passed onto a motor controller in some fashion.

Let's look at the TurtleBot as an example:

• This callback is the callback that actually listens to the Twist message on the cmd_veltopic. They convert the data in the twist into a tuple representing the translational velocity of each wheel that gets stored as self.req_cmd_vel
• Then at the very end of the TurtlebotNode.spin method they call self.drive_cmd with this tuple of wheel velocities.
• Because we are in "twist control mode", the self.drive_cmd actually calls the Turtlebot.direct_drive method defined in the create_driver script. This method transmits a command to the robot base over a UART connection using PySerial. The protocol for this communication was originally defined by iRobot.

If you are interested in studying the kinematics of mobile robots, I'd recommend looking at Siegwart's Introduction to Autonomous Mobile Robots. Section 3.2 of Correll's Introduction to Autonomous Robots (available as a pdf) has a good walkthrough of diff drive robots and car-like steered robots.