Package/ program to run in ROS for raspberry pi of robot

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This is not what rosserial is designed for, you'll need to use a different approach.

The way to solve your problem using ROS is not to connect to the PIs USB port from your PC but to run a ROS node on the PI that collects the sensor data you need, then publishes via a topic to other nodes (and computers) on the network.

If you follow the python publisher tutorial you'll see how to publish data to a ROS topic. If you combine this code with the code you're using now to read data on the PI then you can publish the data on a topic to all ROS nodes.

Hope this helps.