About ROS

Robot Operating System(ROS) is a framework being developed by ROS.org for writing robot software. You can find tutorials and learn more about ROS in ROS wiki.

We think about programs as follows:

• Sensor A/B: Read sensor values and send them to Controller.
• Controller: Calculate motor speed values from the sensor ones, and send them to Motor.
• Motor: Outputs control signals to motors from the motor speed values.

To realize these programs without ROS, it becomes concern how to send/receive data to/from another program. If you write another program Controller2 instead of Controller and want to swap them, it is necessary to reconnect its data passes.

ROS can help to resolve these problems.

The Model of ROS

The following figure illustrates the implementation of ROS from Sensor A/B to Controller.

The terms in the figure are explained below:

Node : The program with some functions.

Message : Nodes send/receive data as Message. Message can include various data structures such as int32, float64, string.

Topic : Topic is like a bulletin board which receives messages. Each node publishes messages to specific topic, and subscribes topic to receive its messages.

Thanks to them, each node can publish/subcribe messages without worrying about the other node.

The implementation of ROS from Controller to motor operation is illustrated below:

Node Controller subscribes topic /sensors and publishes to topic /motor. If you want to swap Controller and Controller2 which subscribes and publishes the same topic, you just run Controller2 instead of Controller.