What are the steps to create complex robot behaviors and state machines in ROS?

1 Nodes and topics

Nodes are the basic units of computation in ROS, and they communicate with each other by sending and receiving messages on topics. Topics are named channels that allow nodes to exchange information without knowing each other's identities or locations. For example, a node that controls a camera can publish images on a topic called /camera/image, and a node that performs object detection can subscribe to that topic and process the images. To create complex robot behaviors, you need to design your nodes and topics in a way that they can perform the tasks and functions that you want, and that they can coordinate with each other through messages.

2 Services and actions

Sometimes, you may need to request a node to perform a specific operation and return a result, or to execute a long-running task and provide feedback and cancellation options. For these cases, ROS offers services and actions. Services are synchronous communication channels that allow a node to call another node and wait for a response. Actions are asynchronous communication channels that allow a node to send a goal to another node and monitor its progress and status. For example, a node that controls a robot arm can offer a service to move the arm to a given position, and an action to follow a trajectory. To create complex robot behaviors, you need to use services and actions when you need to invoke or monitor operations that have a clear start and end point.

3 Parameters and dynamic reconfigure

Parameters are global variables that store configuration values for nodes and topics. They are stored on a parameter server, which is a special node that allows other nodes to read and write parameters. Parameters can be used to set or modify the behavior of nodes and topics at runtime, such as changing the frame rate of a camera or the speed of a motor. Dynamic reconfigure is a tool that allows you to change parameters interactively using a graphical interface. To create complex robot behaviors, you need to use parameters and dynamic reconfigure when you need to adjust or tune the parameters of your nodes and topics without restarting them.

4 State machines and SMACH

State machines are a way of modeling the logic and transitions of robot behaviors. A state machine consists of a set of states, each representing a condition or an action, and a set of transitions, each representing a change from one state to another based on some events or conditions. For example, a state machine for a robot that delivers packages can have states like idle, navigate, pick, deliver, and drop, and transitions like start, arrived, grasped, delivered, and dropped. SMACH is a Python library that allows you to create and execute state machines in ROS. It integrates with nodes, topics, services, actions, and parameters, and provides visualization and introspection tools. To create complex robot behaviors, you need to use state machines and SMACH when you need to define and control the flow and logic of your robot behaviors.