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State Machines

State machines are a powerful way to manage complex robot behaviors by breaking them down into distinct states and defining transitions between those states. Every subsystem is a state machine, and the robot itself is one more state machine on top of them: the super state machine in RobotContainer. Buttons and autonomous routines ask the robot for a state, and the robot asks each subsystem for the state it needs. Anything that uses more than one subsystem is written once, in one place.

Presentation​

Do this lesson in Scriptum

The exercises below are the State Machines lesson in Scriptum. Open the lesson, start the simulated robot and follow its README. Each step is checked with the Verify button.

New Java in this lesson: &&, || and !, boolean helper methods, and lambdas.

Exercises​

State Machines​

  • Write a State Machine in ShooterSubsystem
    • IDLE State: System idle
    • SPIN_UP State: Flywheel spinning up to desired speed
    • SHOOT State: Index balls into flywheel
    • Add an automated transition from SPIN_UP -> SHOOT when the flywheel is at speed
    • Add an automated transition from SHOOT -> SPIN_UP when the flywheel slows down
    • Prevent IDLE -> SHOOT transition

The Robot's State Machine​

  • RobotContainer extends MwSuperStateMachine<RobotStates, RobotConstants>, with the robot states IDLE, DRIVE and SHOOT
    • In updateLogic(), write a switch on the robot's state that asks the drivetrain and the shooter for the state each robot state needs
    • Robot.java asks for IDLE when disabled and DRIVE when teleop starts
  • Bind a button in OI so the robot wants SHOOT while it is held and DRIVE when it is let go
    • Commands.startEnd(() -> robot.setWantedState(RobotStates.SHOOT), () -> robot.setWantedState(RobotStates.DRIVE))
  • Bind a button to a Commands.runOnce(...) that simulates the load from launching a ball
    • Call the shooter's simulateBallLaunch(), which you used in Control Theory

There are no command files: buttons only ask the robot for a state.

Software​

  • State Machines: Basics of state machines, and the robot's super state machine.
  • Subsystems: Overview of subsystem-based programming in FRC robots.
  • Commands: How buttons and autos ask the robot for states.