State Machine

The state machine node (state_machine_node.py) is the central safety and control authority for the kart. It determines what drives the wheels by muxing cmd_vel based on two independent variables: the mission (what the kart should do) and the AS state (whether the autonomous system is allowed to drive).

Source: src/kart_bringup/scripts/state_machine_node.py

Two Independent Variables

Mission

The mission is selected from the dashboard. It determines the operating mode:

Mission	Type	Description
`manual`	Manual	Direct control via joystick/dashboard
`remote_control`	Manual	Same as manual (dashboard remote)
`throttle_test`	Test	Fixed 50% throttle for hardware debugging
`acceleration`	Autonomous	FS acceleration event
`skidpad`	Autonomous	FS skidpad event
`autocross`	Autonomous	FS autocross event
`trackdrive`	Autonomous	FS trackdrive event
`ebs_test`	Autonomous	Emergency braking system test
`inspection`	Autonomous	Technical inspection demo

AS State (Autonomous System State)

The AS state tracks the autonomous system lifecycle per Formula Student rules (T14.8):

State	ID	Description
`AS_OFF`	0	Autonomous system inactive
`AS_READY`	1	Armed and waiting for "Go" signal
`AS_DRIVING`	2	Autonomous driving active
`AS_FINISHED`	3	Mission complete, vehicle stopped
`AS_EMERGENCY`	4	Emergency braking activated

cmd_vel Mux Logic

The state machine selects which velocity command reaches the actuators:

Mission	AS State	Output	Why
`manual` / `remote_control`	any	Manual cmd_vel	Operator has direct control
`throttle_test`	any	Fixed 50% throttle	Hardware test, no perception
Any autonomous	`AS_DRIVING`	Autonomous cmd_vel	Controller drives the kart
Any autonomous	anything else	Zero (stopped)	Safety: no motion unless AS_DRIVING

Key insight

AS state only matters for autonomous missions. Manual and throttle_test bypass it entirely -- the operator is always in control.

State Transitions

stateDiagram-v2
    classDef off fill:#6c757d,color:#fff
    classDef ready fill:#ffc107,color:#000
    classDef driving fill:#28a745,color:#fff
    classDef finished fill:#17a2b8,color:#fff
    classDef emergency fill:#dc3545,color:#fff

    [*] --> AS_OFF

    AS_OFF --> AS_READY : Select autonomous mission
    AS_READY --> AS_DRIVING : "start" command
    AS_DRIVING --> AS_FINISHED : "finish" command
    AS_DRIVING --> AS_READY : "stop" (auto mission)
    AS_READY --> AS_OFF : Switch to manual

    AS_FINISHED --> AS_OFF : "reset" command
    AS_EMERGENCY --> AS_OFF : "reset" command

    AS_READY --> AS_EMERGENCY : "ebs" command
    AS_DRIVING --> AS_EMERGENCY : "ebs" command
    AS_FINISHED --> AS_EMERGENCY : "ebs" command

    AS_DRIVING --> AS_READY : Select new auto mission
    AS_FINISHED --> AS_READY : Select new auto mission

    class AS_OFF off
    class AS_READY ready
    class AS_DRIVING driving
    class AS_FINISHED finished
    class AS_EMERGENCY emergency

Transition Details

Trigger	From	To	Notes
Select autonomous mission	`AS_OFF`, `AS_DRIVING`, `AS_FINISHED`	`AS_READY`	Auto-arms the system
Switch to manual	Any (except `AS_OFF`)	`AS_OFF`	Fully disarms
`start`	`AS_READY`	`AS_DRIVING`	Begin autonomous driving
`stop` (auto mission active)	`AS_READY`, `AS_DRIVING`, `AS_FINISHED`, `AS_EMERGENCY`	`AS_READY`	Stays armed
`stop` (manual mission)	Any	`AS_OFF`	Fully disarms
`ebs`	Any (except `AS_OFF`)	`AS_EMERGENCY`	Emergency braking
`finish`	`AS_DRIVING`	`AS_FINISHED`	Mission complete
`reset`	`AS_FINISHED`, `AS_EMERGENCY`	`AS_OFF`	Clear error/completion

ASSI (Autonomous System Status Indicator)

The ASSI communicates the AS state visually, as required by FS rules:

State	ASSI Signal
`AS_OFF`	Off
`AS_READY`	Yellow continuous
`AS_DRIVING`	Yellow flashing (2-5 Hz)
`AS_FINISHED`	Blue continuous
`AS_EMERGENCY`	Blue flashing (2-5 Hz)

Formula Student Competition Context

These states map to the FS T14.8 AS Status (Figure 15 in FS Rules 2026). See also the rules reference for the full FS decision tree and hardware requirements.

In a real competition, additional checks gate each transition:

AS_READY requires: mission selected + ASMS on + ASB checks OK + TS active + brakes engaged
AS_DRIVING (R2D) is triggered only via the RES (Remote Emergency System) "Go" signal, after 5 seconds in AS_READY
The vehicle must not move until 3 seconds after entering AS_DRIVING

Current limitations

We simulate the RES "Go" signal with the dashboard "start" button since we don't have RES hardware yet. The 5-second AS_READY hold time and the 3-second AS_DRIVING standstill requirement are not yet implemented.

ROS Topics

Subscriptions (inputs)

Topic	Type	Source	Purpose
`/dashboard/mission`	`String`	Dashboard	Mission selection
`/dashboard/state_cmd`	`String`	Dashboard	Commands: start, stop, ebs, finish, reset
`/kart/cmd_vel`	`Twist`	Controller	Autonomous velocity command
`/kart/cmd_vel_manual`	`Twist`	Joystick / Dashboard	Manual velocity command

Publications (outputs)

Topic	Type	Rate	Purpose
`/kart/cmd_vel_muxed`	`Twist`	100 Hz	Final muxed velocity command
`/kart/state`	`String`	10 Hz	Current AS state name (heartbeat)
`/orin/machine_state`	`Frame`	On change	AS state ID to ESP32
`/orin/mision`	`Frame`	On change	Mission ID to ESP32
`/orin/steer_mode`	`Frame`	On change	Steering mode (PID/PWM) to ESP32

Implementation Notes

The node runs two timers:

100 Hz mux timer (_mux_tick): Reads the current mission and state, selects the appropriate cmd_vel source, and publishes to /kart/cmd_vel_muxed.
10 Hz heartbeat (_publish_state): Publishes the current state name so other nodes and the dashboard can monitor it.

State transitions are triggered by ROS topic callbacks, not by the timers. Each transition logs the change and publishes the new state to both ROS topics and ESP32 frames.

When an autonomous mission is selected, the node forces PID steering mode via /orin/steer_mode to ensure the ESP32 uses closed-loop steering control.