# Bt Condition Nodes Nav2 > --- name: bt-condition-nodes-nav2 description: 'Use Nav2 BT condition nodes. Use when checking GoalReached, GoalUpdated, IsBatteryLow, TimeExpired, or IsPathValid conditions.' --- # Nav2 Built-in Condition BT Nodes Condition nodes are leaf nodes that return **SUCCESS** or **FAILURE** only—never RUNNING. They inspect the world state and are typically placed as the first child of a `ReactiveSequence` or `ReactiveFallback` to gate action execution. ## GoalUpdated **The most critical condition node for replanning behavior trees.** Returns SUCCESS if the navigation goal has been updated since the last tick. Used in recovery sub-trees to short-circuit recovery when a new goal arrives. ```xml ``` No ports. Reads from the internal goal tracking in the BT navigator. This node is essential inside recovery fallbacks: ```xml ``` When `GoalUpdated` returns SUCCESS, the ReactiveFallback immediately returns SUCCESS without executing any recovery actions, allowing the main navigation branch to re-plan with the new goal. ## GoalReached Returns SUCCESS if the robot is within tolerance of the current goal. Uses the goal checker plugin's tolerance values: ```xml ``` | Port | Direction | Type | Description | |---------------|-----------|-------------|--------------------------| | goal | input | PoseStamped | Goal to check against | | server_name | input | string | Navigator server | | server_timeout| input | int (ms) | Timeout | Typically used to terminate navigation loops early. ## IsBatteryLow Subscribes to a `sensor_msgs/msg/BatteryState` topic and returns SUCCESS when the battery percentage is below a threshold: ```xml ``` | Port | Direction | Type | Description | |----------------|-----------|--------|--------------------------------------------------| | battery_topic | input | string | Topic publishing BatteryState | | min_battery | input | double | Threshold (0.0–1.0 for percentage, or voltage) | | is_voltage | input | bool | If true, compare against voltage instead of % | Returns SUCCESS when battery is LOW (below threshold). This is a condition to trigger charging behavior: ```xml ``` **Note the inversion**: SUCCESS means the battery IS low. To use it as a "battery OK" guard, wrap it in an Inverter: ```xml ``` ## IsPathValid Checks if a stored path is still traversable by querying the costmap. Returns FAILURE if any point on the path is now in collision: ```xml ``` | Port | Direction | Type | Description | |---------------|-----------|------|--------------------------------| | path | input | Path | Path to validate | | server_name | input | string | Planner server for validation | | server_timeout| input | int | Timeout | Used in a ReactiveSequence to detect when obstacles invalidate the current path, triggering replanning: ```xml ``` If an obstacle appears on the path, `IsPathValid` returns FAILURE, halting `FollowPath` and propagating failure up the tree where a recovery or replanning branch handles it. ## DistanceTraveled Returns SUCCESS when the robot has moved at least `distance` meters since this node was last reset. Used to trigger periodic replanning: ```xml ``` | Port | Direction | Type | Description | |-------------------|-----------|--------|------------------------------| | distance | input | double | Distance threshold in meters | | global_frame | input | string | Global reference frame | | robot_base_frame | input | string | Robot base frame | Common pattern—replan every 2 meters: ```xml ``` ## TimeExpired Returns SUCCESS when `seconds` have elapsed since this node was first ticked: ```xml ``` Use for periodic operations. Note: the timer resets when the parent node resets this child (e.g., after the enclosing Sequence restarts). ## TransformAvailable Returns SUCCESS if a TF transform between two frames is available: ```xml ``` Use in startup sequences to wait for localization to initialize before attempting navigation. ## InitialPoseReceived Returns SUCCESS once an initial pose has been set (e.g., via RViz or the `/initialpose` topic). Useful for gating navigation until localization is bootstrapped: ```xml ``` ## IsBatteryCharging Returns SUCCESS when the robot is currently charging. Subscribes to the same `BatteryState` topic: ```xml ``` Use to hold the robot at the charging station: ```xml ``` ## Custom Condition Patterns ### GlobalUpdatedGoal A custom condition (not built-in) that detects if the global goal has changed from an external replanning request versus user input. Implement by comparing the current goal blackboard entry against a cached value: ```xml ``` ### PathExpiringTimer A custom condition that tracks path age and triggers replanning: ```xml ``` ## Usage Pattern Summary The typical condition placement in Nav2 trees follows this hierarchy: ```xml ``` Conditions placed earlier in a ReactiveSequence have **higher priority**—they are checked first on every tick and can preempt later children.