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docs(failsafe): clarify Offboard proof-of-life and COM_OBL_RC_ACT docs (#27636)
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Sijia
@@ -257,14 +257,13 @@ Loss of a single GPS when none are required is handled by other GPS health check
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## Offboard Loss Failsafe
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The _Offboard Loss Failsafe_ is triggered if the offboard link is lost while under [Offboard control](../flight_modes/offboard.md).
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Different failsafe behaviour can be specified based on whether or not there is also an RC connection available.
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The relevant parameters are shown below:
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| Parameter | Description |
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| -------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------- |
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| [COM_OF_LOSS_T](../advanced_config/parameter_reference.md#COM_OF_LOSS_T) | Delay after loss of offboard connection before the failsafe is triggered. |
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| [COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT) | Failsafe action if RC is available: Position mode, Altitude mode, Manual mode, Return mode, Land mode, Hold mode. |
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| Parameter | Description |
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| -------------------------------------------------------------------------- | ------------------------------------------------------------------------- |
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| [COM_OF_LOSS_T](../advanced_config/parameter_reference.md#COM_OF_LOSS_T) | Delay after loss of offboard connection before the failsafe is triggered. |
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| [COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT) | Flight mode to switch to if offboard control is lost. |
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## Traffic Avoidance Failsafe
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@@ -17,15 +17,15 @@ A good understanding of [PX4 controller diagrams](../flight_stack/controller_dia
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The vehicle obeys position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints provided by some source that is external to the flight stack, such as a companion computer.
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The setpoints may be provided using MAVLink (or a MAVLink API such as [MAVSDK](https://mavsdk.mavlink.io/)) or by [ROS 2](../ros2/index.md).
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PX4 requires that the external controller provides a continuous 2Hz "proof of life" signal, by streaming any of the supported MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) message.
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PX4 enables switching to offboard control mode only after receiving the signal for more than a second, and will failsafe (controlled by [COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT)) if the signal stops.
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PX4 requires that the external controller provides a continuous "proof of life" signal by streaming any of the supported MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) message.
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The stream should be active before switching to Offboard mode, and PX4 will trigger the configured Offboard-loss failsafe action ([COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT)) if proof-of-life messages are not received within the timeout configured by [COM_OF_LOSS_T](#COM_OF_LOSS_T).
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::: info
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- This mode requires position or pose/attitude information - e.g. GPS, optical flow, visual-inertial odometry, mocap, etc. depending on the type of offboard setpoints that the external controller sends.
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- Manual control is disabled except to change modes (you can also fly without any manual controller at all by setting the parameter [COM_RC_IN_MODE](../advanced_config/parameter_reference.md#COM_RC_IN_MODE) to `4: Disable manual control`).
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- The vehicle must already be receiving a stream of MAVLink setpoint messages or ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) messages before arming in offboard mode or switching to offboard mode when flying.
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- The vehicle will exit offboard mode if MAVLink setpoint messages or `OffboardControlMode` are not received at a rate of > 2Hz.
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- The vehicle will exit Offboard mode if MAVLink setpoint messages or `OffboardControlMode` messages stop being received for longer than the timeout configured by [COM_OF_LOSS_T](#COM_OF_LOSS_T).
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- Not all coordinate frames and field values allowed by MAVLink are supported for all setpoint messages and vehicles.
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Read the sections below _carefully_ to ensure only supported values are used.
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@@ -34,10 +34,9 @@ PX4 enables switching to offboard control mode only after receiving the signal f
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## Description
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Offboard mode is used for controlling vehicle movement and attitude, by setting position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints.
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PX4 must receive a stream of MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) at 2 Hz as proof that the external controller is healthy.
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The stream must be sent for at least a second before PX4 will arm in offboard mode, or switch to offboard mode when flying.
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If the rate falls below 2Hz while under external control PX4 will switch out of offboard mode after a timeout ([COM_OF_LOSS_T](#COM_OF_LOSS_T)), and attempt to land or perform some other failsafe action.
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PX4 must receive a stream of MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) message as proof that the external controller is healthy.
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The stream should be active before PX4 arms in Offboard mode or switches to Offboard mode when flying.
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If MAVLink setpoint messages or `OffboardControlMode` messages stop being received for longer than the timeout configured by [COM_OF_LOSS_T](#COM_OF_LOSS_T), PX4 will switch out of Offboard mode and attempt to land or perform some other failsafe action.
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The action depends on whether or not RC control is available, and is defined in the parameter [COM_OBL_RC_ACT](#COM_OBL_RC_ACT).
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When using MAVLink the setpoint messages convey both the signal to indicate that the external source is "alive", and the setpoint value itself.
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@@ -358,7 +357,7 @@ _Offboard mode_ is affected by the following parameters:
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| Parameter | Description |
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| -------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
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| <a id="COM_OF_LOSS_T"></a>[COM_OF_LOSS_T](../advanced_config/parameter_reference.md#COM_OF_LOSS_T) | Time-out (in seconds) to wait when offboard connection is lost before triggering offboard lost failsafe (`COM_OBL_RC_ACT`) |
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| <a id="COM_OBL_RC_ACT"></a>[COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT) | Flight mode to switch to if offboard control is lost (Values are - `0`: _Position_, `1`: _Altitude_, `2`: _Manual_, `3`: _Return_, `4`: _Land_). |
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| <a id="COM_OBL_RC_ACT"></a>[COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT) | Flight mode to switch to if offboard control is lost. |
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| <a id="COM_RC_OVERRIDE"></a>[COM_RC_OVERRIDE](../advanced_config/parameter_reference.md#COM_RC_OVERRIDE) | Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to [Position mode](../flight_modes_mc/position.md). This is not enabled for offboard mode by default. |
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| <a id="COM_RC_STICK_OV"></a>[COM_RC_STICK_OV](../advanced_config/parameter_reference.md#COM_RC_STICK_OV) | The amount of stick movement that causes a transition to [Position mode](../flight_modes_mc/position.md) (if [COM_RC_OVERRIDE](#COM_RC_OVERRIDE) is enabled). |
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| <a id="COM_RCL_EXCEPT"></a>[COM_RCL_EXCEPT](../advanced_config/parameter_reference.md#COM_RCL_EXCEPT) | Specify modes in which RC loss is ignored and the failsafe action not triggered. Set bit `2` to ignore RC loss in Offboard mode. |
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