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83 lines
5.0 KiB
Markdown
83 lines
5.0 KiB
Markdown
# Control Allocation (Mixing)
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:::info
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Control allocation replaces the legacy mixing approach used in PX4 v1.13 and earlier.
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For PX4 v1.13 documentation see: [Mixing & Actuators](https://docs.px4.io/v1.13/en/concept/mixing), [Geometry Files](https://docs.px4.io/v1.13/en/concept/geometry_files) and [Adding a New Airframe Configuration](https://docs.px4.io/v1.13/en/dev_airframes/adding_a_new_frame).
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:::
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PX4 takes desired torque and thrust commands from the core controllers and translates them to actuator commands which control motors or servos.
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The translation depends on the physical geometry of the airframe.
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For example, given a torque command to "turn right" (say):
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- A plane with one servo per aileron will command one of servo high and the other low.
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- A multicopter will yaw right by changing the speed of all motors.
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PX4 separates this translation logic, which is referred to as "mixing" from the attitude/rate controller.
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This ensures that the core controllers do not require special handling for each airframe geometry, and greatly improves reusability.
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In addition, PX4 abstracts the mapping of output functions to specific hardware outputs.
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This means that any motor or servo can be assigned to almost any physical output.
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<!-- https://docs.google.com/drawings/d/1Li9YhTLc3yX6mGX0iSOfItHXvaUhevO2DRZwuxPQ1PI/edit -->
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## Actuator Control Pipeline
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Overview of the mixing pipeline in terms of modules and uORB topics (press to show full-screen):
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<!-- https://drive.google.com/file/d/1L2IoxsyB4GAWE-s82R_x42mVXW_IDlHP/view?usp=sharing -->
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Notes:
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- The rate controller outputs torque and thrust setpoints
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- the `control_allocator` module:
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- handles different geometries based on configuration parameters
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- does the mixing
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- handles motor failures
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- publishes the motor and servo control signals
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- publishes the servo trims separately so they can be added as an offset when [testing actuators](../config/actuators.md#actuator-testing) (using the test sliders).
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- the output drivers:
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- handle the hardware initialization and update
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- use a shared library [src/libs/mixer_module](https://github.com/PX4/PX4-Autopilot/tree/main/src/lib/mixer_module).
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The driver defines a parameter prefix, e.g. `PWM_MAIN` that the library then uses for configuration.
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Its main task is to select from the input topics and assign the right data to the outputs based on the user set `<param_prefix>_FUNCx` parameter values.
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For example if `PWM_MAIN_FUNC3` is set to **Motor 2**, the 3rd output is set to the 2nd motor from `actuator_motors`.
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- output functions are defined under [src/lib/mixer_module/output_functions.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/mixer_module/output_functions.yaml).
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- if you want to control an output from MAVLink, set the relevant output function to **Offboard Actuator Set x**, and then send the [MAV_CMD_DO_SET_ACTUATOR](https://mavlink.io/en/messages/common.html#MAV_CMD_DO_SET_ACTUATOR) MAVLink command.
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## Adding a new Geometry or Output Function
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See [this commit](https://github.com/PX4/PX4-Autopilot/commit/5cdb6fbd8e1352dcb94bd58918da405f8ff930d7) for how to add a new geometry.
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The QGC UI will then automatically show the right configuration UI when [CA_AIRFRAME](../advanced_config/parameter_reference.md#CA_AIRFRAME) is set to the new geometry.
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[This commit](https://github.com/PX4/PX4-Autopilot/commit/a65533b46986e32254b64b7c92469afb8178e370) shows how to add a new output function.
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Any uORB topic can be subscribed and assigned to a function.
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Note that parameters for control allocation are defined in [src/modules/control_allocator/module.yaml](https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/control_allocator/module.yaml)
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The schema for this file is [here](https://github.com/PX4/PX4-Autopilot/blob/main/validation/module_schema.yaml#L440=) (in particular, search for the key `mixer:`
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## Setting the Default Frame Geometry
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When [adding a new frame configuration](../dev_airframes/adding_a_new_frame.md), set the appropriate [CA_AIRFRAME](../advanced_config/parameter_reference.md#CA_AIRFRAME) and other default mixer values for the geometry.
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You can see this, for example, in the airframe configuration file [13200_generic_vtol_tailsitter](https://github.com/PX4/PX4-Autopilot/blob/main/ROMFS/px4fmu_common/init.d/airframes/13200_generic_vtol_tailsitter)
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```sh
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...
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param set-default CA_AIRFRAME 4
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param set-default CA_ROTOR_COUNT 2
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param set-default CA_ROTOR0_KM -0.05
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param set-default CA_ROTOR0_PY 0.2
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...
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```
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## Setting up Geometry and Outputs
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The broad geometry and default parameters for a vehicle are set (from the frame configuration file) when selecting the airframe in QGroundControl: [Basic Configuration > Airframe](../config/airframe.md).
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The geometry parameters and output mapping for the specific frame and flight controller hardware are then configured using the QGroundControl **Actuators** setup screen: [Basic Configuration > Actuator Configuration and Testing](../config/actuators.md).
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