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105 lines
6.3 KiB
Markdown
105 lines
6.3 KiB
Markdown
# Optical Flow
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_Optical Flow_ uses a downward facing camera and a downward facing distance sensor for velocity estimation.
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It can be used to determine speed when navigating without GNSS — in buildings, underground, or in any other GNSS-denied environment.
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The video below shows PX4 holding position using the [Ark Flow](../dronecan/ark_flow.md) sensor for velocity estimation in [Position Mode](../flight_modes_mc/position.md):
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<lite-youtube videoid="aPQKgUof3Pc" title="ARK Flow with PX4 Optical Flow Position Hold"/>
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<!-- ARK Flow with PX4 Optical Flow Position Hold: 20210605 -->
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The image below shows an optical flow setup with a separate flow sensor ([PX4Flow](../sensor/px4flow.md)) and distance sensor ([Lidar-Lite](../sensor/lidar_lite.md)):
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## Setup
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An Optical Flow setup requires a downward facing camera and a downward facing [distance sensor](../sensor/rangefinders.md) (preferably a LiDAR).
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These can be combined in a single product, such as the [ARK Flow](../dronecan/ark_flow.md), [ARK Flow MR](../dronecan/ark_flow_mr.md) and [Holybro H-Flow](https://holybro.com/products/h-flow), or they may be separate sensors.
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The sensor(s) can be connected via MAVLink, I2C or any other bus that supports the peripheral.
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::: info
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If connected to PX4 via MAVLink the Optical Flow camera sensor must publish the [OPTICAL_FLOW_RAD](https://mavlink.io/en/messages/common.html#OPTICAL_FLOW_RAD) message, and the distance sensor must publish the [DISTANCE_SENSOR](https://mavlink.io/en/messages/common.html#DISTANCE_SENSOR) message.
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The information is written to the corresponding uORB topics: [DistanceSensor](../msg_docs/DistanceSensor.md) and [ObstacleDistance](../msg_docs/ObstacleDistance.md).
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:::
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The output of the flow when moving in different directions must be as follows:
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| Vehicle movement | Integrated flow |
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| ---------------- | --------------- |
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| Forwards | + Y |
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| Backwards | - Y |
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| Right | - X |
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| Left | + X |
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Sensor data from the optical flow device is fused with other velocity data sources.
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The approach used for fusing sensor data and any offsets from the center of the vehicle must be configured in the [estimator](#estimators).
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### Scale Factor
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For pure rotations the `OPTICAL_FLOW_RAD.integrated_xgyro` and `OPTICAL_FLOW_RAD.integrated_x` (respectively `integrated_ygyro` and `integrated_y`) have to be the same.
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If this is not the case, the optical flow scale factor can be adjusted using [SENS_FLOW_SCALE](../advanced_config/parameter_reference.md#SENS_FLOW_SCALE).
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:::tip
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The low resolution of common optical flow sensors can cause slow oscillations when hovering at a high altitude above ground (> 20m).
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Reducing the optical flow scale factor can improve the situation.
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:::
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## Flow Sensors/Cameras
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### ARK Flow & ARK Flow MR
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[ARK Flow](../dronecan/ark_flow.md) is a [DroneCAN](../dronecan/index.md) optical flow sensor, [distance sensor](../sensor/rangefinders.md), and IMU.
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It has a PAW3902 optical flow sensor, Broadcom AFBR-S50LV85D 30 meter distance sensor, and Invensense ICM-42688-P 6-Axis IMU.
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[ARK Flow MR](../dronecan/ark_flow_mr.md) is a [DroneCAN](../dronecan/index.md) optical flow sensor, [distance sensor](../sensor/rangefinders.md), and IMU, for mid-range applications.
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It has a PixArt PAA3905 optical flow sensor, Broadcom AFBR-S50LX85D 50 meter distance sensor, and Invensense IIM-42653 6-Axis IMU.
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### Holybro H-Flow
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The [Holybro H-Flow](https://holybro.com/products/h-flow) is a compact [DroneCAN](../dronecan/index.md) optical flow and [distance sensor](../sensor/rangefinders.md) module.
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It combines a PixArt PAA3905 optical flow sensor, a Broadcom AFBR-S50LV85D distance sensor, and an InvenSense ICM-42688-P 6-axis IMU.
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An all-in-one design that simplifies installation, with an onboard infrared LED enhances visibility in low-light conditions.
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### PMW3901-Based Sensors
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[PMW3901](../sensor/pmw3901.md) is an optical flow tracking sensor similar to what you would find in a computer mouse, but adapted to work between 80 mm and infinity.
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It is used in a number of products, including some from: Bitcraze, Tindie, Hex, Thone and Alientek.
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### Other Cameras/Sensors
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It is also possible to use a board/quad that has an integrated camera.
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For this the [Optical Flow repo](https://github.com/PX4/OpticalFlow) can be used (see also [snap_cam](https://github.com/PX4/snap_cam)).
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## Range Finders
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You can use any supported [distance sensor](../sensor/rangefinders.md).
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However we recommend using LIDAR rather than sonar sensors, because of their robustness and accuracy.
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## Estimators
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Estimators fuse data from the optical flow sensor and other sources.
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The settings for how fusing is done, and relative offsets to vehicle center must be specified for the estimator used.
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The offsets are calculated relative to the vehicle orientation and center as shown below:
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Optical Flow based navigation is enabled by both [EKF2](#ekf2) and LPE (deprecated).
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### Extended Kalman Filter (EKF2) {#ekf2}
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For optical flow fusion using EKF2, set [EKF2_OF_CTRL](../advanced_config/parameter_reference.md#EKF2_OF_CTRL).
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If your optical flow sensor is offset from the vehicle centre, you can set this using the following parameters.
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| Parameter | Description |
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| -------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------- |
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| <a id="EKF2_OF_POS_X"></a>[EKF2_OF_POS_X](../advanced_config/parameter_reference.md#EKF2_OF_POS_X) | X position of optical flow focal point in body frame (default is 0.0m). |
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| <a id="EKF2_OF_POS_Y"></a>[EKF2_OF_POS_Y](../advanced_config/parameter_reference.md#EKF2_OF_POS_Y) | Y position of optical flow focal point in body frame (default is 0.0m). |
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| <a id="EKF2_OF_POS_Z"></a>[EKF2_OF_POS_Z](../advanced_config/parameter_reference.md#EKF2_OF_POS_Z) | Z position of optical flow focal point in body frame (default is 0.0m). |
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See [Using PX4's Navigation Filter (EKF2) > Optical flow](../advanced_config/tuning_the_ecl_ekf.md#optical-flow) for more information.
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