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205 lines
14 KiB
Markdown
205 lines
14 KiB
Markdown
# PMW3901-Based Flow Sensors
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PMW3901 is an [optical flow](../sensor/optical_flow.md) ASIC that computes the flow internally and provides a difference in pixels between each frame.
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It uses a tracking sensor that is similar to what you would find in a computer mouse, but adapted to work between 80 mm and infinity.
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The PMW3901 is used in several products, including some from Bitcraze, Tindie, Hex, Thone, and Alientek
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This topic provides links to sensors that have been tested, along with information about mounting and PX4 configuration (this is common to all sensors of this type).
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## Boards using PMW3901
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The following table shows some of the boards that use this sensor, listing the number of interfaces, the number of sensors, the input voltage, and the size.
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The board name links to board-specific sections that include wiring and purchase information.
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| Manufacture | Board | Interface | Flow | 测距仪 | Gyro | Voltage (V) | Size (mm) | Max Height (m) |
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| ----------- | --------------------------------------------------------------------------------- | --------- | ---- | --- | ---- | ----------------------------------------- | ----------------------------------------- | --------------------------------- |
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| Bitcraze | [Flow breakout](#bitcraze-flow-breakout) | SPI | Y | Y | - | 3 - 5 | 21x20 | 1 |
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| Tindie | [PMW3901 Optical Flow Sensor](#tindie-pmw3901-optical-flow-sensor) | SPI | Y | - | - | 3 - 5 | AxB | - |
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| Hex | [HereFlow PMW3901 Optical Flow Sensor](#hex-hereflow-pmw3901-optical-flow-sensor) | CAN | Y | Y | Y | 3 - 5 | AxB | 4 |
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| Holybro | [Holybro PMW3901 Optical Flow Sensor](#holybro-pmw3901-optical-flow-sensor) | UART | Y | - | - | 3 - 5 | AxB | - |
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| Thone | [ThoneFlow-3901U](#thone-thoneflow-3901u) | UART | Y | - | - | 3 - 5 | AxB | - |
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| Alientek | [ATK-PMW3901](#alientek-atk-pmw3901) | SPI | Y | - | - | 3.3 - 4.2 | 27.5x16.5 | 1 |
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## External Rangefinders
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An external rangefinder/distance sensor is _required_ for the sensors that don't have a rangefinder (e.g. _Tindie_ or _Thone_) and _recommended_ for the other boards (as their range is quite limited).
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The range needed is dependent on the application:
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- Indoor flight: ≈4 metres
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- Outdoor flight: ≥10 metres (e.g. to support the position control in environments where GPS might have issues)
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Any [rangefinder/distance sensor](../sensor/rangefinders.md) supported by PX4 may be used.
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The sensor can be mounted anywhere but must point down, and should be connected/configured as usual.
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:::tip
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The PX4 team mainly tested using the [Lidar Lite V3](../sensor/lidar_lite.md) on larger vehicles and the [Lanbao CM8JL65](../sensor/cm8jl65_ir_distance_sensor.md) (Discontinued) on smaller vehicles.
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:::
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## Mounting/Orientation
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Flow modules are typically mounted near the centre of the vehicle.
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If mounted off-centre you will need to set offsets: [Optical Flow > EKF2](../sensor/optical_flow.md#ekf2).
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The flow modules may be mounted with any yaw orientation relative to the vehicle body frame, but you must set the value used in [SENS_FLOW_ROT](../advanced_config/parameter_reference.md#SENS_FLOW_ROT).
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The "zero" rotation is when the sensor board and [vehicle](../getting_started/px4_basic_concepts.md#heading-and-directions) X-axes are aligned (i.e. the "front" of the vehicle and the board are in the same direction), with rotations increasing in a clockwise direction.
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PMW3901 modules have a small notch indicating the direction of the **back** of the board.
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The diagram shows the relative board and vehicle orientations that correspond to `SENS_FLOW_ROT=0` (note the notch at the back).
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The diagram above shows the Bitcraze board.
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You can use the notch to find the orientation of the other boards in the same way:
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| ---------------------------------------------------- | ----------------------------------------------------------------------------------------------- |
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| Tindie<br>![PMW3901 Tindie Notch][tindie_notch] | Hex Hereflow<br>![PMW3901 Hereflow Notch][hereflow_notch] |
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| Thone<br>![PMW3901 Thoneflow Notch][thoneflow_notch] | Alientek (also has an arrow indicating the front!)<br>![PMW3901 Alientek Notch][alientek_notch] |
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[tindie_notch]: ../../assets/hardware/sensors/pmw3901/tindie_notch.jpg
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[thoneflow_notch]: ../../assets/hardware/sensors/pmw3901/thoneflow_3901u_notch.jpg
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[alientek_notch]: ../../assets/hardware/sensors/pmw3901/alientek_pmw3901_notch.jpg
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[hereflow_notch]: ../../assets/hardware/sensors/pmw3901/hex_hereflow_notch.jpg
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## PX4 配置
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The PX4 configuration that is common to all PMW3901-based boards:
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- [Optical Flow > EKF2](../sensor/optical_flow.md#ekf2) explains how to fuse optical flow data in the EKF2 estimator and set position offsets for the mounting position of the flow sensor.
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- [SENS_FLOW_ROT](../advanced_config/parameter_reference.md#SENS_FLOW_ROT) sets the orientation of the flow sensor relative to vehicle heading.
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In addition for:
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- SPI-connected sensors you must set [SENS_EN_PMW3901](../advanced_config/parameter_reference.md#SENS_EN_PMW3901) to `1` in order to enable the sensor driver.
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- UART-connected sensors (e.g. ThoneFlow-3901UY) you must [configure the associated serial port](../peripherals/serial_configuration.md) by setting the parameter [SENS_TFLOW_CFG](../advanced_config/parameter_reference.md#SENS_TFLOW_CFG) to the value of the connected UART port (for example, if you connected this sensor to `TELEM 2`, you need to set `SENS_TFLOW_CFG` to `102`).
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- DroneCAN sensors you must set `UAVCAN_ENABLE` appropriately.
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For more information see [DroneCAN Peripherals](../dronecan/index.md) (and the [HereFlow PMW3901 docs below](#dronecan_wiring)).
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Individual flow sensors are further setup/configured as described in the sections below.
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## Bitcraze Flow Breakout
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The [Bitcraze Flow breakout](https://www.bitcraze.io/products/flow-breakout/) directly exposes the [SPI interface](#spi-wiring) from the PMW3901 module.
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The board also incorporates a distance sensor [wired to the Pixhawk I2C port](#i2c-wiring).
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This distance sensor is the VL53L0x ToF sensor from STMicroelectronics.
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The sensor range is minimal (2 metres) and will be reduced when flying in the sunlight.
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We therefore highly recommend that you use an [external distance sensor](#external-rangefinders).
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[PX4 configuration](#px4-configuration) and [mounting/orientation](#mounting-orientation) instructions are provided above.
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### SPI Wiring
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The PMW3901, if connected to the SPI port on a Pixhawk 4, will automatically detect the Bitcraze flow module.
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This device's driver was explicitly written to be plugged into the SPI port using the chip select 1.
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No parameters will have to be configured other than the [orientation and position of the sensor](#mounting-orientation).
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The pinout mapping for the Pixhawk SPI port to Bitcraze Flow Board is shown below (the port mapping is the same for all Pixhawk FMU versions).
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| Pixhawk SPI Port (from left to right) | Bitcraze flow board |
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| -------------------------------------------------------- | ------------------- |
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| 1 (VCC) | VCC |
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| 2 (SCK) | CLK |
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| 3 (MISO) | MISO |
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| 4 (MOSI) | MOSI |
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| 5 (CS1) | CS |
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| 6 (CS2) | Do not connect |
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| 7 (GND) | GND |
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To connect the bitcraze flow board to the Pixhawk, you will need to solder the wires of the Pixhawk SPI cable to the flow board.
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An SPI cable has 7 wires, from which you need to connect 6 to the flow board.
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The following diagram shows how to wire the sensor to a Pixhawk 4.
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### I2C Wiring
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The I2C wiring is the same for any other distance sensor.
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Simply connect the SLA, SLC, GND, and VCC to the corresponding (same) pins on the Pixhawk and the sensor.
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## Tindie PMW3901 Optical Flow Sensor
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The Tindie [PMW3901 Optical Flow Sensor](https://www.tindie.com/products/onehorse/pmw3901-optical-flow-sensor/) exposes the SPI interface from the PMW3901 module exactly as on the Bitcraze module (see [SPI Wiring](#spi-wiring)).
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The sensor doesn't have a distance sensor onboard, so you will need to use an [external distance sensor](#external-rangefinders).
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[PX4 configuration](#px4-configuration) and [mounting/orientation](#mounting-orientation) instructions are provided above.
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## AlienTek ATK-PMW3901
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The AlienTek [ATK-PMW3901](https://www.aliexpress.com/i/32979605707.html) exposes the SPI interface from the PMW3901 module in the same way as the Bitcraze module (see [SPI Wiring](#spi-wiring)).
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The board also incorporates a distance sensor (we recommend that you use an [external distance sensor](#external-rangefinders)).
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You can wire the internal sensor to the Pixhawk I2C port [in the same way as any other I2C peripheral.](#i2c-wiring)
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A screenshot showing the I2C pins (SLA, SLC, GND, and VCC) is provided below.
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[PX4 configuration](#px4-configuration) and [mounting/orientation](#mounting-orientation) instructions are provided above.
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## Hex HereFlow PMW3901 Optical Flow Sensor
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The Hex [HereFlow PMW3901 Optical Flow Sensor](https://docs.cubepilot.org/user-guides/flow-senor/here-flow) is a tiny board containing the PMW3901 flow module, VL53L1X distance sensor, and an IMU (used to synchronize the flow data with the gyro data).
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An onboard microcontroller samples the three sensors and publishes two DroneCAN messages containing all the information needed for the flow and distance sensor calculations.
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The board can be connected to any CAN port on any Pixhawk board (see [DroneCAN wiring](#dronecan_wiring)).
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As for the other optical flow boards, we recommend that you use an [external distance sensor](#external-rangefinders).
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[PX4 configuration](#px4-configuration) and [mounting/orientation](#mounting-orientation) instructions are provided above.
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### DroneCAN Wiring/Setup {#dronecan_wiring}
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The diagram below shows how to connect the sensor to the Pixhawk 4 CAN bus.
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In addition to any other configuration, you will need to set the parameter [UAVCAN_ENABLE](../advanced_config/parameter_reference.md#UAVCAN_ENABLE) to either 2 or 3, depending on your system:
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- `UAVCAN_ENABLE=2`: DroneCAN sensors but no motor controllers.
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- `UAVCAN_ENABLE=3`: DroneCAN sensors and motor controllers.
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For general information about DroneCAN wiring and configuration see: [DroneCAN Peripherals](../dronecan/index.md).
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## Holybro PMW3901 Optical Flow Sensor
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The [Holybro PMW3901 Optical Flow Sensor](https://holybro.com/products/pmw3901-optical-flow-sensor) exposes a PMW3901 optical flow module via a UART interface.
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The board doesn't include a distance sensor onboard, so you will need to use an [external distance sensor](#external-rangefinders).
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<img src="../../assets/hardware/sensors/pmw3901/holybro-pmw3901-1.jpg" width="350px" title="HB-pmw3901-1" />
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<img src="../../assets/hardware/sensors/pmw3901/holybro-pmw3901-2.jpg" width="350px" title="HB-pmw3901-2" />
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It comes with a 3D printed case and has a pre-soldered TX, Voltage, and GND 4 pin connector that can be plugged into the telemetry port of your flight controller, as shown below (for the Pixhawk 6C).
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## Thone ThoneFlow-3901U
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The Thone [ThoneFlow-3901U](https://www.seeedstudio.com/ThoneFlow-3901U-UART-Serial-Version-PMW3901-Optical-Flow-Sensor-p-4040.html) exposes a PMW3901 optical flow module via a UART interface.
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The board doesn't include a distance sensor onboard, so you will need to use an [external distance sensor](#external-rangefinders).
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<!-- note, this will be set using SENS_TFLOW_CFG
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Wiring is also required.
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-->
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[PX4 configuration](#px4-configuration) and [mounting/orientation](#mounting-orientation) instructions are provided above.
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In addition, you must also set the parameter [SENS_TFLOW_CFG](../advanced_config/parameter_reference.md#SENS_TFLOW_CFG) to the value of the UART port you connected (e.g. if the sensor is connected to `TELEM 2` then set `SENS_TFLOW_CFG=102`.
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For more information see [Serial Port Configuration](../peripherals/serial_configuration.md).
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