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Motion Capture

Motion capture allows for non-GPS navigation by sending pose estimation data to the flight controller's EKF. ROS makes this easy as there are drivers and packages that allow us to publish pose topics into the ROS framework. We can then relay those topics directly into MAVROS which will then send the poses to the flight controller.

Qualisys Motion Capture​

This assumes that you have already set up your Qualisys motion capture system and configured a rigid body for your UAV.

QTM is only supported via Windows, so we will use another computer utilizing a Linux distro to act as our ROS pose publisher. The best practice is to connect these two machines via Ethernet to avoid latency and packet loss.

Docker​

Setting up Qualisys ROS on Linux is made easy via Docker.

This tutorial will assume you are running on a Linux OS, but Docker can be used on both Mac and Windows via Docker Desktop.

First, make sure you installed Docker Engine. It is also recommend you follow the Linux Post-Installation Steps as well to manage Docker as a non-root user.

We can verify our Docker installation via:

$ docker run hello-world

We can now build our Docker image via the following commands:

$ cd ~/mavrospy/docker/mocap
$ bash run_docker.sh

This script will automatically build the Docker image and open it in a Docker container.

PX4 Parameters​

If you wish to use a motion caputure system as your means for navigating with out a GPS, you must configure the following PX4 parameters.

    EKF2_HGT_REF = Vision
    EKF2_EV_DELAY = 50.0ms
    EKF2_GPS_CTRL = 0
    EKF2_BARO_CTRL = Disabled
    EKF2_RNG_CTRL = Disable range fusion
    EKF2_REQ_NSATS = 5
    MAV_USEHILGPS = Enabled
    EKF2_MAG_TYPE = None

Timesync​

For the pose topic to be understood properly between computers, it's important that both machines are in sync. On both systems, run:

$ timedatectl

In the output, you should see:

System clock synchronized: yes
NTP service: active

If the output indicates that the NTP service isn't active, turn it on via:

$ sudo timedatectl set-ntp on

After this, run timedatectl again to confirm the network time status.

Motion Capture Demo​

warning

If you are following this tutorial after the GPS Navigation Tutorial please remove the outdoor MAVROSPY container:

$ docker container rm -f mavrospy-rpi-outdoor-container

On your companion computer, make sure your running the MAVROSPY Docker container:

$ cd ~/mavrospy/docker/rpi
$ bash run_docker.sh

We must modify the mocap.launch file to direct your rigid body name to the relay topic node. Open the file:

$ nano ~/mavrospy/launch/mocap.launch

Find this line:

<node pkg="topic_tools" type="relay" name="relay" output="screen" args="/qualisys/My_Quad/pose /mavros/vision_pose/pose"/>

Replace My_Quad with the label of your rigid body created in QTM.

You can now start our motion capture control node on our companion computer via:

$ roslaunch mavrospy mocap.launch
info

You can specify the flight pattern via the pattern argument. If unspecified, it will default to square.

$ roslaunch mavrospy mocap.launch pattern:=figure8

Available Patterns: square, square_head, circle, circle_head, figure8, figure8_head, spiral, and spiral_head. Where _head will specify the drone to face in the direction of motion.

In order to publish ROS pose topics from our computer to the companion computer, we must establish a ROS Master. In our case, we will choose the companion computer as the master.

On your publishing computer, export the ROS_MASTER_URI environment variable. Make sure to replace master_ip_address with the IP address of the companion computer:

$ export ROS_MASTER_URI=http://master_ip_address:11311

Now start publishing. On your motion capture pose publishing computer, enter:

$ roslaunch mocap_qualisys qualisys.launch server_address:=qtm_server_address

Where qtm_server_address is the IP address to your QTM server.

To verify your pose topics are being published, run the following on another terminal on your companion computer:

$ cd ~/mavrospy/docker/rpi
$ bash run_docker.sh
$ rostopic echo /qualisys/rigid_body_name/pose

Where My_Quad is the label of your rigid body created in QTM. CTRL+C to exit.

You can verify that your companion computer is receiving our pose topic via:

$ rostopic echo /mavros/vision_pose/pose

CTRL+C to exit.

Additionally, you can check that the PX4 flight contoller is able to calculate a global position estimage via:

$ rostopic echo /mavros/global_position/global

CTRL+C to exit.

You can now switch to OFFBOARD mode and watch it fly!