Skip to content

Latest commit

 

History

History
76 lines (61 loc) · 4.7 KB

advanced-camera-trigger.md

File metadata and controls

76 lines (61 loc) · 4.7 KB
Raw

Camera Trigger

The camera trigger driver allows the use of the AUX ports to send out pulses in order to trigger a camera. This can be used for multiple applications including timestamping photos for aerial surveying and reconstruction, synchronizing a multi-camera system or visual-inertial navigation.

In addition to a pulse being sent out, a MAVLink message is published containing a sequence number (thus the current session's image sequence number) and the corresponding time stamp.

Three different modes are supported:

  • TRIG_MODE 1 works like a basic intervalometer that can be enabled and disabled by calling in the system console camera_trigger enable or camera_trigger disable, respectively. Repeated enabling time-shifts the intervals to match the latest call.
  • TRIG_MODE 2 switches the intervalometer constantly on.
  • TRIG_MODE 3 triggers based on distance. A shot is taken every time the set horizontal distance is exceeded. The minimum time interval between two shots is however limited by the set triggering interval.

In TRIG_MODE 0 the triggering is off.

The full list of parameters pertaining to the camera trigger module can be found on the parameter reference page.

**Info ** If it is your first time enabling the camera trigger app, remember to reboot after changing the TRIG_MODE parameter to either 1, 2 or 3.

Camera-IMU sync example

In this example, we will go over the basics of synchronizing IMU measurements with visual data to build a stereo Visual-Inertial Navigation System (VINS). To be clear, the idea here isn't to take an IMU measurement exactly at the same time as we take a picture but rather to correctly time stamp our images so as to provide accurate data to our VI algorithm.

The autopilot and companion have different clock bases (boot-time for the autopilot and UNIX epoch for companion), so instead of skewing either clock, we directly observe the time offset between the clocks. This offset is added or subtracted from the timestamps in the mavlink messages (e.g HIGHRES_IMU) in the cross-middleware translator component (e.g Mavros on the companion and mavlink_receiver in PX4). The actual synchronisation algorithm is a modified version of the Network Time Protocol (NTP) algorithm and uses an exponential moving average to smooth the tracked time offset. This synchronisation is done automatically if Mavros is used with a high-bandwidth on-board link.

For acquiring synchronised image frames and inertial measurements, we connect the trigger inputs of the two cameras to a GPIO pin on the autopilot. The timestamp of the inertial measurement from mid-exposure, and a image sequence number is recorded and sent to the companion computer (CAMERA_TRIGGER message), which buffers these packets and the image frames acquired from the camera. They are matched based on the sequence number, the images timestamped (with the timestamp from the CAMERA_TRIGGER message) and then published.

The following diagram illustrates the sequence of events which must happen in order to correctly timestamp our images.

{% mermaid %} sequenceDiagram Note right of px4 : Time sync with mavros is done automatically px4 ->> mavros : Camera Trigger ready Note right of camera driver : Camera driver boots and is ready camera driver ->> mavros : mavros_msgs::CommandTriggerControl mavros ->> px4 : MAVLink::MAV_CMD_DO_TRIGGER_CONTROL loop Every TRIG_INTERVAL milliseconds px4 ->> mavros : MAVLink::CAMERA_TRIGGER mavros ->> camera driver : mavros_msgs::CamIMUStamp camera driver ->> camera driver : Match sequence number camera driver ->> camera driver : Stamp image and publish end {% endmermaid %}

Step 1

First, set the TRIG_MODE to 1 to make the driver wait for the start command and reboot your FCU to obtain the remaining parameters.

Step 2

For the purposes of this example we will be configuring the trigger to operate in conjunction with a Point Grey Firefly MV camera running at 30 FPS.

  • TRIG_INTERVAL: 33.33 ms
  • TRIG_POLARITY: 0, active low
  • TRIG_ACT_TIME: 0.5 ms, leave default. The manual specifies it only has to be a minimum of 1 microsecond.
  • TRIG_MODE: 1, because we want our camera driver to be ready to receive images before starting to trigger. This is essential to properly process sequence numbers.
  • TRIG_PINS: 12, Leave default.

Step 3

Wire up your cameras to your AUX port by connecting the ground and signal pins to the appropriate place.

Step 4

You will have to modify your driver to follow the sequence diagram above. Public reference implementations for IDS Imaging UEye cameras and for IEEE1394 compliant cameras are available.