Calibrating the VI Sensor

This page will guide you through the calibration of the VI-Sensor (visual-inertial sensor). The intrinsics and extrinsics of the camera system and the transformation of each camera w.r.t. the IMU will be estimated.

More information about the VI-Sensor can be found here.

##Procedure

1. prepare the sensor
2. setting the focus
3. collect calibration data
   1. in-/extrinsics calibration (static calibration)
   2. imu-camera calibration (dynamic calibration)
4. run the calibration
   1. in-/extrinsic camera calibration
   2. imu-camera calibration
5. collect results

##Requirements

• ROS sensor driver is running (image/imu data)
• good Aprilgrid target (pdf, yaml)
• Siemens star (or similar camera focus test pattern)
• IMU configuration for ADIS16448 (yaml)

##1) Sensor preparation

1. make sure the sensor publishes all image and imu streams to ROS

2. minimize the motion blur with a good light source and by reducing the shutter times.

   Shutter times can be set using the following commands:

   rosrun dynamic_reconfigure dynparam set /slam_sensor "{'cam0_agc_enable': 0, 'cam0_aec_enable': 0, 'cam0_coarse_shutter_width': 300}"
   rosrun dynamic_reconfigure dynparam set /slam_sensor "{'cam1_agc_enable': 0, 'cam1_aec_enable': 0, 'cam1_coarse_shutter_width': 300}"

   Observe the result on an image window and tweak the shutter until you get a good image:

   rosrun image_view image_view image:=/cam0/image_raw &
   rosrun image_view image_view image:=/cam1/image_raw &

##2) Setting the focus

1. point the cameras on a Siemens star (or similar pattern)

2. start the focus tool

   kalibr_camera_focus --topic /cam0/image_raw /cam1/image_raw

3. set the focus of both cameras by:

   1. reducing the interference visible around the center of the Siemens star
   2. minimizing the focus measure provided by the tool

Make sure a Teflon band or thread-locking glue prevents unintentional focus changes after this step.

##2) Collect calibration data In this step we need to collect two calibration datasets with the following properties:

1. static dataset (in-/extrinsic calibration of the cameras)

   • attach the sensor somewhere and move the target
   • limit the camera streams to ~4 Hz
   • make sure to cover the entire field of view of the camera
   • use skewed views and varying distances to the calibration target

   view images with:

   rosrun image_view image_view image:=/cam0/image_raw &
   rosrun image_view image_view image:=/cam1/image_raw &

   record bag with:

   rosbag record /cam0/image_raw /cam1/image_raw -O static.bag

2. dynamic dataset (spatial camera-imu calibration)

   • move the sensor (target is fixed)
   • cameras should run at 20 Hz and IMU at 200 Hz
   • try to excite all rotation and acceleration axes of the IMU
   • avoid shocks (e.g. while picking up the sensor)
   • good illumination and shutter times are crucial here (to avoid motion blur while exciting the IMU)

   view images with:

   rosrun image_view image_view image:=/cam0/image_raw &
   rosrun image_view image_view image:=/cam1/image_raw &

   record bag with:

   rosbag record /cam0/image_raw /cam1/image_raw /imu0 -O dynamic.bag

##3) Run the calibration

1. calibration of camera in/extrinsics

   1. run calibration

   kalibr_calibrate_cameras --models pinhole-equi pinhole-equi --topics /cam0/image_raw /cam1/image_raw --bag static.bag --target aprilgrid_6x6.yaml

   1. inspect the result plots
   2. verify calibration on the live image stream
      reprojection errors should be in a normal range (0.1-0.2 px for a good calibration)

   kalibr_camera_validator --chain chain.yaml --target aprilgrid_6x6.yaml

2. camera-imu calibration

   1. run calibration

   kalibr_calibrate_imu_camera --cam chain.yaml --target aprilgrid_6x6.yaml --imu imu0.yaml --bag dynamic.bag

   1. inspect the result plots
      • make sure the predicted accelerations & angular velocities fit the IMU measurements
      • reprojection errors should be in a normal range (0.1-0.2 px for a good calibration)

##4) Collect results Both calibrators have written reports to the working directory containing the plots shown during the calibration. Further a camchain.yaml has been written by the camera calibrator and is extended by the imu-camera calibrator with imu-camera transformations to the file camchain_cimu.yaml. Please refer to YAML formats page for the format.