Extrinsic calibration

This a short tutorial for extrinsic calibration of Pepper's Depth and RGB cameras.

Setup

First of all, print a calibration pattern (a chessboard) if you do not have any. We would recommend A3 size. Once you have it, count the number of rows and columns and measure a size of a square. Adjust these parameters in launch/calibration_cameras_pepper.launch:

• define the numbers of columns -1
• define the numbers of rows -1
• define the size of one square in m

Data recording

• turn on your robot, wake up, and move the head to the "zero" pose, for example

ssh nao@<PEPPER_IP>
qicli call ALMotion.wakeUp
qicli call ALMotion.setAngles "HeadPitch" 0.0 0.3

• take two post-it (one on the top of enougher) and fix it on the right eye of Pepper (to cover this eye with the camera inside)

• start Naoqi Driver, ensuring that color and IR images are enabled in share/boot_config.json

roslaunch naoqi_driver naoqi_driver.launch nao_ip:=<PEPPER_IP>

• create an empty folder for data recording and provide its name in launch/extrinsic_calibration_pepper.launch
• update ROS topics for color and IR images in launch/extrinsic_calibration_pepper.launch
• launch the code:

roslaunch extrinsic_calibration extrinsic_calibration_pepper.launch

• if the software can subscribe to ROS topics, then you should see an image from a color camera
• place the calibration pattern in front of the robot
• once the pattern is detected (you can see all detected crossings), press "s" on your keyboard to save images from both cameras
• repeat 2 previous steps each time changing a distance and/or an angle to cover different views (at 60cm-80m distance) until you reach a sufficient number of image pairs (for example, 30 times or more)

Calibration based on recorded data

• launch the code to compute extrinsic parameters based on recorded images:

roslaunch extrinsic_calibration extrinsic_calibration_pepper.launch

• press "c" on your keyboard to compute calibration values
• check if the resulted XYZ and RPY for last processed image pair correspond to the reality

The extrinsic parameters obtained for our Pepper are:

XYZ: 0.0708098 0.02 0.118115
RPY: -1.5936 -0.0161345 -1.57777

How to update the extrinsic parameters in URDF of your robot

There are two possible ways to applying the extrinsic parameters:

• using ROS static_transform_publisher
• updating the position of the camera in the URDF

Using static_transform_publisher

An example of using static_transform_publisher is shown in register_depth.launch of Naoqi Driver. You can update the values in register_depth.launch in your workspace. The order of the values is: x y z yaw pitch roll. Once updted, launch this file in addition to naoqi_driver:

roslaunch naoqi_driver naoqi_driver.launch nao_ip:=<PEPPER_IP>
roslaunch naoqi_driver register_depth.launch

Using URDF

Alternatively, you can update the parameters directly in the URDF file:

• update RPY values in https://github.com/nlyubova/pepper_robot/blob/master/pepper_description/urdf/pepper1.0_generated_urdf/pepper_sensors.xacro#L23
• update XYZ values in https://github.com/nlyubova/pepper_robot/blob/master/pepper_description/urdf/pepper1.0_generated_urdf/pepper_sensors.xacro#L120
• generate a new pepper.URDF file, for example

roscd pepper_description
cd urdf/pepper1.0_generated_urdf/
rosrun xacro xacro.py -o pepper.urdf pepper_robot.xacro

• start Naoqi Driver as usual