IMU

In this tutorial, we will show you how to connect and use IMU module on your Rover.

Prerequisites

First, make sure you have a compatible IMU module. We recommend Grove - IMU 9DOF v2.0 but any module with MPU-9250 sensor should work.

Grove - IMU 9DOF v2.0

IMU functionality was introduced in leo_firmware version 0.5, so make sure to stay updated.

Mounting and wiring the sensor

By default, the IMU will work on hSens2 port. You can change it to hSens1 by modifying the params.h file in leo_firmware.

hSens2 port location

Connect the sensor pins according to the CORE2 manual.

hSens pin

IMU pin

3

SCL

4

SDA

5

+5V

6

GND

You can use female jumper cables, or create your own IDC cable, like this one:

If you use the Grove IMU, you can 3D print the mounting plate for the MEB cover which contains holes for the self-tapping screws (00189_imu part). The files can be found here:

Integrating the sensor with the system

Turning on the IMU functionality

Log in to the Rover's console via SSH

To set IMU functionality on or off, you need to send message on the /core2/set_imu topic.

rostopic pub /core2/set_imu std_msgs/Bool true

Now, you need to reset the board to apply changes. You can do this by turning on and off the whole Rover, or by sending an empty message on the /core2/reset_board topic.

rostopic pub /core2/reset_board std_msgs/Empty

After the board reset, new topics should spawn: /imu/gyro, /imu/accel, /imu/mag on which IMU gyroscope, accelerometer and magnetometer readings are published. (you can check available topics with rostopic list)

Check if the readings are correct with rostopic echo, for example:

rostopic echo /imu/gyro

sensor axes and units

The sensor's accelerometer and gyroscope X, Y and Z axes should be printed on the board. If they are not, you can check MPU9250 IC orientation and identify the axes with this drawing:

accelerometer and gyroscope axes

The magnetometer axes were also transformed to these axes to comply with the North-West-Up world frame.

The gyroscope data (imu/gyro topic) represents angular velocity around sensor's axes in rad/s (radians per second). The accelerometer data (imu/accel topic) represents linear acceleration along sensor's axes in m/s2 (meters per second squared). The magnetometer data (imu/mag topic) represents magnetic field along sensor's axes in G (Gauss).

Extending the URDF model

The system should be aware of where the module is located on the robot, in order to be able to use the sensor readings. To specify the location, you can extend the URDF model of the Rover by editing the /etc/ros/urdf/robot.urdf.xacro. Simply open the file in nano editor by typing:

sudo nano /etc/ros/urdf/robot.urdf.xacro

Add these lines somewhere between the <robot> tags:

<link name="imu"/>
<joint name="imu_joint" type="fixed">
<parent link="base_link"/>
<child link="imu"/>
<origin xyz="0.027 0 -0.07" rpy="0 0 0"/>
</joint>

imu link represents the reference frame of the sensor readings. base_link is the origin of the robot. imu_joint specifies the transformation between these two links.

You might need to modify the transformation (<origin> tag) depending on how you mounted the module.

base_link -> imu transform visualization

For more information, visit ROS Development -> Adding additional functionality to the rover -> expanding the URDF model

Calibrating the sensor

The firmware also provides services that perform sensor calibration and store the results in a persistent storage.

calibrate gyroscope and accelerometer biases

To calibrate gyroscope and accelerometer biases, just place the Rover on a flat surface, parallel to the ground and call the calibration service:

rosservice call /imu/calibrate_gyro_accel

calibrate magnetometer

Calibrating the magnetometer is a bit more difficult task, as it requires collection of a whole range of measurements on each axis. The method that works best is to move the sensor in a 3-dimensional figure eight, however, such motion can be hard to accomplish with the whole Rover.

To start the calibration, just call the service:

rosservice call /imu/calibrate_mag

After 4 seconds, the firmware should start data collection. After another 15 seconds, the service should return success message. Try to move the sensor in a varied motion for the whole duration of the service call.

When you change the sensor position on the Rover, the hard-iron distortion may change and invalidate previous calibration data, so, before performing the magnetometer calibration, make sure the sensor is located at the designated place.