Docking

Ultrasonic Sensors on the bot

On the rear side of the bot, you can find 2 ultrasonic sensors. Here is the representation of it.

The ros2 topic for this sensors are /ultrasonic_rl/scan for left and /ultrasonic_rr/scan for right, using msg type sensor_msgs/msg/Range

  • To enable or disable the visualization of the sensors you can go to ebot_description/models/ebot/ebot.gazebo
  • Under ebot.gazebo, go to the Ultrasonic section
   <!-- Ultrasonic -->
   <xacro:macro name="ultrasonic_sensor" params="name">
   <gazebo reference="${name}">
   <sensor type="ray" name="${name}">
   <pose>0 0 0 0 0 0</pose>
   <visualize>true</visualize> ```

Concept

Consider a scenario like below,

here you can see we can first align the orientation of the bot wrt the rack,

  • Aligning orientation: We can do this using the IMU sensor directly or using Ultrasonic sensors reading and doing simple coordinate geometry.

    1. Using IMU directly, the orientation data of the ebot can be directly accessed using topic /imu and msg-type sensor_msgs/msg/Imu

    Doing ros2 topic echo /imu --once

       header:
 stamp:
   sec: 823
   nanosec: 847000000
 frame_id: ebot_base_link
orientation:
 x: 0.000953526911620944
 y: 0.00021382570450593477
 z: -0.00048188256696325623
 w: 0.9999994064269182

    Angles are in quaternion, GOOGLE it !!!

    Here you can see, that we get the orientation of the ebot and we already know the orientation of the rack. Taking the difference will give us angular distance.

    1. Using Ultrasonic sensors reading, on topic /ultrasonic_rl/scan for left and /ultrasonic_rr/scan for right

    Doing ros2 topic echo /ultrasonic_rl/scan --once

       header:
stamp:
   sec: 1369
   nanosec: 404000000
frame_id: ebot_base_link
radiation_type: 0
field_of_view: 0.26179999113082886
min_range: 0.029999999329447746
max_range: 4.099999904632568
range: 1.163640022277832   <<<--- Here is the range we need!

    Using the readings from both sensors, you can find the equation of Line A and you already know the equation of Line B

  • Aligning Distance: We can do this using the Ultrasonic sensors reading to find the rear distance and keep moving back, Easy PIsy !!

Steps

  1. We have provided a sample custom service msg pkg under ebot_docking/srv/DockSw, either you can use it or else you can make your custom service msg pkg for this.

    ### Request
bool linear_dock          >> Linear Correction
bool orientation_dock     >> Angular Correction
float64 distance          >> Optional param for distance
float64 orientation       >> Goal Orientation 
string rack_no            >> Rack number
---
### Response
bool success   # indicates successful run of triggered service
string message # informational, e.g. for error messages

  2. Make a Docking service using the above pkg, which will be docking the bot using the above methods

  3. A boilerplate is been provided in the latest repo, under ebot_docking/scripts/ebot_docking_boilerplate.py

  4. In the boilerplate, you have to write the controller logic for docking. Here, for example, the P-Controller is feasible and easy. What is P-Controller, go explore the internet for this and learn.

  5. To write ros2 service server and client refer here.

Here is the example video of docking using the custom service type ebot_docking/srv/DockSw,