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(Citadel ign gazebo+Foxy) How to control a single joint using diff driver?

asked 2021-09-01 12:41:03 -0500

kak13 gravatar image

I was learning how to use Foxy with Citadel. I noticed that this generates 4 topics in the launch

 # Bridge
bridge = Node(
    package='ros_ign_bridge',
    executable='parameter_bridge',
    arguments=['/model/vehicle_blue/cmd_vel@geometry_msgs/msg/Twist@ignition.msgs.Twist',
               '/model/vehicle_blue/odometry@nav_msgs/msg/Odometry@ignition.msgs.Odometry',
               '/model/vehicle_green/cmd_vel@geometry_msgs/msg/Twist@ignition.msgs.Twist',
               '/model/vehicle_green/odometry@nav_msgs/msg/Odometry@ignition.msgs.Odometry'],
    output='screen'
)

But then I decided to add double diff drivers like this in the robot model:

  <plugin
    filename="ignition-gazebo-diff-drive-system"
    name="ignition::gazebo::systems::DiffDrive">
    <left_joint>rear_left_wheel_joint</left_joint>
    <right_joint>rear_right_wheel_joint</right_joint>
    <wheel_separation>1.25</wheel_separation>
    <wheel_radius>0.3</wheel_radius>
  </plugin>

   <plugin
    filename="ignition-gazebo-diff-drive-system"
    name="ignition::gazebo::systems::DiffDrive">
    <left_joint>front_left_wheel_joint</left_joint>
    <right_joint>front_right_wheel_joint</right_joint>
    <wheel_separation>1.25</wheel_separation>
    <wheel_radius>0.3</wheel_radius>
  </plugin>

I don't see any specific joint. Let's say, I want to control the front right wheel only. How do I do that?

I wrote the simple code and it kinda focus on forward, left, right and backward. I can't find a way to control one joint only. Here is the code I wrote:

#!/usr/bin/env python3

import sys
import time

import geometry_msgs.msg
import rclpy

if sys.platform == 'win32':
    import msvcrt
else:
    import termios
    import tty


rclpy.init()
node = rclpy.create_node('teleop_twist_keyboard')
pub = node.create_publisher(geometry_msgs.msg.Twist, '/model/vehicle_green/cmd_vel', 10)

def backward():
    twist = geometry_msgs.msg.Twist()
    twist.linear.x = 2.0  # positive goes backward, negative goes forward
    twist.angular.z = 0.0
    print("Backward.")
    pub.publish(twist)
    time.sleep(0.5)
    twist.linear.x = 0.0
    twist.angular.z = 0.0
    pub.publish(twist)


if __name__ == '__main__':
    backward()

I tried to change from geometry_msgs.msg.Twist, '/model/vehicle_green/cmd_vel' to `geometry_msgs.msg.Twist, '/model/vehicle_green/front_left_wheel_joint/cmd_vel' and it didn't work. Do you have any suggestion?

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1 Answer

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answered 2021-09-26 21:13:20 -0500

A differential drive vehicle by definition is controlled by sending different velocities to each of their 2 driving wheels. So I don't think you want a diff drive controller to be driving a single joint.

Try the JointController plugin instead.

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Comments

Thank you very much for your time! I truly appreciate you answered my post second time now!

kak13 gravatar image kak13  ( 2021-09-27 11:22:07 -0500 )edit

Question, what is the bridge for this jointcontroller?

Like,

'/head@sensor_msgs/msg/JointState@ignition.msgs.JointCmd',

For example in the launch file

kak13 gravatar image kak13  ( 2021-09-27 13:23:22 -0500 )edit

Disregard this, I found the answer

kak13 gravatar image kak13  ( 2021-09-27 14:33:13 -0500 )edit

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Asked: 2021-09-01 12:41:03 -0500

Seen: 60 times

Last updated: Sep 26 '21