Controlling the Turtlesim

To control/move the turtle we make use of the rostopic cmd_vel (command_velocity). This is standard rostopic that is used in practically every robot that uses ROS. With this command we can command the linear and angular speed of the robot.

Get info turtlesim

As reminder we first check the info off the turtlesim node.

Make sure you execute this in the container terminal.

rosnode info /turtlesim 

Output:

Node [/turtlesim]
Publications: 
 * /rosout [rosgraph_msgs/Log]
 * /turtle1/color_sensor [turtlesim/Color]
 * /turtle1/pose [turtlesim/Pose]

Subscriptions: 
 * /turtle1/cmd_vel [unknown type]

Services: 
 * /clear
 * /kill
 * /reset
 * /spawn
 * /turtle1/set_pen
 * /turtle1/teleport_absolute
 * /turtle1/teleport_relative
 * /turtlesim/get_loggers
 * /turtlesim/set_logger_level


contacting node http://127.0.0.1:33755/ ...
Pid: 252
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound (47461 - 127.0.0.1:38932) [16]
    * transport: TCPROS

The turtlesim node is subscribed to the cmd_vel topic. To actually send this topic rostopic pub can be used, see later on.

Get info rostopic cmd_vel

To get the message type of the rostopic /turtle1/cmd_vel we use the argument type below.

rostopic type /turtle1/cmd_vel 

Output

geometry_msgs/Twist

To get the specific info of the rosmsg we can extend the command with rosmsg show

rostopic type /turtle1/cmd_vel | rosmsg show 

Output

geometry_msgs/Vector3 linear
  float64 x
  float64 y
  float64 z
geometry_msgs/Vector3 angular
  float64 x
  float64 y
  float64 z

The requirement is for two vectors with 3 elements each. The message type is geometry_msgs/Twist.

To get a list of messages for ROS of geometry_msgs

http://wiki.ros.org/geometry_msgs (opens new window)

This displays a verbose list of topics to publish to and subscribe to and their type:

rostopic list -v 

Output

Published topics:
 * /turtle1/color_sensor [turtlesim/Color] 1 publisher
 * /rosout [rosgraph_msgs/Log] 1 publisher
 * /rosout_agg [rosgraph_msgs/Log] 1 publisher
 * /turtle1/pose [turtlesim/Pose] 1 publisher

Subscribed topics:
 * /turtle1/cmd_vel [geometry_msgs/Twist] 1 subscriber
 * /rosout [rosgraph_msgs/Log] 1 subscriber

Move the turtle

The following command will send a single message to turtlesim telling it to move with a linear velocity of 2.0, and an angular velocity of 1.8. It will move from its starting position along a circular trajectory for a distance and then stop.

rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]' 

• -1, --once: publish one message and exit
• -r RATE, --rate=RATE publishing rate (hz). For -f and stdin input, this defaults to 10.

TIP

With tab completion it is easy to find the data formats

rostopic (Tab) pub -1 /tur (Tab) cm (Tab) geo (Tab)  (Tab) (Tab) …..	

Output:

rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist 
"linear: 
    x: 0.0
    y: 0.0
    z: 0.0
angular: 
    x: 0.0
    y: 0.0
    z: 0.0" 

Now backspace to fill in the values x=0.0 (linear) and z= 1.8 (angular) (Not yet executed)

If ENTER the rostopic will be publish once due to -1

WARNING

Depending on the programming style of the used robot it can be a very dangerous control with one msg. It is better to work with a stream.

To check what is sent with rostopic /turtle1/cmd_vel use the echo argument. Off-course this has to be done in an extra terminal.

rostopic echo /turtle1/cmd_vel

Output:

linear: 
  x: 1.0
  y: 0.0
  z: 0.0
angular: 
  x: 0.0
  y: 0.0
  z: 1.8
---

To check where the turtle is read the /turtle1/pose topic.

rostopic echo /turtle1/pose 

---
x: 0.332792580128
y: 4.38678216934
theta: -0.580785274506
linear_velocity: 0.0
angular_velocity: 0.0
---

Use CTRL+c to stop the output of position, orientation and velocity.

From the ROS tutorial, a geometry_msgs/Twist msg (cmd_vel topic)has two vectors of three floating point elements each: linear and angular.

'[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]' 

In this case, '[2.0, 0.0, 0.0]' becomes the linear value with x=2.0, y=0.0, and z=0.0, and '[0.0, 0.0, 1.8]'is the angular value with x=0.0, y=0.0, and z=1.8.

These arguments are actually in YAML syntax, which is described more in the YAML command line documentation.

You will have noticed that the turtle has stopped moving; this is because the turtle requires a steady stream of commands at 1 Hz to keep moving. We can publish a steady stream of commands using rostopic pub -r

rostopic pub /turtle1/cmd_vel geometry_msgs/Twist -r 1 -- '[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]' 

Here we publish the topic /turtle1/cmd_vel with the message to repeat the message at 1 second intervals with linear velocity 2 and angular velocity 1.8.

Before we go further reset the location of the turtle with a rosservice

rosservice call /reset 

You will see the turtle is running in a circle.

To Show the rate in Hz of the published topic /turtle1/pose (CTRL-C to stop data stream):

rostopic hz /turtle1/pose 

Output:

subscribed to [/turtle1/pose]
average rate: 62.537
        min: 0.015s max: 0.017s std dev: 0.00046s window: 59
average rate: 62.535
        min: 0.015s max: 0.017s std dev: 0.00046s window: 121
average rate: 62.507
        min: 0.015s max: 0.017s std dev: 0.00048s window: 184
average rate: 62.499
        min: 0.015s max: 0.017s std dev: 0.00047s window: 246

To show the rate in Hz of the published topic /turtle1/cmd_vel (CTRL-C to stop data):

rostopic hz /turtle1/cmd_vel

Output:

subscribed to [/turtle1/cmd_vel]
no new messages
average rate: 1.001
        min: 0.999s max: 0.999s std dev: 0.00000s window: 2
average rate: 1.000
        min: 0.999s max: 1.000s std dev: 0.00019s window: 3
average rate: 1.000
        min: 0.999s max: 1.001s std dev: 0.00051s window: 4
average rate: 1.000
        min: 0.999s max: 1.001s std dev: 0.00048s window: 5
average rate: 1.000
        min: 0.999s max: 1.001s std dev: 0.00044s window: 6

Move the turtle with teleop

If you want to control the turtle quickly by keyboard then you can use the turtlesim_teleop_key.

WARNING

Make sure that all other nodes that publish the /turtle1/cmd_vel are deactivated. Otherwise you could get some funny movements. 😃

Check if the following commands are running in seperate container terminals:

roscore
rosrun turtlesim turtlesim_node 

In a third window, we execute a node that allows keyboard control of the turtle. roscore is running in one window and turtlesim_node in another.

If needed open a new connection with container.

docker exec -it turtlesim_cont bash

rosrun turtlesim turtle_teleop_key 

Output:

Reading from keyboard
---------------------------
Use arrow keys to move the turtle. 'q' to quit.

WARNING

To move turtle with arrow keys, be sure the focus is on the window that started turtle_teleop_key. Otherwise the turtle won't move.

TIP

With space you can stop the turtle.

With the argument list we can check which rosnodes are running.

rosnode list 

Output:

/rosout
/rostopic_1893_1644500426715
/rqt_gui_cpp_node_888
/rqt_gui_py_node_1229
/rviz_1644486378997723646
/teleop_turtle
/turtlesim

We can see that the active rosnode for the teleop controlling is named /turtleop_turtle and not the turtle_teleop_key name used in rosrun command.

To get the info of the /teleop_turtle node:

rosnode info /teleop_turtle 

Output

Node [/teleop_turtle]
Publications: 
 * /rosout [rosgraph_msgs/Log]
 * /turtle1/cmd_vel [geometry_msgs/Twist]

Subscriptions: None

Services: 
 * /teleop_turtle/get_loggers
 * /teleop_turtle/set_logger_level


contacting node http://127.0.0.1:43149/ ...
Pid: 2350
Connections:
 * topic: /rosout
    * to: /rosout
    * direction: outbound (51105 - 127.0.0.1:39512) [13]
    * transport: TCPROS
 * topic: /turtle1/cmd_vel
    * to: /rostopic_1893_1644500426715
    * direction: outbound (51105 - 127.0.0.1:39510) [12]
    * transport: TCPROS
 * topic: /turtle1/cmd_vel
    * to: /turtlesim
    * direction: outbound (51105 - 127.0.0.1:39514) [10]
    * transport: TCPROS