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Using Time and TF

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Using Time and TF on the Arduino

The rosserial_arduino package contains libraries for generating timestamps on the Arduino which are synchronized with the PC/Tablet on which the roscore instance is running. This tutorial shows how to access time, using an example of publishing a tf transform.

The Code

If you have followed the Arduino IDE Setup tutorial, you'll be able to open the sketch below by choosing ros_lib -> TimeTf from the Arduino examples menu.

This should open the following code in your IDE:

   1 /*
   2  * rosserial Time and TF Example
   3  * Publishes a transform at current time
   4  */
   5 
   6 #include <ros.h>
   7 #include <ros/time.h>
   8 #include <tf/transform_broadcaster.h>
   9 
  10 ros::NodeHandle  nh;
  11 
  12 geometry_msgs::TransformStamped t;
  13 tf::TransformBroadcaster broadcaster;
  14 
  15 char base_link[] = "/base_link";
  16 char odom[] = "/odom";
  17 
  18 void setup()
  19 {
  20   nh.initNode();
  21   broadcaster.init(nh);
  22 }
  23 
  24 void loop()
  25 {
  26   t.header.frame_id = odom;
  27   t.child_frame_id = base_link;
  28   t.transform.translation.x = 1.0;
  29   t.transform.rotation.x = 0.0;
  30   t.transform.rotation.y = 0.0;
  31   t.transform.rotation.z = 0.0;
  32   t.transform.rotation.w = 1.0;
  33   t.header.stamp = nh.now();
  34   broadcaster.sendTransform(t);
  35   nh.spinOnce();
  36   delay(10);
  37 }

The Code Explained

Now, let's break the code down.

Error: No code_block found We need to include our typical ROS stuff, as well as the transform broadcaster.

Error: No code_block found Next, we instantiate a TransformStamped message to use, and a broadcaster. We also need to specify the names of the frames we are publishing a transform for.

Error: No code_block found Inside the setup() function, we have to call init() on the TransformBroadcaster with the node handle as a parameter. Without doing this, the broadcaster will not publish correctly.

Error: No code_block found Inside the loop() function, we fill in the fields of our transform. The frame IDs are set to the correct string names, and the values of the translation and rotation are set.

Error: No code_block found Calling nh.now() returns the current time, as a ros::Time instance, just like when using roscpp's ros::Time::now().

Error: No code_block found Finally, we publish the transform, and then spinOnce and wait a bit before doing it again. Transforms should always be published at a regular rate, although usually the data fields will be filled in from real data.

Uploading the Code

To upload the code to your Arduino, use the upload function within the Arduino IDE. This is no different from uploading any other sketch.

Running the Code

Now, launch the roscore in a new terminal window:

roscore

Next, run the rosserial client application that forwards your Arduino messages to the rest of ROS. Make sure to use the correct serial port:

rosrun rosserial_python serial_node.py /dev/ttyUSB0

rosrun rosserial_python serial_node.py _port:=/dev/ttyUSB0

rosrun rosserial_python serial_node.py /dev/ttyUSB0

Finally, you can check the transform using:

rosrun tf tf_echo odom base_link

This by default will echo the transform being sent from the Arduino approximately once per second. Note that the Arduino is likely broadcasting at a higher rate (Mega2560 publishes at about 57hz when connected at 57600)

Or, by running:

rosrun tf view_frames

This last command will run for about 5 seconds and generate a 'frames.pdf' file in the directory you ran the command in.

Further Reading

Please see rosserial/Overview for more information using Time instances.