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Sync problems between ROS and Gazebo

asked 2018-06-20 07:34:12 -0600

schizzz8 gravatar image

updated 2018-06-20 12:56:12 -0600

Hi all,

I built a simple simulated environment and a diff-drive robot (with an Hokuyo and a Kinect) in Gazebo to perform some mapping experiments. All the config files are in this repository and you may test it by running:

roslaunch lucrezio_simulation_environments empty_world_with_apartment_and_robot.launch

After running the experiments I noticed strange behaviors. This is what happens when I try to localize the robot on a map that I previously built and, at the same time, on RViz I want to visualize the colored point cloud returned by the Kinect:

The only interpretation I can give is that there is some delay in the published transforms.

To check this, I wrote a minimal node that does the following:

  • listens to /scan topic to receive laser messages

  • uses tf::TransformListener to receive the TF between /odom and /base_footprint frames

this is the code:

void laserCallback(const sensor_msgs::LaserScan::ConstPtr &laser_msg){
  ros::Time stamp = laser_msg->header.stamp;
  ROS_INFO("Laser msg timestamp: %f",stamp.toSec());

  tf::TransformListener odom_listener;
  tf::StampedTransform odom_tf;

  } catch (tf::TransformException ex){
  ROS_INFO("Odom tf timestamp: %f",odom_tf.stamp_.toSec());

when I execute it, this is what I get:

dede@srrg-02:~$ rosrun lucrezio_semantic_mapper pose_broadcaster_node 
[ INFO] [1529497453.859222896, 17.178000000]: Laser msg timestamp: 17.145000
[ERROR] [1529497454.891297562, 18.207000000]: Lookup would require extrapolation into the past.  Requested time 17.145000000 but the earliest data is at time 17.356000000, when looking up transform from frame [base_footprint] to frame [odom]
[ INFO] [1529497454.891428167, 18.207000000]: Odom tf timestamp: 0.000000
[ INFO] [1529497454.902422031, 18.218000000]: Laser msg timestamp: 18.196000
[ERROR] [1529497455.937621922, 19.248000000]: Lookup would require extrapolation into the past.  Requested time 18.196000000 but the earliest data is at time 18.356000000, when looking up transform from frame [base_footprint] to frame [odom]
[ INFO] [1529497455.937735340, 19.248000000]: Odom tf timestamp: 0.000000

if, instead of stamp, I pass ros::Time(0) as argument to waitForTransform() and lookupTransform() I get no error but I see that the laser message stamp and the tf stamp have a not negligible difference:

dede@srrg-02:~$ rosrun lucrezio_semantic_mapper pose_broadcaster_node 
[ INFO] [1529497759.281287706, 321.114000000]: Laser msg timestamp: 321.107000
[ INFO] [1529497759.484725587, 321.317000000]: Odom tf timestamp: 321.316000
[ INFO] [1529497759.499681070, 321.331000000]: Laser msg timestamp: 321.308000
[ INFO] [1529497759.685763872, 321.516000000]: Odom tf timestamp: 321.516000

Please, can someone explain me what is happening and how to tackle this problem?


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1 Answer

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answered 2018-06-20 13:04:40 -0600

tfoote gravatar image

You are getting a lookup error because you are creating your TransformListener inside the callback. The transform listener requires building up a cache of known transforms. When you construct the listener it will be initialized with an empty cache. In that case it's likely that you'll never be able to transform.

Note in the tutorial how the listener is initialized outside the loop not inside.

The lifetime requirements are explained below:

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Asked: 2018-06-20 07:34:12 -0600

Seen: 492 times

Last updated: Jun 20 '18