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I create a public class variable called double last_cmd_vel_received that gets initialized in my constructor to last_command_vel_received = 0. Then, in the main() function of my node I read in a ROS parameter called double watchdog_timeout. For the main while loop I use ros::spinOnce() and rate.sleep() (where rate is another ROS parameter value). In my class I have a public function called zeroVelocities() that just sets all the actuator outputs to safe values. So, my main while loop looks something like the following:

...
while (nh.ok())
{
    if (ros::WallTime::now().toSec() - my_class->last_cmd_vel_received > watchdog_timeout)
    {
        my_class->zeroVelocities();
    }
    ros::spinOnce();
    rate.sleep();
}
...

In my callback function for the cmd_vel topic (where the function is part of my class) I set last_cmd_vel_received = ros::WallTime::now().toSec(). You can play with timeout values however you want, but I have found that 0.3 or so works well with my robots.