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Best way of publishing to /clock topic type rosgraph_msgs/Clock?

asked 2020-03-27 01:33:13 -0500

Craigstar gravatar image

updated 2020-03-27 01:38:46 -0500

Ultimately I need to publish an accelerated time to the /clock topic which the rest of my simulation nodes are using. I have managed to concoct two separate launch files;

  1. the first launch file includes all my nodes which are driven from the /clock topic which has type rosgraph_msgs/Clock, and
  2. the second launch file instantiates a python coded node that is driven by the system (wall) clock in real time.

I wish to use this python node to accelerate time, and publish to the /clock topic, thereby driving all the simulated nodes at faster than real time.

My problem is that the /clock topic uses type rosgraph_msgs/Clock which contains a field of type time called clock. If I simply try to define and calculate an accelerated time using the Time type available in std_msgs.msg and push this into the /clock, roscore understandably detects the type mismatch as an error. If I try to access the definition of rosgraph_msgs.msg and the clock type, and push a Time type into that, again I fail.

Sample code follows below. 'pub' is fine, but 'pub1' is the version of time_dat that I want to produce as if it was of type rosgraph_msgs/Clock.

If I can appropriately get this fixed, I will be writing up a tutorial on this subject, as it appears to be a common desire to create this sort of facility. Thanks for any/all feedback, Curlyfrond

#!/usr/bin/env python
import rospy
from std_msgs.msg import Time
from rosgraph_msgs.msg import Clock

def simtime_talker():
    pub = rospy.Publisher('timey',Time, queue_size=10)
    pub1 = rospy.Publisher('clockey',Clock, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    sim_speed_multiplier = 10

   zero_time = rospy.get_time()

   while not rospy.is_shutdown():

       timepassed_dat = rospy.get_time() - zero_time
       timepassedaccelerated_dat = sim_speed_multiplier*timepassed_dat
       time_dat = rospy.Time.from_sec(timepassedaccelerated_dat)

       pub.publish(rospy.get_time())
       pub1.publish(time_dat)
       rate.sleep()

if __name__ == '__main__':
try:
    simtime_talker()
except rospy.ROSInterruptException:
    pass
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Comments

My problem is [..] roscore understandably detects the type mismatch as an error. If I try to access the definition of rosgraph_msgs.msg [..] again I fail.

This is going to sound snarky, but really: please always show us any errors or warnings or other such information, as without that, we cannot help you.

What does "I fail" mean?

gvdhoorn gravatar image gvdhoorn  ( 2020-03-27 04:52:06 -0500 )edit

Thanks, I will remember to add the error messages next time. However, the error was simply in my rather "inept" misunderstanding of Python, and I've fixed it. I needed to simply declare the Clock object I was using, and suitably fill it in with a Time. Below I have included the fixed code that works, for those interested in doing this. I will also try to write up a short example ('tutorial') for how to set the simulation clock like this, as it is a common question on ROS. Thanks gvdhoom for taking the time to read the query.

Craigstar gravatar image Craigstar  ( 2020-03-29 21:42:57 -0500 )edit

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answered 2020-03-29 21:46:47 -0500

Craigstar gravatar image

Fixed it myself :-)

Here's the working code sample.

import rospy
from rosgraph_msgs.msg import Clock

def simtime_talker():
    pub1 = rospy.Publisher('clock',Clock, queue_size=10)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    sim_speed_multiplier = 10    
    sim_clock = Clock()
    zero_time = rospy.get_time()

    while not rospy.is_shutdown():
       sim_clock.clock = rospy.Time.from_sec(sim_speed_multiplier*(rospy.get_time() - zero_time))
       rospy.loginfo(sim_clock)
       pub1.publish(sim_clock)
       rate.sleep()

if __name__ == '__main__':
    try:
        simtime_talker()
    except rospy.ROSInterruptException:
        pass
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Asked: 2020-03-27 01:33:13 -0500

Seen: 1,822 times

Last updated: Mar 29 '20