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Non-synchronized nodes in different machines. What to do?

asked 2012-12-19 07:11:25 -0600

updated 2012-12-19 07:19:20 -0600

I have two different machines connected by wifi sharing a ROS_MASTER. One of them is the erratic robot which is sending odom messages continuously. The point is that those messages are being received in the other machine with header.stamp > ros::Time::now() ... when it should be header.stamp <= ros::Time::now(). The time distance is ~0.001s. The machines's clocks clearly aren't synchronized (a past reading can never came from the future).

I don't know in depth how NTP works but I expected it would be working properly. What should I do? Is there any typical way to afford this problem?

In the case of the erratic robot I've been diving a bit in its driver and we get that the stamp comes from the internal player code:

   player_msghdr_t* hdr = msg->GetHeader();

        if ((hdr->type == PLAYER_MSGTYPE_DATA) &&
            (hdr->subtype == PLAYER_POSITION2D_DATA_STATE) &&
            (hdr->addr.interf == PLAYER_POSITION2D_CODE))
        {
            // Cast the message payload appropriately
            player_position2d_data_t* pdata = (player_position2d_data_t*) msg->GetPayload();

[...]

odom.header.frame_id = tf::resolve(tf_prefix_, odom_frame_id);
            odom.header.stamp.sec = (long long unsigned int) floor(hdr->timestamp);
            odom.header.stamp.nsec = (long long unsigned int) ((hdr->timestamp - floor(hdr->timestamp)) * 1000000000ULL);

Regards.

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answered 2012-12-19 07:26:10 -0600

updated 2012-12-19 07:26:39 -0600

Just install ntpd server and checking they share the same synchronization servers (/etc/ntpd.conf) looks to solve the problem. I'll keep you informed.

sudo apt-get install ntp

ref: https://help.ubuntu.com/8.04/serverguide/NTP.html

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Comments

1

On the PR2 and a couple of other robots I know about, chrony is used.

Lorenz gravatar imageLorenz ( 2012-12-19 07:30:21 -0600 )edit

Right. I'll take it into account. It looks great https://wiki.archlinux.org/index.php/Chrony

Pablo Iñigo Blasco gravatar imagePablo Iñigo Blasco ( 2012-12-19 07:50:27 -0600 )edit
2
1

But I must add that I tried this, and have not gotten the two laptops on my custom-made robot to time sync, maybe because their wireless link is through a router

weiin gravatar imageweiin ( 2012-12-19 13:35:13 -0600 )edit

weiin. Did you use ntp or chrony?

Pablo Iñigo Blasco gravatar imagePablo Iñigo Blasco ( 2013-01-13 05:56:23 -0600 )edit

I followed that faq I linked above. chrony.conf to slowly correct drifts, ntpdate to do immediate corrections. But so far I'm usually still getting around 0.2s discrepancy after a while (that's why I use ntpdate to correct instantly)

weiin gravatar imageweiin ( 2013-01-14 12:36:17 -0600 )edit

How much is it a while? more or less?? seconds, minutes, hours?

Pablo Iñigo Blasco gravatar imagePablo Iñigo Blasco ( 2013-01-14 12:55:51 -0600 )edit

I have not done any quantification for 'a while'. I believe it depends on the connection between the two laptops since by definition in chrony.conf, the second laptop needs to sync to the server. As mentioned, my laptops are connected through a wireless router, so connection can get flaky

weiin gravatar imageweiin ( 2013-01-14 15:46:57 -0600 )edit

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Asked: 2012-12-19 07:11:25 -0600

Seen: 1,822 times

Last updated: Dec 19 '12