ros::Time between samples

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Hi,

I am trying to calculate time difference between data in consecutive sensor messages through the subscriber callbacks. For example: I subscribe to sensor_msgs::Imu for the imu data. I get 5 samples per 100ms. So each sample should have a time difference of 20ms with respect to the previous one.

So if the first one arrived at: 20.092s from the start of simulation, I expected the second one to show me a time stamp of 20.112s. I am collecting the time information by using: 'ros::Time::Now()'.

However, the time stamp for all the five samples shows me : 20.092s. The rate at which my publish loop is running is 10Hz i.e. 100ms.

What should I be doing to improve the granularity of the time in the subscribed data?

int main(int argc, char **argv)
{    
    ros::NodeHandle nh;
    geometry_msgs::Twist msg;
    ......//some code here

    sub_odom = nh.subscribe("odom", 100, &velCallback);

    ros::Publisher pub=nh.advertise<geometry_msgs::Twist>("cmd_vel", 100);
    ...//some other code here
    ros::Rate rate(10);

    while(ros::ok()) 
    {      
        ros::spinOnce();

       //do some work

       //publish velocity message.
       publish.pub(msg) 

       rate.sleep();
   }
   ros::spin();
   return 0;
}

void velCallback(const nav_msgs::Odometry::ConstPtr& msg)
{ 
    if(!odomfile.is_open())
    {
        odomfile.open(name.c_str());
    }

    //do somework.

    odomfile << ros::Time::now().toSec() << ","      <-- Collecting timestamp
             << msg->pose.pose.position.x << "," 
             << msg->pose.pose.position.y << ","
             << msg->pose.pose.position.z
             << std::endl;
}

For example, some data from the imu is like this:

20.092, 0.517179, 0.00451321, 9.80055
20.092, -0.0139723, 0.00388159, 9.80482
20.092, -0.0058133, 0.00101918, 9.80735
20.092, 0.0031343, -0.00360347, 9.80619
20.092, 0.00066209, -0.00248975, 9.80001
20.192, 0.485803, 0.00368372, 9.80928

Please advise. Regards, rsmitha.