How to use ira_laser_tools package to merge laser datas [closed]

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I want to merge two different types laser data, and the two laser I used is :

1. rplidar_a2(plug in my computer by usb serial );

2. sick_tim561(communicate by Ethercat);

and I launch the laserscan_multi_merger node, the configuration of my launch file is as follows:

<launch>
     <node pkg="ira_laser_tools" name="laserscan_multi_merger" type="laserscan_multi_merger" output="screen">
       <param name="destination_frame" value="base_link" />
       <param name="cloud_destination_topic" value="/merged_cloud" />
       <param name="scan_destination_topic"  value="/scan_multi" />
       <param name="laserscan_topics" value="/scan2 /scan" />
    </node>
</launch>

The "/scan2" topic is published by the rplidar-a2 sensor,and "/scan " topic is published by the sick laser.By the way the sick laser is mount in front of my robot, and the ranges of this sensor is 270 degrees, rplidar-a2 is mount on the right of my robot,and it's scan area is 360 degrees.
The frame_id of rplidar-a2 is "laser_sl",and sick_tim561's frame_id is ''laser" .I also publish static tf transforms from "laser_sl->base_link" and "laser->base_link".

Above is the descriptions of my merge process, at the beginning the error is:

[ERROR] [1508481764.405939825]: Lookup would require extrapolation at time 1508481764.325456918, but only time 1508481764.422122593 is in the buffer, when looking up transform from frame [laser_sl] to frame [base_link]          
[ERROR] [1508481764.418808129]:Lookup would require extrapolation at time 1508481764.366851540, but only time 1508481764.440297316 is in the buffer, when looking up transform from frame [laser] to frame [base_link]

And then I find a solution to solve the error problem under the instruction of: ( http://answers.ros.org/question/22518... )

But it also have problem, the obstacles of robot's back is not show on the topic of /scan_multi,but I don't know how to solve this problem. I really want to fuse two laser data as soon as possible.If anyone can help me, I really thanks to him/her.