How to use tf in more than one robot?

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try setting the tf_prefix rosparam in your launch file.

http://ros.org/wiki/geometry/CoordinateFrameConventions#tf_prefix

as a word of caution: having multiple robots on a single master consumes a lot of network bandwidth (especially if they share /tf!), you may run into seemingly strange issues as the result of dropped packets. If you try and scale beyond two robots without throttled topics over a standard 802.11g wifi connection you will definitely run into bandwidth constraints and may be forced to run a single master per robot and foreign relay between them.