Robot following person

Hi, I am working on a project , so that it can follow the person that is specified. So i want some help about that, what sensors should i use that are not too much expensive. Also i am not expert so i want suggestions which are at intermediate level.

Note: that the robot will follow one single person among the croud etc.

PLEASE HELP!

This has been answered in the last few days.

http://forum.arduino.cc/index.php?topic=344510.msg2375523#msg2375523

The answer is that it is not practical project, engineers have spend a lot of time and huge amounts of money to do it.

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