Actually, PIR should work if you use 2 PIR sensors, and arrange them so there is a small
overlap of their receptive fields - out at the edge. Then, you will have a "binocular"
system with 2 wide peripheral pickup areas [approx 120 deg each, depending on the
sensors], and a common area of 10-degrees or so where the edges overlap.
When you walk into the room, one of the 2 sensors will pick you up, and the servo (I
presume there's a servo) will pan over until both PIRs are picking you up. Then, as you
move it will continue to follow you (you will write good s.w. for this, I presume).
Once it's locked on, it will stay locked on. If someone else enters the room, it won't
budge, as long as it's already locked on (assuming the s.w. is written this way).
I've done this, it works. My robot could track me out to 20-feet or so. These are the
sensors I used, AMP PIRM 180-200, although bought years ago and elsewhere,
http://www.ebay.com/itm/NIB-ATO-Atochem-910404-Motion-Detector-Detection-Module-PIRM-180-200-Gnd-5V-/281065243211?pt=BI_Security_Fire_Protection&hash=item4170cb424b
or something else from here,