Some help would be appreciated, We've thought about using PIR combined with PID, but we need to know whether or not it will work.
Welcome to the forum!
It's likely you will need to be much more specific about what you hope to achieve if you want to get help.
What kind of robot? Follow human under what conditions? Single person in an empty room or a particular human amongst a crowd? How far from the robot to the person? Will the robot have to firstly find the person or is it just expected to follow along?
PIR assumes person is warmer than surrounds- is that going to be the case?