[...] we bought an Ultrasonic Sensor , is it gonna work? or what alternative can we use that is cheap?
The problem with most of this methods, US, (P)IR etc. is that they have no loyalty and will follow anyone catching their attention (assuming you got something able to track people in the first place).
If you have an US emitter on the person (at the back of a belt or similar), sending out bursts/pings. You can use two receivers mounted as far apart on the vehicle as possible, to get the direction that it has to follow.
A possible weakness of this approach is, if the user turns sideways, as the shadow effect from his/her body may get one of the receivers to be temporarily "blind" - this could be handled somewhat by clever software routines though.
Use other means (like PSD or PIR) to keep the vehicle from ramming your legs.
The major weakness of PIR in this kind of apps is that the PIR only register movement perpendicular to the sensor, so if the person don't move sideways, the PIR unit will have to.
PSD's have very narrow beam angles, so will have to be scanned left right to cover the angle needed.