Hi, i worked in a similar project for robot arm. I used the ultrasonic sensor at transport band and the prismatic articulation for my arm. i created one trajectory that depended of a inductive sensor and ultrasonic sensor, when the indutive sensor is high the arm did a particular trajectory for put the object and when the ultrasonic sensor is high, so, the arm put the object with diferent trajectory.
if you need my help, so write me.