A technique, which is often required in motion control applications, is referencing. This technique can be used in one of two ways: to provide a zero position signal for "homing" and also to ensure correct position synchronisation to objects whose positions vary. This example explains both uses and is based on a labelling machine.

The function of the machine is to apply adhesive labels centrally to randomly arriving products on a conveyor belt. Referencing is used to monitor the position of the product and each label (a through beam photocell for the product and a reflective photocell for the label).