Hi,
@panic mode has described it perfectly. This us how I have done at one of the machines :-
encoder gives you a fixed PPR > u set electronic gear ratio of servo to feed the exact amount of film required per revolution of encoder. > make the servo in external position mode.
The above had one cutter roller (VFD controlled) , connected to encoder , driving a servo which pulls the film.
that project didn't had eye sensor. However, delta ASDA-A2 servo has a "capture" function. The document they made for this has a detailed explanation for how to use this function.
Hope it helps.
