PLCs Help with photoeyes (photosensors)

I need to design a program a pick and place system using a robot to remove cases from a conveyor to a pallet. the system has 3 conveyors. The first two conveyors each have a photo eye. The part I'm having trouble with is that I must maintain spacing by controlling the conveyors. I'm not sure how to do this

here is the drawing i have to go off of

Hmm,
You must stop conveyors when photoeye detect the object.
To "on" conveyor 2 photoeye'3 = 1 and photoeye'1 = 0
To "on" conveyor 1 photoeye'1 = 0 and photoeye'2 = 1