I'm trying to write the logic for the following process:
This is a single conveyor system that has 4 camera stations in sequential order used for inspection. Each station gives a result for each part that passes under it. The conveyor runs at a constant speed but the parts are loaded at different intervals or timing so the spacing will always vary.
When the the part reaches the end of the conveyor I have to pick the the part and place it according to the result of each inspection station. I might have 6 parts or 2 parts on the conveyor at a time and at different locations.

I'm familiar with bit shift command but that only works for a controlled index. Can someone provide me a correct solution in ladder logic for this application? I'm using Unilogic PLC.

 

Is there always a decent gap between parts? If so, look into building a FIFO (First In First Out) array.

Unfortunately, I'm not familiar with Unilogics, so I can't help you build it on that platform.

 

A FIFO or queue of part records still sounds like the right approach here, especially since the spacing between parts isn't fixed.

I would keep one structure per part and update it as the part reaches each camera instead of relying on a timed bit shift. The key is having a real sensor or station event advance the part through the queue, otherwise the PLC's tracking will eventually drift from the physical conveyor.

I've been looking through some Unitronics hardware while trying to understand how people normally build these machine-control setups, and I came across this:
https://iqelectro.com/products/v570-57-t20b-j-unitronics-colorvision-programmable-logic-controller

It's a Vision-series controller rather than the UniStream/UniLogic platform you're using, so I'm not suggesting it as a replacement. I just found the Unitronics hardware side useful for understanding the kind of expandable I/O and machine-control architecture these systems are built around.

For your existing UniLogic setup, I'd keep the current PLC and solve this in software with a FIFO/array of structures tied to the physical camera/sensor events.