In industry, there are many cells used throughout the factory. The  control sys

In industry, there are many cells used throughout the factory. The  control system for those cells is your PLC. Some of the most common ones  are:
Distributing
Testing
Assembly
Pick & Place
Sorting
For the proper operation of those cells, we need sensors and actuators to allow the PLC to execute the programmed logic.  

 
Let’s consider the scenario of the sorting station from FESTO that you will work on in your future PLC courses:
The  Sorting station sorts the workpieces onto three chutes. The detection  module is capable of verifying the presence of red, black, and metallic  workpieces. 
Workpieces placed onto the beginning of the conveyor belt are detected.
The workpieces are stopped by a pneumatic stopper to determine their characteristics.
The sensors in the detection module ascertain the workpieces’ material and color (black, red, metallic).
The workpieces are sorted onto the appropriate chutes by means of electrically operated deflectors.
A retroreflective sensor monitors the fill levels of the chutes.
Think  about how many and what types of sensors are needed for that system.  These sensors will provide the input signals to the PLC, which will  generate the output to turn on the conveyor belt, pneumatic stopper, or  deflectors. 
In your original post, describe the process flow step by step. For example, the initial check would be similar to:
Sequence
1. Workpiece detected at the beginning of the conveyor belt
2. Conveyor motor on
3. Color/material detection