The primary objective of the PLC lab is to familiarize students with how signals from input devices (i.e. switches, sensors, etc.) can interface with PLC units in order to invoke certain actions (i.e. start/stop an electric motor, turn on a light, etc.). A PLC functions similar to your computer. A computer processes input (i.e. keyboard entries, sounds from microphones, etc.) and invokes a certain output (i.e. showing the letter you typed on the screen, sending a request through the internet, etc.). Similarly, a PLC unit processes input and invokes output, but the inputs to a PLC unit are usually devices such as sensors, buttons, switches, etc. The outputs are usually signaling to start/stop motors, turn on/off lights, sound a horn, etc.
One example of where PLCs are used is in traffic lights. The inputs for traffic lights are induction loops that are installed into the streets themselves. When your car goes over the loop, it changes the inductance of the loop, which can be detected by a sensor and signaled to the PLC unit. The PLC unit can then react to that signal by processing it based on a specific ladder logic program that it is running. For example, the ladder logic can be programmed to set a timer to change all the other lights to red and then change the light for the lane your car is in to green. Other examples include controlling conveyor belts and other devices prevalent in manufacturing plants and other industrial settings.