What is the function of sensor on PLC system?

simply from it's name sensor it means to sense and send alarm to the plc that the action is done and do your steps

sensors is a device that convert the physical quantity such as pressure, temperature.
flow , speed ..etc into analog electrical signal to be converted into digital signal and supplying digital signal to the PLC.

Sensors send incoming signals either digital or analog to PLC.
There are a lot of types of sensors: inductive, capacitive, temperature, optical, etc.

Even in PLC's, the basic principle of relay contacts extensive to basic electrical controls still are applied.
Sensors for pressures, flow, temperatures utilized with transducers that activates micro-relays and micro-switches.
Level and limit switches, electromagnetic and actuators that emits movement that will be converted to signal are classified as sensors that will also activate the micro-relays and micro-switches utilized in PLC's.

Sensor is nothing but the system's window to the world.
It can be defined as a device which converts physical stimulus into a readable output.

Sensors are the main inputs for plcs,the only role they got is to introduce the parameters of the external world to the plc .

Sensor provides data to P.L.C in digital signal and sense analog data i.e temperature , humidity, capacitence, induction , speed , voltage etc.
The difference between a sensor and a transducer is that the former convert that analog information specifically in to digital so that it can be understood by the P.L .
C device.

the sensors give electrical signals( pulses), to the PLC , as  answers. Then the PLC will decide the next action to do.

Sensors are the main inputs for the PLC which they convert the physical quantity to analog electrical signal which is then converted to digital signal as to be read by the PLC.

the sensors in the plc  is the signals that limits the flow or volume of liquid and activate pumps to supply liquids up to maximum setting .

Sensors send incoming signals either digital or analog to PLC.
There are a lot of types of sensors: inductive, capacitive, temperature, optical, etc.