What is PLC?
Hi everyone,
PLCs are commonly used in industrial and commercial applications. For instance, in a production line, a PLC detects product positions through sensors and controls their movement. It can also monitor operating conditions and errors, automatically adjusting the system and sending error messages when necessary.
PLCs are flexible and easily programmable, making them suitable for various applications like managing automated doors or home heating and cooling systems.
What is a PLC? Interactive Simulation (for DERELL Plc)
Reads inputs → runs the program → updates outputs. The simulation below shows this cycle at a high‑school level.
This box simulates the PLC scan cycle. On the left is a ladder diagram; on the right are inputs/outputs and the animation.
Ladder Program
I0.0 Start (NO)
+
M0 Seal (NO)
I0.1 Stop (NC)
Q0.0 Motor
Rung 1: (Start or Seal) and (Stop closed) → Motor
I0.2 Sensor (NO)
Q0.1 Lamp
Rung 2: Sensor → Lamp
I0.2 Sensor (NO)
TON T1 (3 s)
Q0.2 Alarm
Rung 3: If Sensor stays active for 3 s → Alarm
I/O and Simulation
Scan (ms)
50
Inputs
I0.0, I0.1, I0.2 states are shown below.
States
I0.0 Start
I0.1 Stop
I0.2 Sensor
M0 Seal
Q0.0 Motor
Q0.1 Lamp
Q0.2 Alarm
T1 (ms)
Scan
Motor
Q0.0 = 0
Lamp
Q0.1 = 0
Alarm
Q0.2 = 0
If the sensor stays ON for 3 seconds, the alarm turns ON.
Quick Experiment
1) Press Start and release: Motor keeps running via seal-in (M0). 2) Hold Stop: Seal breaks, motor stops. 3) Toggle Sensor: Lamp turns on. 4) Keep Sensor ON for 3 s: Alarm turns on. 5) Change scan ms: see the PLC cycle speed.
PLC Scan Indicator
PLCs typically support the following programming languages:
Ladder Diagram: Represents connections between contacts and relays, making it easy to understand.
Function Block Diagram: Shows connections between function blocks that represent operations.
Structured Text: A structured text format that includes various statements and loops.
Sequential Function Chart: Displays the sequence of operations in a chart format.
Main Components of a PLC:
CPU (Central Processing Unit): The "brain" of the PLC, responsible for executing the control program and managing communication.
I/O Modules: Connect the PLC to sensors and actuators.
Programming Device: Used to create and edit the control program.
Power Supply: Provides electrical power to the PLC.
Enclosure: Protects the PLC from environmental factors.
PLC Operation Process:
Connect the PLC to the system and devices (e.g., sensors, actuators).
Load the control program, which dictates how the PLC operates and manages devices.
The PLC continuously monitors device measurements and controls them according to the program.
As system conditions change, the PLC updates its operations accordingly.
The PLC optimizes device operations, ensuring the system functions as intended.