5.3 PLC Programming Basics
Key Concepts
- PLC (Programmable Logic Controller)
- Ladder Logic
- Input/Output (I/O) Configuration
- Programming Software
- Basic Instructions
PLC (Programmable Logic Controller)
A PLC is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines. It reads input signals from sensors, processes them according to a stored program, and controls output devices like motors and valves.
Example: In an automotive assembly line, a PLC might control the robotic arms that weld car parts together. The PLC reads signals from proximity sensors to determine the position of the parts and then sends signals to the robotic arms to perform the welding.
Ladder Logic
Ladder Logic is the most common programming language used for PLCs. It is based on the concept of relay logic, where electrical components are represented as symbols connected in a ladder-like diagram. Each rung of the ladder represents a logic operation.
Example: Imagine a simple light switch circuit. In ladder logic, the switch (input) and the light (output) would be represented as symbols connected in a single rung. When the switch is closed, the light turns on, mimicking the behavior of a real-world circuit.
Input/Output (I/O) Configuration
I/O Configuration refers to the setup of input and output devices connected to the PLC. Inputs are sensors that provide information to the PLC, while outputs are actuators that the PLC controls. Proper configuration ensures that the PLC can correctly interpret inputs and control outputs.
Example: In a conveyor belt system, the PLC might be configured with input devices like photoelectric sensors to detect the presence of items on the belt. Output devices could include motors that drive the belt. The I/O configuration ensures that the PLC knows which sensor signals to read and which motor signals to send.
Programming Software
Programming software is used to write, edit, and upload programs to the PLC. It provides a user-friendly interface for creating ladder logic diagrams and configuring I/O devices. Common software includes Siemens TIA Portal, Allen-Bradley RSLogix, and Schneider Electric Unity Pro.
Example: A technician uses Siemens TIA Portal to create a ladder logic program for a PLC controlling a water treatment plant. The software allows the technician to drag and drop symbols, configure I/O devices, and simulate the program before uploading it to the PLC.
Basic Instructions
Basic instructions are the fundamental operations that can be performed in PLC programming. These include instructions for reading inputs, writing outputs, performing logic operations (AND, OR, NOT), and controlling timers and counters.
Example: A basic instruction in ladder logic might be an "AND" operation. If two switches are connected in series, the output will only turn on if both switches are closed. This mimics the behavior of a logical AND gate in digital electronics.