Introduction to Programmable Logic Controllers
Programmable Logic Controllers (PLCs) have become one of the key elements of automation since their emergence in the late 1960s. The first PLCs were developed by Modicon for General Motors and have since revolutionized the industry by replacing hours of hard wiring, relays, and timers on factory floors.
Functions and Components of PLCs
These devices serve as central management systems for various automated systems. PLCs are equipped with numerous inputs and outputs that implement the functions of switches and relays using transistors and other electronic circuits. PLCs can be programmed through standard computer interfaces, utilizing both proprietary languages and network protocols.
Specifications of PLCs
The specifications for the input and output channels of PLCs include:
- Number of Points: The total number of inputs and outputs of the controller.
- Number of Inputs and Outputs: PLCs can have various combinations of values for inputs and outputs.
- Expansion Capability: The availability of the possibility to add new channels to the system.
- Maximum Number of Channels: This is the maximum number of input and output channels in an expanded system.
- Scanning Time: The time required to check the states of inputs and outputs.
- Data Memory: The amount of data that can be stored in the system.
Types of Inputs and Outputs
The inputs that a PLC can support include:
- AC (alternating current)
- DC (direct current)
- Analog signals
- Thermocouples
- RTD (resistance temperature detectors)
- Frequency and pulse signals
- Transistor and interrupting inputs
The outputs of a PLC can be represented by the following categories:
- Alternating current
- Direct current
- Relays
- Analog signals
- Frequency or pulses
- Transistors
- Triacs
Programming and Languages
PLCs can be programmed using various interfaces, such as a front panel, manual control, and computer. There are numerous programming languages that allow for programming PLCs from different manufacturers. The hardware in PLCs can control elements such as sensors, actuators, valves, etc., using Boolean logic, where different voltage values are interpreted as: 1 - ON state and 0 - OFF state.
Selecting a PLC
When choosing a PLC, it is important to consider:
- Number of points
- Maximum number of channels
- Number of instructions executed
- Scanning time
- Memory capacity
- Power supply and mounting options
- Operating conditions
These factors affect the efficiency and reliability of the programmable logic controller's operation in automated systems.