Programmable Logic Controllers: Architecture and Functionality

Programmable Logic Controllers (PLCs) Explained

A Programmable Logic Controller (PLC) is an electronic device capable of processing analog, digital, numeric, and alphanumeric information. It is designed to monitor real-time automated industrial processes. Basically, a PLC consists of a control unit, memory, and input/output modules. PLC manufacturers establish communication with external circuits. These circuits are constituted by sensors that send information from the industrial process to the PLC, and actuators that implement program orders.

Automation Approaches: Wired vs. Programmed

Wired Automation Limitations

Traditional wired automation presents several challenges:

• Requires large conduits and control panels.
• Lacks flexibility; variations do not allow for the same wiring configurations.
• Difficulty in maintenance and troubleshooting.
• High installation costs, requiring highly qualified staff.

Advantages of Programmed Automation (PLCs)

Programmed automation, primarily through PLCs, offers significant benefits:

• Modifications can be introduced without altering the physical installation.
• Reduces the space occupied by control equipment.
• Lower installation costs.
• Easier maintenance.
• Improved fault diagnosis and self-localization of failures.
• Ability to control multiple processes with a single equipment.

Key Components of a PLC

PLC Memory Types

PLCs utilize different types of memory for various functions:

• RAM (Random Access Memory): This memory can be both read from and written to. In the absence of power, information stored in RAM volatilizes away. To retain this information, the PLC must be provided with a battery backup.
• ROM (Read-Only Memory): This type of memory can only be read. Data is permanently stored and cannot be altered by the user during normal operation.

PLC Power Supply

The power supply's mission is to convert alternating current (AC) to direct current (DC) to power the integrated circuits and electronic components. PLCs normally operate with 24V DC voltages. When passive sensors are located close to the PLC, they can often be directly connected to the PLC's internal power source.

Input and Output (I/O) Modules

I/O modules are formed by a set of modules and a support structure for connections. Their main functions are:

• To adapt the working voltage of sensors and actuators for the PLC's electronic devices, which often operate at different voltages.
• To electrically isolate the control circuits from the power circuits.

Sensor Types for PLCs

Passive Sensors

These sensors have two distinct states: activated or not activated, typically triggered by a mechanical action.

Active Sensors

These are electronic devices that require a voltage supply. They vary their state to produce a logical signal based on the measured parameter.

External Features of PLCs

Common external features found on PLCs include:

• Power supply terminals
• Input/Output (I/O) connection terminals
• Interface expansion slots
• LED indicators
• Battery cartridge accommodation
• EEPROM connector
• Run/Stop switch
• Analog potentiometers

PLC Peripherals and Classification

PLC Peripherals

Programming devices are elements that allow user communication with the PLC, enabling tasks such as:

• Introducing programs into memory.
• Editing and modifying existing programs in the PLC's memory.
• Detecting anomalies in the program format display.
• Real-time display and input of output states.

PLC Classification

Programmable Logic Controllers can be broadly classified into:

• Compact PLCs
• Semi-compact PLCs
• Modular PLCs