The Basics of Programmable Logic Controllers

Programmable Logic Controllers

Programmable Logic ControllersProgrammable logic controllers provide automatic control of processes so that very little human intervention is required. Yet, this results in a well-coordinated, repetitive process.

These systems automatically start and shut down critical equipment in large and complex manufacturing processes. When in operation, programmable logic controllers ensure that all conditions are met before starting up a certain machine or process. In addition, the PLC continues monitoring the entire system using a variety of sensors.

Consequently, this ensures that everything runs as programmed. The PLC will trip if it detects a malfunction or abnormality in any of the monitored parameters.

The system requirements

In the first step, define what requires control and the tasks that need to be accomplished. By analyzing the processes and breaking them into individual tasks, you identify the inputs and outputs. With an understanding of these functions, the operator knows which program instructions are required to achieve these goals.

Speed of Operation

The PLC should have the capacity to operate at the speed of the equipment and processes it controls. In addition, it should be able to perform other functions such as counting pulses from a flow meter or an encoder. So, it responds fast enough without delaying other processes or interfering with the operation of the machinery.

Communication with the PLC system and devices

Programmable Logic ControllersCommunication between the PLC and external devices remains highly important. Access or share the status or application data using devices such as a computer. Furthermore, most PLC systems include options like Ethernet, wired or radio modem option. An Ethernet port allows connection to computers and other standard systems. Therefore, the PLC or equipment status can be accessed from a control room or remotely using the internet.

The PLC operator interface

The level of interaction with the system determines the type of interface that the PLC should have. Especially relevant, the operators access and observe real-time equipment status.

Modern Programmable logic controllers contain interfaces to connect external devices such as computers and displays. Hence, most of the traditional systems provide electronic operator interfaces. These may include pilot lights, push buttons, numeric displays etc. Hence, check the ability of the PLC to interface with other systems and devices.

Physical environment to locate the PLC system

The operating environment, size of the processes, ease of access by operators and maintenance personnel influence the choice of PLC.

A harsh environment with dust or small particles will require a PLC system with an appropriately IP rated enclosure. This prevents ingress of foreign particles into the system. The available space and layout also influence the choice. Therefore, most users require easy access for reprogramming, maintenance, or troubleshooting.

The Types of Programmable Logic Controllers

Unitary PLC

The simplest form of a PLC, all the basic components of a PLC include the I/O points, CPU, and software program. It is typically housed in a single enclosure. These PLCs are attached directly to the equipment being controlled.

Modular PLC

Modular PLCs consists of several components on a common bus within a base unit to allow extending of the input and output. These units work on an expandable base unit. Therefore, it provides all the basic components of a PLC, i.e., a power supply, I/O points, memory and the CPU, and slots for additional cards.  These modules, which may be from the same or different manufacturers, are plugged together into a base unit.

Rack mounting PLC

This follows the same modular concept, but each of the modules plugs into a rack. The add-on modules are contained in racks which are then linked together through a network of connections. Since the rack mounting PLC is highly scalable, it allows very large systems to be assembled.

Advantages of PLCs

  • They have easy and flexible programs and allow maximum flexibility of the device applications.
  • PLC programming eases the process of rewiring of the relays and devices. The PLC does not eliminate the hard wiring; however, it makes it less intensive.
  • Scalability and modular form allow adding and mixing of a variety of input and output devices to suit every application. Depending on the requirements, a flexible PLC allows additional input and output modules to expand its functionality.
  • Enhancing control of processes, machine functions, and complete production lines.
  • PLCs communicate with external devices, hence allowing connection to sophisticated interfaces. Remote access or programming is possible as well.
  • Low power consumption.
  • Ability to handle complex logic operations.

Typical applications of PLCs

In energy management, the PLC monitors and controls equipment such as boiler systems. It helps the organization increase energy efficiency and safety. PLCs are also used in numerous everyday activities. They are found in factories, homes, commercial buildings, airports, unmanned car parks, and more.

Programmable Logic ControllersFinally, some practical applications include:

  • Automatic car wash stations
  • Automatic transfer switches in electrical circuits
  • Washing machines
  • Conveyer belts
  • Elevators
  • Roller coasters
  • Automatic doors
  • Industrial processes
  • Traffic signals
  • Automated welders
  • Industrial robots
  • Glass washing robots for skyscrapers