Buying guides to PLCs

What is a Programmable Logic Controller?

A Programmable Logic Controller (PLC) is a computer control system commonly used for the automation of electromechanical processes in applications such as the control of machinery in diverse industries such as factory-based automation of assembly lines, among many others. The PLC system continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices. A PLC is an example of a hardwired real-time system as results in the output must be produced in response to input conditions within a limited time. The programs that control machine operation are typically stored in battery-backed RAM memory or in non-volatile memory.

Prior to the creation of the earliest PLCs in the late 1960s and early 1970s, most machine control and automation tasks were controlled using a combination of electromechanical components such as simple relay logic, timers, counters and other discrete control components. The early PLCs were simple devices with limited functions and memory, but their capabilities have significantly increased over time. The original Boolean-type operations such as AND and OR functions have been supplemented with mathematical functions and higher-level output capabilities such as pre-configured pulse-width modulation (PWM) blocks, in addition to an increase in input/output (I/O) count and significantly improved external communications options.

Unlike general-purpose computers or PCs, the PLC is designed for multiple I/O configurations, extended temperature ranges, immunity to electrical noise, and also resistance to vibration and impact. Production lines and machine functions or processes can be significantly enhanced using this type of control system. A major benefit in using a PLC is its ability to change and replicate the operation or process while collecting and communicating vital information. The PLC-based system is also a modular one and different levels of I/O can be implemented to suit specific applications.


What are the different types of PLC?

Over the past 40 years or so, PLCs have become increasingly differentiated and a number of categorisations of product are now in common use.

The simplest applications are covered by logic controllers, which are sometimes called smart relays and typically provide a relatively small number of I/O, implement basic Boolean logic control and do not require high-speed operation. They are generally used for processes that follow a pre-defined sequence with limited or no deviation in applications such as car park barriers, car washes, vending machines and simple packing equipment.

The second more complex type is the compact or brick PLC, which offers increased processing speed and I/O capability. There is also support for additional I/O expansion and a more flexible sequence of operation – for instance in response to operator intervention or as a result of monitoring external conditions. Typical applications include programmable cutting machines and batch control of bottling equipment.

The third category is the advanced PLC, which services more complex applications involving large amounts of data, requiring a modular build approach and needing to function at high speed with further increased levels of I/O and also networking capability. One example is RFID-based sorting and routing of products in a conveyor system and high-speed label printing.

A fourth and much more recent category is the programmable automation controller (PAC), which is a largely interchangeable term with PLC or advanced PLC. In many ways it combines capabilities of a PC and a PLC and offers many of the benefits of both in a single package. Some PLC vendors position higher-end PLCs as PACs as these products tend to offer greater flexibility in programming, larger memory capacity, better interoperability and include more connectivity options and broader control capabilities. Generally, a PAC is more suitable for complex automation system architectures that are often composed of a number of PC-based software applications, including HMI (human machine interface) functions, asset management and advanced process control requirements.


What are the different PLC programming methods?

As PLCs have become more capable and differentiated, the task of programming has become more complex over time. Originally designed to be easy to program, early PLCs were programmed directly via a front panel or a special-purpose terminal and a restricted range of functions meant it was often possible to include a dedicated key to represent each logical element of the program. The traditional ‘programming language’ associated with PLCs is ladder logic, which depicts the program graphically, based on an equivalent circuit diagram of relay logic hardware. Ladder logic from individual manufacturers is generally incompatible, so it can be misleading to think of ladder logic as a language as it is closer to a programming style or family of rule-based languages.

To achieve higher efficiency during application development, while also increasing software quality, attempts have been made to introduce a degree of standardisation to PLC programming systems via the IEC 61131 standard. The standard recognises four programming languages: ladder diagram (LD) and function block diagram (FBD) programming are both graphical styles; whilst structured text (ST) and instruction list (IL) are textual types. In addition, IEC 61131-3 defines a sequential function chart (SFC), which includes elements to organise programs for sequential and parallel control processing.


What about HMI?

An increasingly important element now of PLCs is the growing integration of HMI (human-machine interface) technology including high-resolution graphics displays and rugged-environment touchscreens as a substitute for traditional devices such as indicators, displays and switches. Automation and industrial control is becoming more and more reliant on HMI capabilities to deliver accurate and real-time control and monitoring information on industrial processes.


Which are the leading suppliers of PLC based equipment?

There are many suppliers in this market, but leading manufacturers include ABB, Mitsubishi, Omron, Schneider Electric and Siemens, which offer solutions from relatively basic logic controllers for small or simple machines to advanced flexible and scalable systems for highly complex and sophisticated control applications, in addition to a range of HMI touchscreen and display panel options and PC-based automation software solutions.


Which PLC should I choose for my automation and control application?

Choosing the right PLC platform for the application and ensuring it meets the requirements of the process today and in the medium term is a challenging one, whether it is a brand new process or simply replacing an existing controller. Understanding the target machine or process is essential and drawing a simple block diagram can help identify the control devices, I/O types and requirements and also their physical locations, in addition to any specific environmental issues to be considered such as the controller and process operating in extremes of temperature or humidity. This process can aid in the planning of the overall system design, calculating the level of I/O needed and processing speeds versus these I/O requirements, in addition to various network and communications protocol options. Flexibility in system upgradability and I/O module expansion capability should also be a major consideration.

As a starting point a logic controller should only be considered for slower processes with an I/O requirement of less than 20 I/O. Beyond this level, a compact or intermediate PLC provides a better solution with increased processing speed available. An advanced PLC or PAC is a more suitable choice for large I/O applications with networking and remote I/O requirements. The amount of memory needed for the program is more difficult to size, but modern PLC processing units feature memory capacities suitable for the majority of applications and have the option of expanding the program memory if required


Can RS Components help?

In addition to providing a very high level of technical support and application know-how, RS stocks a wide range of equipment from all of the above-mentioned suppliers including all the control devices and components required to build a complete system such as control (CPU) units, I/O expansion and displays and other HMI devices, Further product information is available via the RS website.