Article from RS Components
Despite predictions of their impending demise, programmable logic controllers (PLCs) continue to be the number one choice for a huge variety of embedded and industrial control applications. Products as diverse as car park barriers, vending machines, high-speed packaging apparatus and all kinds of factory automation equipment commonly rely on PLC-type devices in their control systems.
A brief history of programmable logic control
Today’s PLCs even go under a variety of different names, including simply ‘logic controller’, programmable automation controller (PAC) and machine automation controller (MAC): but they remain recognisably descendants of the original PLCs that emerged in the late 1960s and early 1970s.
A number of factors have led to this continuing success – often in the face of keen competition from alternatives such as industrial PCs, which are more familiar in the home and elsewhere in the workplace, and boast economies of scale to which the PLC can never aspire.
The first success factor is undoubtedly that PLCs are designed to tightly fit a particular subset of applications. They evolved initially in the factory automation context to replace hard-wired electromechanical control devices such as switches and relays, and despite recent evolution, they continue to excel in applications that involve managing a sequence of control tasks in a predictable and timely fashion in response to a number of input signals.
Just as important as their inherent suitability for control tasks, PLCs also benefit from the availability of an ecosystem of systems integrators, specialist distributors and factory engineers who know and understand the products and systems they are used to build. A distributor such as RS will stock not only the necessary control (CPU) units, but also I/O expansion, displays and other human machine interface (HMI) components, and all the control devices required to build a complete system.
The very long service life of a typical PLC system contributes to the strength and depth of this ecosystem. Even engineers who are relatively new to the industry need to become familiar with PLC principles because the installed base is so extensive.
Taking the logical next step in machine control
Prior to the creation of the ‘PLC’ most machine control and automation tasks were controlled using a combination of simple relay logic, timers, counters and other discrete control components.
While the early PLCs were simple, with limited functions and memory, as time has passed manufacturers have increased the products’ capabilities. At a core level, the original Boolean-type operations have been supplemented with mathematical functions and higher-level output capabilities such as pre-configured PWM blocks. On the periphery, I/O count has increased and external communications have improved.
PLCs have therefore become increasingly differentiated, with three common categorisations of product now in use. The simplest applications are covered by logic controllers, sometimes called ‘smart relays’. These typically provide a relatively small number of I/Os, 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. Typical examples are car park barriers, car washes, vending machines and simple packing equipment.
The next stage of complexity, the ‘compact’ or ‘brick’ PLC, offers increased processing speed and I/O capability, typically combined into a single unit. There will be 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.
Finally, advanced PLCs service more complex applications that involve large amounts of data, require a modular build approach and need to function at high speed with increased levels of I/O and networking capability. Examples would be RFID-based sorting and routing of products in a conveyorised system, and high-speed label printing.
Logic controller or PLC – the choice is yours
Within the PLC hierarchy specific capabilities vary between manufacturers and their specific ranges. A modern logic controller family such as the Mitsubishi Alpha 2 includes a program capacity of up to 200 function blocks, three times that available in previous product generations. The series also illustrates the increase in the diversity of available function blocks: its instruction set contains fifteen new blocks, including mathematics calculations.
Logic controller series like the Mitsubishi Alpha 2 and the Siemens Logo! have evolved to incorporate the ability to integrate with HMI panels providing operate messages and other status information, a function which a few years ago was only the preserve of compact or advanced PLCs.
I/O count has also improved: most logic controllers now offer expansion up to 28 I/O and often accommodate AC, DC and transistor inputs with a variety of voltage ranges. Integrated PWM (pulse width modulation) output for motor control task is now also a common feature.
Your choice of logic controller or compact / advanced PLC will ultimately be determined by your application, but with the functions and capability of logic controllers increasing every year the line between logic controller and PLC is becoming increasingly blurred.
A PLC is nothing without programming
As PLCs have become more capable and differentiated, the task of programming them has grown correspondingly more complex. Originally designed to be easy to program by electrical engineers – not software engineers, early devices were programmed directly via a front panel or a special-purpose terminal. With a restricted range of functions, 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, a simple system that depicts the program graphically based on an equivalent circuit diagram of relay logic hardware (the name is chosen purely because of the visual resemblance of this representation to a ladder). Unfortunately the ladder logic languages of individual manufacturers are incompatible. It is therefore rather misleading to think of ladder logic as a language: it is closer to a programming style or family of (rule-based) languages.
In fact, some entry-level PLCs retain ladder logic style programming as their program method. The CP1E-E and CP1E-N CPUs from Omron are an example, being designed for simple, cost-sensitive standalone applications.
However, with complexity and hence software costs increasing (and in line with programmers’ native desire for rigour), attempts have been made more recently to introduce a degree of standardisation to PLC programming systems, primarily through the organisation PLCopen, which has been the driving force behind the definition of the IEC 61131 standard.
The stated intention of IEC 61131-3 is to harmonise the way people design and operate industrial controls via standardisation of the programming interface, allowing a team-based approach to project specification, design, implementation, testing, installation and maintenance.
The standard recognises four programming languages: ladder diagram (LD) and function block diagram (FBD) programming are both graphical styles; while structured text (ST) and instruction list (IL) are textual types. In addition, IEC 61131 defines a sequential function chart (SFC), which includes elements to organise programs for sequential and parallel control processing.
A controller such as ABB’s AC500 platform supports a variety of programming paradigms along these lines. In addition to contact plan (its term for LD), IL and ST, it implements a function block language (FBS) and process language (SFC).
A scalable, compact, entry-level version of the company’s AC500 series, the AC500-eco is fully software compatible with the company’s larger CPU modules, and can be programmed using the same PS501 Control Builder Software. The company’s CoDeSys programming environment is custom made for easy integrated network configuration. The user program can be downloaded via an SD card without the need for programming tools: a subsidiary benefit is the ability to perform software updates and data logging via the SD slot.
Other manufacturers are starting to embrace the concept and benefits of offering an IEC 61131 compliant programming environment. Examples include Unity Pro development software, used by Schneider Electric’s Modicon M340™ programmable automation controller (PAC) series, and Mitsubishi Electric’s GX IEC Developer software which can be used with their FX and Q series PLCs
Schneider’s Unity suite offers a choice of five IEC languages, graphic programming and advanced online help. In addition, users can reuse software to obtain maximum cost efficiencies and quality.
PLCopen’s continuing work includes the definition of a set of extensions to standard programming tools and structures, such as an XML specification, motion control library, re-usability and conformity levels.
Communication choice increasing
One further common trend in the PLC market: the move towards the use of familiar – often consumer-market – communications interfaces such as USB.
In the past most PLCs were programmed via an RS-232 or RS-485 serial port, but with most modern laptops failing to incorporate these legacy communication ports, PLC manufacturers are responding by enabling programming via serial (CANopen / DH485), USB and increasingly Ethernet.
PLCs like the Siemens S7-1200 were launched with only Ethernet communication and separate options of other proprietary communication interfaces such as Profibus because Ethernet is now accepted as the future of industrial communication.
The PLC has remained a robust and competent solution for industrial automation applications for over 40 years – today’s controllers are a far cry from their progenitors, while remaining distinctly recognisable in terms of both function and mode of use.
With increasing processing speed and I/O capabilities, as well as improved HMI and networking capabilities, these most enduring of products look set to continue to satisfy the needs of industrial control systems for some time to come.