Programmable Logic Controllers

A Practical Approach to IEC 61131-3 using CoDeSys

Dag H. Hanssen

• Editore: Wiley
• Anno: 2015
• ISBN:
  9781118949221
• Edizione: 1
• Argomento: Computer Science
• Categoria: Computer Engineering

Widely used across industrial and manufacturing automation, Programmable Logic Controllers (PLCs) perform a broad range of electromechanical tasks with multiple input and output arrangements, designed specifically to cope in severe environmental conditions such as automotive and chemical plants.

Programmable Logic Controllers: A Practical Approach using CoDeSys is a hands-on guide to rapidly gain proficiency in the development and operation of PLCs based on the IEC 61131-3 standard. Using the freely-available* software tool CoDeSys, which is widely used in industrial design automation projects, the author takes a highly practical approach to PLC design using real-world examples. The design tool, CoDeSys, also features a built in simulator/soft PLC enabling the reader to undertake exercises and test the examples.

Key features:

• Introduces to programming techniques using IEC 61131-3 guidelines in the five PLC-recognised programming languages.
• Focuses on a methodical approach to programming, based on Boolean algebra, flowcharts, sequence diagrams and state-diagrams.
• Contains a useful methodology to solve problems, develop a structured code and document the programming code.
• Covers I/O like typical sensors, signals, signal formats, noise and cabling.
• Features Power Point slides covering all topics, example programs and solutions to end-of-chapter exercises via companion website.

No prior knowledge of programming PLCs is assumed making this text ideally suited to electronics engineering students pursuing a career in electronic design automation. Experienced PLC users in all fields of manufacturing will discover new possibilities and gain useful tips for more efficient and structured programming.

* Register at www.codesys.com

www.wiley.com/go/hanssen/logiccontrollers

Part One
Hardware

1
About PLCs

The programmable logic controller (PLC) has its origin in relay-based control systems, also called hard-wired logic.¹

Before PLCs became common in industry, all automatic control was handled by circuits composed of relays,² switches, clocks and counters, etc (Figure 1.1). Such controls required a lot of wiring and usually filled large cabinets full of electromagnetic relays. Electricians had to assemble controls or use a prepared relay wiring diagram. The relay wiring diagrams showed how all the switches, sensors, motors, valves, relays, etc. were connected. Such relay wiring diagrams are the forerunners for the ladder diagram (LD) programming language, which is still a common programming language used in programming PLCs.

Figure 1.1 Example of a relay and a timer (mounted on a connector board)

There were many disadvantages with these mechanical controls. In addition to taking up a lot of room, they demand time and labor to implement them and to make any changes in such equipment. A relay control usually consists of hundreds of relays connected together with wires running in every direction. If the logical function needs to be changed or expanded, the entire physical unit must be rewired, something that is obviously expensive in terms of working time. Since the relays are electromechanical devices, they also had a limited service life, something that led to frequent operational interruptions with subsequent disruption.

There also was no way of testing before the control was wired up. Testing therefore had to take place by running the unit. If there was a small failure in the schematic diagram or if an electrician had connected a wire wrong, this could result in dramatic events.

1.1 History

The first PLC came into commercial production when General Motors was looking for a replacement for relay controls. Increased competition and expanded demands on the part of customers meant a demand for higher efficiency, and the natural step was to design a software-based system that could replace the relays. The requirement was that the new system should be able to:

• Compete on price with traditional relay controls
• Be flexible
• Withstand a harsh environment
• Be modular with respect to the number of inputs and outputs³
• Be easy to program and reprogram

Several corporations started work on providing a solution to the problem. Bedford Associates, Inc. from Bedford, Massachusetts, suggested something they called a “modular digital controller” (MODICON). MODICON 084⁴ was the first PLC that went into commercial production. The key to its success was probably the programming language, LD, which was based on the relay diagrams that electricians were familiar with. Today there is no question about the use of programmable controls; the question is rather what type to use.

The first PLCs were relatively simple in the sense that their function was to replace relay logic and nothing else. Gradually, the capabilities improved more and more and functions such as counters and time delays were added. The next step in development was analog input/output and arithmetic functions such as comparators and adders.

With the development of semiconductor technology and integrated circuits, programmable controls became widely used in industry. Particularly when microprocessors came on the market in the beginning of the 1970s, development proceeded at a rapid pace.

The PLCs of today come with development tools in the form of software with every imaginable ready-to-use function. Examples are program codes for managing communications as well as processing functions such as proportional integrator/derivative regulators, servo controls, axial control, etc. In other words, there is the same pace of development as with the PC (Figures 1.2, 1.3, and 1.4).

Figure 1.2 Omron Sysmac C20—Nonmodular PLC with digital I/O and programming terminal

Figure 1.3 PLCs from Telemecanique come in different sizes

Figure 1.4 Newer generation PLC from Wago with Profibus coupler and I/O

The communications side also experienced rapid development. Demand grew quickly for PLCs that could talk to one another and that could be placed away from the actual production lines. Around 1973 Modicon developed a communications protocol that they called Modbus. This made it possible to set up communications between PLCs, and the PLCs could therefore be located away from production. Modicon’s Modbus also provided for management of analog signals. As there became more and more manufacturers of PLCs and associated equipment, there also developed more proprietary⁵ and nonproprietary communications protocols. The lac...