Hello SPS! Part 1: Basics of programmable logic controllers

The Programmable Logic Control (PLC) or Programmable Logic Control (PLC) has developed into an indispensable component in the world of automation systems. This three-part article series is dedicated to this topic in detail.

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Part 1 of the series first introduces the basics of PLCs. The second part explains how the open source PLC OpenPLC can be used in practice. Part 3 shows the environment in which PLCs work together with other automation components. What is a PLC or PLC anyway?

Before we get started, the question that remains to be clarified is what lies behind the terms PLC and PLC. A PLC (Programmable Logic Control) or in English PLC (Programmable Logic Control) is an important component in automation systems.

Prof. Dr. Michael Stal has been working at Siemens Technology since 1991. His research interests include software architectures for large complex systems (distributed systems, cloud computing, IIoT), embedded systems, and artificial intelligence. He advises business areas on software architecture issues and is responsible for the architectural training of senior software architects at Siemens.

These controllers contain code for reading data (e.g. state of a switch, sensors), pre-programmed processing elements (e.g. library blocks such as flip-flops and timers), user-defined code for processing, and code for writing data (e.g. for motor control, open/close). a valve). The input therefore includes the sensors, while the output is responsible for the actuators. Both can be done either analog or digital. In this context, it is important that every cycle-controlled PLC in a system works in a clocked manner. This means it must complete its processing within a predefined time window, for example in 20 ms.

A simple example of a PLC program could be:

Check the status of two connected switches (open, closed). If one of the two switches is closed (i.e. there is current at the respective output), activate the light intensity function block. If the light intensity is below a predefined threshold, pass current through the output of the function block and thereby activate the LED via the output interface

The example in the figure below is written in the PLC programming language LD (Ladder Diagram):

If the normally open switch1 (NO) or the open switch2 is closed and it is dark enough, an LED lights up. There is a reusable component (function block) for measuring the light intensity. Of course you wouldn’t need a PLC for this. A trivial PLC program for illustrative purposes only.

The left ladder in this ladder diagram can be interpreted as the plus pole (anode) of a circuit, the right one as the minus pole (cathode). So the current flows from left to right.

After the program has finished, the PLC starts again with the first instruction of the executed program. If the program of such a PLC exceeds the cycle time, the CPU stops. An example of an event-controlled and therefore not cycle-controlled PLC follows in the next section.

The term PLC scan time is often used for this cycle, which defines the time that a PLC needs to read the inputs, implement the logic specified by the user, and then write to the outputs. In reality there is a fourth step in which the PLC checks its own health.

In each cycle, the PLC reads the input ports, processes the user-defined logic, writes to the output ports, and checks its own state.

If processing requires less than the parameterized cycle time, the PLC remains in an idle state until the end of the time interval. So she always sticks to the set rhythm. This is also important because several cooperating PLCs can synchronize using the same clock. A PLC must therefore never step out of line.

But why are PLCs actually so cool? In 1969, the Modicon 084 (today Schneider Electric) appeared as the very first PLC – Modicon, by the way, stands for Modular Digital Controller. Before this first PLC existed, controls and regulations had to be carried out via connection programming, which required, among other things, relays and logic modules. This leads to a lot of effort and little flexibility. Simply switching vehicle production from one model to another would involve increased effort.

A PLC, on the other hand, can be used flexibly and is much easier to configure, program and adapt for a desired purpose. Using bus systems such as Profinet or Modbus, it is also possible to integrate PLCs into comprehensive solutions or to connect them with one another. No wonder that companies like Siemens and Allen-Bradley (now Rockwell) soon followed Modicon’s example.

A modern PLC does not necessarily have to be a single box – i.e. a monolith – but can be modular. A modular PLC, in contrast to a non-modular one, consists of different assemblies that combine to form a PLC. This can be, among other things, a CPU module, an interface module, a power supply module, an input module or an output module. If the user needs more interfaces or functionalities, he adds additional components. This also includes more complex control assemblies, sensor assemblies, and many other modules.

The S7-300 is a Siemens product from the Simatic S7 series for industrial automation.

(Bild: siemens.com)

The advantage of a modular PLC is that it can be equipped exactly for the desired purpose. This is possible, for example, in a Simatic PLC from Siemens. PLCs like those from the Siemens S7 series (Simatic) are mainly available as PLCs made of modular hardware components, such as components for industrial control cabinets or plug-in cabinets.

As a power source, a PLC typically uses a converter that converts 220V (or in the USA 110V) AC voltage into 5V DC (for CPU, RAM, ROM) and 24V DC (for digital input & output) as well as 10V for analog I/O.

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