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Kuzma Vladimirov
Kuzma Vladimirov

Programmable Logic Controllers Fifth Edition


A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis. Dick Morley is considered as the father of PLC as he had invented the first PLC, the Modicon 084, for General Motors in 1968.




Programmable Logic Controllers Fifth Edition



PLCs were first developed in the automobile manufacturing industry to provide flexible, rugged and easily programmable controllers to replace hard-wired relay logic systems. Since then, they have been widely adopted as high-reliability automation controllers suitable for harsh environments.


PLC originated in the late 1960s in the automotive industry in the US and were designed to replace relay logic systems.[2] Before, control logic for manufacturing was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers.[3]


Programmable logic controllers are intended to be used by engineers without a programming background. For this reason, a graphical programming language called Ladder Diagram (LD, LAD) was first developed. It resembles the schematic diagram of a system built with electromechanical relays and was adopted by many manufacturers and later standardized in the IEC 61131-3 control systems programming standard. As of 2015[update], it is still widely used, thanks to its simplicity.[24]


PLC programs are typically written in a programming device, which can take the form of a desktop console, special software on a personal computer, or a handheld programming device.[27] Then, the program is downloaded to the PLC directly or over a network. It is stored either in non-volatile flash memory or battery-backed-up RAM. In some programmable controllers, the program is transferred from a personal computer to the PLC through a programming board that writes the program into a removable chip, such as EPROM.


The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking. The data handling, storage, processing power, and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allows a general-purpose desktop computer to overlap some PLCs in certain applications. Desktop computer controllers have not been generally accepted in heavy industry because desktop computers run on less stable operating systems than PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs. Operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the controller may not always respond to changes of input status with the consistency in timing expected from PLCs. Desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical.[citation needed]


The most basic function of a programmable controller is to emulate the functions of electromechanical relays. Discrete inputs are given a unique address, and a PLC instruction can test if the input state is on or off. Just as a series of relay contacts perform a logical AND function, not allowing current to pass unless all the contacts are closed, so a series of "examine if on" instructions will energize its output storage bit if all the input bits are on. Similarly, a parallel set of instructions will perform a logical OR. In an electromechanical relay wiring diagram, a group of contacts controlling one coil is called a "rung" of a "ladder diagram ", and this concept is also used to describe PLC logic. Some models of PLC limit the number of series and parallel instructions in one "rung" of logic. The output of each rung sets or clears a storage bit, which may be associated with a physical output address or which may be an "internal coil" with no physical connection. Such internal coils can be used, for example, as a common element in multiple separate rungs. Unlike physical relays, there is usually no limit to the number of times an input, output or internal coil can be referenced in a PLC program.


In his book from 1998, E. A. Parr pointed out that even though most programmable controllers require physical keys and passwords, the lack of strict access control and version control systems, as well as an easy-to-understand programming language make it likely that unauthorized changes to programs will happen and remain unnoticed.[34]


Prior to the discovery of the Stuxnet computer worm in June 2010, the security of PLCs received little attention. Modern programmable controllers generally contain a real-time operating systems, which can be vulnerable to exploits in a similar way as desktop operating systems, like Microsoft Windows. PLCs can also be attacked by gaining control of a computer they communicate with.[21] Since 2011[update], these concerns have grown as networking is becoming more commonplace in the PLC environment connecting the previously separate plant floor networks and office networks.[35]


In more recent years,[when?] small products called programmable logic relays (PLRs) or smart relays, have become more common and accepted. These are similar to PLCs and are used in light industries where only a few points of I/O are needed, and low cost is desired. These small devices are typically made in a common physical size and shape by several manufacturers and branded by the makers of larger PLCs to fill out their low-end product range. Most of these have 8 to 12 discrete inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Most such devices include a tiny postage-stamp-sized LCD screen for viewing simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus four more separate push-buttons, similar to the key buttons on a VCR remote control, and used to navigate and edit the logic. Most have a small plug for connecting via RS-232 or RS-485 to a personal computer so that programmers can use simple applications in general-purpose OS like MS Windows, macOS or Linux, that have user-friendly (G)UIs, for programming instead of being forced to use the tiny LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC, and they still offer robust design and deterministic execution of the logic.


The XC152 programmable logic controllers feature a powerful PLC combined with an integrated SmartWire-DT master and a variety of additional interfaces that allow users to implement innovative automation solutions.


The XC300 series of programmable logic controllers (PLCs) have extensive features such as VISU servers (HTML5) and OPC-UA servers. Integrated CAN and Ethernet interfaces enable the segmented access to the most varied networks


-- a contact module 20 including manually actuable electrical contacts (see Figure 6), an electrical communication terminal 70, 71 and 72 in electrical communication with the contacts and an associated contact operator 74, 75 (see Figure 4 and column 3, lines 9 to 17) and electrical communication means (sockets 18, circuit board 14 and serial I/O scanner module 22) for delivery of an electrical signal to an output device (programmable logic controller 62) remote from the control panel 12 for controlling said output device in response to manual operation of the contact operator 74, 75 by said operator interface device 32 (see Figure 3).


5.2 In the judgement of the Board, it would be obvious to a person skilled in the art to apply the general teaching of D22 regarding the formation of a contact module comprising a logic circuit to the manufacture of a manual control panel assembly according to D42, which also involves the connection of switches with actuators via a separate logic controller (see D42, Figures 2 and 3). In particular, in the light of the disclosure in D22, the skilled person would realise that the manual control panel assembly known from D42 could be further developed by providing the contact blocks with logic circuits which could take over at least some of the functions performed by the separate programmable logic controller 62 and the input/output scanning circuit 22. 041b061a72


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