How does PLC meet the requirements of market and technological development?

At the moment, simply comparing PLC and PC control does not make much sense. What we need to figure out is how to make PLC and PLC-based PC control meet the requirements of market and technological development after more than two decades of rapid progress in hardware, software and communications.

In the 1990s, the School of Mechanics at the University of Michigan in the United States had a doctoral thesis that specifically reviewed dozens of automation control programming languages that had appeared, including the five graphic and text languages of the IEC 61131-3 specification, and the flow chart language that was once popular in the United States. Today, can precipitate and accumulate decades of experience in the industrial control industry, really have vitality to form ecology, that is, only a limited number of LD, SFC, FBD, STL. Even the assembly-like IL instruction table language, which is being used less and less, is about to be removed from the new version of IEC 61131-3. The object-oriented programming method has been introduced IEC 61131-3, and even high-level languages such as C, C++ have actually become commonly used programming languages for PLC. The following figure shows the results of a joint survey conducted by an American website called automation.com and PLCopen International in 2020, which also corroborates the above conclusions. This is also a demonstration that PLC is progressing day by day. IEC 61131-8, which details the actual background and reasons for the selection of these five languages in IEC 61131-3.

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Figure 1. automation.com survey results

The first PLC, born in 1970, was put into industrial application at General Motors GM to control metal cutting, drilling, material handling and assembly. The success of the world’s first launch of PLC is manifested in two aspects: first, the use of computers to analyze the logic control achieved by relays in the past; The second is the use of ladder diagram logic programming, so that the original electrical engineering technicians can use computer programming on the basis of their original technology.

This should be a hallmark of early industrial software applications, but so far few people talk about this important contribution when talking about industrial software.

Known as the father of the PLC, Richard Morley, then an engineer at Bedford Associates, pioneered the innovative initial design, and then with his team developed the solid-state sequential logic solver for factory automation and continuous processing applications, the first practical programmable logic controller. It was designated Modicon 084. In November 1969, after learning of GM’s requirements, they presented the Modicon 084 to the company’s hydraulics division, which received GM’s favor.

Why are programming languages classified as industrial software?

Ladder logic, as a control language, was first used in silicon device built controllers around 1969. To support the use of this control language, a hardware platform consisting of three parts was built, namely, a dual-port memory, a logic solver (later named PLC), and a general-purpose computer. The greatest advantage of this language is that it can be understood by all electrical engineers in the world. Later, the ladder diagram was expanded to multiple nodes, and some additional functions were added. The functionality of ladder logic and the applicability of PLCS quickly became widely adopted in all industries.

Ladder diagram logic programming is very popular with electrical engineering technicians, and its advantages are self-evident. First, the symbols associated with ladder logic programming, developed by Bedford, come from electrical engineering to describe the sequential operation function, which enables electrical engineers and electricians to program PLCS with computers in a very easy to understand way. This embodies the most basic characteristics of industrial software – the basic electrical industry derived from industry. Second, the language was one of the first examples of programming an industrial controller using a general-purpose computer as a tool. Used so far, it not only reflects the characteristics of “industrial software is the mainstream of tool software”, but also has been repeatedly tempered by electrical engineering and industrial automation practitioners for decades, and is widely used in all industrial occasions. These two points alone should establish its position in the development of industrial software. Unfortunately, we do not find it in many articles discussing industrial software, and people seem to have forgotten it.