The computer takes the place of much of the external wiring required for control of a process. The PLC will operate any system that has output devices that go on and off (known as discrete, or digital, outputs). It can also operate any system with variable (analog) outputs. The PLC can be operated on the input side by on-off devices (discrete, or digital) or by variable (analog) input devices.

Today, the big unit growth in the PLC industy is at the low end-where small keeps getting smaller. When a few years ago the micro PLC entered the market, some thougt that these devices had “bottomed out”. Now,nano PLCs-generally defined as those with 16 or fewer I/O-are spreading. Some can fit into your shirt pocket, being no larger than a deck of cards and at the time of this writing, PLC Direct plants to introduce a PLC the size of a box of Tic-Tac candy that will include many features of current micro models.

The first PLC systems evolved from conventional computers in the late 1960s and early 1970s. These first PLCs were installed primarily in automotive plants.  Traditionally, The auto plants had to be shut down for up to a month at model changeover time. The early PLCs were used along with other new automation techniques to shorten the changeover time. One of the major time-consuming changeover procedures had been the wiring of new or revised relay and control panels. The PLC keyboard reprogramming procedure replaced the rewiring of a panel full of wires, relays, timers and other components. The new PLCs helped reduce changeover time to a matter of few days.

There was a major problem with these early 1970s computer/PLC reprogramming procedures. The programs were complicated and required a highly trained programmer to make the changes. Through the late 1970s, improvements were made in PLC programs to make them somewhat more user friendly; in 1978, the introduction of the microprocessor chip increased computer power for all kinds of automotion systems and lowered the computing cost. Robotics, automotion devices, and computers of all types, including the PLC, consequently underwent many improvements. PLC programs, written in high-level language, became more understandable to more people, and PLCs became more affordable.

In the 1980s, with more computer power per dollar available, the PLC came into exponentially increasing use. Some large electronics and computer companies and some perse corporate electronics pisions found that the PLC had become their greatest volume product. The market for PLCs grew from a volume of $80 million in 1978 to $1 billion per year by 1990 and is still growing. Even the machine tool industry, where compute numerical controls (CNCs) have been used in the past, is using PLCs. PLCs are also used extensively in building energy and security control systems. Other nontraditional uses of PLCs, such as in the home and in medical equipment, haved exploded in the 1990s and will increase as we enter the new millennium.

A person knowledgeable in relay logic systems can master the major PLC functions in a few hours, These functions might include coils, contacts, timers and counters. The same is true for a person with a digital logic background. For persons unfamiliar with ladder diagrams or digital principles, howere, the learning process takes more time.

A person knowledgeable in relay logic can master advanced PLC functions in a few days with proper instruction. Company schools and operating manuals are very helpful in mastering these advanced functions. Advanced functions in order of learning might include sequence/drum controller, register bit use, and more functions.

Following are 8 major advantages of using a programmable controller.

①Flexibility. In the past, each different electronically controlled production machine required own controller; 15 machines might require 15 different controllers. Now it is possible to use just one model of a PLC to run any one of the 15 machines. 

②Implementing Change and Correcting Errors. With a wired relay-type panel, any program alterations require time for rewriting of panels and devices. When a PLC program circuit or sequence design change is made, the PLC program can be changed from a keyboard sequence in a metter of minutes. No rewiring is required for a PLC-controlled system. 液压疲劳试验机电动机无级调速英文文献和中文翻译(5):http://www.youerw.com/fanyi/lunwen_62808.html