1、Plc control system design elements Originally, the PLC was represented by the acronym PC. There was some confusion with using this acronym as it is commonly accepted to represent personal computer. Therefore, PLC is now commonly accepted to mean programmable logic contoller. A PLC is a user-friendly
2、, microprocessor-based specialized computer that carries out control functions of many types and leves of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. It can be programmed, controlled, and operated by a person unskilled in operating comp
3、uter. Essentially, a PLCs operator draws the lines and devices of ladder diagrams with a keyboard onto a display screen. The resulting drawing is converted into computer machine language and run as a user program. The computer takes the place of much of the external wiring required for control of a
4、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) inpu
5、t 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 ca
6、n 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 1
7、960s 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 ti
8、me-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 w
9、as 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 introduct
10、ion 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
11、 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 diverse corporate electronics divisions found that the PLC had become t
12、heir 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 buildi
13、ng 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, Thes
14、e 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 PL
15、C 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 u
16、sing a programmable controller. Flexibility. 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. Implementing Change
17、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
18、PLC-controlled system. Large quantities of contacts. The PLC has a large number of contacts for each coil available in its programming.suppose that a panel-wired relay has four contacts and all are in use when a design change requiring three more contacts is made. Time would have to be taken to proc
19、ure and install a new relay or relay contact block. Using a PLC, howere, only three more contacts would be typed in. the three contact would be automatically available in the PLC. Indeed, a hundred contacts can be used from one relay-if sufficient computer memory is available. Lower cost. Increased
20、technology makes it possible to condense more functions into smaller and less expensive packages. Now you can purchase a PLC with numerous relays, timers and counters, a sequencer, and other function for a few hundred dollars. Pilot Running. A PLC programmed circuit can be evaluated in the office or
21、 lab. The program can be typed in, tested, observed, and modified if needed, saving valuable factory time. In contrast, conventional relay systems have been tested on the factory floor, which can be very time consuming. Visual Observation. A PLC circuits operation can be seen during operation direct
22、ly on a CRT screen. The operation or misoperation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done more quickly during visual observation. In advanced PLC systems, an operator message can be programmed for each possible
23、 malfunction. In advanced PLC systems, an operator message can be programmed for each possible malfunction. The malfunction description appears on the screen when the malfunction is detected by the PLC logic. Advanced PLC systems also may have descriptions of the function of each circuit component.
24、Speed of Operation. Relays can take an unacceptable amount of time to actuate. The operational speed for the PLC program is very fast. The speed for the PLC logic operation is determined by scan time, which is a matter of milliseconds. Ladder or Boolean Programming Method. The PLC programming can be
25、 accomplished in the ladder mode by an electrician or technician. Alternatively, a PLC programmer who works in digital or Boolean control systems can also easily perform PLC programming. In modern industrial production equipment, and large numbers of simulations of the control devices, such as the c
26、ease electrical, electromagnetic devices to turn, the number of products, temperature, Ap power, and set the flow control, the automatic control of the industrial site, If a programmable controller (PLC) to resolve the issue has become a cybernetic one of the most effective tools, this article describes Plc
