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1、中文3475字Control Devices and PLCJoseph La FauciFrom Wikipedia, the free encyclopediaSeveral types of control devices are used in industry to satisfy the following control needs.Mechanical Control、Pneumatic Control、Electromechanical Control、Electronic Control、Computer Control、Programmable Logic Control
2、 (PLC)Mechanical control includes cams and governors. Although they have been used for the control of very complex machines, to be cost effectively, today they are used for simple and fixed-cycle task control. Some automated machines, such as screw machines, still use cam-based control. Mechanical c
3、ontrol is difficult to manufacture and is subject to wear.Pneumatic control is still very popular for certain applications. It uses compressed air, valves, and switches to construct simple control logic, but is relatively slow. Because standard components are used to construct the logic, it is easie
4、r to build than a mechanical control. Pneumatic control parts are subject to wear.As does a mechanical control, an electromechanical control use switches, relays, timers, counters, and so on, to construct logic. Because electric current is used, it is faster and more flexible. The controllers using
5、electromechanical control are called relay devices.Electric control is similar to electromechanical control, except that the moving mechanical components in an electromechanical control device are replaced by electronic switches, which works faster and is more reliable.Computer control is the most v
6、ersatile control system. The logic of the control is programmed into the computer memory using software. It not only can be for machine and manufacturing system control, but also for data communication. Very complex control strategies with extensive computations can be programmed. The first is the i
7、nterface with the outside world. Internally, the computer uses a low voltage (5 to 12 volts) and a low current (several milliamps). Machine requires much higher voltages (24, 110, or 220 voltages) and currents (measured in amps). The interface not only has to convert the voltage difference, but also
8、 must filter out the electric noise usually found in the shop. The interface thus must be custom-built for each application.In order to use the advantages of all those controllers and eliminate the difficulties, the programmable logic controllers were invented. A PLC was a replacement for relay devi
9、ces. They are programmed using a ladder diagram, which is standard electric wiring diagram. As PLCs become more flexibility, high-level as well as low-level languages are available to PLC programmers. PLCs have the flexibility of computers as well as a standard and easy interface with processes and
10、other devices. They are widely accepted in industry for controlling from a single device to a complex manufacturing facility.Automatic of many different processes, such as controlling machines or factory assembly lines, is done through the use of small the computers called a programmable logic contr
11、oller (PLC), PLCs were first created to serve the automobile industry, and the first PLC project was developed in 1968 for General Motors to replace hard-wired relay systems with an electronic controller. Since the advent of PLCs, the ability to centralize factory processes, especially in the automo
12、tive industry, has improved greatly. Automatic control has become an important consideration in most industrial processes where certain repetitive operations are performed. This applies to situations such as the automatic assembly of modules and products where a cycle of events is conducted in a con
13、sistent and uniform manner. Applications generally include a combination of feeding, handing, drilling, cutting, assembling, discharging, inspecting, packaging and transporting by conveyor.Prior to the introduction of computer-based control systems the automation of such events was achieved by using
14、 either electrical relay logic circuits or pneumatic logic circuits. Although these are conceptionally simple and easy to maintain, they are somewhat bulky and can be expensive. More important is the fact that the resulting control circuits are inflexible and do not lend themselves to easy system co
15、ntrol alterations. The late 1960s saw the introduction of the programmable logic controller (PLC) as a direct replacement for the relay sequence controllers. In essence the PLC replaces the hardwired relay or pneumatic logic with a more flexible programmable logic. It offers a simple, flexible and l
16、ow-cost means of implementing a sequence control strategy where outputs for switching devices on and off are set according to input conditions as read from digital sensor states. It should be noted that, particularly in the USA, the PLC is often referred to as a programmable controller with the abbreviation of PC. It should not be confused with the personal computer PC or IBM-PC.The PLC is composed of the same ingredients as a microcomputer such as a microp
