外文翻译-控制装置和可编程逻辑控制.doc
190 浏览量
2023-06-28
06:00:58
上传
评论
收藏 63KB DOC 举报
中文 3475 字
Control Devices and PLC
Joseph La Fauci
From Wikipedia, the free encyclopedia
Several 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 (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 control 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 easier 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 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 versatile 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 interface 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 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 devices.
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 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 controller
(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
automotive 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 consistent 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 either electrical relay logic circuits or pneumatic logic circuits. Although
资源评论
