外文翻译-关于PLC的外文及翻译

PLCs --Past, Present and FutureEveryone knows there's only one constant in the technology world， and that's change. This is especially evident in the evolution of Programmable Logic Controllers (PLC) and their varied applications。 From their introduction more than 30 years ago, PLCs have become the cornerstone of hundreds of thousands of control systems in a wide range of industries. At heart， the PLC is an industrialized computer programmed with highly specialized languages, and it continues to benefit from technological advances in the computer and information technology worlds。 The most prominent of which is miniaturization and communications.The Shrinking PLCWhen the PLC was first introduced， its size was a major improvement — relative to the hundreds of hard-wired relays and timers it replaced. A typical unit housing a CPU and I/O was roughly the size of a 19 television set. Through the 1980s and early 1990s, modular PLCs continued to shrink in footprint while increasing in capabilities and performance （see Diagram 1 for typical modular PLC configuration）. In recent years， smaller PLCs have been introduced in the nano and micro classes that offer features previously found only in larger PLCs。 This has made specifying a larger PLC just for additional features or performance， and not increased I/O count， unnecessary, as even those in the nano class are capable of Ethernet communication, motion control， on-board PID with autotune， remote connectivity and more. PLCs are also now well-equipped to replace stand-alone process controllers in many applications， due to their ability to perform functions of motion control， data acquisition， RTU （remote telemetry unit） and even some integrated ...