可编程逻辑控制器电气工程及其自动化毕业论文中英文翻译

可编程逻辑控制器电气工程及其自动化毕业论文中英文翻译内容摘要：

xample As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank. Using only digital signals, the PLC has two digital inputs from float switches (Low Level and High Level). When the water level is above the switch it closes a contact and passes a signal to an input. The PLC uses a digital output to open and close the inlet valve into the tank. When the water level drops enough so that the Low Level float switch is off (down), the PLC will open the valve to let more water in. Once the water level rises enough so that the High Level switch is on (up), the PLC will shut the inlet to stop the water from overflowing. This rung is an example of seal in logic. The output is sealed in until some condition breaks the circuit. | | | Low Level High Level Fill Valve | |[/]|[/](OUT)| | | | | | | | | | | Fill Valve | | |[ ]| | | | | | An analog system might use a water pressure sensor or a load cell, and an adjustable (throttling) dripping out of the tank, the valve adjusts to slowly drip water back into the tank. In this system, to avoid 39。 flutter39。 adjustments that can wear out the valve, many PLCs incorporate hysteresis which essentially creates a deadband of activity. A technician adjusts this deadband so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear. A real system might bine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates and prevent water hammer. Backup and maintenance methods can make a real system very plicated. Programming PLC programs are typically written in a special application on a personal puter, then downloaded by a directconnection cable or over a work to the PLC. The program is stored in the PLC either in batterybackedup RAM or some other nonvolatile flash memory. Often, a single PLC can be programmed to replace thousands of relays. Under the IEC 611313 standard, PLCs can be programmed using standardsbased programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Recently, the International standard IEC 611313 has bee popular. IEC 611313 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical anization of operations. While the fundamental concepts of PLC programming are mon to all manufacturers, differences in I/O addressing, memory anization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly patible. History Origin The PLC was invented in response to the needs of the American automotive manufacturing industry. Programmable controllers were initially adopted by the automotive industry where software revision replaced the rewiring of hardwired control panels when production models changed. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was acplished using hundreds or thousands of relays, cam timers, and drum sequencers and dedicated closedloop controllers. The process for updating such facilities for the yearly model changeover was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians.。