工科毕业设计英文翻译-plc

工科毕业设计英文翻译-plc内容摘要：

ay 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 (tank empty and tank full). 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 tank empty float switch is off (down), the PLC will open the valve to let more water in. Once the water level raises enough so that the tank full switch is on (up), the PLC will shut the inlet to stop the water from overflowing. 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. System scale A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model does not have enough I/O. PLCs used in larger I/O systems may have peertopeer (P2P) munication between processors. This allows separate parts of a plex process to have individual control while allowing the subsystems to coordinate over the munication link. These munication links are also often used for HMI (HumanMachine Interface) devices such as keypads or PCtype workstations. Some of today39。 s PLCs can municate over a wide range of media including RS485, Coaxial, and even Ether for I/O control at work speeds up to 100 Mbit/s. Programming Early PLCs, up to the mid1980s, were programmed using proprietary programming panels or specialpurpose programming terminals. they were designed to replace relay logic systems. These PLCs were programmed in ladder logic, which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams. 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 manufacture。