PLC电梯的控制英文文献五千字翻译(适用于本科毕业设计).ppt

* 1 The Programmable Logic Controller John R. Hackworth, Frederick D. Hackworth, Jr. Programmable Logic Controllers:Programming Methods and Applications. 2.5~2.8,The Programmable Logic Controller, John R. Hackworth, Frederick D. Hackworth, Jr. First Edition.London: Prentice Hall,2003.35~50 2.5 System Block Diagram A Programmable Controller is a specialized computer.Since it is a computer, it has all the basic component parts that any other computer has; a Central Processing Unit,Memory, Input Interfacing and Output Interfacing.A typical programmable controller block diagram is shown in Figure 2.5. Figure 2.5 Programmable Controller Block Diagram The Central Processing Unit (CPU) is the control portion of the PLC. It interprets the program commands retrieved from memory and acts on those commands.In present day PLCs this unit is a microprocessor based system.The CPU is housed in the processor module of modularized systems. Memory in the system is generally of two types; ROM and RAM.The ROM memory contains the program information that allows the CPU to interpret and act on the Ladder Logic program stored in the RAM memory.RAM memory is generally kept alive with an on-board battery so that ladder programming is not lost when the system power is removed. This battery can be a standard dry cell or rechargeable nickel-cadmium type. Newer PLC units are now available with Electrically Erasable Programmable Read Only Memory (EEPROM) which does not require a battery.Memory is also housed in the processor module in modular systems. Input units can be any of several different types depending on input signals expected as described above.The input section can accept discrete or analog signals of various voltage and current levels.Present day controllers offer discrete signal inputs of both AC and DC voltages from TTL to 250 VDC and from 5 to 250 VAC.Analog input units can accept input levels such as ±10 VDC, ±5 VDC and 4-20 ma. current loop values.Discrete input units p