英語翻譯
It is interesting to observe the progress that has been made in the past 15 years in the programmable controller area.The pioneer products of the late 1960s must have been confusing and frightening to a great number of people.For example,what happened to the hardwired and electromechanical devices that maintenance personnel were used to repairing with hand tools?They were replaced with ‘computers’ disguised as electronics designed to replace relays.Even the programming tools were designed to appear as relay equivalent presentations.We have the opportunity now to examine the promise,in retrospect,that the programmable controller brought to manufacturing.
All programmable controllers consist of the basic functional blocks shown in Fig.10.1.We’ll examine each block to understand the relationship to the control system.First we look at the center,as it is the heart ( or at least the brain ) of the system.It consists of a microprocessor,logic memory for the storage of the actual control logic,storage or variable memory for use with data that will ordinarily change as a function power for the processor and memory.Next comes the I/O block.This function takes the control level signals for the CPU and converts them to voltage and current levels suitable for connection with factory grade sensors and actuators.The I/O type can range from digital (discrete or on / off),analog (continuously variable),or a variety of special purpose ‘smart’ I/O which are dedicated to a certain application task.The programmer is shown here,but it is normally used only to initially configure and program a system and is not required for the system to operate.It is also used in troubleshooting a system,and can prove to be a valuable tool in pinpointing the exact cause of a problem.The field devices shown here represent the various sensors and actuators connected to the I/O.These are the arms,legs,eyes,and ears of the system,including push buttons,limit switches,proximity switches,photosensors,thermocouples,RTDS,position sensing devices,and bar code reader as input; and pilot lights,display devices,motor starters,DC and AC drives,solenoids,and printers as outputs.
No single attempt could cover its rapidly changing scope,but three basic characteristics can be examined to give classify an industrial control device as a programmable controller.
(1) Its basic internal operation is to solve logic from the beginning of memory to some specified point,such as end of memory or end of program.Once the end is reached,the operation begins again at the beginning of memory.This scanning process continues from the time power is supplied to the time it it removed.