英文文献-可编程逻辑控制器(PLC)英语

Programmable Logic Controllers

Programmable

logic

controller

(plc)

is

a

solid-state

device

used

to

control

machine motion or process operation by means of a stored program. The PLC sends

output control signals and receives input signals through input/output (I/O) devices. A

PLC

controls

outputs

in

response

to

stimuli

at

the

inputs

according

to

the

logic

prescribed

by

the

stored

program.

The

inputs

are

made

up

of

limit

switches,

pushbuttons,

thunbwheels.

Switches,

pulses, analog signals,

ASCLL serial

data,

and

binary

or

BCD

data

from

absolute

position

encoders.

The

outputs

are

voltage

or

current levels to drive end devices such as solenoids, motor starters, relays, lights, and

so

on.

Other

output

devices

include

analog

devices,

digital

BCD

displays,

ASCII

compatible devices, servo variable-speed drives, and even computers.

Programmable controllers were developed (circa in

1968) when General

Motors

Corp, and other automobile manufacturers were experimenting to see if there might be

an

alternative

to

scrapping

all

their

hardwired

control

panels

of

machine

tools

and

other

production

equipment

during

a

model

changeover.

This

annual

tradition

was

necessary because rewiring of the panels was more expensive than buying new ones.

The

automotive

companies

approached

a

number

of

control

equipment

manufacturers and asked them to develop a control system that would have a longer

productive

life

without

major

rewiring,

but

would

still

be

understandable

to

and

repairable

by

plant

personnel.

The

new

product

was

named

a

“pr

ogrammable

controller

”

.

The processor part of the PLC contains a central processing unit and memory

。

The central processing unit (cpu) is the

“

traffic director

”

of the processor, the memory

stores information. Coming into the processor are the electrical signals from the input

devices, as conditioned by the input module to voltage levels acceptable to processor

logic. The processor scans the state of

I/o and updates outputs based on instructions

stored in the memory of the plc. For example, the processor may be programmed so

that

if

an

input

connected

to

a

limit

switch

is

true

(limit

switch

closed),

then

a

corresponding output wired to an output module is to be energized. This output might

be a solenoid, for example on each scan to see if that limit switch is, in fact, closed. If

it is closed, the processor energizes the solenoid by turning on the output module.

The

output

device,

such

as

a

solenoid

or

motor

starter,

is

wired

to

output

module

’

s

terminal,

and

it

receives

its

shift

signal

from

the

processor,

in

effect,

the

processor

is

performing

a

long

and

complicated

series

of

logic

decisions.

The

PLC

performs

such

decisions

sequentially

and

in

accordance

with

the

stored

program.

Similarly, analog I/O allows the processor to make decisions based on the magnitude

lf

a

signal.

Rather

than

just

if

it

is

on

or

off.

For

example,

The

processor

may

be

programmed to increase or decrease the steam flow to a boiler (analog output ) based

on a comparison of the actual temperature in the boiler (analog input) to the desired

temperature.

This

is

often

performed

by

utilizing

the

built-in

PID

(proportional,

integral, derivative ) capabilities lf the processor.

Because a PLC is

“

software based

”

, its control logic functions can e changed by

reprogramming

its

memory.

Keyboard

programming

devices

facilitate

entry

of

the

revised program, which can be designed to cause an existing machine or process to

operate in a different sequence or to respond to different levels of, or combinations of

stimuli. Hardware modifications are needed only if additional, changed, or relocated

input/output devices are involved.

All

programmable

controllers

consist

of

the

basic

functional

blocks

shown

in

Figure 10.1. We will examine each block to understand the relationship to the control

system. First we looked at the center, as it is the heart 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

of

the

control

program

execution,

and

a

power

supply

to

provide

electrical

power

for

the

processor

and

memory.

Next

comes

the

I/O

bloke.

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, analog, or a variety of special purpose

“

smart

”

I/O which are dedicated to a

certain application task. The programmer 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

pushbuttons,

limit

switches,

proximity

switches,

photosensors,

thermocouples, position sensing devices, and bar code reader as input; and pilot light,

display

devices,

motor

starters,

DC

and

AC

drivers,

solenoids,

and

printers

as

outputs .

An adaptive control system is one whose parameters are automatically adjusted

to

compensate

for

corresponding

variations

in

the

properties

of

the

process.

The

system is, in a word,

“

adapted

”

to the needs of the process. Naturally there must be

2