自动化英文文献翻译

2021年1月25日发(作者：tyer)
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Programmable logic controller
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PLC & input/output arrangements
A
programmable
logic
controller
(PLC)
or
programmable
controller
is
a
digital
computer
used
for
automation
of
electromechanical
processes,
such
as
control
of
machinery
on
factory
assembly
lines,
control
of
amusement
rides,
or
control
of
lighting
fixtures.
PLCs
are
used
in
many
different
industries
and
machines
such
as
packaging
and
semiconductor
machines.
Unlike
general-purpose
computers,
the
PLC
is
designed
for
multiple
inputs
and
output
arrangements,
extended
temperature
ranges,
immunity
to
electrical
noise,
and
resistance to vibration and impact. Programs to control machine operation are typically stored
in battery-backed or
non- volatile

memory
. A PLC
is an example of a
real time
system since output
results
must
be
produced
in
response
to
input
conditions
within
a
bounded
time,
otherwise
unintended operation will result.
Features
Control
panel
with
PLC
(grey
elements
in
the
center).
The
unit
consists
of
separate
elements, from left to right;
power supply
, controller,
relay
units for in- and output
The
main
difference
from
other
computers
is
that
PLCs
are
armored
for
severe
conditions
(dust,
moisture,
heat,
cold,
etc)
and
have
the
facility
for
extensive
input/output

(I/O)
arrangements.
These
connect
the
PLC
to
sensors

and
actuators
.
PLCs
read
limit
switches
,
analog
process variables (such as temperature and pressure), and the positions of complex positioning
systems. Some even
use
machine vision
. On the actuator side, PLCs operate
electric motors
,
pneumatic

or
hydraulic

cylinders,
magnetic
relays

or
solenoids
,
or
analog
outputs.
The
input/output
arrangements
may
be
built
into
a
simple
PLC,
or
the
PLC
may
have
external
I/O
modules
attached to a computer network that plugs into the PLC.
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.

Modular
PLCs
have
a
chassis
(also
called
a
rack)
into
which
are
placed
modules
with
different
functions.
The
processor
and
selection
of
I/O
modules
is
customised
for
the
particular application. Several
racks can be administered by a single processor, and
may
have
thousands of
inputs and outputs.
A special
high speed serial I/O link
is
used so that racks can
be distributed away from the processor, reducing the wiring costs for large plants.

User interface
See also:
List of human-computer interaction topics


PLCs
may need to
interact
with people
for the purpose of configuration, alarm
reporting
or everyday control.
A
Human-Machine Interface
(HMI)
is employed
for this purpose.
HMIs are also referred
to as
MMIs (Man Machine Interface) and GUI (Graphical User Interface).
A
simple
system
may
use
buttons
and
lights
to
interact
with
the
user.
Text
displays
are
available as well as
graphical touch
screens. More complex systems
use a programming and
monitoring
software
installed
on
a
computer,
with
the
PLC
connected
via
a
communication
interface.
Communications
PLCs
have
built
in
communications
ports
usually
9-Pin
RS232
,
and
optionally
for
RS485

and
Ethernet
.
Modbus
or
DF1
is usually included as one of the
communications protocols
. Others' options
include
various
fieldbuses
such as
DeviceNet
or
Profibus
. Other communications protocols that
may
be used are listed in the
List of automation protocols
.
Most
modern
PLCs
can
communicate
over
a
network
to
some
other
system,
such
as
a
computer
running
a
SCADA

(Supervisory
Control
And
Data
Acquisition)
system
or
web
browser.
PLCs
used
in
larger
I/O
systems
may
have
peer-to- peer
(P2P)
communication
between
processors.
This
allows
separate parts of a complex process to
have
individual control
while
allowing
the
subsystems
to
co-ordinate
over
the
communication
link.
These
communication
links
are
also
often
used
for
HMI

(Human-Machine
Interface)
devices
such
as
keypads
or
PC
-type
workstations.
Some
of
today's
PLCs
can
communicate
over
a
wide
range
of
media
including
RS-485,
Coaxial,
and
even
Ethernet

for
I/O
control
at
network
speeds
up
to
100
Mbit/s.

PLC compared with other control systems
PLCs
are
well- adapted
to
a
range
of
automation

tasks.
These
are
typically
industrial
processes
in
manufacturing
where
the
cost
of
developing
and
maintaining
the
automation
system
is
high relative to the total cost of the automation, and where
changes to the system
would
be
expected
during
its
operational
life.
PLCs
contain
input
and
output
devices
compatible
with
industrial
pilot
devices
and
controls;
little
electrical
design
is
required,
and
the design problem centers on expressing the desired sequence of operations
in
ladder logic

(or
function chart
) notation. PLC applications are typically
highly customized systems so
the cost of
a packaged PLC
is
low compared
to the cost of a specific custom- built controller design. On
the other hand, in the case of mass-produced goods, customized control systems are economic
due to the lower cost of the components, which can be optimally chosen instead of a
solution,
and
where
the
non-recurring
engineering
charges
are
spread
over
thousands
or
millions of units.
For
high
volume
or
very
simple
fixed
automation
tasks,
different
techniques
are
used.
For
example,
a
consumer
dishwasher

would
be
controlled
by
an
electromechanical
cam
timer

costing only a few dollars in production quantities.
A
microcontroller
-based design
would be appropriate where
hundreds or
thousands of
units
will
be
produced
and
so
the
development
cost
(design
of
power
supplies
and
input/output
hardware) can be spread over
many sales, and where the end-user would not
need to alter the
control.
Automotive
applications
are
an
example;
millions
of
units
are
built
each
year,
and
very
few
end-users
alter
the
programming
of
these
controllers.
However,
some
specialty
vehicles
such
as
transit
busses
economically
use
PLCs
instead
of
custom- designed
controls,
because the volumes are low and the development cost would be uneconomic.

V
ery
complex
process
control,
such
as
used
in
the
chemical
industry
,
may
require
algorithms
and
performance
beyond
the
capability
of
even
high-performance
PLCs.
V
ery
high-speed or precision controls
may also require customized solutions;
for example, aircraft
flight controls.
Programmable
controllers
are
widely
used
in
motion
control,
positioning
control
and
torque control. Some
manufacturers produce
motion control
units
to be
integrated with PLC
so that
G-code
(involving a
CNC
machine) can be used to instruct machine movements.
[1]


PLCs may include logic for single-variable feedback analog control loop, a
integral, derivative
PID controller
.
manufacturing
process,
for
example.
Historically
PLCs
were
usually
configured
with
only
a
few analog control loops; where processes required hundreds or thousands of loops, a
distributed
control system
(DCS) would instead be used. However, as PLCs have become more powerful, the
boundary between DCS and PLC applications has become less clear-cut.
PLCs
have
similar
functionality
as
Remote Terminal Units
.
An
RTU,
however,
usually
does
not support control algorithms or control
loops. As
hardware rapidly becomes
more powerful
and
cheaper,
RTUs
,
PLCs
and
DCSs

are
increasingly
beginning
to
overlap
in
responsibilities,
and
many
vendors
sell
RTUs
with
PLC-like
features
and
vice
versa.
The
industry
has
standardized on the IEC 61131-3
functional block
language
for creating programs
to
run on
RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated
development environments.
Digital and analog signals
Digital or discrete signals behave as binary switches, yielding simply an On or Off signal
(1
or
0,
True
or
False,
respectively).
Push
buttons,
limit
switches,
and
photoelectric sensors

are
examples of devices providing a discrete signal. Discrete signals are sent using either
voltage
or
current
,
where
a
specific
range
is
designated
as
On
and
another
as
Off.
For
example,
a
PLC
might
use
24
V
DC
I/O,
with
values
above
22
V
DC
representing
On,
values
below
2VDC
representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.

Analog
signals
are
like
volume
controls,
with
a
range
of
values
between
zero
and
full-scale. These are typically
interpreted as
integer
values (counts) by the PLC, with
various

ranges of accuracy depending on the device and the number of bits available to store the data.
As PLCs typically
use 16-bit signed binary processors, the
integer values are
limited between
-32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog
signals. Analog signals can
use
voltage
or
current
with a
magnitude proportional to
the
value of
the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into
an integer value of 0 - 32767.
Current
inputs
are
less
sensitive
to
electrical
noise
(i.e.
from
welders
or
electric
motor
starts) than voltage inputs.

Example
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
raises 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.
|




























































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Low Level





High Level
















Fill V
alve








|

|------[/]------|------[/] ----------------------(OUT)---------

















|
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|












































|

|














|












































|

|


Fill V
alve

|














































|

|------[ ]------|

















































|
|



























































|

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|

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
'flutter'
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 combine 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 complicated.

Programming
PLC programs are typically written in a special application on a personal computer, then
downloaded by a direct-connection cable or over a network to the PLC. The program is stored
in
the
PLC
either
in
battery-backed-up
RAM

or
some
other
non-volatile
flash memory
.
Often,
a
single PLC can be programmed to replace thousands of
relays
.
Under the
IEC 61131-3
standard, PLCs can be programmed
using
standards-based programming
languages.
A
graphical
programming
notation
called
Sequential
Function
Charts

is
available
on
certain programmable controllers.
Recently,
the
International
standard
IEC
61131-3

has
become
popular.
IEC
61131-3
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 organization of operations.
While the fundamental concepts of PLC programming are common to all manufacturers,
differences
in
I/O
addressing,
memory
organization
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 compatible.

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
re-wiring
of
hard-wired
control
panels
when
production models changed.
Before
the
PLC,
control,
sequencing,
and
safety
interlock
logic
for
manufacturing
automobiles
was
accomplished
using
hundreds
or
thousands
of
relays
,
cam
timers
,
and
drum
sequencers
and dedicated closed-loop controllers. The process for updating such facilities for the
yearly
model
change-over
was
very time consuming and expensive, as
the relay systems
needed
to be rewired by skilled electricians.
In
1968
GM
Hydramatic
(the
automatic
transmission
division
of
General Motors
)
issued
a

request for proposal for an electronic replacement for hard-wired relay systems.
The
winning
proposal
came
from
Bedford
Associates
of
Bedford, Massachusetts
.
The
first
PLC,
designated
the
084
because
it
was
Bedford
Associates'
eighty- fourth
project,
was
the
result.
Bedford
Associates
started
a
new
company
dedicated
to
developing,
manufacturing,
selling,
and servicing this
new product: Modicon,
which
stood
for MOdular
DIgital CONtroller. One
of the people who worked on that project was
Dick Morley
, who is considered to be the

of
the
PLC.
The
Modicon
brand
was
sold
in
1977
to
Gould Electronics
,
and
later
acquired
by
German Company
AEG
and then by French
Schneider Electric
, the current owner.
One
of
the
very
first
084
models
built
is
now
on
display
at
Modicon's
headquarters
in
North Andover, Massachusetts
. It was presented to Modicon by
GM
, when the
unit
was
retired after
nearly twenty years of uninterrupted service.
The
automotive
industry
is
still
one
of
the
largest
users
of
PLCs,
and
Modicon
still
numbers some of its controller models such that they end with eighty-four.
Development
Early PLCs 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
V
ery
High
Level
Programming
Language
designed to program PLCs based on
State Transition Diagrams
.
Many of the earliest PLCs expressed all decision making logic in simple
ladder logic
which
appeared similar to electrical schematic diagrams. The electricians were quite able to trace out
circuit
problems
with
schematic
diagrams
using
ladder
logic.
This
program
notation
was
chosen to reduce training demands for the existing technicians. Other early PLCs used a form
of
instruction list
programming, based on a stack-based logic solver.
Programming
Early
PLCs,
up
to
the
mid-1980s,
were
programmed
using
proprietary
programming
panels
or
special-purpose
programming
terminals
,
which
often
had
dedicated
function
keys
representing the
various
logical
elements of PLC programs. Programs were stored on cassette
tape
cartridges.
Facilities
for
printing
and
documentation
were
very
minimal
due
to
lack
of
memory capacity
. The very oldest PLCs used non- volatile
magnetic core memory
.