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本科毕业生外文文献翻译
学生姓名：
史衍彬
指导教师：
荣丽红
所在学院：
信息技术学院
专
业：
农业电气化与自动化
中
国
·大<
/p>
庆
2010
年
4
月
SCM profile
Introduction of
Programmable controllers
From a simple heritage, these
remarkable systems have evolved to not only
replace
electromechanical devices, but
to solve an ever-increasing array of control
problems in both
process and nonprocess
industries. By all indications, these
microprocessor powered giants
will
continue to break new ground in the automated
factory into the 1990s.
HISTORY
In the 1960s,
electromechanical devices were the order of the
day ass far as control was
concerned.
These devices, commonly known as relays, were
being used by the thousands
to control
many sequential-type manufacturing processes and
stand-along machines. Many
of these
relays were in use in the transportation industry,
more specifically, the automotive
industry. These relays used hundreds of
wires and their interconnections to effect a
control
solution. The performance of a
relay was basically reliable - at least as a
single device. But
the common
applications for relay panels called for 300 to
500 or more relays, and the
reliability
and maintenance issues associated with supporting
these panels became a very
great
challenge. Cost became another issue, for in spite
of the low cost of the relay itself, the
installed cost of the panel could be
quite high. The total cost including purchased
parts,
wiring, and installation labor,
could range from $$30~$$50 per relay. To make
matters worse,
the constantly changing
needs of a process called for recurring
modifications of a control
panel. With
relays, this was a costly prospect, as it was
accomplished by a major rewiring
effort
on the panel. In addition these changes were
sometimes poorly documented, causing
a
second-shift maintenance nightmare months later.
In light of this, it was not uncommon to
discard an entire control panel in
favor of a new one with the appropriate components
wired
in a manner suited for the new
process. Add to this the unpredictable, and
potentially high,
cost of maintaining
these systems as on high-volume motor vehicle
production lines, and it
became clear
that something was needed to improve the control
process
–
to make it more
reliable, easier to troubleshoot, and
more adaptable to changing control needs.
That something, in the late
1960s, was the first programmable controller. This
first
‘revolutionary’ system wan
developed as a specific response to the needs of
the major
automotive manufacturers in
the United States. These early controllers, or
programmable
logic controllers (PLC),
represented the first systems that 1 could be used
on the factory floor,
2 could have
there ‘logic’ changed without extensive rewiring
or component changes, and 3
were easy
to diagnose and repair when problems occurred.
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
1
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 bl
ocks 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 c
ertain
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.
(2) The programming logic
is a form of a relay ladder diagram. Normally
open, normally
closed contacts, and
relay coils are used within a format utilizing a
left and a right vertical rail.
Power
flow (symbolic positive electron flow) is used to
determine which coil or outputs are
energized or deenergized.
(3) The machine is designed for the
industrial environment from its basic concept;
this
protection is not added at a later
date. The industrial environment includes
unreliable AC
power, high temperatures
(0 to 60 degree Celsius), extremes of humidity,
vibrations, RF
noise, and other similar
parameters.
General
application areas
The
programmable controller is used in a wide variety
of control applications today, many of
which were not economically possible
just a few years ago. This is true for two general
reasons: 1 there cost effectiveness
(that is, the cost per I/O point) has improved
dramatically
with the falling prices of
microprocessors and related components, and 2 the
ability of the
controller to solve
complex computation and communication tasks has
made it possible to
use it where a
dedicated computer was previously used.
Applications for
programmable controllers can be categorized in a
number of different ways,
including
general and industrial application categories. But
it is important to understand the
2
framework in which controllers are
presently understood and used so that the full
scope of
present and future evolution
can be examined. It is through the power of
applications that
controllers can be
seen in their full light. Industrial applications
include many in both discrete
manufacturing and process industries.
Automotive industry applications, the genesis of
the
programmable controller, continue
to provide the largest base of opportunity. Other
industries, such as food processing and
utilities, provide current development
opportunities.
There are
five general application areas in which
programmable controllers are used. A
typical installation will use one or
more of these integrated to the control system
problem.
The five general areas are
explained briefly below.
Description
The AT89C51 is a low-power, high-
performance CMOS 8-bit microcomputer with 4K bytes
of
Flash programmable and erasable read
only memory (PEROM). The device is manufactured
using Atmel’s high
-density
nonvolatile memory technology and is compatible
with the
industry-standard MCS-51
instruction set and pinout. The on-chip Flash
allows the program
memory to be
reprogrammed in-system or by a conventional
nonvolatile memory
programmer. By
combining a versatile 8-bit CPU with Flash on a
monolithic chip, the Atmel
AT89C51 is a
powerful microcomputer which provides a highly-
flexible and cost-effective
solution to
many embedded control applications.
Function characteristic
The AT89C51 provides the following
standard features: 4K bytes of Flash, 128 bytes of
RAM,
32 I/O lines, two 16-bit
timer/counters, a five vector two-level interrupt
architecture, a full
duplex serial
port, on-chip oscillator and clock circuitry. In
addition, the AT89C51 is designed
with
static logic for operation down to zero frequency
and supports two software selectable
power saving modes. The Idle Mode stops
the CPU while allowing the RAM, timer/counters,
serial port and interrupt system to
continue functioning. The Power-down Mode saves
the
RAM contents but freezes the
oscillator disabling all other chip functions
until the next
hardware reset.
Pin Description
VCC
：
Supply
voltage.
GND
：
Ground.
Port
0
：
Port 0 is an 8-bit open-drain bi-
directional I/O port. As an output port, each pin
can sink eight
TTL inputs. When 1s are
written to port 0 pins, the pins can be used as
highimpedance
0 may also be configured
to be the multiplexed loworder address/data bus
during
accesses to external program and
data memory. In this mode P0 has internal 0
also receives the code bytes during
Flash programming,and outputs the code bytes
during
programverification. External
pullups are required during programverification.
Port 1
3
Port 1 is an 8-bit bi-directional I/O
port with internal Port 1 output buffers can
sink/source four TTL 1s are written to
Port 1 pins they are pulled high by the
internal pullups and can be used as
inputs. As inputs,Port 1 pins that are externally
being
pulled low will source current
(IIL) because of the internal 1 also receives the
low-order address bytes during Flash
programming and verification.
Port 2
Port 2 is
an 8-bit bi-directional I/O port with internal
Port 2 output buffers can
sink/source
four TTL 1s are written to Port 2 pins they are
pulled high by the
internal pullups and
can be used as inputs. As inputs,Port 2 pins that
are externally being
pulled low will
source current, because of the internal 2 emits
the high-order
address byte during
fetches from external program memory and during
accesses to external
data memory that
use 16-bit addresses. In this application, it uses
strong internal
pullupswhen emitting
1s. During accesses to external data memory that
use 8-bit addresses,
Port 2 emits the
contents of the P2 Special Function 2 also
receives the
high-order address bits
and some control signals during Flash programming
and verification.
Port 3
Port 3 is an 8-bit bi-
directional I/O port with internal Port 3 output
buffers can
sink/source four TTL 1s
are written to Port 3 pins they are pulled high by
the
internal pullups and can be used as
inputs. As inputs,Port 3 pins that are externally
being
pulled low will source current
(IIL) because of the 3 also serves the functions
of
various special features of the
AT89C51 as listed below:
Port 3 also receives some control
signals for Flash programming and verification.
RST
Reset input. A high on this pin for two
machine cycles while the oscillator is running
resets
the device.
ALE/PROG
Address
Latch Enable output pulse for latching the low
byte of the address during accesses
to
external memory. This pin is also the program
pulse input (PROG) during Flash
normal
operation ALE is emitted at a constant rate of 1/6
the oscillator
frequency, and may be
used for external timing or clocking purposes.
Note, however, that
one ALE pulse is
skipped during each access to external Data
Memory.
If desired, ALE
operation can be disabled by setting bit 0 of SFR
location 8EH. With the bit
set, ALE is
active only during a MOVX or MOVC instruction.
Otherwise, the pin is weakly
pulled
high. Setting the ALE-disable bit has no effect if
the microcontroller is in external
execution mode.
PSEN
Program
Store Enable is the read strobe to external
program the AT89C51 is
executing code
from external program memory, PSEN is activated
twice each machine cycle,
except that
two PSEN activations are skipped during each
access to external data memory.
4