PLC
A programmable logic
controller (PLC) or programmable controller is a digital computer used for automation ofelectromechanical processes, such as control
of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs
are used in many industries and 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-up or non-volatile
memory. A PLC is an example of a hard real
time system since output results must be produced
in response to input conditions within a limited time, otherwise unintended
operation will result.
History
Before the PLC, control,
sequencing, and safety interlock logic for manufacturing automobiles was
accomplished using hundreds or thousands of relays, cam timers, 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 electricians needed to individually
rewire each and every relay.
Digital computers, being
general-purpose programmable devices, were soon applied to control of
industrial processes. Early computers required specialist programmers, and
stringent operating environmental control for temperature, cleanliness, and
power quality. Using a general-purpose computer for process control required
protecting the computer from the plant floor conditions. An industrial control
computer would have several attributes: it would tolerate the shop-floor
environment, it would support discrete (bit-form) input and output in an easily
extensible manner, it would not require years of training to use, and it would
permit its operation to be monitored. The response time of any computer system
must be fast enough to be useful for control; the required speed varying
according to the nature of the process.[1]
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.[2] 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 "father" of the PLC.[3] 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. Modicon used
the 84 moniker at the end of its product range until the 984 made its
appearance.
The automotive industry is
still one of the largest users of PLCs.
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. This program
notation was chosen to reduce training demands for the existing technicians.
Other early PLCs used a form of instruction listprogramming,
based on a stack-based logic solver.
Modern PLCs can be
programmed in a variety of ways, from the relay-derived ladder logic to
programming languages such as specially adapted dialects of BASICand C. Another method is State Logic, a very high-level programming language designed to program PLCs
based on state transition diagrams.
Many early PLCs did not
have accompanying programming terminals that were capable of graphical
representation of the logic, and so the logic was instead represented as a
series of logic expressions in some version of Boolean format,
similar to Boolean algebra. As
programming terminals evolved, it became more common for ladder logic to be
used, for the aforementioned reasons and because it was a familiar format used
for electromechanical control panels. Newer formats such as State Logic and
Function Block (which is similar to the way logic is depicted when using
digital integrated logic circuits) exist, but they are still not as popular as
ladder logic. A primary reason for this is that PLCs solve the logic in a
predictable and repeating sequence, and ladder logic allows the programmer (the
person writing the logic) to see any issues with the timing of the logic
sequence more easily than would be possible in other formats.
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.[2] Programs were stored on cassette tape cartridges. Facilities for printing and
documentation were minimal due to lack of memory capacity. The very oldest PLCs
used non-volatile magnetic
core memory.
More recently, PLCs are
programmed using application software on personal computers. The computer is
connected to the PLC through Ethernet, RS-232, RS-485 or RS-422 cabling. The programming
software allows entry and editing of the ladder-style logic. Generally the
software provides functions for debugging and troubleshooting the PLC software,
for example, by highlighting portions of the logic to show current status
during operation or via simulation. The software will upload and download the
PLC program, for backup and restoration purposes. In some models of
programmable controller, the program is transferred from a personal computer to
the PLC through a programming board which writes the program
into a removable chip such as an EEPROM or EPROM.
Functionality
The functionality of the
PLC has evolved over the years to include sequential relay control, motion
control, process control, distributed control systems andnetworking. The data
handling, storage, processing power and communication capabilities of some
modern PLCs are approximately equivalent to desktop computers.
PLC-like programming combined with remote I/O hardware, allow a general-purpose
desktop computer to overlap some PLCs in certain applications. Regarding the
practicality of these desktop computer based logic controllers, it is important
to note that they have not been generally accepted in heavy industry because
the desktop computers run on less stable operating systems than do PLCs, and
because the desktop computer hardware is typically not designed to the same
levels of tolerance to temperature, humidity, vibration, and longevity as the
processors used in PLCs. In addition to the hardware limitations of desktop
based logic, operating systems such as Windows do not lend themselves to
deterministic logic execution, with the result that the logic may not always
respond to changes in logic state or input status with the extreme consistency
in timing as is expected from PLCs. Still, such desktop logic applications find
use in less critical situations, such as laboratory automation and use in small
facilities where the application is less demanding and critical, because they
are generally much less expensive than PLCs.
In more recent years, small
products called PLRs (programmable logic relays), and also by similar names,
have become more common and accepted. These are very much like PLCs, and are
used in light industry where only a few points of I/O (i.e. a few signals coming
in from the real world and a few going out) are involved, and low cost is
desired. These small devices are typically made in a common physical size and
shape by several manufacturers, and branded by the makers of larger PLCs to
fill out their low end product range. Popular names include PICO Controller,
NANO PLC, and other names implying very small controllers. Most of these have
between 8 and 12 digital inputs, 4 and 8 digital outputs, and up to 2 analog inputs.
Size is usually about 4" wide, 3" high, and 3" deep. Most such
devices include a tiny postage stamp sized LCD screen for viewing simplified
ladder logic (only a very small portion of the program being visible at a given
time) and status of I/O points, and typically these screens are accompanied by
a 4-way rocker push-button plus four more separate push-buttons, similar to the
key buttons on a VCR remote control, and used to navigate and edit the logic.
Most have a small plug for connecting via RS-232 or RS-485 to a personal
computer so that programmers can use simple Windows applications for
programming instead of being forced to use the tiny LCD and push-button set for
this purpose. Unlike regular PLCs that are usually modular and greatly
expandable, the PLRs are usually not modular or expandable, but their price can
be twoorders
of magnitude less than a PLC and they still offer robust
design and deterministic execution of the logic.
PLC topics
Features
The main difference from
other computers is that PLCs are armored for severe conditions (such as dust,
moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These
connect the PLC to sensors andactuators. PLCs read limit switches,
analog process variables (such as temperature and pressure), and the positions
of complex positioning systems. Some use machine vision.[4] On the actuator side, PLCs
operate electric motors,pneumatic or hydraulic cylinders, magnetic relays, 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.
Scan time
A PLC program is generally
executed repeatedly as long as the controlled system is running. The status of
physical input points is copied to an area of memory accessible to the
processor, sometimes called the "I/O Image Table". The program is
then run from its first instruction rung down to the last rung. It takes some
time for the processor of the PLC to evaluate all the rungs and update the I/O
image table with the status of outputs.[5] This scan time may be a few
milliseconds for a small program or on a fast processor, but older PLCs running
very large programs could take much longer (say, up to 100 ms) to execute the
program. If the scan time was too long, the response of the PLC to process
conditions would be too slow to be useful.
As PLCs became more
advanced, methods were developed to change the sequence of ladder execution,
and subroutines were implemented.[6] This simplified programming
and could also be used to save scan time for high-speed processes; for example,
parts of the program used only for setting up the machine could be segregated
from those parts required to operate at higher speed.
Special-purpose I/O
modules, such as timer modules or counter modules, can be used where the scan
time of the processor is too long to reliably pick up, for example, counting
pulses and interpreting quadrature from a shaft encoder. The relatively slow
PLC can still interpret the counted values to control a machine, but the
accumulation of pulses is done by a dedicated module that is unaffected by the
speed of the program execution.
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 has insufficient 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 are customized 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
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 man-machine interfaces (MMIs) and graphical user interface
(GUIs). 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 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 RS-232, but optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually included as one
of thecommunications protocols. Other 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 devices such as keypads or PC-type
workstations.
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.[7]
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. Initially most PLCs utilized Ladder Logic Diagram
Programming, a model which emulated electromechanical control panel devices
(such as the contact and coils of relays) which PLCs replaced. This model
remains common today.
IEC 61131-3 currently
defines five programming languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text(ST;
similar to the Pascal programming language), instruction list (IL; similar to assembly language)
and sequential function chart (SFC).[8] These techniques emphasize
logical organization of operations.[7]
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.
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. 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 economical. This is due to the lower cost of the
components, which can be optimally chosen instead of a "generic"
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
consumerdishwasher 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, input/output
hardware and necessary testing and certification) 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 buses economically use PLCs instead of custom-designed
controls, because the volumes are low and the development cost would be
uneconomical.[9]
Very complex process
control, such as used in the chemical industry, may require algorithms and
performance beyond the capability of even high-performance PLCs. Very
high-speed or precision controls may also require customized solutions; for
example, aircraft flight controls. Single-board computers using semi-customized or
fully proprietary hardware may be chosen for very demanding control
applications where the high development and maintenance cost can be supported.
"Soft PLCs" running on desktop-type computers can interface with
industrial I/O hardware while executing programs within a version of commercial
operating systems adapted for process control needs.[9]
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.[citation needed]
PLCs may include logic for
single-variable feedback analog control loop, a "proportional, integral,
derivative" or "PID
controller". A PID loop could be used to control the
temperature of a 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. As PLCs have become more powerful, the boundary between DCS and PLC
applications has become less distinct.
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.
In recent years
"Safety" PLCs have started to become popular, either as standalone
models (Pilz PNOZ Multi, Sick etc.) or as functionality and safety-rated
hardware added to existing controller architectures (Allen Bradley Guardlogix,
Siemens F-series etc.). These differ from conventional PLC types as being
suitable for use in safety-critical applications for which PLCs have
traditionally been supplemented with hard-wired safety relays. For example, a
Safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown
response to an emergency stop on a conveyor production line. Such PLCs
typically have a restricted regular instruction set augmented with
safety-specific instructions designed to interface with emergency stops, light
screens and so forth. The flexibility that such systems offer has resulted in
rapid growth of demand for these controllers.
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, andphotoelectric
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
representingOff, 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 0 -
10 V input or 4-20 mA would beconverted 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 by controlling the valve that refills the tank. Shown is a
"ladder diagram" which shows the control system. A ladder diagram is
a method of drawing control circuits which pre-dates PLCs; it is also a type of
PLC user interface which uses the same method of representing a circuit. Shown
are:
§
Two inputs (from the low and high level switches)
represented by contacts of the float switches
§
An output to the fill valve, labelled as the fill valve
which it controls
§
An "imaginary" contact operated by the fill
valve which is created in software and the program.
§
A logical control scheme created by the interconnection
of these items in software
The PLC has two digital
inputs from float switches (Low Level and High Level).
When the water level is below both switches, the switch contacts are closed
providing signals to both the "Low Level" and "High Level"
inputs to the PLC. The PLC uses a digital output to open the inlet valve to fill the tank. The
software-created "Fill Valve" contact latches the circuit so that
even when the "Low Level" input goes away as the water begins to
rise, the fill valve remains on. Once the water level rises enough so that the
"High Level" switch is off (up), the PLC will shut the inlet to stop
the water from overflowing; This is an example of seal-in (latching) logic. The
output is sealed in until a high level condition breaks the circuit. After that
the fill valve remains closed (inactive) until the level drops so low that the
low level switch is activated, and the described process is repeated.
|
|
| Low
Level High Level Fill Valve |
|------[/]------|------[/]----------------------(OUT)---------|
| | |
|
| |
| | |
|
Fill Valve | |
|------[ ]------| |
|
|
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