I am a freelance
PLC programmer with over 20 years of experience
programming all
types of
PLC.
I am based in the Midlands of the UK and so ideally situated to
help where ever you are in the UK or the rest of the world.
I have been a
programmer in a lot of industries including
steel, rubber &
tyres, food & beverage, power generation, chemical process,
automatically guided vehicles,
steel
& copper
tube manufacture,
packaging, material handling, industrial cleaning, special
purpose machinery, conveyors, automated warehouse.
If you need
a PLC Programmer give us a call
Any
Application, Any Industry, Anywhere!
Automation & Controls or
PLC programming,
with an extensive range of test equipment and
software tools, I can help with
Our engineer has just returned from
a successful installation of an OEE data collection system in Dubai.
The touch screen PC based system collects information via sensors on a
folding machine in a major printing comapny. Data such as Good Copy, Bad
Copy, Setting / Good Production is collated by the system and transformed
into statistical data which is used to monitor the efficiency of the machine
and operators. The system also integrates into the businesses management software allowing
the ordering of raw materials and the scheduling of jobs to be automated.
Wolf Automation - Any Application - Any Where !
Our
engineer has just completed the commissioning of the software for the
PLC Control system we
created, for the automatic fuel changeover system on two oil tanker vessels
in Amsterdam. New European legislation means that all ships have to
burn Low Sulpher Deisel Oil rather than the standard Heavy Fuel Oil while in
port. New tanks pipework and valves have to be added as well as
modifications to the boiler burners. In this case automation was added so
that the changeover can be made safely from the engine control room at the
push of a button. A Mitsubishi FX3u was implemented along with FX2n-2AD
modules to enable the reading of flow meters in the fuel lines. The control
is operated and monitored from a Mitsubishi E1100 HMI. Feedback from valves
and motor starters as well as fuel flow can be easily monitored on the
screen. An alarm list has also been generated to integrate with the existing
vessel management system. Wolf Automation - Any Application - Any Where !
Wolf were called in to
setup a 'special effect' in the largest show venue in Athens Greece. The effect involved dropping two large show curtains and 'flying'
them through the venue, over the heads of the audience for Nikos Vertis 2009
show. The unfinished control panel was already on site when we arrived, unfortunately there was
no software and the Mitsubishi inverters (FR-A520--5.5) required configuration. We programmed the
PLC Control System comprising a Koyo DL06 PLC to orchestrate the effect sequence and
control the speed of the
5.5kw winches, Chabuki (kabuki) release devices were used to drop the curtains
before the winches accelerated to full speed in half a second. The curtains
fly over the heads of the audience at a speed of 20ft/second. Obviously
the timing of the drop and wind up is importan to avoid dropping the curtains
onto
the audience. Using our industrial automation experience we were able to hit the ground
running and even though there was only a couple of days available, the effect was ready for
opening night. Once again our engineer displayed the versatility of Wolf Automation - Any Application - Any Where !
Wolf Automation replaced the controls and drive system on an existing
tube cutting and forming machine. The machine takes long lengths of stainless steel tube and cuts and
shapes them into various different styles of heater modules. Wolf engineers realised that the problems with the machine were mostly
generated by the control system and so the mechanical system was refurbished
saving many thousands of pounds on the job. The existing stepper motors were
replaced with DC servo motors and gearheads which improved the reliability
and repeatability of the system. A motion control system and HMI were added
bringing the machine into the 21st century and easing operation.
Due to the success of the the refurbishment, the same is now pending on 2 more
machines for the same customer, a multinational manufacturer of special
heaters and controls. Wolf Automation specialise in retro fitting control
systems to existing machines improving quality, production, reliability and
saving thousands of pounds against buying a new machine.
Wolf Automation - Any Application - Any Where !
Wolf were asked to commission a copper tube straightening machine in Revda Sverdlovskaya
Oblast, Russia. When we arrived on site to
setup the Bronx machine, we found that it hadn't even been wired. We were able to source cable locally and connect the
machine to the manufacturers drawings before commissioning the S7-300 based
PLC control system that utilised a WinCC front end, the positioning system
utilised profibus mounted absolute encoders. We stayed on site during a whole week of production
trials. Wolf Automation - Any Application - Any Where !
Wolf Automation have completed on a brand new control system for a Cold Roll Forming
line in the Midlands of Great Britain. The PLC control system utilises a Mitsubishi FX PLC, Indramat (Bosch Rexroth)
servos, Control Techniques Mentor II 350Kw DC drive and CT commander SK
inverters. The system controls and automates
all parts of the line, Decoiler, Rollfeed, Pierce Punching Press, 2 Accumulator pits,
Rolling Mill, Flying Cutoff with High speed hydraulics, Runoff conveyer and
Product Ejector system to deliver the products to the operator.
Wolf designed, built, programmed, installed and commissioned the line, on
time and on budget. We deliver High-spec,
High-quality, value for money solutions. Wolf Automation - Any
Application - Any Where !
Wolf
were tasked with producing a servo control system to control two actuators
which divert steel section from a conveyor to a packing station. Wolf used the new Bosch Rexroth Indradrive with integrated PLC along with
a VCP08 HMI. This simple point to point application gave us the chance
to acquaint ourselves with some of the latest technology form Bosch Rexroth.
Once again Wolf designed, built, programmed, installed and commissioned
the system. Wolf Automation - Any Application - Any Where !
Wolf Automation visited Lloyds TSB in Bristol recently to fault find
on their backup generators. The system has two generators aswell as a UPS
system and the whole thing is monitored by 3 24v Mitsubishi F2-60MR PLC's.
We
were able to interrogate the software with the old MEDOC software F2-20 GF1
and SC-03 interfaces to quickly find the cause of the problem on the system.
Due to the importance of this system to the entire banking system we had to
carry out all works between 6pm and midnight on a Saturday evening.
Wolf Automation - Any Application - Any Where !
Wolf
completed the installation of a high speed hydraulic punching unit for
a blue chip manufacturer of strut products. The unit controls the impression
of the companies name into the product synchronising with the crank press
that pierces the product. The hydraulic system supplied by H&L Hydraulics (Voith
Turbo) is able to apply 20 tonnes of stamping pressure and cycles at a rate
of over 5hz. Wolf designed the control system to drive and monitor the
hydraulic control card and hydraulic power pack, we also integrated the
system to the existing press and Indramat roll feed system. Plans are in
place to duplicate the system later this year. Wolf Automation -
Any Application - Any Where !
PLC
Control Systems
Industrial control systems
Industrial control system (ICS) is a general term that encompasses
several types of control systems, including supervisory control and data
acquisition (SCADA) systems, distributed control systems (DCS), and other
smaller control system configurations such as skid-mounted programmable logic
controllers (PLC) often found in the industrial sectors and critical
infrastructures. ICSs are typically used in industries such as electrical, water, oil and gas,
data. Based on information received from remote stations, automated or
operator-driven supervisory commands can be pushed to remote station control
devices, which are often referred to as field devices. Field devices control
local operations such as opening and closing valves and breakers, collecting
data from sensor systems, and monitoring the local environment for alarm
conditions.
A historical perspective
Industrial control system technology has evolved over the past three to four
decades. DCS systems generally refer to the particular functional distributed
control system design that exist in industrial process plants (e.g., oil and
gas, refining, chemical, pharmaceutical, some food and beverage, water and
wastewater, pulp and paper, utility power, mining, metals). The DCS concept came
about from a need to gather data and control the systems on a large campus in
real time on high-bandwidth, low-latency data networks. It is common for loop
controls to extend all the way to the top level controllers in a DCS, as
everything works in real time. These systems evolved from a need to extend
pneumatic control systems beyond just a small cell area of a refinery. The PLC (programmable logic controller) evolved out of a need to replace
racks of relays in ladder form. The latter were not particularly reliable, were
difficult to rewire, and were difficult to diagnose. PLC control tends to be
used in very regular, high-speed binary controls, such as controlling a
high-speed printing press. Originally, PLC equipment did not have remote I/O
racks, and many couldn't even perform more than rudimentary analog controls. SCADA's history is rooted in distribution applications, such as power,
natural gas, and water pipelines, where there is a need to gather remote data
through potentially unreliable or intermittent low-bandwidth/high-latency links.
SCADA systems use open-loop control with sites that are widely separated
geographically. A SCADA system uses RTUs (remote terminal units, also referred
to as remote telemetry units) to send supervisory data back to a control center.
Most RTU systems always did have some limited capacity to handle local controls
while the master station is not available. However, over the years RTU systems
have grown more and more capable of handling local controls. The boundaries between these system definitions are blurring as time goes on.
The technical limits that drove the designs of these various systems are no
longer as much of an issue. Many PLC platforms can now perform quite well as a
small DCS, using remote I/O and analog control loops, and are able to
communicate supervisory data. It is not uncommon to have telecommunications
infrastructure that is so responsive and reliable that some SCADA systems
actually manage closed loop control over long distances. With the increasing
speed of today's processors, many DCS products have a full line of PLC-like
subsystems that weren't offered when they were initially developed. This has led to the concept of a PAC (programmable automation controller or
process automation controller). It is an amalgamation of these three concepts.
Time and the market will determine whether this can simplify some of the
terminology and confusion that surrounds these concepts today.
DCSs
DCSs are used to control industrial processes such as electric power
generation, oil and gas refineries, water and wastewater treatment, and
chemical, food, and automotive production. DCSs are integrated as a control
architecture containing a supervisory level of control overseeing multiple,
integrated sub-systems that are responsible for controlling the details of a
localized process. Product and process control are usually achieved by deploying feed back or
feed forward control loops whereby key product and/or process conditions are
automatically maintained around a desired set point. To accomplish the desired
product and/or process tolerance around a specified set point, specific
programmable controllers are used ONLY.
PLCs
PLCs provide boolean logic operations, timers, and (in some models)
continuous control. The proportional, integral, and/or differential gains of the
PLC continuous control feature may be tuned to provide the desired tolerance as
well as the rate of self-correction during process upsets. DCSs are used
extensively in process-based industries. PLCs are computer-based solid-state
devices that control industrial equipment and processes. While PLCs can control
system components used throughout SCADA and DCS systems, they are often the
primary components in smaller control system configurations used to provide
regulatory control of discrete processes such as automobile assembly lines and
power plant soot blower controls. PLCs are used extensively in almost all
industrial processes.
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 list
programming, based on a stack-based logic solver. 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 very high-level programming language designed to program PLCs based on
state transition diagrams.
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.
Functionality
The functionality of the PLC has evolved over the years to include sequential
relay control, motion control, process control, distributed control systems and
networking. 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. 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 and
actuators. PLCs read limit switches, analog process variables (such as
temperature and pressure), and the positions of complex positioning systems.
Some use machine vision. 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.
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 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
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. 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.
Initially most PLC's utilized Ladder Logic Diagram Programming, a model which
emulated electromechanical control panel devices (such as the contact and coils
of relays) which PLC's replaced. This model remains common today. 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.
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 economic 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 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, 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 busses economically use PLCs
instead of custom-designed controls, because the volumes are low and the
development cost would be uneconomic. 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. 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. 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.
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
rises 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. 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.
About PLC Programmer
I began working with PLC's at
a major Japanese manufacturer of office equipment in based in Shropshire.
There were many PLC's installed in various production lines, assembly
equipment and robots. I installed Omron PLC's into a fleet of Automatically
Guided Vehicles that I designed. The AGV's carried photocopiers around the
plant, automatically transferring between one production line and the next.
The PLC installed on board took care of managing the route to take and what
to do when it got there. The AGV PLC communicated to the production line
PLC's in order to instigate and manage transfer from the vehicle to the
conveyor, the PLC also commanded the motion controllers which took
care of the drive and differential steering. Another PLC was statically
based and kept track of each AGV in the fleet, effectively managing the
whole system. This was quite a first PLC project, eventually saving the
company over £300,000 against a similar system bought from their usual
supplier. After 10 years and studying ONC and HNC I moved on to a new
position. PLC based Special purpose machinery for the rubber and plastic
industries. Most equipment went into tyre (tire) plants all over the
world. I designed PLC control systems , wrote the PLC and motion
control software, installed it and commissioned in house and on site all
over the world. This was an interesting position with the great opportunity
to travel the world while still being involved with PLC control systems.
Some small machines such as Tube splicers were installed with various brands
of brick PLC as specified by the customer, the larger machines such as Tire
builders generally had modular PLC's such as Allen Bradley SLC505. The fully
automated bias cutter machines with PLC I/O counts of over 400 had modular
PLC's with distributed I/O and SCADA systems. I installed and serviced PLC
based tire machinery in the UK, USA, Canada, India, China, Indonesia and got
to meet some great people. When I started my own PLC control company I
continued to work all over the world but in many different industries, I
have installed PLC control systems in Breweries, Power stations, Potato
processing plants, steel plants, rubber plants, chemical processing plants
and many more, all over the world. As with any technology PLC's progress and
PLC's installed 10 or 15 years ago may not be operating your machinery
to the optimum, with increased flexibility in good PLC systems such as
integrated motion control, increases in quality and efficiency can be
achieved. Replacing an outdated control system, PLC with an upto
date PLC control system can yield significant benefits.
About PLC's
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.PLCs may need to
interact with people for the purpose of configuration, alarm reporting or
everyday controlA 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.CommunicationsPLCs 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 systemsPLCs 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
"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
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.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.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.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. 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 signalsDigital 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.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 rises 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.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.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.hile 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.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 "father" 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. 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.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 Very 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. 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.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.The functionality of
the PLC has evolved over the years to include sequential relay control,
motion control, process control, distributed control systems and networking.
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.
PLC Programmer services include, PLC Programming, SCADA / HMI Programming, Motion Control Programming, Industrial Control Systems, SCADA Control Systems, Control System Refurbishing, Fault Finding & machine repair Wolf Automation charge by the hour, by the day or we
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