plc based fire monitoring system
DESCRIPTION
final year project of PLC Based Fire Monitoring SystemTRANSCRIPT
PLC Based Petrochemical Fire Monitoring and Control Station via GSM
Batch 2004
NAME OF STUDENTS ROLL NUMBERS
1. Shafiqullah D-04-ES-382. Imran Ahmed D-04-ES-423. Abdul Rauf Khan D-04-ES-124. Shah Faisal D-04-ES-085. Rizwan Faridi D-04-ES-376. Muhammad Wasiq Ansari D-04-ES-32
SUPERVISORSXxxXxx
DEPARTMENT OF ELECTRONIC ENGINEERINGDawood College of Engineering and Technology Karachi
Submitted in partial fulfillment of the requirementfor the Degree of Bachelor of ELECTRONIC Engineering.
JUNE 2009
CERTIFICATE
This is to certify that the work presented in this project report on PLC Based
Petrochemical Fire Monitoring and Control Station via GSM used in DCET is
entirely written by the following students themselves under supervision of Mr. xxx
NAME OF STUDENTS ROLL NUMBERS
1) Shafiqullah D-04-ES-382) Imran Ahmed D-04-ES-423) Abdul Rauf Khan D-04-ES-124) Shah Faisal D-04-ES-085) Rizwan Faridi D-04-ES-376) Muhammad Wasiq Ansari D-04-ES-32
________________________ _________________________________Internal / Thesis Supervisor External Examiner / Exam Committee
________________________________Chairman
Department of Electronic Engineering
Dated: _____________
GROUP MEMBERS
NAME OF STUDENTS ROLL NUMBERS
1) Shafiqullah D-04-ES-38
2) Imran Ahmed D-04-ES-42
3) Abdul Rauf Khan D-04-ES-12
4) Shah Faisal D-04-ES-08
5) Rizwan Faridi D-04-ES-37
6) Muhammad Wasiq Ansari D-04-ES-32
DAWOOD COLLEGE OF ENGINEERING AND TECHNOLOGYKARACHI
DEDICATION
Dedication goes here…
ACKNOWLEDGEMENT
Acknowledgment goes here…
ABSTRACT
Abstract goes here…
PREFACE
This thesis covers the results of more than four years of our work (2005-2009) in the field
of Electronics. Half of the work has been research oriented and the other half project
oriented to transform research results into the real world and test their feasibility.
SUMMARY OF CONTENTS
Chapter Title Page No
Chapter # 1 Introduction 1
Chapter # 2 Automation 15
Chapter # 3 Programmable Logic Controllers 28
Chapter # 4 Human Machine Interface (HMI) 61
INDEX
CHAPTER 1____________________________________________________________12Automation_________________________________________________________________12
Programmable Logic Controller_______________________________________________14
Artificial Intelligence_________________________________________________________15
Relay______________________________________________________________________16
Human Machine Interface____________________________________________________171. Effectiveness___________________________________________________________________182. Efficiency______________________________________________________________________193. Satisfaction_____________________________________________________________________19
History of HMI_____________________________________________________________19
CHAPTER 2________________________________________________________20What is Automation_________________________________________________________20
Importance of Automation____________________________________________________21Gantry (Cartesian) Robot____________________________________________________________22SCARA Robots___________________________________________________________________22Articulated Robots_________________________________________________________________22
Impact of Automation________________________________________________________23
Advantages and Disadvantages________________________________________________24Controversial Factors_______________________________________________________________25
Unemployment____________________________________________________________25 Environment______________________________________________________________25 Human Being Replacement___________________________________________________26
Automation Tools___________________________________________________________26Artificial Neural Network___________________________________________________________26Distributed Control System__________________________________________________________27Human Machine Interface___________________________________________________________27Programmable Logic Controller______________________________________________________27Programmable Automation Controller_________________________________________________28Instrumentation___________________________________________________________________28Motion Control____________________________________________________________________29Robotics_________________________________________________________________________29
Supervisory Control and Data Acquisition (SCADA)______________________________30
Future of Automation________________________________________________________31New Technology Directions_________________________________________________________32The Automated factory_____________________________________________________________33Hard Truths about Globalization______________________________________________________33The Winning Differences____________________________________________________________34
Types of Automation_________________________________________________________341. Fixed Automation_______________________________________________________________342. Programmable Automation________________________________________________________35
3. Flexible Automation_____________________________________________________________35
Industrial Automation Mechanism_____________________________________________36
CHAPTER 3____________________________________________________________38What is PLC________________________________________________________________38
Theory of Operation_________________________________________________________39The Guts Inside___________________________________________________________________39How It Works?____________________________________________________________________40Response Time____________________________________________________________________42Effects of Response Time___________________________________________________________43Relays___________________________________________________________________________45
Replacing Relays____________________________________________________________46Basic Instructions__________________________________________________________________48
Load_________________________________________________________________________48LoadBar_______________________________________________________________________48Out___________________________________________________________________________49Outbar________________________________________________________________________50
Basic Program Example_____________________________________________________________50PLC Registers____________________________________________________________________51A Level Application________________________________________________________________53How a Ladder is Scanned___________________________________________________________55
Features of PLC_____________________________________________________________57
PLC compared with other Control Systems______________________________________57
Programming Of PLC________________________________________________________61
History Of PLC_____________________________________________________________61Development_____________________________________________________________________62Programming_____________________________________________________________________63Functionality_____________________________________________________________________63
User interface_______________________________________________________________63
Communications____________________________________________________________64
CHAPTER 4____________________________________________________________65About Human Machine Interface______________________________________________65
Overview of HMI____________________________________________________________66ISO Definition of Quality of Use______________________________________________________68Usability Engineering______________________________________________________________68
Why Is It Important?__________________________________________________________68
CHAPTER 1
Automation
Industrial automation is the use of robotic devices to complete manufacturing tasks. In
this day and age of computers, industrial automation is becoming increasingly important
in the manufacturing process because computerized or robotic machines are capable of
handling repetitive tasks quickly and efficiently. Machines used in industrial automation
are also capable of completing mundane tasks that are not desirable to workers. In
addition, the company can save money because it does not need to pay for expensive
benefits for this specialized machinery. There are both pros and cons for a company when
it comes to industrial automation.
On the plus side, with soaring healthcare costs, paid days off, vacation time, and other
costly employee benefits, companies can save money with industrial automation. While
robotic machinery can initially be extremely expensive, the loss of monthly wages for
production workers leads to incredible savings for the company. While machinery used
for industrial automation can break down, it does not happen often if it does, only a
handful of maintenance or computer engineers are needed to handle repairs and get lines
running smoothly again.
In addition, many plants hire dozens of production workers for a variety of shifts and
need to close on certain days. Industrial automation, however, allows a company to run
the plant twenty-four hours a day, 365 days a year, without paying overtime. This fact
alone can add up to significant savings.
A company that employs forty-eight factory workers on three different shifts and closes
on weekends, for example, can save thousands of dollars with industrial automation. This
is particularly true if weekend work is necessary, which means overtime pay of time and
a half must be paid for Saturday work and double-time for Sunday. This equates to an
additional twelve hours of pay per employee. Of course, life insurance, 401K benefits,
dental insurance, health insurance, pension coverage, and disability also contribute to the
expense.
Industrial automation can eliminate the need for all forty-eight jobs. The robotic
machinery used for industrial automation may only involve a monthly payment until the
machinery is paid for, a couple technicians to keep the robotic machinery running, and
electricity costs. Unfortunately for workers, industrial automation can eliminate
thousands of jobs. As the workforce decreases and the cost of living increases, many
families struggle to make ends meet as their jobs are replaced by high-tech machines.
Programmable Logic Controller
A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the
necessary sequential relay circuits for machine control. The PLC works by looking at its
inputs and depending upon their state, turning on/off its outputs. The user enters a
program, usually via software, that gives the desired results.
PLCs are used in many "real world" applications. If there is industry present, chances are
good that there is a plc present. If you are involved in machining, packaging, material
handling, automated assembly or countless other industries you are probably already
using them. If you are not, you are wasting money and time. Almost any application that
needs some type of electrical control has a need for a plc.
For example, let's assume that when a switch turns on we want to turn a solenoid on for 5
seconds and then turn it off regardless of how long the switch is on for. We can do this
with a simple external timer. But what if the process included 10 switches and solenoids?
We would need 10 external timers. What if the process also needed to count how many
times the switches individually turned on? We need a lot of external counters.
As you can see the bigger the process the more of a need we have for a PLC. We can
simply program the PLC to count its inputs and turn the solenoids on for the specified
time.
Artificial Intelligence
Artificial Intelligence (AI) is the intelligence of machines and the branch of computer
science which aims to create it. Major AI textbooks define the field as "the study and
design of intelligent agents," where an intelligent agent is a system that perceives its
environment and takes actions which maximize its chances of success. John McCarthy,
who coined the term in 1956, defines it as "the science and engineering of making
intelligent machines."
The field was founded on the claim that a central property of human beings, intelligence
—the sapience of Homo sapiens—can be so precisely described that it can be simulated
by a machine. This raises philosophical issues about the nature of the mind and limits of
scientific hubris, issues which have been addressed by myth, fiction and philosophy since
antiquity. Artificial intelligence has been the subject of breathtaking optimism, has
suffered stunning setbacks and, today, has become an essential part of the technology
industry, providing the heavy lifting for many of the most difficult problems in computer
science.
AI research is highly technical and specialized, so much so that some critics decry the
"fragmentation" of the field. Subfields of AI are organized around particular problems,
the application of particular tools and around longstanding theoretical differences of
opinion. The central problems of AI include such traits as reasoning, knowledge,
planning, learning, communication, perception and the ability to move and manipulate
objects. General intelligence (or "strong AI") is still a long-term goal of (some) research.
Relay
A relay is an electrically operated switch. Current flowing through the coil of the relay
creates a magnetic field which attracts a lever and changes the switch contacts. The coil
current can be on or off so relays have two switch positions and they are double throw
(changeover) switches.
Circuit symbol for a relay Relay
Relays allow one circuit to switch a second circuit which can be completely separate
from the first. For example a low voltage battery circuit can use a relay to switch a 230V
AC mains circuit. There is no electrical connection inside the relay between the two
circuits, the link is magnetic and mechanical.
The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it
can be as much as 100mA for relays designed to operate from lower voltages. Most ICs
(chips) cannot provide this current and a transistor is usually used to amplify the small IC
current to the larger value required for the relay coil. The maximum output current for the
popular 555 timer IC is 200mA so these devices can supply relay coils directly without
amplification.
Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts,
for example relays with 4 sets of changeover contacts are readily available. For further
information about switch contacts and the terms used to describe them please see the
page on switches.
Most relays are designed for PCB mounting but you can solder wires directly to the pins
providing you take care to avoid melting the plastic case of the relay.
The supplier's catalogue should show you the relay's connections. The coil will be
obvious and it may be connected either way round. Relay coils produce brief high voltage
'spikes' when they are switched off and this can destroy transistors and ICs in the circuit.
To prevent damage you must connect a protection diode across the relay coil.
The animated picture shows a working relay with its coil and switch contacts. You can
see a lever on the left being attracted by magnetism when the coil is switched on. This
lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground
and another behind them, making the relay DPDT.
The relay's switch connections are usually labeled COM, NC and NO:
COM = Common, always connect to this; it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on.
Connect to COM and NO if you want the switched circuit to be on when the relay coil is
on.
Connect to COM and NC if you want the switched circuit to be on when the relay coil is
off.
Human Machine Interface The term HMI is used mostly in a manufacturing environment. The human-machine
interface (HMI) is where people and technology meet on the plant floor. The HMI can be
as simple as the grip on a drill or as complex as the controls of an automated packing
line.
Or
The Human-Machine Interface is quite literally where the human and the machine meet.
It is the area of the human and the area of the machine that interact during a given task.
Interaction can include touch, sight, sound, heat transference or any other physical or
cognitive function. It is also known as Man-Machine Interface.
This layer where humans and technology meet actually separates the human operating the
machine from the machine(s) itself. A good HMI should aid the human in mentally
mapping out the task needing to be performed with the machine. The use of the HMI
itself should be self evident or intuitive.
In manufacturing, a typical HMI could be a button that initiates a sequence of events or a
process. Or, a Windows based computer that uses software to interface the human and the
PLC, which initiates, stops or somehow controls actions of a machine.
The HMI effectiveness is measured by a number of components. One of them is
learnability and productivity. These components are brought together under the title of
"usability", also known as quality of use. Usability is mainly a characteristic of the user
interface, but is also associated with the functionalities of the product. It describes how
well a product can be used for its intended purpose by its target users with efficiency,
effectiveness, and satisfaction.
Standard Components of HMIThe ISO 9241 standard defines three components of quality of use applicable to the
design of HMIs:
1. Effectiveness: Does the product do what the users expect. Does it do the right
thing?
2. Efficiency: Can the users learn the interface quickly? Can they carry out their
tasks with minimum effort without any supervision. Does it improve the
productivity/effort ratio?
3. Satisfaction: Do users express satisfaction with the product and does the new
product reduce stress. Are thee end users now have a more satisfying job?
History of HMI The prominence of various interfaces can be divided into the following phases according
to the dominant type: Batch interface. This interface was dominant during 1945-1968.
Command-line user interface. This interface was dominant during 1969-1980.
Graphical user interface. This interface was dominant during 1981 - Till date.
Touch User Interface. This interface was dominant during 2002 - Till date.
Examples of HMIA typical computer station will have four human-machine interfaces, the keyboard
(hand), the mouse (hand), the monitor (eyes) and the speakers (ears).
CHAPTER 2
What is Automation
Automation is the use of control systems (such as numerical control, programmable logic
control, and other industrial control systems), in concert with other applications of
information technology (such as computer-aided technologies [CAD, CAM, CAx]), to
control industrial machinery and processes, reducing the need for human intervention. In
the scope of industrialization, automation is a step beyond mechanization. Whereas
mechanization provided human operators with machinery to assist them with the physical
requirements of work, automation greatly reduces the need for human sensory and mental
requirements as well. Processes and systems can also be automated.
Automation plays an increasingly important role in the global economy and in daily
experience. Engineers strive to combine automated devices with mathematical and
organizational tools to create complex systems for a rapidly expanding range of
applications and human activities.
Many roles for humans in industrial processes presently lie beyond the scope of
automation. Human-level pattern recognition, language recognition, and language
production ability are well beyond the capabilities of modern mechanical and computer
systems. Tasks requiring subjective assessment or synthesis of complex sensory data,
such as scents and sounds, as well as high-level tasks such as strategic planning, currently
require human expertise. In many cases, the use of humans is more cost-effective than
mechanical approaches even where automation of industrial tasks is possible.
Specialised hardened computers, referred to as programmable logic controllers (PLCs),
are frequently used to synchronize the flow of inputs from (physical) sensors and events
with the flow of outputs to actuators and events. This leads to precisely controlled actions
that permit a tight control of almost any industrial process.
Human-machine interfaces (HMI) or computer human interfaces (CHI), formerly known
as man-machine interfaces, are usually employed to communicate with PLCs and other
computers, such as entering and monitoring temperatures or pressures for further
automated control or emergency response. Service personnel who monitor and control
these interfaces are often referred to as stationary engineers.
Importance of Automation
The importance of automation and robots in all manufacturing industries is growing.
Industrial robots have replaced human beings in a wide variety of industries. Robots out
perform humans in jobs that require precision, speed, endurance and reliability. Robots
safely perform dirty and dangerous jobs. Traditional manufacturing robotic applications
include material handling (pick and place), assembling, painting, welding, packaging,
palletizing, product inspection and testing. Industrial robots are used in a diverse range of
industries including automotive, electronics, medical, food production, biotech,
pharmaceutical and machinery.
The ISO definition of a manipulating industrial robot is "an automatically controlled,
reprogrammable, multipurpose manipulator". According to the definition it can be fixed
in place or mobile for use in industrial automation applications. These industrial robots
are programmable in three or more axes. They are multi-functional pieces of equipment
that can be custom-built and programmed to perform a variety of operations.
The major advantages of industrial robots is that they can be programmed to suit industry
specific requirements and can work continuously for years, consistently meeting high
manufacturing quality standards. The economic life span of an industrial robot is
approximately 12-16 years. Due to their persistent accuracy industrial robots have
become an indispensable part of manufacturing.
Industrial robots are classified into different categories based on their mechanical
structure. The major categories of industrial robots are:
Gantry (Cartesian) Robot: They are stationary robots having three elements of
motion. They work from an overhead grid with a rectangular work envelope. They are
mainly used to perform 'pick and place' actions. Gantry robots have all their axes above
the work making them also ideal for dispensing applications.
SCARA Robots: (Selectively Compliant Articulated Robot Arm) These robots have 4
axes of motion. They move within an x-y-z coordinated circular work envelope. They are
used for factory automation requiring pick and place work, application and assembly
operations and handling machine tools.
Articulated Robots: An Articulated Robot has rotary joints. It can have from two to
ten or more interactive joints. Articulated robots are well suited to welding, painting and
assembly.
Basic industrial robot designs can be customized with the addition of different
peripherals. End effectors, optical systems, and motion controllers are essential add-ons.
End effectors are the end-of-arm-tooling (EOAT) attached to robotic arms. Grippers or
wrenches that are used to move or assemble parts are examples of end effectors. End
effectors are designed and used to sense and interact with the external environment. The
end effectors' design depends on the application requirements of the specific industry.
Machine Vision systems are robotic optical systems. They are built-on digital
input/output devices and computer networks used to control other manufacturing
equipment such as robotic arms. Machine vision is used for the inspection of
manufactured goods such as semiconductor chips. Motion controllers are used to move
robots and position stages smoothly and accurately with sub-micron repeatability.
Impact of Automation
Automation has had a notable impact in a wide range of highly visible industries beyond
manufacturing. Once-ubiquitous telephone operators have been replaced largely by
automated telephone switchboards and answering machines. Medical processes such as
primary screening in electrocardiography or radiography and laboratory analysis of
human genes, sera, cells, and tissues are carried out at much greater speed and accuracy
by automated systems. Automated teller machines have reduced the need for bank visits
to obtain cash and carry out transactions. In general, automation has been responsible for
the shift in the world economy from agrarian to industrial in the 19th century and from
industrial to services in the 20th century.
The widespread impact of industrial automation raises social issues, among them its
impact on employment. Historical concerns about the effects of automation date back to
the beginning of the industrial revolution, when a social movement of English textile
machine operators in the early 1800s known as the Luddites protested against Jacquard's
automated weaving looms — often by destroying such textile machines— that they felt
threatened their jobs. One author made the following case. When automation was first
introduced, it caused widespread fear. It was thought that the displacement of human
operators by computerized systems would lead to severe unemployment.
Critics of automation contend that increased industrial automation causes increased
unemployment; this was a pressing concern during the 1980s. One argument claims that
this has happened invisibly in recent years, as the fact that many manufacturing jobs left
the United States during the early 1990s was offset by a one-time massive increase in IT
jobs at the same time. Some authors argue that the opposite has often been true, and that
automation has led to higher employment. Under this point of view, the freeing up of the
labor force has allowed more people to enter higher skilled managerial as well as
specialized consultant/contractor jobs (like cryptographers), which are typically higher
paying. One odd side effect of this shift is that "unskilled labor" is in higher demand in
many first-world nations, because fewer people are available to fill such jobs.
At first glance, automation might appear to devalue labor through its replacement with
less-expensive machines; however, the overall effect of this on the workforce as a whole
remains unclear. Today automation of the workforce is quite advanced, and continues to
advance increasingly more rapidly throughout the world and is encroaching on ever more
skilled jobs, yet during the same period the general well-being and quality of life of most
people in the world (where political factors have not muddied the picture) have improved
dramatically. What role automation has played in these changes has not been well
studied.
Advantages and Disadvantages
Industrial automation is changing the face of production. It is ideal for robotic
applications like arc welding, material handling, and plasma cutting.
The main Advantages of Automation are:
Replacing human operators in tedious tasks.
Replacing humans in tasks that should be done in dangerous environments (i.e.
Fire, space, volcanoes, nuclear facilities, under the water, etc)
Making task that are beyond the human capabilities such as handle too heavy
loads, too large objects, too hot or too cold substances or the requirement to make
things too fast or too slow.
Economy improvement. Sometimes and some kinds of automation implies
improves in economy of enterprises, society or most of the humankind. For
example, when an enterprise that have invested in automation technology recover
its investment; when a state or country increase its incomes due to automation like
Germany or Japan in the XX Century or when the humankind can use the internet
which in turn use satellites and other automated engines.
The main disadvantages of automation are:
Technology limits. Nowadays technology is not able to automatist all the desired
tasks.
Initial costs are relative high. The automation of a new product required a huge
initial investment in comparison with the unit cost of the product, although the
cost of automation is spread in many product batches. The automation of a Plant
required a great initial investment too, although this cost is spread in the products
to be produced.
Controversial Factors
Unemployment: It is commonly thought that automation implies
unemployment due to the fact that the work of a human being is replaced in part
or completely by a machine. Nevertheless, the unemployment is caused by the
economical politics of the administration like dismissing the workers instead of
changing their tasks. Since the general economical policies of most of the
industrial plants are to dismiss people, nowadays automation implies
unemployment. In different scenarios without workers, automation implies more
free time instead of unemployment like the case with the automatic washing
machine at home. Automation does not imply unemployment when it makes tasks
unimaginable without automation such as exploring mars with the Sojourner or
when the economy is fully adapted to an automated technology as with the
Telephone switchboard.
Environment: The costs of automation to the environment are different
depending on the technology, product or engine automated. There are automated
engines that consume more energy resources from the Earth in comparison with
previous engines and those that do the opposite too.
Human Being Replacement: In the future there is a possibility that the
Artificial intelligence could replace and improve a human brain and the robots
would become not only fully automated but fully autonomous from the human
beings (Technological singularity)
Automation Tools
Many different types of devices used for automation exist. These devices include a range
from SCADA, used for data and control, to HMI, in which users adapt a program.
1) ANN - Artificial Neural Network.
2) DCS - Distributed Control System.
3) HMI - Human Machine Interface.
4) SCADA - Supervisory Control and Data Acquisition.
5) PLC - Programmable Logic Controller.
6) PAC - Programmable Automation Controller.
7) Instrumentation – Instrumentation.
8) Motion Control - Motion Control.
9) Robotics – Robotics.
Artificial Neural Network
An artificial neural network (ANN), usually called "neural network" (NN), is a
mathematical model or computational model that tries to simulate the structure and/or
functional aspects of biological neural networks. It consists of an interconnected group of
artificial neurons and processes information using a connectionist approach to
computation. In most cases an ANN is an adaptive system that changes its structure based
on external or internal information that flows through the network during the learning
phase.
In more practical terms neural networks are non-linear statistical data modeling tools.
They can be used to model complex relationships between inputs and outputs or to find
patterns in data.
Distributed Control System
A distributed control system (DCS) refers to a control system usually of a manufacturing
system, process or any kind of dynamic system, in which the controller elements are not
central in location (like the brain) but are distributed throughout the system with each
component sub-system controlled by one or more controllers. The entire system of
controllers is connected by networks for communication and monitoring. DCS is a very
broad term used in a variety of industries, to monitor and control distributed equipment
Human Machine Interface
The term user interface is often used in the context of computer systems and electronic
devices. The user interface of a mechanical system, a vehicle or an industrial installation
is sometimes referred to as the Human-Machine Interface (HMI). HMI is a modification
of the original term MMI (Man-Machine Interface). In practice, the abbreviation MMI is
still frequently used although some may claim that MMI stands for something different
now. Another abbreviation is HCI, but is more commonly used for Human-computer
interaction than Human-computer interface. Other terms used are Operator Interface
Console (OIC) and Operator Interface Terminal (OIT).
Programmable Logic Controller
A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control
system that continuously monitors the state of input devices and makes decisions based
upon a custom program to control the state of output devices.
Almost any production line, machine function, or process can be greatly enhanced using
this type of control system. However, the biggest benefit in using a PLC is the ability to
change and replicate the operation or process while collecting and communicating vital
information.
Another advantage of a PLC system is that it is modular. That is, you can mix and match
the types of Input and Output devices to best suit your application.
Programmable Automation Controller
A programmable automation controller (PAC) is a compact controller that combines the
features and capabilities of a PC-based control system with that of a typical
programmable logic controller (PLC). A PAC thus provides not only the reliability of a
PLC, but also the task flexibility and computing power of a PC. PACs are most often
used in industrial settings for process control, data acquisition, remote equipment
monitoring, machine vision, and motion control. Additionally, because they function and
communicate over popular network interface protocols like TCP/IP, OLE for process
control (OPC) and SMTP, PACs are able to transfer data from the machines they control
to other machines and components in a networked control system or to application
software and databases. A PAC at the core of an automation system can integrate
multiple fieldbus networks like RS-485, RS-232, RS-422, CAN, Ethernet, EtherNet/IP,
and others.
Instrumentation
Instrumentation is the branch of science that deals with measurement and control. An
instrument is a device that measures or manipulates variables such as flow, temperature,
level, or pressure. Instruments include many varied contrivances which can be as simple
as valves and transmitters, and as complex as analyzers. Instruments often comprise
control systems of varied processes. The control of processes is one of the main branches
of applied instrumentation.
Control instrumentation includes devices such as solenoids, Valves, breakers, relays, etc.
These devices are able to change a field parameter, and provide remote and/or automated
control capabilities.
Transmitters are devices which produce an analog signal, usually in the form of a 4-20
mA electrical current signal, although many other options are possible using voltage,
frequency, or pressure. This signal can be used to directly control other instruments, or
sent to a PLC, DCS, SCADA system or other type of computerized controller, where it
can be interpreted into readable values, or used to control other devices and processes in
the system.
Instrumentation plays a significant role in both gathering information from the field and
changing the field parameters, and as such are a key part of control loops.
Motion Control
Motion control is a sub-field of automation, in which the position and/or velocity of
machines are controlled using some type of device such as a hydraulic pump, linear
actuator, or an electric motor, generally a servo. Motion control is an important part of
robotics and CNC machine tools, however it is more complex than in the use of
specialized machines, where the kinematics are usually simpler. The latter is often called
General Motion Control (GMC). Motion control is widely used in the packaging,
printing, textile, semiconductor production, and assembly industries.
Robotics
Robotics is the science and technology of robots, and their design, manufacture, and
application.[1] Robotics is related to electronics, mechanics, and software.
Stories of artificial helpers and companions likewise attempts to create them have a long
history, but fully autonomous machines only appeared in the 20th century. The first
digitally operated and programmable robot, the Unimate, was installed in 1961 to lift hot
pieces of metal from a die casting machine and stack them. Today, commercial and
industrial robots are in widespread use performing jobs cheaper or more accurately and
reliably than humans. They are also employed for jobs which are too dirty, dangerous, or
dull to be suitable for humans. Robots are widely used in manufacturing, assembly and
packing, transport, earth and space exploration, surgery, weaponry, laboratory research,
safety, and mass production of consumer and industrial goods.
Supervisory Control and Data Acquisition (SCADA)
SCADA stands for Supervisory Control And Data Acquisition. It generally refers to an
industrial control system: a computer system monitoring and controlling a process. The
process can be industrial, infrastructure or facility based as described below:
Industrial processes include those of manufacturing, production, power
generation, fabrication, and refining, and may run in continuous, batch, repetitive,
or discrete modes.
Infrastructure processes may be public or private, and include water treatment and
distribution, wastewater collection and treatment, oil and gas pipelines, electrical
power transmission and distribution, civil defense siren systems, and large
communication systems.
Facility processes occur both in public facilities and private ones, including
buildings, airports, ships, and space stations. They monitor and control HVAC,
access, and energy consumption.
A SCADA System usually consists of the following subsystems:
A Human-Machine Interface or HMI is the apparatus which presents process data
to a human operator, and through this, the human operator monitors and controls
the process.
A supervisory (computer) system, gathering (acquiring) data on the process and
sending commands (control) to the process.
Remote Terminal Units (RTUs) connecting to sensors in the process, converting
sensor signals to digital data and sending digital data to the supervisory system.
Programmable Logic Controller (PLCs) used as field devices because they are
more economical, versatile, flexible, and configurable than special-purpose RTUs.
Communication infrastructure connecting the supervisory system to the Remote
Terminal Units.
There is, in several industries, considerable confusion over the differences between
SCADA systems and Distributed control systems (DCS). Generally speaking, a SCADA
system usually refers to a system that coordinates, but does not control processes in real
time. The discussion on real-time control is muddied somewhat by newer
telecommunications technology, enabling reliable, low latency, high speed
communications over wide areas. Most differences between SCADA and DCS are
culturally determined and can usually be ignored. As communication infrastructures with
higher capacity become available, the difference between SCADA and DCS will fade.
Future of Automation
Automation has experience many advancements over the years it has been around.
Inventions continue to advance the technology such as, nanotech sensors. Automation
needs to be versatile and will be more useful with remote control.
The Since the turn of the century, the global recession has affected most
businesses, including industrial automation.
Because of the relatively small production volumes and huge varieties of
applications, industrial automation typically utilizes new technologies developed
in other markets.
Automation companies tend to customize products for specific applications and
requirements. So the innovation comes from targeted applications, rather than any
hot, new technology.
Over the past few decades, some innovations have indeed given industrial automation
new surges of growth:
The programmable logic controller (PLC) designed to replace relay-logic; it
generated growth in applications where custom logic was difficult to implement
and change.
The PLC was a lot more reliable than relay-contacts, and much easier to program
and reprogram.
Growth was rapid in automobile test-installations, which had to be re-
programmed often for new car models. The PLC has had a long and productive
life and (understandably) has now become a commodity.
Mini-computers replaced large central mainframes in central control rooms, and
gave rise to "distributed" control systems (DCS). These were not really
"distributed" because they were still relatively large clumps of computer hardware
and cabinets filled with I/O connections.
The arrival of the PC brought low-cost PC-based hardware and software, which
provided DCS functionality with significantly reduced cost and complexity.
The plethora of manufacturing software solutions and services will yield
significant results, but all as part of other systems.
Innovation and technology can and will reestablish growth in industrial
automation.
The automation industry does NOT extrapolate to smaller and cheaper PLCs,
DCSs, and supervisory control and data acquisition systems; those functions will
simply be embedded in hardware and software. Instead, future growth will come
from totally new directions.
New Technology Directions
Industrial automation can and will generate explosive growth with technology
related to new inflection points:
Nanotechnology and Nan scale assembly systems.
MEMS and nanotech sensors (tiny, low-power, low-cost sensors) which can
measure everything and anything; and the pervasive Internet, machine to machine
(M2M) networking.
Real-time systems will give way to complex adaptive systems and multi-
processing. The future belongs to nanotech, wireless everything, and complex
adaptive systems.
The Automated factory
Automated factories and processes are too expensive to be rebuilt for every
modification and design change so they have to be highly configurable and
flexible.
To successfully reconfigure an entire production line or process requires direct
access to most of its control elements switches, valves, motors and drives down to
a fine level of detail.
The vision of fully automated factories has already existed for some time now.
The promise of remote-controlled automation is finally making headway in
manufacturing settings and maintenance applications.
Communications support of a very high order is now available for automated
processes: lots of sensors, very fast networks, quality diagnostic software and
flexible interfaces all with high levels of reliability and pervasive access to
hierarchical diagnosis and error-correction advisories through centralized
operations.
The factory of the future will be small, movable (to where the resources are, and
where the customers are).
Hard Truths about Globalization
Innovation is the true source of value, and that is in danger of being dissipated
sacrificed to a short-term search for profit, the capitalistic quarterly profits
syndrome. Countries like Japan and Germany will tend to benefit from their
longer-term business perspectives.
The Winning Differences
In a global market, there are three keys that constitute the winning edge:
Proprietary products: developed quickly and inexpensively (and perhaps
globally), with a continuous stream of upgrade and adaptation to maintain
leadership.
High-value-added products: proprietary products and knowledge offered through
effective global service providers, tailored to specific customer needs.
Global yet local services: the special needs and custom requirements of remote
customers must be handled locally, giving them the feeling of partnership and
proximity.
Types of Automation
Three types of automation in production can be distinguished:
1. Fixed Automation
2. Programmable Automation
3. Flexible Automation.
1. Fixed Automation
Fixed automation is a system in which the sequence of processing operation is fixed by
the equipment configuration.
Main Features:
1) Hard Wire Logic
2) Electronic Cards
3) For Mass Production
2. Programmable Automation
In Programmable Automation the machine is designed with the capability to change the
sequence of operation to accommodate different product configuration. A Program
controls the sequence.
Main Features:
1) Microcontroller based programming.
2) Not easy access to Programming.
3. Flexible Automation
Flexible automation (FA) is a type of manufacturing automation, which exhibits some
form of "flexibility." Most commonly this flexibility is the capability of making different
products in a short time frame. This "process flexibility" allows the production of
different part types with overlapping life cycles. Another type of flexibility that comes
with flexible automation is the ability to produce a part type through many generations.
Clearly, there are several other manifestations of flexibility. Flexible automation allows
the production of a variety of part types in small or unit batch sizes. Although FA
consists of various combinations of technology, flexible automation most typically takes
the form of machining systems, that is, manufacturing systems where material is removed
from a workpiece. The flexibility comes from the programmability of the computers
controlling the machines. Flexible automation is also observed in assembly systems. The
most prominent form of flexible assembly is observed in the electronics industry, where
flexible machines (automated surface mount technologies) are used to populate printed
circuit boards with integrated circuits and other componentry. In this instance,
manufacturers have found the machines far superior accuracy and reliability to be
sufficient to warrant the significant investment. Overall, however, manufacturer tend to
use automation for fabrication, and leave assembly to human operators who can adapt to
a greater variety of changing circumstances more rapidly and easily than machines. In
this article, the discussion of flexible automation is primarily focused on machining
systems.
Flexible automation is an extension of programmable automation. The disadvantage with
programmable automation is the time required to reprogram and change over the
production equipment for each batch of new product. This is lost production time, which
is expensive. In flexible automation, the variety of products is sufficiently limited so that
the changeover of the equipment can be done very...
Main Features:
1) Controlling through PLC.
2) Easy modification.
3) Reduces engineering time.
4) Reduces Man power.
5) Data Management.
6) Visualization.
Industrial Automation Mechanism
The use of control system or computers to control industrial process and industrial
machinery is called industrial automation or numerical automation. In the mechanism the
human labor is replaced by sophisticated computers. It is different from mechanization.
In mechanization human operators run the machinery as per the requirements of the
work. But in numerical automation most of the works are facilitated by the highly
improved computers. The whole system and the processes of the industry are automated
under the automated system. Industrial automation requires several automation tools.
There are several automation tools that make industrial possible. These tools include
Distributed control system, human machine interface, laboratory information
management system, manufacturing executive system. In addition to it in manufacturing
units some other tools applied are programmable logic controller, programmable
automation controller, supervisory control and data acquisition, and simulation and field
bus.
Industrial automation has got wide application in the today's economic world order. the
automation engineers are playing a very important role in the maximizing the efficiency
of industries. They create a complex system of human computer interface to facilitate a
wide range of application. But industrial automation is not flawless. First, it makes the
industry over dependent on the automation system which sometimes backfires. There is
always a probability of affecting computers with a number of viruses turning the whole
system in fiasco. The potential danger always looms large over the industries.
Social issues involved in the industrial automation have also wide ramifications. It raises
many social issues which are very much relevant for the populous countries. Industrial
automation affects employment to a great extent. It is a sheer myth that it leads to higher
employment. The truth is that it affects unskilled labors most. There is no space for those
who are unskilled. This is because automated industries require only skilled labors. But
this unleashes a cathartic effect on the labor society. It forces more and more people to
take up skilled jobs or makes them try for it. Nowadays the automation is very much
advanced. It has replaced even many skilled jobs also. It is advancing rapidly throughout
the world. In future it may hamper more and more skilled jobs.
In spite of all these repercussions there is a greater emphasis on the industrial automation.
Primarily its main purpose was to maximize the production of the industry. Now the
focus has been shifted to enhance the quality of the products which is the need of the
hour in the present scenario. Industrial automation has made workforce more flexible and
now it is quite feasible to switch over from one product to the other product in an
automated system.
CHAPTER 3
What is PLC
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, amusement rides, or lighting fixtures. PLCs are used in many
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.
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.
Theory of Operation
The Guts Inside
The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive
input/output data. We can actually consider the PLC to be a box full of hundreds or
thousands of separate relays, counters, timers and data storage locations. Do these
counters, timers, etc. really exist? No, they don't "physically" exist but rather they are
simulated and can be considered software counters, timers, etc. These internal relays are
simulated through bit locations in registers.
What does each part do?
INPUT RELAYS-(contacts): These are connected to the outside world. They physically
exist and receive signals from switches, sensors, etc. Typically they are not relays but
rather they are transistors.
INTERNAL UTILITY RELAYS-(contacts): These do not receive signals from the
outside world nor do they physically exist. They are simulated relays and are what
enables a PLC to eliminate external relays. There are also some special relays that are
dedicated to performing only one task. Some are always on while some are always off.
Some are on only once during power-on and are typically used for initializing data that
was stored.
COUNTERS: These again do not physically exist. They are simulated counters and they
can be programmed to count pulses. Typically these counters can count up, down or both
up and down. Since they are simulated they are limited in their counting speed. Some
manufacturers also include high-speed counters that are hardware based. We can think of
these as physically existing. Most times these counters can count up, down or up and
down.
TIMERS: These also do not physically exist. They come in many varieties and
increments. The most common type is an on-delay type. Others include off-delay and
both retentive and non-retentive types. Increments vary from 1ms through 1s.
OUTPUT RELAYS-(coils): These are connected to the outside world. They physically
exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or
triacs depending upon the model chosen.
DATA STORAGE: Typically there are registers assigned to simply store data. They are
usually used as temporary storage for math or data manipulation. They can also typically
be used to store data when power is removed from the PLC. Upon power-up they will
still have the same contents as before power was removed. Very convenient and
necessary!!
How It Works?
A PLC works by continually scanning a program. We can think of this scan cycle as
consisting of 3 important steps. There are typically more than 3 but we can focus on the
important parts and not worry about the others. Typically the others are checking the
system and updating the current internal counter and timer values.
Step 1-CHECK INPUT STATUS: First the PLC takes a look at each input to determine
if it is on or off. In other words, is the sensor connected to the first input on? How about
the second input? How about the third... It records this data into its memory to be used
during the next step.
Step 2-EXECUTE PROGRAM: Next the PLC executes your program one instruction at
a time. Maybe your program said that if the first input was on then it should turn on the
first output. Since it already knows which inputs are on/off from the previous step it will
be able to decide whether the first output should be turned on based on the state of the
first input. It will store the execution results for use later during the next step.
Step 3-UPDATE OUTPUT STATUS: Finally the PLC updates the status of the outputs.
It updates the outputs based on which inputs were on during the first step and the results
of executing your program during the second step. Based on the example in step 2 it
would now turn on the first output because the first input was on and your program said
to turn on the first output when this condition is true.
After the third step the PLC goes back to step one and repeats the steps continuously.
One scan time is defined as the time it takes to execute the 3 steps listed above.
Response Time
The total response time of the PLC is a fact we have to consider when shopping for a
PLC. Just like our brains, the PLC takes a certain amount of time to react to changes. In
many applications speed is not a concern, in others though...
If you take a moment to look away from this text you might see a picture on the wall.
Your eyes actually see the picture before your brain says "Oh, there's a picture on the
wall". In this example your eyes can be considered the sensor. The eyes are connected to
the input circuit of your brain. The input circuit of your brain takes a certain amount of
time to realize that your eyes saw something. (If you have been drinking alcohol this
input response time would be longer!) Eventually your brain realizes that the eyes have
seen something and it processes the data. It then sends an output signal to your mouth.
Your mouth receives this data and begins to respond to it. Eventually your mouth utters
the words "Gee, that's a really ugly picture!".
Notice in this example we had to respond to 3 things:
INPUT: It took a certain amount of time for the brain to notice the input signal from the
eyes.
EXECUTION: It took a certain amount of time to process the information received from
the eyes. Consider the program to be: If the eyes see an ugly picture then output
appropriate words to the mouth.
OUTPUT: The mouth receives a signal from the brain and eventually spits (no pun
intended) out the words "Gee, that's a really ugly picture!"
Effects of Response Time
Now that we know about response time, here's what it really means to the application.
The PLC can only see an input turn on/off when it's looking. In other words, it only looks
at its inputs during the check input status part of the scan.
In the diagram, input 1 is not seen until scan 2. This is because when input 1 turned on,
scan 1 had already finished looking at the inputs.
Input 2 is not seen until scan 3. This is also because when the input turned on scan 2 had
already finished looking at the inputs.
Input 3 is never seen. This is because when scan 3 was looking at the inputs, signal 3 was
not on yet. It turns off before scan 4 looks at the inputs. Therefore signal 3 is never seen
by the plc.
To avoid this we say that the input should be on for at
least 1 input delay time + one scan time.
But what if it was not possible for the input to be on this long? Then the plc doesn't see
the input turn on. Therefore it becomes a paper weight! Not true... of course there must be
a way to get around this. Actually there are 2 ways.
Pulse Stretch function: This function extends the length of the input signal until the plc
looks at the inputs during the next scan.( i.e. it stretches the duration of the pulse.)
Interrupt function: This function interrupts the scan to process a special routine that you
have written. i.e. As soon as the input turns on, regardless of
where the scan currently is, the plc immediately stops what its
doing and executes an interrupt routine. (A routine can be
thought of as a mini program outside of the main program.)
After its done executing the interrupt routine, it goes back to
the point it left off at and continues on with the normal scan process.
Now let's consider the longest time for an output to actually turn on. Let's assume that
when a switch turns on we need to turn on a load connected to the plc output.
The diagram below shows the longest delay (worst case because the input is not seen
until scan 2) for the output to turn on after the input has turned on.
The maximum delay is thus 2 scan cycles - 1 input delay time.
Creating Programs
Relays
Now that we understand how the PLC processes inputs, outputs, and the actual program
we are almost ready to start writing a program. But first lets see how a relay actually
works. After all, the main purpose of a plc is to replace "real-world" relays.
We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a
magnetic field is generated. This magnetic field sucks the contacts of the relay in, causing
them to make a connection. These contacts can be considered to be a switch. They allow
current to flow between 2 points thereby closing the circuit.
Let's consider the following example. Here we simply turn on a bell (Lunch time!)
whenever a switch is closed. We have 3 real-world parts. A switch, a relay and a bell.
Whenever the switch closes we apply a current to a bell causing it to sound.
Notice in the picture that we have 2 separate circuits. The bottom(blue) indicates the DC
part. The top(red) indicates the AC part.
Here we are using a dc relay to control an AC
circuit. That's the fun of relays! When the switch is
open no current can flow through the coil of the
relay. As soon as the switch is closed, however,
current runs through the coil causing a magnetic
field to build up. This magnetic field causes the
contacts of the relay to close. Now AC current
flows through the bell and we hear it. Lunch time!
A Typical IndustiralRelay
Replacing Relays
Next, lets use a plc in place of the relay. (Note that this might not be very cost effective
for this application but it does demonstrate the basics we need.) The first thing that's
necessary is to create what's called a ladder diagram. After seeing a few of these it will
become obvious why its called a ladder diagram. We have to create one of these because,
unfortunately, a plc doesn't understand a schematic diagram. It only recognizes code.
Fortunately most PLCs have software which convert ladder diagrams into code. This
shields us from actually learning the plc's code.
First Step: We have to translate all of the items we're using into symbols the plc
understands. The plc doesn't understand terms like switch, relay, bell, etc. It prefers input,
output, coil, contact, etc. It doesn't care what the actual input or output device actually is.
It only cares that its an input or an output.
First we replace the battery with a symbol. This symbol is common to all ladder
diagrams. We draw what are called bus bars. These simply look like two vertical bars.
One on each side of the diagram. Think of the left one as being + voltage and the right
one as being ground. Further think of the current (logic) flow as being from left to right.
Next we give the inputs a symbol. In this basic example we have one real world input.
(i.e. the switch) We give the input that the switch will be connected to, to the symbol
shown below. This symbol can also be used as the contact of a relay.
A Contact Symbol
Next we give the outputs a symbol. In this example we use one output (i.e. the bell). We
give the output that the bell will be physically connected to the symbol shown below.
This symbol is used as the coil of a relay.
A Coil Symbol
The AC supply is an external supply so we don't put it in our ladder. The plc only cares
about which output it turns on and not what's physically connected to it.
Second Step: We must tell the plc where everything is located. In other words we have to
give all the devices an address. Where is the switch going to be physically connected to
the plc? How about the bell? We start with a blank road map in the PLCs town and give
each item an address. Could you find your friends if you didn't know their address? You
know they live in the same town but which house? The plc town has a lot of houses
(inputs and outputs) but we have to figure out who lives where (what device is connected
where). We'll get further into the addressing scheme later. The plc manufacturers each do
it a different way! For now let's say that our input will be called "0000". The output will
be called "500".
Final Step: We have to convert the schematic into a logical sequence of events. This is
much easier than it sounds. The program we're going to write tells the plc what to do
when certain events take place. In our example we have to tell the plc what to do when
the operator turns on the switch. Obviously we want the bell to sound but the plc doesn't
know that. It's a pretty stupid device, isn't it!
The picture above is the final converted diagram. Notice that we eliminated the real
world relay from needing a symbol. It's actually "inferred" from the diagram.
Basic Instructions
Now let's examine some of the basic instructions is greater detail to see more about what
each one does.
Load
The load (LD) instruction is a normally open contact. It is sometimes also called examine
if on.(XIO) (as in examine the input to see if its physically on) The symbol for a load
instruction is shown below.
A LoaD (contact) symbol
This is used when an input signal is needed to be present for the symbol to turn on. When
the physical input is on we can say that the instruction is True. We examine the input for
an on signal. If the input is physically on then the symbol is on. An on condition is also
referred to as a logic 1 state.
This symbol normally can be used for internal inputs, external inputs and external output
contacts. Remember that internal relays don't physically exist. They are simulated
(software) relays.
LoadBar
The LoaDBar instruction is a normally closed contact. It is sometimes also called
LoaDNot or examine if closed. (XIC) (as in examine the input to see if its physically
closed) The symbol for a loadbar instruction is shown below.
A LoaDNot (normally closed contact) symbol
This is used when an input signal does not need to be present for the symbol to turn on.
When the physical input is off we can say that the instruction is True. We examine the
input for an off signal. If the input is physically off then the symbol is on. An off
condition is also referred to as a logic 0 state.
This symbol normally can be used for internal inputs, external inputs and sometimes,
external output contacts. Remember again that internal relays don't physically exist. They
are simulated (software) relays. It is the exact opposite of the Load instruction.
*NOTE- With most PLCs this instruction (Load or Loadbar) MUST be the first symbol
on the left of the ladder.
Logic State Load LoadBar
0 False True
1 True False
Out
The Out instruction is sometimes also called an OutputEnergize instruction. The output
instruction is like a relay coil. Its symbol looks as shown below.
An OUT (coil) Symbol
When there is a path of True instructions preceding this on the ladder rung, it will also be
True. When the instruction is True it is physically On. We can think of this instruction as
a normally open output. This instruction can be used for internal coils and external
outputs.
Outbar
The Outbar instruction is sometimes also called an OutNot instruction. Some vendors
don't have this instruction. The outbar instruction is like a normally closed relay coil. Its
symbol looks like that shown below.
An OUTBar (normally closed coil) symbol
When there is a path of False instructions preceding this on the ladder rung, it will be True. When
the instruction is True it is physically On. We can think of this instruction as a normally closed
output. This instruction can be used for internal coils and external outputs. It is the exact opposite
of the Out instruction.
Logic State Out OutBar
0 False True
1 True False
Basic Program Example
Now let's compare a simple ladder diagram with its real world external physically
connected relay circuit and SEE the differences.
In the above circuit, the coil will be energized when there is a closed loop between the + and -
terminals of the battery. We can simulate this same circuit with a ladder diagram. A ladder
diagram consists of individual rungs just like on a real ladder. Each rung must contain one or
more inputs and one or more outputs. The first instruction on a rung must always be an input
instruction and the last instruction on a rung should always be an output (or its equivalent).
Notice in this simple one rung ladder diagram we have recreated the external circuit
above with a ladder diagram. Here we used the Load and Out instructions. Some
manufacturers require that every ladder diagram include an END instruction on the last
rung. Some PLCs also require an ENDH instruction on the rung after the END rung.
PLC Registers
We'll now take the previous example and change switch 2 (SW2) to a normally closed
symbol (loadbar instruction). SW1 will be physically OFF and SW2 will be physically
ON initially. The ladder diagram now looks like this:
Notice also that we now gave each symbol (or instruction) an address. This address sets
aside a certain storage area in the PLCs data files so that the status of the instruction (i.e.
true/false) can be stored. Many PLCs use 16 slot or bit storage locations. In the example
above we are using two different storage locations or registers.
REGISTER 00
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
1 0
REGISTER 05
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
0
In the tables above we can see that in register 00, bit 00 (i.e. input 0000) was a logic 0
and bit 01 (i.e. input 0001) was a logic 1. Register 05 shows that bit 00 (i.e. output 0500)
was a logic 0. The logic 0 or 1 indicates whether an instruction is False or True.
*Although most of the items in the register tables above are empty, they should each
contain a 0. They were left blank to emphasize the locations we were concerned with.
LOGICAL CONDITION OF SYMBOL
LOGIC BITS LD LDB OUT
Logic 0 False True False
Logic 1 True False True
The plc will only energize an output when all conditions on the rung are TRUE. So,
looking at the table above, we see that in the previous example SW1 has to be logic 1 and
SW2 must be logic 0. Then and ONLY then will the coil be true (i.e. energized). If any of
the instructions on the rung before the output (coil) are false then the output (coil) will be
false (not energized).
Let's now look at a truth table of our previous program to further illustrate this important
point. Our truth table will show ALL possible combinations of the status of the two
inputs.
Inputs Outputs Register Logic Bits
SW1(LD) SW2(LDB) COIL(OUT) SW1(LD) SW2(LDB) COIL(OUT)
False True False 0 0 0
False False False 0 1 0
True True True 1 0 1
True False False 1 1 0
Notice from the chart that as the inputs change their states over time, so will the output.
The output is only true (energized) when all preceding instructions on the rung are true.
A Level Application
Now that we've seen how registers work, let's process a program like PLCs do to enhance
our understanding of how the program gets scanned.
Let's consider the following application:
We are controlling lubricating oil being dispensed from a tank. This is possible by using
two sensors. We put one near the bottom and one near the top, as shown in the picture
below.
Here, we want the fill motor to pump lubricating oil into the tank until the high level
sensor turns on. At that point we want to turn off the motor until the level falls below the
low level sensor. Then we should turn on the fill motor and repeat the process.
Here we have a need for 3 I/O (i.e. Inputs/Outputs). 2 are inputs (the sensors) and 1 is an
output (the fill motor). Both of our inputs will be NC (normally closed) fiber-optic level
sensors. When they are NOT immersed in liquid they will be ON. When they are
immersed in liquid they will be OFF.
We will give each input and output device an address. This lets the plc know where they
are physically connected. The addresses are shown in the following tables:
Inputs Address Output Address Internal Utility Relay
Low 0000 Motor 0500 1000
High 0001
Below is what the ladder diagram will actually look like. Notice that we are using an
internal utility relay in this example. You can use the contacts of these relays as many
times as required. Here they are used twice to simulate a relay with 2 sets of contacts.
Remember, these relays DO NOT physically exist in the plc but rather they are bits in a
register that you can use to SIMULATE a relay.
We should always remember that the most common reason for using PLCs in our
applications is for replacing real-world relays. The internal utility relays make this action
possible. It's impossible to indicate how many internal relays are included with each
brand of plc. Some include 100's while other include 1000's while still others include 10's
of 1000's! Typically, plc size (not physical size but rather I/O size) is the deciding factor.
If we are using a micro-plc with a few I/O we don't need many internal relays. If
however, we are using a large plc with 100's or 1000's of I/O we'll certainly need many
more internal relays.
If ever there is a question as to whether or not the manufacturer supplies enough internal
relays, consult their specification sheets. In all but the largest of large applications, the
supplied amount should be MORE than enough.
How a Ladder is Scanned
Let's watch what happens in this program scan by scan.
Initially the tank is empty. Therefore, input 0000 is TRUE and input 0001 is also TRUE.
Scan 1 Scan 2-100
Gradually the tank fills because 500(fill motor) is on.
After 100 scans the oil level rises above the low level sensor and it becomes open. (i.e.
FALSE)
Scan 101-1000
Notice that even when the low level sensor is false there is still a path of true logic from
left to right. This is why we used an internal relay. Relay 1000 is latching the output
(500) on. It will stay this way until there is no true logic path from left to right.(i.e. when
0001 becomes false)
After 1000 scans the oil level rises above the high level sensor at it also becomes open
(i.e. false)
Scan 1001 Scan 1001
Since there is no more true logic path, output 500 is no longer energized (true) and
therefore the motor turns off.
After 1050 scans the oil level falls below the high level sensor and it will become true
again.
Notice that even though the high level sensor became true there still is NO continuous
true logic path and therefore coil 1000 remains false!
After 2000 scans the oil level falls below the low level sensor and it will also become true
again. At this point the logic will appear the same as SCAN 1 above and the logic will
repeat as illustrated above.
Features of PLC
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.
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 "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. 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.
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.
| |
| Low Level High Level Fill Valve |
|------[/]------|------[/]----------------------(OUT)---------|
| | |
| | |
| | |
| Fill Valve | |
|------[ ]------| |
| |
| |
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 Of PLC
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 Of PLC
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.
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 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.
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.
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.
Communications
PLCs have built in communications ports usually 9-Pin RS232, and optionally for RS485
and Ethernet. Modbus, BACnet 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 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.
CHAPTER 4About Human Machine InterfaceHuman machine interfaces (HMI) are operator interface terminals with which users
interact in order to control other devices. Some human machine interfaces include knobs,
levers, and controls. Others provide programmable function keys or a full key pad.
Devices that include a processor or interface to personal computers (PCs) are also
available. Many human machine interfaces include alphanumeric or graphic displays. For
ease of use, these displays are often backlit or use standard messages. When selecting
human machine interfaces, important considerations include devices supported and
devices controlled. Device dimensions, operating temperature, operating humidity, and
vibration and shock ratings are other important factors.
Many human machine interfaces include flat panel displays (FPDs) that use liquid crystal
display (LCD) or gas plasma technologies. In LCDs, an electric current passes through a
liquid crystal solution that is trapped between two sheets of polarizing material. The
crystals align themselves so that light cannot pass, producing an image on the screen.
LCDs can be monochrome or color. Color displays can use a passive matrix or an active
matrix. Passive matrix displays contain a grid of horizontal and vertical wires with an
LCD element at each intersection. In active matrix displays, each pixel has a transistor
that is switched directly on or off, improving response times. Unlike LCDs, gas plasma
displays consist of an array of pixels, each of which contains red, blue, and green
subpixels. In the plasma state, gas reacts with the subpixels to display the appropriate
color.
Human machine interfaces differ in terms of performance specifications and I/O ports.
Performance specifications include processor type, random access memory (RAM), and
hard drive capacity, and other drive options. I/O interfaces allow connections to
peripherals such as mice, keyboards, and modems. Common I/O interfaces include
Ethernet, Fast Ethernet, RS232, RS422, RS485, small computer system interface (SCSI),
and universal serial bus (USB). Ethernet is a local area network (LAN) protocol that uses
a bus or star typology and supports data transfer rates of 10 Mbps. Fast Ethernet is a 100
Mbps specification. RS232, RS422, and RS485 are balanced serial interfaces for the
transmission of digital data. Small computer systems interface (SCSI) is an intelligent I/O
parallel peripheral bus with a standard, device-independent protocol that allows many
peripheral devices to be connected to the SCSI port. Universal serial bus (USB) is a 4-
wire, 12-Mbps serial bus for low-to-medium speed peripheral device connections.
Human machine interfaces are available with a variety of features. For example, some
devices are web-enabled or networkable. Others include software drivers, a stylus, and
support for a keyboard, mouse, and printer. Devices that provide real-time clock support
use a special battery and are not connected to the power supply. Power-over-Ethernet
(PoE) equipment eliminates the need for separate power supplies altogether. Human
machine interfaces that offer shielding against electromagnetic interference (EMI) and
radio frequency interference (RFI) are commonly available. Devices that are designed for
harsh environments include enclosures that meet standards from the National Electronics
Manufacturers’ Association (NEMA).
In other words:
The human-machine interface (HMI) is where people and technology meet. This people-
technology intercept can be as simple as the grip on a hand tool or as complex as the
flight deck of a jumbo jet.
Overview of HMI This thesis introduces the underlying principles of good HMI design and outlines a core
usability engineering lifecycle.
In addition, the thesis guides the reader to relevant international standards and sources of
further information.
The target audiences for this tutorial are as follows:
Engineers and designers with an interest in meeting end users' wants and needs.
Individuals who have heard about usability or user-centered design and would
like to find out more.
Nowadays humans interact more with computer-based technology than with hammers
and drills. Unlike tools, the visible shape and controls of a computer do not communicate
its purpose. The task of an HMI is to make the function of a technology self-evident.
Much like a well-designed hammer fits the user's hand and makes a physical task easy, a
well-designed HMI must fit the user's mental map of the task he or she wishes to carry
out.
In nearly every technological solution, the effectiveness of the HMI can predict the
acceptance of the entire solution by the intended users. Often, as far as consumers are
concerned, the HMI is the product—the user's experience with the interface is far more
important than the architecture of the internal workings.
HMI effectiveness is measured by a number of components, such as learnability and
productivity. These components are sometimes brought together under the title of
"usability," also known as quality of use.
ISO Definition of Quality of Use The ISO 9241 standard defines three components of quality of use applicable to the
design of HMIs:
Effectiveness: Does the product do what the users require? Does it do the right
thing?
Efficiency: Can the users learn the HMI quickly? Can they carry out their tasks
with minimum expended effort, including a minimum of errors? Does it improve
the productivity/effort ratio? Does it do things right?
Satisfaction: Do users express satisfaction with the product? Does the new
product reduce stress? Do the end users now have a more satisfying job?
Why Is It Important?In an increasingly competitive marketplace, those products and services that do not meet
customer needs fail. Ease of use is a real user need and most product reviews, whether in
consumer magazines or professional journals, usually compare products based on their
usability. Research also shows that about 50 percent of the code in new software
applications is devoted to the user interface, making it a significant cost component.
Finally, applications have become increasingly complex—and delivering this increased
complexity while maintaining ease of use is a challenging endeavor.
The importance of usability will vary from product to product. For safety-critical
applications such as nuclear power-station management or air-traffic control, usability is
crucial. Where a high value is placed on productivity, or perhaps a high cost is associated
with human error (such as a financial dealing room or telecommunications network
management center), usability is essential as well.
In marketing terms, where significant additional revenue will be generated from
increased usage (for example, telephony services), usability is also a priority.
Usability EngineeringThese components are not intrinsic qualities of a product. The designer cannot take a tape
measure to an HMI and measure its effectiveness, efficiency, or satisfaction scores.
Effectiveness depends on the users' intentions, goals, or tasks; efficiency depends on
users' understanding of the product and on their previous experience; and satisfaction can
only be expressed by the users.
As a result, the unifying principle of design techniques that deliver usable products is that
each recognizes the need to keep users at the center of the process. The overall design
process that brings these techniques together is known as "usability engineering."
Principle #1: Know Your UsersThree simple design principles that underlie the development of products and services
(know your users, involve users early and continuously, and rapid and frequent iteration
towards measurable usability targets) will be outlined in this and the following two
modules. The next module discusses the usability engineering lifecycle and shows how
these three design principles are kept in focus by specific activities during development.
The best way to meet users' needs—including usability—is to understand the users
intimately. A user-centered approach assumes that although people vary widely, they all
have particular needs that must be met. For example, where a business process is being
automated, instead of automating whatever can be automated and leaving the remaining
tasks to the users, a user-centered approach will assign specific tasks to the users and the
system, taking into account the users' needs.
Users can often be a source of product improvement and innovation—especially lead
users and early adopters. The most difficult and demanding customers often become the
best partners for product improvement. Users will often use products in ways that were
never intended or expected—these uses and abuses, and the problems users have as a
result, are often sources of inspiration for improvement or differentiation.
Users are experts in their requirements—they understand their goals and their tasks, and
they know the objects and artifacts they produce and use, the work-arounds they invent
(not just the official, formal procedures), and the problems they have. However, users are
not always good at describing, explaining, or predicting their behavior. Because users do
not often make good designers, they must be involved in effective ways.
In particular, users develop their own conceptual model of their work. This conceptual
model is never the same as the designer's model. Users always behave in surprising ways,
which is why they must be involved in the design process. A successful HMI maps the
users' conceptual model directly onto the software or hardware so that users may not even
be aware of the HMI components.
Who Are the Users?
The first issue to be resolved is how to choose which users will be involved in the design.
For consumer products, the answer will lie in the demographics of the target user
population. For business productivity applications, target users are often easier to define
but sometimes more difficult to access or to involve in the design process.
For almost all products, there will not be a single user or user role. Although the end user
may be the primary person affected by a design, there will also be secondary users—
people who have requirements that must be taken into account in the design and who are
affected by the design even if they do not actually press the keys. The task of identifying
the users and their different requirements is known as "stakeholder analysis."
For example, in a call center for customer assistance and queries, the call-center operators
are the primary users, but the call-center managers and the customers who call are other
stakeholders. Similarly, the people who decide to buy a videoconferencing service for a
major corporation are often not the people who have to use it, but both of these
stakeholders—the users and the choosers—have important needs that must be met.
During the early gathering of information, designers will start to understand the users'
range of concerns, goals, and priorities. It is often helpful to develop a series of
stereotypes—imaginary individuals whose life details and images are representative of
the main user population. Developers and users alike can often relate to these portraits
more easily than to dry statistics. These imaginary individuals can also star in the
storyboards and scenarios that are used to gather users' requirements and explore
solutions.
Principle #2: Involve Users Early and ContinuouslyEarly design decisions are usually those concerning the product concept, its architecture,
and its priorities. As a result, these are the decisions that are the most costly to change
later. If these decisions are not user-centered, the end product will not be usable. For
example, adding well-designed icons to a flawed menu structure will not rescue a poor
product.
For this reason, users should be involved as early as possible in the design process. Users
usually contribute to early efforts to gather information through observation,
questionnaires, focus groups, interviews, or more detailed task analysis. At this stage,
designers will build models of the users' domain and establish task priorities and
relationships. As was noted, users may not be good at articulating their requirements or
describing or predicting their own behavior. For this reason, field observation of user
behavior is often most effective.
Users are also better at critiquing an existing HMI than designing one from scratch.
However, user involvement must be cost-effective. Simply placing users in front of a new
application and asking for improvements will lead to an unprioritized wish list. This is
why prototyping is crucial. Users should be given a number of alternative designs—
whether high-level or detailed—to compare and critique. The alternatives will help them
generate more ideas and also show that their comments are welcome and useful.
As the design becomes relatively stable, user activities are aimed at refining and
validating detailed design. Usability testing becomes most effective when measuring
performance and productivity—including error rates and causes—and validating
terminology and icons. Typically, users will be asked to carry out specific tasks designed
to test parts of the interface or to address particular design issues that could not be
resolved earlier. Well-designed user trials will get maximum results for the time and
effort invested by designers and users.
Principle #3: Rapid and Frequent Iteration Toward Measurable Usability TargetsThe key to involving users is to take an iterative approach. Each iteration is an
opportunity to bring in real users and evaluate different aspects of the evolving product.
Early iteration prevents major architectural decisions from leading the development down
erroneous and costly paths. Iteration should start as early in the development cycle as
possible, with lo-fi prototypes, often pencil and paper designs that can be changed
quickly. If a physical product is being designed, then card, foam, or plastic models should
be built to test acceptability of size, weight, and shape and to ascertain the location of
main components.
One potential downfall is that with iteration it is difficult to know when to stop. Each
iteration must be focused on a desirable target and should improve the design. Usability
metrics are the key, derived from an understanding of the users' priorities, the main tasks
they will carry out, and the desired productivity. Even if a development is time-boxed,
with a prespecified number of main iterations, there should be some minimum usability
acceptance criteria.
REFERENCES
http://www.plcs.net/
http://www.iec.org/
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