Automation and robotics

Chapter – 01 1.1 Automation “The control of an industrial process manufacturing, production etc by automatic rather than manual means is often called automation”.. Automation or Industr

C hapter – 0 1 Automation

1.1 Automation 1.2 Industrial Automation 1.3 Building Blocks of Automation

1.3.1 Building Block Systems 1.3.1.1 Processing Systems 1.3.1.2 Multi-Microprocessor Systems 1.3.1.3 Local Area Networks

1.3.1.4 Analog and Digital I/O Modules 1.3.1.5 Supervisory Control and Data Acquisition Systems (SCADA) 1.3.1.6 Remote Terminal Unit

1.3.1.7 PID Controllers 1.3.2 Building Block Components 1.3.2.1 Sensors

1.3.2.2 Analyzers 1.3.2.3 Actuators 1.3.2.4 Drives

Chapter – 01

1.1 Automation

“The control of an industrial process (manufacturing, production etc) by automatic

rather than manual means is often called automation” Automation is prevalent in

the chemical, electric power, paper, automobile and steel industries, among

others The concept of automation is central to our industrial society

In its modern usage, automation can be defined as a technology that uses

programmed commands to operate a given process, combined with feedback of

information to determine that the commands have been properly executed

Automation is often used for processes that were previously operated by

humans When automated, the process can operate without human assistance or

interference A semi automated process is one that incorporates both humans

and robots For instance, many automobile assembly line operations require

cooperation between a human operator and an intelligent robot

Examples of manufacturing automation include:

1 Automatic machine tools to process parts

2 Industrial robots

3 Automatic material handling

4 Automated storage and retrieval, and inspection systems

5 Feedback control systems

6 Computer systems for designing, automatically transforming designs into

products, planning and decision making to support manufacturing

1.2 Industrial Automation

From the moment people started doing work, they began trying to find methods

of automating the work Progress in such methods can be seen in the use of

automated machines, computer-aided designs, computer-aided manufacturing,

computer-aided robotics, and industrial robots Programmable controllers and

robots are key components for industrial automation These systems have

enabled our factories to increase productivity, decrease costs and increase the

quality of manufactured goods

Automation or Industrial Automation is the use of computers to control

industrial machinery and processes, replacing human operators It is a step

beyond mechanization, where human operators are provided with machinery to

help them in their jobs The most visible part of automation can be said to be

industrial robotics Some advantages are repeatability, tighter quality control,

waste reduction, integration with business systems, increased productivity and

reduction of labour Some disadvantages are high initial costs and increased dependence on maintenance

By the middle of the 20th century, automation had existed for many years on a small scale, using mechanical devices to automate the production of simply shaped items However the concept only became truly practical with the addition

of the computer, whose flexibility allowed it to drive almost any sort of task

Computers with the required combination of power, price, and size first started to appear in the 1960s, and since then have taken over the vast majority of assembly line tasks (some food production/inspection being a notable exception)

In most cases specialised hardened computers refered to as PLCs (Programmable Logic Controllers) are used to synchronize the flow of inputs from sensors and events with the flow of outputs to actuators and events This leads

to precisely controlled actions that permit a tight control of the process or machine

HMIs (Human-Machine Interfaces) are usually employed to communicate to PLCs e.g.: To enter and monitor temperatures or pressures to be maintained

Social issues of automation

Automation raises several important social issues Among them is automation's impact on employment/unemployment

Some argue automation leads to fuller employment One author made that case here: When automation was first introduced, it caused widespread fear It was thought that the displacement of human workers by computerized systems would lead to unemployment (this also happened with mechanization, centuries earlier)

In fact the opposite was true, the freeing up of the labor force allowed more people to enter information jobs, which are typically higher paying One odd side effect of this shift is that "unskilled labor" now is paid very well in most industrialized nations, because fewer people are available to fill such jobs leading

to supply and demand issues

Some argue the reverse, at least in the long term First, automation has only just begun and short-term conditions might partially obscure its long-term impact For instance many manufacturing jobs left the United States during the early 1990s, but a massive upscaling of IT jobs at the same time offset this as a whole

It appears that automation does devalue unskilled 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 in the "western world" is quite advanced, yet during the same period the general wellbeing of its

Chapter – 01

citizens has increased dramatically What role automation played in these

chanes has not been well studied

1.3 Building Blocks of Automation

One by one, better methods are being found to sense, move, position, orient,

fabricate, and assemble products using a wide variety of ingenious basic

components – the building blocks of automated systems Unless and until a

manufacturing workstation has been analyzed thoroughly and fitted out with the

basic components of automation, it usually is not ready for more exotic hardware

such as industrial robots Indeed, the industrial robots themselves are

constructed of some of these same basic components of automation

To make sense out of the diversity of automation components, some sort of

crude classification is needed The classification is rough because some of the

most useful components find their way into several of the categories, depending

upon how they are used Generally, automation is distributed into two main

categories; namely,

 Building Block Systems

o Processing systems

o Multi-microprocessor systems

o Local area networks

o Analog and Digital I/O modules

o Supervisory Control and Data Acquisition Systems (SCADA)

o Remote Terminal Unit

It should be noted that, the operator of an automated system is again a human

himself; the industrial robot is the part of the automated system The industrial

robot is actually an integrated system made up of all four of the basic automation

component categories – sensors, analyzers, actuators and drives

1.3.1 Building Block Systems

The development in the field of automation and in the field of intelligent

machines, (i.e., starting from computers, microprocessors to present day expert

systems and neural networks) which constitutes the basic building block systems, were almost simultaneous It is a known fact that growth in computer and microprocessor technology was one single big cause for the growth in automation techniques like Direct Digital Control (DDC), Distributed Control, and Adaptive Control etc

1.3.1.1 Processing system Computers and microprocessors: The computers are predecessors to

microprocessors The basic concepts of computers were evolved before the

dawn of the microprocessor

By the early 1970’s, small integrated circuits (TTL logic) were well established while MOS integrated circuits, such as calculator components had started to appear The use of a microcomputer or control processor (as against the use of general purpose computers) was also well known It was clear to some semiconductor engineers that if the calculator chip could become more general, it would have wider application

Figure 1-1: Von Neumann organization of computer

Also, the mini-computer users were confident that if it could be made more compact and cheaper its application areas will become further wide These were the two mainstreams that led to the microprocessor development Undoubtedly, what made the processor possible was MOS technology and the remarkable properties of silicon, providing that, the microprocessor was inevitable

Figure 1-1 shows the block diagram of computer showing different units and is known as Von Neumann Organization of Computers The computer organization proposed by Von Neumann envisages that binary number systems are used for

Chapter – 01

both data as well as instructions There is a direct correlation between the

microprocessor organization and the organization proposed by Von Neumann for

computers Due to advancements in micro-miniaturization, the arithmetic logic

unit and control unit have been put on a single chip known as microprocessors

Microcomputers and microcontrollers: Microcomputers are microprocessors

with on-chip memory Some microcomputer chips contain timer/counter, interrupt

handling, also along with processor and memory Timer/counter and interrupts

are useful for control application and these microcomputers are called

microcontrollers To be compatible with analog and digital world in some cases

analog to digital converter and digital to analog converter have also been

integrated on the chip There is a variety of microcomputers/microcontrollers from

different manufacturers like 8048, 8051 and 8086 series of Intel, Z8 from Zilog,

M6801 and 68HC11 from Motorola,1650 series from General Instruments,

IM61000 from Internal Inc etc In general, these chips come in three versions

namely “on-chip ROM version”, “on-chip EPROM version” and “ROM-less

version” The later two versions are used for development purposes After the

development is complete, the large number of ‘on-chip ROM’ version chips can

be obtained by getting the program fused at source, at low cost

The Transputer: The principle behind the design of the transputer is to provide

the system designer with a building block component which can be used in large

numbers to construct very high performance systems The transputers have

been specifically developed for concurrent processing The on-chip local memory

assists in eliminating processor to memory bottlenecks and each transputer

supports a number of asynchronous high speed serial links to other transputer

units The efficient utilization of processors time slices is carried out by a micro

coded scheduler

The transputer to transputer links provide a combined data communications

capacity of 5 Mega bytes per sec, and operate concurrently with internal process

This is a radical difference from the shared bus concept employed in the majority

of multi-microprocessor architectures It allows parallel connection without

overhead because of the complex communication between conventional parallel

processors The advantages over multiprocessor are as follows:

 No contention for communication

 No capacitive load penalties as transputers are added

 The bandwidth does not become saturated as system increases in size

The system supports high level concurrent programming language Occam (high

level concurrent programming language) specifically designed to run efficiently

on transputer systems The Occam allows access to machine features and

removes the need for a low level assembly language

1.3.1.2 Multi-microprocessor Systems

The architecture proposed by Von Neumann was a Single Instruction Single Data (SISD) stream A number of computers have been designed around this structure The single instruction and single data stream computers are easy to conceptualize and design since the computer is executing only one instruction at

a time The data flow is from/to only from one input/output unit at an instant

Multitasking concept was used for increasing the speed of program execution

This allows a number of programs resident in the computer’s memory at one time The computer switches from current task to other task as and when an I/O instruction is encountered Since I/O units are comparatively slower than CPU, the computer on encountering the I/O instruction initiates its execution and then starts executing another program On completion of I/O, the computer gets signal

to switch back to the original program This optimizes the CPU time However, the branch and return addresses as well as the status of various programs are to

be maintained by CPU A number of new concepts have been introduced both in computers and microprocessors with the aim of increasing the speed, by incorporating parallelism in memory and processing These concepts are:

It was however clear that parallelism is necessary for increased speed which is measured by millions instructions per seconds (MIPS) executed by the CPU It was thought that instead of SISD architecture, data and instruction stream can be increased The classification of computer architecture with respect to data stream and instruction stream is shown in Figure 1-2 Other equally important reasons for introducing parallelism were reliability through redundancy in control systems and geographically or functionally distributed control systems

Chapter – 01

Figure 1-2: Classification of computer architecture.

1.3.1.3 Local Area Networks

Local area networks generally called LANs are basically loosely coupled systems

having autonomous microprocessors with local memories interconnected via I/O

circuits The transfer of information requires Input-Output operations (Figure 1-3)

Both serial and parallel interconnections are possible

Local area networks are privately-owned networks, and not subjected to Federal

Communication Commission (FCC), within a single building or campus of up to a

few kilometers in size They are widely used to connect personal computers and

work stations in such a way that every device is potentially able to communicate

with every other device in company offices and factories to share resources (e.g.,

printers) and exchange information Traditional LANs run at speeds of 10 to 100

Mbps, have low delay (tens of microseconds), and make very few errors Newer

LANs may operate at higher speeds, up to hundreds of megabit/sec LANs are

distinguished from other kind of networks by three characteristics: (1) their size,

(2) their transmission technology, and (3) their topology

Figure 1-3: Local area network interconnection.

The LAN puzzle

 Workstation: personal computer or a processor that is able to run PC

software Individual workstations on the LAN are usually computers or PCs with some degree of intelligence A LAN workstation may run application software on its own processor using its own ROM, but may also store and retrieve programs and data files elsewhere in the network

micro- Server: a station in the network that handles special functions, such as

disc storage or programs, documents or data files, for the purpose of printing The server station provides a shared resource device for programs, documents, and data files available to other users To be suitable for use as a server, the PC must have a hard disc for storage of information and the ability to load the appropriate server control software

 Networking software: the file server runs a series of programs known

as the network software

 Topology: the arrangement of the workstations in relation to each other

 Network interface card (NIC): the circuit board or hardware device

which connects or permits the attachment of each workstation to the transmission media The most common type of interface between a PC

and the transmission media is through an interface card This is a

printed circuit board designed to fit inside the PC cabinet The cable will attach directly to the card Besides serving as the physical connections, the interface controls the signaling method incorporated by the workstations as they attempt to transmit messages throughout the network

Chapter – 01

Another type of interface (transceiver) is called a T-connector This is a

hardware device in the shape of a “T” which permits the connections of the workstation directly to the coaxial cabling

 Patch Cord - (Also called a Patch Cable) This is the cable that usually

connects computers to computers (NIC to NIC), computers to Hubs, or computers to Transceivers

 Protocol: the access method (protocol) is the technique under whose

control the network determines the order of message transmission among the participating workstations

 Repeaters and amplifiers: to overcome distance limitations of the basic

network, vendors may supply repeaters or analog amplifiers to boost the signal strength on the transmission media, and which can effectively allow for the connection of two segments of a LAN

LAN Topologies

There are a number of different LAN topologies, as shown in Figure 1-4, each

suited to particular application environments Issues such as reliability, speed,

cost, and distance influence the choice of LAN In many of these topologies,

communicating devices share a common transmission medium instead of being

connected by individual point-to-point links The major point becomes how you

gain access to the medium that you share with others

Figure 1-4: LAN Topologies

Star Topology: A star network has a central or controlling workstation with

branches to various slave devices The controlling workstation is often a

computerized switch Each slave device is linked to the central workstation with a

point-to-point serial connection

Any protocol or code conversions may be centralized in the controlling workstation, thus eliminating expensive and complex network interfaces

However, in a star network, the controlling workstation is a critical node and if it fails, the associated terminals and other devices are rendered inoperative

Ring Topology: The Ring or loop topology is a pattern of computing

workstations arranged in a circle Establishing a primary controlling workstation can centralize control Or control can be distributed by assigning an equal or peer-to-peer status to all of the workstations In this arrangement, a message passes from workstation to workstation, being examined by each intermediate

workstation, until it reaches its destination

In a ring arrangement each station assumes an active role and is therefore potentially critical node Failure of one relay point in the ring (depending upon the physical connections) may halt the flow of data

Bus Topology: A bus network has a backbone cable with two endpoints Each

workstation is connected to the cable using a tap box or T-connector (transceiver) There are no critical nodes on a bus arrangement because each workstation interfaces with the bus separately Failure of one device does not effect the entire network

As with the ring network, all messages are broadcast to all of the workstations

The intelligence in each station only recognizes the message destined for it

Combined Topologies: In large computer networks, such as in a University

Campus, often a number of types of LAN topologies will exist in the same network

1.3.1.4 Analog and Digital I/O Modules

After having discussed the computers and microprocessors, we shall now be dealing with the modules which connect the process to the data processing unit

Analog input signals are received from sensors and signal conditioner and represent the value of measured like flow, position, displacement, temperature etc The signal conditioner takes as input the output of sensor and suitably conditions it to be acceptable to real-time systems The signal may be amplified, filtered or even digitized for some applications

Digital input signals refer to the ON – OFF states of various valves, limit switches, etc One digital input signal represents status of the limit switch or valve and is represented by one bit of information for real-time systems Normally digital input signals are compatible to real-time systems and can be inputted directly In some cases signal amplification or attenuation is required

Chapter – 01

Interrupt input signals draw the attention of real-time system towards certain

abnormal situations in the environment or the process controlled by real-time

system The real-time system on receipt of interrupt signal attends to the

abnormality pointed out and resumes its normal work from the point where it was

suspended The abnormalities may be excess flow, temperature, power failure,

or some process faults which must be notified immediately One interrupt signal

will correspond to only one particular abnormality which needs to be attended

Real-time systems attend to abnormalities by executing special programs called

Interrupts Servicing Routines Thus there is one to one correspondence between

an abnormality – interrupt signal and interrupt servicing routine

Figure 1-5: An inside look of real-time systems.

In order to enable the real-time system to suspend its current program, execution

of the interrupt servicing routine and restarting of the suspended program is needed The facility to store the status of the program and fetch the same afterwards is essential

Timer/counter input signals are important part of any real-time system Through these signals the concept and measure of real-time is derived These signals are used as clock input to timer/counter in real-time system or gate input to enable/disable different timers/counters The timer circuit may be used to initiate events at defined intervals The counter circuit on the other hand, may be used to count the occurrences of any defined event The output of timer/counter may be used as interrupt signal to real-time system

Display output signals are used to derive the display devices like LED, LCD, Audio Alarms etc The display of status of process, various control valves etc is very important to the operator Apart from this, the limits set for various parameters at different places in the process are also displayed for the benefit of operators The display output signals carry the information which is displayed on one or more display devices Some of the real-time systems do not control the process but display the various parameter values, their variations, limits set etc

for the benefit of operators, who eventually control the process by manually

operating various control valves Such real-time systems are called Data Acquisition Systems

Control output signals are required to derive the control valves, motors etc to perform the control action decided by the real-time systems The control action desired may be simple ON – OFF control of valves/motor or fine control of motor speed, position, displacement, flow and level through control valves The control output signals are analog signals which can derive various actuators However with the emergence of digital actuators, these analog signals will soon be replaced by digital signals

The major hardware subsystems of real-time control systems are shown in Figure 1-5

1.3.1.5 Supervisory Control and Data Acquisition Systems

After having profound knowledge of the basic modules of a real-time system, let

us now proceed to the understandings of Supervisory Control and Data Acquisition (SCADA) system, since it is the first step towards automation The basic functions carried out by an SCADA system are:

 Channel scanning

 Conversion into engineering units

 Data processing

Chapter – 01

Figure 1-6 shows the block schematic of SCADA

Channel Scanning: There are many ways in which microprocessor can address

the various channels and read the data

Polling: The microprocessor scans the channel to read the data, and this

process is called polling In polling, the action of selecting a channel and

addressing it is the responsibility of processor The channel selection may be

sequential or in any particular order decided by the designer The channel

scanning and reading of data requires the following actions to be taken:

 Sending channel address to multiplexer

 Sending start convert pulse to ADC

 Reading the digital data

Figure 1-6: Supervisory control and data acquisition system

Interrupt Scanning: Another way of scanning the channels may be to provide

some primitive facility after transducer to check for violation of limits It sends interrupt signal request to processor when the analog signal from transducer is not within High and Low limits boundary set by Analog High and Analog Low

signals This is also called Scanning by Exception When any parameter exceeds

the limits then the limit checking circuit would sent interrupt request to microprocessor which in turn would monitor all parameters till the parameter values come back within pre-specified limits This allows a detailed analysis of the system and the problems by the SCADA system

Conversion to Engineering Units: The data read from the output of ADC

should be converted to equivalent engineering units before any analysis is done

or the data is sent for display or printing For an 8-bit ADC working in unipolar mode the output ranges between 0 and 255 An ADC output value will

corresponds to a particular engineering value based on the following parameters

 Calibration of transmitters

 ADC mode and digital output lines

Depending on the input range of measurand value for transmitter, a calibration factor is determined If a transmitter is capable of measuring parameter within the input range X1 and X2 and provides 0 – 5 V signal at output then calibration factor

is

units X

5

X Volt

5

255 Volt

1 

Thus the conversion factor is

units g engineerin 5

X 5

255 output 2 X1

Chapter – 01

units g engineerin 255

X 1 output 2 X1

therefore is

factor Conversion

255

) Y(X

1 2

1 2

X

X





The conversion of ADC output to engineering units, therefore, involves

multiplication by conversion factor The conversion factor is based on the ADC

type, mode and the transmitter range This multiplication can be achieved by shift

and add method in case of 8-bit microprocessor For 16-bit microprocessor, a

single multiplication instruction will do the job

Data Processing: The data read from the ADC output for various channels is

processed by the microprocessor to carry out limit checking and performance

analysis

Distributed SCADA System: In any application, if the number of channels is

quite large then in order to interface these to processor, one approach is to use

multiplexers at different levels The alternative approach for such conditions is to

use more than one SCADA system and distribute the channels among them

1.3.1.6 Remote Terminal Unit

The remote terminal units (RTUs) are basically distributed SCADA based

systems used in remote locations in applications like oil pipelining, irrigation

canals, oil drilling platforms etc They are rugged and should be able to work

unattended for a long duration There are two modes in which Remote Terminal

Unit work

1 Under command from central computer

2 Stand alone mode

Since these RTUs have to operate for a long duration unattended, the basic

requirements would be that they consume minimum power and have

considerable self-diagnostic facility Following are the main parts of remote

terminal units

Figure 1-7: Block diagram of Remote Terminal Unit.

( t K e t

Chapter – 01

The controller gain is adjusted to increase or decrease the sensitivity of the

controller output to the deviations between setpoint and the controlled variable

Taking the Laplace transforms gives the following transfer function:

(1.2)

c

The advantage of a proportional-only control is its simplicity If offsets can be

tolerated, the use of a proportional controller may be optimal However, it will not

eliminate the steady-state errors that occur after a set-point change or a

sustained load disturbance

Integral Control

Integral control depends on the integral of the error signal over time The integral

time constant,I, is the adjustable controller parameter with units of time

(1.3)

*

*) ( ) / 1 ( ) (

The primary advantage of integral control is that it eliminates offset This

happens because p (t ) will change until the error signal is zero, thus eliminating

a deviation between the controlled variable and setpoint in the steady-state The

disadvantage of integral-only control is that the controller will not respond until

the error signal has persisted

To counteract this problem, controllers have been developed that combine the

use of proportional and integral control The result is a proportional-integral (PI)

controller, which is commonly used because of the immediate acting proportional

control coupled with the corrective acting integral control The PI controller

Derivative control is used to anticipate the future behavior of the error signal by

using corrective action based on the rate of change in error signal

 /  (1.5) )

p  D

Derivative action is used to stabilize the controlled process When the error signal is increasing greatly, the controller output is large The error signal decreases, and the process is eventually stabilized A disadvantage of derivative control is that controller output is zero when the error signal is constant

To counteract this problem, proportional-derivative (PD) controllers have been developed to improve the dynamic response of the controlled variable The transfer function of a PD controller is

(1.6) ) 1 ( s K

Gc  c  D

Proportional-Integral-Derivative Control

A three mode proportional-integral-derivative (PID) controller combines the advantages of each individual mode of control The ideal PID controller output equation is

(1.7) )

/ (

*

*) ( ) / 1 ( ) ( )

Note: A PID controller is not used for highly noisy control variables like flow control, because the derivative response will amplify the random fluctuations in the system

1.3.2 Building Block Components

Another approach to classify the automation is based on the components used in the automation industry Generally, there are four basic components which constitute an automatic system One of them must be present in one form or another if the system is to be termed as the automatic system

Chapter – 01

1.3.2.1 Sensors

Sensors are the first link between the typical automated system and the

conventional process Sensors convey information from the manufacturing

process equipment and from the human operator, if any

Manual Switch: the most familiar sensor of all is the manual switch An

automation system is linked through a manual switch to the operator, who may

desire to turn the system on or off or make adjustments to the automated cycle

Virtually, all manual switches are electric, but they are actuated mechanically

Most switches have two stable states: on and off However, many switches have

only single stable state That stable state can either be the open position or the

closed position, which leads to the terms normally open (NO) and normally

closed (NC) used to describe switches

 Single-pole, single-throw (SPST): the ordinary wall switch is an example

of toggle switch Such switches are designated pole, throw (SPST) and are illustrated in Figure 1-8(a) The term single-pole,

single-single-throw implies that it merely opens or closes a single circuit

 Single-pole, double-throw (SPDT): toggle switches are usually

two-position switches, but may be three two-position switches, with the center position designated as off This makes it possible for the switch to complete two different circuits Toggled to the left makes circuit A;

toggled to the right closes circuit B; and the center position holds both circuits A and B open Even when a toggle switch has only two positions,

it may be wired to throw two circuits Figure 1-8(b) diagrams such a switch in which one lead to the switch is common but the position of the switch determines whether circuit A or circuit B is made Such a switch is

designated single-pole, double-throw (SPDT) The switch is single pole

because of the common lead on one side of the switch, but it is double throw because it can complete either of two circuits on this same common pole

 Double-pole, single-throw (DPST): it is also possible to have two leads

on both sides of the switch, which enables the switch to make two different circuits that do not have common pole In effect, such a switch

is closing two different circuits with a single mechanical throw Such a

switch is called a double-pole, single-throw (DPST) switch and is

illustrated in Figure 1-8(c)

 Double-pole, double-throw (DPDT): in still another configuration, each

contact on one side of the switch can be connected to either of two

contacts each on the other side of the switch Such a switch is a pole, double-throw (DPDT) switch and is illustrated in Figure 1-8(d)

double-Figure 1-8: Examples of electric switch

It is possible to add as many poles as desired, but to go beyond two throws the

use of a toggle switch becomes impractical For multiple throws, a rotary switch

becomes appropriate Figure 1-9 illustrates a rotary switch that could be classified as a single-pole, five-throw switch, although it is usually designated simply as a six position rotary switch A mechanical detent is placed at each end

of the six positions Note that there are six positions but only five throws, because one detent is used for the off position to represent “all circuits broken” A manual control knob without detent is usually a continuous control used for some variable in the automated process that can take on any value over a continuous range

The safety of robots and other automated systems demands that most operator control switches have only one stable state: off Thus, a positive operator action must be maintained to keep the switch on The beauty of single-stable-state switches is that control can be exercised whenever and wherever it is necessary

to change the operational state of the system To kill the system in an emergency, the operator would have to find that one switch and turn it off But

with a momentary switch, the system could be switched on at one location and

then switched off by other momentary switches at any of several convenient locations around the machine In the automation industry, these momentary off switches located at various points about the equipment are called Emergency Power Offs or simply EPOs

Figure 1-9: Rotary Switch

Chapter – 01

The most popular physical

configuration of a momentary switch is

the familiar pushbutton Pushbuttons

can be either “making” or “breaking” –

i.e., normally open or normally closed,

respectively Figure 1-10 illustrates the

standard momentary pushbutton

diagram in both configurations Figure 1-10: Momentary (spring-return)

There is another type of pushbutton switch that has two stable states; the action

of the pushbutton is to switch the circuit to the opposite state, whatever that state

might have been Such a switch resembles a toggle switch in function but may

have a disadvantage in failing to display which of the two feasible circuit states

(open or closed) currently exists

Limit Switches: like manual switches, limit switches are actuated mechanically,

but limit switches are automatic i.e., inputs are applied from the manufacturing

process, the material, or the automated system itself, without intervention by the

operator There are literally thousands of styles and models of limit switches,

because they are designed to be exactly correct in size for the specific

automation application Levers, toggles, pushbuttons, plungers, rollers, “cat

whiskers” actuate limit switches and just about anything the inventor can devise

to make an automation application feasible

Robot switches employ limit switches both in

the construction of the robot itself and in the

peripheral equipment Limit switches can be

used to limit the travel of a robot arm on any

of its axes of motion When the limit is

reached, a circuit is opened (or closed) that

removes power from that axis of motion

either directly or via the robot controller Figure 1-11: limit switches

Proximity switches: some switches do not require physical contact or light

radiation to “feel” or sense an object Such switches are called proximity

switches, as shown in Figure 1-12, because they can sense the presence of a

nearby object without touching it Proximity switches sense the approach of a

metallic machine part either by a magnetic or high-frequency electromagnetic

field Some types of proximity switches act only upon presentation of ferrous

metal object within its sensing range The other types of switch are capable of

sensing both ferrous and nonferrous metal objects

There are physical basis for proximity switches that can respond to any object – metal or nonmetal One type uses an electromagnetic (radio frequency) antenna specifically designed and placed to fit the application

Figure 1-12: Proximity Switch

The antenna receives a signal transmitted by another strategically placed antenna, but the reception of the signal is distributed by the intrusion of any object into the field The antenna that trips a switch when the disturbance reaches a specified level detects this disturbance

Another type of proximity switch that works for nonmetallic object is the sonar type Sonar systems transmit and receive elections of pressure waves to detect

object presence These pressure waves are commonly called sound waves when their frequencies are within the audible range

Figure 1-13: Three types of reflective surfaces for photoelectric systems

Chapter – 01

A sophisticated proximity switch system employs the Hall Effect, in which a

small voltage is generated across a conductor carrying current in an external

magnetic field The amount of Hall voltage is proportional to the flux density of

the magnetic field, which is perpendicular to the flow of current This

proportionality enables Hall Effect proximity to detect not only presence but also

relative distance to a sensed object

Photoelectric Sensors: In wider use than proximity switches are sensors that

are sensitive to light radiation: photoelectric sensors Two basic approaches for

employing photoelectrics are in use The first approach merely uses a photocell

to detect the presence of light radiating naturally from some object in the

process The second approach to photoelectrics employs a beam of light emitted

by an artificial light source The principal purpose of this approach is to detect the

presence or absence of objects in the path of the beam The beam emitter can

be a separate unit or can be incorporated into the sensor The combination

variety requires some type of natural or artificial reflector to direct the light beam

back to the sensor

Reflective surfaces for photoelectric systems are of three types: diffuse, specular

reflective, and retroreflective, as shown in Figure 1-13

 Diffuse reflector: the diffuse reflective surface is the lowest in cost and

describes most reflective surfaces Even an ordinary white object acts as

a diffuse reflective surface in that it reflects light but not images Diffuse reflector scatters so much light that only a small fraction makes its way back to the photoelectric sensor Photoelectric systems that use diffuse reflectors are also more susceptible to stray signals

 Specular reflector: specular reflective surfaces are most often

associated with the word reflective and include mirrors and very shiny surfaces Specular reflective surfaces obey the physical law that the angle of incidence equals the angle of reflection For systems in which the emitter and sensor are mounted in the same unit, the plane of the specular reflective surface must be perpendicular to the direction of the incident beam or the reflected beam will be lost

 Retroreflector: retroreflective surfaces are the most complex and

expensive of the three types They are capable of reflecting back to the source a large percentage of the light beam regardless of the angle of incidence Basically, the retroreflector violates the physical principle that angle of incidence equals angle of reflection, except when the plane of the surface is perpendicular to the incident beam Red reflector on the rear of the bicycle is the example of retroreflector Retroreflectors somewhat combine the advantages of diffuse and specular reflective surfaces, but at a price – the retroreflective surface is the more expensive of the three types

As useful as photoelectrics are, we should be aware of conditions that can ruin the photoelectric system like stray ambient light, high temperatures, vibrations from adjacent machines, and dust or condensation may cause the sensor to be damaged To overcome these problems, mirrors, air jets, and infrared or fiber optics are used

Infrared sensors: Infrared sensors respond to radiation in range of wavelengths

just beyond the visible spectrum at the red end Hot objects emit infrared radiation, and thus infrared sensors are useful for locating heat sources in a process Infrared sensors are virtually unaffected by stray ambient light – with obvious advantages

Fiber optics: fiber optic light tubes are flexible pipes of glass or plastic that can

be used to bend light beams around corners When bundles of fibers are used together, whole images can be transmitted However, the typical automation application is to use one fiber to transmit a light beam that is sensed by the system as being either present or absent

Lasers: lasers (Light Amplification Stimulant for Emission Radiation) are

concentrated, amplified beams of collimated light They are capable of delivering over a distance a large amount of energy into a tiny spot and thus have obvious industrial applications In automated systems, the laser is useful in providing very long, precise light beams Figure 1-14 shows the precision available from laser systems The precision of these beams make them excellent for detecting tiny objects that are capable of breaking the beam at large and varying distances

Figure 1-14: Laser light source

1.3.2.2 Analyzers

Once information is sensed up by an automated system, it must be registered and analyzed for content, and then a decision must be made by the systems to what action should be taken The function can be quite complex, and the system components that perform it are also generally complicated

Chapter – 01

Computers: digital computers are the primary means of analyzing automation

system inputs Computers are extremely versatile in that the ways they can be

programmed to manipulate data are limitless The continuing miniaturization of

computer circuits along with decreasing costs made possible by technological

breakthroughs have created a continuing increase in the number of feasible

applications of manufacturing automation

Counters: it is frequently useful for an automated system to determine how

many various items are present or pass through an automated system This

function can be handled either internally by a computer or programmable

controller or externally by a separate device called counter The counter can be

mechanical, but most automatic systems employ solid-state electronic counters

Counters can count-up or down The quantity counted is usually a series of

voltage pulses Another useful feature of many counters is bi-directionality, which

enables them to count up or count down This can be useful in automatic

industrial quality control applications and in material handling

Timers: if precise clock pulses are available, a counter that counts these pulses

becomes timer, basically a clock When elapsed time becomes equal to preset

value, an output signal is generated Like counters, industrial timers can be

bi-directional – i.e., time up and time down

Timers often have the additional feature of being interruptible – that is, they can

be cumulative in summing the various periods of voltage up time interrupted by

various periods of voltage down time The applications of industrial timers to

robots and automation are even greater than that of counters Besides being

available as separate units, industrial timers can be internal to programmable

controllers and online process control computers

Bar Code Readers: although it can be considered a sensor, a bar code reader

is an analyzing system that incorporates conventional photoelectric or laser

scanner along with timers and counters Successive bars of varying width are

scanned and counted The scan is orthogonal to the bars, and thus voltage

pulses from the photoelectric sensor can be compared to determine individual

bar widths

Optical encoders: the capability of rapidly scanning a series of bars makes

possible additional automation opportunities when light and dark bars are placed

in concentric rings on a disk The assembly consisting of optical sensors for each

ring is called an optical encoder and is useful for automatically detecting shaft

rotation

Optical encoders can be either incremental or absolute The incremental types

transmit series of voltage pulses proportional to the angle of rotation of the shaft

The control computer must know the previous position of the shaft in order to

calculate the new position Absolute encoders transmit pattern of voltages that describes the position of the shaft at any given time The innermost ring switches from dark to light every 180, the next ring every 90, the next 45, and so on, depending upon the number of rings on the disks The resulting bit pattern output

by the encoder reveals the exact angular position of the shaft

Case study

An absolute optical encoder disk has eight rings and eight LED sensors, and in turn provides 8-bit outputs Suppose the output pattern is 10010110 What is the angular position of the shaft?

Solution:

Encoder Ring Angular Value

Observed Pattern

Computed Value (Degrees)

iA m A

Where i = ring number

i

m = 0 if the ring is white, and 1 if the ring is black

i

A = angular value of ring i

n = total number of rings

1.3.2.3 Actuators

Once a real world condition is sensed and analyzed, something may need to be done about it Actuation may be a direct physical action upon the process, such

Chapter – 01

as a physical making of an electrical circuit, which in turn has effect upon the

process

Actuation may be a direct physical action upon the process, such as sweep bar

that sweeps items off a conveyer belt at the command of a computer or other

analyzer In other cases, an actuator is simply a physical making of an electrical

circuit, which in turn has a direct effect upon the process An example would be

an actuator (relay) that turns on power to an electric furnace heating circuit

Actuators take on many diverse forms to suit the particular requirements of

process-control loops

Cylinders: when linear movement is required in an automation application, a

cylinder usually is chosen to accomplish it The most popular are the pneumatic

types because of the convenience of the piping compresses air throughout a

manufacturing plant Valves that may be electrical impulses or air logic devices

accomplish the control of air cylinder

When the manufacturing process requires forces to be applied automatically in

excess of 200 pounds, the more powerful hydraulic cylinder is usually selected

over the pneumatic cylinders Hydraulic pressures in excess of 2000 psi are

readily available; compare these pressures with the 80 to 100 psi commonly

used in pneumatic systems Given the mechanical advantage of a large enough

cylinder, pneumatics can deliver as large force as the hydraulics, but

convenience tend to favor hydraulics for the large forces Hydraulic actuators

drive the most powerful industrial robots

A caution to observe in the design of either pneumatic or hydraulic actuators is

that both pressure and volume requirements must be met A system may have

sufficient pressure to actuate cylinders or other actuators, but may not be able to

maintain that pressure during high-speed operation

Solenoids: A solenoid is an elementary device that converts an electrical signal

into mechanical motion, usually rectilinear (in a straight line) When a small, light

quick linear motion is desired in an automated system, an electrical solenoid is a

logical selection In basic physics, we learned that the principle of the solenoid

operation is the creation of a magnetic field set up by passing an electrical

current through a coil Thus, the core of the solenoid can be selectively drawn

into the coil in response to an electrical current In the absence of the coil current,

the core can be automatically returned by spring action The stroke motion of a

solenoid is not very controlled in comparison, for example, with a hydraulic

cylinder – but many automation applications require only a short, quick, discrete

action, not a smooth controlled stroke

Relays: the most popular solenoid of all is one that is used to switch an electrical

circuit – i.e., the common relay Switching-type circuits usually operate at lower

voltages and especially at lower amperage than power circuits The output of the switching logic network can be used to trip one or more relays to close or open a power circuit

A relay can be described as either latching or non-latching A latching relay

needs only an electrical impulse to pull and hold the power circuit closed

Another impulse is needed on a different switching circuit to release the latch

Non -latching relay hold only while the switching relay is energized and thus requires a continuous electrical signal

So far, we have described relays that make circuits when energized, but relays can also break circuits when they receive an electrical signal When the energization of the relay coil makes a circuit, the relay is designated “normally open.” Conversely, the relay that breaks circuit when energized is designated

“normally closed.” It follows, then that the normal state of an electric relay is the de-energized state

The typical relay and solenoids in general operate on low-voltage direct current

But the convenience and availability of 110-Volt alternating current (ac), have given rise to the ac relays and ac solenoids As the amperage level of the power

circuits increases, the nomenclature for relays changes to the power relays At higher amperage the relay may be called a contractor Still the basic principle of

the simple relay is being employed, and the automation engineer should not be confused by these terms

A special need for a relay is in the tripping of power circuits for electric motors

The automation engineer will hear reference to “motor starters”; these devices are either contractors or relays that in addition provide overload protection to open the motor circuit if heavy mechanical load begins to cause the motor to carry too much current

1.3.2.4 Drives

Like actuators, drives take some action upon the process at the command of a computer or other analyzer For purpose of classification, the distinction being made here between actuators and drives is that actuators are used to effect short, complete, discrete motion – usually linear – and drives execute more continuous movements typified by, but not limited to, rotation Actuators may turn drive on and off, and drives may provide the energy for the movement of actuators Some automation devices, such as Geneva’s and walking beams, seem to belong to both categories

Motors: An automation engineer must have a broad perspective of the term

motor to include not only electric motors but hydraulic and pneumatic motors as

Chapter – 01

well Hydraulic and pneumatic motors are the converse of their corresponding

pumps Hydraulic motors are capable of delivering a large amount of power in a

confined space Compared to hydraulic motors, pneumatic motors are noisier

and less powerful, but they may be more practical in many automation

applications Both pneumatic and hydraulic motors have some advantages over

electric motors in systems in which electric motors may be hazardous either from

an electrocution standpoint or from the ignition of the flammable vapors or gases

Figure 1-15: PWM control of servo motors

Stepper Motors: for several reasons, the stepper motor is a very useful drive in

the automation applications It is driven by discrete dc voltage pulses The

stepper motor is also ideal for executing a precise angular advance as may be

required in indexing or other automation applications Stepper motors are ideal

for open-loop operations, but it is possible to feedback loops to monitor the

position of the driven components An analyzer in the loop compares the actual

position with desired position, and the difference is considered error The driver

can then issue voltage pulses to the stepper motor until the error is reduced to

zero

Servo Motors: Servo motors have more torque and capabilities than stepping

motors but also cost about twice as much as stepping motors Years ago, servo

motors were difficult to work with because you had to tune the motors and

controllers Most servo controllers now automatically tune the motors and their controller It is important to tune motors after the load is attached so that the controller can see the effects of the load on the system Servo motors are known

as very accurate, fast, high torque, precise control

Inside the servo is a control board, a set of gears, a potentiometer (a variable resistor) and a motor The potentiometer is connected to the motor via the gear set A control signal gives the motor a position to rotate to and the motor starts to turn The potentiometer rotates with the motor, and as it does so its resistance changes The control circuit monitors its resistance, as soon as it reaches the appropriate value the motor stops and the servo is in the correct position

Servos are positioned using a technique called pulse width modulation This is a continuous stream of pulses sent to the servo The pulse normally lasts for between 1ms and 2ms, depending on the positioning of the servo The pulse has

to be continually repeated for the servo to hold its position, usually around 50 to

60 times a second It is the actual pulse that controls the position of the servo, not the number of times it is repeated every second

A 1ms pulse will position the servo at 0 degrees, where as a 2ms pulse will position the servo at the maximum position that it can rotate to A pulse of 1.5ms will position the servo half way round its rotation The diagram in Figure 1-15 shows three typical pulses

The diagram is not to scale but hopefully demonstrates that each pulse must be the same length That is the combined time that the pulse is on and off

DC Servo Motors are useful in numerically controlled machine tools and industrial robots for the control of motion By using a feedback loop, the controller can deliver dc voltage that is proportional to the observed error When the error is reduced to zero, the voltage goes to zero and the motor stops One important characteristic of the dc servomotor that is also true for the stepper motor is that both hold their torque when they come to rest under power Therefore, the power

is useful not only, for rotating the shaft but also for holding it motionless when no movement is desired

Kinematic Linkages: when applying automation to the workstation of a

manufacturing machine, it is easy to forget that an ample and ready source of mechanical power usually exists from the machine itself When machine speed is increased, the kinematic linkages attached to it speeds up right along with the machine Gears, cams, levers, and ratchets are the components of the kinematic off-machine linkages

Chapter – 01

Two mechanical linkages however are of particular importance to manufacturing

automation in that they impart intermittent motion to automated flow lines They

are:

 Genevas

 Walking Beams

A geneva mechanism is used to drive an indexing table intermittently and walking

beams are a mean of intermittently indexing a linear type of automated line and

are thus analogous to the genevas used to drive rotary indexing tables A walking

beam is driven by an actuating cylinder but has an advantage over the geneva in

permitting arbitrary setting of the index and dwell times by varying the cylinder

stroke and return times

Review Questions

Choose the Appropriate Answer

1 are the first link between the typical automated system and the conventional process

a Voltage

b Current

c Power

a Present

b Previous

c Next

Fill in the blanks

11 The building blocks of any automation system are _,

_, and

12 The type of switch making two different circuits with a single mechanical

throw, and do not have common pole is known as _

13 Switches that are more sensitive to light are known as

- _

14 sensors are useful to detect electromagnetic radiation

outside the visible range

15 A relay can be described as either or _

16 LASER is an abbreviation of

17 A dc motor uses a _ to produce a static magnetic field

across two pole pieces

18 Optical encoders can be either _ or _

19 The diffuse reflective surface is the in cost

20 The action of the pushbutton is to switch the circuit to the

state

True False

21 Unlike the manual switches, limit switches are actuated electrically

22 Proximity switches always require physical contact to sense an object

23 The diffuse reflective surface is the lowest in cost and describes most

reflective surfaces

24 A convenient supplement to photoelectric or infrared sensor systems is

fiber optic light tubes

25 Compared to hydraulic motors, pneumatic motors are noisier and less

powerful

26 Limit switches are type of manual switches

27 Sonar type of proximity switches works for nonmetallic objects

28 Relays can also break circuits when they receive an electrical signal

29 The stepper motors are ideal fro open loop operations

30 A solenoid is an elementary device that converts a mechanical force into

an electrical signal

C hapter – 0 2

2.1 Sensors 2.2 Motion Sensors

2.2.1 Types of Motion 2.2.2 Accelerometer Principles 2.2.3 Types of Accelerometer 2.2.4 Applications

2.5 Light sensors

2.5.1 Photo Detectors 2.5.2 Photo Conductive Detectors 2.5.3 Photo Voltaic Detectors 2.5.4 Photo Diode Detectors 2.5.5 Photo Emissive Detectors

2.6 Humidity sensors

Chapter – 02

2.1 Sensors

As described earlier, sensors are the first link between the typical automated

system and the conventional process Sensors convey information from the

manufacturing process equipment and from the human operator, if any For an

automated system to perform tasks, the system must have a sensing capability

The basic sensor used in control systems are transducers i.e., “the transducers

convert energy from one form to another”

Any sensor requires calibration in order to be useful as a measuring device

Calibration is the procedure by which the relationship between the measured

variable and the converted output signal is established

Sensors are classified in many types, according to the applications and the type

of the signals they are required to sense Some of the important sensors used in

the modern age industries are described below

2.2 Motion Sensors

Motion sensors are special class of transducers (i.e., a device used to convert

one form of energy into other) that are used to measure the velocity or

acceleration of objects in industrial processing and testing Often, these variables

are not under specific control but are used to evaluate the performance, durability

and failure modes of manufactured products and the processes that produce

them

Motion sensors are designed to measure the rate of change of position, location

or displacement of an object that is occurring If the position of an object is a

function of time x(t), then the first derivative gives the speed of the object, v(t),

which is called the velocity if a direction is also specified If the speed of the

object is also changing, then the first derivative of the speed gives the

acceleration This is also the second derivative of the position The equations for

the velocity and the accelerations are given as:

(2.2)

/

) ( /

) ( ) (

(2.1)

/

) ( ) (

2 2

dt t x d dt t dv t a

dt t dx t v







The primary form of motion sensor is the accelerometer This device measures

the acceleration of an object In accelerator we have the sensors that can provide

the acceleration, speed, and velocity and position information The accelerometer

can also be used to measure the speed and position of the object as well

Thus, in the accelerometer we have a sensor that can provide acceleration,

speed (or velocity), and position information The design of a motion sensor to

measure motion is often tailored to the type of motion that is to be measured A

few types of motion are discussed below

Rectilinear Motion: This type of motion is characterized by velocity and

acceleration, which is composed of straight-line segment Thus, objects may

accelerate forward to a certain velocity, deaccelerate to a stop, reverse, and so

on There are many types of sensors designed to measure this type of motion If

a vehicle motion is to be measured two transducers may be used, one to

measure the motion in the forward direction of the vehicle motion and other

perpendicular to the forward axis of vehicle

Figure 2-1: An object in periodic motion about an equilibrium at x =0 The peak

displacement is x 0.

Angular Motion: The sensors designed to measure only rotation about some

axis such as the angular motion of the shaft of the motor Such devices cannot

be used to measure the physical displacement of whole shaft, but only its

rotation

Vibration Motion: Often, vibrations are somewhat random in both the frequency

of periodic motion and the magnitude of displacements from equilibrium For

Chapter – 02

analytical treatments, vibration is defined in terms of a regular periodic motion

where the position of an object in time is given by

x  0sin  …… (2.3)

Where x   t = object position in meters, m

0

x =peak displacement from equilibrium in meters, m

 = angular frequency in radian/seconds

The definition of  as angular frequency is consistent with reference to  as

angular speed An angular rate of one revolution per second corresponds to an

angular velocity of 2π rad/s, because one revolution sweeps out 2π radians

From this argument, we see that f and  are related by

f



  2

Because f and  are related by a constant, we refer to  as both angular

frequency and angular velocity

Now we can find the vibration velocity as a derivative of Equation 2.3:

v    0cos  …… (2.4) and we can get the vibration acceleration from a derivative of Equation 2.4:

a   2 0sin  …… (2.5)

Vibration position, velocity, and acceleration are all periodic functions having the

same frequency Of particular interest is the peak acceleration:

0

2x

apeak  

We see that the peak acceleration is dependent on2 This may result in very

large acceleration values, even with modest peak displacements

Shock:

A special type of acceleration occurs when an object that may be in uniform

motion or modestly accelerating is suddenly brought to rest, as in a collision

Such phenomena are the result of very large accelerations, or actually

decelerations, as when an object is dropped from some height onto a hard

surface The name shock is given to the declarations that are characterized by

very short time, typically in few milliseconds with peak acceleration over 500g (1g

= 9.8 m/s2)

In Figure 2-2, we have typical acceleration graph as a function of time for a shock

experiment This graph is characterized by a maximum or peak

decelerationapeak, shock durationTd , and bouncing We can find an average

shock by knowing the velocity of the object and the shock duration

Figure 2-2: Shock

2.2.2 Accelerometer Principles

As described earlier, accelerometer or accelerator is a device that is used to

measure the velocity and acceleration of an object In accelerator we have the

sensors that can provide the acceleration, speed, velocity, and position

information Accelerometer works on two principles:

 Spring mass system

 Natural frequency and damping system

Chapter – 02

Figure 2-3: Spring-Mass system

Spring-mass system: in Figure 2-3(a) we have a mass that is free to slide on a

base The mass is connected to the base by a spring that is in its unextended

state and exerts no force on the mass In Figure 2-3(b), the whole assembly is

accelerated to the left, as shown Now the spring extends in order to provide the

force necessary to accelerate the mass Springs (within their linear region) are

governed by a physical principle known as Hooke's law Hooke's law states that

a spring will exhibit a restoring force which is proportional to the amount it has

been stretched or compressed Specifically, F=kx, where k is the constant of

proportionality between displacement (x) and force (F) The other important

physical principle is that of Newton's second law of motion which states that a

force operating on a mass which is accelerated will exhibit a force with a

magnitude F=ma

x k

Equation 2.6, allows the measurement of acceleration to be reduced to a

measurement of spring extension (linear displacement) because

m x k

If the acceleration is reversed, the spring is compressed instead of extending

The spring-mass principle applies to many common accelerometer designs The

mass that converts the acceleration to spring displacement is referred to as the

test mass or seismic mass