me2028 notes rejinpaul (1).pdf

8/18/2019 me2028 notes rejinpaul (1).pdf

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UNIT 3

SENSORS AND MACHINE VISION

Prepared byS. Senthil Kumar, M.TechAssistant professor Department of Mechanical EngineeringSri Vidya College of Engineering and TechnologyVirudhunagar

1

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Transd cers and Sensors www rejinpaul com


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Transducers and Sensors

Transducers:• Converts one type of physical variable into another form

Sensors:• Is a transducer which is used to make a measurement of a physical variable

of interest

• Some common sensors and transducers include strain gauges, thermocouple,speedometers and pitot tubes

Types of sensors and transducers:1. Analog transducers2. Digital transducers

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Desirable features of sensors Accuracy:

• True value can be sensed with no systematic positive or negative errors

Precision:• Little or no random variability in the measured variableOperating range:• Sensor should provide a wide operating range

Speed of response:• Capable of responding to changes in the sensed variable in minimum time

Calibration:• Should be easy to calibrate

Reliability:•

Should not be subjected to frequent failures during operationCost and ease of operation:• Cost must be low• Installation and operation of the device would not require a specially

trained or highly skilled operator 3

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Types of Sensors

• Position sensors – Piezo electric sensors – LVDT – Resolvers – Optical encoders – Pneumatic position sensors

• Range Sensors – Triangulation principle –

Structured lighting approach – Time of flight range finders – Laser range meters

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Types of Sensors

• Proximity sensors – Inductive and capacitive – Hall effect sensors – Ultrasonic proximity sensors – Optical proximity sensors

• Touch sensors – Binary sensors – Analog sensors

• Wrist sensors

• Compliance sensors

• Slip sensors 5

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Position sensors www rejinpaul com


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Position sensors• Device that permits position measurement.• Either be an absolute position sensor or a relative

Types: Piezoelectric sensor:

• Piezo electric materials when stretchedor compressed generates electric charges withone face of the material becoming positively charged and another

face become negatively charged•

As a result a voltage is produced• The net charge on a surface is proportional to the amount by

which the charges have been displaced

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Position sensors LVDT:

• Linear Variable Differential Transformer •

Consists of 3 coils – 1 primary coil – 2 secondary coils

• Central coil is primary and other 2 are identical secondarycoils which are connected in series, such a way their outputoppose each other

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Position sensors www.rejinpaul.com


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Position sensors

• Magnetic core is moved through the central tube as a result of displacement being monitored

• When there is an alternating voltage input to the primary coil,alternating e.m.f.s induced in secondary coils

• With the magnetic core central, the amount of magneticmaterial in each coil are the same, since their output oppose

each other the net result is zero output• When the core is displaced from the central position there is a

greater amount of magnetic core in one coil than the other,resulting in greater e.m.f. is induced in one coil than the other

•

Operating range is from ±2 to ±400 mm with non-linearityerrors of ±0.25%

• The free end of the rod may be spring loaded for contact withthe surface being monitored

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Position sensors

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Position sensors Pneumatic position sensors:

• Low pressure air allowed to escape through a port in front of

the sensor • This escaping air, in the absence of any close by object,

escapes and in doing so also reduces the pressure in nearbysensor output

•

If there is a close by object, the air cannot so readily escapesand this increases the sensor output port• Such sensors are usedfor the measurement

of displacements of fractions of mm in arange of about 3 - 12mm

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Position sensors Encoders:

• Gives a digital output as a result of linear or angular position –

Incremental encoders• Detects changes in rotation from some datum position

– Absolute encoders• Gives actual angular position

• A beam of light passes through slots in a disc and it is detected

by a suitable light sensor • When the disc is rotated, a pulsed output is produced by the

sensor with the number of pulses being proportional to theangle through which the disc rotates

• Thus, angular position of the shaft is determined by thenumber of pulses produced since some datum position

• The resolution is determined by the number of slots in the disc• If there are 60 slots in the disc, then resolution = 6 °

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Position sensors

Encoders:

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Resolver:•

Type of rotary electrical transformer used for measuringdegrees of rotation.• It is considered as an analog device• For position evaluation, resolver to digital converters are

commonly used• They convert sine and cosine signal into binary signal that can

be more easily be used by the controller

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http://en.wikipedia.org/wiki/File:ResolverPrinzip.pnghttp://en.wikipedia.org/wiki/File:ResolverPrinzip.png


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Position sensorsResolver Encoder

Angle measurement

Absolute resolution

Incremental resolution

Accuracy (arc minutes)

Electronic interface

Noise immunity

Output signal

Construction materials

Weight

Inertia

Longevity

Shock/vibration

Temperature

Contamination

Implementation

Interchangeability

Retrofit/upgrade

absolute

16 bits

N/A

4 to 40

R/D converter

sensitive

analog

robust

heavy

high

very high

rugged

+150 ºC

immune

complex

limited

fixed

absolute or incremental

13 bits

10,000 lines/revolution

.25 to 6

direct

best

analog or digital

fragile

lighter

low

high

limited

+100 ºC

vulnerable

simple

vast

open 13

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Range sensors www.rejinpaul.com


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Range sensorsTriangulation principle:• Laser is directed to the surface at an oblique angle• By means of camera the displacement of the bright laser lines

can be determined in the image• If the angle between the camera and laser and the line

displacement is known, the component height can be

calculated

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Range sensors

• Even differences of tenths of millimeters can be detected easilyin practice without special effort• In case of obtuse angle, the measuring accuracy is low, butlarge differences in height can be detected

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Structured lighting approach:• Used for measuring 3-D shape of an object using projected

light patterns and a camera system• Goal is to produce a depth edge of the real world scene• Projection of patterns consisting of many stripes at once• Applied in precision shape measurement for production

control(turbine blades), capturing environments for augmentedreality gaming

16Get useful study materials from www.rejinpaul.com



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Time of flight range finders:Time of flight:• Method that measures the time taken by the object, particle or

wave to travel through a particular medium

• Used for the measurement of distance• As time-of-flight measurements are preferentially used for

large distances, the beam quality of the laser source is crucial• Applied in safety sensors, single spot parking sensor, distance

measurement gauge• Very high photosensitivity allows the operating ranges up to

several meters and a accuracy down to a centimeter dependingon the lens and illumination power




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Laser range meter:• Uses laser beam to determine the distance of object• Operates on time of flight principle by sending a laser pulse in

a narrow beam towards the object and measuring the timetaken by the pulse to be reflected off the target and returned tothe sender

• Due to high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where other techniques are used


Proximity Sensors www.rejinpaul.com


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Proximity Sensors

• Able to detect the presence of nearby objects without any physical contact

• Proximity sensor may often emit an electromagnetic field or a beam of electromagnetic radiation and looks for changes in thefield or return signal

• Maximum distance that this sensor can detect is defined asnominal range

• They are highly reliable• Used in parking systems, vibration measurement systems,

conveyor systems




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y

Inductive and capacitive proximity sensors:• Operating principle is based on a high frequency oscillator that

creates a field in the close surroundings of the sensing surface• The presence of metallic and any material in the operating area

causes a change of oscillation amplitude• The rise or fall of such oscillation is identified by a threshold

circuit that changes the output state of sensor • Operating distance of the sensor depends on the actuator’s

shape and size and is strictly linked to the nature of material• Sensitivity regulation is useful in applications, such as

detection of full containers and non-detection of emptycontainers


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y Hall effect sensors: Hall effect:

•

When a beam of charged particles passes through a magneticfield, forces act on the particles and the beam is deflected fromthe straight line path

• A current flowing in the conductor is like a beam of movingcharges and thus can be deflected by a magnetic field

Working:• Consider electrons moving in a conductive plate with a

magnetic field at right angles to the plane of the plate• As a consequence of the magnetic field, the moving electrons

are deflected to one side of the plate and thus that side becomes negatively charged, while the opposite side becomes positively charged since the electrons are directed away from it

• This charge separation produces an electric field in thematerial




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• The charge separationcontinues until the forces onthe charged particles fromthe electric field just balancethe forces produced by themagnetic field




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Ultrasonic proximity sensors:• Evaluates the attributes of a target by interpreting the echoes

from radio or sound waves respectively• Converts energy into ultra sound or sound waves above the

normal range of human hearing

• Components are emitter and detector • Emitter continuously emits the light• When the target comes with in the operating range of the

sensor the light from the emitter is reflected off the target anddetected by the detector




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Optical proximity sensors:• Costs more than other types of proximity sensors• Widely used in automated systems• Commonly known as light beam sensors


Touch Sensors www.rejinpaul.com


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• Can be generated by robot’s arm movements• Enables the robot to predict the resulting sensor signals of its

internal motions• Measures force, contact time and repetition• Robots uses touch signals to map the profile of a surface in

hostile environment such as water pipe

Types:• Analog Sensors• Digital Sensors




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Analog Sensors:• Gives the change in electric property to signify the change in

environment• Sensor circuits are designed to monitor these changes and

provide a voltage difference




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Digital Sensors:• Output of this sensor can be either ON or OFF or it can be 1 or

0


Wrist Sensors www.rejinpaul.com


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• Several different forces exist at the point where a robot arm joins the end effectors

• Wrist force sensor can detect and measure these forces• Consists of specialized pressure sensors known as strain gauges• Strain gauges convert the wrist forces into electric signals,which go to the robot controller


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• Slip can be defined as relative movement of one object’ssurface over an other when in contact

• In an assembly operation, it is possible to test the occurrenceof slip to indicate some pre determined contact forces betweenthe object and assembled part

• To implement this grasping motion with the robot hand,

sensors have been proposed that detect an incipient slip with inthe contact surface or stick-slip


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UNIT 4Robot kinematics and Robot

Programming

Prepared by

S. Senthil Kumar AP/MECH

SVCET


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Robot Programming

• Teach Pendant Programming – Programmer’s pendant – Operator’s pendant

• Lead Through programming – Powered lead through – Manual lead through


Teach pendant Programming www.rejinpaul.com


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p g g• Serves as a primary point of control for initiating andmonitoring operations• Guides the robot while teaching the locationsTypes:• Programmer’s pendant• Operator’s pendant


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• Programmer’s pendant is designed for using while anapplication is being written and debugged

• Operator’s pendant is designed for use during normal systemoperation

• Operator pendant has a palm operated switch, which isconnected to the remote emergency stop circuitry of thecontroller

• Major areas of teach pendant are• Data entry buttons

– Used to input data normally in response to prompts that appear on the pendant display

• Emergency stop switch – Halts program execution and turns off power


Teach Pendant ProgrammingU LED

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• User LED: – When it is not lit, none of the predefined functions are being used – When it is lit application program is being in use

•

Mode control buttons: – When it is in manual mode, these buttons select which robot joint will

move, or the co-ordinate axis along which the robot will move• Manual state LED’s :

– Indicated the type of manual motion that has been selected•

Speed bars: – Used to control robot’s speed and direction

• Slow button: – Selects between two different speed ranges of speed bars

• Pre-defined function button: – display of co-ordinates, clear error etc.,

• Programmable function button: – Used in custom application programs

• Soft button: – Depends on application program being run, or the selection made from the

pre-defined function buttonsGet useful study materials from www.rejinpaul.com

Lead Through programming www.rejinpaul.com


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• Robot is moved through the desired motion path in order torecord the path into controller memory

•

Two types – Powered lead through – Manual lead through

Powered lead through:• Makes use of teach pendant to control various joint motors• Helps in driving the robot arm and wrist through series of

points in space• Each point is recorded in memory for subsequent playback

during work cycle• Largely limited to point to point control options, since it is

difficult to regulate complex geometric motions in space• Includes part transfer tasks, machine loading and unloading

and spot welding


Lead Through programming www.rejinpaul.com


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Manual lead through:• Also called as walk through method• Used for continuous path programming where the motion

cycle involves smooth complex curvilinear movements of arm• Most widely used application is Spray painting, where the gun

is attached at the end effector must execute a smooth, regular

motion pattern to pain in an even manner over the entiresurface to be coated• Continuous arc welding is another example• The programmer physically grasps the robot arm and manually

moves it through the desired motion cycle• If the robot is large, it will be difficult to moveTwo modes of control systems for both lead through procedures:1. teach mode : used to program the robot

2. Run mode : used to execute the programGet useful study materials from www.rejinpaul.com

Robot programming Languageswww.rejinpaul.com


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• Textual robot languages possess a variety of structures andcapabilities

Generation of programming languages:• First generation language• Second generation language• Future generation language First generation language:

• Uses a combination of command statements• Developed largely to implement motion control with a textual

programming language• Also called as ‘motion level’ languages• Typical features include

– Ability to define manipulator motions – Straight line interpolation – Branching –

Elementary sensor commands involving binary signalsGet useful study materials from www.rejinpaul.com



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• VAL programming is an example of first generation robot programming language

• Limitations include – Inability to specify complex arithmetic computations for use during

program execution – Inability to make use of complex sensors and sensor data – Limited capacity to communicate with other computers

Second generation languages:• Overcome many of the limitations of the first generation

languages• Enables the robot to accomplish more complex tasks• Also called as ‘structured’ programming languages• Commercially available languages include

– AML, RAIL, MCL, VAL-II• Programming in these languages is similar to computer

programming




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• One limitation is that computer programmer’s skills arerequired to accomplish the programming

• Features and capabilities are – Motion control

• Basically same as for the first generation languages – Advanced sensor capabilities

• Capacity to deal with more than simple binary signals like analog signals• Capacity to control devices by means of sensory data

– Limited intelligence• Ability to utilize information received about the work environment to modify

system behavior in a programmed manner – Communications and data processing

• Provisions for interacting with computers and computer data bases

• Sensors can be able to measure the forces• Limited intelligence is provided• Error recovery is enhanced




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Future generation languages:• Involves a concept called ‘work modeling’• Robot possesses a three dimensional world and it is capable of

developing its own step by step procedure to perform a task based on a stated objective of what is to be accomplished

• Two basic ingredients – First , is robot system has its control memory a 3-D model – This model includes the robot manipulator itself, the work table,

fixtures, tools etc., – At times the robot develops its own three 3-D model of work space –

Second, is the capacity of automatic self programming – In effect, the human programmer gives the system an objective, and the

system develops its own program of actions required to accomplish theobjective


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• Victor’s Assembly Language, named after Victor Scheinman in1979 by Unimation, Inc., for its PUMA robot series

•

The language is then upgraded to VAL II in 1984


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Unit VImplementation and Robot Economics

Prepared by

S. Senthil Kumar AP/MECHSVCET


Steps involved in implementation of robots are www.rejinpaul.com


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• Initial familiarization with the technology• Plant survey to identify potential applications• Selection of the application• Selection of the robot• Detailed economic analysis and capital authorization• Planning and engineering the installation• Installation


Robot implementation www.rejinpaul.com


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Initial familiarization with the technology:• It is important that the management should provide continuous

support for the implementation f new robot• It is important that the manager responsible for using the

system also be committed to its success• Another way to key success is to include production personnel

in the project• Without the acceptance of workers the problem of installing

and operating a robot cell in the plant becomes much moredifficult




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Plant survey to identify potential applications:• Hazardous or uncomfortable working conditions• Repetitive operations• Difficult handling jobs• Multi shift operationSelection of best application:• Operation is simple and repetitive• Cycle time of operation• Delivery of parts•

Part weight• Inspection requirement• Replacement of persons• Setups and changeovers are frequent




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Selection of robot:• Material transfer • Machine loading• Spot welding• Arc welding• Spray coating• Assembly

Detailed economic analysis and capital authorization:• The engineer must describe the project in terms of its

application features, required change to existing equipment,

new equipment that must be acquired, fixtures and tooling,anticipated production rates, effects on labor, potential problem areas




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Planning and engineering the installation:• Study the operation method• Design of robot workcell• Workcell control• Safety considerations designed into the cell• End effector design• Design of other tools and fixtures for the cell

Installation:• start up•

Debugging• Trial production runs• Fine tuning of workcell


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There are three occasions when humans are close enough tomachine to be exposed to danger

• During programming of robot• During operation of the robot cell when humans work in the

cell• During maintenance of robotWork design consideration for safety:


Safety Consideration for Robot Operationswww.rejinpaul.com


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Safety sensors and safety monitoring:• Level 1 – perimeter penetration detection• Level 2 – intruder detection inside workcell• Level 3 – intruder detection in the immediate vicinity of the

robotTraining:• Awareness• Justification• Application•

Operations and maintenance


Economic Analysis www.rejinpaul.com


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Basic data required:• Type of robot installation• Cost data required for analysis

– Direct cost assosiated with robot project• Investment cost• Operating cost and savings

Method of Economic Analysis:• Pay back method• Equivalent uniform annual cost method• Return on investment method

me2028 notes rejinpaul (1).pdf

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