METROLOGY AND MEASUREMENTS UNIT III

3 MECH A & C

ASSIGNMENT DETAILS

35 & BELOW MARKS 1 TIME ALL DIAGRAM ( 1.5 UNIT)

30 & BELOW MARKS - 1 TIME ALL DIAGRAM ( 1.5 UNIT)

1 TIME WITHOUT CHOICE Q & A

25 & BELOW MARKS – 1 TIME ALL DIAGRAM ( 1.5 UNIT)

3 TIME WITHOUT CHOICE Q & A

UNIT III

ADVANCES IN METROLOGY

Basic concept of lasers Advantages of lasers – laser Interferometers – types – DC and AC Lasers interferometer – Applications – Straightness – Alignment. Basic concept of CMM – Types of CMM – Constructional features – Probes – Accessories – Software – Applications – Basic concepts of Machine Vision System – Element – Applications.

UNIT III

ADVANCES IN METROLOGY

Laser & Interferometers

CMM

Machine Vision System

LASER &

INTERFEROMETERS

LASER

LIGHT AMPLIFICATION BY STIMULATED

EMISSION OF RADIATION.

Why Laser instruments used ?

Laser Instruments are devices to produce

powerful, monochromatic collimated beam of

light in which the waves are coherent.

PRINCIPLE OF LASER

PRINCIPLE: When photon emitted during

stimulated emission has the same energy, phase &

frequency as the incident photon.

Photon comes in contact with another atom or

molecule in high energy level E2, then it will cause the

atom to return to ground state energy level E1 by

releasing another photon.

The sequence of triggered identical photon from

stimulated atom is known as stimulated emission.

PRINCIPLE OF LASER

This multiplication of photon through

stimulated emission leads to coherent,

powerful monochromatic, collimated beam

of light emission. This light emission is called

LASER.

BASIC CONCEPTS OF LIGHT

Distance between two trough

or crest are called

WAVELENGTH λ.

Time taken to travel one

wavelength λ is called time

period T.

Maximum disturbance of wave

is called amplitude (A).

Velocity of transmission is (

λ/T )

(1/T) is called Frequency.

LASER

Helium-Neon Laser source produces a 1 to 2 mm diameter

beam of pure red light having power of 1 MW. So, Very High

Intensity.

Laser is used for Interferometry.

Upto a great distance beam has no divergence , then it begins

expand at rate of about 1 mm/m.

Is visible & it can be observed easily. Its accuracy is of 1 mm

over 2 m.

Used for very accurate instruments in the order of 0.1

micron in 100m.

LASER METROLOGY

Commercially, It is projected onto position detector which is used for the method of alignment.

It is very convenient, collimated & high intensity source used, because for its Precision, Accuracy, No Contact and Hot moving parts.,

Laser diodes, Semiconductor lasers – More advantages, Lower cost.

Laser instruments – In surface Inspection & Dimensional Measurements.

LASER TELEMETRIC SYSTEM

Is a non contact gauge with a collimated laser

beam.

Measuring Rate – 150 Scans Per Second.

Three Components – Transmitter, Receiver,

Processor Electronics

LASER TELEMETRIC SYSTEM

TRANSMITTER – Produces a collimated parallel

scanning laser moving at high constant linear

speed. It appears as red line after scanning.

TRANSMITTER Components :

Low power helium neon gas laser.

Synchronous motor.

Collimating lens.

Reflector prism.

Synchronous pulse photo detector.

Replaceable Window.

SCHEMATIC DIAGRAM OF LASER

TELEMETRIC SYSTEM

ADVANTAGES

Possible to detect changes in dimensions when product

is in continuous processes.

There is no need to wait for measuring in hot

conditions.

Applied on production machines & controlled with

closed feedback loops.

Possible to write programs for microprocessor to take

care of smoke, dust & other airborne interference

around the work piece being measured.

LASER & LED BASED DISTANCE

MEASURING INSTRUMENTS

Can measure 1 to 2 m with accuracy of order of 0.1 to 1 % of measuring range.

It has two distance meter at equal distance on either side of object & control unit to measure thickness of object.

The angle at which the light enters the detector will change the distance between measuring head & object is changed.

Change in angle of deviation is measured & calibrated in terms of distance.

LASER & LED BASED DISTANCE MEASURING

INSTRUMENTS - ADVANTAGES

These types of instruments are very reliable

because there is no moving part.

Instrument response time is in milliseconds.

The output is provided

as 0 – 20 mA.

SCANNING LASER GAUGE

Used for Dimensional Measurements.

Has Transmitter, Receiver & Processor Electronics.

ADVANTAGES :

Accuracy of ±0.25 micron for 10-50mm diameter

objects.

It is used for objects of 0.05mm to 450mm diameter.

SCANNING LASER GAUGE

PHOTODIODE ARRAY IMAGING

Used for dimensional measurements.

Stationary part shadow is projected on a

solid state diode array image sensor.

Has four parts :

Laser source

Imaging optics

Photodiode array

Signal processor & Display unit

PHOTODIODE ARRAY IMAGING

ADVANTAGES

Used for large parts, two arrays.

Measurement accuracy is as high as

±0.05 micron.

DIFFRACTION PATTERN

TECHNIQUE

Used in dimensional measurements.

Parallel coherent laser beam is diffracted by

a small part & resultant pattern is focused by

lens on a linear diode array.

Distance between alternating light & dark

bands in diffraction pattern is direct function

of wire diameter, laser beam wavelength &

Lens focal length.

DIFFRACTION PATTERN

TECHNIQUE

ADVANTAGES

Used to measure small gaps & small

diameter parts.

Measurement accuracy is more for smaller

parts.

DISADVANTAGES

Not suitable for large diameters.

LASER TRIANGULATION SENSORS

For Dimensional

Measurements.

Quick Deviation

Measurement due to

Surface Change.

Perform Automatic

Calculation on Shell

Metal Stampings.

Can measure Bore

Diameter using 2 Sensor.

INTERFEROMETRY - BASICS

When light is made to interfere, it produces a pattern

of dark bands, which corresponds to a very accurate

scale of divisions.

A ray having a single frequency and wavelength is

produced generally known as monochromatic light.

This monochromatic light is used for measuring

flatness and determining the length of slip gauges.

It is the basic principle of interferometry.

LASER USES IN INTERFEROMETRY

To find accurate measurement of length.

Reduces time & Skill requirement like other

methods used for finding the length.

The Laser Measurement accuracy is order of

0.1µm in 100m.

In modified laser designs, a single frequency is

selected from the coherent beam and used for

interferometric measurement.

TWO FREQUENCY LASER

INTERFEROMETER

Two Frequency Laser Head

Beam Directing and Splitting Optics

Measurements optics

Receivers

Wavelength Compensators

Electronic Receivers

TWO FREQUENCY LASER

INTERFEROMETER

TWO FREQUENCY LASER

INTERFEROMETER - ADVANTAGES

For Measuring Linear Positioning, Straightness

in two planes.

High Sensitivity.

Free from Noise Disturbances ( Light, Electrical

Noise, Air turbulence).

Use of Single photo detector / Measurement

axis, Sensitivity to Optical Alignment is Less.

GAUGING WIRE DIAMETER FROM

DIFFRATION PATTERN USING LASER

LENGTH MEASUREMENT USING

FRINGE COUNTING

Incoherent Laser Beam illuminates three slits at

a time in the first plane which form Inference

fringes.

Fringe movement is determined by detector on

the other side of short length of grating.

The number of slits in the first plane is governed

by length over which measurement is required.

LENGTH MEASUREMENT

USING FRINGE COUNTING

LENGTH MEASUREMENT

USING FRINGE COUNTING

Using He-Ne laser at 0.63µm a fringe

spacing of 1µm will be obtained at 1.4µm

from the slits, if a separation of 1µm is used.

The spacing between the slits & distance of

the slit to the plane of grating depend on the

wavelength of the light used.

ADVANTAGES

Accurate measurements over relatively

short distances.

Wire diameters from 0.005 to 0.2mm

can be measured.

LASER INTERFEROMETRY

COMPONENTS

Two frequency laser source.

Optical elements.

Laser heads measurement receiver.

Measurement display.

TWO FREQUENCY LASER SOURCE

Generally, It becomes He-Ne type that generates

stable coherent light beams of two frequencies.

One is Polarized vertically and other one is

polarized horizontally relative to the plane of the

moving feet.

Laser slightly oscillates at two frequencies by a

cylindrical permanent magnet around the cavity.

TWO FREQUENCY LASER SOURCE

Beam containing both frequencies passes

through a quarter wave & half wave plates

which change the circular polarization, or

vertical and other horizontal.

Linearly polarized beam is expanded in a

collimating telescope, after which most of the

beam is transmitted through a 45 degree beam

splitter and one of the laser head.

OPTICAL ELEMENTS

The various optical elements are :

Beam Splitters

Beam Benders.

Retro reflectors.

BEAM SPLITTERS

Used to divide the laser beam into separate

beams along different axes.

Possible to adjust the splitted laser’s output

intensity by having a choice of beam splitter

reflectivities.

BEAM SPLITTERS USAGE ( DIVIDING EQUAL LASER OUTPUT )

BEAM BENDERS

Used to deflect the light beam around

corners on it path from the laser to each axis.

Beam benders are just flat mirrors, but having

absolutely flat and very high reflectivity.

Normally, the 90 degree beam deflection is

avoided for not to disturb the polarizing

vectors.

RETRO REFLECTORS

They are plane mirrors, roof prisms or cube

corners.

Cube corners are three mutually perpendicular

plane mirrors, and reflected beam is always

parallel to the incident beam in these devices.

In AC laser interferometer measurements, two

retro reflectors are used.

When plane mirror is used as retro reflectors in

plane mirror interferometer, it must be flat with

in 0.06 micron / cm.

LASER HEAD’S MEASUREMENT

RECEIVER

Used to detect the part of the returning beam

as F1 – F2 and a Doppler shifted frequency

component del f.

MEASUREMENT DISPLAY

Has microcomputer to compute & display results.

Signals from reference receiver & measurement

receiver located in the laser head are counted in two

separate pulse counters and subtracted.

Other input signals for correction are temperature

co-efficient of expansions.

Air velocity is also displayed.

AC LASER INTERFEROMETER

TYPES

Standard Interferometer

Single Beam Interferometer

STANDARD INTERFEROMETER

STANDARD INTERFEROMETER

The displacement is measured between the

interferometer & cube corner.

Measurement retro reflector for this

interferometer is a cube corner.

It is least expensive and can be used wherever

it is possible.

SINGLE BEAM INTERFEROMETER

SINGLE BEAM INTERFEROMETER

• As same as standard interferometer.

• Has outgoing and returning beam super

imposed on each other, giving the appearance if

only one beam travelling between the

interferometer and the retro reflector.

• Very much suitable when space for optics and

beam path is minimum.

LASER INTERFEROMETER

Uses AC Laser as light source.

Enable to measure over longer distance.

Laser – Monochromatic optical energy, (

Collimated into a directional beam ) – Exact

wavelength – Pure for highly accurate

measurements.

Utilize the principles of both optical

techniques & digital electronics.

AC LASER INTERFEROMETER

COMPONENTS – LASER

INTERFEROMETER

Two frequency zeeman laser.

Beam splitters.

Fixed internal cube corners.

External cube corners.

Photo detectors.

Amplifiers.

Pulse converter.

COMPONENTS – LASER

INTERFEROMETER

Two Frequency Zeeman Laser :He-Ne type

Beam Splitters : Adjust Output based on Reflectivities

Cube corners : Fixed & Movable external –Used to reflect

the laser beam

Photo detectors : To change beam splitters signal to electrical

signal

Amplifiers : Used to separate frequency difference.

Pulse converter : Used to extracts change in frequency.

MICHELSON INTERFEROMETER

Consists of monochromatic light source, a beam

splitter & two mirrors.

PRINCIPLE : Constructive & Destructive

Interference when one mirror remains fixed and

the other is moved.

Monochromatic Light from an extended source

falls on a beam splitter which splits the rays into

two equal rays of same intensity at right angles.


If both mirrors are kept at same distance from

beam splitter, the observer will see bright spot

due to constructive interference.

If Movable mirror M1, shifted by quarter ( half )

wavelength, then the beam will be returned to

observer 180 deg out of phase ( 360 deg in phase )

and darkness will observed due to destructive

interference.


IMPROVING CONDITIONS

Use of laser light for measuring longer distance.

Instead of using mirror the cube corner

reflector is suited for reflecting the light.

Photocells can be employed to convert light

intensity variation in voltage pulses to given

direction of position change.

DUAL FREQUENCY LASER

INTERFEROMETER

Used to measure displacement, high

precision measurement of lengths, angles and

refractive indices as well as derived static

and dynamic quantities.


INTERFEROMETER - CONSTRUCTION


INTERFEROMETER - WORKING

Laser source generates two frequencies. F1 & F2

Reference signal F1-F2 is detected in Photo detector

P2.

Measuring signal F1-F2 +- Del F for X(Y) direction by

photo detector P3(P4).

Environment correction is made by photo detector P1.


INTERFEROMETER


INTERFEROMETER -

APPLICATIONS

Used for both incremental displacement and

angle measurements.

Resolution 2nm in 10m measuring range.

TWYMAN – GREEN

INTERFEROMETER

Used as polarizing interferometer with variable

amplitude balancing between sample and reference

waves.

For exact test surface measurement, the instrument

error can be determined by absolute measurement.

This error is compensated by storing the same in

microprocessor system & subtracting from the test

surface measurement.

TWYMAN – GREEN

INTERFEROMETER ADVANTAGES

Permits testing of surface with wide varying

reflectivity.

Avoids undesirable feed back of light reflected

of the tested surface & instrument optics.

Enables utilization of the maximum available

energy.

Polarisation permits phase variation to be

effected with the necessary precision.

LASER INTERFEROMETER APPLICATIONS

LINEAR MEASUREMENT

LASER INTERFEROMETER APPLICATIONS

ANGULAR MEASUREMENT

MACHINE TOOL METROLOGY

Component parts accuracy depends on machine tool

accuracy.Parts quality depends on,

Rigidity & Stiffness of machine tool & its components.

Alignment of various components in relation to one

another.

Quality & Accuracy of the control devices and the

driving mechanism.

ALIGNMENT ACCURACY OF

MACHINE TOOL CHECKED BY

Geometrical Test :

Dimensions, Position and Displacement of

Component relative to one another are checked.

Static Test

Dynamic Test

Practical Test :

Test pieces are machined in machines. Test

pieces must be approximate to fundamental

purpose for which machine has been designed.

PURPOSE OF MACHINE TOOL

TESTING

Dimensions of any work piece, its surface &

geometry depend on machine tool accuracy

for its manufacture.

In mass production, various components

produced should be of high accuracy &

assemble to be on non-sensitive basis.

GEOMETRICAL CHECKS ON MACHINE

TOOL Straightness.

Flatness.

Parallelism, Equidistance & Coincidence.

Squareness of straight line & plane.

Rotations

Out of round.

Eccentricity.

Run out.

Periodical axial slip.

Camming.

GEOMETRICAL CHECKS ON

MACHINE TOOL

Movement of all the working components.

Spindle test for

Concentricity.

Axial slip.

Accuracy of axis & position.

LASER EQUIPMENT FOR ALIGNMENT

TESTING

Particularly suitable in aircraft production , shipbuilding etc., ( Flatness testing, Squareness checking etc., )

Consist of Laser of 10mm dia, Auto reflector.,

Laser has Helium-Neon Plasma Tube in heavy aluminum cylindrical housing.

Laser beam comes from its centre & parallel to housing within 10” of arc & alignment stability is of order of 0.2” of arc/hour.

LASER EQUIPMENT FOR ALIGNMENT

TESTING

Auto reflector consists of detector head & read

out unit.

No. of photocell are arranged to compare laser

beam in each half horizontally & vertically.

Detect the alignment of flat surfaces

perpendicular to a reference line of slight.

MACHINE TOOLS TEST

Test for level of installation of machine tool in

horizontal & vertical planes.

Tests for perpendicularity of guide ways to other

guide ways.

Test for flatness of machine bed & for

straightness & parallelism of bed ways on bearing

surface.

Test for true running of main spindle & its axis

movements.

MACHINE TOOLS TESTS

Test for line of movement of various members

like spindle, tables & cross slides.

Test for parallelism of spindle axis to guide

ways or bearing surfaces.

ALIGNMENT TESTS ON LATHE

Levelling of the machine.

True running of locating cylinder of main spindle.

Axial slip of main spindle & true running of

shoulder face of spindle nose.

True running of headstock center.

Parallelism of main spindle to saddle movement.

True running of taper socket in main spindle.

ALIGNMENT TESTS ON LATHE

Movement of upper slide parallel with main spindle

in vertical plane.

Parallelism of tailstock guide ways with the

movement of carriage.

Parallelism of tailstock sleeve taper socket to saddle

movement.

Pitch accuracy of lead screw.

Alignment of lead screw bearings with respect to

each other.

LEVELLING OF THE MACHINE

Done by sensitive spirit level.

Saddle is approximately kept in centre of the

bed support feet.

TRUE RUNNING OF LOCATING CYLINDER

OF MAIN SPINDLE

• Locating cylinder is provided to locate the chuck / face plate.

• Locating surface should be in cylindrical shape.

• Dial indicator is fixed to carriage & indicator feeler touches locating surface.

• Surface is rotated on its axis & indicator should not show any spindle movement.

ALIGNMENT TESTS ON MILLING

MACHINE

Cutter spindle axial slip or float.

Eccentricity of external diameter.

True running if internal taper.

Surface parallel with longitudinal movement.

Traverse movement parallel with spindle axis.

Centre T-slot Square with the arbor.

Tests on column.

Over arm parallel with spindle.

ALIGNMENT TESTS ON PILLAR

TYPE DRILLING MACHINE

Flatness of clamping surface of base.

Flatness of clamping surface of table.

Perpendicularity of drill head guide to the base plate.

Perpendicularity of drill head guide with plate.

True running of spindle taper.

Parallelism of the spindle axis with its vertical

movement.

Squareness of clamping surface of table to its axis.

Total deflection.

ACCEPTANCE TESTS FOR

SURFACE GRINDERS

Table top parallel to its movement by attaching the dial gauge in the stationary spindle and dial feeler touching the table top and traversing the table.

Spindle axis parallel to table top by fitting dial gauge on redial arm of the spindle & its feeler touching the square testing on table.

Slots parallel to table movement by fitting the dial gauge in the stationary spindle & dial feeler touching the slot & traversing the table.

ACCEPTANCE TESTS FOR

SURFACE GRINDERS

Vertical movement of the spindle square with

table top by fitting dial gauge in spindle & its

feeler touching the square resting on table.

Spindle axis square with the slot by fitting dial

gauge on an arm fitted to spindle & rotating

the spindle & rotating the spindle by 180 deg.

Practical tests.

CO ORDINATE

MEASURING MACHINE

CONTENTS

INTRODUCTION

TYPES OF MEASURING MACHINE

CMM AND TYPES OF CMM

PROBE

FEATURES

APPLICATION

ADVANTAGES

DISADVANTAGES

INTRODUCTION

Measuring machines are used for measurement

length over the outer surface of the length bar or

any long member.

The member may be either rounded or flat or parallel.

Measuring are more useful and advantages that

vernier calipers, micrometer etc..,

TYPES OF MEASURING MACHINE

Length bar measuring machine

New all measuring machine (spirit level)

Universal measuring machine (all)

Co-ordinate measuring machine

Computer controlled co-ordinate measuring machine

(inspect & measure)

CO ORDINATE MEASURING

MACHINE

Co-ordinate measuring machine is also called as CMM.

CMM is used for contact inspection the parts. When it is used for

computer integrated manufacturing, the CMM by computer

numerical control method.

A coordinate measuring Machine is a 3D device For measuring the

Physical geometrical Characteristics of an object.

CMM has movements in three axis .

This machine may be manually controlled by an operator or it may

be computer controlled.

BLOCK DIAGRAM OF THE

ELEMENTS OF A CMM

CO-ORDINATE

MEASURING

MACHINE

DIGITAL

READ OUT COMPUTER

SOFTWARE PROBING

SYSTEM

ELEMENTS OF CMM

A basic Co-ordinate Measuring Machine consists of four

elements.

The machine structure which is basically an X-Y-Z

positioning device.

The Probing system use to collect raw data on the part and

provide input to the control system.

Machine Control and Computer hardware.

The Software for three dimensional analysis.

CMM – BASIC PARTS

Description The typical 3 "bridge" CMM is composed of three axes, an X, Y and Z. These axes are orthogonal to each other in a typical three dimensional coordinate system. Each axis has a scale system that indicates the location of that axis. The machine will read the input from the touch probe, as directed by the operator or programmer. The machine then uses the X,Y,Z coordinates of each of these points to determine size and position with micrometer precision typically. Reference

FEATRUES OF CNC - CMM

According to control system

Manual CMM

CNC CMM or DCC CMM

According to design of main structure

CANTILEVER

MOVING BRIDGE

FIXED BRIDGE

HORIZONTAL ARM

GANTRY

COLUMN

According to mounting style

Bench top

Free Standing

Portable and Hand Held

TYPES OF CMM

Cantilever type

• A vertical probe moves in the z-axis • Carried by a cantilevered arm that moves in

the y-axis • This arm also moves laterally through the x-

axis • Two configuration: fixed table and movable

table

• Advantage- a fixed table allows good accessibility to the work piece

• Required small floor space

• Disadvantage- the bending caused by the cantilever design

• Suitable for measuring long, thin part

Moving bridge type

• Most widely used

• Has stationary table to support work piece to be measured and a moving bridge

• Stationary table and movable bridge: Allows high table load

• Advantage- reduce bending effect

• Disadvantage- with this design, the phenomenon of yawing can occur- affect the accuracy

Fixed bridge type

• Most widely used CMM

• In the fixed bridge configuration, provides better accuracy and rigidity.

• Disadvantage : Limited accessibility caused by the bridge

• Stationary bridge and movable table Leads to small measuring uncecertainty

Column type

• Often referred to as universal measuring machine instead of CMM

• The column type CMM construction provides exceptional rigidity and accuracy

• These machines are usually reserved for gauge rooms rather than production floor.

Horizontal arm type

• Have very large measuring range and low power consumption.

• Maximum accessibility has been achieved • Also referred to as layout machine

• Type of CMM: Moving arm and Moving Table

• Advantage- provides a large area, unobstructed

work area

• Ideal configuration for measurement of automobile parts

Gantry type

• supported by four vertical columns rising from the floor

• Employed three movable component moving along mutually perpendicular giudeways

• This setup allows you to walk along the work piece with the probe, which is helpful for extremely large pieces

• Used in measuring car body and wind tunnel Model

Gantry configuration with dual linear motor drives, laser scales an online compensation

According to mounting style

Bench top

Mounted on a bench or deck. Most CMM are this type

Free Standing

These machines can support themselves and do not required mounting

Portable and Hand Held

Moved freely and used almost anywhere

ADVANTAGES OF CMM

Flexibility

Reduced Setup Time

Single Setup

Improved Accuracy

Reduced Operator Influence

Improved Productivity

DISADVANTAGES OF CMM

The probe may not be in perfect alignment.

The probe may have run out.

The probe may moving in Z-axis may have

some perpendicular error.

There may have errors in digital system.

Common Applications

• Dimensional measurement

• Profile measurement

• Angularity or orientation

• Depth mapping

• Digitizing or imaging

• Shaft measurement

FEATURES CMM SOFTWARE

Measurement of the diameter, center

distance, length

Measurement of plane and spatial curves.

Minimum CNC program

Data communication

Digital input and output command

Interface CAD software

PROBE or Sensors

The measuring head that contact the job to

inspect and measure is called as probe.

The different kinds of probes like taper tip, ball

tip etc..,

The function of a CMM is to acquire information

about the measurand, usually in the form of

Cartesian coordinates. The devices used to explore

the area to generate this information are the

"sensors".

126

PROBE or Sensors • Two types of sensors:

“Tactile” sensors.

“Noncontact” sensors or "optoelectronic" sensors

• Tactile sensors distinguish:

“Rigid” or hard or fixed sensors .

“Point to point” or touch trigger sensors.

“Continuous” or measuring type or displacement sensors

• In non contact sensors ,Two main principles are used:

Triangulation 3D

Time of Flight

127

Sensors Tactile Sensor

Characteristic elements of a generic, tactile, fixed probing system

1. Co-ordinate measuring

machine ram (or spindle).

2. Probe extension.

3. Probe changing system.

4. Probe.

5. Stylus changing system

6. Stylus extension

7. Stylus shaft.

8. Stylus.

9. Stylus tip.

10. Tip diameter.

11. Generic, fixed probing system.

12. Stylus system (composed of

stylus system components).

128

Sensors The Rigid Sensor

Point measurement by means of a spherical rigid sensors

• Rigid sensor: “Touch finger” constituted by a shaft for the fitting of the touch

finger in the CMM probe holder.

• On the opposite extremity has either a sphere or a cone.

129

Sensors Point to Point Tactile Sensors

• The point to point type of measurement determines the position of points by

bringing in physical contact the tip of the sensors with the part and then

retracting.

• The point to point measurement represents a fast ideal solution to define

Dimension and Position.

Probing (point measurement) sequences

Legend:

1......: Starting point.

2......: Point probing.

3......: Disengagement

after point probing

130

Point to Point Tactile Sensors • Tactile sensors for point to point measurement are based on “Isostatic

support” concept.

• The spring still keeps the contact closed. the force of contact balances the force of the spring itself.

• Just before that the balance between the forces is reached, the force of contact between part and sensor causes a slight displacement of the stylus and the contact is opened.

• A signal is generated that allows the recording of the co-ordinates of the sphere centre at the moment of contacting the part surface. The point has been measured.

Isostatic support tactile sensor

The CMM starts to decelerate, while the spring preloaded mechanism follows the machine movement with a slight over-travel. Finally the machine retracts and the Isostatic support assumes its resting position in an extremely repeatable mode, ready to measure more points.

131

Sensors Continuous Tactile Sensors

Probing sequence in continuous measurement

• In the point to point measurement the sensor gets in contact with the part to be measured.

• In continuous measurement the sensor remains in contact with the part

• follows its profile and measuring points according to pre-determined laws in a single measuring path.

• Very accurate and relatively larger than point to point, the continuous sensors, can supply very complete information on the form of the measured feature.

132

Sensors Non Contact Sensors

• The non-contact sensors are based on optoelectronic techniques. These

types of sensors can measure an object without physical contact with it.

• the non-contact measurement technology allows:

To measure very small pieces not otherwise measurable.

For a faster measurement than a touch sensor.

To measure very soft pieces of material not otherwise measurable.

• Noncontact sensors are based on two principles

3D Triangulation Principle

Time of Flight Principle

133

Sensors Non contact sensor:3D Triangulation Principle

• It is a principle based on noncontact sensor.

• When object point C is lit by the laser, the

point becomes visible to the camera. Distance d between the laser source and the camera .

• the orientation a of the laser are known.

• line CB is determined by connecting the lens center to the image of point C in the image plane and finding its angle with line AB.

• In triangle ABC, by knowing d and angles a and b the position of point C on the object can be determined.

• Increasing angle g will increase depth accuracy, but that will increase self-occlusion, making determination of depth impossible.

The triangulation principle.

134

Continued… • Angle gamma determines the quantization accuracy of the scanner.

• When this angle is zero, objects at different depths cannot be distinguished from each other.

135

Sensors Non contact sensor: Time of Flight Principle

• The Time of Flight scanner is used to scan and investigate with laser light.

• The scanner rangefinder determines the distance from work surface by noting down the timing of round trip of the light pulse.

• The laser when emits light pulse detector has timed the reflected light.

• If c is known speed of light. If round-trip time t is known, then distance is (c* t)/2.

•

line of reference shown applications

MACHINE VISION

SYSTEM

MACHINE VISION SYSTEMS

Machine Vision::Computer Vision::Intelligent Vision

Automated Inspection of Manufactured products for Quality

and Process Control.

Definition:

Means of electro-optically simulating the image

recognition capabilities of the human eye/brain.

PRINCIPLE OF WORKING

Image Formation

Processing of image

Defining and Analysis of Image

Image interpretation and Decision Making

OVERVIEW OF MACHINE VISION PROCESS

FUNCTIONING OF

MACHINE VISION

SYSTEM

STEPS IN MACHINE VISION

SYSTEM Image Formation

Back Lighting

Diffused Front Lighting

Processing of image

Windowing

Image restoration

Defining and Analysis of Image

Distance measurement

Object orientation

Defining Object position

Image interpretation and Decision Making

CCD CAMERA

A CHARGE-COUPLED DEVICE (CCD) is a

sensor for recording images, consisting of an

integrated circuit containing an array capacitors.

MACHINE VISION SYSTEM

APPLICATION FIELDS

Inspection

Part Identification

Guidance and Control

HUMAN VS MACHINE VISION

HUMAN VISION

Recognition

Hand-eye coordination

Inspection

Find Position

Gather Information

Safety

MACHINE VISION

Identification

Robot guidance

Inspection

Find position

Measure

HUMAN VS MACHINE VISION

HUMAN VISION

Human Vision

High image resolution

Interprets complex scene

quickly

Operates in visible light

spectrum

MACHINE VISION

Machine Vision

Consistent, tireless

Operate in visible Infrared,

X-ray.

Operates in hostile

environments

Follows program precisely

MACHINE VISION APPLICATIONS

IN-LINE

Checking each item that passes the camera

RANDOM

Checking one item out of a series as it passes the camera on

the production line, and

OFF-LINE

Take an item off the production line for a test in a quality

assurance laboratory.

WHERE IS MACHINE VISION USED?

Semiconductor

Electronic

Automotive

Container

Pharmaceutical

Medical device

Plastic

Chemicals

Food

Footwear

Textiles

Printing

Wood / Forest

Fabricated Metal

WHY USE MACHINE VISION?

High speed production lines

Microscopic inspection

Clean room environments

Hazardous environments

Closed -loop process control

Robot guidance

Precise non-contact measurement

WHAT CAN MACHINE VISION DO?

PRESENCE - ABSENCE CHECKING,

ASSEMBLY VERIFICATION

Verify that part components are present and in the correct

locations

Dimensional Gauging

Calculate the distance between two or more points on an object

Defect detection

Identify defects and calculate defect characteristics such as position and size

ASSEMBLY VERIFICATION

DEFECT DETECTION

Golden Template Image Image with Defects Difference Image

GAUGING

ADVANTAGES

Lower inventories.

Fewer production overruns.

Reduced labor costs.

Less rework and production.

Avoidance of inspection bottlenecks.

Elimination of adding value to scrap conditions

ELIMINATES

Cost of Recruiting and Training.

Scrap - Rework created while learning a new job.

Average workers' compensation paid for injuries.

Average Educational grant per employee.

Personnel - Payroll department costs per employee

RESULT

Increased Market share.

Improved Customer satisfaction and lower warranty costs.

Penetration of New Niche Markets.

Pricing flexibility and participation in price Elastic markets.

METROLOGY AND MEASUREMENTS UNIT III

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Transcript of METROLOGY AND MEASUREMENTS UNIT III