This document provides information on advances in metrology using laser technology. It discusses the principles and working of lasers, including spontaneous and stimulated emission. It describes applications of lasers such as laser interferometers, laser scanning gauges, and laser telemetric systems. It also covers coordinate measuring machines, the different types of CMMs, and probes used in CMMs like trigger probes and measuring probes. The document presents information concisely on various laser-based measurement techniques.

1. Unit-3
Advances in Metrology

2. LASER
• The word LASER stands for Light Amplification by Stimulated
Emission of Radiation.
Properties of Laser
• Highly monochromatic
• Highly directional
• Coherent
• Highly powerful

3. Principle of LASER: Spontaneoues and Stimulated Emission
• According to the quantum mechanics, an electron within an atom can have
only certain values of energy, or energy levels. There are many energy levels
that an electron can occupy.
• If an electron in the excited state with the energy E2 it may spontaneously
decay to the ground state, with energy E1, releasing the difference in
energy between the two states as a photon. This process is
called spontaneous emission, producing fluorescent light. The phase and
direction of the photon in spontaneous emission are completely random due
to Uncertainty Principle.

4. • Conversely, a photon with a particular frequency would be absorbed by an
electron in the ground state. The electron remains in this excited state for a
period of time typically less than 10-6 second. Then it returns to the lower
state spontaneously by a photon. These common processes of absorption and
spontaneous emission cannot give rise to the amplification of light. The best
that can be achieved is that for every photon absorbed, another is emitted.
• Alternatively, if the excited-state atom is perturbed by the electric field of a
photon with frequency ω, it may release a second photon of the same
frequency, in phase with the first photon. The atom will again decay into the
ground state. This process is known as stimulated emission.

5. • Advantages of laser
• High precision and accurate measurements are possible
• Improvement in quality of manufacturing is ensured as absolute references
are used for measurements
• Fast data acquisition is possible without the potential distortion
• It has very minimum signal leakage.
• Laser instruments have few moving parts, hence it leads to less wear and tear.

6. AC LASER INTERFEROMETER (ACLI)
Components
• Two frequency zeeman laser:
• It is generally He-Ne type which generates the stable coherent light beams of two
frequencies; one is vertically polarized and the other is horizontally polarized relative
to the plane of the mounting.
• Beam splitters
• It divides the laser beam into two separate beams along different axes
• It is possible to adjust the splitted laser’s output intensity by having a choice of beam
splitter reflectiveness
• To avoid attenuation, it is essential that the beam splitters must be oriented so that
the reflected beam forms a right angle with the transmitted beam
• Beam benders
• These are flat mirrors with high reflectivity used to deflect the light beam around
corners on its path from the laser to each axis.

7. • Retro reflectors
• These are plane mirrors, roof prisms or cube corners. Cube corners are three
mutually perpendicular plane mirrors and in these devices the reflected
beam is always parallel to the incident beam
• Each ACLI transducer needs at least two retroreflectors.
• All measurements are made by sensing differential motion between
retroreflectors relative to an interferometer
• Laser head’s measurement receiver
• During measurement, the laser beam is directed through optics in the
measurement path and then returned to the laser head’s measurement
receiver which will detect part of the returning beam as doppler shifted
component
• Measurement display
• It contains a microcomputer to do necessary calculations and the computed
values are displayed

9. Working
• Two frequency zeeman laser generates light of two slightly different
frequencies with opposite circular polarisations.
• These beams get split up by beam splitter B1: one part travels towards B2
and from there to external cube corner where the displacement is to be
measured.
• Beam splitter B2 optically separates the frequency f1 which alone is sent to
the movable cube corner reflector. The second frequency f2 from B2 is sent
to a fixed reflector which then rejoins f1 at the beam splitter B2 to produce
alternate light and dark fringes
• Now if the movable reflector moves, then the returning beam frequency
will be doppler shifted slightly up or down by Δf1
• Thus the light beams moving towards photo detector P2 have frequencies
f2 and f1±Δf1 and p2 changes this frequencies into electric signal

10. • Photo detector P1 receives signal from beam splitter B1 and changes
the reference beam frequencies f1 and f2 into electric signal
• An AC amplifier A1 separates frequency difference signal f2-f1 and A2
separates frequency difference signal f2-(f1±Δf)
• The pulse converter extracts Δf, one cycle per half wave length of
motion. The up-down pulses from the pulse converter are counted
electronically and displayed in analog or digital form on the indicator.

11. • It uses single frequency circular polarized laser beam
• On reaching the polarising beam splitter, the beam splits into two components, one
being vertically polarized light and the transmitted beam being horizontally
polarized light.
• These two beams referred to as reference arm and measurement arm respectively
travel to their retroreflectors and are then reflected back towards the beam splitter.
• The recombined beam at beam splitter consists of two superimposed beams of
different polarization. These two beams being differently polarized do not interfere.
• The recombined beam then passes through a quarter wave plate which causes the
two beams to interfere with one another to produce a beam of plane polarized
light.
• The beam after quarter wave plate is split into three polarization sensitive
detectors. As the plane of polarized light spins each detector produces a sinusoidal
output waveform
DC LASER INTERFEROMETER

12. • The output of three detectors can be used to distinguish the direction of
movement and also the distance moved by the moving retroreflector
attached to the surface whose displacement is to be measured.

13. LASER APPLICATIONS
Laser Telemetric System
• Laser telemetric system is a non-
contact gauge that measures with a
collimated laser beam. It measure at
the rate of 150 scans per second.
• It basically consists of three
components, a transmitter, a receiver
and processor electronics.
• The transmitter module produces a
collimated parallel scanning laser
beam moving at a high, constant,
linear speed. The scanning beam
appears as a red line. The receiver
module collects and photoelectrically
senses the laser light transmitted past
the object being measured. The
processor electronics takes the
received signals to convert them to a
convenient form and displays the
dimension being gauged.

14. • The transmitter contains a low-power helium-neon gas laser and its power
supply, a specially designed collimating lens, a hysteresis synchronous
motor, a multi-faceted reflector prism, a synchronous pulse photodetector
and a protective replaceable window.
• The high speed of scanning permits on-line gauging and thus it is possible
to detect changes in dimensions when components are moving or a
continuous product such as in rolling process moving at very high speed.
• This system can also be applied on production machines and control them
with closed feedback loops.
• Since the output of this system is available in digital form, it can run a
process controller, limit alarms can be provided and output can be taken on
digital printer.
• It is possible to write programs for the microprocessor to take care of
smoke, dust and other airborne interference around the workpiece being
measured.

15. Laser Scanning gauge
• Metrology lasers are low-power instruments. Most
are helium-neon continuous-wave output lasers that
emit visible or infrared light.
• He-Ne lasers produce light at a wavelength of 6328 A
(0.6 pi) that is in phase, coherent, and a thousand
times more intense than any other monochromatic
source.
• Laser inspection systems enable measurement of a
part as it is produced, thus permitting 100% quality.
Laser systems have wide dynamic range and high
contrast.
• Lasers find applications in dimensional
measurements and surface inspection because of
the properties of laser light (bright, unidirectional,
collimated beam, with a high degree of temporal
and spatial coherence).

16. Working
• It basically utilizes a transmitter, receiver and processor electronics.
• A thin band of scanning laser light is made to pass through a linear
scanner lens to render it parallel beam.
• The object placed in a parallel beam, casts a time-dependent shadow.
Signals from the light entering the photo cell (receiver) are processed
by a microprocessor to provide display of the dimension represented
by the time difference between the shadow edges.
• It can provide results to an accuracy of + 0.25 um for 10-50 mm
diameter objects. It can be used for objects 0.05 mm to 450 mm
diameter and offers repeatability of 0.1 um.

17. • Laser Triangulation Sensors
• In this method, the laser hits the target, light reflected off the target is concentrated
through the receiving lens and is focused onto the light receiving element.
• If the distance from the sensor to the target changes, the angle of the reflected light
changes causing the position of the received light to change on the light receiving
element.
• This change is proportional to the movement amount of the target, as the distance
between each position on the light receiving element is able to determine
displacement.
Advantages
 A quick measurement of
deviations can be ensured.
 Can perform automatic
calculation on shell metal
stampings.
 Able to measure inside
diameter of bores if two
sensors are used.

18. • Gauging wide diameter
• Fig. shows a method of measuring the diameter of thin
wire using the interference fringes resulting from
diffraction of light by the wire
• A measure of the diameter can be obtained by moving
the photodetector until the output is restored to its
original value
• Fig shows the method of length measurement by fringe
counting
• The laser output illuminates three slits at a time in the
first plane which form interference fringes. The
movement of the interference fringes is determined by a
detector on the other side by moire grating
• The total number of slits in the first plane is governed by
the length over which measurement is required
• The spacing between the slits and distance of the slit to
the plane of the grating depend on the wavelength of the
light used
• This method is capable of accurate measurements over
relatively short distances of the order of 100mm

19. COORDINATE MEASURING MACHINES
• The term measuring machine generally refers to a single axis
measuring instrument. Such an instrument is capable of measuring
one linear dimension at a time
• The term coordinate measuring machine refers to the instrument that
is capable of measuring in all three orthogonal axes. Such a machine
is popularly abbreviated as CMM
• It simultaneously captures both dimensions and orthogonal
relationships. Another remarkable feature of CMM is its integration
with a computer. The computer provides additional power to
generate 3D objects as well as to carry out complex mathematical
calculations

20. • Its potential as a sophisticated measuring machine can be exploited
under the following conditions
• Multiple features: The more the number of features(dimensional and
geometric) that are to be controlled the greater the value of CMM
• Flexibility: it offers flexibility in measurement, without the necessity to use
accessories such as jigs and fixtures
• Automated inspection: whenever inspection needs to be carried out in a fully
automated environment, CMM can meet the requirements quite easily.
• High unit cost: If rework or scrapping is costly, the reduced risk resulting from
the use of CMM becomes a significant factor

21. • Structure of CMM
• The basic version of a CMM has three axes along three mutually
perpendicular directions. Thus the work volume is cuboidal
• Each axis is fitted with a precision measuring system which continuously
records the displacement of the carriage from a fixed reference.
• The third axis carries a probe
• When the probe makes contact with the workpiece the computer captures
the displacement of all the three axes.

23. TYPES OF CMM
• The five basic types of CMM are
• Cantilever
• Bridge
• Column
• Horizontal arm
• Gantry
• Cantilever type
• The vertically positioned probe is carried by a cantilever
arm.
• The probe moves up and down along the z axis whereas
the cantilever arm moves in and out along the y axis.
• The longitudingal movement is provided by the x axis,
which is basically the work table
• This configuration provides easy access to the workpiece
and a relatively large work volume for a small floor
space.

24. • Bridge type CMM
• A bridge type configuration is a good choice if
better rigidity in the structure is required.
• The probe is mounted on a bridge structure that is
moved relative to a stationary table on which the
part to be measured is positioned.
• This provides a more rigid structure than the
cantilever design, and its advocates claim that this
makes the moving bridge CMM more accurate.
• However, one of the problems encountered with
the moving bridge design is yawing (also known
as walking), in which the two legs of the bridge
move at slightly different speeds, resulting in
twisting of the bridge.
• It is well suited to the size range of parts
commonly encountered in production machine
shops.

25. • Column type CMM
• This configuration provides exceptional rigidity and
accuracy
• It is quite similar in construction to a jig boring
machine.
• Machines with such a configuration are often referred
to as universal measuring machine
• The x- and y-axis movements are achieved by moving
the worktable, while the probe quill is moved vertically
along a rigid column to achieve z-axis motion.

26. • Horizontal arm
• In this type of configuration, the probe is carried by
the horizontal axis. The probe assembly can also move
up and down along a vertical axis
• It is used for gauging large workpieces since it has a
larger work volume. It is often referred as layout
• To achieve x-axis motion, either the column is moved
horizontally past the worktable (called the moving ram
design), or the worktable is moved past the column
(called the moving table design).
• The cantilever design of the horizontal arm
configuration makes it less rigid and therefore less
accurate than other CMM structures.
• On the positive side, it allows good accessibility to the
work area.
• Large horizontal arm machines are suited to the
measurement of automobile bodies

27. • Gantry
• In this type, the support of the workpiece is
independent of the X and Y axis. Both these axes are
overhead and supported by four vertical columns
from the floor.
• This construction, is generally intended for inspecting
large objects.
• The probe (z-axis) moves relative to the horizontal
arm extending between the two rails of the gantry.
• The workspace in a large gantry type CMM can be as
great as 25 m (82 ft) in the x-direction by 8 m (26 ft) in
the y-direction by 6 m (20 ft) in the z-direction
• The operator can walk along the probe, which is
desirable for large workpieces

28. CMM – Modes of Operation
• Manual
• Semi automated
• Computer controlled
• Manual
• The manual CMM has a free floating probe that the operator moves along the machine’s three
axes to establish contact with part features
• The difference in the contact positions are the measurements
• Semi automatic
• A semi automatic machine is provided with an electronic digital display for measurement.
• Many functions such as setting the datum, change of sign and conversion of dimensions from
one unit to another are done electronically
• Computer controlled
• A computer controlled CMM has an onboard computer, which increases versatility, convenience
and reliability
• Such machines are quite similar to CNC machines in their control and operation
• Computer assistance is utilized for three major functions
• Programming software –directs the probe to data collection points
• Measurement commands- enable comparison of the distance traversed to the datum
• Computational capability-processing of data and generating results

29. Probes used in CMM
• Trigger type probe system
• The buckling mechanism is a three point bearing, the
contacts of which are arranged at 120O around the
circumference
• These contacts act as electrical micro switches.
When being touched in any probing direction one or
more contacts is lifted off and the current is broken,
thus generating a pulse.
• When the circuit is opened, the current co-ordinate
positions are read and stored.
• After probing, a prestressed spring ensures the
perfect zero position of the three point bearing.
• With this probe system data acquisition is always
dynamic and therefore the measuring time is shorter
than in static principle

30. • Measuring type probe system
• The buckling mechanism of this system consists of parallel guideways. At the moment
of probing the spring parallelograms are deflected from their initial position.
• As the entire system is free from torsion, play and friction, a defined parallel
displacement of probes as compared to their original arrangements can be measured.
• If the components of the CMM are assumed as rigid bodies, the position deviations of
the carriage can be described by three displacement deviations parallel to the axis (1),
(2) & (3) and three rotational deviations about the axes (4), (5) and (6)

31. Non contact probes
• Non-contact sensors are based on opto electronic techniques where no
stylus is used to detect the part surface
• Depending on the measuring task, they can be as much as three hundred
percent faster than contact systems. In addition to this, problems such as
stylus compensation, friction angles or changing contact forces are not
encountered by these systems.
• Laser probes
• A popular laser based device used on CMMs is the single spot laser
triangulation method
• This method uses a low powered laser beam for distance measurement. The
laser beam directed perpendicular to the part surface reflects on the surface
and it is received by a light sensitive detector at an angle of approx. 25O
• A change of standoff distance from the sensor to the part surface results in a
lateral shift of the detected light spot on the detector which is directly
related to the standoff distance

32. • Vision probes
• The features of the part surface are measured by computer count of the pixels of
the electronic image.
• A solid state camera consists of imaging optic and large number photosensitive
elements whose signals are accessed and stored in a computer
• For the photosensitive image sensor, a charge coupled device is usually
employed in this kind of camera. It contains matrix arrays of small and accurately
spaced photosensitive elements.
• When light passing through the camera optic, it strikes the array of each
photosensitive element which converts the portion of light falling on it into an
analog electrical signal
• The entire image is thus broken into an array of individual picture elements
called pixels
• In the image processor, the analog voltage values of each pixel are converted
into corresponding digital values which can be processed by a computer

33. Machine vision
Machine vision can be defined as the acquisition of image data of an object
of interest followed by processing and interpretation of data by a computer
program for useful applications
Machine vision system enables the identification and orientation of a work
part within the field of vision and has far reaching applications.
Stages of Machine Vision
• Image generation and digitization
• Image processing and analysis
• Image interpretation
• Generation of actual signals

35. Image generation and digitization
• The primary task in a vision system is to capture a 2D or 3D image of the
work part. A 2D image captures either the top view or a side elevation of
the work part
• While the 2D image is captured using a single camera, the 3D image
requires at least two cameras positioned at different locations
• The work part is placed on a flat surface and illuminated by suitable
lighting, which provides good contrast between the object and the
background
• The camera is focused on the work part and a sharp image is obtained.
The image comprises a matrix of discrete picture elements popularly
referred to as pixels
• The intensity value for each pixel is converted to its equivalent digital
value by an analog to digital converter. The digitized frame of the image
is referred to as the frame buffer

36. • The choice of camera and proper lighting of the scene are important to
obtain a sharp image having a good contrast with the background
• Two types of cameras are used in machine vision applications namely vidicon
cameras and solid state cameras
• Vidicon cameras are analog cameras quite similar to the ones used in
conventional television pictures. The scanning is done in a systematic
manner covering the entire area of the screen in a single scan
• Solid state cameras are more advanced and function in digital mode. The
image is focused on to a matrix of equally spaced photosensitive elements
called pixels
• An electric charge is generated in each element depending on the intensity
of light striking element. The charge is accumulated in a storage device. The
status of every pixel comprising either the grey scale or the colour code is
thus stored in a frame buffer.
• Solid state cameras have become more popular because they adopt more
rugged and sophisticated technology and generate much sharper images

37. • Image processing and analysis
• A number of techniques are available to analyse the image data. However, the
information available in the frame buffer needs to be refined and processed to facilitate
further analysis
• The most popular technique for image processing is called segmentation. It involves two
stages
• Thresholding
• Edge detection
• Thresholding converts each pixel value into either of the two values white or black
depending on whether the intensity of light exceeds a given threshold value
• If necessary, it is possible to store different shades of grey in an image called the grey
scale system
• If the computer has a higher main memory and a faster processor an individual pixel can
also store colour information

38. • Edge detection is performed to distinguish the image of the object from its
surroundings. Computer programs are used which identify the contrast in light
intensity between pixels bordering the image of the object and resolve the boundary
of the object
• In order to identify the work part, the pattern in the pixel matrix needs to be
compared with the templates of known object. Since the template is quite high, one
to one matching at the pixel level within a short time duration demands high
computing power and memory
• An easier solution to this problem is to resort to a technique known as feature
extraction. In this technique, an object is defined by means of its features such as
length, width, diameter, perimeter and aspect ratio.

39. • Image interpretation
• Once the features have been extracted, the task of identifying the object becomes
simpler, since the computer program has to match the extracted features with the
features of templates already stored in the memory. This matching task is popularly
referred as template matching
• Whenever a match occurs, an object can be identified and further analysis can be
carried out. This interpretation function that is used to recognize the object is known
as pattern recognition
• Many computer algorithms have been developed for template matching and pattern
recognition.
• In order to eliminate the possibility of wrong identification when two objects have
closely resembling features, feature weighting is resorted to. In this technique, several
features are combined into a single measure by assigning a weight to each feature
according to its relative importance in identifying the object. This adds an additional
dimension in the process of assigning scores to features and eliminate wrong
identification of an object.

40. • Generation of actual signals
• Once the object is identified, the vision system should direct the inspection station
to carry out the necessary action.
• In a flexible inspection environment, the work cell controller should generate the
actuation signals to the transfer machine to transfer the work part from matching
stations to the inspection station and vice versa
• Clamping, declamping, gripping etc., of the work parts are done through actuation
signals generated by the work cell controller