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Visual Inspection

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Title: Visual Inspection


1
Visual Inspection
2
Outline
  • Introduction
  • Basic principles
  • Manual Vision Inspection
  • Human Vision
  • Common Inspection applications
  • Equipment
  • Automated or Machine Vision Inspection
  • Machine Vision
  • Common Inspection Applications
  • Equipment
  • Advantages and Limitations
  • Glossary of terms

3
Introduction
  • Visual inspection is commonly defined as the
    examination of a material, component, or product
    for conditions of nonconformance using light and
    the eyes, alone or in conjunction with various
    aids.
  • Visual inspection often also involves, shaking,
    listening, feeling, and sometimes even smelling
    the component being inspected.
  • Visual inspection consists of at least two major
    processes.
  • The first is a search process.
  • The second is a process of combining relevant
    knowledge, sensory input, and pertinent logical
    processes to provide an identification that some
    anomaly or pattern represents a flaw that poses a
    risk to the performance of the part.
  • Visual inspection is commonly employed to support
    other NDT methods.
  • Digital detectors and computer technology have
    made it possible to automate some visual
    inspections. This is known as machine vision
    inspection.

4
Introduction
  • Visual inspection is the most basic and most
    commonly employed NDT method.
  • It is applicable to a wide variety of material
    types and product forms.
  • Several characteristics about the part being
    examined may be determined, which include
    dimensional conformance, the presence of
    discontinuities, general fit and wear, and simple
    cosmetic compliance.
  • It can be performed by direct or indirect methods
    during various stages of manufacturing or after
    the component has been placed in-service.

5
Introduction
  • The quality of an inspection are affected
    primarily by four factors.
  • The quality of the detector (eye or camera).
  • The lighting conditions.
  • The capability to process the visual data.
  • The level of training and attention to detail.

6
Introduction Manual Versus Automated Inspection
  • The majority of visual inspections are completed
    by an inspector, but machine vision is becoming
    more common.
  • The primary advantage of an inspector is their
    ability to quickly adapt to a variety of lighting
    and other non-typical conditions, and their
    ability to use other senses.
  • The primary advantage of a machine vision
    inspection system is their ability to make very
    consistent and rapid inspections of specific
    details of a component.
  • Machine vision is primarily used in production
    applications where a large number of components
    require inspection and the inspection conditions
    can be closely controlled.

7
Basic Principles The Human Eye
  • Light enters the eye through the pupil and an
    image is projected on the retina.
  • Muscles move the eyeball in the orbits and allow
    you to focus the image on the central retina or
    fovea.

8
Basic Principles The Human Eye
The retina is a mosaic of two basic types of
photoreceptors, rods and cones.
  • Rods are sensitive to blue-green light and are
    used for vision under dark or dim conditions.
  • Cones operate only in relatively bright light,
    but they provide us with our sharpest images and
    enable us to see colors. There are three types
    of cones
  • L-cones are red absorbing cones or those that
    absorb best at the relatively long wavelengths
    peaking at 565 nm
  • M-cones are green absorbing cones with a peak
    absorption at 535 nm
  • S-cones are blue absorbing cones with a peak
    absorption at 440 nm.

9
Basic Principles Visual Acuity
  • Cones provide us with our sharpest images because
    most of the 3 million cones in each retina are
    confined to a small region just opposite the lens
    called the fovea. The maximum concentration is
    about 180,000 cones per square mm.
  • Our sharpest and most colorful images are
    produced in the fovea.
  • Outside of this region our vision is relatively
    poor but, since we can quickly redirect our eyes
    we tend not to be aware of our poor peripheral
    vision.

10
Basic Principles Visual Acuity
There is a limit to what the unaided eye can see.
  • Normal visual acuity or 20/20 vision is defined
    as the ability to resolve a spatial pattern
    separated by a visual angle of one minute or 1/60
    of a degree of arc.
  • One degree of a scene is projected on about 290
    micrometers of the retina.
  • In 290 micrometers there are 123 cones and in
    1/60 of a degree there 2 cones which is the
    number required to resolve an object.
  • The size of an object that can be seen at a given
    distance can be calculated using the following
    formula X (d tan q/2)2

When visually inspecting an object for a defect,
a comfortable viewing distance d might be
around 12 inches. At 12 inches, the normal
visual acuity of the human eye is 0.0035 inch.
(It must be noted that this value is for the
situation where there is good lighting and high
contrast between the objects being viewed.)
11
Basic Principles Contrast Sensitivity
  • Contrast sensitivity is a measure of how faded or
    washed out an object can be before it becomes
    indistinguishable from a uniform field
  • It has been experimentally determined that the
    minimum discernible difference in gray scale
    level that the eye can detect is about 2 of full
    brightness
  • Contrast sensitivity varies with
  • the size or spatial frequency of a feature
  • The lighting conditions
  • Whether the object is lighteror darker than the
    background

The graph to the right plots thevisibility of a
spot as a function of theabove variables
12
Basic Principles Contrast Sensitivity
It should be noted, however, that larger objects
are not always easier to see than smaller objects
as contrast is reduced.
  • In this image
  • The luminance of pixels is varied sinusoidally
    in the horizontal direction. The spatial
    frequency increases exponentially from left to
    right.
  • The contrast also varies logarithmically from
    100 at the bottom to about 0.5 at the top.
  • The luminance of peaks and troughs remains
    constant along a given horizontal path through
    the image.

Campbell, F. W. and Robson, J. G. (1968)
Application of Fourier analysis to the visibility
of gratings. Journal of Physiology (London) Image
Courtesy of Izumi Ohzawa, Ph.D. University of
California School of Optometry
If object visibility was dictated solely by image
contrast, the alternating bright and dark bars
should appear to have equal height everywhere in
the image. However, the bars seem to be taller in
the middle of the image.
13
Basic Principles Light Levels
  • Under normal lighting conditions the cones are
    operating and the eye has good visual acuity and
    is most sensitive to greenish yellow color, which
    has a wavelength around 555 nanometers (photopic
    curve).
  • When the light levels drop to near total
    darkness, the response of the eye changes
    significantly as shown by the scotopic response
    curve on the left.
  • At this level of light, the rods are most active
    and the human eye is more sensitive to any amount
    of light that is present, but is less sensitive
    to the range of color.
  • At this very low light level, sensitivity to
    blue, violet, and ultraviolet is increased, but
    sensitivity to yellow and red is reduced.

14
Basic Principles Light Intensity Measurement
  • Effective visual inspection requires adequate
    lighting.
  • The type of inspection will dictate the lighting
    requirements. Inspection of components with fine
    detail and low contrast will require greater
    illumination than components with large details
    and high contrast.
  • Light intensity may be measured with a suitable
    light meter. The unit of measure for white light
    is foot-candles (fc).
  • A foot-candle is equal to the amount of direct
    light thrown by one standard candle at a distance
    of 1 foot.
  • Inspection of components with fine detail and low
    contrast may require 100 foot-candles or more.
  • Specification requirements for lighting should be
    reviewed prior to performing an inspection.

15
Basic Principles Light Directionality
  • The directionality of the light is a very
    important consideration.
  • For some applications, flat, even lighting works
    well.
  • For other applications, directional lighting is
    better because it produces shadows that are
    larger than the actual flaw and easier to detect.

16
Basic Principles Perspective
The eye/brain need visual clues to determine
perspective.
Is the book facing towards or away from you?
17
Basic Principles Optical Illusions
Sometime the eye/mind has trouble correctly
processing visual information.
Are the horizontal lines parallel or do they
slope?
How many black dots do you see?
18
Basic Principles Vision
  • When evaluations are made by an inspector, eye
    examinations must be done at regular intervals to
    assure accuracy and sensitivity. These
    examinations may consist of the following
  • Near Vision (Jaeger)
  • Far Vision (Snellen)
  • Color Differentiation
  • When using machine vision, different but similar
    performance checks must be performed.

19
Basic Principles
  • For best results the inspector or machine vision
    operator must have
  • A basic knowledge of material processing,
    forming, machining and joining processes.
  • A general understanding of design features,
    application and service requirements.
  • Specific instructions on what to look for and
    specific accept/reject criteria.

20
Inspection Applications
Applications for visual inspection and many and
range from looking a product over for obvious
defect to performing detailed inspections. Some
of the common applications include
  • Detection of surface anomalies such as scratches,
    excess surface roughness, and areas void of paint
    or plating.
  • Crack, porosity, corrosion or other flaw
    detection.
  • Dimensional conformance.
  • Precision measurements.
  • Foreign object detection.
  • Component location.

21
Inspection Applications Flaw Detection
  • Visual inspection of manufactured materials and
    components is a cost effective means of
    identifying flaws.
  • Visual inspection of a casting reveals a crack
    between a threaded opening and a pressed fit.
  • The aluminum sand casting has hot tears and
    shrinkage at the transition zones.

22
Inspection Applications Flaw Detection
In-service inspections of existing components and
structures is commonly accomplished visually.
  • In this example, visual inspection of a fire
    escape reveals a failure in a handrail tube.
  • The failure is in the tube seam and is likely the
    result of ice expansion.

23
Inspection Applications Flaw Detection
  • Normal inspection practices for highway bridges
    rely almost entirely on visual inspection to
    evaluate the condition of the bridges.

24
Inspection Applications Flaw Detection
Over 80 percent of all aircraft inspections are
performed visually.
25
Inspection Applications Flaw Detection
  • Weld quality requirements are commonly determined
    through visual inspection.
  • Many standards have established acceptance
    criteria for welds.

Transverse weld crack
Slag rolled into toe of weld
26
Dimensional Conformance
  • Visual inspection is commonly employed for
    general dimensional conformance, assembly fit,
    and alignment between components.
  • Common applications include determining
  • Weld size and tolerance.
  • Component dimensions.
  • Material alignment and allowable distortion.

27
Dimensional Conformance
  • Welds are commonly inspected for dimensional
    tolerance.
  • There are several types of gages used to inspect
    welding fit up and finished weldments.
  • These gages are intended for general inspection
    where close tolerances are not required.
  • The gage used is determined by the application.

Palmgren gage
Fillet gage set
VWAC gage
Cambridge gage
28
Dimensional Conformance
  • Visual inspection is commonly used to determine
    weld size and tolerances according to standards
    and engineering specifications.

Throat measurement using a Palmgren gage.
Leg size determination with fillet gage.
Convexity measurement with VWAC gage.
29
Dimensional Conformance
  • Undercut in a weld is readily seen visually. In
    many cases its depth must be measured to
    determine if it exceeds code requirements.

Measurement of undercut depth with VWAC gage.
30
Dimensional Conformance
  • Component finish dimensions are checked with the
    use of measurement devices, such as transferring
    gages and precision measurement gages.

The finished depth of a machined mold is
determined with a depth micrometer.
Small hole gage used in determining hole diameter.
31
Dimensional Conformance Alignment/Distortion
  • Visual inspection frequently involves checking
    materials and components for fit and alignment.
  • Many standards establish allowable tolerances for
    fit and distortion.
  • Structural fabrication requires dimensional
    inspection of finished components prior to
    shipment to the field site.
  • Basic tools are used for the inspection. An
    inspector will set up string lines at known
    distances and plum them using a tape measure.
    Measurements are then taken at various locations
    and compared to code requirements.

In this image a fabricated girder is being
inspected for distortion, sweep and web flatness.
32
Equipment
  • Visual inspection equipment includes a variety of
    different tools. These may range from basic
    rulers, tape measures and spring type calipers to
    rigid or flexible borescopes and remote crawlers
    with cameras.
  • Many tools have been designed for specific
    applications such as the various weld gauges.
  • Some of the specialized tools such as crawlers
    have been designed to satisfy the inspection
    needs in applications where conventional
    techniques are not feasible.

33
Equipment Basic Measurements
  • One of the most common tools used in visual
    inspection is the rule or scale.
  • Used to measure linear dimensions, when properly
    used will measure within 0.015 or 1/64 and
    smaller.
  • Rules are made in a variety lengths, widths, and
    thicknesses.
  • They are graduated in common fractions, decimal
    units, and metric units, or combinations of both.
  • The specific type of rule is typically chosen
    relative to the application.

34
Equipment Precision Measurements
  • Sliding calipers are a precision refinement of
    the common rule, which results in greater
    accuracy of measurements.
  • They may incorporate either a dial indicator or
    digital readout.
  • Sliding-type calipers are commonly used to check
    dimensional tolerances of machined components,
    wear on components, and fit between components.

35
Equipment Precision Measurements
  • Micrometers are precise measurement instruments
    used to make accurate direct readings in contact
    measurements.
  • Micrometers are designed for inside, outside, and
    depth measurements, and are available in a wide
    variety of shapes and sizes.
  • Micrometers may be either thousandth inch
    (.001) or ten thousandth inch (.0001)
    measurement capable.

36
Equipment Precision Measurements
  • Micrometers operate on the principle that a
    precision made screw with a pitch of forty
    threads per inch will advance one fortieth of an
    inch (.025) with each complete turn.
  • On a one inch micrometer, the sleeve is marked
    longitudinally with forty lines to the inch which
    corresponds to the number of threads on the
    spindle.

37
Equipment Precision Measurements
  • The reading line on the sleeve is divided into
    forty equal parts by vertical lines, each
    designates 1/40th or .025 and every fourth line
    denotes hundreds of thousandths and is numbered 1
    0.
  • The beveled edge of the thimble is divided into
    twenty five equal parts with each representing
    .001, with every line numbered from 0 -24.

38
Equipment Precision Measurements
  • Example
  • One major division on the sleeve is visible,
    representing one tenth of an inch.
  • Two minor divisions are visible, which each
    represent an additional 25 thousandths.
  • Line 15 on the thimble coincides with the reading
    line on the sleeve indicating that fifteen one
    thousandths of an inch should be added to the
    measurement.
  • By adding all three values, the micrometer
    reading is obtained.

0.100 (2 X 0.025) 0.050 (15 X .001)
0.015 0.165
39
Equipment Precision Measurements
  • Reading the ten thousandths micrometer.
  • The ten thousandths micrometer incorporates a
    vernier scale.
  • The vernier consists of ten divisions the sleeve,
    which occupies the same space as nine divisions
    on the beveled edge of the thimble.
  • The difference between one of the ten spaces on
    the sleeve and one of the nine spaces on the
    thimble is 1/10th of a division or 0.0001.
  • Example
  • The second major divisions (line 2) on the sleeve
    is visible (2 x 0.100 0.200)
  • Three minor divisions after line 2 are visible (3
    X 0.025 0.075)
  • The beveled edge of the thimble is between 0 and
    1 (0 x 0.001 0)
  • The vernier scale has the sixth line on the
    sleeve perfectly lined up with one of the marks
    on the thimble ( 6 x 0.0001 0.0006)
  • The micrometer reading is 0.2756

0.200 0.075 0.000 0.0006 0.2756
40
Equipment Transferring Gauges
  • Transfer instruments are used to take
    measurements which are transferred to direct
    measurement devices.
  • They consist of calipers, dividers, telescoping
    gages and small hole gages.

41
Equipment Transferring Gauges
  • Spring type calipers are available for contact
    measurements of inside and outside dimensions.
  • They are useful for measuring distances between
    and over surfaces.
  • They are commonly used to transfer dimensions or
    sizes between the work piece and standard
    measuring devices, such as graduated rules.
  • The size of a linear or rounded indication of a
    discontinuity may be measured with dividers.

42
Equipment Transferring Gauges
  • Small hole gages are a type of transfer
    instrument used to measure small holes or slots.
  • They are generally supplied in sets with a range
    of 1/8 - 1/2.
  • The actual measurement is determined by
    transferring a properly adjusted gage to a
    micrometer.

43
Equipment Transferring Gauges
  • Telescoping gages make inside measurements such
    as hole diameter and slot width.
  • They are designed to be measured by a micrometer
    after being set to the hole or slot size.
  • To make accurate measurements it is important to
    make sure the telescoping gage is aligned
    properly in the measuring faces of the micrometer.

44
Equipment Screw Pitch Gage
  • The screw pitch gage is a basic visual aid for
    checking the number of threads per inch and rough
    inaccuracies of threads.
  • The gage consists of a steel case with a number
    of folding leaves at each end.
  • Each leaf is number and contains teeth
    corresponding to a specific thread pitch.

45
Direct and Remote Visual Inspection
  • Many codes refer to direct visual examination as
    a visual inspection which requires that access to
    the area is sufficient to place the eye within 24
    inches of the surface to be examined and at an
    angle of not less than 30º to that surface.
  • If these requirements cannot be met, then remote
    visual inspection may be used.
  • Remote visual inspection may be accomplished with
    the use of a number of optical aids such as,
    mirrors, magnifiers, and rigid or flexible
    borescopes.

46
Optical Aids
  • Mirrors are valuable aids in visual inspection,
    they allow the inspection of threaded and bored
    holes, inside surfaces of pipes and fittings, as
    well as many others.
  • Magnifiers assist the visual inspector by
    enlarging the size of the object being examined.
  • Comparators are a magnifier with a measuring
    capability. The comparator has interchangeable
    reticles which provide measurements for threads,
    angles, linear measurement, diameters and radii.

47
Optical Aids
  • Borescopes are visual aids used for the
    inspection of internal surface areas.
  • They are designed for remote viewing in difficult
    to reach areas such as jet engines, cylinders,
    tanks, and various enclosed chambers.
  • Borescopes are available in many different
    diameters and lengths, and are classified as
    rigid or flexible.

48
Visual Inspection With A Borescope
Clean Surface
Corrosion Damage
49
Optical Aids
  • Advances in technology has allowed video
    equipment to be adapted to portable and robotic
    devices.
  • Portable video probes allow inspectors to
    remotely perform examinations in closed chambers
    which are inaccessible by convention inspection
    means.
  • Robotics have been developed whereby cameras can
    be affixed to crawlers and submersibles.
  • Retrieval tools can be affixed to robotics to
    remove foreign objects.
  • Conventional recording techniques may be
    employed for image capture and storage with many
    of the remote video inspection methods.

50
Machine Vision Inspection
51
Machine Vision Basic Principles
  • Machine vision technology uses an imaging system
    and a computer to analyze an image and to make
    decisions based on that analysis.
  • In inspection applications, the machine vision
    optics and imaging system enable the processor to
    "see" objects precisely and thus make decisions
    about which component meet a specific inspection
    criteria.
  • Machine vision can eliminate human factor error
    that might result from doing difficult, tedious,
    or boring tasks. It also allows process
    equipment to be utilized 24 hours a day.

52
Machine Vision Basic Principles
  • The following process steps are common to all
    machine vision applications
  • Image acquisition An optical system gathers an
    image, which is then converted to a digital
    format and stored into computer memory.
  • Image processing A computer processor uses
    various algorithms to enhance elements of the
    image that are of specific importance to the
    process.
  • Feature extraction The processor identifies and
    quantifies critical features in the image (e.g.,
    the position of holes on a printed circuit board,
    the number of pins in a connector, the
    orientation of a component on a conveyor) and
    sends the data to a control program.
  • Decision and control The processor's control
    program makes decisions based upon the data. Are
    the holes within specification? Is a pin missing?

53
Machine Vision - Applications
As mentioned previously, machine vision is
primarily used in production applications where a
large number of components require inspection and
the inspection conditions can be closely
controlled. Uses include
  • Assembly verification (caps, fasteners,
    electronic board components, etc.)
  • Surface inspection (dents, scratches,
    porosity.and other undesirable features)
  • Verification of colors, gradients, patterns in
    fabrics and labels.
  • Confirmation of proper labeling for medications,
    foods and other products.
  • Inspection of coating coverage.
  • Feature measurements.

Assembly Verification
Spark Gap Measurement
54
Machine Vision - Equipment
  • Key System Elements
  • A variety of components are included in a
    machine vision system, which depend on the
    environment, the application, and the budget.
    However, the following components are common to
    all vision systems
  • Front-end optics this includes the lighting, the
    lens, and the camera.
  • Frame grabber this is a computer processor board
    that accepts the video input from the camera,
    digitizes it, and stores it for analysis.
  • Processor A computer processor is required to
    control the vision application.
  • Control Software Computer software is used for
    controlling and executing vision tasks.

55
Advantages of Visual Inspection
  • Readily used on almost all materials.
  • Simple to perform.
  • Low in cost, (application dependent).
  • Relatively quick.
  • Results may be permanently recorded.
  • Can be automated.

56
Limitations of Visual Inspection
  • Direct inspections are limited to surfaces only.
  • Indirect inspections require greater inspector
    knowledge and training.
  • Inspector dependent, knowledge of materials and
    processing, eye sight.
  • Standards (workmanship) may be difficult to
    obtain.

57
Glossary of Terms
  • Borescope A visual inspection aid used for the
    inspection of internal surfaces. Borescopes are
    rigid and flexible.
  • Corrosion The deterioration of a metal by
    chemical or electro-chemical reaction with its
    environment.
  • Defect A discontinuity which interferes with
    the usefulness of a part. A fault in any
    material or part which is detrimental to its
    serviceability.

58
Glossary of Terms, Cont.
  • Direct Visual Inspection Visual inspection
    conducted where the eye can be place within 24
    of the area to be inspected at an angle of not
    less than 30.
  • Discontinuity An interruption in the normal
    physical structure or configuration of a part.
  • Foot Candle The amount of direct light thrown
    by one standard candle on a surface one foot
    away.

59
Glossary of Terms, Cont.
  • Inherent Discontinuity Discontinuities which
    are ordinarily normal to the material at the time
    it originally solidifies from the molten state.
    They consist of porosity, inclusions, and pipe.
  • Magnifier An optical glass which enlarges the
    real size of an object being inspected
  • Micrometer A precise measuring instrument used
    to take accurate readings in contact measurement.

60
Glossary of Terms, Cont.
  • Pocket Comparator A magnifier with measuring
    capability through a system of interchangeable
    reticles.
  • Power of Magnification The amount that the real
    size of an object is enlarged. The power if
    designated by X.
  • Remote Visual Inspection Inspection conducted
    where the eye cannot be placed within 24 of the
    area to be inspected, or at an angle less than
    30.

61
Glossary of Terms, Cont.
  • Reticle A series of lines, dots, cross hairs,
    or wires in the focus of the eyepiece of and
    optical instrument.
  • Rule or Scale A measuring device used to make
    linear measurements.
  • Screw Pitch Gages A visual aid used to check
    the number of threads per inch and rough
    inaccuracies in threads.

62
Glossary of Terms, Cont.
  • Secondary Processing Metal removal processes,
    heat treatment and plating processes used to
    produce and article of the desired shape and
    finish from the formed material.
  • Service Discontinuity Discontinuities induced
    in components after they have been placed in
    service.
  • Transfer Instruments A group of instruments
    used to take measurements which are transferred
    to precise direct measuring devices. They
    consist of calipers, dividers, telescoping gages,
    and small hole gages.

63
Glossary of Terms, Cont.
  • Vernier Scale A device for indicating a
    fraction of a whole division of a scale reading
  • Visual Scale The ability to see. The keenness
    of perception.
  • Wear Wear is deterioration due to use. Wear
    will cause a reduction in cross section and
    strength.
  • Weld Inspection Gages A group of measuring
    devices used to inspect welding fit up and
    finished weldments.
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