cims2009 ch22 p5 insptechnology

7/31/2019 CIMS2009 Ch22 P5 InspTechnology

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Inspection Technologies

Sections:

1. Inspection Metrology

2. Contact vs. Noncontact Inspection Techniques

3. Conventional Measuring and Gaging Techniques4. Coordinate Measuring Machines

5. Surface Measurement

6. Machine Vision

7. Other Optical Inspection Techniques

8. Noncontact Nonoptical Inspection Technologies


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Inspection Metrology

Measurement - a procedure in which an unknown

quantity is compared to a known standard, using an

accepted and consistent system of units

The means by which inspection by variables is

accomplished

Metrology the science of measurement

Concerned with seven basic quantities: length,

mass, electric current, temperature, luminous

intensity, time, and matter

From these basic quantities, other physical

quantities are derived


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Characteristics of

Measuring Instruments

Accuracy how closely the measured value agrees

with the true value

Precision a measure of the repeatability of the

measurement process

Rule of 10 the measuring instrument must be

ten time more precise than the specified tolerance

Resolution the smallest variation of the variable that

can be detected

Speed of response how long the instrument takes

to measure the variable

Others: operating range, reliability, cost


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Accuracy vs. Precision

(a) (b) (c)

(a) High accuracy but low precision, (b) low accuracy buthigh precision, and (c) high accuracy and high precision


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Analog vs. Digital Instruments

Analog measuring instrument output signal variescontinuously with the variable being measured

Output signal can take on any of an infinite number ofpossible values over its operating range

Digital measuring instrument can assume any of adiscrete number of incremental values corresponding tothe variable being measured

Number of possible output values is finite

Advantages: Ease of reading the instrument

Ease of interfacing to a computer


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Two Basic Types of

Inspection Techniques

1. Contact inspection

Makes contact with object being inspected

2. Noncontact inspection

Does not make contact with object being inspected


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Contact Inspection Techniques

Uses a mechanical probe that makes contact with the object

being measured or gaged

Principal techniques:

Conventional measuring and gaging instruments,manual and automated

Coordinate measuring machines

Stylus type surface texture measuring machines


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Noncontact Inspection Techniques

Uses a sensor or probe located a certain distance away fromthe object being measured or gaged

Two categories:

Optical uses light to accomplish the inspection

Nonoptical - uses energy form other than light

Advantages of noncontact inspection:

Avoids possible damage to surface of object

Inherently faster than contact inspection

Can often be accomplished in production withoutadditional part handling

Increased opportunity for 100% inspection


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P-Q Chart for Inspection Technologies


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Conventional Measuring and

Gaging Techniques

Measuring instruments - provide a quantitative value of the

part feature of interest

Examples:

Steel rules, calipers, micrometer, dial indicator,protractor

Gages - determines whether a part feature falls within a

certain acceptable range

Examples: Snap gages for external dimensions, plug gages for

hole diameters, thread gages


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Coordinate Metrology

Concerned with the measurement of the actual shape

and dimensions of an object and comparing these

with the desired shape and dimensions specified on a

part drawing

Coordinate measuring machine (CMM) an

electromechanical system designed to perform

coordinate metrology

A CMM consists of a contact probe that can be

positioned in 3-D space relative to workpart features,and the x-y-z coordinates can be displayed and

recorded to obtain dimensional data about geometry


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Coordinate Measuring Machine


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CMM Components

Probe head and probe to contact workpart surfaces

Mechanical structure that provides motion of the probe in

x-y-z axes, and displacement transducers to measure the

coordinate values of each axis

Optional components (on many CMMs):

Drive system and control unit to move each axis

Digital computer system with application software


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Contact Probe Configurations

(a) (b)

(a) Single tip and (b) multiple tip probes


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CMM Mechanical Structure

Six common types of CMM mechanical structures:

1. Cantilever

2. Moving bridge

3. Fixed bridge4. Horizontal arm

5. Gantry

6. Column


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CMM Structures

(a) (b)

(a) Cantilever and (b) moving bridge structure


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CMM Structures

(c) (d)

(c) Fixed bridge and (d) horizontal arm (moving ram type)


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CMM Structures

(e) (f)

(e) Gantry and (f) column


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CMM Operation and Controls

Four Main Categories

1. Manual drive CMM human operator physically moves

the probe and records x-y-z- data

2. Manual drive and computer-assisted data processing

can perform calculations to assess part features3. Motor-driven CMM with computer-assisted data

processing uses joystick to actuate electric motors to

drive probe

4. Direct computer control (DCC) operates like a CNCmachine tool and requires part program


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DCC Programming

Manual leadthrough

Operator leads the CMM probe through the various

motions in the inspection sequence, indicating points

and surfaces to be measured and recording these intocontrol memory

Off-line programming

Program includes motion commands, measurement

commands, and report formatting commands and isprepared off-line


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CMM Software

The set of programs and procedures used to operate the

CMM and its associated equipment

Example: part programming software for DCC machines

Other software divide into following categories:1. Core software other than DCC programming

2. Post-inspection software

3. Reverse engineering and application-specific software


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Core Software Other than

DCC Programming

Minimum basic programs for the CMM to function, whichapplies only to DCC machines

Examples:

Probe calibration defines probe parameters so thatCMM can automatically compensate for probedimensions

Part coordinate system definition instead of aligningthe part with the CMM axes, axes are aligned to part

Geometric feature construction e.g., hole center

Tolerance analysis compares measurements withpart drawing dimensions and tolerance


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Post-Inspection Software

Programs applied after the inspection procedure

Statistical analysis used to accomplish various

statistical analyses

Process capability Statistical process control

Graphical data representation displays data

collected during CMM inspection in a graphical or

pictorial way, permitting easier visualization of formerrors and other data


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Reverse Engineering and

Application-Specific Software

Reverse engineering

CMM explores part surface and constructs 3-D model

Application-specific software:

Gear checking Thread checking

Cam checking

Automobile body checking


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Advantages of using CMMs

over Manual Inspection

Reduced inspection cycle time translates to higher

throughput rate

Especially with DCC, approximately 90% reduction in

certain tasks

Flexibility CMMs are general-purpose machines

Reduced operator errors in measurement and setup

Greater inherent accuracy and precision

Avoidance of multiple setups in general allmeasurements of a given part can be made in one setup


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Inspection Probes on Machine Tools

Mounted on toolholders

Stored in the tool drum

Handled by the automatic tool-changer the same waycutting tools are handled

Inserted into the machine tool spindle by the automatictool-changer

When mounted in the spindle the machine tool iscontrolled very much like a CMM

Sensors in the probe determine when contact is madewith part surface so that required data processing isperformed to interpret the sensor signal


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Portable CMMs

In the conventional application of a CMM, parts must be

removed from the production machine and taken to the

inspection department where the CMM is located

New coordinate measuring devices allow the inspection

procedures to be performed at the site where the parts are

made

Example: Faro gage, a.k.a. Personal CMM, is a six-

jointed articulated arm

At the end of the arm is a touch probe to perform

coordinate measurements, similar to a CMM


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Advantages of In-Situ Inspection

Necessity to move parts from the machine tool to the CMM

and back is eliminated

Material handling is reduced

Inspection results are immediately known The machinist who makes the part performs the inspection

Because part is still attached to machine tool during

inspection, any datum reference locations established

during machining are not lost Any further machining uses the same references

without the need to refixture the part


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Surface Measurement

Most surface measuring devices use a contacting stylus

Therefore, classified as contact inspection

Cone-shaped diamond stylus with point radius = 0.005

mm (0.0002 in) and 90 degree tip angle As stylus is traversed across surface, tip also moves

vertically to follow the surface topography

Movement is converted to electronic signal that can be

displayed as either1. Profile of the surface

2. Average roughness value of the surface


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Stylus-Type Surface Measurement


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Deviations from a Nominal Surface

Surface measurement can be displayed as the profile of thesurface as indicated by the stylus trace or the average of

the surface deviations during the trace


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Machine Vision

Acquisition of image data, followed by the processing andinterpretation of these data by computer for some useful

application

Also called computer vision

2-D vs. 3-D vision systems:

2-D two-dimensional image adequate for many

applications (e.g., inspecting flat surfaces, presence or

absence of components)

3-D three-dimensional image requires structuredlight or two cameras


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Operation of a Machine Vision System

1. Image acquisition and digitization

2. Image processing and analysis

3. Interpretation


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Image Acquisition and Digitization

With camera focused on subject, viewing area is

divided into a matrix of picture elements (pixels)

Each pixel takes on a value proportional to the

light intensity of that portion of the scene and is

converted to its digital equivalent by ADC

In a binary system, the light intensity is reduced

to either of two values, white or black

In a gray-scale system, multiple light intensities

can be distinguished

Each frame is stored in a frame buffer (computer

memory), refreshed 30 times per second


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Dividing the image into a Matrix of

Picture Elements (Pixels)

(a) The scene

(b) 12 x 12 matrix

superimposed on the

scene

(c) Pixel intensity values,

either black or white, in

the scene


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Types of Cameras

Vidicon camera

Focus image on photoconductive surface followed

by EB scan to determine pixel value

Have largely been replaced by Solid-state cameras

Focus image on 2-D array of very small, finely

spaced photosensitive elements that emit an

electrical charge proportional to the light intensity Smaller and more rugged

No time lapse problem


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Illumination Techniques

(a) (b) (c)

(a) Front lighting, (b) back lighting, (c) side lighting


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More Illumination Techniques

(d) Structured lighting using a planar sheet of light


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More Illumination Techniques

(e) Strobe lighting


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Image Processing and Analysis

Segmentation techniques to define and separate

regions of interest in the image

Thresholding converts each pixel to a binary

value (white or black) by comparing the intensity

level to a defined threshold value

Edge detection determines location of

boundaries between an object and its background,

using the contrast in light intensity between

adjacent pixels at the boundary of an object Feature extractiondetermines an objects features

such as length, area, aspect ratio


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Interpretation

For a given application, the image must be interpreted

based on extracted features

Concerned with recognizing the object, called pattern

recognition - common techniques:

Template matching compares one or more

features of the image object with a template (model)

stored in memory

Feature weighting combines several features into

one measure by weighting each feature according

to its relative importance in identifying the object


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Machine Vision Applications

1. Inspection:

Dimensional measurement

Dimensional gaging

Verify presence or absence of components in anassembly (e.g., PCB)

Verify hole locations or number of holes

Detection of flaws in printed labels

2. Identification for parts sorting or counting3. Visual guidance and control for bin picking, seam

tracking in continuous arc welding, part positioning


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Other Optical Inspection Methods

Conventional optical instruments

Optical comparator

Conventional microscope

Scanning laser systems

Linear array devices

Optical triangulation techniques


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Scanning Laser Device


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Linear Array Measuring Device


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Optical Triangulation Sensing

Range Ris desired to

be measured

Length L and angle A

are fixed and known

Rcan be determined

from trigonometric

relationships as follows:

R= L cot A

N N i l


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Noncontact Nonoptical

Inspection Techniques

Electrical field techniques

Reluctance, capacitance, inductance

Radiation techniques

X-ray radiation

Ultrasonic inspection methods

Reflected sound pattern from test part can be

compared with standard

Parts must always be presented in the same positionand orientation relative to the probe

cims2009 ch22 p5 insptechnology

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