Inspection Metrology

By Prof N D Sadaphal

Assistant Professor

Sanjivani College of Engineering, Kopargaon (Maharashtra State) 423601

Mechanical Engineering

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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 Metrology – the science of measurement

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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(L.C.)– 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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Characteristics of Measuring Instruments

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

(a) (b) (c)

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• Automated inspection can be defined as the automation of one or more of the steps involved in the inspection procedure.

• There are a number of alternative ways in which automated or semi- automated inspection can be implemented:

1. Automated presentation of parts by an automatic handling system with a human operator will performing the examination and decision steps.

2. Automated examination and decision by an automatic inspection machine, with manual loading (presentation) of parts into the machine.(bottle filling plant)

3. Completely automated inspection system in which parts presentation, examination, and decision are all performed automatically.

Automated Inspection

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Where and When to Inspect

• Inspection can be performed at any of several places in production:

1. Receiving inspection, when raw materials and parts are received from suppliers.

2. At various stages of manufacturing, and

3. Before shipment to the customer.

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Off-Line and On-Line Inspection

• The timing of the inspection procedure in relation to the manufacturing process is an important consideration in quality control.

• Two alternative situations can be distinguished:

1. Off-line inspection.

2. On-line inspection.

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Off-Line Inspection

• Off-line inspection is performed away from the manufacturing process, and there is generally a time delay between processing and inspection.

• Manual inspection is common.

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On-Line Inspection

• The alternative to off-line inspection is on-line inspection, in which the procedure is performed when the parts are made, either as

An integral step in the processing or assembly operation, or

Immediately afterward.

• Two on-line inspection procedures can be distinguished:

On-line/in-process.

On-line/post-process.

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On-Line/ in-process Inspection

• This is achieved by performing the inspection procedure during the manufacturing operation.

• As the parts are being made, the inspection procedure is measuring or gauging the parts simultaneously.

• Technologically, automated on-line/in-process insection of the product is usually difficult and expensive to implement, As an alternative, on-line/ post-process procedures are often used. 9

On-Line/ post-process Inspection • The measurement or gauging procedure is accomplished

immediately following the production process.

• Even though it follows the process, it is still considered an on-line method because it is integrated with the manufacturing workstation, and the results of the inspection can immediately influence the production operation of the next part.

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Contact and Non-contact Inspection Techniques

• Inspection techniques can be divided into two broad categories:

1. Contact Inspection.

2. Non-contact Inspection.

• In contact inspection, physical contact is made between the object and the measuring or gauging instrument.

• In non-contact inspection no physical contact is made.

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Laser light beam is:

Highly monochromatic - the light has a single wave length

Highly collimated - the light rays are parallel

These properties have motivated many applications in measurement and inspection

Lasers Metrology

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Fig: Scanning laser system for measuring diameter of cylindrical workpart; Time of interruption of light beam is proportional to diameter D.

Scanning Laser Systems

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1. Laser beam deflected by a rotating mirror to sweeps a beam of light past an object.

2. Photodetector on far side of the object senses the light beam during its sweep except for the short time while it is interrupted by the object.

3. This time period can be measured quickly with great accuracy.

4. A microprocessor system measures the time interruption related to the size of the object in the path of the laser, and converts it to a linear dimension.

Scanning Laser Systems

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Inspection Techniques • Conventional techniques and CMMs compete with each other in the

measurement and inspection of part dimensions. The general application ranges for the different types are presented in the PQ chart below

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Coordinate Measuring Machines (CMM)

• Coordinate metrology is concerned with the measurement of the actual shape and dimensions of an object and comparing these with the desired shape and dimensions.

• In this connection, coordinate metrology consists of the evaluation of the location, orientation, dimensions, and geometry of the part or object.

• A Coordinate Measuring Machine (CMM) is an electromechanical system designed to perform coordinate metrology.

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Coordinate Measuring Machines • A CMM consists of a constant probe that can be positioned in 3D

space relative to the surface of a workpart, and the x, y, and z coordinates of the probe can be accurately and precisely recorded to obtain dimensional data concerning the part geometry

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Video 1 Video 2 Video 3

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(a) Single tip (b) multiple tip probes

Coordinate Measuring Machines

• CMM is composed of the following components:

Probe head and probe

Mechanical structure that provides motion to probe in three Cartesian axes

displacement transducers to measure the coordinate values of each axis.

Drive system and control unit to move each of the three axes

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CMM Mechanical Structure

(a) Cantilever (b) Moving bridge (c) Fixed bridge 21

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CMM Mechanical Structure

(d) Horizontal Arm (e) Gantry (f) Column

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CMM Controls

• The methods of operating and controlling a CMM can be classified into four main categories:

1. Manual drive,

2. Manual drive with computer-assisted data processing,

3. Motor drive with computer-assisted data processing, and

4. Direct Computer Control with computer-assisted data processing

(DCC CMM).

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•Higher productivity - a CMM can perform complex inspection procedures in much less time than traditional manual methods. •Greater inherent accuracy and precision than conventional methods. •Reduced human error. •Versatility - a CMM is a general purpose machine that can be used to inspect a variety of part configurations.

CMM Advantages

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In human being vision system Eye – senses the image. Brain- analysis the information & take the action on the basis of analysis. Machine vision system • It gives accurate result & take timely corrective action to control

the process. • Mass Production.

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Machine vision for Inspection

• 3D system Only for special purpose Application include 3D analysis of scenes.

• 2D system --- Most commonly using system. --- For measuring dimensions of parts. Verifying presence of components. Checking features of Flat or semi flat

surfaces.

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1. Image acquisition and digitization 2. Image processing and analysis 3. Interpretation

Functions of Machine Vision:-

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1. Image acquisition system- capture of image by video camera & dividing it into very small elements called as pixel.

digitization system- store the image data for subsequent analysis. 2. Image processing and analysis – involves segmentation (edge

location), Thresholding. Edge location -Determining location of boundaries of object. Thresholding –conversion of each pixel level into binary value. 3. Interpretation- it concerned with recognizing the object or

pattern recognition. identify the object & comparing it with predefined models,

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Image acquisition system-

capture of image/video -(image acquisition) then digitize it by ADC( Analog to digital converter) and store analysis. (dividing it into very small elements called as pixel.)

There are mainly five categories of lighting systems. Front lighting Back lighting Side lighting Structured lighting Strobe Lighting.

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Front lighting: -Light source is located at the same side of the camera. -Produces a reflected light from the object that allow inspection of surface features.

Back lighting: -Light source is placed between behind the object being viewed by the camera. -This create dark image of the object that contrasts sharply with the light background. -This type is used for inspect parts dimension and distinguish between part outlines.

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dark image/Silhouette

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Side lighting -Light source is placed at the side of the surface to be illuminated. -For finding out surface irregularities, flaws, defects on the surface.

Structured lighting -Projection of special light pattern onto the object. -Usually planer sheet of highly focused light are used.

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Strobe Lighting. - The scene is illuminated by short pulse of high

intensity light which causes moving object appear to be stationary.

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It involves segmentation (consist of two different technique) Thresholding Edge detection

Thresholding- -Conversion of each pixel intensity level into a binary value, representing black or white. -There is a threshold value of intensity -If the value of the pixel intensity of the image is less than the threshold value then the pixel value is Zero(Black) otherwise One( White).

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Edge detection -Determines the location of boundaries between an object and its surroundings in an image.

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-It concerned with recognizing the object or pattern recognition.

- Identify the object & comparing it with predefined models Template matching:- Compare one or more feature of an image with the

corresponding feature of model or template stored in computer memory.

Feature Weighting. Several features like area, length and perimeter are combined

into a single measure by assigning a weight to each feature according to the relative importance in the identifying the object.

The score of the object in the image is compared with the score of the image in the computer memory to achieve proper identification. 38

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Inspection

Identification

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