1、Visual Inspection,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,Introduction,Visual inspe
2、ction 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
3、 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
4、 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.”,Introduction,Visual inspection is the most basic an
5、d 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 comp
6、liance. It can be performed by direct or indirect methods during various stages of manufacturing or after the component has been placed in-service.,The quality of an inspection are affected primarily by four factors. The quality of the detector (eye or camera). The lighting conditions. The capabilit
7、y to process the visual data. The level of training and attention to detail.,Introduction,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 abili
8、ty 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 prima
9、rily used in production applications where a large number of components require inspection and the inspection conditions can be closely controlled.,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 an
10、d allow you to focus the image on the central retina or fovea.,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 o
11、f 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.,Basic Principles The Human Eye,The retina is a
12、mosaic of two basic types of photoreceptors, rods and cones.,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 sharpe
13、st 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.,Basic Principles Visual Acuity,Normal visual acuity or 20/20 vision is defined as the ability
14、 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 objec
15、t. 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,Basic Principles Visual Acuity,When visually inspecting an object for a defect, a comfortable viewing distance “d” might be around 12 inches. At 12 inches, the normal visual
16、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.),There is a limit to what the unaided eye can see.,Basic Principles Contrast Sensitivity,Contrast sensitivity is a measure o
17、f 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 sp
18、atial frequency of a feature The lighting conditions Whether the object is lighter or darker than the background,The graph to the right plots the visibility of a spot as a function of the above variables,In this image: The luminance of pixels is varied sinusoidally in the horizontal direction. The s
19、patial 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.,It should be noted, however, that larger object
20、s are not always easier to see than smaller objects as contrast is reduced.,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,Basic
21、Principles Contrast Sensitivity,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.,Under normal lighting conditions the cones are
22、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
23、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 i
24、s reduced.,Basic Principles Light Levels,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 con
25、trast. 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 req
26、uire 100 foot-candles or more. Specification requirements for lighting should be reviewed prior to performing an inspection.,Basic Principles Light Intensity Measurement,Basic Principles Light Directionality,The directionality of the light is a very important consideration. For some applications, fl
27、at, 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.,Is the book facing towards or away from you?,Basic Principles Perspective,The eye/brain need visual clues to determine perspecti
28、ve.,Are the horizontal lines parallel or do they slope?,How many black dots do you see?,Sometime the eye/mind has trouble correctly processing visual information.,Basic Principles Optical Illusions,When evaluations are made by an inspector, eye examinations must be done at regular intervals to assur
29、e 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.,Basic Principles Vision,For best results the inspector or machine vision
30、 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.,Basic Principles,Inspection Applic
31、ations,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.,Applications for visual inspection
32、 and many and range from looking a product over for obvious defect to performing detailed inspections. Some of the common applications include:,Visual inspection of manufactured materials and components is a cost effective means of identifying flaws. Visual inspection of a casting reveals a crack be
33、tween a threaded opening and a pressed fit. The aluminum sand casting has hot tears and shrinkage at the transition zones.,Inspection Applications Flaw Detection,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 th
34、e result of ice expansion.,Inspection Applications Flaw Detection,In-service inspections of existing components and structures is commonly accomplished visually.,Normal inspection practices for highway bridges rely almost entirely on visual inspection to evaluate the condition of the bridges.,Inspec
35、tion Applications Flaw Detection,Over 80 percent of all aircraft inspections are performed visually.,Inspection Applications Flaw Detection,Weld quality requirements are commonly determined through visual inspection. Many standards have established acceptance criteria for welds.,Slag rolled into toe
36、 of weld,Transverse weld crack,Inspection Applications Flaw Detection,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 dimens
37、ions. Material alignment and allowable distortion.,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 re
38、quired. The gage used is determined by the application.,Fillet gage set,Palmgren gage,VWAC gage,Cambridge gage,Dimensional Conformance,Visual inspection is commonly used to determine weld size and tolerances according to standards and engineering specifications.,Throat measurement using a Palmgren g
39、age.,Leg size determination with fillet gage.,Convexity measurement with VWAC gage.,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.,Dimensional Conformance,Dimensional Co
40、nformance,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.,Dimensional Conformance A
41、lignment/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.
42、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
43、 flatness.,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 vari
44、ous 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.,Equipment Basic Measurements,One of the most common tools used in visual inspection is the rule or scale. Used to measure
45、 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 relati
46、ve to the application.,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, w
47、ear on components, and fit between components.,Equipment Precision Measurements,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 ar
48、e available in a wide variety of shapes and sizes. Micrometers may be either thousandth inch (.001”) or ten thousandth inch (.0001”) measurement capable.,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
49、”) 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.,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.,
