3D MACHINE VISION SYSTEMS AS SHOP FLOOR METROLOGY TOOL full report
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3D MACHINE VISION SYSTEMS AS SHOP FLOOR METROLOGY TOOL
Machine vision refers to applications in which the PC automatically makes a decision based on visual input from a camera. Machine vision is a term typically used in industrial manufacturing, where applications range from culling blemished oranges from a conveyor belt to saving lives by inspecting to ensure that the correct drug capsule has been placed in the package before the product is shipped to the pharmacy. Three dimensional vision based measurement systems have made their presence into production metrology applications, notably in the electronics field. However, in the more traditional fields of durable goods now dominated by hard gauges and CMMs, 3D optical systems has been hindered by perceptions and real limitations. This paper will review where 3D vision is today, and what advances have been made to enable more quantitative, shop floor metrology applications. The field of 3D machine vision is a less established field, but one that is actively growing today. Three dimensional vision based measurements have come a long way in the past few years, moving from purely visualization tools that generate attractive color pictures, to serious measurement tools. These 3D systems include laser scanning, structured light, stereo viewing, and laser radar just to name a few.
Modern day durable goods manufacturing have begun to embrace the concepts of digitization as a means to improve productivity and quality. Moving away from expensive hard gages made for specific parts; manufacturers are seeking the means to measure parts in a flexible manner, and capture the resulting measurements by digital means. For higher volume parts, such as from forging or fast machining operations, speed of gauging is still an issue.
This is the area where machine vision based tools start to excel. Machine vision in general has been used for everything from guiding the insertion of electronic chips on circuit boards to inspecting bottles at several per second in bottling lines. A natural extension of machine vision inspection is to provide programmable measurements for machined parts. In many applications, these measurements can be made in two dimensions for which there is an established based of machine vision tools working in the sub-thousandth of an inch range at multiple measurements per second. Each of these methods has their strong points and weak points for a given application.
The key performance parameters needed for durable good manufacturing include:
Â¢ Resolution in the mil, and sub-mil range
Â¢ Speeds sufficient to complete all measurements in a few seconds
Â¢ Ability to look at a wide range of surface types and finishes
This last point, the ability to look at a wide range of surface finishes has perhaps been the biggest limitation of 3D machine vision technology. In many cases, the surface needs to be diffuse to permit reflected light to be easily seen to achieve a good signal to noise ratio.
Three dimensional optical sensors can perhaps be broken into a few basic types:-
Point scanning sensors measure only the specific points of interest, typically in a serial fashion,
Line sensors provide a single line of points in the form of a cross section of the contour of interest,
Full-field sensors provide an X, Y, Z map of all the points in the scene, which must then be analyzed down to the information of interest.
Each of these types of sensors has developed through technology which is suited to the application. In some cases, the technology is capable of multiple modes of operation (finding point on a surface, or defining the full surface) as well, but this often stretches the technology into a field overlapping other technologies. There has not to date been any single sensor in industrial applications which does everything.
DISCUSSION OF TECHNOLOGIES
There are currently three basic approaches to three-dimensional machine vision:
Â¢ range finding including structured lighting
Â¢ stereo or binocular vision,
Â¢ gray scale or range finding methods.
The most popular commercial versions of range finding use the triangulation method where a beam of light is project and implimentationed onto the object's surface at some angle and the image of this spot or line of light is viewed at some other angle (see Figure 1). As the object distance changes a spot of light on the surface will move along the surface by:
(change in spot position) = (change in distance) x (tan(incident angle) + tan(viewing angle))
Figure 1. A triangulation based system using a point of light to obtain distance.
Good resolution of a few tens of microns have been realized with these systems. Most triangulation gages today use laser light. When a laser beam is incident on an opaque, rough surface, the micro-structure of the surface can act as though it is made of a range of small mirrors, pointing in numerous directions. These micro-mirrors may reflect the light off in a particular direction, as generally machine marks do, or may direct the light along the surface of the part. Depending on how random or directional the pointing of these micro-mirrors may be, the apparent spot seen on the surface will not be a direct representation of the light beam as incident on the part surface. The effects that may be seen from a laser beam reflecting off a rough surface include:
Â¢ directional reflection due to surface ridges
Â¢ skewing of the apparent light distribution due to highlights
Â¢ expansion of the incident laser spot due to micro surface piping
The result of this type of laser reflection or "speckle" is a noisy signal from some surfaces. In like manner, there can be a problem with translucent surfaces, as the laser light will scatter through the medium and produce a false return signal. For a laser based sensor, a smooth, non-mirror like, opaque surface produces the best results. An active variation of restricting the view uses synchronized scanning. In the synchronized scanning approach (see figure 2), both the laser beam and viewing point is scanned across the field. In this manner, the detector only looks at where the laser is going.
Figure 2. A synchronized scanning system, limited the range of view.
A more extreme variation of the synchronized scanning type of approach is to use an active point seeking triangulation system (see figure 3). With an active point seeking triangulation system, a single point detector is limited to a very narrow view, at a constant angle, which is then translated across the path of the laser beam.
Figure 3. Active triangulation system, seeking laser point, providing extended range.
The detector now has the advantage that it can resolve the distribution of light seen along that particular angle, and potentially decide on which signals are the correct ones. Reflections that do not go along the view axis are not seen at all. The limitations of this approach can be more time consumed in seeking each point, and low light collection to maintain high angle separate. If as opposed to a single spot of light, a line is project and implimentationed onto the surface by imaging or by scanning a beam, as shown in Figure 1, the line will deform as it moves across a contoured surface as each point of the line move as described above. The effect is to provide an image of a single profile of the part (see figure 4). In applications requiring only a profile measurement, these techniques offer good speed of measurement.
Figure 4. Laser radar sensor scanning a box from a single view point
The individual laser probes have seen a near 10 fold improvement in resolution in the past few years. The application of such probes in the electronics industry has been a great benefit to making high speed electronics assembly possible. Scanning and fixed triangulation systems have been used to contour large structures such as airplanes, logs or airfoil shapes, and metal rolling mill flatness control. The large area systems primarily have used one or multiple lines of light to obtain a cross sectional profile at a time. In many cases, these line sensors are connected with a CMM type device to extend the working range of the sensor. The resolutions of such systems need typically be less than a millimeter, and more typically is around 2.5 microns (0.0001 inch). Full-field structured light systems, based upon project and implimentationed grids by direct sensing of the grid or related to moire are also commercially available. The primary application of this type of sensor has been the contouring of continuously curved non-prismatic parts such as turbine airfoils, sheet metal, clay models, and human anatomy. Special compact sensors for use on CMMs are also available with this technology. Most of the applications of this technology has been on applications requiring dense data, but have also been engineered to enhance video data for the purpose of 3D "comparator" type measurements on objects ranging from rolled sheet metal (for flatness) to car bodies.
The techniques described so far are based on some form of active illumination of the surface, and active sensing of that specific illumination. When people view an object, they receive 3-D information based upon the principles of parallax and shadings seen on the surface.
Stereo or binocular machine vision methods work on the same principle as human vision by obtaining parallax information by viewing the object from two different perspectives as is shown in Figure 5. (as our two eyes do).
Figure 5. Two cameras see different perspectives to provide stereo parallax information.
The two views must be matched up and then the difference in the position of a correlated feature gives the depth information from the geometry of the two view points(similar to the triangulation methods). These methods can be full-field, thereby keeping the data acquisition time to a minimum, and they do provide all the 2-D feature information. In fact, stereo depends on the surface features to the extent that a featureless, smooth surface (such as the turbine blade), can not be contoured directly with stereo views.
Many machine vision systems have a simple stereo matching capability, made for tracking individual points in space, but not for providing full surface contours as would be desired for reverse engineering applications. Resolutions of a few millimeters at certain points, and processing times of a few seconds has been typical for the current industrial applications. There have been dedicated hardware systems made, but stereo has not be able to provide the flexibility of scanning type systems for CAD data generation applications.
As has already been stated, the key operational parameters needed for production machine vision include speed, resolution, and robustness especially to changing part surface conditions. Many systems that provide the best resolution are not the fastest, so a tradeoff must be made. Just as with touch probes, there are certain types of features or surfaces that optical 3D methods can be expected to work good on, and others where there may be problems. If has been pointed out that shiny, but not specular surfaces have offered one of the biggest challenges. In like manner, when a surface changes from a shiny area to a dull, many sensors may generate a bias error. In the simple case of triangulation, the measurement is based upon finding the centroid of a light spot of some finite size. If half that spot is on an area that reflects back to the sensor well, and the other half is not, the center of brightness of the spot will not be the geometric center, but rather weighted toward the brighter region. Testing the sensor on edge and surface transition features is a valuable first test to consider (such as in Figure 6).
Figure 6. Measuring the effect across an edge boundary.
The next area of concern is the surface texture itself. A surface with machining marks has a texture which may scatter light into long lines that may confuse the sensor. In a similar manner, if the surface is translucent, the spot of light may spread out in an unpredictable manner, again offsetting the spot center, and hence the measurement made. Therefore, testing the sensor on a surface texture that matches the one to be measured is important. A final important feature consideration for many optical gages is the slope of the surface. A standard way to test such effects is to scan a diffuse sphere and fit the data to the known diameter (see Figure 7).At some angle around the sphere, the surface can be expected to lift off the true surface, before the signal is lost entirely. As with any gage, comparison of the optical gage against other measurement tools provides valuable information regarding confidence of the measurements. This is not always an easy comparison, as the repeatability of many traditional gages may not be as good as the optical gage.
Figure 7.Measurement on a sphere
Anytime one is trying to improve a process, one encounters the challenge of demonstrating a capability against a tool with less capability. The very high speeds and data densities available from optical gages offer some significant advances. Yet trying to compare those advantages against a gage which stakes everything on a half dozen points is a difficult road. However, the comparisons and qualification of the new process must be done. Comparisons against inadequate measurement tools can prove very frustrating, so at least one independent means of comparison is desirable. The use of reference artifacts that can be verified by independent means, preferable with primary standard reference (e.g., the laser distance interferometer is the international standard of length measurement), is a valuable aid in these comparison.
Machine vision systems can be used in various fields for numerous purposes as stated below:
Industrial inspection (Inspecting machine parts, Adaptive inspection systems)
Vision for Autonomous Vehicles (Detecting obstructions, Exploring new surroundings, AV surveillance)
Transport (Traffic monitoring, Aerial navigation, Transport Safety)
Surveillance(Intruder monitoring, Number plate identification, Tracking people)
Vision and Remote Sensing (Land Management, Crop classification, Surveying by satellite)
Medical (Planning radiation therapy, Chromosome analysis, Merging medical images)
As with any technology of this nature, the performance changes with the component technology. The primary advance that has made machine vision systems feasible for shop floor gauging applications has been the speed up in computing power. This has brought the processing times from 15 or 20 minutes on an expensive workstation to seconds on a standard PC. The other technologies that are influencing performance today include lower cost, digital cameras than provide better light range and pixel resolution with lower noise, and better light sources such as higher power laser diodes well as higher brightness and resolution LCD project and implimentationors. The consumer market largely influences all of these technologies, which is currently a much bigger driver than any manufacturing support system. However, as system prices decrease and performance improves, there is a wide range of new markets these systems will likely address ranging from dentistry to home 3D pictures.
1. ASME Journal of Machine Design
2. "Machine Vision Based Gaging of Manufactured Parts," K. Harding, Proc. SME Gaging '95, Detroit, Nov. 1995.
3. The Promise of Machine Vision, K. Harding, Optics and Photonics News, p. 30 May 1996 Optical Society of America
4. "Light Source Design for Machine Vision", Eric J. Sieczka and Kevin G. Harding, SPIE Proceedings Vol. 1614, Optics, Illumination and Image Sensing for Machine Vision VI, Boston, Nov. 1991.
5. L. Bieman, "Three-Dimensional Machine Vision," Photonics Spectra, May 1988, p. 81.
6. E. L. Hall and C. A. McPherson, "Three Dimensional Perception for Robot Vision," SPIE Proceedings Vol. 442, p. 117 (1983).
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