This report evaluates some of the challenges faced with 2D camera based on-machine metrology and potential options with using 3D sensors for such Direct Write applications. Specifically, in order to fully exploit 3D direct write technology to surfaces in excess of 45 degree to the print direction, non-planar motion employing 4th and 5th rotary axes are often necessary. This report will outline a procedure for doing high accuracy rotary axis calibration. Furthermore, the use of online metrology solution to enable tuning of the rotary axis as well as for online print characterization will be detailed. These efforts will provide a fresh impetus to the use of 3D sensors for on-machine monitoring applications in additive manufacturing.
Visual Inspection is the most common means for inspecting manufactured parts for random defects such as pits, scratches, breaks, corrosion or general wear. The reason for the need for visual inspection is the very random nature of what might be a defect. Some defects may be very rare, being seen once or twice a year, but May still be critical to part performance. Because of this random and rare nature, even the most sophisticated image analysis programs have not been able to recognize all possible defects. Key to any future automation of inspection is obtaining good sample images of what might be a defect. However, most visual check take no images and consequently generate no digital data or historical record beyond a simple count. Any additional tool to captures such images must be able to do so without taking addition time. This paper outlines the design of a potential visual inspection station that would be compatible with current visual inspection methods, but afford the means for reliable digital imaging and in many cases augmented capabilities to assist the inspection. Considerations in this study included: resolution, depth of field, feature highlighting, and ease of digital capture, annotations and inspection augmentation for repeatable registration as well as operator assistance and training.
One form of additive manufacturing is to use a laser to generate a melt pool from powdered metal that is sprayed from a nozzle. The laser net-shape machining system builds the part a layer at a time by following a predetermined path. However, because the path may need to take many turns, maintaining a constant melt pool may not be easy. A straight section may require one speed and power while a sharp bend would over melt the metal at the same settings. This paper describes a process monitoring method that uses the intrinsic IR radiation from the melt pool along with a process model configured to establish target values for the parameters associated with the manufacture or repair. This model is based upon known properties of the metal being used as well as the properties of the laser beam. An adaptive control technique is then employed to control process parameters of the machining system based upon the real-time weld pool measurement. Since the system uses the heat radiant from the melt pool, other previously deposited metal does not confuse the system as only the melted material is seen by the camera.
The making of composite parts involves laying down multiple layers of tape in an organized manner. Misplaced ends, wrinkles or other factors can cause the part being built to have weaknesses or other imperfections. However, the actual edges of the tape do not stand out well with each layer reacting differently to lighting. The fiber nature of the tape will make the surface appear bright in some orientations and very dark in other. To complicate the problem, each layer of tape needs to be laid down at different angles, so can be dark, light or in-between, and at positions to fairly tight tolerances. This paper presents a study of several methods for determining the tape position and flaws, as well as details of a structured light method for determining the tape position. Considerations of tolerances, experimental results and how such a system might be implemented will be presented.
Due to the high temperatures and stresses present in the high-pressure section of a gas turbine, the airfoils
experience creep or radial stretching. Nowadays manufacturers are putting in place condition-based maintenance
programs in which the condition of individual components is assessed to determine their remaining lives. To
accurately track this creep effect and predict the impact on part life, the ability to accurately assess creep has become
an important engineering challenge. One approach for measuring creep is using moiré imaging. Using pad-print
technology, a grating pattern can be directly printed on a turbine bucket, and it compares against a reference pattern
built in the creep measurement system to create moiré interference pattern. The authors assembled a creep
measurement prototype for this application. By measuring the frequency change of the moiré fringes, it is then
possible to determine the local creep distribution. However, since the sensitivity requirement for the creep
measurement is very stringent (0.1 micron), the measurement result can be easily offset due to optical system
aberrations, tilts and magnification. In this paper, a mechanical specimen subjected to a tensile test to induce plastic
deformation up to 4% in the gage was used to evaluate the system. The results show some offset compared to the
readings from a strain gage and an extensometer. By using a new grating pattern with two subset patterns, it was
possible to correct these offset errors.
Characterization of a surface shape and finish has been vital for the manufacture of precision parts. Overall profile, surface finish and waviness of a part can be measured in two ways, contact and non-contact. In the contact method a stylus is dragged on the surface of a part to measure the profile and texture of the part for quantifying the surface characteristics. Non-contact methods applied z precision metrology include: microscopy, interferometry, chromatic confocal microscopy and laser profiling such as structured light methods. The chromatic confocal method offers flexibility because of its fiber optics probes that can be manipulated to accommodate many sample geometries. This flexibility provides a wide range of possible analysis dimensions such as cylindrical shapes of holes and the potential to provide both surface roughness and shape. This paper will discuss the setup and testing of a system specifically for measuring cylindrical shaped parts and present the performance of the technology as a precision metrology tool.
Moiré imaging has been used to measure creep in the airfoil section of gas turbine blades. The ability to accurately
assess creep and other failure modes has become an important engineering challenge, because gas turbine
manufacturers are putting in place condition-based maintenance programs. In such maintenance programs, the
condition of individual components is assessed to determine their remaining lives. Using pad-print technology, a
grating pattern was printed directly on a turbine blade for localized creep detection using the spacing change of
moiré pattern fringes. A creep measurement prototype was assembled for this application which contained a lens,
reference grating, camera and lighting module. This prototype comprised a bench-top camera system that can read
moiré patterns from the turbine blade sensor at shutdown to determine creep level in individual parts by analyzing
the moiré fringes. Sensitivity analyses and noise factor studies were performed to evaluate the system. Analysis
software was also developed. A correlation study with strain gages was performed and the measurement results
from the moiré system align well with the strain gage readings. A mechanical specimen subjected to a one cycle
tensile test at high temperature to induce plastic deformation in the gage was used to evaluate the system and the
result of this test exhibited good correlation to extensometer readings.
Measurement of surface finish in industrial manufacturing has traditionally been done by means of either visual
comparison with reference plates or by the use of contact stylus based profilers. There are many challenges associated with contact profilers such as stability during measurement in an industrial environment, damage and wear of the tip, measurement in tight spaces or on curved surfaces and just the limited amount of data obtained by a linear scan of the stylus. Many alternative methods have become available such as white light interferometry, focus based systems, and even laser scatter. This paper will present the result of testing of the commercially available methods with particular emphasis on the fine surface finishes demanded in today’s manufacturing, then presents some alternative methods that show strong potential to address some of the challenges mentioned above that are not in wide use today. The analysis will specifically explore some of the physical mechanisms that affect the stylus based measurement, as well as the limitations of many of the optical approaches related to view angle and diffraction limited resolution consequences. The area of confocal imaging will be specifically explored as to how it might be used to obtain more complete data on very fine surface finishes.
In some applications such as field stations, disaster situations or similar conditions, it is desirable to have a contactless,
rugged means to collect fingerprint information. The approach described in this paper enables acceleration of the
capture process by eliminating an otherwise system and finger cleanup procedure, minimizes the chance of the spread of
disease or contaminations, and uses an innovative optical system able to provide rolled equivalent fingerprint
information desirable for reliable 2D matching against existing databases. The approach described captures highresolution
fingerprints and 3D information simultaneously using a single camera. Liquid crystal polarization rotators
combined with birefringent elements provides the focus shift and a depth from focus algorithm extracts the 3D data. This
imaging technique does not involve any moving parts, thus reducing cost and complexity of the system as well as
increasing its robustness. Data collection is expected to take less than 100 milliseconds, capturing all four-finger images
simultaneously to avoid sequencing errors. This paper describes the various options considered for contactless
fingerprint capture, and why the particular approach was ultimately chosen.
Pattern projection using physical gratings or interference effects has successfully been used to perform 3D measurements of parts. However, such systems lack the flexibility to adjust light levels over the area illuminated, leaving some areas too dark or too light to measure, or the ability to mask out parts of the illumination field, often creating spurious reflections and noise from areas not of interest. LCD/DMD digital projection systems have been used to create flexible projection of patterns, but they are limited in their resolution and ability to accurately reconstruct a smooth light pattern such as a sine wave, creating more a binarized approximation. This paper describes a method that combines together a computer-interfaced projector, such as an LCD or DMD based projector, with a high-resolution pattern projection system. The result is a system that has high depth resolution, but with the added flexibility of a programmable light source to control light levels and areas of illumination. This paper will discuss the pros and cons of this method, and suggest ways this approach might be applied to difficult part measurement problems.
SC583: How to Use Machine Vision In Your Industrial Inspection Application
This course will provide the potential user, developer, or current practitioner with a broad understanding of the tools available for the application of machine vision and optical metrology methods as applied to industrial inspection and measurement needs. Material covered in this course will include the selection of optical components such as lenses, filters, cameras and light sources to achieve specific performance objectives. In addition, this course will review a wide range of standard and specialized methods of lighting, imaging and analyzing industrial parts using current off-the-shelf systems and software packages. Finally, this course will provide case study examples of successful applications and the steps needed to reach such success. Attendees are encouraged to bring their application problems for a hands-on workshop session with actual machine vision hardware and software at the end of the course.