March 2009 - Level 3 Inspection

Archive for March, 2009

What is White Light Interferometry Scanning (WLS)?

White Light Interferometry Scanning (WLS)

While white light interferometry is certainly not new, in fact the development of scanning white-light interferometry is in many ways a back-to-basics scenario. As interferometry progressed from using white light to monochromatic light to lasers to computerized fringe analysis to phase shifting techniques, the path has actually led right back to white light. Scanning white-light interferometry combines the power of modern high-speed computers with the vast amount of surface information produced by white-light interferometry. This permits WLS-based systems to measure surface features far more accurately than those measurable with conventional phase-measuring interferometry techniques.
White-light interferometry scanning (WLS) systems capture intensity data at a series of positions along the vertical axis, determining where the surface is located by using the shape of the white-light interferogram. The white light interferogram actually consists of the superposition of fringes generated by multiple wavelengths, obtaining peak fringe contrast as a function of scan position, that is, the red portion of the object beam interferes with the red portion of the reference beam, the blue interferes with the blue, and so forth. In other words, a prodigious amount of data is available in white-light interferograms.
Conventional WLS systems use fringe contrast to yield surface information. Frequency domain analysis (FDA) is an alternate approach that uses all of the information available in the interferogram. This Fourier analysis method is used to convert intensity data to the spatial frequency domain, allowing production of an extremely accurate surface map.
In a WLS system, an imaging interferometer is vertically scanned to vary the optical path difference. During this process, a series of interference patterns are formed at each pixel in the instrument field of view. This results in an interference function, with interference varying as a function of optical path difference. The data are stored digitally and Fourier-transformed into frequency space.
At this point the original intensity data are expressed in terms of interference phase as a function of wavenumber. Wavenumber k is just a representation of wavelength in the spatial frequency domain, defined by k = 2p/l. If phase is plotted versus wavenumber, the slope of the function corresponds to the relative change in group-velocity optical path difference DG by Dh = DG/2nG where nG is group-velocity index of refraction. If this calculation is performed for each pixel, a three-dimensional surface height map emerges from the data.
In the actual measuring process, the optical path difference is steadily increased by scanning the objective vertically using a precision piezoelectric positioner. Interference data are captured at each step in the scan. In effect, an interferogram is captured as a function of vertical position for each pixel in the detector array. To sift through the large amount of data acquired over long scans, a patented technique involving both acquisition and processing algorithms is used. This method allows the instrument to reject raw data that do not exhibit the intensity variations that indicate white-light fringes.
Using discrete Fourier-transform techniques, the intensity data as a function of the optical path difference are converted to the spatial frequency domain.

Six Sigma and CAI

Computer-Aided Inspection (CAI):

Six Sigma is a highly disciplined process that focuses on developing and delivering near-perfect products and services. Sigma is a statistical term that measures how far a given process deviates from perfection. The central idea behind Six Sigma is that if you can measure how many defects you have in a process, you can systematically figure out how to eliminate them and get as close to zero defects as possible. Manufacturing companies around the world are implementing the Six Sigma process to:
  • Improve customer satisfaction
  • Maximize process efficiencies
  • Increase competitive advantage and market share
  • Save millions of dollars in operating expenses
Jack Welch, former CEO of GE, says that Six Sigma is “the most important initiative GE has ever taken…it is part of the genetic code of our future leadership.”
Many companies implementing Six Sigma use a process called DMAIC (define, measure, analyze, improve and control) for continued improvement. DMAIC is a systematic, scientific and fact-based process that eliminates unproductive steps. It helps companies fulfill the vision of Six Sigma.
New computer-aided inspection (CAI) technologies can drastically improve the “measure” and “analyze” part of the DMAIC process, making them valuable tools in the Six Sigma approach to quality.

The Bottleneck

It is well documented that three-dimensional verification is frequently the slowest, most expensive and disruptive element of the manufacturing process. Advances in CAD/CAM/CAE and digital technologies continue to improve other manufacturing processes, but the dimensional inspection bottleneck has remained intact.
Manufacturing companies spend a significant percentage of their resources to develop 3D CAD models that define the design of a part. But, the efficiencies of the 3D CAD model do not carry over into the verification and inspection processes.
Designers break the 3D models into 2D drawings and apply constraints to determine product configuration. Geometric design and tolerance (GD&T) assigned in 2D drawings are then evaluated in the 3D models, yielding a complex web of dependencies. Changing and maintaining the 2D drawings requires considerable effort, and the process contains a great deal of duplication.

Today’s Inspection Process

Most inspection today is done by contact metrology, the science of physically determining an object’s dimensions. Contact metrology includes everything from simple manual tools to coordinate measurement machines (CMM).
A CMM system consists of a platform on which the desired measurement object is fixed. The station has an arm where a touch probe is attached. The machine makes direct contact with the object being measured at a rate of approximately one contact measure per second. CMM systems vary in size depending on the objects they need to measure. They are slow, large, expensive and susceptible to environmental temperature variation.

The CAI Alternative

CAI aims to remove the bottleneck of dimensioning verification in the manufacturing process. It is not intended to replace metrology, but rather to introduce a faster and easier process for directly comparing as-built parts with their 3D CAD definition.
There are two widely used non-contact measurement systems for CAI: 3D scanners and laser trackers. 3D scanners are commonly used for range finding. They work by projecting a laser beam onto an object and measuring its reflected image with a positioning sensor. 3D scanners are fast and can be mounted onto an existing CMM.
White Light Scanning is also known as interferometry. By knowing the wavelength of the emitted light and the distance between fringes, the distance between two points can be computed.
Non-contact white light measurement systems collect data much faster than CMM’s. It is common to collect tens of thousands of points per second. With massive amounts of data being transmitted to a computer for processing, software based on complex mathematics and sophisticated algorithms must be used to interpolate the data quickly.

The CAI Wish List

The new wave of CAI systems combine fast, non-contact measurement machines with software that aligns, compares, evaluates and reports the deviation between as-built parts and the 3D CAD reference model. Market research has determined that customers want the following attributes in a CAI solution:
  • Accuracy of 0.001mm to 0.005mm
  • Speed of more than 10,000 points per second
  • Portability for in-process measurement
  • An open system for rapid conversion to standard or native CAD formats
  • Automatic align, compare, evaluate and report process
  • Repeatable setup, process and results
  • Ease of use so it can be deployed on the shop floor by inspectors with no CAD training
  • Go/no-go displays that enable an automatic pass/fail check with pre-defined tolerances

Fulfilling the CAI Promise

CAI software will become a mainstream application and an essential part of the digital product development cycle. Pushing quality inspection into earlier phases of the development cycle will speed product development and ensure greater quality. As supplier networks continue to expand and products are increasingly differentiated according to quality, how well a company implements the inspection process can be a make or break factor in the marketplace.
CAI is beginning the same way as most new technologies. The innovators are relatively small companies without extensive marketing resources to penetrate deeper into large manufacturing operations. System integrators and consultants are needed to customize the new CAI technologies and integrate them into existing digital manufacturing processes.
CAI can eliminate the requirement for generating 2D drawings for parts and tools, which can save hundreds of engineering hours. It places the verification and inspection process into the 3D digital realm, where it can be fully integrated into existing CAD/CAM/CAE processes. This will allow manufacturers to reap greater benefits from their CAD/CAM/CAE investments and bring them closer to the promise Six Sigma.
The biggest obstacle to CAI acceptance is the inertia caused by familiar ways of doing things. Unwillingness to take a risk is always a deterrent to innovation. But, the difference between CAI and earlier digital processes is that there is much less risk. CAI technologies complement, rather than replace, the CAD/CAM/CAE systems manufacturers already have in place. Early adopters of CAI will get a head start in establishing processes whose benefits will only multiply with time.