Zirong Zhai

Effects of light wavelength and coherence in structured light sensors

Structured light methods are used by many commercial products on the market today. Many such systems using white light projectors while many line gages use standard red laser diodes. However, in recent years there has been much claimed about using blue light, polarized light and partially coherent systems to obtain better performance. Unlike interferometers, moving from red to blue light for a system using only geometric shape information does not gain an automatic advantage from the shorter wavelength. The sensitivity metric does not have a wavelength component to it. But there are other factors that can improve gage performance. The ability to measure some feature is also a function of other parameters such as signal to noise ratio, reflectivity variations, and depth-of-field over which a clear pattern can be seen. This paper will explore the theoretical and experimental data relating to what works and what can be expected from variations on the old methods.

Super finished surface roughness measurement sensor for hard access area

The accurate measurement of surface roughness is essential in ensuring the desired quality of machined parts. Today the most common method to check roughness is to use a contact stylus profiler. There are many challenges for the application of a contact stylus for measuring the surface roughness of machined parts especially for the hard to access areas such as slots, deep holes and curved pockets. This study proposes a surface roughness measurement technique for these hard to access areas that are based on the measurement of the statistical analysis of the specular and scattered light intensity. In order to solve the accessibility issue, a miniaturized roughness measurement sensor with an Ø3mm diameter was designed using a dual laser emitting configuration. This dual laser approach makes the system more durable and reliable for different materials and machining processes such as polishing, lapping and precision grinding. This paper will present initial research results that demonstrate the applications of this approach to real industrial parts. Tests were performed to evaluate the sensitivity and feasibility of the proposed technique with various machined surfaces and materials.

Industrial surface finish method comparison for fine finish measurements

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.