Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1999-04-21
2001-04-24
Rosenberger, Richard A. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
Reexamination Certificate
active
06222628
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of surface measurements and metrology. More specifically, the invention relates to optical devices for optically measuring characteristics of a surface such as roughness, waviness and/or form error.
2. Description of Related Art
The global economy and the ever increasing demands of competition has led to ever increasing quality of manufactured products. In this quest for quality, manufacturing methods and machines have been created to attain the engineering requirements specified in these products. As manufacturers seek to produce products with more desirable surface characteristics and to tightly control quality of these surfaces, technology related to the field of surface characteristics measurement and metrology have continued to develop and evolve. For example, in the metal processing industry, the measurement of surface characteristics is critical in determining the quality of ground or rolled metal products. In addition, surface characteristics measurement must be made on the mills, rollers, molds and other processing equipment which are used to manufacture metal products in order to ensure that the quality of the products meet or exceed the engineering design specifications. Of course, surface characteristics measurement is critical in many other industries in addition to the metal processing industry such as plastics, textiles, paper, composites, silicon processing and glass industries to name a few.
Various surface characteristics may be described and quantified to describe the physical attributes of any given surface. Such characteristics of particular interest include surface roughness, waviness and form error which are all currently measured and monitored in the above noted industries as well as others. These three surface characteristics all describe the irregularities which are present in all surfaces. These surface characteristics are related terms of art and are differentiated primarily by the wavelength and the amplitude of a particular irregularity such as a peak or a valley on the surface. In this regard, a reference parameter “G”, has been established in order to allow this differentiation. The reference parameter G is defined as the ratio between the amplitude of the irregularity and one wavelength of the irregularity (i.e. distance between consecutive irregularities). Thus, surface roughness is generally characterized by 0.01<G<0.2; waviness is generally characterized in that 0.001<G<0.01; and form error is generally characterized in that G<0.001. In absolute numeric terms, surface roughness in metals and metal manufacturing is generally considered to have a wavelength &lgr;<500 &mgr;m. In these industries, waviness is generally considered to have a wavelength &lgr; between 500 &mgr;m and 1 cm whereas form error generally has a wavelength &lgr;>1 cm. As described, it should be understood that these three surface characteristics are differentiated primarily by the size of the wavelength. Thus, the above cited measurements are general ranges only and may differ between applications and various industries.
Surface roughness has been of particular interest to various industries including the steel and machine industries. In these industries, the surface roughness is quantified by measuring an “Ra” value which is defined as the arithmetical average profile deviation of the surface irregularities with respect to a hypothetical perfect surface established by an arithmetical averaged line. Because of the importance in obtaining accurate surface roughness measurements, many devices have been developed to measure the Ra values of a surface. For instance, mechanical devices have been developed including profilometers that have a probe such as a stylus which is brought in contact with the surface of the object being measured. The stylus is then horizontally moved across the surface for a predetermined distance. During this horizontal movement, the stylus is moved in a vertical direction following the peaks and valleys of the irregularities on the surface thereby providing a profile of the surface being measured. This vertical displacement generates an electrical signal which may then be used with the known horizontal displacement to determine the surface roughness. Such profilometers are known in the art and is generally described in U.S. Pat. No. 5,778,551 to Herklotz et al.
Although these profilometers have gained substantial popularity in industry, there are several disadvantages which limit their applicability. These disadvantages include the fact that the object being measured must be physically contacted by the probe in order to obtain the roughness measurement. This contact can cause scratches and additional irregularities on the surface being measured. Other disadvantages include limitations on accuracy and repeatability since the probes have a physical dimension and will alter the surface as it is moved across the measured surface. In addition, the profilometer is not practical for use in many manufacturing settings such as in a production line because the object to be measured must be stopped and the measurement process itself takes a relatively long time. Furthermore, many manufacturing environments are subject to vibrations which can render the profilometer measurements inaccurate and useless. For these reasons, profilometers are commonly used in laboratory environments and have not been effectively implemented in manufacturing environments.
Optical devices which allow non-contact measurement of surfaces have been developed in order to avoid the above noted disadvantages of mechanical designs. These optical devices detect the image of an illuminated point such as those created by a laser beam on the surface to be measured. Two categories of such optical devices known and used in industry are light scattering systems and triangulation systems.
The light scattering systems measure a surface characteristic by measuring the amount of a light beam scattered by the surface; or conversely, by measuring the intensity of light beam not scattered by the surface. Such light scattering systems generally operate by deflecting a laser beam at a predetermined angle off the surface to be measured. This deflected laser beam is somewhat scattered by the surface irregularities thereby creating a diffused field where the light is deflected in various directions depending upon the surface irregularities. This scattering of the laser beam correspondingly decreases the intensity of the deflected specular beam. The deflected specular beam is then directed on to a photodiode which generates a signal in proportion to the intensity of the deflected specular beam. Since the intensity of the light beam initially emitted by the laser is known, the desired surface characteristic can be determined by processing the signal from the photodiode. More specifically, the signal which corresponds to the intensity of the deflected specular beam (or conversely, the reduction of the initial laser beam) may be correlated with known surface characteristics such as roughness. In other systems, the diffused fields of the laser beam may be detected by photodetectors to provide a signal corresponding to the intensity of these fields in order to determine surface characteristics. Such light scattering systems are illustrated and discussed in U.S. Pat. No. 3,771,880 to Bennett, U.S. Pat. No. 4,364,663 to Gardner et al. and U.S. Pat. No. 5,608,527 to Valliant et al. Another related light scattering system is disclosed in U.S. Pat. No. 5,661,556 to Schiff et al. which utilizes a hollow sphere to measure the total laser light scattered on a surface to determine the correlated roughness of the surface.
In contrast to the light scattering systems described above, the triangulation systems measure surface characteristics by detecting a position of diffused light on a position sensing device (PSD). More specifically, such triangulation systems operate by focusing a laser beam on a point at a
Corallo Valeriano
Docchio Franco
Minoni Umberto
Zorzella Emidio
Leedom Jr. Charles M.
Nixon & Peabody LLP
Rosenberger Richard A.
Song Daniel S.
Techint Compagnia Tecnica Internazionale S.p.A.
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