Steinbeis experts compare optical confocal and white light measurement technology with tactile roughness measurement
The Steinbeis experts working at the Steinbeis Transfer Center in Herzogenaurach are specialists in the field of bearings. For some time now, they have been looking closely into optical measurement technology used with rolling bearing components. The reason for this is that there has been a noticeable rise in quality requirements in the rolling bearing industry, and demand for high-precision measurement technology is intensifying. Optical measurement methods make it possible to analyze materials with incredible accuracy, even down to the nanometer, and some technologies based on contactless measurement can now be integrated into existing production processes. But how closely do tactile roughness measurements correspond with measurements taken using optical methods? This was the question investigated in research conducted by the Steinbeis experts.
The normal way to ascertain the quality of a surface is to use tactile measurement methods – involving contact. But over the last 30 years optical methods have become more and more prevalent. These are capable of capturing features without making any kind of contact with the surface. To assess the potential offered by optical measurement, results are compared and contrasted to standardized tactile measurements according to DIN standard EN ISO 4287.
The researchers conducted a series of experiments with a multisensory device called the FRT MicroProf® 200. The equipment, supplied by the metrology specialist FRT, comprises a confocal chromatic white light sensor (CWL) and a confocal microscope (CFM) . It is capable of determining surface topography, roughness, and contours. Using optical measurement techniques even makes it possible to analyze components with complex geometries. As well as measuring simple lines, optical procedures allow surfaces to be measured down to less than one millimeter.
The white light sensor integrated into the measurement unit is based on a combination of confocal measurement principles and the principle of chromatic depth scanning. The chromatic effect of the lenses fans out white light along optical axes, creating focal points of different wavelengths. The confocal capability of this measurement technique means that the spectrometer only detects points that are in focus. Each individual wavelength is given a height coordinate. The sensor is capable of continually determining a plethora of points to provide measurements across a wide area . To measure an area 5 x 5mm, the system scans along 5,000 lines, each containing 5,000 points on the surface of an outer bearing ring (“race”).
This technique using a confocal microscope in the measurement unit is based entirely on confocal measurement principles. Light emitted by the microscope lands “in one go” on the focal plane of an object. Rays are then reflected and picked up by a detector. Recording the intensity of reflected light makes it possible to assess the height of confocal measurements. The system includes an integrated Nipkow disk which allows images to be captured across an area measuring up to 890 x 655μm . The measurement system requires surfaces consisting of several layers one on top of the other, such that only one confocal point is measured within a single layer. To allow the measuring device to determine new confocal points on new layers, each current focal point is taken one step away from the surface using an adjuster. The measurement method developed by the Steinbeis experts involved merging individual image fields into a measuring range of 5 x 5mm.
To take tactile measurements, the researchers from Herzogenaurach used a roughness measuring device called the MarSurf LD 130, which was supplied by Mahr. This offers a profile method using a contact stylus according to DIN standard EN ISO 3274. The device has a conical diamond mounted on the tip of a contact arm with a radius of 2μm at an angle of 60°. The probe is drawn over surfaces and resulting deflections of the arm are evaluated to provide surface parameters .
The question is: What are the correlations between the tactile readings and the optical readings? To answer this question, the experts working with the two Steinbeis directors, Prof. Dr.-Ing. Stephan Sommer and Dominik Helfrich, measured the outer surface of four outer races (bearing rings) using both tactile and optical methods. Races #1 and #2 both had a homogeneously milled surface. Grooves had been added to the surface of races #3 and #4 in order to observe any effects resulting from the different measurements. The benchmarks used for each of the measurements included average roughness (Ra), arithmetic range in roughness (Rz), and skewness (Rsk). These are all in keeping with DIN standard EN ISO 4287. The Ra and Rz readings were used because they are frequently looked at in industry to assess the surface quality of rolling components. The Rsk value was added because it can be used by the rolling bearing specialists as an additional assessment parameter. If it’s negative, it indicates a plateau-like profile on the surface. Such profiles are generally good in terms of load-bearing properties . The percentile differences between the tactile and optical values are highlighted in color in the correlation table. Tactile results are shown in green to serve as a reference.
“Aside from the measurements themselves, we ascertained a tendency toward a close correlation between the tactile and confocal procedures,” explains Stephan Sommer, “especially with races #3 and #4, where there’s a good match between the confocal readings and the tactile readings.” With the white light measurements, the results showed that the Rsk values had a more pronounced deviation versus the tactile values. With races #1 and #2, the Rsk values for the white light measurements differ significantly from the tactile values. Many of the differences between the confocal and the tactile readings are between +/-30% and +/-50%. The only significant difference was found with the Rsk value from the confocal reading on Ring #2. As the tables show, the Ra and Rz parameters for the two optical methods correspond closely to the readings of the tactile methods. In comparison with the tactile sensor, however, the white light sensor does deliver a more positive value when determining Rsk. Since the value it returns is almost zero, one interpretation could be that the surface has less effective load-bearing properties than is actually the case.
Bottom line, what conclusions did the scientists come to following their measurements? As the correlation table clearly shows, the confocal measurement technique has an almost 80% correlation to the tactile method. On the other hand, the white light approach has only a 66% correlation to the tactile method. The lower percentages are due to the fact that the Rsk values of the white light measurements deviate significantly from the tactile values. This compares to the values of the confocal microscope, which generally delivers better results with the Rsk value. Not too much should be read into these deviations, however. These differences are only in the order of nanometers, so on a general level it can be assumed that there is a correlation between the two optical methods and the tactile method. In the rolling bearing industry, values in the micrometer range are considered acceptable.
“Optical measuring is not just intended for use in roughness measurement. It also offers other special characteristics. Measuring areas also makes it possible to assess several points at the same time. Information on peaks can help with things like displaying surface structures in two or three dimensions,” explains Dominik Helfrich, underscoring the benefits of the measurement technique. Measuring areas using optical methods is especially common in dentistry because it can be carried out without coming into contact with surfaces. In addition, white light sensors can be integrated into live production processes. Because the sensors can take measurements along lines, they make it possible to run 100% checks fully automatically within the shortest possible time.