WM | RS-C - Optical, short-coherent roughness measurement

In addition to dimensional accuracy and positional deviation, roughness is increasingly becoming the focus of quality assurance in the production of components. Functional properties and structures of component surfaces must be tested in order to guarantee properties such as sealing, lubrication, friction or wear behaviour. Therefore, microscopic structures are increasingly measured in the CMM environment and characterised with the help of roughness parameters.

Published in the inVISION in September 2020

However, the known, profile-based roughness parameters Ra and Rz only provide limited information due to the calculation method. Therefore, it is necessary to additionally derive function-oriented parameters from measurement data, which are based on material distribution analysis (e.g. Rk, Rpk and Rvk), and to additionally consider 3D parameters (e.g. Sa or Sz). The latter have a clear advantage over the profile-based measuring principle of tactile probes due to the two-dimensional measurement and better statistical evaluation. This is one reason why optics is gaining more and more acceptance in industrial applications.

In order to take this trend into account in the CMM environment, it is advantageous not only to determine form and position deviations, but also to be able to evaluate the surface quality microscopically as part of automated measurement processes. With the aid of CMMs, a 3D reference to the workpiece or the manufacturing process is created and thus a reference to the component geometry is established. The new WENZEL sensor WM | RS-C is ideally suited for use in this application area.

Figure 1: White light interferometer WM | RS-C for measuring roughness and microstructures in detail view

For the non-destructive, optical measurement of topographies, the sensor has interesting unique selling points: The WM | RS-C is an area-measuring interferometer with FULL HD resolution (1920 X 1080) that combines the extreme, vertical resolution of an interferometer (in the nanometre range) with an extreme lateral resolution of only 55 nm (100X objective). The sensor is optionally operated with a piezo-electric drive or an external actuator and can optically resolve the finest microstructures down to the physical diffraction limit. Such fine structures cannot be detected by far with destructive measuring probes due to the Hertzian pressure and the morphological profile filtering (caused by comparatively large probe tip radii of 2 to 5 µm). This fact makes the sensor interesting for numerous applications, such as the measurement of ground, polished, lapped or honed surfaces. Applications from semiconductor technology, wafer production, technical microstructuring and medical technology round off the sensor's range of use, even in very demanding areas of application. Due to the two-dimensional measuring principle, the sensor also enables a far better statistical evaluation of surfaces compared to tactile measuring systems. This knowledge is gradually gaining acceptance in industrial applications and is also influencing roughness standardisation, for example (e.g. the standardisation of 3D roughness parameters).

The WM | RS-C enables the measurement and export of topographies, point clouds and triangulated STL meshes with more than 4 million triangles per single measurement. The measurement data can be provided in a measurement time of less than 30 seconds. Finally, 2D and 3D roughness analyses according to DIN EN ISO are carried out specifically on the basis of the measurement data and roughness parameters are output as a report.

Figure 2: Exemplary representation of microscopic surfaces measured with the new WENZEL roughness sensor WM | RS-C. Top left: Superfine roughness standard. Top right: Depth adjustment standard (groove standard with a groove depth of 75 nm). Bottom left: Ground aluminum. Bottom right: Glass scale with periodic structure.

Another special feature of the WM | RS-C is its size, which was achieved by optimizing the beam path: The sensor is no larger than a standard Smart phone and, thanks to its industry-compliant Gigabit Ethernet interface, can be quickly and easily adapted to existing CMMs. New evaluation algorithms enable measurement even in the presence of machine or environmental vibrations, which promises enormous potential, especially in the manufacturing environment. This means that the sensor can also be operated on CMMs such as the WENZEL LH, the WENZEL CORE or on robot arms in the store floor area. Thanks to optional CMM adaptation, it is also possible to establish a global coordinate reference between the microscopic topography and the coordinate system of the machine or workpiece. Consequently, multi-sensor applications in different scale ranges are also possible.

In addition to the technical aspects, the control and evaluation software of the WM | RS-C is another central element in practical sensor use: The sensor is operated with the WM | PointMaster software. This provides its own module for data acquisition and sensor control. The software also includes its own roughness and evaluation tool, which can be used to analyse surfaces for roughness in accordance with standards. With the help of the software, profile sections and spline curves can be defined interactively on the surface. Accordingly, roughness analyses are possible that go far beyond purely linear evaluations. The implemented standards include DIN EN ISO 16610 (for standard-compliant filtering), DIN EN ISO 4287 (for calculating roughness parameters such as Ra and Rz), as well as DIN EN ISO 13565 (for calculating Abbott-based parameters such as Rk, Rvk and Rpk). Furthermore, analyses of the measured point clouds or the STL meshes (for example of micro surfaces) are possible without restrictions.