The quality control of braking systems is essential, as criteria such as durability, performance, reliability, and cost savings are among the most important factors relevant for safety and economy. Quality inspection of friction surfaces can help to detect potential problems such as surface irregularities, defects, or wear at an early stage. Therefore, the durability of braking systems can be increased, and the risk of accidents can be reduced. In addition, metrological quantification in development and production can help to ensure optimal functionality of braking systems. Problems can be identified in an early stage, allowing direct intervention in development or production processes, which in turn contributes to more cost-efficient production. Important components of brake systems are the brake discs, which can be made from a wide variety of materials, e.g., grey cast iron or steel alloy. Different manufacturing steps (e.g., the friction surface is finished by turning or grinding machines) are used to create a suitable surface characteristic that has an influence on friction, braking force and wear behaviour. The requirements for a measuring system are manifold: it must measure large components quickly and efficiently, with high accuracy, traceable and repeatable, and preferably automatically directly in the production. Additionally, several areal surface and profile surface characteristics must be determined. Optical metrology became more and more important in industry where the requirements on dimensions, manufacturing tolerances and the quality of surface finishing are continuously increasing. Optical metrology offers many advantages in terms of contact-less measurement, accuracy and repeatability. It is one of the most important tools for quality assurance in many areas of automotive or aircraft industry and is used research as well as directly during the manufacturing process. Here, an automatic measurement solution based on focus variation is presented. Focus variation is an optical measurement principle which computes topographical and colour information through focus variation. In order to achieve this, focus variation measurement devices exploit the small depth of focus of an optic with a vertical scanning process. The result is a 3D model of the measured part which can further be used for roughness and form measurement. Due to its ability to measure surface topographies with an Ra less than 9nm and to measure steep flanks with a slope angle of 90 degree and more, focus variation can be used for high resolution form and roughness measurement. Here, a novel measurement solution for the highly efficient automatic optical metrological quantification of brake disc surfaces is presented. The measurement platform uses two high-resolution focus variation sensors in combination with a lateral axis and a rotational axis to measure simultaneously both sides of the brake disk. The lateral axis is used for highly accurate large imagefield measurements across the friction ring. The rotational axis is used to automatically rotate the disk so that the measurements can be repeated at several positions along the friction ring. The metrological quantification includes profile roughness measurement according to ISO 21920 as well as areal surface measurement according to ISO 25178. By combining two focus variation sensors that measure both friction surfaces simultaneously and an automation software the brake discs can be measured in a highly efficient way. This measurement solution opens the possibilities for smart manufacturing by directly integrating the measurement platform into the production process. In order to demonstrate the capabilities of the measurement system, the strength of the focus variation sensor for areal and profile roughness measurement and its use for quality inspection on brake disks is demonstrated.
Dr. Kerstin Zangl, Software Development, R&D, Bruker Alicona; Mr. Franz Helmli, Head of R&D, Bruker Alicona; Mr. Reinhard Danzl, Head of Software Development, Bruker Alicona; Mr. Florian Schwimmer, Marketing Director, Bruker Alicona