Extensive microstructural quality control inside a machine tool using multiwavelength digital holography

Abstract

With state-of-the-art 3D measurement systems, short-wave structures such as tool marks cannot be resolved directly inside a machine tool chamber. Up to now, measurements had to be performed outside the machine tool. We present an interferometric sensor that carries out such measurements inside the machine tool, which saves time-consuming and expensive setup procedures. Our sensor HoloCut uses digital holography as measurement principle. By the use of multiple wavelengths, we get a large unambiguous axial measurement range of up to 2 mm and achieve micron repeatability, even in the presence of laser speckles. With a lateral resolution of 7 µm across the entire 20 x 20 mm² field of view, both macro-and microstructures (such as tool marks) are measured with an axial resolution of 1 µm. Consequently, this qualifies HoloCut for in-situ measurements and integration in a machine tool. In this paper, the boundary conditions of integrating interferometers inside a machine tool are evaluated. Occurring vibrations and limited available space are particularly challenging constraints: The optical and mechanical design of HoloCut is introduced along with numerical correction algorithms: A piezo-stage setup is used to induce known displacements. Using these algorithms, measurements even with a closed-loop control of the machine tool head activated are demonstrated on a coin measurement. The use of HoloCut is motivated on the base of the daily operation of a 5-axis machine tool: We present an evaluation of an exemplary ISO 25178 parameter Sq using HoloCut measurements and compare those with reference, yet not inline-capable systems.

Industrial applications of digital holography

“…In conclusion, holography is a serious alternative to time-consuming sample testing with coordinate measurement devices in a temperate-controlled measurement room. In a next step, we are aiming at realization of a miniaturized wireless sensor to be used within a tooling machine [6] and to perform holographic measurements of continuously moving objects [7] .…”

Multiwavelength digital holography in the presence of vibrations: laterally resolved multistep phase-shift extraction

“…Multiwavelength digital holography is an established technology for nondestructive testing in industrial applications [3]. However, as an interferometric measurement principle, digital holography is very sensitive to vibrations [4]. In order to compensate imperfect phase shifts in temporal phase-shifting, Cai et al introduced a generalized approach of phase shift extraction for unknown phase shifts [5].…”

“…Temporal phase-shifting is applied by a piezo actuator in open-loop mode: For each wavelength, three interferograms are recorded with phase shifts of approximately 2π/3. In addition, a combination of lateral [4] and critical axial oscillations (orange arrow in Figure 1 right) due to the closed-loop position controller of the machine tool superimposes the phase shift introduced by the piezo. Cai et al describes calculation of global phase shift between single interferograms using the mean modulation between images [5].…”