Radial scan form measurement for freeform surfaces with a large curvature using stylus profilometry

Abstract

Stylus profilometry is one of the most significant technologies to measure freeform surfaces. However, the lateral force on a stylus usually leads to invalid measurements, especially for surfaces with a large curvature, due to the structure of the cantilever. Thus, we propose a novel radial scanform measurement for such surfaces, with an aim to reduce measurement errors. The precise 3D form of the workpiece was obtained using a series of profiles measured individually along the radial direction via a rotary mechanism. The lateral slope and force on the stylus for every profile are therefore much lower than those measured by traditional raster scan form measurements. We also propose a stable and accurate two-step registration procedure to evaluate the freeform surface form error; this eliminates the need to calibrate the spindle of both the workpiece and the rotary mechanism. A freeform surface with a 30 mm diameter aperture and 24 mm curvature radius was measured on a Form Taylorsurf PGI 1250A profilometer. Compared to traditional raster scan 3D form measurements, the peak-to-valley and root mean squared values of the form error decrease from 409.415 µm and 67.490 µm to 4.967 µm and 0.658 µm, respectively, via the proposed method. Results show that our method solves the problems of measuring surfaces with a large curvature.

Advances in the design and manufacturing of novel freeform optics

“…This instrument is functionable for slopes up to 50 • and resolution down to 0.8 nm. Contact-type radial form measurement technique is proposed to reduce the measurement errors [207]. An aluminium alloy mould of large curvature freeform surface is measured using Form Taylorsurf PGI 1250A profilometer with stylus of 400 µm radius ball tip of part number 112-3450, as shown in figure 25.…”

Exploring a Feedback-Oriented Design Process Through Curved Folding

“…With this technique, they were able to obtain highly accurate physical data of freeform surfaces. Various other methods have been suggested for computing and improving the accuracy of complex curved surfaces (e.g., via a rotary scanning configuration) [46] in the design and engineering fields [26].…”

Precise surface form measurements of off-axis aspheric freeform optics

“…They provide direct and highly repeatable profiles at a high resolution regardless of optical reflection or surface textures. It is noted that the structure of the cantilever and the tangential slope of the off-axis optical surface makes it inevitable that profile errors will be introduced on a stylus by lateral force [5]. As for the optical probe, the large tangential slope leads to the over range of the angular property of the probe, which brings the challenge of the high-precision measurement.…”