Curvature effect on surface topography and uniform scallop height control in normal grinding of optical curved surface considering wheel vibration

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. High-precision optical components with complex shapes or microstructures have been extensively used in numerous fields such as biomedicine, energy and aerospace. In order to accurately achieve the specific functions of the components, the form accuracy and uniform surface quality need to reach an ever-high level. To achieve this, ultra-precision normal grinding is used for machining various types of complex optical surfaces. However, the intricate variation of the workpiece curvature and grinding wheel vibration gives rise to great challenges to obtain higher precision and uniform surface conditions. In this study, the influence of curvature on surface topography generation has been investigated and a novel model of scallop height has been developed for surface topography generation in the normal grinding of the curved surface. In addition, the relative influence of the curvature is analyzed experimentally, in which the micro-waviness generation as a consequence of the unbalanced vibration of the grinding wheel is modeled and validated by experiments. Finally, the micro sinusoidal array with the setting value for scallop height is achieved by controlling the feed speed, which is determined by the local curvature of surface profile. The results indicated that the curvature variation posed a significant effect on surface uniformity and the model is valid to achieve surface scallop height control in the normal grinding effectively

Research on profile compensation, on-machine metrology and precision monitoring of ultrahigh precision machining process using real-time position capture system

東京電機大学202

DEVELOPMENT AND APPLICATION OF ON-MACHINE SURFACE MEASUREMENT FOR ULTRA-PRECISION TURNING PROCESS

Optical freeform components, featured with high functional performance, are of enormous demand in advanced imaging and illumination applications. However, the geometrical complexity and high accuracy demand impose considerable challenges on the existing ultra-precision freeform machining technologies. Surface measurement and characterisation become the key to further improving machining performance. In order to further increase the metrology availability and efficiency, a shift in the approach of surface metrology from offline lab-based solutions towards the use of metrology upon manufacturing platforms is needed. On-machine surface measurement (OMSM) will not only allow the assessment of manufactured surfaces just-in-time without transportation and repositioning, but also provide feedback for process optimization and post-process correction with consistent coordinate frame. In the thesis, a single point robust interferometer is integrated onto a diamond turning lathe to establish the metrology-embedded ultra-precision manufacturing platform. To extract a priori information for the subsequent OMSM, a theoretical and experimental study of surface generation was carried out for ultra-precision turning of optical freeform surfaces. With the proposed machining methodology and surface generation simulation, two freeform surfaces (sinusoidal grid and micro-lens arrays) were successfully fabricated using the slow tool servo technique. The machined topography of freeform surfaces was uniformly distributed and in agreement with simulated results. Since it operates in the manufacturing environment, the machine tool effects on the OMSM were comprehensively evaluated, including on-machine vibration test, machine kinematic error mapping and linearity error calibration. A systematic calibration methodology for single point OMSM was proposed. Both theoretical and experimental investigation have been conducted to prove the validity of the proposed calibration methodology and the effectiveness of OMSM. With the aid of OMSM, potential applications were explored to exploit the integration benefits to further enhance the ultra-precision machining performance. OMSM integration will increase the automation level of the manufacturing. As OMSM preserves the coordinate system between the machining and measurement, the process investigation can be carried out in a more deterministic manner. The effect of process parameters on the surface form errors was investigated for ultra-precision cylindrical turning process. An empirical model based on response surface methodology has been established and validated with the experimental results. Moreover, a corrective machining methodology was proposed to further improve the accuracy of diamond turned surfaces with OMSM. According to different correction tasks, corresponding OMSM data processing methods were presented. Profile and surface correction experiments were performed to validate the proposed corrective machining methodology and 40% improvement of surface accuracy was achieved

Study and Performance Enhancement of Fast Tool Servo Diamond Turning of Micro-structured Surfaces

Ph.DDOCTOR OF PHILOSOPH

Examining the relationship of variables related to litigation regarding students with significant cognitive disabilities

Non-null interferometry offers a viable alternative to traditional interferometric testing of aspheric micro-lenses since computer generated holograms or null optics whose fabrication and testing are very expensive, are not required. However, due to the violation of the Nyquist sampling theorem these non-null tests provide limited dynamic range. The dynamic range of these non-null tests can be extended by implementing an index liquid which allows the measurement of micro-lenses with several microns of departure from a sphere. The first objective of this dissertation was to test important micro-lens properties such as the sag, radius of curvature and form errors for a micro-lens by using an index liquid. The results compared favorably to measurements taken on a Twyman-Green interferometer, a contact profilometer and an optical non-contact profilometer. Also, retrace errors, which are aberrations caused by altered ray paths of the test beam through a micro-lens were investigated. Reverse ray-trace and reverse optimization techniques are typically used to calibrate retrace errors, but in depth knowledge of the interferometer optics is assumed, and hence cannot be used for systems containing commercial optics. In this dissertation, re-trace errors are quantified and a novel calibration procedure derived to experimentally compensate for these errors. This retrace error calibration led to agreement of within 1% for the sag values between the index liquid technique and a profilometer. The second objective of this dissertation was to enable measurements of arbitrary geometries and to reduce testing time compared to profilometry. The index liquid technique was applied to faceted microstructured optical products which are becoming more widespread due to advances in manufacturing. Many of these structures contain faceted surfaces with steep slopes. Adequate metrology for such surfaces is lacking. The use of the index liquid technique achieved high quality, high speed measurements of such faceted microstructures. Refraction is accounted for at the interfaces, rather than consider only optical path length changes due to the index liquid, and this significantly improves the facet angle measurement. The technique is demonstrated with the measurement of an array of micro-pyramids and show that our results are in good agreement with measurements taken on a contact profilometer. The index liquid measurements took approximately five seconds to complete compared to a measurement time of six hours for the contact profilometer. The technique was also extended to measure opaque micro-corner cubes by implementing an intermediate replication step. This allowed a measurement of the angle between facets of a nickel micro-corner cube hexagonal array, a combination not previously demonstrated in the literature. A first order uncertainty analysis was carried out on the index liquid technique to determine any limiting factors that need to be taken into account when assessing such parameters as the sag and facet angle. The uncertainties in the sag and facet angle were found to be well below 1%. Lastly secondary factors such interferometer bias, refraction, masking effects and pixel calibration were investigated to understand the possible implications on the sag and facet angle calculation

A basis for the representation, manufacturing tool path generation and scanning measurement of smooth freeform surfaces

Freeform surfaces find wide application, particularly in optics, from unique single-surface science programmes to mobile phone lenses manufactured in billions. This thesis presents research into the mathematical and algorithmic basis for the generation and measurement of smooth freeform surfaces. Two globally significant cases are reported: 1) research in this thesis created prototype segments for the world’s largest telescope; 2) research in this thesis made surfaces underpinning the redefinition of one of the seven SI base units – the kelvin - and also what will be the newly (and permanently) defined value for the Boltzmann constant. Theresearchdemonstratestwounderlyingphilosophiesofprecisionengineering, the critical roles of determinism and of precision measurement in precise manufacturing. The thesis presents methods, and reports their implementation, for the manufacture of freeform surfaces through a comprehensive strategy for tool path generation using minimum axis-count ultra-precision machine tools. In the context of freeform surface machining, the advantages of deterministic motion performance of three-axis machines are brought to bear through a novel treatment of the mathematics of variable contact point geometry. This is applied to ultra-precision diamond turning and ultra-precision large optics grinding with the Cranfield Box machine. New techniques in freeform surface representation, tool path generation, freeform tool shape representation and error compensation are presented. A comprehensive technique for very high spatial resolution CMM areal scanning of freeform surfaces is presented, with a new treatment of contact error removal, achieving interferometer-equivalent surface representation, with 1,000,000+ points and sub-200 nm rms noise without the use of any low-pass filtering

Development of an ultraprecision shaping machine for manufacturing of Stavax lens molds

The production of high-precision aspheric microlenses has become increasingly difficult due to an increase in the complexity of the profile, the decrease in the lens’ size, and the demand for tighter tolerances. Machines built to fabricate these lenses generally include several expensive components due to the stringent stiffness, resolution, and bandwidth requirements necessary for proper machining. This thesis deals with reducing the cost of production by building an ultraprecision shaping machine that is comprised of three reasonably priced custom made axes that meet the requirements needed for ultraprecision machining. These three axes are (1) a flexure-based, single DOF axis driven by a voice coil actuator, (2) an inchworm axis driven by an assembly of five piezoelectric actuators, and (3) a long range fast tool servo driven by a large piezoelectric actuator. These three axes were developed individually to meet a set of requirements determined necessary for the machining of a microlens mold array in Stavax, a stainless steel variant. Each axis was designed such that it would not fail due to fatigue failure, was capable of achieving a high resolution ( 200 N/µm). The X-axis needed a range greater than 250 µm, the Y-axis needed a range greater than 3 mm, and the Z-axis needed a range greater than 35 µm. The X-axis needed to be capable of following a low frequency sine wave, while the Z-axis needed to be capable of following high frequency wave forms (200 Hz). Simulations were performed to determine if the designs would meet all the requirements set. All the designed axes have met the requirements, but only the X- and Y-axes have been manufactured for testing. Preliminary testing has shown that the X-axis has at least a stiffness of 60 N/µm in both the degrees of constraint. Movement in the parasitic directions while the axis was being actuated was also tested and showed that the only movement in the parasitic directions is when the X-axis crosses the zero point. Most likely, this is due to the electronics being used, which are also making it difficult to determine the full range of the axis and close the loop. Testing on the Y-axis has revealed that it has a stiffness of at least 125 N/µm in the direction of motion and stiffnesses between 60 N/µm and 100 N/µm in the degrees of constraint. The axis is capable of running at a speed of 150 µm/s, which is only limited by the amplifiers being used. Closed loop testing has shown that the axis is capable of 10 nm steps

Interferometric Metrology Using Reprogrammable Binary Holograms

Interferometric methods for surface metrology have been widely used for many years due to their speed, accuracy and versatility. It is frequently necessary however to produce a known comparison reference surface to minimise the optical path difference and hence enhance the dynamic range. An alternative to this is to use a computer generated hologram to act as the reference wave, or to correct a spherical reference wave to match a highly aspheric optic in order to achieve a null test. This thesis shall present a novel method of producing such holograms through the use of a binary ferroelectric liquid crystal on silicon spatial light modulator (FLCOS SLM) rather than using the more common lithographically produced plates. One of the primary advantages this could introduce is the ability for arbitrarily reprogrammable holograms to be created upon demand rather than needing to produce a series of holographic plates, saving both time and money in the testing of surfaces. We present results characterising the ability of a FLCOS SLM to produce increasingly large Zernike aberrations as well as quantifying the resulting errors, before using the device to reduce interferometric fringe density allowing us to measure aberrated optics and reveal low amplitude surface variations on the scale of 0.045 waves RMS

Computational Microscopy at 5 Meters Using Symmetric Fourier Sampling

Robotic planetary exploration relies upon a suite of scientific instruments to measure and record the environment under study, with the most ubiquitous instrument being some form of imager. This work describes the development of a microscope that can be mounted to the mast of planetary rover and obtain images with 10 μm spatial resolution at an unprecedented 5 meter distance. Rather than using traditional optics to generate images on a 2D focal plane array, this “remote microscope” uses a computation imaging technique to reconstruct images of targets. A set of four electronically programmable, frequency-shifted collimated laser beams that are symmetric about the axis of the optical system are projected to overlap at a distance of 5 meters and generate moving interference fringes which are used to probe the matched spatial Fourier components of the 2D intensity reflectivity function of the target surface. By probing and collecting a large set of these Fourier measurements, an image of the target is reconstructed using Fourier synthesis. This document provides a detailed description of the optical designs, electronic control requirements, opto-mechanical structures, operational conditions and algorithmic techniques used to generate a functioning computational remote microscope.I describe and analyze a novel optical design capable of achieving the operational requirements of the system and derive the optical parameters and relevant aberrations. A novel optical surface testing technique useful for high departure aspheres is derived and demonstrated with experimental measurements. I describe in detail the optical procedures and electronics components of the laboratory implementation of the computational microscope. I report the images obtained using the microscope of scattering and reflective targets. Finally, calculation of the effects of a turbulent atmosphere on the operation of the microscope are derived and demonstrated with experimental data, and a new approach to measuring the turbulent atmosphere was developed