Design for metrology for freeform optics manufacturing

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29th CIRP Design 2019 (CIRP Design 2019) 29th CIRP Design 2019 (CIRP Design 2019)

Design for for optics manufacturing Design 28th for metrology metrology for freeform freeform optics manufacturing CIRP Design Conference, May 2018, Nantes, France Kristen Vendittiaa, Chris Evansa* , Konstantinos Falaggisaa, Alex Blumaa, Romita Chaudhuribb, Jeremy Kristen , Chris Evans , Konstantinos Falaggis , Alex Blum , Romita architecture Chaudhuri , Jeremy b physical A newVenditti methodology toa*analyze functional and of Goodsellbb the and Jannick P. Rolland b Goodsell and Jannick P. Rolland

existing products for assembly oriented product family identification University of Northan Carolina at Charlotte, 9201 University City Blvd, Charlotte, NC 28223-0001,USA a a b University of North Carolina at Charlotte, University City Institute of Optics, University of Rochester, 4809201 Intercampus Drive, b

Blvd, NC 28223-0001,USA River Charlotte, Campus, Rocheter, NY 14627, USA Institute of Optics, University of Rochester, 480 Intercampus Drive, River Campus, Rocheter, NY 14627, USA * Corresponding author. Tel.: +1-704-687-5869; fax: +1-704-687-8255. E-mail address: [email protected] * Corresponding author. Tel.: +1-704-687-5869; fax: +1-704-687-8255. E-mail address: [email protected] École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France

Paul Stief *, Jean-Yves Dantan, Alain Etienne, Ali Siadat

*Abstract Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected]

Abstract

Freeform optical surfaces offer significant design opportunities but pose new challenges in metrology and manufacturing. Evolution in optics Freeform optical surfaceshave offerchanged significant but pose challenges in metrology and manufacturing. Evolutiontoinbeoptics manufacturing processes the design surfaceopportunities spatial frequencies thatnew must be measured. Optical surface definition is expected with manufacturing processes have changed the surface spatial frequencies that must be measured. Optical surface definition is expected to in besome with respect to fiducials and datums which must be realizable at all stages of manufacture; uncertainty in that realization becomes important Abstract respect to fiducials and datums which must be realizable at all stages of manufacture; uncertainty in that realization becomes important in some cases. Concurrent engineering is required, but appropriate data has not been collated for use by optical designers. One approach to providing Concurrent engineering required, but appropriate data has not been collated for useis by optical designers. approach totheproviding such data is described. Incases. today’s business environment,is the trend towards more product variety and customization unbroken. Due to thisOne development, need of such data is described. agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production © The Authors. Published Elsevier B.V. systems well as to choose theby optimal product © 2019 2019 as The Authors. Published by Elsevier B.V. matches, product analysis methods are needed. Indeed, most of the known methods aim to © 2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee the Designfamilies, Conference 2019 may differ largely in terms of the number and analyze a product or one product family on the physical level.of Different product however, Peer-review under under responsibility responsibility of of the the scientific scientific committee of the CIRP CIRP Design 2019. Peer-review committee of the CIRP DesignofConference Conference nature of components. This fact impedes an efficient comparison and choice appropriate2019 product family combinations for the production Keywords: freeform optics, metrology, concurrent engineering, design for metrology, system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster Keywords: freeform optics, metrology, concurrent engineering, design for metrology, these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the 1. Introduction organizations. A significant similarity between product families by providing design support to both,integrated production optics system manufacturing planners and product designers. An illustrative 1. Introduction integratedofoptics manufacturing organizations. A significant state-of-the-art optics areof steering developed using example of a nail-clipper is used to explain the proposed methodology. Anfraction industrial case study on two product families columns of fraction ofproposed state-of-the-art optics are designs developed using Freeform optical rapidlyoutemerging design form, evaluation thyssenkrupp Presta surfaces France is are thenacarried to give a first industrial of theprocesses; approach. sequential “completed” optical are passed Freeform optical surfaces arebyafreedom rapidly B.V. emerging design form, sequential processes; “completed” designs are passed allows new design and opportunities to ©which 2017 The Authors. Published Elsevier off to “opto-mechanical engineers”optical who may negotiate with which allows new design freedom and opportunities to off to “opto-mechanical engineers” who may negotiate with Peer-review under responsibility the scientific committee 28th CIRP Conference 2018. develop optical systems withof better performance in ofa the given theDesign optical designers before turning – often -- to small,

develop optical systemsvolume with better performance in aoptical given volume or a significant reduction for a given

the optical designers to small, specialized fabricators.before The turning transition– often from --systems of specialized fabricators. The transition from systems of spherical and plano optics, dominantly produced by classical function [1]. Starting in the 1980s, evolutiontobegan from spherical and plano optics, dominantly produced by classical load controlled full aperture optics an fabrication increasing polishing processes, to aspheric and freeform optics is a load aperture optics fabrication machines to increasing polishing processes, to aspheric and freeform optics is a use ofcontrolled small toolfull processes using ultra-precision and driving force for change. use of small tool processes ultra-precision and driving force for change. displacement control (suchusing as diamond turningmachines and milling From the perspective manufacturing, arrays of aspheric control (suchofasposition diamond and milling 1.displacement Introduction of the product range and of characteristics manufactured and/or From the perspective of manufacturing, arrays of aspheric machines) or combinations andturning dwell time control micro-optics and off-axis aspherics share many of the machines) or combinations of position and dwell time control assembled in this system. In this context, the main challenge in (such as magnetorheological finishing and computermicro-optics and off-axis fabrication. aspherics share manyhowever, of the characteristics of freeform The latter, (such as magnetorheological finishing and computerDue to the fast development in the domain of modelling and analysis is now not only to cope with single controlled polishing). These processes enabled cost-effective characteristics of freeformchallenges, fabrication. latter, pose additional metrology theThe topic of thishowever, paper. controlled polishing). processes enabled cost-effective communication and and anThese ongoing trend of optical digitization and products, a limited productchallenges, range or existing product fabrication of onoff-axis aspheric surfaces. pose additional metrology the topic of thisfamilies, paper. fabrication of onand off-axis aspheric optical surfaces. digitalization, manufacturing enterprises facing enable important but also to be able to analyze and to compare products to define Further advances in these processes and are in design use 2. Datums, tolerances ISO 10110 Further advances inwith these processes and in design enable use challenges in today’s market a off continuing new product assembly, families. Itfiducials, can be observed thatand classical existing of optical surfaces no axis ofenvironments: invariance on or the part 2. Datums, assembly, fiducials, tolerances and ISO 10110 of optical surfaces with no axis of invariance on or off the part tendency product families are regrouped in function of clients or features. [2,3]. towards reduction of product development times and [2,3]. Elements of optical systems comprising andtoplano shortened product lifecycles. In addition, there is annot increasing assembly oriented product familiesspherical are hardly find. Manufacturing of state-of-the-art optics does conform However, Elements ofhave optical systems comprising spherical andby plano Manufacturing of state-of-the-art optics does not conform (flat) surfaces an “optical axis: typically defined the demand customization, being at same time in a global On the product family level, products differ mainly in to the ofconceptual structures of themainstream production. (flat) surfaces have an part “optical axis: typically defined bytwo the to the conceptual structures of mainstream production. outside diameter of the acting as a datum after “centering” competition with competitors the world. This trend, characteristics: (i) part the number ofa components and (ii) the Concurrent engineering, taughtalltoover mechanical engineers often main outside diameter of the acting as datum after “centering” Concurrent engineering, taught mechanical engineers often (see of forcomponents example [5]). Aspheric opticselectrical, have an electronical). axis (on or off which is inducing development from macro to micro (e.g. mechanical, in sophomore year the in the USAto (see for example [4]), is a type (see for example [5]). Aspheric optics have an axis (on or off in sophomore year in the USA (see for example [4]), is a the part) defined by the prescription and typically realizable in markets, in diminished inlotmost sizes optics due to and augmenting Classical methodologies considering mainly single products foreign results topic academically physics the part) defined by the prescription and typically realizable in foreign topic academically in most optics and physics measurement of the surface. product varieties (high-volume low-volume [1]. or solitary, already existing product families analyze the faculties, although one that isto practiced in production) some vertically measurement of the surface. faculties, although one that isvariety practiced in as some To cope with this augmenting as well to bevertically able to product structure on a physical level (components level) which identify possible optimization potentials in the existing causes difficulties regarding an efficient definition and 2212-8271 © 2019 The Authors. Published by Elsevier B.V. 2212-8271 ©under 2019responsibility The it Authors. Published Elsevier B.V.of the Peer-review of the scientific committee CIRP Design Conference 2019 of different product families. Addressing this production system, is important tobyhave a precise knowledge comparison volume a significant volume reduction for a given Keywords: Assembly; Design Family identification functionor [1]. Starting inmethod; the 1980s, an evolution beganoptical from

Peer-review under responsibility of the scientific committee of the CIRP Design Conference 2019

2212-8271©©2017 2019The The Authors. Published by Elsevier 2212-8271 Authors. Published by Elsevier B.V. B.V. Peer-review under responsibility of scientific the scientific committee theCIRP CIRP Design Conference 2019. Peer-review under responsibility of the committee of the of 28th Design Conference 2018. 10.1016/j.procir.2019.04.255

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The ISO standard on preparation of drawings (ie specification) of optical elements and systems has separate parts for aspheric surfaces [6] and general (including freeform) surfaces [7]. It should be noted that the standard applies to optical elements as well as assemblies and systems. Hence, the designer’s intent (if any) regarding assembly is not necessarily captured in an element drawing. Since a freeform does not have an “optical axis”, a “reference axis” is defined [7] as a “theoretical axis given by the optical designer which does not depend on the symmetries of the surface and usually represents the center of the optical path for the main function”. The standard continues “… the position and orientation of the reference axis is defined by measurable references at and/or on the general surface…”. Hence, according to the standard, acceptance of an element’s surface shape is limited in part by the uncertainty in the realization of the coordinate system using fiducials and other references and the tolerances in positioning the surface with respect to the reference axis. Positional compensators (adjustments) allow optimization of the system wavefront error for both element placement and fabrication errors. These adjustments enable relaxation of tolerances on the optical surface at the price of increased system complexity. One key difference between on-axis aspheric systems and freeforms is the limited ability with freeforms to use rotation of the optical elements about their reference axes to minimize rotationally varying system wavefront error arising from manufacturing errors in individual element surfaces. A related limitation arises in some optical methods of measuring surfaces. For tight tolerance rotationally invariant surfaces, rotations are used to separate errors in the part from errors in the test set-up in an analogous manner to roundness testing. This is not possible for freeforms. Clearly concurrent engineering – specifically concurrent optical design, design for manufacture, design for metrology, and design for assembly – is required for appropriate allocation of tolerances. In the early 1990s some work was done to enable “snap together” aspheric optical systems, i.e. systems in which no adjustments are provided [8]. This approach was limited to IR systems, primarily by the tolerances associated with machining of locating features and the capabilities of the machine tools available. The absence of adjustments tightens the tolerances on surface form and location. More recent advances in machine design, including 4- and 5-axis ultraprecision systems where coordinate axis turning and micromilling can be implemented in a single set-up, has resulted in revived interest in “snap together” systems (see [9,10] for example). Recent publications describe snap together freeform systems [11], multiple optical surfaces on a single substrate [12] and monolithic systems [13]. One attraction of two separate surfaces on a single substrate is that the relative position location accuracy is determined by the (small) error motions of the precision machine. However, thermal effects scale with the substrate size, not the element size.

Coordinate systems should be realizable both in manufacturing steps as well as metrology – a key task in design for metrology. Further, design for metrology should be part of concurrent engineering that allocates tolerances based on system requirements and manufacturing and metrology capabilities. 3. Metrology method capabilities One vision of concurrent engineering in advanced optics development would provide summary data to optical designers on the manufacturing capabilities integrated with optical design tools. For example, “soft” constraints in the optimization code might flag elements with apertures larger than can be fabricated or measured using equipment available in-house or tolerances tighter than the maximum permissible error on an available coordinate measuring machine. A less ambitious approach, being developed within the Center for Freeform Optics (CeFO), is to develop a data hierarchy in which the capabilities of different metrology tools appropriate for measurement of form, mid-spatial frequencies (waviness) and surface finish of freeform optical surfaces. In addition to optics designers, the same data may be useful to fabricators. Target data characteristics include:  “Coherent”: the same data representations (from standards where possible) are used for different types of instrument;  “Stratified”: layers of information, with active links. The highest level data shows measurement technologies and capabilities to help down selection of approaches relevant to the most general description of the part to be tested. The lowest level gives detailed discussion of individual commercially available instruments and instruments in development, which could be used to measure the part under test; and  “Curated”: The source of the data used, at every level, will be reported. The user of the information can weigh manufacturer reported performance against data from other sources. For some of the generic instruments, examples are installed at CeFO sites. Performance test data and experience on those instruments are reported and the source of the data cited. In some cases, newer models have been developed, and appropriate data from the manufacturer are given (and sources cited). This tool specific information on the capabilities of different instruments is akin to the Quality Information Framework (QIF) [14] although not planned to be integrated into enterprise management systems. The current implementation uses a text document (.docx or .pdf) with navigation links, viewed as a precursor to implementation in HTML. Currently, there are 3 levels of information. Level 1 shows broad instrument capabilities by category (scanning white light interferometers (SWLI), contact profilometers, Fizeau interferometers, and coordinate measuring machines.



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format. It is possible to plot a metrology capability diagram for a SWLI system, for example, showing multiple objectives (each discontinuity in the slope limit line represents a change of objective), the trade-off between measurement time and the noise floor, etc (Figure 2). Similarly, it is possible to show the effect of trace length and area measured in a sequential instrument such as a 3D profilometer. Industry feedback, however, suggested that the increased fidelity impeded understanding. Hence, we have focussed on the simplified diagram at the highest level of the information hierarchy, giving increased detail in subsequent levels, and using simple industry terminology.

Figure 1: Generic instrument capability diagram

3.1 Generic instrument capabilities Basic capabilities (and more detailed information at lower levels) are shown on a “Freeform Optics Metrology Capability Diagram” (FOMCD). This is based on the approach pioneered by Stedman [e.g. 15, 16] using amplitudewavelength space (i.e. considering the optical surface prescription and topography in the Fourier domain). Notably, this representation is global; in some instruments better performance may be achieved over smaller apertures. Here we use “aperture” (projected maximum dimension of the optic under test) and “sag” (departure from the reference) which are more familiar to optical designers and optical fabricators and hence more appropriate for this use. Note also that the FOMCD assumes smooth, continuous surfaces, especially for optical measurement methods. The FOMCD should be used with caution (if at all) when considering surfaces which give stronger diffraction effects [18]. Figure 1 shows a generic capability diagram; in the tool we are developing, there is an accompanying table with numeric values for the limits, the source of the information and/or how it was calculated. Note that the limits plotted appear as “hard limits”. In practice, these limits are more nuanced. For example, min (the minimum sag or vertical resolution) in Figure 1 for a SWLI system is, in practice, noise limited. Trading-off resolvable amplitude with increased measurement time pushes down the minimum vertical resolution. At the same time,the minimum spatial wavelength “measurable” is a function of spatial wavelength. At the highest spatial frequencies, the instrument transfer function shows that the ratio of “true” to measured amplitude decreases as the spatial frequency approaches the Nyquist limit [17]. The original Stedman approach implies that instruments are limited at 1 cycle per aperture. This is clearly not true, for example when measuring optical surfaces with large base radii or designed astigmatism, which are measurable but noise limited. In previous work [19], we attempted to capture these and other nuances, as well as retaining the original Stedman

Figure 2: Adding information hinders comprehension

Figure 1 shows limits for a full aperture figure measuring interferometer. These limits are, in fact, specific to a particular measurement configuration. Fizeau interferometers are commercially available with a variety of “accessories” (transmission flats and transmission spheres) that allow for measurement of a range of departures from specified spherical base radii. Leaving aside the limitations imposed by “nulls”, commercial interferometers have limitations on radii that can be measured around the in-cavity focus[20] as well as limitation posed by the depth of focus in the field . We are working to develop an “angular capability diagram” which captures these limits in a manner analogous to the Stedman based capability diagram. 3.2 Generic technology details Once the user of the tool has selected a “generic technology”, Level 2 provides a “Use case” and a “Discussion”. The use case is specific to measurement of optical surfaces. For a Scanning White Light Interferometer (also known by a number of other names including Coherence Scanning Interferometer), the stated use case is “Area measurement of finish and mid-spatial frequencies (MSF) with stitching as required. Form metrology over limited aperture sizes (slope dependent) using stitching.”. This use case deliberately ignores the many other applications of this type of instrument. The discussion provides a brief description of the physical operating principle of the instrument (with references) and

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information on general characteristics and limitations of this type of instrument. At the end of the discussion are links to data on specific instruments; both commercially available instruments and instruments under development within CeFO are discussed. 3.3 Specific instrument data The specific capability data are collected under the generic categories used in the higher levels. For example, the section on “Profilometers” currently contains details on five different instruments from 3 manufacturers. One is a custom instrument, based on a commercial profiler that is at a CeFO University site and two are at CeFO Affiliate member’s sites. Data on the remaining two are from their respective manufacturers. In addition to “capability”, data resulting from experience with an instrument in a specific environment are included. Mostly, this data is based on work at the CeFO University sites. Examples are surface topography repeatability as a function of surface topography and surface damage (if any) introduced by contact profilometers. 4. Concluding remarks Concurrent engineering is an important facet of cost effective, agile manufacturing and measurement of state-of the-art freeform optics systems. This process will be eased if optical designers, fabricators and system integrators have, at their fingertips, data on manufacturability, including metrology. In the context of “design for metrology”, the approach developed here allows optical designers, fabricators and metrologists to evaluate metrology capabilities in familiar language. At the earliest stages of the processes, they can check if metrology tools exist for the aperture (ie part lateral size) and sag (peak deviation of the optical prescription from the instrument’s reference surface). The data structure allows rapid navigation from selecting a generic method to detailed information on specific instruments and experience data on those tools. Acknowledgements The authors acknowledge helpful discussions with Nick Horvath, Jonathan Papa, Aaron Michalko, Di Xu, and Martin Tangari. This research was partially supported by the National Science Foundation I/UCRC Center for Freeform Optics (IIP1338877, IIP-1338898, IIP-1822049 and IIP-1822026) and by the UNC Charlotte Center for Precision Metrology

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