To meet stringent automotive safety requirements, car taillights typically incorporate retroreflective elements. In addition to their retroreflective role, these structures are also used for lighting/aesthetic/styling purposes. The most common type of automotive retroreflector (RR) – also known as reflex reflector – is characterized by a corner-cube (CC) geometry that has been fabricated for more than 60 years through a conventional pin-bundling technology. While accurate, this manufacturing approach remains time-consuming, expensive and over-constraining in terms of the RR design. To address this, alternate RR fabrication pathways have been developed and this study outlines the capabilities of a novel approach including milestones, setbacks, advantages and disadvantages. Corner-cube geometry includes three mutually orthogonal facets that meet at a common vertex/apex. This configuration precludes the use of most material removal techniques involving rotational tools. To address this, an alternate RR shape called right triangular prism (RTP) was proposed. This geometry is amenable to diamond-based single point cutting approaches, but its optical performance proved to not be identical with that conventional CC RR. The successful fabrication of RTP RRs was demonstrated in acrylic and quality/functionality of the prototype were assessed through both metrological and optical means. Surface quality Ra of less than 20 nm was achieved through an adequate combination of multi-axis machine tool kinematics and ultraprecise single point tool geometry. This cutting technique worked well on non-ferrous, but not on ferrous materials. Nevertheless, an alternative strategy involving micromilling has been developed for cutting RTPs in ferrous substrates. The successful fabrication of tooling inserts has been completed such that injection molded replicas of RTP RRs will be produced in the future. It is expected that the development of cutting-based RR fabrication strategies along with the associate knowledge on the underlying cutting mechanics will enable a broader diversity of RR designs in the future.
The direct write laser-machining process is based on CNC technology and therefore it is assumed that the laser beam focal spot scans the workpiece surface in conformance with the predetermined tool path. However, laser- machining experiments indicate that the geometric quality of the machined part is not only defined by the pre- determined tool path but is also largely influenced by the dynamics of the laser-material interactions. The geometry and surface profile of the machined part is the result of a combined effect of the complex dynamic processes accompanying the machining process. All laser-machining systems have unique dynamic characteristics, represented by random variations of process parameters, e.g., pulse energy, travel speed, etc. Thus, accurate prediction of part quality becomes very difficult, requiring a systematic experimental study for each specific material and the laser-machining system. The objective of this work is to experimentally investigate the effect of variations in laser pulse energy on the geometric quality of the machined parts in terms of accuracy, precision, and surface quality.
Large area polydimethylsiloxane (PDMS) flexible optical light guide sheets can be used to create a variety of passive light harvesting and illumination systems for wearable technology, advanced indoor lighting, non-planar solar light collectors, customized signature lighting, and enhanced safety illumination for motorized vehicles. These thin optically transparent micro-patterned polymer sheets can be draped over a flat or arbitrarily curved surface. The light guiding behavior of the optical light guides depends on the geometry and spatial distribution of micro-optical structures, thickness and shape of the flexible sheet, refractive indices of the constituent layers, and the wavelength of the incident light. A scalable fabrication method that combines soft-lithography, closed thin cavity molding, partial curing, and centrifugal casting is described in this paper for building thin large area multi-layered PDMS optical light guide sheets. The proposed fabrication methodology enables the of internal micro-optical structures (MOSs) in the monolithic PDMS light guide by building the optical system layer-by-layer. Each PDMS layer in the optical light guide can have the similar, or a slightly different, indices of refraction that permit total internal reflection within the optical sheet. The individual molded layers may also be defect free or micro-patterned with microlens or reflecting micro-features. In addition, the bond between adjacent layers is ensured because each layer is only partially cured before the next functional layer is added. To illustrate the scalable build-by-layers fabrication method a three-layer mechanically flexible illuminator with an embedded LED strip is constructed and demonstrated.
The controlled guidance of light rays through a mechanically flexible large area polymer optical waveguide sheet is investigated using Zemax OpticStudio software. The geometry and spatial distribution of micro-optical features patterned on the waveguide sheet determines whether the surface acts as a light concentrator or diffuser. To illustrate the concept, incident light is collected over a large center area and then transmitted to the border where it is emitted through an illumination window covered by an array of photo-cells. The efficiencies of the collector and illuminating regions of the hybrid PDMS collector-diffuser waveguide sheet are discussed. Initial analysis of the waveguide design demonstrates an ideal efficiency of over 90% for the concentrating region of the waveguide and over 80% efficiency for the diffusing region of the waveguide. The Zemax simulation of the ideal design of the hybrid concentrator-diffuser waveguide exhibited an efficiency of up to 75%. However this efficiency significantly decreased when examining the waveguide’s performance as a flexible sheet. The necessary design modifications, to mitigate these losses in efficiency, are discussed, and future work will focus on analyzing and optimizing the waveguide design for performance as a fully flexible concentrator-diffuser membrane.
Soft-lithography techniques can be used to fabricate mechanically flexible polydimethylsiloxane (PDMS) optical waveguide sheets that act as large area light collectors (concentrators) and illuminators (diffusers). The performance and efficiency of these optical sheets is determined by the position and geometry of micro-optical features embedded in the sheet or imprinted on its surface, thickness and shape of the waveguide, core and cladding refractive indices, and wavelength of the incident light source. The critical design-for-manufacturability parameters are discussed and a scalable method of fabricating multi-layered PDMS optical waveguides is introduced. To illustrate the concepts a prototype waveguide sheet that acts a combined light collector and illumination panel is fabricated and tested. The region of the waveguide sheet that acts as the light collector consists of two superimposed PDMS layers with slightly different indices of refraction. The top layer is patterned with micro-lenses that focus the incident light rays onto the micro-wedge features that act as reflectors on the bottom of the second layer and, due to total internal reflection, redirect the light rays to the light diffuser region of the waveguide sheet. The bottom face of the diffuser PDMS layer is patterned with angled triangular wedge micro-features that project the light out of the waveguide sheet forming an illuminating pattern. The proposed fabrication technique utilizes precision machined polymethylmethacrylate (PMMA) moulds with negative patterned PDMS inserts that transfer the desired micro-optical features onto the moulded waveguide.
A major limitation in terahertz (THz) imaging applications is the relatively poor diffraction limited spatial resolution. A common approach to achieve subwavelength resolution is near-field imaging using a subwavelength aperture, but the low transmission efficiency through the aperture limits the sensitivity of this method. Bullseye structures, consisting of a single subwavelength circular aperture surrounded by concentric periodic corrugations, have been shown to enhance transmission through subwavelength apertures. At optical wavelengths, the fabrication of bullseye structures has been traditionally achieved by lithographic or chemical processes. Since the scale of plasmonic structures depends on the incident wavelength, precision micromilling techniques are well suited for THz applications. In this paper we describe a diamond micromilling process for the fabrication a plasmonic lenses operating at 325 GHz. Theoretical simulations are obtained using an FDTD solver and the performance of the lens is measured using a customized THz test bed.
Edge-lit light guide panels (LGPs) with micropatterned surfaces represent a new technology for developing small- and medium-sized illumination sources for application such as automotive, residential lighting, and advertising displays. The shape, density, and spatial distribution of the micro-optical structures (MOSs) imprinted on the transparent LGP must be selected to achieve high brightness and uniform luminance over the active surface. We examine how round-tip cylindrical MOSs fabricated by precision micromilling can be used to create patterned surfaces on low-cost transparent polymethyl-methacrylate substrates for high-intensity illumination applications. The impact of varying the number, pitch, spatial distribution, and depth of the optical microstructures on lighting performance is initially investigated using LightTools™ simulation software. To illustrate the microfabrication process, several 100×100×6 mm 3 LGP prototypes are constructed and tested. The prototypes include an “optimized” array of MOSs that exhibit near-uniform illumination (approximately 89%) across its active light-emitting surface. Although the average illumination was 7.3% less than the value predicted from numerical simulation, it demonstrates how LGPs can be created using micromilling operations. Customized MOS arrays with a bright rectangular pattern near the center of the panel and a sequence of MOSs that illuminate a predefined logo are also presented.
The solar energy industry strives to produce more and more efficient and yet cost effective photovoltaic (PV) panels.
Integration of specific micro/nano optical structures on the top surface of the PV panels is one of the efficient ways to
increase their PV performance through enhancing light trapping and in-coupling. In this study, periodic triangular
gratings (PTGs) in polymethyl methacrylate (PMMA) were numerically simulated and optimized. The goal of this study
is to enhance the ability of solar panels to convert maximum obtainable amount of solar energy by improving the optical
in-coupling of light to PV material. Initial optical simulation results shown that a flat PV panel (without any enhancing
micro-optical structures) exhibits an average incident light power of 0.327 W over a range of the incident light angles
between 15º and 90º. Introduction of the PTG allows capturing the incoming sunlight and reflecting it back onto the PV
material for a second or more chances for absorption and conversion into electricity. The light trapping and redirection is
achieved through the total internal reflection (TIR) phenomenon. Geometry of the PTG was initially optimized with
respect to an incident sunlight orientation of 15º, 30º, 45º, 60º, 75º, and 90º. Optical performance of the particular
optimized PTGs was analyzed over daylight conditions and several optical parameters, such as average incident power
and intensity, were calculated when sunlight orientation angle was changing from 15º to 90º. By adding the PTG
optimized for 15º incidents light, an average incident power of 0.342 W was achieved (4.6% improvement of optical
performance). Functional PTG prototypes were fabricated with optical surface quality (below 10 nm Ra). The simulation
results allow understanding how the overall daytime photovoltaic performance of solar panels can be improved.
Laser micro-polishing (LμP) is a new laser-based microfabrication technology for improving surface quality during a
finishing operation and for producing parts and surfaces with near-optical surface quality. The LμP process uses low
power laser energy to melt a thin layer of material on the previously machined surface. The polishing effect is achieved
as the molten material in the laser-material interaction zone flows from the elevated regions to the local minimum due to
surface tension. This flow of molten material then forms a thin ultra-smooth layer on the top surface. The LμP is a
complex thermo-dynamic process where the melting, flow and redistribution of molten material is significantly
influenced by a variety of process parameters related to the laser, the travel motions and the material. The goal of this
study is to analyze the impact of initial surface parameters on the final surface quality. Ball-end micromilling was used
for preparing initial surface of samples from H13 tool steel that were polished using a Q-switched Nd:YAG laser. The
height and width of micromilled scallops (waviness) were identified as dominant parameter affecting the quality of the
LμPed surface. By adjusting process parameters, the Ra value of a surface, having a waviness period of 33 μm and a
peak-to-valley value of 5.9 μm, was reduced from 499 nm to 301 nm, improving the final surface quality by 39.7%.
Edge-lit backlighting has been used extensively for a variety of small and medium-sized liquid crystal displays (LCDs).
The shape, density and spatial distribution pattern of the micro-optical elements imprinted on the surface of the flat
light-guide panel (LGP) are often "optimized" to improve the overall brightness and luminance uniformity. A similar
concept can be used to develop interior convenience lighting panels and exterior tail lamps for automotive applications.
However, costly diffusive sheeting and brightness enhancement films are not be considered for these applications
because absolute luminance uniformity and the minimization of Moiré fringe effects are not significant factors in
assessing quality of automotive lighting. A new design concept that involves micromilling cylindrical micro-optical
elements on optically transparent plastic substrates is described in this paper. The variable parameter that controls
illumination over the active regions of the panel is the depth of the individual cylindrical micro-optical elements.
LightTools™ is the optical simulation tool used to explore how changing the micro-optical element depth can alter the
local and global luminance. Numerical simulation and microfabrication experiments are performed on several
(100mmx100mmx6mm) polymethylmethacrylate (PMMA) test samples in order to verify the illumination behavior.
This paper presents design, fabrication, and performance testing of an innovative, laser machined structure to act as a microgripper. The proposed design constitutes of a pair of identical, cascaded actuation structures oriented in a face-to-face direction, to act as microtweezers. Each microactuator consists of five actuation units joined together horizontally in a consecutive order to build the cascaded structure. The actuation unit incorporates an internal constrainer and two semi-circular-shaped actuation beams. The actuation principle is based on the electro-thermal effect. On application of electrical potential at the backends, the conductive, geometrically complex structure of the microgripper produces non-uniform resistive heating and uneven thermal expansion generating tweezing displacements and force through a cumulative effect of all the individual actuation units within the cascaded microactuators. High-precision laser micromachining process was employed in the fabrication of the copper and nickel-based prototype microgrippers with overall dimension of 1.4 mm(L)×2.8 mm(W) and with relative accuracy within 1%. The geometrical parameters of the prototypes were evaluated in terms of accuracy and precision to demonstrate the fabrication capabilities. Challenges involved and the solutions to develop functional microparts with high aspect ratio (dimensional) with respect to the local elements and overall dimensions were described. The performance of the two microgripper prototypes was analyzed and compared. These microgrippers are useful in micromanipulating and microhandling operations for micro-electro-mechanical systems, biological, medical, chemical, and electro-opto-mechanical engineering applications.
The actual performance of a miniature mechanism significantly depends on the geometric quality of the machined part and specific features therein. To fabricate functional parts and features with accuracy and precision within +/- 1 μm or less, the laser micromachining system requires the capabilities of following the desired toolpath trajectories with minimum dynamic errors, high positional repeatability, and synchronization of laser firing events at precise time-and-location to ablate the material. The major objectives of this study are to fabricate miniature functional mechanisms using precision laser micromachining method, explore the machining challenges and evaluate the geometrical quality of the machined parts in terms of accuracy, precision and surface quality. Two functional mechanisms based on electro-thermal actuation have been studied. Several machining challenges related to the corner accuracy, the asynchronization of motions and, the laser-on/off events in space and time with respect to the part geometry have been addressed. The source of inaccuracies primarily stems from the geometric complexity of the mechanism that consists of several features, such as, arcs, radii, lines, curvatures, segments and pockets, along with their dimensional aspect ratio. Such a complex design requires a large number of inconsecutive trajectories to avoid thermal deformations. Copper and nickel foils with a thickness of 25 and 12.5 µm respectively were used in the fabrication of the prototypes. The machining challenges were successfully tackled and the geometrical performance of the fabricated prototypes was evaluated. Local feature accuracies within 0.1 - 0.2 µm have been recorded.
Industrial lasers are used extensively in modern manufacturing for a variety of applications because these tools provide a highly focused energy source that can be easily transmitted and manipulated for micro-machining. The quantity of material removed and the roughness of the finished surface are a function of the crater geometry formed by a laser pulse with specific energy (power). Laser micro-machining is, however, a complex nonlinear process with numerous stochastic parameters related to the laser apparatus and the material specimen. Consequently, the operator must manually set the process control parameters by trial and error. This paper describes how an artificial neural network can be used to create a nonlinear model of the laser material-removal process in order to automate micro-machining tasks. The multi-layered neural network predicts the pulse energy needed to create a crater of specific depth and average diameter. Laser pulses of different energy levels are impinged on the surface of the test material in order to investigate the effect of pulse energy on the resulting crater geometry and volume of material removed. Experimentally acquired data from several sample materials are used to train and test the network's performance. The key system inputs for the modeler are mean depth of crater and mean diameter of crater, and the system outputs are pulse energy, variance of depth and variance of diameter. The preliminary study using the experimentally acquired data demonstrates that the proposed network can simulate the behavior of the physical process to a high degree of accuracy. Future work involves investigating the effect of different input parameters on the output behavior of the process in hopes that the process performance, and the final product quality, can be improved.
In this paper, an analysis of the dynamic characteristics of machine tool spindle-bearing systems is presented. The research utilized the force impact-response testing method. The results are applied to the analysis and modeling of the dynamic performance of machine tool spindle-bearing systems. As an indicator of dynamic performance, the impulse response matrices are experimentally obtained. Two types of impulse response matrices are considered: (1) with respect to (wrt) acceleration; which describes the space-coupled relationship between the vectors of the force (impact) and measured acceleration (response) and (2) wrt displacement; which describes the space-coupled relationship between vectors of the force and simulated displacement. The results indicate an interrelation between different directions of displacements, and lay a foundation for the dynamic modeling of spindle-bearing systems in view of the transfer matrix with nonzero non-diagonal elements. From an engineering point of view, the transfer function matrix can be considered a `dynamic imprint', or `signature' of system performance. As a practical example, the dynamic properties (the impulse and frequency response matrices) of the spindle-bearing system of a Barer-Proteo D/94 high precision machining center are obtained, identified and investigated. The developed approach for modeling and parameter identification appears promising for a wide range of industrial applications, including rotary systems.
This report deals with the analysis and the design of a conventional machining system by means of mathematical modeling of the machining process. The paper considers the machining process as a dynamic interaction between the cutting tool and workpiece in space and time that additionally involves the dynamic properties of the machine tool mechanical subsystem, the cutting motions. An ideal machining system provides an accurately controlled tool path. However, machining experience has shown that the geometric qualities of the machined part are not only defined by the uniformity of the too path, but are also influenced by the dynamics of the machine tool and cutting process and by the external and internal disturbances. Developed herein, is a systematic approach tying together the four main factors associated with the dynamic processes that play an important role during machining and influence the quality of the machined workpiece. These factors are (a) the kinematic/dynamic disturbances within the cutting/feed motion subsystem, (b) the dynamics of the machine tool mechanical subsystem, (c) the tool-workpiece interaction as a dynamic process, and (d) the forming of the workpiece surface as a dynamic surface as a dynamic process. The generalized mathematical model of the machining process is developed based on the dynamic relationships between those above-mentioned aspects of the process. The approach performs the dynamic analysis of a machining process for diagnostics, control, and process optimization purposes.
A method for studying the problem of modeling, identification and analysis of mechanical system dynamic characteristic in view of the impulse response matrix for the purpose of adaptive control is developed here. Two types of the impulse response matrices are considered: (i) on displacement, which describes the space-coupled relationship between vectors of the force and simulated displacement, which describes the space-coupled relationship between vectors of the force and simulated displacement and (ii) on acceleration, which also describes the space-coupled relationship between the vectors of the force and measured acceleration. The idea of identification consists of: (a) the practical obtaining of the impulse response matrix on acceleration by 'impact-response' technique; (b) the modeling and parameter estimation of the each impulse response function on acceleration through the fundamental representation of the impulse response function on displacement as a sum of the damped sine curves applying linear and non-linear least square methods; (c) simulating the impulse provides the additional possibility to calculate masses, damper and spring constants. The damped natural frequencies are used as a priori information and are found through the standard FFT analysis. The problem of double numerical integration is avoided by taking two derivations of the fundamental dynamic model of a mechanical system as linear combination of the mass-damper-spring subsystems. The identified impulse response matrix on displacement represents the dynamic properties of the mechanical system. From the engineering point of view, this matrix can be also understood as a 'dynamic passport' of the mechanical system and can be used for dynamic certification and analysis of the dynamic quality. In addition, the suggested approach mathematically reproduces amplitude-frequency response matrix in a low-frequency band and on zero frequency. This allows the possibility of determining the matrix of the static stiffness due to dynamic testing over the time of 10- 15 minutes. As a practical example, the dynamic properties in view of the impulse and frequency response matrices of the lathe spindle are obtained, identified and investigated. The developed approach for modeling and parameter identification appears promising for a wide range o industrial applications; for example, rotary systems.