Yunpeng Feng

Optimal strategy for fabrication of large aperture aspheric surfaces

Aspheric surfaces are widely used because of their desirable characteristics. Such a surface can obtain nearly perfect imaging quality with fewer optical elements and reduce the size and mass of optical systems. Various machine systems have been developed based on modern deterministic polishing technologies for large aperture aspheric surfaces. Several factors affect the final precision of large aperture aspheric surfaces, such as the velocity limit of the machine and the path design. Excess velocity, which will be truncated automatically by the computer numerical control system, may cause the dwell time to deviate from the desired time. When a path designed on a two-dimensional surface map with equidistant pitch is projected onto an aspheric surface, the pitch changes as a result of the varied curvature of the aspheric surface. This may affect the removal map and cause some ripple errors. A multiregion distribution strategy, which includes velocity checking, is proposed in this study to avoid exceeding the velocity limits. The strategy can be used to modify local errors and edge effects. A three-dimensional spiral path generation method is also presented using an iterative method to ensure uniformity in the space length of the adjacent circle of the spiral path. This process can reduce the ripple error caused by the overlapping of tool paths. A polishing experiment was conducted, and the results proved the validity of the proposed strategies.

Study of a wheel-like electrorheological finishing tool and its applications to small parts

A wheel-like electrorheological finishing (ERF) tool for small parts polishing is proposed and thoroughly studied. First, the electrorheological polishing fluid is tested, and its properties suggest usability for electrorheological fluid-assisted finishing. Then, the mathematical removal model of the ERF tool is built employing the conformal mapping method and high-order multipolar moment theory. Finally, a micropattern of trough is fabricated on a slide glass (7 mm wide and 1 mm thick). The trough is 70 nm deep, and its flat bottom is 1.5 m wide (peak to valley of 3.16 nm and root mean square of 1.27 nm); the surface roughness finally achieves 0.86 nm. The results demonstrate the stable machining capability of the ERF tool for miniature parts.

Correction of remounting errors by masking reference points in small footprint polishing process

The remounting accuracy of optical components between measurement and polishing affects the polishing results, especially for small polishing footprint processes such as magnetorheological jet polishing (MJP). In this paper, two important remounting errors (translation and rotation errors) are discussed, and a masking method is proposed to correct these errors. A mathematical model that describes the relationship between remounting errors and reference points is constructed. A mask is created to provide reference points on a sample because such points are important for identifying remounting errors. The remounting errors are then used as bases in correcting the parameters used for actual polishing. Experiments are conducted on a K9 optical sample to validate the proposed approach. After the reference points are obtained by measuring the mask on the sample, the remounting errors are derived. The translation errors are 5.61 mm in the X direction and 6.08 mm in the Y direction; the rotation error is 4.1°. Deviations from the desired positions are eliminated and the desired surface smoothness is obtained after parameter correction. Results indicate that the proposed method is suitable for high-precision polishing.

Tool removal function modeling and processing parameters optimization for bonnet polishing

ABSTRACT Since computer controlled optical surfacing (CCOS) processes were proposed in the 1960s, many processes were developed for precision optics successfully. In this present work, a novel approach, the precessions process, is proposed and used for large segments fabrication. The removal function of the bonnet polisher based on velocity and pressure distribution, which are obtained from the geometry of the process tool-motion and Hertzian contact theory respectively, are simulated. A finite element analysis (FEA) model is constructed to optimize process parameters. At last, detailed experimental studies are carried out to verify the optimal parameters.

Large field distributed aperture laser semiactive angle measurement system design with imaging fiber bundles

A type of laser semiactive angle measurement system is designed for target detecting and tracking. Only one detector is used to detect target location from four distributed aperture optical systems through a 4×1 imaging fiber bundle. A telecentric optical system in image space is designed to increase the efficiency of imaging fiber bundles. According to the working principle of a four-quadrant (4Q) detector, fiber diamond alignment is adopted between an optical system and a 4Q detector. The structure of the laser semiactive angle measurement system is, we believe, novel. Tolerance analysis is carried out to determine tolerance limits of manufacture and installation errors of the optical system. The performance of the proposed method is identified by computer simulations and experiments. It is demonstrated that the linear region of the system is ±12°, with measurement error of better than 0.2°. In general, this new system can be used with large field of view and high accuracy, providing an efficient, stable, and fast method for angle measurement in practical situations.

Calibrating system errors of large scale three-dimensional profile measurement instruments by subaperture stitching method.

This study presents a subaperture stitching method to calibrate system errors of several ∼2  m large scale 3D profile measurement instruments (PMIs). The calibration process was carried out by measuring a Φ460  mm standard flat sample multiple times at different sites of the PMI with a length gauge; then the subaperture data were stitched together using a sequential or simultaneous stitching algorithm that minimizes the inconsistency (i.e., difference) of the discrete data in the overlapped areas. The system error can be used to compensate the measurement results of not only large flats, but also spheres and aspheres. The feasibility of the calibration was validated by measuring a Φ1070  mm aspheric mirror, which can raise the measurement accuracy of PMIs and provide more reliable 3D surface profiles for guiding grinding, lapping, and even initial polishing processes.

A calculation method on dynamic force feedback in complex free-form contour pre-generation

In this paper, an approximate inverse algorithm is described to calculate the dynamic cutting force applied on the complex free-form contour pre-generation. In traditional contour milling process, cutting force can be generally determined by installing force transducers or sensors. However, conveniently and easily controllable tool is one of important considerations in complex contours generation since its non-gentle slopes and curvatures. The proposed algorithm focuses on the calculation of a dynamic feedback force by dealing with both minimum and non-minimum phase systems. According to the information of the output tool position and input driving force, the dynamic cutting force information can be calculated out by the approximate inverse approach. Therefore, force transducers or sensors are eliminated and the disadvantages are prevented accordingly. Experiments have been carried out under empty-/rough-/fine-generating conditions, showed that depth of feed is one of the important factors affects the dynamic force, and it is feasible to calculate cutting force effectively in complex contour generation without force transducers or sensors installment.

Luminous intensity correction method for misalignment-induced measurement error of light-emitting diode arrays

To eliminate axial misalignment-induced measurement errors of discrete light-emitting diode arrays in the far-field condition, a robust and effective method for correcting the measured luminous intensity distribution is proposed. The precision of the correction can be determined beforehand by setting a criterion which can also be used to determine the required test distance. To validate the feasibility and practicability of the proposed approach, numerical simulations of light-emitting diode arrays with three kinds of typical luminous intensity distributions were performed. In addition, the test distances as a function of the light-emitting diode luminous intensity distribution, packing density and dimensions under translational misalignment were analysed. Some beneficial operating methods and rules for practical application are summarised. Finally, physical measurements of several experimental examples were collected. The correction results agreed with the desired data and again proved the utility of the presented method.

Non-Newtonian hydrodynamic modeling of electrorheological finishing feature based on wheel-like finishing tool

Electrorheological finishing method is a promising method for small parts fabrication. The small footprint of the tool can provide material removal by virtue of numerical control machine. The characteristics of electrorheological polishing fluid is tested by Haake CV20 rheometer. The dependence of shear stress and viscosity of the electrorheological polishing fluid under different supply voltage, as well as the field dependence of shear stress of ER polishing fluid at different shear rate, was obtained. Hydrodynamic pressure model in working area was studied based on the phenomenon of hydrodynamic lubrication theory of Bingham fluid, and it is proved that the pressure distribution is the dominant factor on the footprint shape. The experiments further showed that the efficiency of material removal is a combined action of hydrodynamic pressure and wheel speed.

An Investigation on the Effect of Curvatures and Corners along Path

The unit removal function (URF) of special tools and tool paths are required for the modern sub-aperture deterministic polishing technologies. Paths may affect the amount of removed material because of the curvatures and corners existing on them. This study proposes a mathematic model along path for analyzing the effects of curvatures and corners on material removal during polishing. A numerical method was used to calculate the integration of segments covered by the URF. The length covered by the URF was also determined to predict the amount of material removed from a given location of a path. For the effects of curvatures, the maximum material removed is a distance of about 0.5 to the normalized radius of the URF. For the effects of corners, the peak removal is a distance between 0.5 and 1 to a corner. The Harris corner detection algorithm was used to estimate corner distribution. Aside from curvatures and corners, the space between adjacent paths is another key parameter that affects the amount of removed material. The combined effect of curvatures, corners, and spaces was comprehensively analyzed, and two typical paths -- spiral and raster paths -- were introduced in polishing experiments. The experimental results are consistent with the numerical prediction.