The classical Preston equation considers that the material removal is linearly related to time, velocity, and pressure. However, in the wheel polishing technology, it is found through experiments that there is a nonlinear relationship between the rotational speed of the polishing wheel and the amount of material removed. In order to accurately control the material removal in the polishing wheel variable speed machining strategy, it is necessary to modify the classical Preston equation. In this paper, the control variable method is used to carry out the sampling experiment: the time and pressure are set as fixed values, and the polishing wheel speed is set as a variable and the value is between 0-4rps. Then the data points were analyzed and a least squares fit was used to obtain a non-linear function between the rotational speed of the polishing wheel and the amount of material removed. Finally, the classical Preston equation is modified to obtain the removal equation suitable for the variable speed machining strategy.
UlE, as a kind of ultra-low thermal expansion glass, has been widely applied in large-aperture optical mirrors and space telescopes. However, hard-brittle material feature of ULE brings certain difficulty for machining. In this paper, rotary ultrasonic vibration-assisted peripheral grinding (RUPG), combining rotary ultrasonic vibratio (UV) and conventional grinding (CG) is proposed, and grinding process and quality characteristic are investigated in terms of surface morphology, grinding force, surface roughness, subsurface damage depth, and subsurface morphology. A serious of comparative experiments for between RUPG and CG were conducted. The results show that rotary UV in RUPG can markedly decrease the grinding force with a factor of 46.78%. The change rule of grinding force with the varying of grinding parameters in RUPG is consistent with that in CG, i.e., grinding force increases as the increasing of grinding depth and feed rate, while it decreases with the growth of spindle speed. Grinding surface marks were found to be obviously weakened by UV and surface consistency was accordingly improved. Besides, measurement results of surface roughness also real that UV plays a positive role in diminishing surface roughness by almost 23.01%, and reducing the subsurface damage depth by a factor of 17.19%.
In this paper, industrial robot is used as motion carrier and self-developed flexible wheel tool is used as polishing tool to realize low-cost, high-efficiency, and high-precision optical processing. Firstly, the mapping formula between the workpiece coordinates and the road point coordinates is deduced, and the position and posture data required for robot programming are obtained. Secondly, a new type of wheel polishing tool is designed, which controls the polishing pressure through a pneumatic floating structure to ensure the stability of the removal function. Finally, an off-axis paraboloid of φ345mm was processed using this technology. After three times of processing for 10 hours, the surface error converged from PV-2.111λ, RMS-0.249λ to PV-0.119λ, RMS-0.01λ. PV and RMS converged by 94% and 96%, respectively. This proves that the technology has the advantages of high efficiency and high precision, and is expected to be widely used in the field of precision optical processing.
In this paper, a new type of wheel polishing tool is designed. Through the bevel gear structure, only one motor is used to realize the revolution and rotation of the polishing wheel, which makes the structure simple, small inertia, and stable operation. Traditional polishing wheels have a three-layer structure: internal rigid hub, middle flexible rubber, and external polishing pad. It is found through experiments that the hardness of rubber has a greater influence on the tool influence function (TIF). Therefore, by optimizing the hardness of the rubber, we obtained a TIF very close to the Gaussian shape, which is conducive to the rapid convergence of the surface error. Finally, the effects of polishing wheel speed, polishing pressure and polishing time on the TIF, as well as the stability of the TIF, are studied through experiments. Experiments show that: (1) There is no linear relationship between the removal efficiency of the polishing wheel, the polishing wheel speed, and the polishing pressure, but as the parameter increases, the increase in the removal efficiency slows down; (2) There is a good linear relationship between the removal amount of the polishing wheel and time; (3) The TIF is very stable, and the stability of the TIF reaches 98%.
Space telescope is widely used nowadays. Reflective mirror is an important part of the space telescope. Many engineers are trying their best to make the reflective mirror lighter and lighter. Carbon fiber composite material is universally used in the field of aerospace. But there is little engineering experience about making a reflective mirror from carbon fiber. This paper will make a structure optimization about reflective mirror which is made of carbon fiber. Traditional way of structure optimization will call numerical simulation software for many times before finding the best solution. This paper uses a black box model to take the place of simulation software to do the optimization. The black box in this paper is response surface. To some extent, it is a simple type of machine learning. The response surface learns some key information and characteristics from the model which is made by simulation software. Then, the black box, response surface, can work like simulation software. The paper makes a structure optimization based on a certain kind of carbon fiber reflective mirror by the response surface. The diameter of reflective mirror which needs to be optimized in the paper is 150mm. The optimization variables are the thickness of carbon fiber plate and the position of supports. The purpose of the optimization is that the reflective mirror has small value of PV, proper fundamental frequency, small value of stress. The paper finds the best structure design at last by the method stated above. This kind of method can be used to make optimization about other similar optical structure.
Computer-generated hologram (CGH) is an effective way to compensate wavefront aberration in null test of aspheric surfaces and freeform surfaces. Our strategies of CGH design for 820mm diameter tertiary mirror surface figure testing are presented, and an experiment demonstrating the compensation test results of CGH is reported. We design a CGH including two sections on the same substrate in order to align the CGH to the incident wavefront: main section for compensating wavefront in null test, alignment section for adjusting the relative position between CGH and interferometer. Because there is no center hole in the mirror, in order to isolate different orders of diffraction, we used tilt carrier to make different orders of diffraction come to focus at different position perpendicular the axis to avoid ghost reflections.
An ABB IRB6640 industrial robot is used as a processing platform for optical polishing. The relationships of coordinate systems are defined, the algorithm of coordinate transformation, Euler angles and quaternion are provided. M-like removal function and Gaussian-like removal function are used to simulation process an off-axis aspheric surface. The surface error after polishing by M-like removal function is 1.5 to 2.5 times bigger than Gaussian-like removal function. This proves that M-like removal function also has good convergence speed. Then, the pentagram polishing head is used to polish a Φ600mm off-axis paraboloid surface. After 15 cycles, about 120 hours processing, PV converges from 5.8μm to 0.836μm, RMS converges from 1.2μm to 0.054μm, PV and RMS respectively converge 85% and 95%. The experiment shows M-like removal function has good convergence speed.
The manufacturing and testing of a surface modified silicon carbide mirror with a bowl-shaped structure was introduced. The entire process flow includes pre-modification silicon carbide substrate processing, silicon carbide substrate surface modification, and silicon modified layer processing. Firstly, before the modification, the conventional processing method of silicon carbide was used, and the effect of the support form on the figure was eliminated by multiple direction rotation testing.At the same time, the self-aligned compensation cross-test was completed and the accuracy of the aspherical surface coefficient was verified. In addition, the polishing process of the silicon modified layer material was studied, and the optimum process parameters suitable for polishing the silicon modified layer material were found out. Based on the above experiments, the modified optical processing adopts a combination of two kinds of polishing technology: flexible chemical mechanical polishing (FCMP)and ion beam figuring (IBF).The surface roughness and surface finish of silicon modified layer are improved by flexible chemical mechanical polishing technology. The high figure accuracy of silicon modified layer is achieved finally by ion beam figuring technology. Finally, the final result of the mirror after IBF is:the RMS values of the figure and roughness in the Φ450 mm aperture is 0.01λ (λ=632.8 nm) and 0.52 nm. The mirror's processing results fully meet the design specifications.
To improve the reflecting properties of all CFRP mirror, a high reflecting coating must be prepared on the mirror surface. In this paper, the effect of the roughness, film material and the deposition temperature on the reflecting rate was discussed. In the experiments, it was observed that the film exhibited higher reflecting rate on the smoother surface; meanwhile, the deposition rate must be controlled below the soften point of the surface replicated resin; if not, pits will generate on the surface and reduce reflecting rate. Ag film system exhibited higher reflecting rate than Al films. Finally, a multilayer film Ag and SiO2 was deposited on CFRP mirror, with a reflecting rate over 95% between 450nm and 800 nm.
A Φ450mm primary mirror subsystem of a space-based astronomy telescope was designed with mass, optical surface distortion and reflectivity requirement. The open-back primary mirror was made of pressure-less sintering silicon carbide, light-weighted at a ratio of approximately 70%. Three side supporting invar flexure bipods were designed to minimize the assembling stress and the thermal stress. The high reflection was obtained from the optical surface cementite. The mirror weighted 7kg and the reflectivity was 97% after optical polishing. The mirror subsystem was precisely assembled under the strict technical condition. The optical test with interferometer showed that the optical surface distortion is less than 1/40λ rms, which met the critical optical requirements for the primary mirror of the space-based astronomy telescope.
Due to low density, high specific stiffness, and low thermal expansion, carbon fiber reinforced plastic (CFRP) is believed to be one of the potential material choices for optical mirrors. But CFRP is one of the two-phase materials that cannot be used as optical surface and must be surface modified. To develop one kind of grid-reinforced CFRP mirror, optical replication technology was used to modify and achieve high-precision surface, and theoretical deformation due to replica resin curing and deformation caused by laminates’ manufacturing errors were studied in detail. Optical replication experiment has shown that λ / 20 root mean square high-precision surface can be achieved for ϕ100-mm grid-reinforced carbon fiber mirrors.
Variable curvature mirror (VCM) can change its curvature radius dynamically and is usually used to correct the defocus and spherical aberration caused by thermal lens effect to improve the output beam quality of high power solid-state laser. Recently, the probable application of VCM in realizing non-moving element optical zoom imaging in visible band has been paid much attention. The basic requirement for VCM lies in that it should provide a large enough saggitus variation and still maintains a high enough surface figure at the same time. Therefore in this manuscript, by combing the pressurization based actuation with a variable thickness mirror design, the purpose of obtaining large saggitus variation and maintaining quite good surface figure accuracy at the same time could be achieved. A prototype zoom mirror with diameter of 120mm and central thickness of 8mm is designed, fabricated and tested. Experimental results demonstrate that the zoom mirror having an initial surface figure accuracy superior to 1/80λ could provide bigger than 36um saggitus variation and after finishing the curvature variation its surface figure accuracy could still be superior to 1/40λ with the spherical aberration removed, which proves that the effectiveness of the theoretical design.
Up to now, traditional materials, such as glass, metal and SiC ceramic, gradually begin to be unsatisfied development of
the future mirrors. Designable carbon fiber reinforced composites became optimized material for large aperture
lightweight mirrors. Carbon/carbon composites exhibit low thermal expansion and no moisture-absorption expansion
problem, therefore, they get particular attention in the space reflector field. Ni was always employed as optical layer in
the mirror, however, the coating behaved poor bond with substrate and often peeled off during optical processing. In
order to solve this problem, slight oxidation was carried on the C/C composites before Ni plated. The Ni coating
exhibited stronger coherence and better finish performance. Finally, a 100mm diameter plane mirror was successful
fabricated.
The advantage of Carbon Fiber Reinforced Polymer (CFRP) is obvious as a common space material for low density, low thermal expansion coefficient and high specific stiffness characteristics, it is the ideal material choice for space optical reflector. Mirror structure with honeycomb can achieve high rates of lightweight, as well as high specific stiffness. For Φ300mm CFRP mirror, accounting of the actual process properties of CFRP, mirror panels laminated based on thermal stability design, honeycomb fabricated using one innovative inlaying-grafting design method. Finally, lightweight structure design of the CFRP primary mirror completed, the thermal stability result of the Φ300mm CFRP mirror achieved is 10nm°C.
Due to low density, high stiffness, low thermal expansion coefficient, duplicate molding, etc., carbon fiber reinforced polymer (CFRP) is one of the potential materials of the optical mirror. The process developed for Φ300mm high precision CFRP mirror described in this paper. A placement tool used to improve laying accuracy up to ± 0.1°.A special reinforced cell structure designed to increase rigidity and thermal stability. Optical replication process adopted for surface modification of the carbon fiber composite mirror blank. Finally, surface accuracy RMS of Φ300mm CFRP mirror is 0.22μm, surface roughness Ra is about 2nm, and the thermal stability can achieve 13nm /°C from the test result. The research content is of some reference value in the infrared as well as visible light applications.
Zoom mirror is a kind of active optical component that can change its curvature radius dynamically. Normally, zoom mirror is used to correct the defocus and spherical aberration caused by thermal lens effect to improve the beam quality of high power solid-state laser since that component was invented. Recently, the probable application of zoom mirror in realizing non-moving element optical zoom imaging in visible band has been paid much attention. With the help of optical leveraging effect, the slightly changed local optical power caused by curvature variation of zoom mirror could be amplified to generate a great alteration of system focal length without moving elements involved in, but in this application the shorter working wavelength and higher surface figure accuracy requirement make the design and fabrication of such a zoom mirror more difficult. Therefore, the key to realize non-moving element optical zoom imaging in visible band lies in zoom mirror which could provide a large enough saggitus variation while still maintaining a high enough surface figure. Although the annular force based actuation could deform a super-thin mirror having a constant thickness to generate curvature variation, it is quite difficult to maintain a high enough surface figure accuracy and this phenomenon becomes even worse when the diameter and the radius-thickness ratio become bigger. In this manuscript, by combing the pressurization based actuation with a variable thickness mirror design, the purpose of obtaining large saggitus variation and maintaining quite good surface figure accuracy at the same time could be achieved. A prototype zoom mirror with diameter of 120mm and central thickness of 8mm is designed, fabricated and tested. Experimental results demonstrate that the zoom mirror having an initial surface figure accuracy superior to 1/50λ could provide at least 21um saggitus variation and after finishing the curvature variation its surface figure accuracy could still be superior to 1/20λ, which proves that the effectiveness of the theoretical design.
A three-dimensional kinematic model of spherical mirror is developed by coordinate transformation theory. Based on the
model and Preston equation, material removal rate for polishing with constant pressure can be obtained. Then the fitted
parameters are determined for special process tools (pentagon tool and Uniform Removal Tool), Conclusions beneficial
to glass polishing in practice are arrived. The simulate results can describe visually the material removal law of mirror
under so many kinematic parameters. The work discussed above is of importance to polish flat mirror, spherical mirror
and aspheric mirror with one-axis polisher.
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