Magnetorheological Finishing (MRF®) is a polishing process with a wide parameter space. Through the selection of various fluid compositions, wheel sizes, and other process settings, a large suite of possible tools is available. Most of the process space has been engineered to satisfy final figuring in which typically hundreds or possibly thousands of nanometers of removal are required. For larger aperture optics and/or higher amplitude corrections, there is interest in efficiently removing larger volumes of material. Continued optimization of the fluid delivery system, specifically the nozzle design, has resulted in dramatically improved removal rates. Initial process optimization has been performed to integrate these improvements into production. A new nozzle design produced improved removal rates of up to 200% relative to the standard nozzle designs. With the new design, several factors needed to be investigated to verify its suitability for production. Foremost among these was the performance around the edges of the polished aperture. Transitioning an MRF tool onto an optical surface typically produces a zone where the tool shape and/or rate differs from the characterized tool. The size of the zone scales with the size of the tool, and within this zone the removal is not well predicted. Varying the removal function (also referred to as the spot) size around the edges to mitigate this effect was assessed with promising results. Finally, the stability of the tool was characterized. The stability of the tool affects global unpredicted removal errors, particularly in higher spatial frequency bands. Preliminary results have shown performance at least comparable to the standard designs.
Magnetorheological Finishing (MRF) is a well-established process for precision optical finishing. The primary benefits of MRF are its determinism, ability to finish complex surface shapes and good removal rate. Higher removal rates can be desirable to reduce cycle times or increase the amount of material removal. Cases include but are not limited to aspherization, harder materials, correcting higher error amplitudes, polishing very large aperture optics, and subsurface damage removal. Novel developments have been made with the fluid nozzle, magnetic field, and wheel geometry to increase the removal rate. Custom nozzle shapes are used to create a very wide MRF ribbon resulting in a much wider removal function (spot). The increase in ribbon width requires the use of higher volumetric flow rates. The magnetic field must be shaped to ensure that the field passes through this wider ribbon. Radius of curvature of the MRF wheel parallel to the axis of rotation is increased to achieve a wider spot. Results show an order of magnitude increase in volumetric removal rate due to a much wider and longer spot. Spot width increases of 3x, and spot length increases of 2x, as compared to a traditional MRF spot, were achieved. Polishing shape corrections were conducted with these spots which resulted in convergence rates over 80%. This development allows MRF to be used in applications previously limited by cycle time
As the demand for higher precision optics grows, commercially available manufacturing options are needed to meet the stringent requirements requested by optical designers. Short radius concave optics have been a challenge for optical manufacturers as sub-aperture polishing tools that are small enough to accommodate the shape of the optic have not been available. Until recently, the smallest MRF wheel was 20 mm in diameter, which allowed polishing a minimum concave radius of approximately 14 mm. With the newly developed 10 mm diameter MRF wheel, we can push the previous MRF boundaries to accommodate even shorter concave radii. Not only does this tool extend the concave radius limits of MRF technology, but it also improves the efficiency of correcting mid-spatial frequency errors. As the removal function, or ‘spot’, becomes smaller we can make corrections to errors with higher spatial frequencies. In addition, the geometry of the wheel and the size of the removal function provide further benefits that will be explained. This module was designed to be compatible with any machine within QED Technologies’ Q-flex family of MRF equipment.
Optical system designers are well-versed in optimizing the performance of a system. The impact of the optical and optomechanical assembly, however, poses a significant challenge to attaining the modelled performance in practice. The system engineers are tasked with designing tooling, fixtures and procedures that minimize such impacts, employing well known modeling and analysis techniques. Despite these efforts the resulting system performance often exhibits errors that can be directly related to the assembly process.
In the face of lost system performance, the optical designer can compensate with more stringent component and alignment specifications. Alternatively, at the risk of a more complex design, she can consider active compensation, or the addition of compensation components. Yet another path is correcting the components after assembly to regain the original optical performance.
MRF is well known for its ability to produce state of the art optical components, lenses, mirrors, etc. In this paper we will explore and demonstrate its application to correcting errors induced by various assembly techniques by reviewing several examples, their respective challenges and the results of the post assembly corrections.
Deterministic subaperture finishing technologies, such as Magnetorheological Finishing (MRF(R)) are becoming the
industry standard for finishing high precision optics with complex shapes, such as aspheres. However, astronomical or
very large optics were beyond the scale of existing capabilities and relied on traditional, artisan-based methods of
manufacture. It is not uncommon for these critical parts to spend a year or more in production. Recent developments
from QED Technologies(R) have expanded MRF technology to enable the manufacture of meter-scale aspheric optics.
QED, in conjunction with the Steward Observatory Mirror Laboratory (SOML) at the University of Arizona,
demonstrated the fabrication of an 840 mm diameter convex asphere with 1.3 mm of aspheric departure from a best-fit
sphere. Long-trace profilometry scans were initially performed at SOML to characterize the surface. A first figure
correction polishing iteration was conducted at QED Technologies in Rochester, NY on a meter-class MRF machine
(Q22-950F). The correction improved the surface to within the capture range of a full aperture interferometric test
performed at the Mirror Lab. A final polishing iteration at QED improved the surface to meet the optic specifications.
New optical designs containing freeform optics have recently begun appearing in systems. Applications have
incorporated parts ranging in size from small (e.g.: ~5 – 10 mm rectangles) to large (e.g.: astronomical applications).
To meet these needs, QED Technologies recently introduced two solutions using its Q22-Y and Q22-950F platforms.
Magnetorheological Finishing® (MRF®) is a production proven technology for deterministically finishing symmetric
parts (flats, spheres, and on-axis aspheres) using a rotational toolpath, and rectangular flats and cylinders using a raster
toolpath. The new freeform toolpath expands the raster capabilities of the Q22-Y and Q22-950F machines to include
spheres, aspheres, off-axis sections, and true freeform geometries.
The freeform raster toolpath was first introduced on a meter-class optic platform, the Q22-950F. As optics grow in size,
the mass typically scales as well. This in turn increases the demands on the machine dynamics to meet rotational
polishing requirements. The raster freeform toolpath solution greatly reduces the machine dynamics and is employed to
polish a wide variety of part shapes, sizes, and geometries. A similar version of the toolpath was subsequently
implemented on the smaller Q22-Y platform. This paper will compare the implementations on each platform, describe
the benefits of the toolpath for existing and new applications, and present results from demonstrations on the two platforms.
Fabrication of large optics has been a topic of discussion for decades. As early as the late 1980s, computer-controlled
equipment has been used to semi-deterministically correct the figure error of large optics over a number of process
iterations. Magnetorheological Finishing, MRF®, was developed and commercialized in the late 1990's to predictably
and reliably allow the user to achieve deterministic results on a variety of optical glasses, ceramics and other common
optical materials. Large and small optics such as primary mirrors, conformal optics and off-axis components are
efficiently fabricated using this approach. More recently, specific processes, MR Fluids and equipment have been
developed and implemented to enhance results when finishing large aperture sapphire windows.
MRF, by virtue of its unique removal process, overcomes many of the drawbacks of a conventional polishing process.
For example, lightweighted optics often exhibit a quilted pattern coincident with their pocket cell structure following
conventional pad-based polishing. MRF does not induce mid-frequency errors and is capable of removing existing quilt
patterns. Further, odd aperture shapes and part geometries which can represent significant challenges to conventional
polish processing are simply and easily corrected with MRF tools. Similarly, aspheric optics which can often present
multiple obstacles-particularly when lightweighted and off-axis−typically have a departure from best-fit sphere that is
not well matched with to static pad-based polishing tools resulting in pad misfit and associated variations in removal.
The conformal subaperture polishing tool inherent to the QED process works as well on typical circular apertures as it
does on irregular shapes such as rectangles, petals and trapezoids for example and matches the surface perfectly at all
points. Flats, spheres, aspheres and off-axis sections are easily corrected. The schedule uncertainties driven by edge
roll and edge control are virtually eliminated with the MRF process.
This paper presents some recent results of the deterministic finishing typified by the QED product line and more
specifically of its large-aperture machines, presently capable of finishing optics up to one meter in size. Examples of
large sapphire windows and meter-class aspheric glass optics will be reviewed. Associated metrology concerns will also
be discussed.
The Electrooptic Beam Scanner is a solid state device developed at Carnegie Mellon's Data Storage Systems Center (DSSC) that is capable of scanning a laser beam approximately +/- 1 - 3 degree(s) at frequencies of at least 200 kHz. The optical group of the DSSC is currently employing the scanner as a fine actuator for high performance optical data storage systems. This paper discusses several aspects of that research, including multi-track scanning for increased data rate, controller for two stage actuator systems, and feedforward control for ultra-fast seeks.
Optical encoders are the most common means for measuring linear or rotary position. A photodetector senses the occlusion, reflection, or diffraction of light by a structure that moves linearly or rotates relative to a fixed light source. Counting the switching between `on' and `off' gives a measure of position, and velocity is determined through numerical differentiation. This paper proposes the use of an electrooptic beam scanner to improve the accuracy of the detection of both position and velocity, by scanning the light source across the moving structure and comparing the phase of the photodetector signal to the phase of the scan. The Doppler effect between the forward and backward scan can provide velocity information without numerical differentiation. A simple experiment for detecting the position of a razor edge demonstrates the concept for position.
Electro-optic (EO) scanners can be used as fine tracking actuators to improve the servo bandwidth in future high density/high data-rate optical disk drives. In this paper we report on the use of an EO scanner in a new optical tracking system. Track following has been accomplished with a servo bandwidth of 200 kHz and we have demonstrated track switching between nine tracks using only an EO scanner. A fine tracking experiment using an EO scanner has been demonstrated in parallel with a voice-coil actuated lens to expand the fine tracking range. Significant improvement in track switching speed and track following are demonstrated with the scanner/lens actuators as compared to tracking with the lens actuator alone.
KEYWORDS: Scanners, Head, Optical tracking, Servomechanisms, Actuators, Digital signal processing, Electro optics, Objectives, Control systems, Data storage
Optical disks are widely used in data storage system as well as in audio disk and video disk systems. Rapid progress is now being achieved in toms of recording density and data capacity. Ultra-high density magneto-optical recording has been obtained using recently developed magnetic field modulation (MFM) recording and magnetic super resolution (MSR) and magnetic domain expansion readout techniques.
A novel materials handling system is being developed at Carnegie Mellon University's Mechanical Engineering Department. This system contains an array of cells, each of which has two actuators. The two actuators are orthogonally oriented motorized roller wheels which, in combination, can generate a vector of motion in any planar direction. This work develops control laws for transporting and manipulating objects which rest on the array. Towards this goal, we consider the dynamics of parcel transport and manipulation. The parcel dynamics are based on an exact discrete representation of the system, unlike other methods where a continuity assumption is made. Two types of contact models are considered. This work extends the previous 1D discrete model into 2D.
KEYWORDS: Head, Servomechanisms, Scanners, Data storage, Feedback control, Signal processing, Magnetism, Computing systems, Magnetic tracking, Systems modeling
As the data storage density continues to grow, high performance tape transports are needed for fine positioning and transport of the media over the read and write heads. This paper presents a survey of digital tape transport servo systems for four popular types of drives: (1) reel to reel drives; (2) single cartridge drives; (3) helical scanning drives; and (4) belt-driven drives. Concepts behind velocity, tension and tracking control employed in production drives are discussed and references to pertinent research cited. Techniques used to measure error in the parameters under servo control are presented.
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