Hexapods Support Precision Measuring of Aspheric Lenses

Aspherical lenses have rotationally symmetrical optics around the optical axis, whose radius of curvature changes radially with the distance from the center. This allows optical systems to achieve high image quality, whereby the number of elements required decreases and that saves on both costs and weight. However, testing the aspheric shape accuracy, which means the quality of these types of lenses, is a considerable challenge to the manufacturer:

This requires measuring the tiniest deviations in shape in the nanometer range and at the same time, making short measuring and setup times possible. The solution is to use a new type of interferometer with multiple tilted wavefronts. As part of the overall system, the hexapod takes over several positioning tasks for calibrating and measuring.


A New Approach: Tilted Wavefront Technology

Without CGH
Without sequential stitching
Measuring within < 30 seconds
With high lateral resolution
Measuring uncertainties of < 50 nm

Several methods have been established for checking the shape of aspherical lenses for accuracy: For example, interferometers with computer-generated holograms (CGHs) generate an aspherical wavefront in the desired shape and therefore make it possible to determine the deviation of the lens. However CGHs need to be created individually for each test object shape and are therefore only economical for series production.
Interferometric measuring of aspheres in circular subsections is another possibility. Finally, each partial measurement is combined to a full-surface interferogram. The process is very flexible compared to CGHs and is also suitable for the production of prototypes and smaller series. However, "stitching" the circular rings is often very time consuming, as in the case of steeper optics, only smaller circular interference pattern rings can be captured and therefrom many interference patterns have to be stitched together.

For this reason, the metrology company Mahr developed a new instrument for precise, fast, flexible measuring of different aspheres directly on the production line, without CGH, classical stitching or tactile contacting. In contrast to existing systems that need several minutes to do the measuring, this tilted wavefront interferometer (TWI) needs only 20 to 30 seconds to measure the entire surface. The next test object can be already measured while the previous one is being evaluated.

Specifications of the new tilted wavefront interferometer TWI 60

Measuring and Referencing Process

The new measuring system does not immediately acquire the entire test object optically in one single image, but in several subapertures that are active at different times.

The individual geometrically distributed subapertures are actively switched and different tilted wavefronts hit the inspection optics without overlapping interference patterns. An undisturbed interference pattern of a local part of the test object surface is obtained from each subaperture and the entire surface of the test object can be measured within a short time.

Finally, the individual interference patterns are combined to form a topography of the test object's surface and the deviation of the test object's actual shape is determined from the nominal shape. The design of the TWI allows measuring of individual surface shapes with high lateral resolution and measuring uncertainties under 50 nm.

For referencing and calibration a highly accurate sphere with known geometrical specifications is moved for each subaperture to a specific position and then measured by this subaperture. Finally, all measurements are evaluated and an algorithm is used to correct the systematic measurement deviations across all subapertures.

As all kind of positioning errors of the calibration sphere affect the correction algorithm of the respective subaperture, the calibration sphere needs to be positioned very exactly. A maximum lateral position error of 5 µm with a repeatability of less than 0.5 µm is required.

In order to meet the high demands on the positioning mechanism in the TWI and after careful testing, we finally made the decision to use the H-824 hexapod.

Dr.-Ing. Jürgen Schweizer, Product Management Marketing at Mahr GmbH

Hexapod Positions Calibration Sphere and Test Object

The Hexapod H-824 positions the calibration sphere and, prior to the actual measuring process, the test object in five degrees of freedom. Also, both the desired and actual position need to be matched exactly. For example, deviations in tilt may not exceed 60 µrad.

The high-performance >> C-887 digital controller controls the hexapod that, thanks to the user-friendly software, enables easy commanding. The positions are specified in Cartesian coordinates, and all transformations to the individual drives are done inside the controller.

Specifications of the hexapod H-824

Advantages of the Parallel-Kinematic Positioning System

High repeatability
Submicron precision
Six degrees of freedom
Central aperture
Freely definable center of rotation

Hexapods are able to position in all degrees of freedom with high accuracy and travel along trajectories with high precision. In the case of hexapods and in contrast to serial kinematics, all six actuators act directly on the same platform. This allows a more compact design than stacked systems. Because hexapods move only one platform, the overall mass is also less, which results in high dynamics in all motion axes.

In contrast to a stacked system, hexapods are also distinguished by their improved path accuracy, higher repeatability, and flatness. Another essential characteristic of hexapods is the freely definable rotation or pivot point, which means it is possible to define various coordinate systems that for example, refer to the position of the workpiece or tool.

Downloads

Hexapod Positioning Systems

Version / Date
CAT136 2018-08
Document language English
pdf - 11 MB

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