repeatability_performance_of_asi_stage
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+ | ====== Repeatability Performance of ASI Stages ====== | ||
+ | Microscope stages are often used in demanding applications where the stage’s ability to return precisely to a given location is a requirement. | ||
+ | ===== Definitions ===== | ||
+ | |||
+ | ==== Unidirectional repeatability ==== | ||
+ | The ability of the stage to return to a given point, always coming from the same previously defined point. | ||
+ | ==== Bi-directional repeatability ==== | ||
+ | The ability of the stage to return to a given point coming from a random previous point. | ||
+ | ==== Backlash ==== | ||
+ | Systematic error created by lost motion in the drive mechanism that appears when changing direction. | ||
+ | ==== Absolute accuracy ==== | ||
+ | The ability of the stage to move precisely the distance commanded. | ||
+ | |||
+ | ===== Test Methods ===== | ||
+ | |||
+ | All ASI stages undergo stringent optical performance testing before they are shipped. Optical testing methods can give the best local repeatability and backlash measurements. | ||
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+ | ==== Comparison with Linear Encoders ==== | ||
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+ | Figures 1 and 2 below show typical test results where the accuracy of the stage is compared with linear encoders that act as the standard. | ||
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+ | [{{ stage_repeatability_1.jpg? | ||
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+ | [{{ stage_repeatability_2.jpg? | ||
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+ | The same stage, tested with the controller’s anti-backlash algorithm turned ON, is shown in Figure 2. The hysteresis present in Figure 1 almost disappears. | ||
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+ | These tests with the linear encoder are best for quantifying the errors due to imperfections in the leadscrew. | ||
+ | |||
+ | ==== Optical Bi-directional Repeatability Test ==== | ||
+ | |||
+ | For this test we choose a suitable dust spec in the video image as the target position on the slide and establish the initial image as the “home” position. | ||
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+ | [{{ stage_repeatability_3.jpg? | ||
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+ | Note how without the backlash correction, the stage “stops short” of its desired location by several microns, depending upon which direction it has come from. This is the characteristic pattern for a fixed amount of lost motion on each axis. The return locations cluster much tighter when the anti-backlash algorithm is used, Figure 3(b). The best results are obtained with linear encoders, as shown in Figure 3(c). | ||
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+ | We quantify the point spread of the return locations with the standard deviation for both the X and Y directions. | ||
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+ | [{{ stage_repeatability_4.jpg? | ||
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+ | [{{ repeat_159.gif? | ||
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+ | It is apparent that the repeatability is better if the moves are smaller. | ||
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+ | [{{ stage_repeatability_5.jpg? | ||
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+ | [{{ repeat_159_le.jpg? | ||
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+ | With the linear encoders, the data now show a difference between the X and Y axis. The X & Y axis repeatability is less 150 nm for all but the largest moves. | ||
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+ | ==== Optical Grid Pattern Test ==== | ||
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+ | This test tracks an imaged dust spec about the field of view of the video microscope and records the position as the stage is scanned in uniform grid pattern. | ||
+ | |||
+ | [{{ stage_repeatability_6.jpg? | ||
+ | The grid pattern shown in Figure 6 is for a standard rotary encoded stage using the anti-backlash routine built into the controller. | ||
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+ | {{tag> |
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