lead_screw_pitch_options_xystage
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+ | ====== Lead Screw Pitch Options for ASI XY Stages: Speed, Resolution, and Repeatability ====== | ||
+ | The following table shows tested specifications for different lead screws pitches, motor gear-head combinations and encoder types. | ||
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+ | With rare exceptions, ASI stages use small DC servomotors with zero-backlash gear-heads and an integrated rotary encoder. | ||
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+ | ^ Standard Configuration using DC Servomotor with 141:1 Zero-Backlash Gearhead (except US-2000 stage) | ||
+ | ^ Pitch (TPI) ^ Pitch (mm) ^ Letter Code ^ Name ^ Approx. Max Speed (mm/ | ||
+ | | ::: | ::: | ::: | ::: | ::: ^ Encoder Resolution | ||
+ | | 1 | 25.40 | E | MM76D_ROT | 50 | 163 | < 6 µm | 0.25 | NA | NA | NA | special 76:1 gearhead | ||
+ | | 1 | 25.40 | D | Ultra-Coarse | ||
+ | | 2 | 12.70 | C | Super-Coarse | ||
+ | ^ 4 ^ 6.35 ^ B ^ Standard | ||
+ | | 8 | 3.18 | M | Medium-Fine | ||
+ | ^ 16 ^ 1.59 ^ A ^ Fine ^ 1.75 ^ 5.5 ^ < 0.5 µm ^ 0.25 ^ 10 ^ < 0.25 ^ ± 3 µm ^ default LS-x ^ | ||
+ | | 40 | 0.635 | U | Extra-Fine | ||
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+ | The US-2000 stage is designed for thermal stability and the dynamic performance is compromised. | ||
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+ | Scan-optimized stages have a special gearbox and other modifications that allow for smoothest possible continuous scanning in one axis. Most often they have 16 TPI leadscrews. | ||
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+ | Note that faster speed means that the stage has less torque and hence the achievable ramp up/down times may be compromised. | ||
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+ | There are subtle but important differences between the definitions of resolution, accuracy, and repeatability. See the page on [[ultimate_resolution_of_microscopes|ultimate resolution limits of microscopes]] for a general description of these terms and how they interplay with optical resolution. | ||
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+ | ===== Resolution ===== | ||
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+ | Resolution typically means the smallest possible stage move and/or the unit of measured position. | ||
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+ | The smallest possible move is limited by both the encoder resolution (measurement resolution) and by the minimum achievable mechanical motion possible (mechanical resolution). | ||
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+ | The unit of measured position is the encoder resolution listed in the table above. | ||
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+ | ===== Repeatability ===== | ||
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+ | Repeatability quantifies how well the stage returns to the exact same position after having been moved away. It is often the most important specification for XY stages in optical microscopy. | ||
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+ | Each stage is tested for bi-directional repeatability before leaving ASI and a plot is shipped with the stage. | ||
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+ | ASI's stage controllers automatically compensate for the residual backlash present in the mechanical system. | ||
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+ | Crossed roller bearing assemblies seem to have a repeatability limit somewhere around 50 to 100 nm which sets a lower bound on repeatability. | ||
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+ | ===== Accuracy ===== | ||
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+ | Accuracy is knowing exactly where the stage is, and by extension, being able to move an exact distance. | ||
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+ | For rotary encoders (position feedback at the motor), positional accuracy hinges upon the pitch accuracy of the lead screw. | ||
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+ | For linear encoders (position feedback at the stage plate), positional accuracy is determined by the accuracy of the optical scale which is +/- 3 μm relative to the center mark anywhere along the entire scale (i.e. the entire stage travel) and in practice is usually even better than the +/- 3 μm specification. | ||
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+ | ===== Speed ===== | ||
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+ | The maximum speed scales linearly with lead screw pitch. | ||
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+ | For applications where extremely constant speed of movement is an important factor, a different motor/ | ||
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+ | The full dynamic average speed range for ASI stages under closed loop velocity control is about a factor of 10, | ||
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+ | ===== Special Large Stages ===== | ||
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+ | For large stages only there is a larger motor / gearbox available. | ||
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+ | ^ Special Configuration using DC Servomotor with 3.71:1 Planetary Gearhead (available only in the MS-8000 and LS-400 stages) | ||
+ | ^ Lead Screw Pitch (mm) ^ Lead Screw Pitch Name ^ Max Speed(mm/ | ||
+ | | ::: | ::: | ::: ^ Encoder Resolution (µm) ^ RMS Bi-directional Repeatability (μm) <wrap lo>(with anti-backlash firmware)</ | ||
+ | | 25.40 | Ultra-Fast | ||
+ | | 12.70 | Super-Fast " | ||
+ | ^ 6.35 ^ Standard " | ||
+ | | 1.59 | Fine | 30 | 0.21 | no data | 0.25? | 50 | no data | ± 3 µm | | ||
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+ | {{tag> |
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