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lead_screw_pitch_options_xystage

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. 4 TPI is a good general-purpose choice for for XY stages. 2 TPI is used to boost XY speed. 16 TPI is default for vertical applications and for scan-optimized stages.

With rare exceptions, ASI stages use small DC servomotors with zero-backlash gear-heads and an integrated rotary encoder. When linear encoders are installed on the stage then the controller is usually set to use those for position feedback, otherwise the rotary encoder is used. Linear encoders utilize a glass scale adhered to the moving stage plate detected by a stationary read head. For rotary encoders, the stage position is inferred by how many revolutions the motor has turned assuming a perfect lead screw as described below. 1)

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/s) Standard Rotary Encoder Standard Linear Encoder Comments
Encoder Resolution (nm)2) RMS Bi-directional Repeat-ability (μm) 3) Lead Screw Accuracy (μm/mm) Encoder Resolution (nm) RMS Bi-directional Repeat-ability (μm) Glass Scale Accuracy 4)
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 28 88 < 3 µm 0.25 10 < 1.0 ± 3 µm
2 12.70 C Super-Coarse 14 44 < 1.5 µm 0.25 10 < 0.4 ± 3 µm
4 6.35 B Standard 7 22 < 0.7 µm 0.25 10 < 0.3 ± 3 µm default XY
8 3.18 M Medium-Fine 3.5 11 < 0.7 µm 0.25 10 < 0.3 ± 3 µm by special request
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 0.7 2.2 < 0.5 µm 0.25 10 < 0.25 ± 3 µm

The US-2000 stage is designed for thermal stability and the dynamic performance is compromised. It is only built with 16 TPI and 40 TPI leadscrews. Its bi-directional RMS repeatability specification is 1.5µm.

Scan-optimized stages have a special gearbox and other modifications that allow for smoothest possible continuous scanning in one axis.

Note that faster speed means that the stage has less torque and hence the achievable ramp up/down times may be compromised. Further note that for relatively small moves – approximately 10% of the leadscrew pitch – the total move time is dominated by the ramp time and not by the maximum stage speed. The upshot is that getting a stage with larger maximum speed may only have a small impact on the time required to execute a particular move pattern and worsen performance in other ways; please consult with ASI if you are trying to perform a certain move pattern as fast as possible. You can also find more details with the documentation on stage accuracy and settling times.

There are subtle but important differences between the definitions of resolution, accuracy, and repeatability. See the page on ultimate resolution limits of microscopes for a general description of these terms and how they interplay with optical resolution.

Resolution

Resolution typically means the smallest possible stage move and/or the unit of measured position.

The smallest possible move is limited by both the encoder resolution (measurement resolution) and by the minimum achievable mechanical motion possible (mechanical resolution). The the fundamental small-move limit is typically one or two rotary encoder counts even if linear encoders are used.

The unit of measured position is the encoder resolution listed in the table above. Note that the resolution with rotary encoders is smaller than with linear encoders for fine-pitch lead screws, and vice-versa for coarse-pitch lead screws.

Repeatability

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. Linear encoders improve repeatability because position measurement is closer to the sample, but ASI stages have excellent repeatability even using rotary encoders.

Each stage is tested for bi-directional repeatability before leaving ASI and a plot is shipped with the stage. The repeatability specification tabulated above is a “red-line” value; a stage that exceeds the specified value on the test will not leave ASI. The test is performed by making moves in a circular pattern and seeing how repeatably it returns to the original position, starting with 20 mm moves and going down to 100 μm moves. The stated values are for all move distances tested, but longer moves have worse repeatability (especially without linear encoders).

ASI's stage controllers automatically compensate for the residual backlash present in the mechanical system. On each commanded move, the anti-backlash algorithm causes the stage to move to the target position by moving first to a point a fixed distance away from the target, ensuring that the approach to the target is always from the same direction. The anti-backlash routine removes almost all of the inherent backlash in the mechanical system so that very repeatable positioning is possible even without linear encoders. When linear encoders are used the anti-backlash routine is disabled by default.

Crossed roller bearing assemblies seem to have a repeatability limit somewhere around 50 to 100 nm which sets a lower bound on repeatability.

Accuracy

Accuracy is knowing exactly where the stage is, and by extension, being able to move an exact distance. Linear encoders are recommended for applications requiring the best possible absolute accuracy.

For rotary encoders (position feedback at the motor), positional accuracy hinges upon the pitch accuracy of the lead screw. The turn-to-turn wobble of the lead-screw is typically 3-10 μm/rev p-p and leads to an corresponding cyclic position inaccuracy of typically 3-10 μm along the travel with period of the leadscrew. Furthermore, the overall pitch may be slightly inaccurate. However both of these effects are constant over time and do not affect repeatability.

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. When using linear encoders imperfections in the lead screw do not affect accuracy because the position is measured at the stage plate directly. 5)

Speed

The maximum speed scales linearly with lead screw pitch. However, the torque generated also scales (in a not quite linear way) with lead screw pitch, so the maximum speed may not be achievable in practice for coarse lead screws, especially with heavy loads.

For applications where extremely constant speed of movement is an important factor, a different motor/gear-head combination can be used which has more mechanical backlash (typically ~7 μm for 1.59 mm lead screws) but eliminates small jumps, but and the anti-backlash algorithm described above means that repeatability is still excellent. This is commonly called “scan-optimized” stages. See information on optical tracking tests.

The full dynamic average speed range for ASI stages under closed loop velocity control is about a factor of 10,000. However, the slowest speeds are not as smooth, so a good working minimum speed is 1000 times less than the maximum speed (for details, see Technical Note on Slow Speed Considerations).

Special Large Stages

For large stages only there is a larger motor / gearbox available. This is a very rare build and has internal designation “GTS”. These can also be implemented with direct drive (no gearbox).

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/s) 512 Counts/Rev Rotary Encoder Linear Encoder
Encoder Resolution (µm) RMS Bi-directional Repeatability (μm) (with anti-backlash firmware) Lead Screw Accuracy (µm/mm) Encoder Resolution (nm) RMS Bi-directional Repeatability (µm) Glass Scale Accuracy (per length of scale)
25.40 Ultra-Fast 4806) 3.3 no data 0.25? 100 no data ± 3 µm
12.70 Super-Fast “GTS_50” 2407) 1.7 no data 0.25? 100 no data ± 3 µm
6.35 Standard “GTS_25” 120 0.84 no data 0.25? 50 no data ± 3 µm
1.59 Fine 30 0.21 no data 0.25? 50 no data ± 3 µm
1)
Other manufacturers use stepper motors that can miss counts, but this is not a problem with ASI stages that use DC servomotors.
2)
512 counts/rev for encoder * 4 for quadrature * 141 gear reduction = 288,768 counts/motor turn / Pitch (mm) = counts/mm
3)
with anti-backlash firmware routine
4)
per length of scale
5)
Note that linear encoders must be aligned properly at the factor to avoid a similar problem of its own. Linear encoders count finely by interpolating the cyclic waveforms derived from the encoder’s optical scale. Small interpolation errors can occur with the period of the encoder scale (typically every 4 to 20 μm.) Careful adjustment of the linear encoder is required to minimize such errors.
6) , 7)
Not verified
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lead_screw_pitch_options_xystage.txt · Last modified: 2023/11/13 14:42 by jon