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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 (and expected in italics) specifications for different lead screws pitches, motor gear-head combinations and encoder types. The most popular combinations are use the 6.35 or 1.59 mm pitch lead screw are high lighted below.

Standard Configuration using DC Servomotor with 141:1 Zero-Backlash Gearhead
Lead Screw Pitch (mm / TPI) Lead Screw Pitch Code / Name Approx. Max Speed (mm/s) 512 Counts/Rev Rotary Encoder Linear Encoder
Encoder Resolution (nm)1) 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 / 1 D / Ultra-Coarse 28 88 < 3 µm 0.25 10 < 1.0 ± 3 µm
12.70 / 2 C / Super-Coarse 14 44 < 1.5 µm 0.25 10 < 0.4 ± 3 µm
6.35 / 4 B / Standard 7 22 < 0.7 µm 0.25 10 < 0.3 ± 3 µm
1.59 / 16 A / Fine 1.75 5.5 < 0.5 µm 0.25 10 < 0.25 ± 3 µm
0.635 / 40 U / Extra-Fine 0.7 2.2 < 0.5 µm 0.25 10 < 0.25 ± 3 µm
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 480 3.3 < 4 µm 0.25 100 < 2.0 ± 3 µm
12.70 Super-Fast 240 1.7 < 2 µm 0.25 100 < 1.0 ± 3 µm
6.35 Standard 120 0.84 < 1 µm 0.25 50 < 0.5 ± 3 µm
1.59 Fine 30 0.21 < 1 µm 0.25 50 < 0.5 ± 3 µm

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 200 times less than the maximum speed (for details, see Technical Note on Slow Speed Considerations).

The smallest possible move is limited by both the minimum encoder resolution (measurement resolution) and by the minimum achievable mechanical motion possible (mechanical resolution). ASI stage controllers can typically move the stage motors to within one or two rotary encoder counts of the desired position, setting the fundamental small-move limit for our stages. Crossed roller bearing assemblies seem to have a repeatability limit somewhere around 50 to 100 nm. The repeatability is often worse for long moves than for short moves. Repeatability errors can come about because of uncertainty in the measured position from the encoder or because of mechanical uncertainty (backlash) in the mechanical components of the stage. Using linear encoders will improve repeatability because position measurement is closer to the sample.

In the standard configuration, ASI stages use small DC servomotors with zero-backlash gear-heads and an integrated rotary encoder. This motor/gear-head/encoder combination provides excellent closed loop control for high precision microscopy stages where modest speeds are acceptable. The zero-backlash gear-head reduces – but does not completely eliminate – mechanical backlash from the stage system.

For applications where speed of movement is an important factor, a different motor /gear-head combination can be used. Mechanical backlash can be around 10 μm for these systems if linear encoders or anti-backlash controls are not used.

ASI's stage controllers can automatically compensate for most of the backlash present in the mechanical system. On each 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 much of the inherent backlash in the mechanical system so that reasonably repeatable and accurate positioning is possible without using linear encoders.

Overall positional accuracy depends upon the pitch accuracy of the lead screw if rotary encoders are used, or the accuracy of the optical scale if linear encoders are used. For best possible positional accuracy linear encoders are recommended because the position of the stage is measured directly, whereas with rotary encoders only the motor rotation is added even though the leadscrew and gearbox sit between the motor and the stage. The turn-to-turn wobble of the lead-screw (frequently of 3-10 μm/rev p-p) leads to similar positional inaccuracy with rotary encoders. The linear encoder eliminates this problem but must be aligned properly to avoid a similar problem of its own. 2)

Each stage is tested for bi-directional repeatability before leaving ASI and a plot is shipped with the stage. The repeatability numbers tabulated above “red-line” values; a stage that exceeds those numbers will not ship. Typical RMS repeatability is better than 400 nm for standard stages without linear encoders and better than 200 nm with linear encoders. 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).

1)
512 counts/rev for encoder * 4 for quadrature * 141 gear reduction = 288,768 counts/motor turn / Pitch (mm) = counts/mm
2)
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.
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lead_screw_pitch_options_xystage.txt · Last modified: 2021/09/23 17:15 (external edit)