LIU BWS follow-up

865/1-D17 - Livia Lungu (CERN)

865/1-D17 - Livia Lungu


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·         Federico, Lars J, Jose Sirvent, Jonathan, Patrik, William, Dimitry, JJ, Ray

General ACTION:

·         Federico to summarise LIU specification table to allow comparison of mechanical specifications (& their impact) with achieved measurements.

Optical Disk Uncertainty (Jose):

·         20um optical spot on disk - pitch pattern?

Sources of uncertainty - systematic errors

·         Disk mounting eccentricity

o    Mounting Error (On-line correction with two focusers)

·         Disk fabrication

o    Pattern Uniformity (Possible to identify on eccentricity correction)

·         Disk imperfections

o    Scratches produce loss-counts (Corrected during processing)

·         Disk rotation run-out

o    Slow and cyclic optical signal power loss (Corrected during processing)

·         Sensor Interferometry

o    Optical signal modulation during runout (Corrected during processing)

·         Disk Ref.-Fork Alignment

o    Incertitude on fork projection angle (Extracted trough calibration)

Sources of uncertainty - random errors - mechanics

·         Disk / Focuser transversal vibrations

o    Linear displacements on the track axis

·         Focusers tilt due to vibrations

o    Eccentricity correction would be affected

·         Disk Fixation Mechanical play

o    Random Disk eccentricity variation

Sources of uncertainty - random errors - mechanics

·         Sampling frequency VS Speed limits resolution on crossing points

o    @ 133rad/s Signal Freq. ~ 600KHz, sampled @ 20 MHz < 0.09um projected

·         Random variation on signal processing algorithm

o    count-loss correction, dynamic threshold crossing


Slit-to-slit projected distance : 26.6 um (Interpolation possible)

·         Projected systematic position error on a scan: ~1.33um



·         Without correction (all speeds)

o    Max Pos. Error 3.8mRad (570um Proj.)

o    Disk off-centre by 177um

·         After correction

o    8m/s Encoder Reproducibility (Precision) ~ 2.9urad à 0.36 um

o    20m/s Encoder Reproducibility (Precision) ~ 3.5urad à 0.43 um

Calibration Bench:

Position reproducibility:

·         20m/s : Projected Precision: 57.3 um

·         8 m/s : Projected Precision: 11 um

·         Difference presumed to be due to vibrations

o    Need to isolate laser assembly from scanner tank to re-test

Relative laser scale on single scan:

·         Precision between 50-100um depends on scan speed

·         not understood why larger than position reproducibility

Speed variation (as calculated)

·         0.1 m/s  @ 20m/s

·         0.04 ms @ 8m/s


·         Re-design test bench to separate laser & scanner (probably not before review)

  • May be possible to make a simple set-up for a single laser position, decoupling laser & photodiodes from the table holding the scanner mechanics

·         Try to understand why 2 laser distance resolution worse than scatter on position

o    Try to analyse single laser to position for both & compare to average

·         Look at reproducibility of start timing to given position


Mechanical Uncertainty (Dimitry)

In mechanical engineering, backlash, sometimes called lash or play, is a clearance or lost motion in a mechanism caused by gaps between the parts.

·         Engineering tolerance is the permissible limit or limits of variation of dimension


Look at which aspects have a significant impact on uncertainty, producing expected values for the following:

·         Fork deformation

o    During acceleration

o    Due to shaft torque

·         Wire vibration

·         Wire deformation

·         Play between

o    optical wheel & shaft

o    optical wheel & optics

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