Investigations of Digital Levels for High Precision Measurements Georg L Gassner Robert E Ruland Brendan Dix Metrology Department Stanford Linear Accelerator Center Work supported by the U.S. Department of Energy under contract number DE-AC03-76SF00515 Thanks chairmen / person SLAC Stanford DOE TITLE Encountered Problems not explainable No test stand in US Started in 2002

Factors influencing the accuracy: Introduction Requirements at SLAC: 150 mm / 300 m 50 mm / single components Factors influencing the accuracy: Scale factor Critical distances and focusing End section of the staff Illumination Tested Equipment: Leica DNA03 Trimble (formerly Zeiss) DiNi12 (Leica NA3000) Digital level + fast and normally accuratly measurements Known sources - scale factor regular basis

Design and Hardware Principle idea is to move the levelling rod controlled by an interferometer and measure the height differences with the level at the same time Only two other in the world Graz and Finland Sounds pretty easy but it is not z. B. the staff is moveable over a range of 3m with an accuracy of about 1micrometer The digital level is mounted in a moveable cage on the ceiling which allows us to measure at sighting distances from 1.8m up to 30m Fully automated Mention tour to SLAC

Determination of the scale factor Scale Determination Determination of the scale factor Repeated regularly – ensures equipment works correctly Explain X axis y axis Two purposes 1.) determination of the scale factor 2.) test of equipment

Critical Distances – Leica 1 code element (2.025mm) is projected onto exactly 1 pixel, or integer multiples Leica NA3000 critical distance at 14.92 m (causes up to 0.4 mm misreading) Leica DNA03 critical distance at 26.70 m At certain distances it appears that digital levels don’t give the same accuracy as usual. Investigation this effects peaple found that this happens when one code element projected on the ccd array has the same size as one pixel. An explanation therefore is that if one code element has a dark image projected on it and the next a light image an effect called crosstalk occures meaning that the not illuminated pixel gets activate by the neighboring pixl A real problem occurs with a instrument no longer in use at SLAC the NA3000 There one code element is projected onto the ccd array with the size of 1 pixel at 14.92 m.

Critical Distances – DiNi12 There are 251 critical distances between 1.5 m and 15 m Size of 1 code element is 20 mm Example: At 10.98 m 1 code element is projected onto 38 pixels

Defocused Measurements Leica DNA03 and Trimble DiNi12: critical distances do not cause deviations > 30 mm Defocused measurements increase these values focused set up defocused set up Easy to take measurements with not accuratly set up instrument

End Section of the Staff (1) Measurements at the end section of the rod Only parts of the staff are visible Inaccurate measurements are the consequence Can also occur if parts of the rod are covered Good practice to avoid lower end of the rod because of atmospheric effects

End Section of the Staff (2) DiNi12 300 mm code section H [mm] = start of visible code on the staff + 150 mm Leica DNA03 DNA03 uses 1.1° code section (moveable) H [mm] = start of visible code + 20 mm + 6.9·sighting distance [m] invisible code section

Illumination Only illumination at a steep angle causes biased measurements of up to 100 mm (only correct for the instruments tested) Measurements in dimly lit environments require artificial illumination

Conclusion Determination of the scale factor on a regular basis gives the confidence of quality checked measurements. The level has to be horizontal and properly focused. When the highest accuracy is desired, the rod end sections and every obstruction have to be avoided. Illumination at a steep angle must be avoided.