Investigations of Digital Levels for High Precision Measurements

1. 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

2. 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)

3. Design and Hardware

4. Scale Determination • Determination of the scale factor • Repeated regularly – ensures equipment works correctly

5. 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

6. 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

7. focused set up defocused set up Defocused Measurements • Leica DNA03 and Trimble DiNi12: critical distances do not cause deviations > 30 mm • Defocused measurements increase these values

8. 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

9. 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

10. 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

11. 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.