1. Hexagon Metrology By: JHorwell Sr. Metrologist July 2013
Alternate Method to Certify Grid Plates and Line Scales using a CMM with Chromatic Confocal Sensor verses Traditional Methods
Hexagon Metrology
By: JHorwell Sr. Metrologist July 2013

2. What may I Learn or Why Listen?
Alternate method for certifying Line Scales
How to apply a white light confocal sensor for edge detection
Difference between displacement and intensity
Density of the data
Selection Criteria
2013 NCSLI International Workshop and Symposium

3. 01 Brief History 02 03 04 05 06 Overview of Requirements Test Piece
AGENDA
Overview of Requirements 01
Brief History 02
Test Piece 03
Results 04 05
Error Budget 06
Lessons Learned
2013 NCSLI International Workshop and Symposium

4. Test the ability of a confocal chromatic sensor to find edges
Overview
Test the ability of a confocal chromatic sensor to find edges
Review existing Hexagon Hardware and firmware to accomplish this task
Compare distance measurements against certified artifact
Determine the feasibility to certify grid plates and glass line scales on a high accuracy CMM with confocal chromatic sensor
2013 NCSLI International Workshop and Symposium

5. Some have combinations of manual and automatic systems
History
Currently most line scales are measured on special purpose hardware consisting of an XY stage, Laser Interferometer feed back for position and an optical measuring device
Others use comparative techniques from a certified master scale to the scale under test
Some have combinations of manual and automatic systems
In the past some used high accuracy CMM with an image analysis system to certify grid plates
2013 NCSLI International Workshop and Symposium

6. 200mm Glass Scale with Chrome Lines
Task is to be able to find the edges with a standard deviation less than 0.09um
Calculate the distances and compare these to a certified value.
2013 NCSLI International Workshop and Symposium

7. Equipment Selection Criteria
Small Measurement Uncertainty ≤ L/1000
Non Contact Sensor Availability
Stability over time
Hardware, Software and Firmware available and capable for finding edges
2013 NCSLI International Workshop and Symposium

8. LSP –S4 Probe head with integrated optional Precitec LR Sensor
Leitz PMM C Infinity
Measuring Error MPE in [µm] according to ISO (2010-6)
E0 ≤ L/1000
E150 ≤ L/1000
R0 ≤ 0.3
Scale Resolution
4nm
LSP –S4 Probe head with integrated optional Precitec LR Sensor
2013 NCSLI International Workshop and Symposium

9. Precitec LR Sensor Fully Integrated into Leitz PMM C Infinity
Numeric aperture
Measuring procedure chromatic confocal
Measurement angle relative to surface 90° ±40°
Measuring distance mm
Diameter of measuring spot µm
2013 NCSLI International Workshop and Symposium

10. 90 degree Sensor
We are continually developing sensors and adaptors for our systems
The sensor to the right is similar to the sensor we used but can measure the line scales in the vertical direction verses horizontal
The sensor can be manually rotated and our controller can handle more than one sensor on a single system
2013 NCSLI International Workshop and Symposium

11. Overview Glass Line Scales
Find the edges of a Chrome Lines on Glass Line Scales using the Precitec LR Chromatic Confocal Sensor on the Leitz infinity in RI
Build a reference system on the Glass Scale using tactile probing
Build an alignment system using the Precitec LR Sensor
Measure each reference line via 3-pts and constructing an axis
Intersect each reference line with the alignment axis
Calculate the distance from each line to the zero line and compare with certificate of certification
Test 2-methods for finding the edge
Test alignment stability
Run repeat test to determine standard deviations
2013 NCSLI International Workshop and Symposium

12. Test Part – Glass Line Scale
Certificate of Calibration No by CIPM MRA
Uncertainty of Measurement: U = 0.10µm + L/2000
The distances between the graduations were measured with a photo mask measuring system consisting of a precision x-y bearing table, a two-axis differential plane mirror interferometer and a microscope. The table moves the graduations to the focus of microscope while the interferometer measures its position. The relative position of a graduation within the field of view the microscope is determined by digital image analysis.
The scale was cleaned, supported at the Bessel-points (distance 121mm) and measured with the graduation on top using episcopic white light illumination. For each graduation the position of the trailing edge was measured at the height of the collinear alignment marks
2013 NCSLI International Workshop and Symposium

13. What was missing from the hardware configuration
While we could find the edge of the lines using traditional analysis by automatically finding extreme points with a standard deviation between 112nm and 250nm using a 30 repeat test. We determined that these values were not as good as we believe our equipment was capable up.
Our Firmware Development Group made a flag within our firmware that allowed us to receive the intensity out verses displacement from the Precitec LR Sensor
This was the break thru that we needed to accomplish our task
2013 NCSLI International Workshop and Symposium

14. Alignment & Fixture Procedure For Glass Scale
The Glass Scale was place on 3-spherical pins which were near the airy points with 2 at one level and a single pin at the other airy point.
2-posts were used to align the scale along its axis and a single pin for the stop on the zero end plane of the Glass Scale
Tactile alignment
Using the LSP-S4 probe head and ruby sphere probe we measured a plane on the top and long side and a single point on the end face. This provided us with the rough alignment before we picked the Precitec LR Sensor from our Probe Change Rack
Optical Alignment
Scanned two lines on the top surface of the Glass Scale, constructed a plane and defined the Z-axis spatial vector and alignment from that.
We measured 3-points on the line between the 0 and 10mm lines and constructed axis
Measure same on the last line between the 190 and 200mm marks
Constructed an axis from these two points which define the rotational portion of our Coordinate system
Measured 3-points and constructed an axis on the back side of the zero line at ±0.5mm from the axis just created
Intersected this zero line axis with the line the defined the rotational portion of our Reference System and this became the X-zero of our Glass Scale
2013 NCSLI International Workshop and Symposium

15. Alignment of Glass Scale using Precitec LR Sensor10
190
200
Found 3-edge points at ±0.5mm from zero alignment axis on the 0-position vertical line and built an axis and the intersection of that axis and the zero alignment line created the XY origin
Found 3-edge points at 0.5mm spacing on each of the 2-horizontal lines and constructed an axis between then which defined the axial alignment
2013 NCSLI International Workshop and Symposium

16. Measurement Sequence
We started our measuring sequence by measuring the Zero Line using 3-points; constructing an axis and intersecting the newly measured axis with the directional axis of our reference system and got a single point for each line measured.
We basically stepped and repeated this process until all lines were measured
We then calculated the distance in X-direction between each line measured and the Zero Line.
We repeated this process 10 times
2013 NCSLI International Workshop and Symposium

17. Finding the edge
What are some of the issues with finding the real edge and then correlating this to traditional methods.
Edge is not perpendicular to the reference plane
Edge is not straight
Does back lighting verses top lighting give two different edges
Can we find the same edge as traditional optical systems and measuring microscopes
2013 NCSLI International Workshop and Symposium

18. Finding extreme points on the edge
Edge Detail
2013 NCSLI International Workshop and Symposium

19. Summary of Results
2013 NCSLI International Workshop and Symposium

20. Current procedure
The 3 slides following this show the results from finding the edge of a single chrome line using ScanOnLine with the following point spacing:
= 10,000pts/mm
= 20,000pts/mm
= 40,000pts/mm
I am hoping to have even lower standard deviations that I originally obtain using the Precitec LR sensor on Infinity
Currently I am finding the edge by selecting points based on light intensity of 0.26 to 0.35
When I am on the chrome surface I get saturation which is near a value of 1
2013 NCSLI International Workshop and Symposium

21. Finding the Edge Alignment Line
Top picture illustrates the edge as seen by the CMM system in red
The blue line is the change in intensity as the system crosses the edge
The small red points on top of the blue line are the points used to define the edge based on intensity value.
Lower picture shows the measured edge from the CMM in Red
The line in black shows the intensity curve from the Crocodile box. The change in intensity is from glass surface to the silver plated surface
2013 NCSLI International Workshop and Symposium

22. Intensity Points Selected
Density at 10,000pts/mm
Chrome Surface
Glass Surface
Measured Curve
Intensity Points Selected
Scan Direction
2013 NCSLI International Workshop and Symposium

23. Results
The red line represents average value from 10 repeats – the certified value. I did not apply temperature compensation at the moment but could and most likely will.
The Blue dots represent the results from the 25 runs
2013 NCSLI International Workshop and Symposium

24. Results - numerical
2013 NCSLI International Workshop and Symposium

25. Noise reduction algorithm affects the edge detection!
Still not Satisfied!
Noise reduction algorithm affects the edge detection!
2013 NCSLI International Workshop and Symposium

26. Raw Data From CMM with out Noise Reduction
2013 NCSLI International Workshop and Symposium

27. Selection based on Intensity Data Shown has NO Noise Reduction
The points inside this window represents selected data based on intensity:
Blue = 0.5 to 0.6 Intensity
Blue + Red = 0.5 to 0.71 Intensity
The points inside this window represents original selection for intensity:
Green = 0.5 to 0.6 Intensity 0.26 to 0.35
1µm
Intensity range
Location (mm)
0.26-0,35
2013 NCSLI International Workshop and Symposium

28. Selection based on Intensity Data Shown has NO Noise Reduction
The original testing presented in this paper used the 0.26 to 0.35 intensity selection criteria and noise reduced data from the CMM
We wanted to test raw data with no noise reduction and a steeper point on the intensity curve.
Selected 0.5 to 0.6 Intensity
Selected 0.5 to 0.71 Intensity
Results are taken weeks apart with completely new setups and the standard deviations for this newest procedure has a maximum σ of 0.55µm
2013 NCSLI International Workshop and Symposium

29. Intensity Selection Comparison
Effect of Intensity Selection
Distance
Sigma (mm)
Δ Cert (um)
(mm)
10.0
-0.15
-0.14
-0.11
20.0
-0.06
-0.05
-0.03
30.0
-0.22
-0.21
-0.20
40.0
-0.26
-0.25
50.0
60.0
-0.08
70.0
0.04
0.05
0.02
80.0
2013 NCSLI International Workshop and Symposium

30. Straightness Plot of the Scan (YZ plane)
2013 NCSLI International Workshop and Symposium

31. Verification results are equal in the + and – Scan Direction
2013 NCSLI International Workshop and Symposium

32. Verification results are equal in the + and – Scan Direction
2013 NCSLI International Workshop and Symposium

33. Optiv Performance 2z443 Standard Scale Resolution
Measuring Error MPE in [µm] according to ISO
EX , EY ≤ L/125
Standard Scale Resolution
0.5µm
Optional Scale Resolution
0.1µm
2013 NCSLI International Workshop and Symposium

34. OPTIV Results
[2.2 + L/125]µm
2013 NCSLI International Workshop and Symposium

35. Error Budget Environment 20°C±0.1°C 10%CTE of 8.3*10-6/°C = 0.016µm
Reproducibility – actual 25 repeats plus realignment = 0.083µm
CMM Accuracy Parallel to X extrapolated from physical measurements [0.2 + L/1500]µm
Other considerations
Quality of edge
Squareness of measured lines to center axis
UC = [0.3 + L/1000]µm with coverage factor of 2
2013 NCSLI International Workshop and Symposium

36. Summary
We wanted to show that one could calibrate a Glass Line Scale and or a grid plate via a non traditional method such as presented within this paper. We hope that the community will consider this alternate method and hope to engage in some round robin testing to verify that a high accuracy CMM with chromatic confocal sensor could be used and that is process could allow users to verify their working standards without the need for special purpose systems. It also adds validity for owning instruments of this caliber.
We are not claiming that this method is faster than a tradition non-contact CMM, but we do feel that we can reduce the measurement uncertainty for calibrating longer artifacts with a significant lower measurement uncertainty and without the need for a dedicated single purpose system.
2013 NCSLI International Workshop and Symposium

37. Quality of the edge has a significant impact to our standard deviation
Lessons Learned
It is possible to find an edge using a chromatic confocal sensor using only displacement
Light intensity changes are a better method to find the edges than using displacement values only
Quality of the edge has a significant impact to our standard deviation
Use of noise reduction algorithms has some impact on the standard deviation, but changing the selection criteria had very little impact
Correlation to alternate calibration methods are easy to reach with the Leitz Infinity with integrated Precitec LR chromatic confocal sensor
2013 NCSLI International Workshop and Symposium

38. The End
Thank you!
2013 NCSLI International Workshop and Symposium