Hexagon Metrology

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

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AGENDA

01

02

03

04

Overview of Requirements

Brief History

Test Piece

Results

Error Budget0506 Lessons Learned


4

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


5

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


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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.


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Equipment Selection Criteria

• Small Measurement Uncertainty ≤ 0.3 + L/1000

• Non Contact Sensor Availability• Stability over time

• Hardware, Software and Firmware available and capable for finding

edges


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Leitz PMM C 12107 Infinity

• Measuring Error MPE in [µm] according to ISO 10360-2 (2010-6)

• E0 ≤ 0.3 + L/1000• E150 ≤ 0.5 + L/1000• R0 ≤ 0.3

• Scale Resolution• 4nm

• LSP –S4 Probe head with integrated optional Precitec LR Sensor


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Precitec LR Sensor Fully Integrated into Leitz PMM C Infinity

• Numeric aperture 0.66• Measuring procedure chromatic confocal• Measurement angle relative to surface 90° ±40°• Measuring distance 6.5 mm• Diameter of measuring spot 1.4 µm


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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


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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 12107 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


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Test Part – Glass Line Scale

• Certificate of Calibration No 115-01538 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


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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


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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 alignment1. 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 Alignment1. 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.2. We measured 3-points on the line between the 0 and 10mm lines and constructed axis3. Measure same on the last line between the 190 and 200mm marks4. Constructed an axis from these two points which define the rotational portion of our

Coordinate system5. Measured 3-points and constructed an axis on the back side of the zero line at ±0.5mm from

the axis just created6. 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


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Alignment of Glass Scale using Precitec LR Sensor

0 10 190 200

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


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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


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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


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Finding extreme points on the edge

Edge Detail

Pos 1 15.17795Pos 2 15.18129Mean 15.17962Δdist 0.00334


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Summary of Results

Element Y1 Point Y2 Point Y3 Point Y4 PointRange 0.000635 0.000428 0.001127 0.000420Stdev 0.000150 0.000112 0.000265 0.000123

Results from 30 Repeats (mm)


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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:• 0.0001 = 10,000pts/mm• 0.00005 = 20,000pts/mm• 0.000025 = 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


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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

Y-Axis Intensity-15.17914 0.24-15.17911 0.24-15.17908 0.23-15.17906 0.23-15.17903 0.24-15.17900 0.24


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Density at 10,000pts/mm

Chrome Surface

Intensity Points Selected

Measured Curve

Glass Surface

Scan Direction


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Results

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200-1.00

-0.50

0.00

0.50

1.00

f(x) = 0.00038692097557322 x

Correlation with Original Certification Tolerance Bands +/- (0.3 + L/1000)um

25 runLinear (25 run)+ Tol-Tol1st 10 runs

Distance (mm)

Del

ta (m

icro

ns)

• 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


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Results - numerical

Nominal (mm) Meas-Cert (µm) 2 sigma (µm)0 0.00 0.123

10.0 -0.10 0.07320.0 -0.02 0.10130.0 -0.19 0.12140.0 -0.23 0.13150.0 -0.07 0.13060.0 -0.08 0.13870.0 0.00 0.13180.0 -0.14 0.12390.0 0.09 0.174

100.0 -0.04 0.151110.0 -0.15 0.179120.0 0.08 0.160130.0 -0.07 0.119140.0 -0.05 0.141150.0 0.03 0.138160.0 0.09 0.132170.0 0.12 0.144180.0 0.20 0.135190.0 0.23 0.165200.0 0.24 0.177

Infinity w/Precitec 23-Apr-2013


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Still not Satisfied!Noise reduction algorithm affects the edge

detection!


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Raw Data From CMM with out Noise Reduction


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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 IntensityBlue + 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)

0.5-0.6 79.9997320.5-0.71 79.999760

0.26-0,35 80.000352


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Selection based on Intensity Data Shown has NO Noise Reduction

-10 10 30 50 70 90

-0.3

-0.2

-0.1

5.55111512312578E-17

0.1

0.2

0.3

Affect of Intensite on Distance

.5-.6.5-.71

Distance (mm)

Del

ta fr

om C

ert (

um)

• 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


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Intensity Selection Comparison

Effect of Intensity SelectionDistance Sigma (mm) Δ Cert (um)

(mm) 0.5-0.6 0.5-0.70.26-0.35

0.5-0.6

0.5-0.7

0.26-0.35

10.0 0.000024 0.000014 0.000025 -0.15 -0.14 -0.1120.0 0.000036 0.000039 0.000053 -0.06 -0.05 -0.0330.0 0.000047 0.000042 0.000046 -0.22 -0.21 -0.2040.0 0.000043 0.000040 0.000049 -0.26 -0.25 -0.2150.0 0.000038 0.000051 0.000061 -0.14 -0.14 -0.0560.0 0.000044 0.000041 0.000056 -0.08 -0.08 -0.0570.0 0.000042 0.000051 0.000047 0.04 0.05 0.0280.0 0.000046 0.000044 0.000066 -0.11 -0.11 -0.11


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Straightness Plot of the Scan (YZ plane)


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Verification results are equal in the + and – Scan Direction


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Verification results are equal in the + and – Scan Direction


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Optiv Performance 2z443

• Measuring Error MPE in [µm] according to ISO 10360-7• EX , EY ≤ 2.2 + L/125

• Standard Scale Resolution• 0.5µm

• Optional Scale Resolution• 0.1µm


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OPTIV Results

0 20 40 60 80 100 120 140 160 180 200

-5

-4

-3

-2

-1

0

1

2

3

4

5

Results from OPTIV

Op-tiv DataOPTIV Uc

Distance (mm)

Delta

from

Cer

t (um

)

[2.2 + L/125]µm


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Error Budget

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98010

0010

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-0.0013

-0.0008

-0.0003

0.0002

0.0007

0.0012

Average difference NIST vs NPL

Avg NISTAVG NPL

Nominal Step

Ave

rage

Dev

iatio

n (m

m)

NPL Calibration Uncertainty NIST Uncertainty Infinity Accuracy Limits

• 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


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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.


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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


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The End

Thank you!


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