measuring through glass windows with laser tracker …...gsi helmholtz centre for heavy ion research...

GSI Helmholtz Centre for Heavy Ion Research

GSI Helmholtz Centre for Heavy Ion Research

Measuring through glass windows with laser tracker under

cold-vacuum conditions: investigations and results

Vasileios Velonas [email protected]

on behalf of the Survey & Alignment Team: I. Pschorn, T. Miertsch, K. Knappmeier, A. Junge

[email protected]

mailto:[email protected]

mailto:[email protected]

GSI Helmholtz Centre for Heavy Ion Research IWAA Grenoble 2016

Outline

2

Motivation

Measuring through air-glass-air

Measuring through air-glass-vacuum-cold

Building a mathematical model (MM) to gain absolute coordinates in a given reference system

Summary and outlook


Motivation

What was the trigger for this investigation...

Measuring through glass window in cold conditions up to 4K Observing targets which are positioned on the cold mass Combination of theodolite and laser tracker Only 2D information gained from this procedure

cryostat fiducials

3


Motivation

... and what we aim at, is ...

The high precise monitoring regarding deformations of the

cold mass along with movements vs. cryostat of superconducting components in 3D!!

4

cryostat fiducials


Step 1 - Measuring through glass windows in air-glass-air

First phase measurements

5

Coplanar glass window (N-BK7) between laser tracker and points

Trying to track through a glass window

Glass window in an extreme slope position regarding the laser beam

Measuring through other glass windows

Autocollimation of the laser beam

• Measurements were not always possible

• Reproducibility of the position behind the windows was chaotic

• Laser tracker head could not find the target by autocollimation

• No tracking was possible through the window

• The reflection of the glass window was too high

Nothing did really work!


Step 2 – Coating of a glass window

The glass window that we released to coat was a 15 mm thickness borosilicate (N-BK7) coplanar glass window

6

Two main parameters are highly important to know before coating a glass window:

1. The max. allowed reflection from both planes of the glass (in

our case 0.5 %)

2. What is the needed wavelength which is allowed to pass through? (in our case for all laser trackers)


Wavelengths of the laser trackers

Leica LTD 500 Leica AT402 FARO SI2 FARO Vantage

[nm] [nm] [nm] [nm]

Absolute Distance Measurement (ADM) 780 795 1550 1550

Laser Interferometer (IFM) 633 * 633 *

Automatically Target Recognition (ATR) * 905 * *

Laser-Pointer 633 635 633-635 653-663

Step 3 - What is the wavelength of our instruments?

7

FARO SI2 FARO Vantage Leica LTD 500 Leica AT402

Source: Leica and Faro


Step 3 - What is the wavelength of our instruments?

Reflection diagram including all our laser trackers

8

The ADM wavelength range of Faro

The laser pointer wavelength range of Leica and Faro

The ADM wavelength range of Leica

Source of the diagram: Prinz Optiks GmbH


Step 4 - Measuring through coated glass window in air-glass-air

Second phase measurements

9

Coated glass window Setup of the instrument-glass window-points (long distance)

Setup of the instrument-glass window-points (short distance)

Autocollimation of the laser beam Extreme position (approx.60°) of the glass window regarding the laser beam

• Different setup of the instruments and the SMRs regarding the glass window

• Measuring the same position of the targets at least five times in two telescope positions

• All laser trackers could find the targets without any problem

• Tracking the laser beam behind the glass window was possible

The position of the points behind the glass window is highly reproducible for all instruments sdXYZ= 5 μm (1σ)

Coating the glass was successful!



Results

10

-8.00

-7.95

-7.90

-7.85

-7.80

-7.75 0 1000 2000 3000 4000 5000 6000

Dis

plac

emen

t of t

he ta

rget

[m

m]

Distance between laser tracker and SMR [mm]

Pseudo position of the target (X-axis) caused by the glass window regarding the real position (air-glass-air)

Leica AT402

FARO SI2

FARO Vantage

Leica LTD500

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view



Results

11

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view

-0.60

-0.40

-0.20

0.00

0.20

0.40

0.60

0 1000 2000 3000 4000 5000 6000

Dis

plac

emen

t of t

he ta

rget

[m

m]


Pseudo position of the target (Y-axis) caused by the glass window regarding the real position (air-glass-air)

Leica AT402

FARO SI2

FARO Vantage

Leica LTD500



Results

12

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 1000 2000 3000 4000 5000 6000

Dis

plac

emen

t of t

he ta

rget

[m

m]


Pseudo position of the target (Z-axis) caused by the glass window regarding the real position (air-glass-air)

Leica AT402

FARO SI2

FARO Vantage

Leica LTD500


Step 5 – Measuring through coated glass window in air-glass-vacuum-cold

13

Using of break resistant reflector (BRR) Vacuum in the chamber 10-3 mbar Cold conditions up to 90 K using liquid nitrogen The angle of incidence (AOI) is up to 5°

LT SMR 2

window vacuum chamber

laser beam

SMR 1

X

Z side view

Angle of incidence (AOI)

The chamber (front side)

Setup of the instruments and conditions for the measurements


Step 5 – Measuring through coated glass window in air-glass-vacuum-cold

Third phase measurements

14

The chamber Set up in a long distance

Set up in a short distance

• All laser trackers could find the target without any problem, in vacuum as well as in cold condition

The insight of the chamber

The position of the points behind the glass window is again highly reproducible for all instruments sdXYZ= 5 μm (1σ)


Step 6 – Recognizing the problem measuring through different optical media by laser trackers. Definition of the absolute position

• Pointing error (top view)

15

• Range error (side view)



The problem of the range error caused by different optical media which the laser tracker can not understand

16

LT SMR


n1 (Temperature, Pressure, Humidity)

laser beam (LB)



The problem of the range error caused by different optical media which the laser tracker can not understand

17

LT SMR


laser beam (LB)

LB: n1=f(T, P, H)

LB: n2=f(Glass material, LT wavelength)

LB: n3=1



For the correction of the pointing error quaternions (mathematical tool) were used to manipulate the rotation in 3D in combination with Snell’s law

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distance from glass to reflector (medium: vacuum) DLT = Dreal *nvacuum

distance through glass DLT = Dreal *nglass

wave front

distance from LT source to the glass (medium: air) DLT = Dreal *nair

Angle of incidence (AOI)


Step 7 – Building a Mathematical Model (MM)

To calculate absolute coordinates to a given reference system a MM is needed to correct the pointing and range errors

The MM is a function of all parameters which are known and measureable:

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Geometry of the window Position of the laser tracker

Angle of incidence Material of the window

Wavelength of the laser tracker Temperature

Pressure Humidity

et al.

MM


Step 8 – Applying the Mathematical Model (MM)

Results in air-glass-air (FARO Vantage)

20

-9.00

-8.00

-7.00

-6.00

-5.00

-4.00

-3.00

-2.00

-1.00

0.00 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Dis

plac

emen

t of t

he ta

rget

[m

m]

Distance between the laser tracker (Vantage) and the SMR [mm]

Pseudo position of the target (X-axis) caused by the glass and its actual position by applying the MM (air-glass-air)

Without the MM

With the MM

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view




21

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Dis

plac

emen

t of t

he ta

rget

[m

m]


Pseudo position of the target (Y-axis) caused by the glass and its actual position by applying the MM (air-glass-air)

Without the MM

With the MM




22

SMR

X

Y

glass window

top view

SMR

X

Z

glass window

side view

-0.20

0.00

0.20

0.40

0.60

0.80

1.00

1.20

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Dis

plac

emen

t of t

he ta

rget

[m

m]


Pseudo position of the target (Z-axis) caused by the glass and its actual position by applying the MM (air-glass-air)

Without the MM

With the MM The geometrical reconstruction of the points behind the glass window is precise for all instruments sdXYZ= 15 μm (1σ) (absolute accuracy)



Results in air-glass-vacuum-cold (all instruments)

23

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

Dis

plac

emen

t of t

he ta

rget

[m

m]

Epoch (Timeline)

Pseudo position of the target (X-axis) caused by the glass-vacuum-cold conditions and its actual position by applying the MM (air-air-conditions)

Leica AT402 without the MM Leica AT402 wih the MM FARO SI2 without the MM FARO SI2 with the MM FARO Vantage without the MM FARO Vantage with the MM Leica LTD500 without the MM Leica LTD500 with the MM

W V

C

WW WW

W

WW: Without Window W: Warm V:Vacuum C:Cold


Conclusion and outlook

The reproducibility of the points behind the glass window is highly accurate in air-glass-air as well as in air-glass-vacuum-cold condition (sdXZY= 5 μm).

Comparing relative coordinates behind the glass window requires a position of the instrument more than 3 m in order to avoid disturbance. Combining the instruments in this case is fatal, do not do this!

Using the mathematical model absolute coordinates can be calculated. The pointing and range errors can be eliminated which are caused by the glass window (sdXZY= 15 μm).

The material of the glass window in combination with the wavelength of the instrument defines the refractive index of the glass window, thus the material of the glass window has to be chosen wisely.

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Conclusion and outlook

The mechanical properties of the window (size of the glass surface,

thickness) has to be taken in account in order to reduce or eliminate the bulge of the window, caused by the vacuum in the chamber, which can produce an error in the mathematical model. Also, as smaller the angle of incidence is, as more negligible this error can be.

Cross checking and optimization of the mathematical model by a practical approach is in progress.

The movement of the SMR center point because of the contraction in cold environment has to be taken in account in order to distinguish it from the displacement of the cold mass.

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Thank you for your

attention!

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measuring through glass windows with laser tracker …...gsi helmholtz centre for heavy ion research...

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