Laser Trackers and Laser Scanners for Large Scale .... Steven Phillips - Laser... · Laser Trackers...

Laser Trackers and Laser Scanners for

Large Scale Coordinate Metrology

NACMA 2012

Steve Phillips

& colleagues:

D. Sawyer, B. Muralikrishnan

C. Blackburn, C. Shakarji

National Institute of

Standards and Technology

Gaithersburg, Maryland USA

Outline

• The Big Picture: Past and Future

• Current Technology: Pros & Cons

• Technology Basics: Error Sources & Discovery

– Range measurement systems & errors

– Volumetric measurement & errors

• Applications Errors – Workpiece and Operator effects

The Big Picture: Past and Future

The Third Industrial

Revolution

“The digitization of

manufacturing will transform

the way goods are made…”

The Economist

April 23, 2012

Revolutions tend to look like evolution

when you live them…

1770-1820: Water and steam replace human energy

in factories…

1850-1900: Interchangeable parts lead to mass production…

1960-2010s: Semiconductors yield computers and

optoelectronics that create CNC machine tools

and 3D coordinate metrology

Current status: We are near the point where design,

production and metrology are entirely digital…

Digital

Manufacturing

1960s 1980s 2000s

Design: Paper prints 2D CAD 3D solid model

Production: Manual CNC CNC & 3D printing

Metrology: Hard Gages CMMs CMMs & Scanners

Optoelectronics & Computers are Driving 3D

Coordinate Metrology at Exponential Rates:

Date Collection Rate Doubling Time: < 2 years

Moore’s Law

Computation: MFLOPS

Maximum 3D Point Coordinate

Collection Rates: Points/sec

1970s 10-1 100 manual CMMs, hard probes

1980s 101 101 CNC CMMs, touch probes

1990s 103 103 Fast scanning analog probes

2000s 105 105 Optical probes; scanning heads

2010s 107 107 – 108 ??? Fast optical; CT;…

High Density Scanning is Needed for

Metrology of Complex Geometry

Complex by

Design:

Complex by

Imperfection:

Current Technology: Pros & Cons

Large Conventional CMMs

Advantages

• Very Accurate: 1-10 µm/m

• Fully Automated

• Contact Probing:

– All types of materials

– All types of features (holes…) Disadvantages • Cost: $5K - $15K / m3 (but usually all useful volume)

• Fixed Location: – Not reconfigurable factory floor space

– Workpieces must be moved to the CMM

• Speed: 1-10 pts/sec (typical); 1000 pts/sec recently

Laser Trackers:

Spherical Coordinate Measurement

Systems That Use Cooperative Targets

X

Y

Z

x

z

Horizontal Axis

R

Ver

tica

l Ax

is

SMR: Spherical

Mounted Retroreflector

Laser Trackers

Disadvantages • Mostly line-of-sight

• Manual measurement (unless used for fixed targets)

Speed: 1 - 100 pts/sec (typical)

Can be difficult or dangerous to position target

Labor intensive

Advantages • Large Measurement Volume

50 m range over 100,000 m3

• Cost:$1-$5 /m3 (not all useful volume)

• Portable (move scanner to workpiece)

• Moderate Accuracy: 10-20 µm/m

• Contact Probing:

– All types of materials

– Most types of features (with special probe…)

Laser Scanners:

Spherical Coordinate Measurement

Systems That Use NonCooperative Targets

Laser Scanners

Disadvantages • Line-of-sight only: many features not accessible

• Moderate to lower accuracy: 20-200 µm/m

• NonContact Probing: Material issues affect accuracy

Optical spot size issues affect accuracy

Reflection issues affect accuracy

Advantages • Large Measurement Volume

5-30 m range over 100,000 m3

• Cost: $5 - $500 /m3 (not all useful

volume)

• Portable (move scanner to workpiece)

• Fully automated measurements

• Fast & Hi density: 103 -106 pts /sec

Technology Basics: Error Sources & Evaluation

Trackers and Scanners

• Range measurement systems & errors

• Volumetric measurement & errors

Measurement

Instrumentation

Workpiece

Properties

Environment Sampling

Strategy

Fitting

Algorithm Result &

Uncertainty

Algorithm

Selection &

Implementation

Measurand

Definition

Understanding

Uncertainty Propagation

Laser Interferometry Lasing results in phase coherence which allows

displacement interferometry via wave interference interference

=

+

/4

Laser

DetectorMeasurement Arm

(moving SMR)

Reference Arm (fixed)

Inside Laser Tracker

=

Detector

Output

/2 2 3/2

Las

er

Measurement Arm

(moving SMR)

A ruler with 3,000,000

graduations per meter ....

Detector

Output

/2 2 3/2

Practical Implementation of Laser Interferometery

= 1 part in 150 =

2 nm

Note: Do Not Break the Beam!

Advantages

• Very Accurate: < 0.1µm/m with good n correction

• Gold standard for traceability to SI meter

Error Sources

• Wavelength depends on index of refraction “n”

• Phase discrimination (small)

• Vacuum wavelength error (unlikely)

Laser Interferometry Ranging

Iodine Stabilized Laser Used to

Calibrate Other HeNe Lasers… • Various stabilization schemes (Zeeman, Polarization…)

• Typical vacuum wavelength 10-7 - 10-8

1D Cooperative Target Range Facility

NIST System Configuration

Interferometer

Instrument

under test

Carriage can accommodate a

variety of targets

1-D Cooperative Target Range Facility

Reference retroreflector

Target retroreflector

Laser Tracker


Range: 60 m (200 feet)

Temperature: 20 ± 0.15 C

Sensors:

U(T) = 0.01 C, spaced @ 10 m

U(P) = 20 Pa

U(RH) = 1 % RH

U(L) = 0.15 m + 2.5 × 10-7


Rail Stability (61 m)

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

0.025

0 1 2 3 4 5 6

Time, Hours

Las

er r

ead

ing

, mm

Static Environmental Compensation

Active Environmental Compensation

1 10-7

Drift Test at 61 m

Showing the effect of index of refraction

mm

1-D Cooperative Target Range Facility Ranging test of an IFM tracker showing vacuum wavelength error

0 10 20 30 40 50 60-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

Displacement (m)

Erro

r (m

m)

Run #1

Corrected run #1

Run #2

Corrected run #2

Error due to incorrect wavelength

1 x 10-6

Wavelength in instrument = 632.990400 nm

Calibrated wavelength (NIST) = 632.991061 nm

Difference = 0.6 10-3 nm

Relative length error = 0.6 10-3 nm / 633 nm = 1 10-6

Absolute Distance Measurement (ADM)

• Simple Time of Flight

Round trip Time T is measured by timing

electronics; 30 GHz 10 mm resolution

While some tricks, e.g. averaging and subpixel

interpolation can help, for < 50 m distances,

resolution is often the limiting factor

Target

D

Laser and

Receiver

2c

D Tn

ADM Basics • Phase based Amplitude Modulated

Continuous Wave Detection A trick to convert a time of flight to a phase measurement

Measure the phase difference between outgoing and returning beam

D

Target

Laser

and

Detector

???

Trick 1:

Make the modulated wavelength longer than the max range of the instrument is a fraction of one .

OR use coarse time of flight measurement to determine interval

Intensity Modulation

Trick 2: Decrease the modulated wavelength, keeping track of

number of wavelengths in the interval; repeat…

ADM Basics • Phase based Amplitude Modulated

Continuous Wave Detection Errors

Modulation frequency of laser beam, limited by electronics and becomes less precise at high frequencies “wavelength limited”

Measurement of (phase discrimination) gets harder at higher frequencies

Ambiguity interval mistakes

Signal to noise ratio

Averaging time

Index of refraction

ADM Basics Frequency Modulation (very schematic)

A trick to convert a time of flight to a frequency measurement

Laser Target

Detector Beat Frequency

Wavelength Vs Time Frequency Vs Time

Beat Frequency

Freq at

Start

Freq at

Return

Round Trip Time

ADM Basics

• Frequency Modulation Continuous Wave Detection Errors

Linearity of frequency ramp

Measurement of beat frequency over a short time

Modulation frequency of laser beam, limited by electronics and becomes less precise at high frequencies

Signal to noise ratio

Index of refraction


Ranging test of an ADM tracker passing specifications

0 10 20 30 40 50 60-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

Displacement (m)

Erro

r (m

m)

Run #1

Run #2

Run #3

MPE

+MPE

MPE

Significantly more

points sampled than

required by B89.4.19

1 10-6

Ranging System Test on ADM #2-2

0.000

0.100

0.200

0.300

0.400

0 5 10 15 20 25

Length, m

Err

or,

mm


Ranging test of an ADM showing large range errors before

compensation & small errors after compensation

Before

Comp

After Comp

Non-Cooperative Target Range for Laser Scanners

• 66 meters (216’)

• 100 mm Dia Spheres with passive reflectance

• 22 Positions

• U(L) = 10 m + L × 10-6

Scanner under Test

Non-Cooperative Target Range for Laser Scanners (Scanner A)

0 2 4 6 8 10 12 14 16 18-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Target position (m)

Range e

rror

(mm

)

Unconstrained case

Ranging error with the scanner placed in-line with targets

Non-Cooperative Target Range for Laser Scanners (Scanner B)

Ranging error with the scanner placed in-line with targets

0 10 20 30 40 50 60-10

-8

-6

-4

-2

0

2

4

6

Range to target (m)

Rangin

g e

rror

(mm

)

Geometric Misalignments

in Laser Scanners and Trackers

• Offsets

– Beam offset

– Transit offset

– Mirror offset

– Cover plate offset

– Vertical index offset

• Tilt

– Beam tilt

– Mirror tilt

– Transit tilt

• Eccentricity

– Horizontal angle encoder eccentricity

– Vertical angle encoder eccentricity

• Scale errors in encoder

Vertical angle

encoder

Horizontal angle encoder

Standing axis

Transit axis

Target

Source in tracker

Head/fiber coupled,

no mirrors

Source in stationary

base

Source attached to

standing axis

(a) API (b) Faro (c) Leica (d) Metris


Different Constructions

VmxVmx

Vmz. xVm n.xVmnxVmzxxRm

Vmx

Rm

Vmnx

Rm

VmzxV

HmxHmx

HmyxHmxxVm

x

Vm

xnx

Vm

Vmzx

VmRm

x

Rm

nx

VmRm

VmzxH

xVmxR

dc

ba

2sin2cos

sin10cos10-cos.6sin.65

cos.2cos.1sin.1

2sin2cos

sin.9cos.9tan

8

sin

76sin

cos.6

sin.

31

sin.

cos.1

sin.2

1212

1212

11

VmxVmxVm

z. x Vm

n.x

HmyxHmxxxRm

nx

Rm

Vmx

Rm

Hmyx

Rm

HmxxV

HmxHmxHmyxHmxx

Vmx

Vm

x

Vm

Hmyx

Vm

Hmxx

VmRm

tx

VmRm

Hmyx

VmRm

HmxxH

xVmxVmxR

dc

ba

sincos2

sin2

cos-

cos.sin.2cos.2

cos.sin.

2sin2cossin.cos.

2tan.

2cos

sin

sin.

sin

cos.

sin.sin.

sin.

sin.

cos.

)2/sin(.2sin.

12121010

6654

22

11

121299

8766411

1132

Geometric error model for the Photon/Surphaser scanners

Geometric error model for the Laser Radar

Kinematic Models of Error

Parameters

Scanner with 18

terms in its error

model

Find test positions (by

simulation) so that there is at

least one test that is sensitive to

each of the terms in the error

model

Note that some terms may be

sensitive to two-face tests also Scanner with 20

terms in its error

model


Some Testing Positions for

Laser Trackers per ASME B89.4.19

D

DD

0 m

3, 3.04 m

9, 9.03 m

15.5 m

22 m

28 m

D

Laser Tracker B89.4.19 Volumetric System Tests

(Diagonal Tests)

Laser Tracker B89.4.19 Volumetric System Tests

B89.4.19 Volumetric System Tests

Volumetric Performance Test on ADM #5

-0.150

-0.100

-0.050

0.000

0.050

0.100

0.150

Err

or,

mm

1st reading 2nd reading 3rd reading +MPE -MPE

Horizontal

Technician: Chris Blackburn

Vertical Right Diagonal Left Diagonal User

0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

3 m

minimum

distance

3 m 3 m 3 m

6 m6 m6 m6 m


Volumetric Performance Test on IFM #4

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

Err

or,

mm

Technician: Chris Blackburn

0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

3m 3m 3m 3m

6m 6m 6m 6m

minimum

distance

Horizontal Vertical Right Diagonal Left Diagonal


Hor

1m Hor

3m Hor

6m Ver

3m Ver

6m RD

3m RD

6m LD

3m LD

6m 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

Beam Tilt

(Ix)

Beam Tilt

(Iy)

Horizontal

Encoder

Eccentricity

(Ex)

Horizontal

Encoder

Eccentricity

(Ey)

Effect of Beam Tilt & Horizontal Encoder Eccentricity on Volumetric Tests

0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

D minimum

3 m

6 m

Low Mid High Low Mid High Low Mid High

0

0.05

0.1

Erro

r (m

m)

1st reading

2nd reading

3rd reading

+MPE

-MPE

(X)

(Y)

(Y)

(X)

(Y)

0 5 10 15 20 25 30 35-200

-150

-100

-50

0

50

0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

D minimum

3 m

6 m

Low Mid High Low Mid High Low Mid High

0

0.05

0.1

Erro

r (m

m)

1st reading

2nd reading

3rd reading

+MPE

-MPE

Beam Tilt (X)

Beam Tilt (Y)

Horizontal Angle

Encoder Eccentricity

(Y)

Horizontal Angle

Encoder Eccentricity

(X)

0 5 10 15 20 25 30 35-0.4

-0.2

0

0.2

0.4

Erro

r (m

m)

Measured errors

Simulated errors

Hor

1m Hor

3m Hor

6m Ver

3m Ver

6m RD

3m RD

6m LD

3m LD

6m 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

Beam Tilt

(Ix)

Beam Tilt

(Iy)

Horizontal

Encoder

Eccentricity

(Ex)

Horizontal

Encoder

Eccentricity

(Ey)


0 5 10 15 20 25 30 35-0.4

-0.2

0

0.2

0.4

Erro

r (m

m)

Measured errors

Simulated errors

0 5 10 15 20 25 30 35-0.4

-0.2

0

0.2

0.4

Erro

r (m

m)

Measured errors

Simulated errors

Hor

1m Hor

3m Hor

6m Ver

3m Ver

6m RD

3m RD

6m LD

3m LD

6m 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270 0 90 180 270

Beam Tilt

(Ix)

Beam Tilt

(Iy)

Horizontal

Encoder

Eccentricity

(Ex)

Horizontal

Encoder

Eccentricity

(Ey)


Simulated and actual errors for the volumetric test

-4000

-2000

0

2000

4000

6000

-3000

-2000

-1000

0

1000

2000

3000

4000

-1500

-1000

-500

0

500

1000

1500

2000

X (mm)Y (mm)

Z (

mm

)

Laser Scanner: Volumetric Test

Target Arrangement


Target Arrangement


Point Coordinate Error (Distance from measured point to calibrated point)

2 2.5 3 3.5 4 4.50.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Target distance from instrument (m)

Coord

inate

err

or

(mm

)

Unconstrained case

A 3 m x 3 m ideally flat plane with center 2 m away from scanner. It is at a

compound angle with respect to the scanner. .

Simulation of Systematic Errors from Laser Scanner

-2000

0

2000

-1500-1000-500050010001500-0.6

-0.4

-0.2

0

0.2

0.4

X (mm)Y (mm)

Appare

nt

out-

of-

flatn

ess (

mm

)

Beam offset 0.0743 mm

Beam offset -0.0735 mm

Transit offset -0.003 mm

Mirror offset 0.0268 mm

Vert index offset 0.0008 rad

Beam tilt -0.0001 rad

Beam tilt -0.0002 rad

Mirror tilt 0.0003 rad

Transit tilt -0.0004 rad

Hor angle eccentricity 0.0001 mm/mm

Hor angle eccentricity 0.0004 mm/mm

Ver angle eccentricity -0.0004 mm/mm

Ver angle eccentricity -0.0008 mm/mm

Bird bath 0.0993 mm

Hor second order scale 0 rad

Hor second order scale 0 rad

Ver second order scale 0 rad

Ver second order scale 0 rad

Error Sources & Evaluation

Operator & Workpiece issues

– Workpiece reflectivity

– Workpiece optical penetration

– Workpiece secondary reflections

– Spot size on workpiece geometry

– Sampling strategy effects

Measurement

Instrumentation

Workpiece

Properties


Strategy

Fitting

Algorithm Result &

Uncertainty

Algorithm

Selection &

Implementation

Measurand

Definition

Understanding


Challenges: Reflections off Workpiece and into Scanner

Polished Steel Matte finish Titanium

Radius error

= -0.010 mm

σ = 0.056 mm

Radius error

=0.267 mm

σ = 6.732 mm

Challenges: Optical Penetration of Materials

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 20

1000

2000

3000

4000

5000

6000

Residuals from fit, Range = 3.7946, Stdev = 0.55956

File: one inch Nylon points.txt 91856 points used

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 20

0.5

1

1.5

2

2.5

3

3.5x 10

4

Residuals from fit, Range = 6.6453, Stdev = 0.11198

File: one inch Polypro points.txt 70393 points used

σ=0.560 mm σ=0.112 mm

R error = -0.575 mm

R error = -0.027 mm

No

min

al R

No

min

al R

Challenges: Secondary Reflections

Potential error in the edge position could be 50 μm

406.6 406.8 407 407.2 407.4

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

Travel (mm)

Heig

ht

(mm

)

Probe

Possible secondary reflection problem

Scatted light from bottom block

Scatted light from top block

Probe reads -5 mm

Probe zero

(b)

Peak from top

surface

Peak from bottom

surface

Top block (block under test)

Bottom block

(a)

H h

(H-h)tanα

Challenges:

Secondary Reflections

Measurement

Instrumentation

Workpiece

Properties


Strategy

Fitting

Algorithm Result &

Uncertainty

Algorithm

Selection &

Implementation

Measurand

Definition

Understanding


Sampling Strategy Effects: Point Coordinate Uncertainty vs.

Task Specific Uncertainty

Measurement

Point

Uncertainty

Sampling Strategy Effects: Ratio of Circle Radius Uncertainty

to Point Coordinate Uncertainty

Disclaimer:

Data on commercial products are only provided for the sake of

describing experimental results. NIST does not endorse or recommend

any commercial products or imply that this equipment is the best for

any particular application.

Laser Trackers and Laser Scanners

for Large Scale Coordinate Metrology

NACMA 2012

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