Laser Trackers and Laser Scanners for Large Scale Coordinate Metrology NACMA 2012

Transcription

1 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

2 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

3 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

4 Revolutions tend to look like evolution when you live them : Water and steam replace human energy in factories : Interchangeable parts lead to mass production s: 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

5 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

6 Optoelectronics & Computers are Driving 3D Coordinate Metrology at Eponential Rates: Date Collection Rate Doubling Time: < 2 years Moore s Law Computation: MFLOPS Maimum 3D Point Coordinate Collection Rates: Points/sec 1970s manual CMMs, hard probes 1980s CNC CMMs, touch probes 1990s Fast scanning analog probes 2000s Optical probes; scanning heads 2010s ??? Fast optical; CT;

7 High Density Scanning is Needed for Metrology of Comple Geometry Comple by Design: Comple by Imperfection:

8 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 / m 3 (but usually all useful volume) Fied Location: Not reconfigurable factory floor space Workpieces must be moved to the CMM Speed: 1-10 pts/sec (typical); 1000 pts/sec recently

9 Laser Trackers: Spherical Coordinate Measurement Systems That Use Cooperative Targets Vertical Ais Z R z Y SMR: Spherical Mounted Retroreflector Horizontal Ais X

10 Laser Trackers Advantages Large Measurement Volume 50 m range over 100,000 m 3 Cost:$1-$5 /m 3 (not all useful volume) Portable (move scanner to workpiece) Moderate Accuracy: µm/m Contact Probing: All types of materials Disadvantages Mostly line-of-sight Manual measurement (unless used for fied targets) Speed: pts/sec (typical) Can be difficult or dangerous to position target Labor intensive Most types of features (with special probe )

11 Laser Scanners: Spherical Coordinate Measurement Systems That Use NonCooperative Targets

12 Laser Scanners Advantages Large Measurement Volume 5-30 m range over 100,000 m 3 Cost: $5 - $500 /m 3 (not all useful volume) Portable (move scanner to workpiece) Fully automated measurements Fast & Hi density: pts /sec Disadvantages Line-of-sight only: many features not accessible Moderate to lower accuracy: µm/m NonContact Probing: Material issues affect accuracy Optical spot size issues affect accuracy Reflection issues affect accuracy

13 Technology Basics: Error Sources & Evaluation Trackers and Scanners Range measurement systems & errors Volumetric measurement & errors

14 Understanding Uncertainty Propagation Measurement Instrumentation Measurand Definition Environment Sampling Strategy Fitting Algorithm Result & Uncertainty Workpiece Properties Algorithm Selection & Implementation

15 Laser Interferometry Lasing results in phase coherence which allows displacement interferometry via wave interference interference Laser Detector Reference Arm (fied) Measurement Arm (moving SMR) Detector Output Inside Laser Tracker = + = /2 3/2 2 /4

16 Laser Practical Implementation of Laser Interferometery Measurement Arm (moving SMR) A ruler with 3,000,000 graduations per meter... Detector Output = 1 part in 150 = 2 nm /2 3/2 2 Note: Do Not Break the Beam!

17 Laser Interferometry Ranging Advantages Very Accurate: < 0.1µm/m with good n correction Gold standard for traceability to SI meter Error Sources Wavelength depends on inde of refraction n Phase discrimination (small) Vacuum wavelength error (unlikely)

18 Iodine Stabilized Laser Used to Calibrate Other HeNe Lasers Various stabilization schemes (Zeeman, Polarization ) Typical vacuum wavelength

19 1D Cooperative Target Range Facility NIST System Configuration Interferometer Carriage can accommodate a variety of targets Instrument under test

20 1-D Cooperative Target Range Facility Target retroreflector Reference retroreflector

21 Laser Tracker 1-D Cooperative Target Range Facility Range: 60 m (200 feet) Temperature: 20 ± 0.15 C Sensors: U(T) = 0.01 C, 10 m U(P) = 20 Pa U(RH) = 1 % RH U(L) = 0.15 m

22 Laser reading, mm mm 1-D Cooperative Target Range Facility Drift Test at 61 m Showing the effect of inde of refraction Rail Stability (61 m) Static Environmental Compensation Active Environmental Compensation Time, Hours

23 Error (mm) 1-D Cooperative Target Range Facility Ranging test of an IFM tracker showing vacuum wavelength error Run #1 Corrected run #1 Run #2 Corrected run #2 Error due to incorrect wavelength Displacement (m) Wavelength in instrument = nm Calibrated wavelength (NIST) = nm Difference = nm Relative length error = nm / 633 nm =

24 Absolute Distance Measurement (ADM) Simple Time of Flight Laser and Receiver D Target Round trip Time T is measured by timing electronics; 30 GHz 10 mm resolution While some tricks, e.g. averaging and subpiel interpolation can help, for < 50 m distances, resolution is often the limiting factor

25 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 Laser and Detector Intensity Modulation Target??? Trick 1: D Make the modulated wavelength longer than the ma range of the instrument is a fraction of one. OR use coarse time of flight measurement to determine interval Trick 2: Decrease the modulated wavelength, keeping track of number of wavelengths in the interval; repeat

26 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 Inde of refraction

27 ADM Basics Frequency Modulation (very schematic) A trick to convert a time of flight to a frequency measurement Detector Beat Frequency Laser Target Frequency Vs Time Wavelength Vs Time Freq at Return Freq at Start Beat Frequency Round Trip Time

28 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 Inde of refraction

29 Error (mm) 1-D Cooperative Target Range Facility Ranging test of an ADM tracker passing specifications Run #1 Run #2 Run #3 MPE +MPE Significantly more points sampled than required by B Displacement (m) MPE

30 Error, mm 1-D Cooperative Target Range Facility Ranging test of an ADM showing large range errors before compensation & small errors after compensation Ranging System Test on ADM # Before Comp After Comp Length, m

31 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

32 Range error (mm) Non-Cooperative Target Range for Laser Scanners (Scanner A) 1.2 Unconstrained case Target position (m) Ranging error with the scanner placed in-line with targets

33 Ranging error (mm) Non-Cooperative Target Range for Laser Scanners (Scanner B) Range to target (m) Ranging error with the scanner placed in-line with targets

34 Geometric Misalignments in Laser Scanners and Trackers Target Vertical angle encoder Standing ais Horizontal angle encoder Transit ais Offsets Beam offset Transit offset Mirror offset Cover plate offset Vertical inde offset Tilt Beam tilt Mirror tilt Transit tilt Eccentricity Horizontal angle encoder eccentricity Vertical angle encoder eccentricity Scale errors in encoder

35 Different Constructions (a) API (b) Faro (c) Leica (d) (a) Metris API (b) Faro (c) Leica (d) Metris Source in tracker Head/fiber coupled, no mirrors Source attached to standing ais Source in stationary base

36 z. n. n z Rm Rm n Rm z V Hm Hm Hm y Hm n z Rm Rm n Rm z H R d c b a sin 2 cos2 sin 10 cos 10 -.cos 6.sin cos.cos 1.sin 1 sin 2 cos2.sin 9.cos 9 tan 8 sin 7 6 sin.cos 6.sin 3 1.sin.cos 1 2.sin z. n. Hm y Hm Rm n Rm Rm Hm y Rm Hm V Hm Hm Hm y Hm Hm y Hm Rm t Rm Hm y Rm Hm H R d c b a sin cos 2 sin 2 cos -.cos.sin 2.cos 2.cos.sin sin 2 cos2.sin.cos 2 tan. 2 cos sin.sin sin.cos.sin.sin.sin.sin.cos 2) /.sin( 2.sin Geometric error model for the Photon/Surphaser scanners Geometric error model for the Laser Radar

37 Kinematic Models of Error Parameters 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 Scanner with 18 terms in its error model Note that some terms may be (a) API (b) Faro Scanner (c) Leica with (d) 20 Metris sensitive to two-face tests also terms in its error model

38 Some Testing Positions for Laser Trackers per ASME B D D 28 m 22 m 15.5 m 9, 9.03 m D D 3, 3.04 m 0 m

39 Laser Tracker B Volumetric System Tests (Diagonal Tests)

40 Laser Tracker B Volumetric System Tests

41 Error, mm B Volumetric System Tests Volumetric Performance Test on ADM #5 Technician: Chris Blackburn Horizontal minimum distance 3 m 6 m Vertical Right Diagonal Left Diagonal User 6 m 6 m 6 m 3 m 3 m 3 m st reading 2nd reading 3rd reading +MPE -MPE

42 Error, mm B Volumetric System Tests Volumetric Performance Test on IFM #4 Technician: Chris Blackburn 0.40 Horizontal Vertical Right Diagonal Left Diagonal minimum distance m 6m 6m 6m 3m 3m 3m 3m

43 Error (mm) Error (mm) Error (mm) 50 B Volumetric System Tests Simulated and actual errors for the volumetric test Effect of Beam Tilt & Horizontal Encoder Eccentricity on Volumetric Tests Beam Tilt (X) (I) Beam Tilt (X) Beam Tilt (Iy) (Y) (Y) Horizontal Encoder Eccentricity (E) (X) Beam Tilt (Y) Horizontal Horizontal Encoder Angle D minimum 3 m 6 m Low Mid High Low Mid Eccentricity High Low Encoder (Ey) Eccentricity (Y) Low Mid High Low Mid High Low Mid High Horizontal Angle Encoder Eccentricity (X) st reading 2nd reading 3rd reading +MPE -MPE Hor Hor Hor Ver Ver RD RD LD LD 1m -200 D minimum 3 m (Y) 6 3m 6m 3m 6m 3m 6m 3m 6m Effect of Beam Tilt & Horizontal Encoder Eccentricity on Volumetric Tests Measured errors Simulated errors Beam Tilt (I) Beam Tilt (Iy) (Ey) 0 Hor 5 Hor Ver 10 Ver 15 RD 20 RD 25 LD LD Hor 1m 3m 6m 3m 6m 3m 6m 3m 6m Horizontal Encoder Eccentricity (E) Horizontal Encoder Eccentricity

44 Z (mm) Laser Scanner: Volumetric Test Target Arrangement Y (mm) X (mm)

45 Laser Scanner: Volumetric Test Target Arrangement

46 Coordinate error (mm) Laser Scanner: Volumetric Test Point Coordinate Error (Distance from measured point to calibrated point) Unconstrained case Target distance from instrument (m)

47 Apparent out-of-flatness (mm) Simulation of Systematic Errors from Laser Scanner Beam offset mm Beam offset mm 0.4 Transit offset Mirror offset mm mm Vert inde offset rad 0.2 Beam tilt Beam tilt rad rad Mirror tilt rad 0 Transit tilt Hor angle eccentricity rad mm/mm Hor angle eccentricity mm/mm -0.2 Ver angle eccentricity Ver angle eccentricity mm/mm mm/mm Bird bath mm Y (mm) X (mm) 2000 Hor second order scale Hor second order scale Ver second order scale Ver second order scale 0 rad 0 rad 0 rad 0 rad A 3 m 3 m ideally flat plane with center 2 m away from scanner. It is at a compound angle with respect to the scanner..

48 Error Sources & Evaluation Operator & Workpiece issues Workpiece reflectivity Workpiece optical penetration Workpiece secondary reflections Spot size on workpiece geometry Sampling strategy effects

49 Understanding Uncertainty Propagation Measurement Instrumentation Measurand Definition Environment Sampling Strategy Fitting Algorithm Result & Uncertainty Workpiece Properties Algorithm Selection & Implementation

50 Challenges: Reflections off Workpiece and into Scanner Matte finish Titanium Polished Steel Radius error = mm σ = mm Radius error =0.267 mm σ = mm

51 Nominal R Nominal R Challenges: Optical Penetration of Materials File: one inch Nylon points.tt points used σ=0.560 mm File: one inch Polypro points.tt points used 3 σ=0.112 mm R error = mm R error = mm Residuals from fit, Range = , Stdev = Residuals from fit, Range = , Stdev =

52 Height (mm) Challenges: Secondary Reflections Probe Possible secondary reflection problem Travel (mm) Potential error in the edge position could be 50 μm

53 Challenges: (a) Secondary Reflections H (H-h)tanα Top block (block under test) h Scatted light from top block Scatted light from bottom block Probe zero Probe reads -5 mm (b) Bottom block Peak from top surface Peak from bottom surface

54 Understanding Uncertainty Propagation Measurement Instrumentation Measurand Definition Environment Sampling Strategy Fitting Algorithm Result & Uncertainty Workpiece Properties Algorithm Selection & Implementation

55 Sampling Strategy Effects: Point Coordinate Uncertainty vs. Task Specific Uncertainty Measurement Point Uncertainty

56 Sampling Strategy Effects: Ratio of Circle Radius Uncertainty to Point Coordinate Uncertainty

57 Laser Trackers and Laser Scanners for Large Scale Coordinate Metrology Disclaimer: NACMA 2012 Data on commercial products are only provided for the sake of describing eperimental results. NIST does not endorse or recommend any commercial products or imply that this equipment is the best for any particular application. Questions?