CMM developments at the National Physical...

CMM developments at theNational Physical Laboratory

Andrew Lewis, NPL

Tercera Reunión Anual del Club Mexicano De Usarios de Máquinas De Medición por Coordenadas

Santiago de Querétaro, Mexico, 26 October 2006

OUTLINE

• Introduction to Length metrology at NPL

• Small-scale CMM developments

• Medium-scale CMM developments

• Large-scale CMM developments

• Optical CMM developments

NPL LENGTH METROLOGY

11 research staff

4 calibration staff

SMALL SCALE CMM

50 x 50 x 50 mm range

50 nm uncertainty

CMM z-stem

3 mirrors on probe carrier

length-angle interferometer

autocollimator

probing system

metrology frame

component mount

Laser interferometers measure:

3 axis displacements

3 axis parasitic rotations

3D analogue probe

Math compensation for errors in machine geometry

Overall schematic (one metrology axis shown)

SMALL SCALE CMM: SCHEMATIC

Capacitance gauges, 20 µm gap

Beryllium-Copperflexure

PROBE HEAD

300 µm probe tip

Mass of moving part: 350 mg

Small CMM and host CMM

Value

Type B, Type ASource of uncertainty

3.5 nm, 4 nmInterferometry

12.4 nm, 2.0 nmProbe carrier, including mirrors

9 nmMetrology frame

51 nm, 6 nmMiniature probe system

52 nm, 7 nmTotal (standard uncertainty, per axis)

UNCERTAINTIES

MEASUREMENT TASK

M2

MICROPROBES

STEP GAUGE SYSTEM

CMM verification

Step gauges

L < 1200 mm

(L > 1200 mm)

STEP GAUGE SYSTEM

2 mirrors attached to probe

Column reference interferometer

Length-angleinterferometer

STEP GAUGE SYSTEM: OPTICAL SCHEMATIC

D1D2

DR

∆D1∆D2

∆DR

Compensation for measurements not on the Abbe line: Length (D) and Angle (∆D)

Measurement from two directions but with common reference

OVERALL PERFORMANCE

Run – to – run repeatability:

– typically 70 nm std dev on any face centre

Uncertainty (95% confidence level):

± (0.1 + 0.3L) µm

where L (m) is the distance from the datum step

Build a medium-size CMM1 m range, 0.3 µm uncertainty

Build metrology system for large CMMs1 to 30 m range, 1 to 30 µm uncertainty

EVOLVING REQUIREMENTS

Error mapping of medium &

large sized CMSs

> 1 m range, > 1 µm uncertainty

Multilateration is defined as “a measuring system that determines either two or three dimensional coordinates

by combining only length measurements made from

fixed points”

SOLUTION: MULTI-LATERATION

Fixed measuringstations Movable

target

Li

Use laser trackers as measuring stations

Multilateration using laser trackers:

ensures that the Abbe criterion is always fulfilled (all displacements are, by definition, along the measurement axes [laser beams]);

uses a ‘virtual’ metrology frame, separate from the motion systems;

uses data redundancy for increased confidence or monitoring:

� after n measurements of target: 10 + 3n (unknown) , 4n (known)

� so for n =10, system is solvable, n > 10 gives data redundancy

MULTILATERATION: ADVANTAGES

IMPROVING LASER TRACKERS

Problems of commerciallaser trackers:

misalignment of steering mirror axes

misalignment of reflecting point of laser on mirror surface

small acceptance angle of retro-reflector

relatively poor angle scale uncertainty

Combined bearing and interferometer reference

Spherical bearing

Integrated interferometer

Datum is virtual centre of sphere:

Mechanical centre =

Optical centre

METROLOGICAL LASER TRACKERS

Hemi-spherical retro-reflector

r1 = (n-1)r2

r1 r

2

RETRO-REFLECTOR IMPROVEMENTS

r

Spherical retro-reflector

r1 = r2 = r, when n = 2

n = 2 retro-reflector

r1 = r2 = r

r

Uncertainty contributions

Form error

Homogeneity

Aberrations

But…Wider angle of acceptance

REDUCTION OF RETRO ERRORS

CMM &

trackers agree

on

displacement to

within

200 nm

MULTILATERATION WITH 4 TRACKERS

Only 1 tracker needed

Mounted in 4 positions

CMM must be repeatable

Combined uncertainty

of the tracker system

and the CMM repeatability is about

200 nm (k = 1)

SEQUENTIAL MULTILATERATION WITH 1 TRACKER

Collaboration with PTB:

Heinrich Schwenke

SEQUENTIAL MULTILATERATION WITH 1 TRACKER

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0 100 200 300 400 500 600 700 800 900 1000

Y (mm)

(mm

)

HORIZONTAL STRAIGHTNESS

Holeplate

Sequential multilateration-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0 100 200 300 400 500 600 700 800 900 1000

Y (mm)

(mm

)

VERTICAL STRAIGHTNESS

Holeplate

Sequential multilateration-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0 100 200 300 400 500 600 700 800 900 1000

Y (mm)

(mm

)

SCALE ERROR

Holeplate

Sequential multilateration

OPTICAL CMM STANDARDS

• NPL Vision CMM

• Laser interferometer

• Grid plate software:

– Error separation

– 4 rotations

– 1 linear shift

• Grid error

• Machine error

• U < 1 µm

OPTICAL CMM STANDARDS

• Grid plates:

– from 60 x 60 mm

– to 600 x 600 mm

OPTICAL CMM STANDARDS: DETAIL OF GRID (TARGETS 30 – 500 µm)

OPTICAL CMM STANDARDS: FORM MEASUREMENT STANDARDS

• Prismatic standards for optical 3D systems

• Calibrated on contact CMM

– 50 mm size

– 100 mm size

– ‘simple’

STANDARDS FOR HUMAN ARTEFACTS: ANTHROPOMETRY

• 3D body scanners – clothing industry, medical, security

• Traceability ?

ANTHROPOMETRY STANDARDS: FIRST GENERATION ARTEFACT

• Anthropometric standard

• 2 m tall

• Prismatic components

• Calibrated to < 50 µm

SUMMARY


– 50 mm CMM to 50 nm


– Step gauge system


– Multi-part artefact

– Multi-lateration


– Grid plates

– Form artefacts for optical CMMs

– Anthropometric standards

THANK YOU!

CMM developments at the National Physical...

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