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Home , Metrology

Some aspects of ultra-precision metrology

Paul Morantz Michael de Podesta • Cranfield University Robin Underwood • Loxham Precision • NPL 10 Fields of metrology

Field Associated Wavelength, Dimension, Angle, Form, Surface quality Temperature Contact and non-contact thermometry, of humidity Time, frequency Vibration Acoustics, Accelerometry, Ultrasound, Mass, force, pressure, density, viscosity Electricity Current, voltage, magnetism Flow Gas flow, liquid flow Radiometry, photometry, colorimetry Radiation Dosimetry, calorimetry Amount Concentration, pH

02/02/2015 2 SI Units quantity unit symbol Definition of the Length m unit Mass kg Time s National primary standards Current A Temperature K Reference Luminous cd standards Intensity Amount mol Industrial standards

Measurements

02/02/2015 3 Kelvin: SI Unit of Temperature

Length Temperature 1/273.16 Mass Chosen so that 1 kelvin ≈ 1 degree Time Electrical Current Mole

The kelvin, the unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water

TTPW reproducible to ~50 mK

02/02/2015 4 Kelvin in the NEW SI Unit System

Energy ∆휈 (joule) e 133Cs

s A

c m kg h

Kcd Cd kB K NA mol

02/02/2015 5 Kelvin in the NEW SI Unit System

Energy ∆휈 (joule) e 133Cs The kelvin, the unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water s A

c h The kelvin, the unitm of thermodynamickg temperature, is such that the has the exact value -23 kB= 1.380 65XX×10 joules per kelvin

Kcd Cd kB K NA mol

02/02/2015 6 Primary thermometers are based on gases

• Molecular motions are simple • We can approach ‘ideal gas’ conditions at low pressure • In an ideal gas the internal energy is just the kinetic energy of the molecules 0.005 0.004 0.003

0.002 Signals Signals V /

0.001

0 7526 7530 7534 7538 Frequency / Hz

Find mass of molecule ퟑ풎 풌 = 풔풑풆풆풅 풐풇 풔풐풖풏풅 ퟐ 퐁 ퟓ푻 do experiment Measure the

at TTPW speed of sound

02/02/2015 7 Measure the Average Radius using Microwaves

100 TM11 Resonance

80 • In a sphere, F0 is inversely proportional to the radius

60

Signal Signal 40 • Requires a triaxial deformation of the sphere to resolve these 20 components 0 2109 2110 2111 2112 Frequency (MHz) • Need to measure:

• F0, F1 and F2 • Average radius ~ 62mm • Shape ~ 31 microns eccentricity (by design) • And to know the measurement

02/02/2015 8 Dimensional measurements

CMM

Pycnometry Microwaves

02/02/2015 9 Pycnometry (for average radius)

Fill with highly pure water (by evaporation)

02/02/2015 10 Pycnometry (for average radius)

And then weigh: infer volume and thus radius

02/02/2015 11 Tactile CMM Measurements

02/02/2015 12 Tactile CMM probing diamond turned copper

Even at low (< 10g) probe forces with large (8mm) stylus tips, significant damage can occur if approach speed is not very low

02/02/2015 13 Tactile CMM probing diamond turned copper

Even at very low approach speeds slight damage occurs This indentation of 50nm depth is caused by 5 repeat measurements in the same location

02/02/2015 14 Metrology in support of manufacture • The ‘spheres’ were made at Cranfield • Three metrology techniques were used in support of the diamond turning • On-machine profilometery (1) was eclipsed by: • (2) Interferometry • (3) CMM Scanning

02/02/2015 15 Tool position 100

Turning programming 80

60

toolx' 40 mm toolz

20

0

-20 -10 0 10 20 30 40 50 60 70 radians

This is a tri-axial ellipsoid, non-rotationally symmetric (freeform) turning, with sub-micron accuracy

02/02/2015 16 Interferometry

Hemisphere

Diamond turning tool

Interferometer

Field of view and fringe spacing limitations require stitching interferometry for full hemisphere metrology

02/02/2015 17 CMM scanning metrology

Scanning measurements (as for single point ones) will damage the surface Final measurement is taken before final machining pass, which removes < 2 μm

02/02/2015 18 Results

-23 Our 2013 publication moved kB from 1.380 65XX×10 joules per kelvin to 1.380 651 56 (98) with a relative of 0.71 x 10-6

02/02/2015 19 Next generation of large telescopes 39m

Hale Keck GMT TMT E-ELT (1948) (1993) (2021) (2022) (2024)

02/02/2015 20 Process chains for ELT segments Stage 2 Stage 3 Stage 1

Fixed abrasive Form/finish Sub-aperture Grinding improvement figure correct <1 mm form >10 µm RMS ~300 nm RMS ~10 nm RMS accuracy form accuracy form accuracy form accuracy

• Differing metrology requirements after each stage of a typical process chain, e.g. • Grinding • Sub-aperture polishing 10.2 nm RMS residuals • Sub-aperture broad ion-beam figuring after active correction (ref: R. Geyl, Sagem)

02/02/2015 21 Surface geometry

Non rotationally symmetric (freeform) Z machining/metrology requirements • Sag up to ~3mm C • Departure from spherical ~ 0.15mm X

02/02/2015 22 On-machine metrology

• On machine metrology is accurate, but slow – high spatial resolution data are not available on machine

02/02/2015 23 CMM Scanning measurement

02/02/2015 24 Areal measurements from tactile scans

Z C X

02/02/2015 25 Complex form compensation high (~106 points) spatial resolution measurements are valuable for components unsuitable for interferometry i.e. non-reflective

02/02/2015 26 Ultra Precision Temperature Controllers

Thermal Control

Loxham Precision’s ultra precision temperature controllers are based on the most advanced thermal management technologies offering: o Sub milli-Kelvin resolution control o Multiple channels o Matched performance temperature sensors o High response cooling technology o Advanced fluid heater technology o Remote heater and sensor positioning o Advanced control functions

0.1°C

1

02/02/2015 27 Ultra Precision Temperature Controllers

• Modular multi-channel configurable approach • Specific miniaturised electronic and fluid control hardware development • Sub milli-Kelvin operating resolution • Specially encapsulated ultra-stable sensors • Remote (wireless) sensor capability • Numerous control options: • Cascade, gain scheduling, adaptive control, alarms etc. • Controllable: • Pumping power • Cooling power • Heating power • Tailored solutions available

02/02/2015 28 Final word on temperature

The definition of the units of temperature (the kelvin and the degree Celsius) is about to change.

From 2018, temperature measurements will be fundamentally linked to the units of energy.

Every temperature measurement you make is linked to our fundamental understanding of the thermal properties of matter - its accuracy will have been founded on dimensional metrology of copper components.

02/02/2015 29