An Automatic Instrument to Measure the Absolute Components of the Earth's Magnetic Field H.-U. Auster, M. Mandea, A. Hemshorn, E. Pulz, M. Korte

An Automatic Instrument to Measure the Absolute Components of the Earth's

Magnetic Field

H.-U. Auster, M. Mandea, A. Hemshorn, E. Pulz, M. Korte

XIIth IAGA Workshop, Belsk 20062

Outline

Fundamentals of the Method Magnetic field along a rotation axis Elimination of systematic measurement errors

Manually performed test of the method in Niemegk Automation

Magnetometers Mechanics & Optics Controlling Set up in Belsk

Outlook


Motivation

Auster H.U., V.Auster, A new method for performing an absolute measurement of the geomagnetic field,Meas. Sci. Technol. 14, 1013-1017, 2003

Manually performed absolute measurement in Hermanus by rotation of a fluxgate magnetometer about two well defined axesOngoing activities to automate

absolute measurement

Automating of DI-Flux

Manipulation of vector proton magnetometer

Automating of rotation of a vector fluxgate magnetometer about two well defined axes


Magnetic field along a rotation axis (1)

Co-ordinate systems Red: arbitrary oriented fluxgate

magnetometer Black: Geophysical reference

system Relation between both: Euler

angles

Rotation about Precession angle i variable

and constant Bz = Bz(υ,φ)


Magnetic field along a rotation axis (2)

Computation of field in rotation axis Three independent measurements with arbitrary I

necessary Magnitude of B in direction of rotation axis becomes

independent from sensor orientation angles and

Matrix MB of measurement results

Unit vector of sensor orientation


Measurement Procedure

Rotation A to adjust mechanical axis to azimuth marks Rotation B to turn the sensor about the mechanical axes

Always 360° forward and backward 6 measurement stops each rotation direction (B and azimuth)

Rotation C for magnetometer calibration

x yy x

Measurement time: 30 minutes


Elimination of systematic measurement errors

Magnetometer errors by scalar calibration Rotation about two axes sufficient for full determination of

linear transfer function (offsets, scale factors, non orthogonality)

Full Earth field magnetometer necessary, high requirements on linearity (10-5)

Orientation of rotation axes Horizontal balance by level tube, misalignment of level tube

eliminated by interchanging of its ends Azimuth by telescope, misalignment of optical axis and

rotation axis eliminated by rotation of telescope


44920

44925

44930

44935

449407 17 23 5

March April May June July Aug. Sept. Oct. Nov. Dec. Jan. Febr. March April May

DIflux measuremnets Prototype instrument merasurements

Measurement Results of one year operation in Niemegk - Z

Standard deviation: 1.0nT

2005 2006


36

37

38

39

40

41

7 17 23 5


DIflux measuremnets Prototype instrument merasurements

Measurement Results of one year operation in Niemegk - D

Standard deviation after trend and readjustment correction: 0.6nT

2005 2006


Measurement Results of one year operation in Niemegk - Magnetometer

Offset in nT

-2

0

2

4

6

8

50 150 250 350 450

X Y Z

Scale Values

1,001

1,002

1,003

1,004

1,005

50 150 250 350 450

X Y Z


Steps to automation

Magnetometers Optical control

Mechanics: 3 Rotations Rotation about measurement axis

- arbitrary angles Turn Table

- arbitrary, but well known angle Sensor rotation

- arbitrary angle

Controlling Hardware Software


Magnetometers

two digital 3-axes fluxgate magnetometers range: 64000nT resolution: 0.01nT Non linearity < 10-5

Serial & Flashcard interface

Proton-Magnetometer Range: 20000-64000nT resolution 0.01nT Serial & Flashcard interface

basketmagnetometer

scalarmagnetometer

variometer


Performance of Magnetometers

Variometer Basket magnetometer Observatory Data

02:00:0014.05.06

07:00:0014.05.06

12:00:0014.05.06

17:00:0014.05.06

22:00:0014.05.06

20

40

60

80

100

620

640

660

680

700


02:00:0014.05.06

07:00:0014.05.06

12:00:0014.05.06

17:00:0014.05.06

22:00:0014.05.06

-4

-2

0

2

4

Performance of Magnetometers

Variometer - Obs Basket - Obs Variometer - Basket


Components of the optical system

Neodym Laser Coupled in by fibre optics

PSD 2cm x 2cm Resolution < 0.1 mm Protected from stray light by

Band-filter (635nm) Black tube

Azimuth Mark Made by ceramics Grounded in concrete

Bild PSD


Performance of optical system

-4

-3

-2

-1

0

1

2

3

-4 -3 -2 -1 0 1 2 3

x/mm

y/m

m

~ 15 m

0.2mm

= atan(0.2mm/15 m) ~ 3''

Stability of azimuth mark: Quarzgut 0.5 ppm/°C Displacement < 0.1 mm

(h=2m, T = 50°C)


Rotation A by Pneumatics

Pointing requirements: 1cm/20m 0.1mm/20cm 2arcmin

Well defined end positions necessary

7kg has to be rotated Low friction by bearing Importance of surface

treatment Pressure supply necessary

(2 bar)


Rotation B & C by Piezo Motors

(1) Application example(2) Linear motor(3) Rotation to linear conversation(4) Final solution using gravitation

Attempts to develop non magnetic motors for sensor rotation


Control Unit

GPS controlled Timing Motor control

rotation about measurement axis by piezo motor

Flip mechanism by piezo motor

Pneumatic control Turn table rotation by two valves

Laser switching and PSD read out

Magnetometer control Pre processing of data


Magnetic Requirements for Accommodation

Subunit magnetic field in a distance of 1m

distance requirement (<0.1nT)

remarks

Proton Magnetometer 15nT 6m polarization Fluxgate Variometer 5nT 4m 6mA x 4m² Fluxgate Basket Magn. 0.4nT 1.6m 4mA x 0.5m² Electronics boxes <0.5nT 2m Piezo electronics <0.1nT 1m PSD electronics <0.5nT 2m PSD sensors <0.5nT 2m Pneumatic valves 4nT 4m Power supply 120nT 12m Pressure supply 80nT 10m


Accommodation in Belsk


Outlook

System has to run permanently in Niemegk

Reliability have to be tested and improved Redesign of laser optics and some

mechanical parts Option: Replacement of pneumatics by

Piezo motor driven system New design for lower latitudes


Acknowledgement

GFZ for personal and financial investigation All the the people designed and manufactured

the mechanics (in Niemegk Potsdam Braunschweig Lindau and Garching)

Magson for magnetometers and software support

Belsk observatory for support to install the facility


18725

18730

18735

18740

18745

7 17 23 5


Measurement Results of one year operation in Niemegk - H

Standard deviation: 1.4nT

Documents

An Automatic Instrument to Measure the Absolute Components of the Earth's Magnetic Field H.-U. Auster, M. Mandea, A. Hemshorn, E. Pulz, M. Korte