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

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H u auster m mandea a hemshorn e pulz m korte

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


Outline
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
Motivation

Manually performed absolute measurement in Hermanus by rotation of a fluxgate magnetometer about two well defined axes

Ongoing 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

Auster H.U., V.Auster, A new method for performing an absolute measurement of the geomagnetic field,

Meas. Sci. Technol. 14, 1013-1017, 2003


Magnetic field along a rotation axis 1
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 ivariable

  •  and  constant

  • Bz = Bz(υ,φ)


Magnetic field along a rotation axis 2
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
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

y

y

x

x

 Measurement time: 30 minutes


Elimination of systematic measurement errors
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


Measurement results of one year operation in niemegk z
Measurement Results of one year operation in Niemegk - Z

Standard deviation: 1.0nT

2005

2006


Measurement results of one year operation in niemegk d
Measurement Results of one year operation in Niemegk - D

Standard deviation after trend and readjustment correction: 0.6nT

2005

2006



Steps to automation
Steps to automation Magnetometer

  • 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


Magnetometer s
Magnetometer Magnetometers

basket

magnetometer

  • 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

variometer

scalar

magnetometer


Performance of magnetometers
Performance Magnetometer of Magnetometers

Variometer

Basket magnetometer

Observatory Data


Performance of magnetometers1
Performance Magnetometer of Magnetometers

Variometer - Obs

Basket - Obs

Variometer - Basket


Components of the optical system
Components of the optical system Magnetometer

  • 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

~ 15 m Magnetometer

0.2mm

 = atan(0.2mm/15 m) ~ 3''

Performance of optical system

  • Stability of azimuth mark:

    • Quarzgut 0.5 ppm/°C

    • Displacement < 0.1 mm

      (h=2m, T = 50°C)


Rotation a by pneumatics
Rotation A by Pneumatics Magnetometer

  • 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
Rotation B & C by Piezo Motors Magnetometer

Attempts to develop non magnetic motors for sensor rotation

(1) Application example

(2) Linear motor

(3) Rotation to linear conversation

(4) Final solution using gravitation


Control unit
Control Unit Magnetometer

  • 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




Outlook
Outlook Magnetometer

  • 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
Acknowledgement Magnetometer

  • 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


Measurement results of one year operation in niemegk h
Measurement Results of one year operation in Niemegk - MagnetometerH

Standard deviation: 1.4nT