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

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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
- Manually performed test of the method in Niemegk
- Automation
- Magnetometers
- Mechanics & Optics
- Controlling
- Set up in Belsk
- Outlook

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)

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)

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

y

y

x

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

Measurement Results of one year operation in Niemegk - D

Standard deviation after trend and readjustment correction: 0.6nT

2005

2006

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

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

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

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

- 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

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

- 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

- 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

Measurement Results of one year operation in Niemegk - H

Standard deviation: 1.4nT

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