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

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

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### 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

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 Magnetometer

- Magnetometers
- Optical control
- Mechanics: 3 Rotations
- Rotation about measurement axis
- arbitrary angles

- Turn Table
- arbitrary, but well known angle

- Sensor rotation
- arbitrary angle

- Rotation about measurement axis
- Controlling
- Hardware
- Software

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

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

~ 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 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 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 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

Magnetic Requirements for Accommodation Magnetometer

Accommodation in Belsk Magnetometer

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 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 - MagnetometerH

Standard deviation: 1.4nT

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