HIGH-RESOLUTION GRADIENT MAGNETOMETER
I. Bolshakova, R. Holyaka, Magnetic Sensor Laboratory, Lviv Polytechnic National University,
1 Kotliarevsky Str., Lviv 79013, Ukraine
M. Kumada, National Institute of Radiological Sciences,4-9-1 Anagawa, Inage, Chiba, 263-8555 Japan

Abstract
The present work is devoted to one of essential problems of modern magnetometry – measurement of magnetic field distribution in space-limited objects, foremostin small gaps of strong magnets. Distinctive features ofthis problem is minimal dimensions of operating gaps ofthe magnets (units of millimeters), considerable values ofmagnetic fields (up to 4 T), non-uniform magnetic fieldwith complex form of its distribution (for instance,caused by special constructions of magnet concentrator
and yoke), strict requirements to the resolution and flexibility of measurement instruments etc. Taking into account that each specific problem like this one has also thewhole number of its own features, the problem of field
mapping in small gaps of magnets could be solved within
the scope of separate R&D projects only.

2 DEVELOPMENT RESULTS OF THE
GRADIENT MAGNETOMETERPerformed analysis of basic approaches and problems
of magnetic field measurements showed that applicationof linear HG matrixes (scanners) is most optimal solution
for field mapping in space-limited objects. In this casecomprehensive field mapping requires development ofseveral various types of linear HG matrixes.
Test version of hardware and software system "MultiScan-1" (fig.1) is developed and produced, which consists of: probe set, basic unit, interface unit, supply unit,
elements for probe mounting and shifting and software.

1 PROBLEM ANALYSIS
Primary transducers for devices of magnetic field distribution measurement in space-limited objects, in particular in narrow gaps of permanent magnets, shouldhave the following features:- minimum dimensions.
- high sensitivity, linearity and temperature stabilityof the transducing function.
- high selectivity to the spatial components of thefield induction, i.e. directive sensitivity (this featureis required due to the field complexity in a studied
volume – three spatial induction component values
could differ by several orders between each other).
- absence of ferromagnetic materials inside the measuring probes, which could distort the studied field.
Taking into account the requirements given above,
Hall generators (HG) are chosen in order to solve thegiven problem [1]. Let us denote the number of basic
problems arising in the process of development of HGbased devices for magnetic field distribution measurements in space-limited objects:- error of field induction averaging over the HG volume.
- HG spatial uncertainty during its shift in studied
volume, which results in inadequacy of measurement results.
- secondary effects when HG operates in strong andnon-uniform magnetic fields, which follows to theparasitic modulation of transducing parameter.

Fig.1. Appearance of gradient magnetometer
MultiScan-1 uses standard voltmeter like Keithley
2000 model, personal IBM-compatible computer withoperating system Windows 9x and LPT port available.
Probe set consists of four various probe types by meansof linear HG matrixes, which consists of: Probe-1 forgeneral scanning; Probe-2 for central scanning; Probe-3
for side scanning; Probe-4 for radial scanning; probeindexing mechanisms. Features of such set are wide possibilities of field mapping, high measurement accuracy
and operating simplicity. This problem is solved bymeans of using the matrixes with different HG spacingsand shiftability with high placement accuracy. Diameterof the probes is equal to 5.8 mm which meets the requirements of field mapping in the permanent magnet
with operating gap diameter of ∅ = 6±0.1 mm. Length ofthe probes is equal to L = 30 mm, sensitive elements arelocated in central part of the probes. Probes have twodegrees of freedom – linear shift along the magnet gap
and angular orientation in the plane perpendicular to thelinear shift. Stability of probe placement in magnet gap isprovided by means of indexing mechanisms located at
the magnet from both gap sides (fig.2 ). Channel isformed by the probe length for their angular orientation.
This channel is coupled with fixing disk of rotation angle,
which could be rotated either freely or with increment of
3 grades.

Fig.3. General scanning probe

Fig.4. Central scanning probe

Fig.2. Magnet with measuring assembly
Probe-1 for general scanning means the line of 5 HG
with spacing of ∆ = 1 mm (Fig.3). This probe allows
performing field induction measurements along the central axis of operating gap of the magnet (r = 0 mm) andsymmetrically to the central axis by the lines of r = +1, –1, +2, –2 mm.
Each of the Probe-2 (Fig.4) for central scanning andProbe-3 (Fig.5) for side scanning are lines of 10 HG withspacings of 0.2 mm. First of them is located symmetrically to the central axis of the gap with scanning zone 1 mm < r < +1 mm, and the second one is shifted in relation to center which provides the scanning zone of0 mm < r < 2 mm.
Probe-4 (Fig.6) for radial scanning consists of 4 parallel HG with the crystal centers located by a gap diameter.
Measurements are performed by the lines of r = +0.5, –0.5, +2, –2 mm.

Fig.5. Side scanning probe
All probe outputs are routed to unified 25-lead connectors. Spatial orientation of the probes in magnet gap isperformed by means of special mounting and shiftingelements.

Example of investigation results for the magnetic field
induction distribution is given at the fig.8, and for its
gradient – at the fig.9.

Fig.6. Radial scanning probe
Basic unit is intended for HG matrix supply, theircommutation and measurement of magnet temperature.
This unit allows commutating 16 channels, among which
there are 4 supply channels for measuring transducers,
10 channels of output voltage of HG matrixes, 1 channel
for reference measurement and 1 channel for temperature
measurement.
Interface unit is intended for transfer of computer instructions into the basic unit and voltmeter like Keithley
2000 model by the IEEE 488 interface protocol. The
same unit transfers data from voltmeter to computer. To
provide flexibility and assembling efficiency of "MultiScan-1" system the interface unit is connected directly to
LPT port of the computer.
Software “MSoft-1” is developed for operating controlof “MultiScan-1” system and visualization of measurement results. Its main operating window is presented atthe fig.7.

Fig.7. Main operating window of MSoft-1
Resolution of gradient magnetometer is equal to0.001 mT, measurement non-reproducibility at a fixed
spatial location of the probes does not exceed 0.03 mT.
During graphical representation of measurement results the array of obtained results is exportable into specialized software products, for example Microsoft Excel,
which allows to enlarge resources for magnetic field distribution visualization.

Fig.8. Example for measurementof magnetic field induction distribution

Fig.9. Example for measurementof magnetic field induction gradient distribution

3 CONCLUSIONS
Analysis of basic approaches to magnetic field mapping in space-limited objects is performed. It is denotedthat devices based on matrixes of Hall Generators areoptimal solution for the problem stated for this R&D
work – measurement of 3D field components in the narrow operating gap of a permanent magnet.
Design is developed and probe set is produced on thebase of HG linear matrixes. The set consists of the probes
for general, central, side and lateral scanning. The
probes have two degrees of freedom – rotation angle
around the central axis of operating magnet gap and coordinate by the length of this axis.
Test version of hardware-software system "MultiScan1" is developed and manufactured for field mapping in
the narrow operating gaps of permanent magnets. The
system consists of probe set, basic unit, interface unit,
supply unit, probe mounting and shifting elements, software "MSoft-1".

REFERENCES[1] I. Bolshakova, "Magnetic microsensors: technology,properties, applications", Sensors & Actuators: A.
Physical, Vol. 68(1998)/1-3, P.282-285.