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Active Magnetic Shield controlled by algorithms without Cross-interference
T. Ueda1, K. Yamazaki2, M. Takenaka1, T. Sato3, A. Haga3
KRI, Inc.1, Takenaka Corp.2, Tohoku Gakuin University3, JapanABSTRACT
We propose new methods to control the active magnetic shield system composed of plural compensation coils, plural broadband magnetometricsensors, and a control unit. The conventional active magnetic shield system suffers from the oscillation problem because a magnetometric sensor
detect environmental magnetic noise and compensating magnetic fields generated by the currents flowing in not only the nearest coil but also othercoils. To prevent this oscillation problem, we propose two methods (Base Transition Method and Genaral Inverse Matrix Method) using thesensitivity co-efficients. In both proposed methods, the stability of controlling the prototype system has been confirmed in the experiments.
KEY WORDS
Active magnetic shield, Magnetic field compensation, Base transition method, Oscillation, MCG, MEG, Magnetization of biomagnetite,
Magnetization of ferumoxides.
INTRODUCTION
Environmental magnetic noises adversely affect measuring equipments in fields such as biomagnetism measurement, localization of magnetic
fluid marker in vivo, study on characteristics of magnetic materials and so on. The earth magnetism is always varying because of the affections of thesolar storm and the core activities in the earth. The magnetic fluxs concentrate to automobiles, elevators, steel doors and other structures made ofmagnetic materials. Therefore the spatial movement of them causes environmental magnetic noises. Electric currents flowing in near catenary cablesof DC electric railcar or supply cables for power-electronics apparatus around and spinning fans for air-conditioning also cause noises. These
environmental magnetic noises generally have spatial inclination. AMS (active magnetic shield) system with plural coils and plural sensors is thought
to be a better technical method to reduce environmental magnetic noises than heavy MSR (magnetic shield room) made from expensive materials
such as Permalloy because AMS could generate homogeneous zero magnetic fields by using plural coils. However conventional AMS system has theoscillation problem because magnetic fields are piled up at each sensor position to compensate environmental noises by plural coils. Here we propose
new control methods to prevent this oscillation problem.
METHODS[Base Transition Method]
In AMS system with m compensationcoils and n magnetometric sensors,
the output Bj of sensor j is predicated as
residual environmental magnetic noise.
(1)
where Sj is true environmental magnetic
noise at sensor position j and Oj isestimated compensation magnetic fieldvalue given by eq. (2)Figure 1. Explanation of Base
(2)
Transition Method
where a sensitivity co-efficient �ij isdefined as the output value of sensor j when unit current is supplied to only
compensation coil i, and ui is electric current flowing in coil i to compensate
environmental noises.
Evaluation function E given by eq. (3) can be translated such as (4),(4’), and (4’’).

Figure 2. Construction of Experimental System

(3)
(4)
(4’)
(4’’)
,,
Here
As evaluation function E is always a parabolic function concerning as aunknown value ui, we use the following recurrence formula eq.(5) in order tominimize evaluation function E eq.(3). Ki is a constant co-efficient previously
calculated by using m×n sensitivity co-efficients.

644a) Systemc) Sigma-Delta A/D Converterd) MR Sensorb) D/A and Voltage/Current converter
Figure 3. Photograph of Experimental System and Boards

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(5)
Here
(6)
Here eq.(5) is given by the fact that the axis ofparabolic function E is given by eq.(7).
(7)
However each sensor must be assigned to the mostappropriate position to generate a homogeneous
zero magnetic field space.
It is very interesting that if m is equal to n and all �ij
is zero when i is not equal to j, equation (5) reduces
to Gauss-Seidel Method, and that Ki is equal to thebest step co-efficient � of adaptive digital filter
algorithm called as LMS (least square mean).

Figure 4. Example of Measured Sensitivity Co-efficients by Experiment andCalculated Inverse Matrix A-[General Inverse Matrix Method]
It is defined that A is sensitivity matrix with m×n sensitivity coefficients �ij as elements, U is coil current matrix, S is trueenvironmental noise, O is compensating magnetic field matrix, andB is sensor output matrix. The condition is given by eq.(15) tominimize evaluation function E eq.(14). We propose algorithmeq.(17). Here A- is general inverse matrix of A.

(8),

(9)

(10),(11)(12),(13)
(14)

(15),

(16),(17)

RESULTSAs shown in figure 2 and 3, our prototype apparatus of active
magnetic shield system was composed of 6 coils, 6 MR sensors, Figure 5. Result of Experiment by using General Inverse
sigma-delta A/D board with 6 input channels, a computer, and D/A Matrix Method
and Voltage/Current converter board with 6 output channels. Asshown in figure 4, sensitivity co-efficients of which total number is 36 could be measured by our prototype system and inverse matrix of them werecalculated. The coil configuration had strong cross-interference condition because of short distance between coils. However it seems that ourproposed active magnetic shield with new control method is very effective as show in figure 5.
DISCUSSION
It has been demonstrated that our proposed control method is effective and that it is possible to achieve active magnetic shield control stably evenon the unstable oscillation condition with strong cross-interference. However the base transition method seems more effective than the inverse matrix
method because of flushing errors to a particular co-efficient at calculating inverse matrix.
In the case of system with Base Transition method, true environmental magnetic noise can be easily known by adding compensation field value tomeasured sensor output value. So artificial magnetic field with arbitrary vector can be generated in the active magnetic shield space. fi(t) is a coilcurrent to generate artificial magnetic field. The true residual environmental magnetic noise is given by eq.(18) and coil current i for generating
arbitrary artificial magnetic fields is given by eq.(19).
(18),
(19)
This function is important in order to magnetize or de-magnetize magnetic materials or particles such as ferumoxides or biomagnetites in active
magnetic shield space by using same coils generating compensation field because Permalloy shield distorts magnetic field.
REFERENCES
Ueda T, Kasai N, Sagane M, Chinone K, Awano N. Visualization for Electric Activities in a Human Heart. Biomag96; 1996, p. 569-572.

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