IMPROVEMENTS IN OR RELATING TO REDUCING THE STRENGTH OF ELECTRO-MAGNETIC FIELDS

Abstract: Apparatus (2) (see Figure 3) for reducing the strength, within a room C, of the magnetic component of an electro-magnetic field, comprises means for creating a second, controlled, magnetic field component which interferes with the first mentioned magnetic field component so as to reduce the strength thereof. The means (2) comprise a plurality of induction winding means (d1, d2, d3) and a plurality of controlling magnetic field sensors (f1, f2, f3, f4), the number of sensors (f1, f2, f3, f4) being greater than the number of winding means (d1, d2, d3). The winding means (d1, d2, d3) encompass the room C about three Cartesian co-ordinate axes (X, Y, Z). Similar apparatus (20) (see Figure 4) is provided for reducing the strength of the electrical component of the electro-magnetic field wherein, inter alia, the windings (d1, d2, d3) are replaced by large area capacitor plates.

Description: I PROVEMENTS IN OR RELATING TO REDUCING THE STRENGTH OF ELECTRO-MAGNETIC FIELDS This invention relates to reducing the strength of electro-magnetic fields and achieves such strength reduction either by reducing the strength of the magnetic components of the fields, or the electrical components thereof, or both. The invention finds application in reducing alternating electro-magnetic field components in enclosed regions, being electro-magnetic field components created by or arising as a result of power lines in the neighbourhood of the region. It is a matter of fact that electro-magnetic field components cause interference with and are liable to damage certain categories of electronic equipment, and it has been suggested, with some supporting evidence, that such fields may constitute a health hazard. Indeed, alternating electro-magnetic field components, created by power cables operating at line frequency, ie 50 Hz in the United Kingdom and its harmonies, are suspected of causing foetal and infantile damage. Low frequency electrical field components are, as is well known, difficult to control even in enclosed regions and with the use of conventional, passive, shielding techniques. According to one aspect of the present invention, a method of reducing the strength, within a region, of at least one component, ( magnetic and/or electrical), of an electro-magnetic field, comprises creating a second electro-magnetic field component which interferes with the said one electro-magnetic field component so as to reduce the strength thereof, by employment of second component generating means and a plurality of controlling sensors, the number of sensors being greater than the number of second component generating means. According to another aspect of the present invention, apparatus for reducing the strength, within a region, of at least one component, (magnetic and/or electrical), of an electro-magnetic field component, comprises means for creating a second, controlled, electro-magnetic field component which interferes with the said one electro-magnetic field component so as to reduce the strength thereof, characterised in that said means comprise a plurality of second component generating means and a plurality of controlling sensors, the number of sensors being greater than the number of second component generating means. In order to .create an interfering magnetic field component, the generating means may comprise inductive winding means. Alternatively, or in addition, in order to create an interfering electrical field component, the generating means may comprise capacitor means. The various aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a di gram of a closed region, namely a room of a house, in relation to an electric power cable-carrying pylon, Figure 2 is a representation of, inter alia, one set of inductive windings encompassing the region, so as to create an interfering, controlled, magnetic field component, Figure 3 is a representation similar to that illustrated by Figure 2, with added features, and Fi gure 4 i s a representati on simi l r to that il l ustrated by Fi gure 3, but i s concerned with apparatus for creating an i nterfering , controlled, electrical field component. With reference to Figure 1, a pylon a carries six power cables b, to b fi respectively, which convey high alternating currents at line frequency. The magnetic fields B, to B β (B β only being illustrated) are circumferential ly distributed about their respective current- carrying conductors b, to b g , falling off inversely in magnitude with distance from the relevant cable. The total magnetic field in a closed region C, comprising a room in building 1, which is due to the power cables b, to bg, is the superposition of the instantaneous electro-magnetic fields due to each cable. The resultant field is an alternating electro-magnetic field at the line frequency whose phase, magnitude and direction depends on the distances from the room C to each cable, and the magnitude and relative phase of the alternating current in each conducting cable. The strength and spatial distribution of the electro-magnetic field in the room C may change with time, as a result of changes in the currents in each of the power lines due to supply demands. If the current in the i'th cable is represented in complex form as I.e J i where C5 = 2τι.50 in this case, and the distance from each cable to an observation point in the room C as r., the alternating magnetic field strength at this observation point is given by the expression: To cal cul ate the order of magni tude of thi s magnetic fi el d strength , assume a singl e conductor cabl e carrying a current of 1 , 000 amperes (rms) at a distance of 20 metres from the observation point in the _5 room C, so that B is about 10 tesla Crms). With reference now to Figure 2 as well, apparatus 2 is provided for reducing the strength of the magnetic component of the electro¬ magnetic field within the closed region defined by the room C. The apparatus 2 comprises means 3 for creating a destructive interfering alternating magnetic field component substantially equal and opposite to that created in the room C by the six power-carrying cables b, to b g , so as to at least reduce the strength of said field component. The apparatus 2 comprises a plurality of inductive winding means and a plurality of magnetic field sensors. The inductive winding means encompass the room C in each of three Cartesian axes X, Y, Z, each winding means being driven by an alternating electric current (provided by alternator 4) at the line frequency of variable amplitude and phase. For reasons of clarity, Figure 2 shows only one set of winding means, in the form of three coils, d, , d 2 , d,, and one set of magnetic field sensors in the form of "search" coils f, , fp, f , f . The two sets illustrated by Figure 2 are associated with axis X only of the three Cartesian axes X, Y, Z. Identical sets are provided for the other two axes Y, Z. it will be noted that the number of field sensors f, to f * of a set is greater than the number of windings d-, to d, associated therewith. In practice, therefore, nine windings and twelve field sensors are provided. However, for simplicity, only windings d, to d 3 and magnetic field sensors f-, to f * will be referred to in the following description. The three windings d, , do, d 3 are driven by alternating currents I ,, I « and I-3 respectively, derived by passing signals at the line frequency through electrical networks N, , N 2 and No_ respectively by which the amplitude and phase of the said currents may be adjusted. The arrangement may be simplified by arranging that the currents through each of the windings are identical, for example, by wiring them in series. The windings d-j , d 2 , are substantially equi-spaced from each other. If the spacing between the windings d-, , d 2 , d is approximately equal to their effective radius, a type of "Helmholtz coils" arrangement is created providing a sensibly constant magnetic field in the region defined by the room C. In practice, the field sensors f-. to f * are not disposed in a line as shown in Figure 3, but are well spaced out within the body of the room C in order to get the best effect. Such distribution, plus the fact that a greater number of sensors f, , f 2 , , is used than the number of control windings d, , d 2 , do, results in substantially uniform magnetic field component cancellation over substantially the whole of the closed region defined by the room C. The interfering magnetic field component achieved by the arrangement illustrated by Figure 2 is approximately the same as that created. in the centre of a flat coil which, if the effective radius is r and the winding has n turns and carries a current of I amperes, is: o c B c = 2a So, for exampl e , assuming a = 3 metres and n = 100 turns, B i s about 10 -5 tesl a for a current in the coi l (I ) of about 1 ampere, a reasonabl e fi gure . In order to adjust the currents in each of the "control" windings d, , d 2 , do, so as to create a substantially equal and opposite magnetic field component interfering with that due to the power line-created electro-magnetic field, the apparatus 2 is provided with an automatic control system which uses the magnetic field detectors f, , f J 3 , f 4 , which are disposed inside the room C, in order to monitor the total magnetic field therein, and to adjust the magnitude and phases of the currents in the windings d, , do, do so as to substantially reduce or even to minimise the sum of the squares of the outputs from the field sensors f-, to f * . The automatic control system is illustrated, again for control in the Cartesian axis X only, in Figure 3, which shows the control system 5 using a controller 6 for adjusting the three control winding currents I ,, I 2 ar >d I c3 so as to substantially reduce and minimise, as far as possible, the sum of the squares of the voltage outputs V, to V, of the four field sensors f, to f , . When the inductive winding means d, , d 2 , d, are wound round all three axes X, Y, Z, of the region defined by the room C, the field sensors f■, , f 2 , fo, f» must also be oriented in a variety of directions to pick up the total magnetic field. It is straightforward, however, to provide a controller 6 which will account for any interaction between each inductive winding means and its associated field sensor in order to produce a single coupled control system rather than three independent control systems controlling the field in the three axes independently. The controller 6 may be implemented in a variety of ways. For example, if the total alternating output voltage at the line frequency from the -2'th field sensor CV-2 j ) is represented in complex form, it can be considered as the sum of the components due to the original field CV-δ ) and due to each of the three control windings, as d, , do, do, (V-β for = 1 to 3), so: 3 m m=l Each V-e is linearly proportional to the complex current in this winding, I say, with a complex constant of proportionality C-2 m m,' so: v .τ ■ % + -2m cm m=l Defining the vectors: = V 1T V 2T V LT ] p - cv 1P v 2P v Lp ] - I cl ! c2 W and the matrix: C 1 1 C 1 2 C = C 21 C 22 'LM we can write : V τ = V + C I — T — p — c The control problem is to minimise the sum of the squared moduli of the total output voltages from the search coils f, to f. by adjusting each s sum of squared output voltages may be wπ tten as V_ j "V_ j where H denotes the conjugate transpose of the vector , by adjusting • * • each Ic _m_, . It can be shown that the result of the standard complex quadratic minimisation problem is a unique optimum set of control winding currents given by: I (opt) = -[£ H ] _1 C H V f By using test currents into each of the windings, the control system can determine each of the elements of the matrix C_, and with the current to each winding set to zero the elements of the vector V„ can be determined. The controller 6 is implemented on a data 5 processor in the form of a microprocessor 7, using analogue to digital converters to link the field sensors f, , f 2 , fo, f * with the microprocessor and to measure each field sensor voltage, V, to V- and digital to analogue converters to link the microprocessor with ■jQ the windings d, , d ? , do and control each control winding current, I . The microprocessor 7 may readily be programmed to seek to minimise the value of a cost function according to a control algorithm relating to the minimisation of the sum of the squares of the outputs from the magnetic field sensors Cf, , fo, fo, f * and, 15 hence, of the magnetic field component of the electro-magnetic field. A number of methods may be used to make the controller 6 responsive to changes in the external magnetic field component. An adaptive 2o method of control applicable to the present invention is disclosed by PCT Application published as W088/02912. The present invention (including the Figure 4 arrangement described below) could adopt the digital control system disclosed by 25 W088/02912, to provide a system 8 whereby the interfering magnetic field component is adaptively adjusted in an automatic manner using adaptive digital filters. A reference signal 9 derived from the mains supply, eg from one of cables b, to b β is employed by the control system 8, preferably stepped down by a transformer. 30 Instead of , or in addition to , the magnetic component reducti on apparatus 2 of Fi gures 2 and 3, means may be provided for creati ng a second, control led el ectrical fi el d component which destructively interferes with the el ectrical component of the el ectro-magnetic 35 fi el d to that created by the cabl es b, to bg of Fi gure 1 . Such means are illustrated by Figure 4, wherein the interfering electrical field component is created by apparatus 20. The apparatus 20 comprises a plurality of capacitor means 21 and a plurality of electrical field sensors ef, , efo, efo, ef * . The capacitor means 21 comprise insulated capacitor plates 22, 23, shown by the shaded areas of the Figure, oppositely disposed at the end surfaces of the room C, on Cartesian axis X. Identical electrode plates are disposed on the floor and roof surfaces of the room C (on axis Z), and on the opposite side surfaces thereof (on axis Y), to form two further capacitor means 21. Control system 25, controller 26, alternator 24 and reference input 29 correspond to items 5, 5, 4 and 9 of Figure 3. The voltage generator means 24 is operable to develop in the capacitor means 21 voltages variable in amplitude and phase, and the controller 26 is responsive to the outputs of the electrical field sensors ef, , efo, efo, ef, (by way of V , ) in order to vary the magnitudes and phases of the voltages developed at the capacitor means 21, so as to substantially reduce (and preferably minimise) the sum of the square of the outputs from the field sensors ef -, ,

Claims: C L A I M S 1. Apparatus for reducing the strength, within a region, of at least one component, (magnetic and/or electrical) of an electro-magnetic field, comprising means for creating a second, controlled, electro-magnetic field component which interferes with the said one electro-magnetic field component so as to reduce the strength thereof, characterised in that said means (2 and/or 20) comprise a plurality of second component generating means (d, , d ? , do and/or 21 ) and a plurality of controlling sensors (f■, , f 2 , fo, f, and/or ef -, , ef-, efo, ef,), the number of sensors (f, , f 2 , fo, f, or efι , efo, efo, ef,) being greater than the number of second component generating means (d-, , do, do and/or 21). 2. Apparatus as claimed in Claim 1, characterised in that the second component generating means (2) comprise a plurality of inductive winding means (d, , do, do) and the sensors comprise magnetic field sensors (f, , f 2 , fo, f.). 3. Apparatus as claimed in Claim 2 characterised in that the winding means Cd, , do, d,) encompass the region (C) about three Cartesion co-ordinate axes (X,Y,Z). 4. Apparatus as claimed in Claim 1, for reducing the strength of an alternating magnetic field component, characterised in that it comprises inductive winding means (d, , d 2 , do) encompassing the region about three Cartesion co-ordinate axes (X, Y, Z), alternating current generator means (4) operable to develop in the winding means alternating currents variable in amplitude and phase, magnetic field sensors (f, , f 2 , f,, f,) operable to sense the magnetic field resultant at each of a plurality of positions distributed within the region (C) and means (6) responsive to the outputs of the said field sensors to vary the magnitudes and phases of the currents developed in the said winding means, so as to substantially reduce the sum of the squares of the outputs from the said sensors. 5. Apparatus as claimed in Claim 1, for reducing the strength of an electrical field component, characterised in that the second component generating means (20) comprises capacitor means (21), voltage generator means (24) operable to develop in the capacitor means (21) voltages variable in amplitude and phase; and means ( 26) responsive to the outputs of the sensors (ef, , efo, ef,, ef,) in order to vary the magnitudes and phases of the voltages developed at the capacitor means so as to substantially reduce the sum of the square of the outputs from the sensors (ef-,, efo, efo, ef,). 6. Apparatus as claimed in Claim 5, characterised in that the said capacitor means (21) each comprise a pair of plates (22, 23) disposed on opposite surfaces of the region (C). 7. Apparatus as claimed in any one ' .of Claims 1 to 6, characterised in that it is provided with a digital data processor (7), with analogue to digital converters linking the sensors (f-, , f 2 , fo, f,) and/or ef, , ef 2 , efo, ef,) to the data processor, and digital to analogue converters linking the data processor to the second component generating means (2 and/or 20). 8. Apparatus as claimed in Claim 7, characterised in that the digital data processor (7) is programmed to seek to minimise the value of a cost function according to a control algorithm relating to the minimisation of the sum of the squares of the outputs from the sensors (f•> , fo, fo, f, and/or ef, , efo, efo, ef,). 9. Apparatus as claimed in any one of Claims 1 to 8, characterised in that it is provided with a digital control system (8) wherein the interfering electro-magnetic field component is adaptively adjusted in an automatic manner using adaptive digital filters. 10. A method of reducing the strength, within a region, of at least one component, (magnetic and/or electrical) of an electro-magnetic field, comprising creating a second, controlling, electro-magnetic field component which interferes with said one electro-magnetic field component so as to reduce the strength thereof, character sed in that it employs a plurality of second component generating means (2 and/or 20) and a plurality of sensors (f,, fo, fo, f, and/or ef■■ , ef 2 , efo, ef,), the number of sensors being greater than the number of second component generating means. 11 . Apparatus for reducing the strength of an el ectro-magnetic fi el d, substantial ly as herei nbefore described, with reference to the accompanyi ng drawings . 12. A method of reduci ng the strength of an el ectro-magnetic fi el d * substanti al ly as herei nbefore described, with reference to the accompanyi ng drawi ngs .

Inventors: ELLIOTT STEPHEN JOHN, ELLIOTT, STEPHEN, JOHN
Applicants: ADAPTIVE CONTROL LTD, ADAPTIVE CONTROL LIMITED
Application year: 1989
Main classification: H05K9
IPC codes: G01R33, H05K9