UA19997U - Method for mechanically balancing a semiconductor magnetic field gradiometer - Google Patents

Abstract

The proposed method for mechanically balancing a semiconductor magnetic field gradiometer consists in installing a balancing element in the middle of the compensation coils of the gradiometer, then displacing the balancing element along the longitudinal axis of the gradiometer within the distance between the compensation coils.

Description

Description of the invention

The useful model belongs to the measuring technique, namely to magnetometry, designed for the registration of ultra-weak low-frequency magnetic fields against the background of external magnetic interference of a much larger amplitude.

More specifically, the utility model refers to a method that ensures the balancing of the gradiometer.

The most difficult problem is the suppression of industrial interference lying in the ICG signal band.

The main method of solving this problem is the spatial selection of interference magnetic fields generated by distant sources, that is, the use of so-called superconducting gradient meters. 70 Due to the fact that the windings of the gradient meter differ from each other, in other words - they are not exactly ideal and parallel, after manufacturing the gradient meter always has a small residual signal generated by a uniform magnetic field, the so-called initial technological mechanical imbalance. Typical unbalance values are equal to 1073, that is, one thousandth part of the uniform field passes to the SQUID input, and the rest is suppressed. For biomagnetic measurements, it is essential that the degree of imbalance does not exceed 107. Such a degree cannot be ensured only by manufacturing. On the other hand, EPP cannot be expected to improve the balance by more than two orders of magnitude. Therefore, to improve the initial technological balance, mechanical balancing of the gradiometer by two to three orders of magnitude is required. In general, all the above ways can be considered and used independently of each other, because none of them is able to provide the necessary degree of balance.

There is a known method of balancing that was developed at the National Bureau of Standards in the USA (project

Mo2750482, report Mo10,736 dated 31.03.1972 "ONgazepzyime ziregsopacysipud tadpeiiis dgadioteyeg", ittegtap and others). This scheme was based on moving superconducting plates and disks to achieve transverse and longitudinal (axial) balance. However, this scheme had two drawbacks: the adjustment was very non-linear and there was also a mutual influence between different plates. -

Another approach, described in patents OB 3 965 411 (06/22/1976) and 5 C 956 690 (05/11/1976), used as trimming elements mutually perpendicular superconducting coils that were connected to a magnetic flux sensor. Trim-rings were wound on a cylinder in a gradient-metric configuration, and the superconducting magnetic shield moving by them 30 provided a change in the connection between the corresponding trim-rings and the magnetic field. The disadvantage of this scheme is that the trim rings must be precisely wound on the cylinder, which (22) increases the price of the device. But the main drawback of this approach is the redistribution of the inductance of the measuring Fu gradient meter between the gradient meter's own inductance and the inductances of the trimming elements. As a result, when the trim rings were introduced, the sensitivity of the SQUID sensor to the measured signal deteriorated. (22) with In other previous methods for balancing the gradient meter, based on mutually orthogonal "superconducting elements", the change of the effective areas of the turns of the gradient meter was used. For example, superconducting disks were used in patents OZ Z 976 938 (24.08.76) and 05 4 320 341 (16.03.82). Similarly, for this purpose, superconducting plates were also used, for example - .). "

A significant drawback of these methods is the deterioration of the symmetry of the gradient meter when adjusting elements are introduced. A balancing device that uses trim discs to achieve balance in combination with the ability to change the distance between the measuring coils is described in patent 05 4 523 147 (11.06.85). 1» The disadvantage of this approach is the more complex and precise design of the gradient meter, hence the significantly higher price of the device.

In patent O5 C 976 9383, a scheme consisting of two stages was proposed. At the 1st stage, 415 rough balancing is carried out. Then the gradiometer with the SQUID sensor is removed from the cryostat, and the disks are mechanically fixed (fixed) in their positions. Next, for precise balancing, the gradiometer is again immersed in liquid helium. But such methods, which require removing and re-immersing the gradient meter in liquid helium (so-called thermocycling), are time-consuming, relatively inaccurate (because the linear dimensions of parts change during thermocycling), and therefore not satisfactory.

The closest analogue is described in patent 5 3 965 411 (22.06.1976), in which mutually perpendicular superconducting coils are used as trimming elements, which are connected to a magnetic flux sensor. The trim rings are wound on a cylinder in a gradient-metric configuration, and a movable superconducting magnetic shield provides a change in coupling between the respective trim rings and the magnetic field. The disadvantage of this method is that the trim rings must be precisely wound on the cylinder, which increases the price of the device. But the main drawback of this method is the redistribution of the inductance of the measuring gradiometer between the inductance of the gradiometer and the inductances of the trimming elements. As a result, when the trim rings were introduced, the sensitivity of the SQUID sensor to the measured signal deteriorated.

The technical task of the useful model is to develop a method of mechanical balancing of a superconducting magnetic field gradiometer, which allows to increase the degree of balance to 2.1079, to simplify its adjustment, and also to reduce the cost of the balancing device.

The set task is a method of mechanical balancing of a superconducting magnetic field gradiometer, which includes moving a balancing element in the magnetic field, the set task is achieved by placing the balancing element in the middle between the compensation coils at the beginning of balancing, and balancing the superconducting magnetic field gradiometer is carried out by moving the balancing element 65 along the axis of the cylindrical body within the distance between the compensating coils.

The technical result is achieved due to the use of one superconducting ring moving along the axis of the gradiometer. One trim ring creates the prerequisites for simplifying the design of the regulating device for mechanical balancing of the gradient meter, increasing the degree of balance, and for the ability to perform balancing taking into account the ambient noise, i.e. directly in the clinic.

Figure 1 shows a general view of a superconducting magnetic field gradiometer; in Fig. 2 - a regulating device for mechanical balancing of the gradient meter; in Fig. 3 - a diagram of the mutual arrangement of the short-circuited ring and the axial gradiometer of the 2nd order connected to the SQUID; Fig. 4 shows the output noise spectrum of the SQUID magnetometer without balancing; Fig. 5 shows the output noise spectrum of the SQUID magnetometer after electronic interference suppression (electronic balance); Fig.b shows the output noise spectrum of the SQUID magnetometer 7/o after mechanical balancing; in Fig. 7 - Spectrum of the output noise of the SQUID magnetometer after mechanical and electronic balancing (EPP).

Design of a highly balanced superconducting gradiometer used as an input antenna

A SQUID magnetometer for measuring magnetic signals of the heart in unshielded conditions is shown in Fig.1.

The specified device is an axial gradiometer of the 2nd order, intended for registration of the 2nd 7/5 spatial derivative of the vertical (axial) component of the field. At the same time, there is a spatial selection of magnetic field signals, namely, for sources located at a distance g close to the value of the base, the attenuation of the useful signal in the lower coil of the antenna is negligible. At the same time, the signals of distant sources are weakened in proportion to 1/g7. However, this ratio is true only for a gradient meter with a high degree of balance, which is achieved by equalizing the effective areas of the gradient meter coils using mechanical balancing.

Structurally, the gradiometer consists of circular coils 1, 2 and Z, and the distance between the coils (gradientometer base B) is 60 mm. The lower coil C is a receiving coil designed to register a useful signal, for example, from the human heart. The middle 2 and upper C coils are compensating coils that compensate for signals from distant sources of magnetic interference. The upper 1 and lower Z coils each have one turn, wound, for example, clockwise, and the middle 2 - two turns, wound against the time arrow - in this way, a 2nd-order gradiometer is formed, which registers the second spatial derivative (gradient) From the magnetic field directed along its axis.

The middle compensating coil 2 is two-turn and placed in the middle of the housing 4, the receiving coil C and the second compensating coil 1 are single-turn, placed at the opposite ends of the housing 4, the same receiving

C is at the bottom, and the second compensation 1 is at the top. All coils are wound with a single length of superconducting wire made of niobium, and connected with the help of straight and reverse wire segments, twisted and inserted into a vertical o-groove (Fig. 3). Gee!

Body 4 has a cylindrical shape and is made of a non-magnetic material (graphite) with a coefficient of linear expansion equal to that of niobium (wire material). The turns of coils 1, 2 and Z are placed in annular grooves with a depth of 0.2 mm, located in the transverse plane. The useful signal is introduced into SQUID 5 by means of "- signal coil 3, which, together with the gradient meter, forms a magnetic flux transformer.

Achieving a high degree of balance of this type of gradiometer is limited by 2 factors: 1) the spiral shape of the grooves and 2) the magnetic connection between the turns of the middle coil 2. The reason is that the grooves in the cylindrical body 4 of the spiral gradiometer are located in a plane inclined to its axes, therefore not « 70 are exactly perpendicular to the axis. As a result, a parasitic magnetic field will penetrate the antenna, which will create a significant imbalance in the transverse component of the field. The presence of a magnetic connection between the turns of the middle coil 2 leads to an increase in its total inductance. Therefore, in order to achieve a high technological balance, the )" gradient meter in this implementation has the following differences: 1) the grooves have an annular shape, 2) the distance between the turns of the middle coil is equal to bmm. As a result, the design provides an initial (technological)

Unbalance along the axial component of the field in the range of 4-8.1071 and transverse 2-4.107. This holder ensures stable fixation of the trim ring in the transverse plane, formed by a cylindrical frame 7, a rod 8 and a slider 9. The ring is wound and glued to a non-magnetic frame 7, which is connected to the slider 9 through a rod 8. Slider 9 s 50 It is attached to the power screw 10 with a precise thread and moves inside the gradiometer in the axial direction due to the rotation of the screw 10. "m The power screw 10 is the main part of the movement drive, which ensures smooth movement of the frame 7 and the trim ring b in the axial direction. In Fig. 2 schematically shows the design of the movement drive, which consists of a screw 10 inserted into the hole of a threaded coupling 11. This coupling 11 is inserted into the hole of the probe body 12, and the screw 10 moves in it. A silicone seal 13 is installed between the screw 10 and the cryostat cover. c The rotation of the screw 10 leads to the movement along the axis up and down the slider 9, the thrust axis 5 and the trim ring 6.

The speed of movement is determined by the pitch of the thread, and in this implementation is 5 revolutions per mm.

In this implementation, a simple method using the trim ring 6 is proposed for suppressing external interference. This method is based on the change in the effective inductance of the gradient meter coils due to the 60 mutual magnetic connection between the trim ring b and the upper 1 and middle 2 coils of the gradient meter.

The effectiveness of the proposed method is ensured by the fact that the trim ring 6 introduces a positive imbalance into the antenna, which is approximately equal in magnitude, but opposite in sign to the technological imbalance. When moving the trim ring down, the effective inductance of the upper coil 1 increases, and the lower C - decreases, so that an additional positive imbalance is introduced into the antenna, and thus, the initial technological balance 65 is compensated.

Therefore, in this useful model to suppress external interference, first the trim ring 6 is moved down until the output signal of the SQUID magnetometer 5 reaches a minimum. If this does not happen, the ring 6 moves down until the specified signal reaches a minimum. The minimum signal corresponds to the best balance of the gradient meter. When the balancing is complete, the trim ring b is securely fixed with a locking varnish (item 14, Fig. 2) applied between the screw 10 and the coupling 11. Kay varnish can be used

Tatreg Emidepi Zea! (EGESTKOG OVE I Admiviop oiNK., MUepijogii I itiea, Viake Noize, U/agogame, VegkKegpPige,

Epadiapa.

In the proposed implementation, antenna balancing is performed by one superconducting short-circuited three-ring b. The trim ring b is made of a thin wire with a diameter of 50 microns, from which the gradient meter is also made, which does not distort the magnetic flux inside the gradient meter. The specified ring 6 during manufacture is placed equidistantly between the upper and middle coils in the plane of turns. To avoid phase distortions, ring 6 is made of the same material from which the gradient meter is made, i.e. niobium.

The reason for the effect of the ring on the turns of the gradiometer is that under the effect of the quantization of the magnetic flux, a shielding current is induced in the ring, which tends to compensate for the magnetic flux introduced by the gradiometer. 7/5 When a ring is inserted between the coils of the gradiometer due to the magnetic connection, mutual shielding of the currents occurs.

According to the proposed method, it is first necessary to move the trim ring down, because, as a rule, the technological imbalance exceeds (in magnitude) the imbalance introduced by the ring. However, the balancing efficiency is based precisely on the fact that in the initial position (in the middle between the coils) the positive 2o imbalance of the trim ring is equal in magnitude and opposite in sign to the residual negative imbalance of the antenna.

Thus, the proposed method of balancing has a number of advantages over the known ones: 1. Balancing is carried out by only one balancing element - a short-circuited ring. 2. Absence of parasitic distortions of the horizontal components of the interference field due to small dimensions

Rings due to the small diameter of the wire, which reduces the mutual influence between the channels.

C. Strong sensitivity to the interference field due to the large diameter of the trim ring, approximately equal to the diameter of the antenna, which determines the small range of its necessary movement (between the middle and upper coils). 4. The high efficiency of the balancing procedure due to the fact that the initial position of the ring in the middle of M zo Between the middle and the upper coil introduces an imbalance into the antenna, opposite to the sign of the residual imbalance of the antenna. b"

Simplicity and unequivocal performance of actions due to the fact that there is a single position of the ring in which Ge! the condition of achieving balance is reached, therefore - in the range of movements there is only one minimum of the interference signal. Me

The essence of the useful model is demonstrated by the graphs of the output noise of the SQUID magnetometer using the EP in the o/r range of 0-17 Hz. The graph in Fig. 4 shows the spectral density of the output noise without any suppression, that is, without mechanical balancing and EPP. In this case, the output signal has maxima at 1/3 subharmonic of the mains frequency (50/3-16.6 Hz) and at low frequencies (several Hertz and below). When using only EPP, the suppression is quite effective (Fig. 5) both on NU and subharmonics of the industrial network (16-17Hz). But the obstacle level is still "

It remains significant, so one EPP is not enough. When using only mechanical balancing, both at 16.6 Hz and at LF, the noise decreased, but the suppression of subharmonics was more effective, because the subharmonic at 16.6 Hz was suppressed almost completely (Fig. b). Therefore, only mechanical balancing is not enough either, it must be combined with EPP.

When using both methods of balancing, the obstacles are reduced to a level so that they are practical

It can be neglected (Fig. 7). Thus, the method of mechanical balancing proposed in the invention allows (in combination with EPP) to almost completely suppress external interference in the entire spectrum of the useful MCG signal, from zero line drift to network interference and its subharmonics. at

Claims (1)

The formula of the invention about 50 "M The method of mechanical balancing of a superconducting magnetic field gradient meter, which includes the movement of a balancing element in a magnetic field, which differs in that at the beginning of balancing, the balancing element is placed in the middle between the compensation coils, and the balancing of the superconducting magnetic field gradient meter is carried out by moving the balancing element element along the Y axis of the cylindrical body within the distance between the compensating coils. 60 b5