EP0170551A1 - X-ray apparatus with servo control of the focus position - Google Patents

Abstract

The invention relates to a radiological device with servo-position of a focus, said focus (f) being formed on the anode (9) of an X-ray tube (3) with magnetic bearings (PM1, PM2). The radiological device (1) comprises a focal point sensor device (14) cooperating with electronic means (12) associated with the magnetic bearings (PM1, PM2), to control the position of said focal point (f) by moving a rotor (8 ) under the action of said magnetic bearings (PM1, PM2).

Description

The present invention relates to an X-ray device with position control of a focus formed on the rotating anode of an X-ray tube with magnetic bearing. The invention is applicable in the general case of radiodiagnostics and in particular in computed tomography.

In radiodiagnostics, the X-ray used is generally produced from an X-ray tube carrying an anode, generally rotating, on which the impact of an electron beam determines a focus. This focal point is the source of X-rays.

The X-ray tube is installed in a protective sheath (with respect to high voltage and X-radiation), with which it constitutes an X-ray emitter assembly. When this assembly is mounted on a radiology device, it is in fact the only sheath which is fixed thereto, on the supports of its reference surfaces provided for this purpose. If the rigidity of this fixation can be fairly easily ensured, relative movements between the hearth of the X-ray tube and the mechanical references of the sheath which contains it are difficult to control.

• - la première est d'origine mécanique : les pièces de fixation qui lient le tube radiogène dans sa gaine sont obligatoirement constitués de matériaux isolants électriquement. Certains de ces matériaux ne sont pas rigides, et ne sont pas à proprement parlé des matériaux mécaniques, ces isolants ayant une trop grande flexibilité et une trop grande compressibilité ;
• - la seconde origine est due à des mouvements thermiques. Au cours de l'utilisation normale d'un ensemble radiogène, les différences de température auxquelles il est soumis sont considérables, particulièrement pour certains éléments du tube radiogène comme le disque d'anode, les différents axes, et le rotor auquel est couplée en rotation l'anode.

These movements have mainly two origins:

• - the first is of mechanical origin: the fixing parts which link the X-ray tube in its sheath must be made of electrically insulating materials. Some of these materials are not rigid, and are not strictly speaking mechanical materials, these insulators having too much flexibility and too much compressibility;
• - the second origin is due to thermal movements. During normal use of an X-ray assembly, the temperature differences to which it is subjected are considerable, particularly for certain elements of the X-ray tube such as the anode disc, the various axes, and the rotor to which is coupled in rotation. the anode.

It follows, between a cold start and a balanced temperature operation, non-negligible expansions (0.5 mm) which lead to deport the hearth, compared to an initial position of the latter at cold start. This displacement of the focal point due to the thermal movement takes place mainly along an axis parallel to that along which the anode disc is linked to the rotor.

The inevitable thermal movements of the focal point are detrimental to the functioning of radiological devices and, particularly in computed tomography where the quality of the image is limited by the defects in the acquisition of analog signals, particularly because of the drifts in the spatial position of the foyer.

The object of the present invention is to allow an improvement in the quality of X-ray imaging, by stabilizing the position of the focal point in space, along a longitudinal axis of the X-ray tube.

To this end, the present invention uses an X-ray tube of the magnetic bearing type, by modifying the organization of the traditional means associated with said magnetic bearings, by which the rotor is led to occupy an equilibrium position along radial axes and an axis. longitudinal. It is known that the magnetic bearings are made up of a set of stators, each stator itself being made up of a certain number of magnetic poles independent of each other and whose harmony of actions, managed by electronic control, leads to stabilize the position of the rotor in space. To do this, the control electronics must receive the information from the rotor position detectors, at all times, in order to correct if necessary by an action on one or more magnetic poles, a fault in the rotor position. It is known mathematically that a rotor (similar to a cylinder) has 5 degrees of freedom in a 3-dimensional space, and for magnetic bearings it is convenient to define 5 axes. These five axes are distributed in 1 longitudinal axis and 4 radial axes contained in the planes of two different straight sections; two radial axes of the same section being generally orthogonal and parallel each to one of the radial axes of the other section.

Thus, it is sufficient to place on each of these axes at least one position detector to be completely informed about the spatial position of the rotor. It is also necessary and sufficient for each position fault identified by a position detector, that an action is possible along the axis corresponding to this position detector by requesting a pole or a combination of poles. Also, the active magnetic bearings are generally organized in radial bearings, axial bearings or taper bearings. The radial and tapered bearings generally have 4 poles (2 poles on each slave axis), and the axial bearings 1 or 2 poles. The radial and axial bearings act only along a single axis while the conical bearings act both along a radial axis and along the longitudinal axis. Thus the 2 most well-known magnetic bearing assemblies are constituted as follows: for the first assembly, two radial bearings (each with 4 poles controlling 2 axes) and an axial bearing; for the second set, two 4-pole taper bearings. Each assembly comprising at least one position detector along the 5 axes of the 5 degrees of freedom previously mentioned.

In the radiological device of the invention, it is the action of the magnetic bearings on the rotor, along the longitudinal axis, which is exploited by means of a new arrangement which makes it possible to control the position of the rotor according to a desired focus position.

According to the invention, a radiological device with focal position control, comprising an X-ray tube with rotating anode on which said focal point is formed, said X-ray tube being of the magnetic bearing type and comprising a rotor coupled in rotation to said rotating anode according to a longitudinal axis around which it causes said anode to rotate, said magnetic bearings being controlled by electronic means, said rotor being capable of displacement along said longitudinal axis under the action of said magnetic bearings, is characterized in that it also includes a focus position sensor device providing a variable signal as a function of a position variation of said focus along a servo axis parallel to said longitudinal axis, and in that said signal is applied to electronic means in order to control, via said magnetic bearings, a movement of the rotor along said longitudinal axis in a opposite direction to that of said focus.

• - la figure 1 montre schématiquement la structure générale d'un dispositif radiologique selon l'invention ;
• - la figure 2 est un schéma montrant une organisation de moyens associés à des paliers magnétiques d'un tube radiogène, dans une première version du dispositif radiologique selon l'invention ;
• - la figure 3 est un schéma montrant les moyens associés aux paliers magnétiques, dans une seconde version du dispositif radiologique de l'invention.

The invention will be better understood thanks to the description which follows, given by way of nonlimiting example, and to the three appended figures among which:

• - Figure 1 schematically shows the general structure of a radiological device according to the invention;
• - Figure 2 is a diagram showing an organization of means associated with magnetic bearings of an X-ray tube, in a first version of the radiological device according to the invention;
• - Figure 3 is a diagram showing the means associated with magnetic bearings, in a second version of the radiological device of the invention.

For clarity, the same elements are given the same references in the three figures.

FIG. 1 schematically shows the general structure of a radiological device 1 according to the invention, the representation of which is limited to the means necessary to understand the invention.

The radiological device 1 comprises a sheath 2, containing an X-ray tube 3 (partially represented in a frame in dotted lines) of the type with magnetic bearings (not shown in FIG. 1). The sheath 2 is assembled with a beam limiting device 4, and it is conventionally supported (not shown) above a patient support panel 5 (partially shown), under which is also conventionally arranged means X-ray receiver 6.

The X-ray tube 2 has a longitudinal axis 7, along which is disposed a rotor 8 integral with a rotating anode 9. The X-ray tube 2 produces, from a focal point f formed on the anode 9, a beam of X-ray radiation 10, to which the limiting device 4 determines limits 11.

As will be explained further in a description which follows in FIG. 2, the rotor 8 is capable of displacement along the longitudinal axis 7, under the action of the magnetic bearings (not shown in the figure). 1), themselves controlled by electronic means 12 associated with these magnetic bearings.

This possibility of displacement of the rotor 8 is used in the radiological device 1 according to the invention, to control the position of the focus f along a control axis 13, parallel to the longitudinal axis 7; the movement of the rotor 8 taking place as a function of an observed movement of the focus f along this servo axis 13.

To this end, the radiological device 1 according to the invention further comprises a focal point sensor device 14, capable of supplying an electrical signal - SD indicating that the focal point f is moving along the servo axis 13 , in either of the opposite directions shown by the first and second arrows 15, 16.

Such a device 14 for detecting the focus position can be constituted as in the example described by detector means 17, 18, sensitive to X-radiation, on which an image (not shown) of the focus f is produced.

In the nonlimiting example described, the sensor device 14 of the hearth position comprises a support structure 19, provided with an opening 20 oriented towards the hearth f; the sensor device 14 being secured by fixing means 21, to the receiving means 6, which receiving means 6 constitutes in the nonlimiting example described, a reference member with respect to which it is desired to control the position of the focus f. The sensor device 14 comprises a pinhole 22, by which the image of the focal point f is produced on input planes 23, 24 of the detector means 17, 18. (We understand by pinhole any means comprising an opening, such as a slot for example or a fine hole, allowing to realize the image of a hearth).

In the nonlimiting example of the description, the two detector means 17, 18 are arranged contiguously, and are separated by a fine partition 27, which projects onto the entry planes 23, 24 along a median line which, being arranged in a plane perpendicular to Figure 1, is coincident with a central point 25. This central point 25 constitutes the center of a zone Z, on which the image of the focus f is produced by a pinhole 22, of a equally distributed on each of the two entry planes 23, 24. The central point 25 or median line, and the pinhole 22 define a reference axis 26, which it suffices to orient so that it passes through the focal point f , to obtain this condition of equal distribution of the image of the focal point f on the two input planes 23, 24. Each detector means 17, 18 delivers an output signal S ₁ , S ₂ , variable as a function of the illumination of its entry plane 23, 24 by the image of the foyer f. Also, the two detector means 17, 18 having in the nonlimiting example described, similar characteristics, the output signals respectively S ₁ , S _{2 which} they each deliver, have the same amplitude when the image of the focus f is also distributed on each entry plane 23, 24.

In this configuration, the comparison of the output signals S ₁ , S ₂ provides information, indicating whether the image of the focus f is also distributed over each of the input planes 23, 24 or not, that is to say whether the reference axis 26 passes through the focal point f or not; the comparison between the output signals S ₁ , S _{2 also} making it possible, according to the direction of their difference, to know the direction 15, 16 according to which the focus f has moved along the servo axis 13, with respect to a point I where the reference axis 26 intersects the servo axis 13.

In the nonlimiting example described, the means used to compare the output signals S ₁ , S ₂ consists of a differential amplifier 28, to which these output signals S ₁ , S ₂ are applied. The differential amplifier 28 delivers a difference signal ^: SD, expressed by a voltage whose value is equal to zero when the reference axis 26 passes through the focus f and whose value is non-zero otherwise; the polarity of the difference signal ± SD being positive or negative, depending on whether the focus f has moved in the first or second direction 15, 16 relative to the point of intersection I.

The radiological device 1, according to the invention, makes it possible to control the position of the focus f relative to the point of intersection h considered as the initial position of the focus f.

To this end, the difference signal ± SD, delivered by the focal point position sensor device 14, is applied to the electronic means 12 which, traditionally are responsible for managing the operation of the magnetic bearings.

The sensor device 14 is arranged so that the reference axis 26 passes through the focal point f; this condition being verified by measuring using conventional means (not shown), the difference signal ± SD, which in this case has a value equal to zero.

The positioning of the reference axis 26 can be obtained at the level of the fixing means 21 for example, by causing them to rotate about a fixing axis 30, according to the third arrow 31, and by moving the pinhole 22 along a second axis 32; this movement of the pinhole 22 can be effected in one or the other of the directions shown by the fourth and fifth arrows 33, 34 by means of displacement 35 of conventional type, by means of which the pinhole 22 is secured to the support structure 19.

It should be noted that the focal point sensor device 14 is arranged at the periphery of the X-ray beam 10 near a limit 11 of the latter, so as to allow normal operation of the radiological device 1 in the context of a radiodiagnostic.

FIG. 2 is a diagram which shows, in a first version of the radiological device 1 of the invention, the way in which the focal point sensor device 14 is associated with the magnetic bearings, by means of the electronic means 12.

The X-ray tube 3 comprises in known manner, in an envelope 38, a cathode 37 held opposite the anode 9 by a support 36. In operation, the rotating anode 9 is rotated about the longitudinal axis 7, in the direction shown by a sixth arrow 40 for example, and comprises the focal point f formed from the cathode 37. The rotating anode 9 is secured by a shaft 41, to the rotor 8 which ensures its rotation by means of a stator 42.

In the nonlimiting example described, the lift of the rotor 8 during its rotation is ensured by magnetic bearings, of the type comprising a first and a second conical bearing PM ₁ , PM ₂₉ located on the same side with respect to the rotating anode 9.

As previously explained in the preamble, the bearings each comprise, in a known manner, 4 magnetic poles arranged two by two along radial axes. In FIG. 2, the first and second bearings PM ₁ , PM ₂ are each represented only by two poles arranged along a first and a second radial axis 47, 47a contained in the same plane as that of the figure: either for the first bearing PM ₁ , a first and a second magnetic pole respectively 43, 44 arranged on either side of the rotor 8 along the first radial axis 47, and for the second bearing PM ₂ , a third and a fourth magnetic pole respectively 45, 46 arranged along the second radial axis 47a. Being arranged along radial axes (not shown) perpendicular to the plane of Figure 2, the other two magnetic poles that each have bearings PM ₁ , PM ₂ are not shown for clarity of this figure.

In the nonlimiting example described, the rotor 8 is hollow and rotates around a fixed shaft 52, provided with two guard bearings 53, on which the rotor 8 comes to bear when it is not kept in equilibrium; a distance (not shown) formed between a longitudinal inner wall 55 of the rotor 8 and, the guard bearings 53, represents a space in which the rotor 8 can move along the radial axes 47, 47a. The rotor 8 can also move along the longitudinal axis 7, between shoulders inner elements 56, which is provided with the rotor 8 and on which the guard bearings 53 come to bear when the rotor 8 is moved in the directions shown by the first and second arrows 15, 16.

Thus conventionally, along the radial axes 47, 47a, the balance of the rotor 8 is obtained by an action of the first and fourth magnetic poles 43, 46 which are simultaneously opposed to the second and third magnetic poles 44, 45; the balance of the rotor 8 along the longitudinal axis 7 being obtained by the action of the first and second magnetic poles 43, 44 which oppose the action of the third and fourth magnetic poles 4 5.46. The action of the magnetic poles 43 to 46 takes place under the control of the electronic means 12.

To this end, the electronic means 12 comprise, in a known manner, comparison and control means 70 which deliver a control signal to each magnetic pole that comprises a bearing. In the nonlimiting example described where, the magnetic poles arranged along radial axes perpendicular to the plane of Figure 2 are not shown, the representation of these commands is limited to four commands C ₁ , C ₂ , C ₃ , C4 applied first, second, third and fourth magnetic poles 43, 44, 45, 46 respectively.

The control signals C ₁ , ..., C4 are generated as a function, on the one hand, of signals produced by radial position detectors (not shown) which the bearings PM ₁ , PM ₂ conventionally comprise, and on the other hand as a function of axial position signals (not shown in FIG. 2) relating to the position of the rotor 8 along the longitudinal axis 7, delivered by an axial position detector 50; the combination of these different means constituting control loops known to those skilled in the art.

In the operation according to the prior art, the position of the rotor 8 is permanently maintained at a so-called reference position P _r . This reference position P is, for example, as shown in FIG. 2 by a line in dotted lines which coincides with the plane of an inner lateral wall 59 of the rotor 8; the rotor 8 being movable along the longitudinal axis 7, on either side of this reference position P, within the limits - P, + P imposed by the shoulders 56, as has been previously explained. The reference position P is generally adjusted by a voltage level, which constitutes a reference voltage V to which an axial position signal (not shown in FIG. 2) delivered by the axial detector 50 is compared. This comparison makes it possible to '' develop an error signal SE from which, via the comparison and control means 70, the magnetic poles 43 to 46 are controlled to cause movement of the rotor 8 in the direction 15, 16 suitable for replacing this last at the reference position P _r .

In this spirit, the electronic means 12 comprise, as in the prior art, a comparator means 72 to which, in this first version of the invention, is applied to a first input +, a reference voltage V supplied by a voltage source 73.

But in the radiological device 1 according to the invention, an important difference compared to the prior art lies in that the sensor device 14 of the focal position participates in the development of the error signal SE which is applied to the means of comparison and order 70.

It should be noted that the focal point sensor device 14 described above has an output voltage, corresponding to the first difference signal - SD, which depends on the amount of X-ray radiation delivered by the X-ray tube 3, except in the case where the focus f is perfectly centered on the reference axis 26; in this case, as previously explained, this voltage has a zero value, which is retained even with variations in the amount of X-ray radiation. It is under these conditions that the invention will be best used if the sensor device 14 is of the type which has been described.

It should also be noted that the main function of the leveling means 75 is to adapt the level of the first signal difference - SD at the required level, that is to say at the level of the signal which would have been produced by the axial detector 50. This leveling means could be omitted, and the first difference signal ± SD could be applied directly to the comparator means 72, in particular if the focal point sensor device 14 is of the type previously described; in this case, the reference voltage V must also be zero.

In the example of the first version of the invention shown in FIG. 1, the difference signal ± SD is applied to a leveling means 75, comprising for example an amplifier (not shown), which delivers a second difference signal SD 'applied to the second input - of the comparator means 72. The signal SD' then has a value similar to the signal which could be delivered by the axial position detector means 50 (not used), when the rotor 8 occupies the along a longitudinal axis 7 a reference position P.

r

In the nonlimiting example described, the second difference signal SD 'is applied to the comparator means 72, by means of a switch means 81, manual or automatic, the function of which is explained in a continuation of the description.

In this configuration, the second difference signal SD 'has an initial value which represents a position of the focal point f in which, as has been explained previously, the reference axis 26 passes through this focal point f at a point I on the servo axis 13; this initial position I of the focal point f corresponding to an initial position PI of the rotor 8 along the longitudinal axis 7, this initial position PI being in the example of FIG. 2, merged with the reference position P.

Thus a displacement Δ _f of the focus f in one or the other of the directions shown by the first or second arrows 15, 16, bringing it to a position P _l or P ₁ 'on the servo axis 13, determines a variation of the second difference signal SD 'applied to the comparator means 72, and a variation of the level of the error signal SE applied to the comparison and communication means 70, which by magnetic poles 43 to 46; determine a displacement Δ of the rotor 8 of the same value as the displacement at the focal point f but in the opposite direction; the movement of the rotor 8 taking place until the focus f returns to the initial position I.

It should be noted that this operation requires the X-ray tube 3 to produce X-ray radiation. Also, to maintain the rotor 8 in a fixed position outside a pose (in which X-ray radiation exists), it is necessary to provide a signal intended to replace the one originating from the focus position sensor device 14. To this end, in the nonlimiting example described, the radiological device 1 also comprises a second voltage source 80 which delivers a replacement signal SR. This replacement signal SR can, for example, have a fixed value representing the position of the rotor 8 "cold", or be constituted by the stored signal (not shown) of the second difference signal SD ', then representing the position of the focus f at the end of the last pose.

The switch means 81 receives the replacement signal SR on a first pad 82, the second difference signal SD 'on a second pad 83, and applies one or the other of these signals SD', SR, to the input - comparator means 72; the control of the switch means 81 being carried out at the start and end of installation, by conventional means not shown.

FIG. 3 shows another embodiment of the invention, in which the axial position detector 50 is used.

The representation of the X-ray tube 3 is limited to the rotating anode 9, linked to the rotor 8 by the shaft 41; the magnetic poles 43, 44, 45, 46 being controlled, as in the previous example, by the comparison and control means 70.

In this second version of the invention, the axial position detector 50 delivers an analog signal SA, linked to the position of the rotor 8 along the longitudinal axis 7; this function being the known function of this detector.

The analog signal SA is applied to the input + of the comparator means 72. The comparator means 72 receives on its other input -, the second difference signal SD 'via the switch means 81; the second difference signal SD 'being, as has been explained above, relating to the position of the focus f along the servo axis 13.

As in the previous example, the focus position sensor device 14 participates in the preparation of the error signal SE, from which the action of the magnetic poles 4 to 46 is generated for positioning the rotor 8 on along the longitudinal axis 7. But in this latter configuration, the second difference signal SD 'constitutes a variable reference voltage, while in the previous example, the reference voltage V is fixed. Under these conditions, the system no longer functions as a comparator with respect to a fixed reference voltage but with respect to a reference voltage SD 'which varies according to the position of the focus f along the servo axis 13. This situation requires a calibration or a calibration of the second difference signal SD 'with respect to the analog control signals C ₁ to C ₄ applied to the magnetic poles 43 to 46 to control the position of the rotor 8, this calibration could consist, for example, in reducing or to increase the amplitude of variations called Δ V _f of the second difference signal SD ', according to which the latter expresses the variations in positions of the focus f along the servo axis 13, with respect to the initial position I of the home f.

kf est un premier coefficient de réponse lié à une première voie de mesure représentée par le capteur de position 14 et le moyen de mise à niveau 75 ; k_r est un second coefficient lié au système de paliers magnétiques, c'est-à-dire à une seconde voie représentée par le détecteur axial 50, les moyens électroniques 12, le rotor 8, les paliers magnétiques PM₁, PM₂.It is thus possible, in addition to maintaining the focus f at the initial position I, a stabilization of the position of the focus f which consists in attenuating an effect of position drift of the focus f, by under-compensation or on the contrary an over-compensation. compensation. Indeed, if the variation at V _f of the second difference signal SD ', relating to the position variation Δ f of the focal point f, is equal to k _f , Δ _f , the system of magnetic bearings will be called upon to act on the rotor 8 in the opposite direction to the movement of the hearth f, the movement Δ of the rotor 8 will be translated by a variation of voltage AV by the detector of axial position 50 equal to:

kf is a first response coefficient linked to a first measurement channel represented by the position sensor 14 and the leveling means 75; k _r is a second coefficient linked to the magnetic bearing system, that is to say to a second channel represented by the axial detector 50, the electronic means 12, the rotor 8, the magnetic bearings PM ₁ , PM ₂ .

When the rotor 8 is returned to the position with a value Δ _r , the anode 9 carrying the focal point f is also displaced by a value Δ _r , producing another variation Δ f 'of the position of the focal point f translated by another variation Δv'f of the difference signal SD ', equal to:

soit:

Balance will be restored when the voltages SA and SD 'are again equal, and therefore:

is:

et si ces deux coefficients sont de même signe + ou -, le rapport

1 et les dérives du foyer f sont sous-compensées.The relative position variations between the rotor 8 and the focus f will be equal to:

and if these two coefficients have the same sign + or -, the ratio

1 and the drifts of the focus f are undercompensated.

It should be noted that the first coefficient kf is preferably very large compared to the second coefficient k _r , and in this case the ratio of the position variation Δ f of the focus f to the position variation Δ of the rotor 8 is little different from 1 ; the signs +, - assigned to each coefficient kf, k _{r which} can be defined for example at the level of the detector means 17, 18 in the sensor 14, and at the level of the inputs of the comparator means 72.

In either of these cases, a variation in the position A f of the focus f accomplished for example in the second direction 16, from the initial position 1 to the position P ₁ ′, is compensated by a displacement Δ of the rotor 8 in an opposite direction, the displacement A of the rotor 8 having a value lower or greater than that of the displacement Δ f depending on whether there is over-compensation or under-compensation. In the example of the latter case, the focus f is brought back, on the servo axis 13 to an intermediate position P3 between the initial position I and the displaced position P ₁ '.

This description of the radiological device 1 according to the invention constitutes a nonlimiting example, the radiological device 1 being able for example to include an X-ray tube 3 provided with magnetic bearings of radial or axial type, the use of which in the spirit of the invention is obvious to those skilled in the art.

The radiological device 1 of the invention makes it possible to control the position of the focal point of a rotating anode of an X-ray tube, along a longitudinal axis of the X-ray tube or parallel to this longitudinal axis, thanks to a new arrangement in which bearings magnetic of the X-ray tube are combined with a focus position sensor device; the focus position sensor device can also be of a different type than that described, the main thing being that it takes part in the preparation of the signals which, in the magnetic bearings, determine the axial position of the rotor.

Claims (14)

1. Radiological device with focal position control, comprising an X-ray tube (3) with a rotating anode (9) on which said focal point (f) is formed, said X-ray tube (3) being of the magnetic bearing type (PM ₁ , PM ₂ ) and comprising a rotor (8) coupled in rotation to said rotary anode (9) along a longitudinal axis (7), around which it causes said anode (9) to rotate, said magnetic bearings (PM ₁ , PM ₂ ) being controlled by electronic means (12), said rotor (8) being capable of displacement along said longitudinal axis (7) under the action of said magnetic bearings (PM ₁ , PM ₂ ), characterized in that it further comprises a focus position sensor device (14) providing a difference signal (± SD, SD ') variable as a function of a position variation (Δf) of said focus (f) along an axis d 'servo (13) parallel to said longitudinal axis (7), and in that said difference signal (t SD, SD') is applied to the electrical means tronic (12) in order to control, via said magnetic bearings (PM ₁ , PM ₂ ), a movement (dr) of the rotor (8) along said longitudinal axis (7) in an opposite direction (15, 16) to that of the displacement (6 f) of said focus (f). 2. Radiological device according to claim 1, characterized in that the electronic means (12) comprise a comparator means (72) intended to generate an error signal (SE) used to determine the action of the magnetic bearings (PM ₁ , PM ₂ ) on the rotor (8) along said longitudinal axis (7), said difference signal ( ⁱ SD, SD ') relating to the position of the focus (f) being applied to said comparator means (72). 3. X-ray device according to claim 2, characterized in that the electronic means (12) comprise a voltage source (73) delivering a reference voltage (V) applied to a first input (+) of the comparator means (72), and in that said difference signal (± SD, SD ') is applied to a second input (-) of said comparator means (72). 4. X-ray device according to claim 2, characterized in that it further comprises an axial position detector (50) delivering a signal (SA) relating to the position of the rotor (8) along the longitudinal axis (7 ), said signal (SA) being applied to the first input (+) of the comparator means (72), and in that said difference signal ( ^f SD, SD ') is applied to the second input (-) of said comparator means (72). 5. X-ray device according to claim 4, characterized in that said focal point (f) and said focal point sensor device (14) represents a first measurement channel having a first coefficient (kf) of given sign (+, -) , and in that the axial position detector (50), the electronic means (12), the magnetic bearings (PM _{l '} PM ₂ ) and the rotor (8) constitute a second measurement channel having a second coefficient (k) of sign (+, -) given. 6. X-ray device according to claim 5, characterized in that the ratio of the first coefficient (kf) to the second coefficient (kf) is greater than 1 ( ^kf _kr > 1). 7. X-ray device according to claim 6, characterized in that the coefficients (kf, k) are of opposite signs (+, -) so as to overcompensate for variations in the position of the focus f. 8. Radiological device according to claim 5, characterized in that the coefficients (kf, k) are assigned the same sign (+, -), so as to under-compensate the variations in position of the focal points (f). 9. X-ray device according to claim 1, characterized in that the electronic means (12) comprise a leveling means (75), by means of which the difference signal (± SD) is applied to the comparator means (72 ) in the form of a second difference signal (SD '). 10. X-ray device according to claim 2, characterized in that the electronic means (12) comprise a second voltage source (80) providing a replacement signal (SR) applied to the comparator means (72) in the absence of X-radiation . 11. Radiological device according to claim 1, characterized in that the sensor device (14) of the focal position comprises a pinhole (22) and two detector means (17, 18) sensitive to X-rays, contiguous, on which is carried out a image of said focus (f). 12. Radiological device according to claim 11, characterized in that said sensor device (14) further comprises a means of comparison (28) of output signals (S ₁ , S ₂ ), delivered by said detector means (17, 18 ) as a function of a distribution of the image of said focal point (f) on their respective entry planes (23, 24). 13. X-ray device according to claim 11, characterized in that the sensor device (14) comprises a reference axis (26) determined by said pinhole (22) and said detector means (17, 18), said reference axis (26 ) passing through said focal point (f) at a point (I) constituting an initial position of said focal point (f). 14. X-ray device according to claim 1, characterized in that it further comprises a reference member (6), to which said sensor device (14) for the focal position is secured.

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