"STABILIZER DEVICE FOR ROTARY MEMBERS"

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FIELD OF THE INVENTION

The present invention concerns a stabilizer device for rotary members substantially applicable to any rotary body able to transmit determinate torques and powers, in order to limit to the utmost the effects of imbalance and dynamic instability which normally occur due to the rotation thereof. In particular, the device according to the present invention has a particular application in the so-called heavy industries, that is, for the production of generators of electric energy, off-shore drilling systems, fast rotation conditioning systems, molecular turbo-pumps, pumps and compressors for the chemical and petrol industries with corrosive impact and lined rotor, fast electro-mandrels, rolling plants with active control systems of the vibrations, rotary machines for the printing and production of paper, or for textile production or other.

The device according to the present invention also finds advantageous application in the aerospace and motor industries, and also in MEMS (Micro Electro Mechanical Systems) in the field of telecommunications electronics. BACKGROUND OF THE INVENTION

It is known that a shaft, or other rotary member, in order to transmit a determinate torque and power, is normally subjected during its rotation to effects of imbalance and dynamic instability which, if not contained, can lead to its collapse. It is known that to compensate for the effects of imbalance and dynamic instability, different construction solutions are normally adopted; that is, whereas in order to contain the imbalance, the rotor is balanced with technologies to perfect assembly and distribution of the masses, during the construction and maintenance thereof, with regard to dynamic instability the remedy practiced is to inhibit the functioning beyond the threshold of instability.

For this last feature, it is also known to adopt suitable stratagems to increase the threshold value, substantially by reducing as much as

possible every type of damping associated with the moving parts and by increasing as much as possible the damping associated with the stator supports.

In practice, however, it is much easier to actuate the increase of the damping associated with the stator supports, since forms of damping such as structural damping, by which the elastic deformation of the rotor produces a dissipation due to hysteresis of the material which makes it up, are naturally associated with the rotating materials, and therefore are not easily controllable. From a constructional point of view, non-rotary damping is currently applied in literature and in trade by means of passive forms of damping devices and stabilizers such as lubricated bearings or mechanical supports with viscous damping, or, in more modern solutions, by means of magnetic devices with induced currents. Stabilizer devices of active form are also known, actuated by means of active magnetic bearings, in which the rotor is normally suspended, balanced and damped by means of opposite pairs of coils passed through by current and controlled by an electronic system outside the device.

Particularly in traditional active magnetic bearings, electromechanical forces are produced with electronic regulation of the rigidity in order to control the imbalance.

In all cases, in order to guarantee maximum effect on the imbalance, the need is considered to feed the coils that make up the magnetic bearings, by means of electric current, whose intensity and direction of circulation in the feed circuit are such as to guarantee electromechanical forces on the rotor, synchronous or slightly sub-synchronous, with respect to the excitation applied by the imbalance, to the speed of rotation of the rotor. This condition repeats what happens in the passive devices cited, where other forms of actuation, for example exerted by fluids, are precisely synchronous or sub-synchronous.

In all the known solutions, however, in the event of the rotation of the rotor at a speed comprised in its sub-critical field, with respect to the so- called critical speed of rotation, every form of damping provided by known

stabilizer devices has as its sole effect the damping of the precessions of the rotor around its axis of rotation.

On the contrary, if rotation occurs in the supercritical field, which in itself is favorable to contain the effects of the imbalance, known stabilizer devices do not guarantee an indefinite dynamic stability for every value of rotation, indeed, beyond the so-called threshold speed of rotation, the dynamic instability of the precession motions of the rotor sets in, due to the sudden growth of the amplitude of the precession, in an exponential manner, which can even cause the rotor to break. The seriousness of the phenomenon is such that beyond the threshold, at whatever speed the rotor rotates, the dynamic instability is active. The value of the threshold speed depends on the ratio between the damping offered by known stabilizer devices, not rotary, and that introduced by the rotating parts, that is, mainly the rotor. EP-A-1 621 785 describes a technique for increasing efficiency in the control of vibrations in molecular turbo-pumps suspended on radial magnetic bearings, equipped with position sensors. This technique consists in applying an appropriate control algorithm which, from the radial position readings arriving from each sensor, automatically detects if the precession of the rotor is direct (concord with the direction of rotation of the shaft) or retrograde (opposite the direction of rotation of the shaft) in order to apply a control force that operates separately on one precession and the other, so as to minimize the energy associated with each intervention. This known technique makes use of the active magnetic suspension also as a stabilizing component and promotes the minimization of the actuation energy, operating separately on the effects of the direct and retrograde precessions. However, a damping force is applied which, following the classical practice, functions as a fixed action in the stator reference system and increases the non-rotary damping contribution made by the stator. In practice, this known technique distinguishes the value of the non-rotary damping contribution useful for stabilizing the direct motions from the value stabilizing the retrograde motions. These latter, in the quite particular case of the molecular turbo-

pump, would be made instable by the effects induced by the vibration motions of the rotor bladings of the molecular turbo-pump, absent in a rotary shaft for other applications.

One purpose of the present invention is to achieve a stabilizer device which allows to increase the value of the threshold speed of the rotor, and which substantially eliminates the instable conditions of rotation in the range of normal working speed, allowing the rotor to rotate in the supercritical field, in an auto-centered condition, and in a condition of stable precession with considerable benefits for the supports. Another purpose of the present invention is to achieve a stabilizer device which requires reduced energy for active stabilization, but gives the same result as known stabilizer devices, guaranteeing minimum consumption.

The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purpose, a stabilizer device according to the present invention is applied to a rotary member provided with at least a shaft that rotates at a determinate speed and in a determinate direction of rotation. The stabilizer device according to the invention comprises means to stabilize the rotation, disposed around the rotary shaft and able to generate an electric or magnetic field to stabilize the rotation of the rotary shaft.

According to a characteristic feature of the present invention, the stabilizer device also comprises transducer means associated with the stabilization means, and able to detect the parameters both of radial displacement, of speed, of the direction of rotation of the rotary shaft, and according to said parameters detected, able to generate an asynchronous electric signal, with respect to the speed of rotation of the stabilized rotor,

able to feed the means to stabilize rotation, so that the latter generates an electric or magnetic field opposite and equivalent to the rotation parameters detected.

The stabilizer device according to the invention is therefore of the active type and automatically and dynamically generates a counter- rotating electric or magnetic field with a frequency and force selectively variable depending on the actual radial displacement of the rotation, the speed and direction of rotation of the rotor, so as to stabilize, on each occasion and selectively, the imbalance and dynamic instability. Applicant has found by experiments that the equivalent counter-rotation of the field generated, with respect to the rotation of the rotor, is extremely effective with regard to the stabilization of the rotation, under both supercritical and also sub-critical conditions, considerably increasing the value of the threshold speed for dynamic instability of the rotor. With the device according to the present invention, in fact, the stronger the opposite rotation or counter-rotation, the smaller the contribution of stabilization necessary to obtain the value of the threshold speed of the rotor, to compensate for the dynamic instability and imbalance due to the rotation. Moreover, we must point out that the technical solution according to the present invention is very different from those of the state of the art, including the one described in EP-A-1 621 785. In fact, whereas the latter describes the technique of stabilizing separately the direct and retrograde precessions by applying in practice a force originating from the coils of the magnetic suspension device in such a manner that it appears solid with the stator of the machine, in the present invention the instable motion of precession is stabilized by a force that rotates in the opposite direction to the rotation of the shaft. This innovative feature differentiates the energy consumption associated with the action of stabilizing the rotor, in practice reducing it considerably and to a degree that increases as the angular speed of counter-rotation applied to the electromechanical force created by the device in question increases. This operation raises the threshold speed of rotation for the dynamic instability of the rotor, and hence its

useful working field.

One advantage of the present invention is that it allows to integrate the commands necessary to stabilize the instability and balance of the rotation. In fact, with the present invention it is possible to operate in such a manner that the balancing component occurs in the synchronous control, and the stabilizing component occurs in the non-synchronous, and counter-rotating control.

In a preferential form of embodiment of the device according to the present invention, the transducer means comprises two or more position sensors disposed along two axes, substantially orthogonal with respect to each other, which intersect with the axis of rotation of the rotor, so as to monitor the radial displacements of the precession of the rotor. The transducer means also comprises a speed sensor, which in a preferential form of embodiment is an optical encoder, which sensor detects not only the value of the speed of rotation of the rotor, but also its direction of rotation.

Advantageously, the device also comprises an asymmetrical reference element, with a double and offset reference, mounted on the rotor member in correspondence with the speed sensor, to allow the latter to detect the direction and speed of rotation.

In a first form of preferential embodiment, the means to stabilize the rotation comprises a plurality of coils disposed angularly offset around the rotor, which are fed normally and as a function of the signal emitted by the transducer means, to generate a determinate counter-rotating magnetic field.

According to a variant, for example optimized for application of the present invention in micrometric scale, the means to stabilize the rotation comprises a plurality of components fed electrically, which generate a desired field of electrostatic forces instead of magnetic forces, while the field generated remains counter-rotating, coordinated with the rotation parameters of the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the present invention will become

apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic view of a rotary member to which a stabilizer device according to the present invention is applied;

- fig. 2 is a schematic view of the stabilizer device in fig. 1.

DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF

EMBODIMENT

With reference to the attached drawings, a stabilizer device according to the present invention, indicated in its entirety by the reference number

10, is applied to actuate the stabilization of rotation of a rotary shaft 11 , so as to limit to the utmost the imbalance and dynamic instability thereof, damping the effects of precession around its axis of rotation Z, which ideally joins the stator supports of the rotary shaft 11. The screened curved arrow in fig. 1 exemplifies the case of a direct precession of the axis of symmetry Z' of the rotary shaft 11 around the axis of rotation Z.

In this case, the stabilizer device 10 according to the present invention comprises four electromagnetic coils 12, 13 disposed in pairs angularly offset with respect to each other by 90° and disposed around the rotary shaft 11 , substantially coaxial with its axis of rotation Z.

It also comes within the scope of the present invention to provide that the stabilizer device 10 can provide a different number of coils 12, 13, other than four, for example eight, sixteen or more, in any case obtaining the same result. The stabilizer device 10 also comprises a transducer unit 15, associated with the rotary shaft 11 and electrically connected with the four electromagnetic coils 12, 13.

In this case, the transducer unit 15 comprises a pair of position sensors 16 and 17 and a speed sensor 19 associated with the rotary shaft 11 , and two command units 20 connected with the sensors 15, 16 and 19.

In particular, each position sensor 16 or 17 is disposed in correspondence with a respective radial axis X or Y, with which a corresponding pair of electromagnetic and opposite coils 12, 13 is

aligned, so that the relative command unit 20 is connected both to the sensors 16 or 17 and 19, and also to the relative pair of electromagnetic coils 12 and 13, so as to effect a proportionate correction for each axis X, Y. In this case, the two axes X and Y are substantially orthogonal to each other and to the axis of rotation Z.

Each of the two position sensors 16, 17 is able to detect a deviation of the position of the rotary shaft 11 from the axis of rotation Z along the relative axis X or Y, that is, to detect the value of radial displacement of the motion of precession of the rotary shaft 11 , and to emit a relative electric deviation signal "x", "y".

According to a variant of the present invention, not shown here, instead of the position sensors 16 and 17 the variation in the magnetic reluctance perceived by the coils 12 and 13 is monitored directly, due to the distancing or approach of the rotary shaft 11. In this technique, also known as "self-sensing", the current signal, which is generated by actuation, is distinguished from the current signal that the counter- electromotive force of the rotary shaft 11 applies, weakly, when it moves in the magnetic gap. The speed sensor in this case is an optical encoder 19 disposed at the side of the rotary shaft 11 in a position of non-interference with the electromagnetic coils 12 and 13. Advantageously, on the shaft 11 , in correspondence with the optical encoder 19, asymmetrical notches are provided, with a double and offset reference, of a known type and not shown here, which allow to recognize the direction of rotation of the rotary shaft 11 , as well as to measure its speed, by means of known techniques including, but not only, those known in the state of the art as "off sensor", "off track" or others.

Each command unit 20 comprises a logic adder 21 , a control operator 22 and a power amplifier 23, assuming that the position sensors 16 and 17 offer a suitably amplified signal compatible with the format of the logic operator concerned. The electric deviation signals "x" or "y" emitted by the relative position

sensor 16 or 17 are introduced into every logic adder 21 , to be processed according to a reference signal "r", which indicates the ideal functioning condition of the rotary shaft 11.

In this way, for every axis X, Y controlled by the position sensors 16, 17, relative error signals "e" are obtained, showing an error position.

After this processing, the error signals "e" are amplified by the relative power amplifier 23, which then feeds the corresponding pair of electromagnetic coils 12 or 13 in the form of asynchronous current, according to the value of deviation detected, the speed and direction of rotation detected, of the rotary shaft 11.

According to a characteristic feature, the asynchronous feed is such as to allow the relative pairs of electromagnetic coils 12 and 13 to generate a magnetic field "C" with a suitable intensity and with rotation parameters

(speed, direction of rotation) opposite those detected of the rotary shaft 1 1.

In this way, the phenomena of imbalance and those of dynamic instability of rotation are substantially stabilized, damping the effects of precession of the rotary shaft 11 around the axis of rotation Z.

The intensity of the magnetic field "C" is able to be selectively modulated, on each occasion, according to the variations in speed and direction of the rotation of the rotary shaft 11 detected by the speed sensor 19, and also according to its deviations of precession around the axis of rotation Z along the axes X and Y as detected instantaneously by the two position sensors 16 and 17. In this way, the stabilizer device 10 according to the present invention can thus constantly guarantee the optimum conditions of functioning and of rotation of the rotary shaft 11.

It is clear, however, that modifications and/or additions of parts may be made to the stabilizer device 10 as described heretofore, without departing from the scope of the present invention.

For example, it comes within the scope of the present invention to provide that the device 10 can be applied to devices in micrometric scale, such as for example the so-called MEMS (Micro Electro Mechanical

Systems). In this case, the electromagnetic coils 12 and 13 can be replaced by capacitors or condensers fed so as to develop electrostatic fields equivalent and opposite the speed and direction of rotation of the rotor to be stabilized. It is also clear that, although the present invention has been described with reference to specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of stabilizer device for rotary members, having the characteristics as set forth in the claims and hence all coming within the scope of protection defined thereby.