UNSTABLE VIBRATION PREVENTION APPARATUS FOR MAGNETIC BEARING SYSTEM

Abstract: An active type magnetic bearing system includes magnetic bearings (112, 113) for suspending a rotary body (1) to permit it to rotate about its rotational axis. A control system (118) supplies the magnetic bearings (112, 113) with signals to control the radial position of the rotary body, and comprises displace­ ment detecting circuits (2, 3, 13, 14) for detecting any displacement of the radial position of the rotary body (1) and a control circuit (118) for producing a control signal which serves to suppress any unstable vibration of the rotary body.

Description: The present invention generally relates to a magnetic bearing system, and more particularly, to an unstable vibration prevention apparatus designed for an active type magnetic bearing system adapted to control a magnetic force by using an electronic circuit. A magnetic bearing is capable of suspending an object with a magnetic force in a completely non-contact state and therefore has a wide variety of features. Such magnetic bearings show the following advantages by virtue of the absence of mechanical contact: (1) no abrasion is created, and hence the life thereof is semipermanent; (2) both vibrations and noises are small with no abrasion occurring; (3) rotational drag (a loss associated with rotation) is small, and high-speed rotation can be attained; (4) magnetic bearings are usable under such special circumstances as operation in a vacuum, at relatively high and low temperatures and in a radioactive atmosphere; and (5) a perfectly oil-free system is practicable due to the absence of any need for lubricants. In addition, a control type magnetic bearing adapted to control a magnetic force has the following advantages: (1) the stiffness and damping characteristics of the bearing are freely adjustable; (2) since a wide bearing-gap is capable of being set, the position of the rotor can be moved with minute precision; and (3) a displacement sensor can be built-in the system, which arrangement facilitates the monitoring of the system's operation. In the conventional active type magnetic bearing system wherein an electronic circuit serves to control the magnetic force to adjust the radial position of a rotary body, instability of the rotary body becomes more marked as the rotational frequency of the rotary body increases. In the range of low stiffness and damping within lower frequencies, whirling (unstable vibration) of the conical mode in which the direction of the whirling is opposite to the rotational direction is in some cases generated. Moreover, in the range of higher frequencies, whirling of the conical mode wherein the whirling is effected in the same direction as that of rotation is likewise produced if the damping force decreases due to internal damping or other causes. This kind of whirling will be explained more fully later. Such unstable vibration is propagated to a casing via a fixed member on which the stator of the magnetic bearing is mounted, thereby causing resonance of the structure. This results in augmentation of the control current of the magnetic bearing, which in turn results in the generation of heat. In addition, the unstable vibration sometimes abruptly diverges and an uncontrollable state is thereby induced. Such unstable vibration can be prevented by increasing the gain of a control system to heighten the stiffness of a bearing and increment the damping. In order to increase the gain, however, the capacity of control circuit must be raised. Such an increase in the capacity of the control circuit raises the problem of increased vibration during the rotation and excited resonance of the structure. Since the above-described unstable vibration corresponds to the self-excited vibration of the rotor, if the natural frequency causing an unstable vibration is set to be within a proper range at the design stage, such unstable vibration can be avoided. However, when a variety of constraints exist in regard to the structure of the rotary body and the like, in most cases it is difficult to arbitrarily set the natural frequency. In addition to the above mentioned prior art attention is further drawn to GB-A-2 129 582 which discloses in substance an active magnetic bearing system as set forth in the preamble of claim 1. The prosent invention has been made in the light of the above-mentioned defects inherent in the prior art and has the object of providing an unstable vibration prevention apparatus capable of effectively attenuating or preventing unstable vibration of a rotary body. After conducting various studies, the inventors have found that in order to prevent or backward vibration (forward rotational nutation or backward rotational precession) of a rotational axis, it is necessary to decide the mode of the unstable vibrations and to apply to a fundamental control circuit a signal which is advanced by 90° in the whirling direction of the unstable vibration - i.e., a signal the phase of which is advanced by 90° in the direction of displacement of the unstable vibration on the rotational axes. In other words, in order to prevent the unstable vibration, it is necessary to take the steps of: assuming two control axes perpendicular to the rotational axis of a rotary body; deciding a whirling direction of the rotational axis in one of the control axes; producing a signal indicating such a whirling direction; and adding this signal to a signal indicating a displacement of the rotational axis detected in the other of the control axis. The signal indicating the whirling direction detected in one axis is considered, with respect to the other axis, as a signal which is advanced by 90° in the direction of the whirling, or, a signal which is advanced by 90° with respect to the displacement detected in the other axis. Since the signal which is advanced by 90° in the direction of unstable vibration displacement corresponds to a signal proportional to the velocity, a control force proportional to this signal can serve as an damping force. Consequently, the signal advanced by 90° is applied to the control circuit and then added to the other signal. It therefore follows that the damping force with respect to the whirling increases. In an active type magnetic bearing according to the present invention, if whirling, viz., unstable vibration, is generated during rotation of a rotary body, the slant angle of the whirling of the rotary body is at first detected from the outputs of two displacement sensors disposed at different positions in the direction of the rotational axis. This step allows a decision to be made as to whether the unstable vibration being generated is in the parallel mode or the conical mode. If the unstable vibration is in the parallel mode, the slant angle of the whirling is 0°, and an output signal of a detecting circuit is also zero. When the whirling is in the conical mode, the output from the detecting circuit will not be zero. The next step is to decide whether the whirling direction of the unstable vibration is the same as the rotational direction of the rotary body (a forward rotation) or is opposite to the rotational direction (a backward rotation). This is determined by passing the output signal of the detecting circuit through a low-pass filter and a band-pass filter. This decision utilizes the fact that, in the output signal of the detecting circuit, a high frequency band region includes the frequency of the whirling that takes place in the same direction as that of the rotary body's rotation (the forward rotation), while a low frequency band region inciudes the frequency of the whirling that occurs in the opposite direction (the backward rotation). Based upon the above-mentioned decisions, a signal is produced indicating whether the unstable vibration is defined as whirling in the conical mode of the forward rotation of whirling in the conical mode of the backward rotation. In the plane perpendicular to the plane that includes the above-mentioned sensors, a decision is also made as to whether the whirling direction of the unstable vibration is forward or backward, and a signal indicating the whirling direction is thus generated. The signal which indicates the whirling direction with respect to one of the planes is added to signals which indicate the vibration displacement of the rotational axis in the other plane. Thus, the signal indicating the whirling direction is considered as a signal which is advanced by 90° in the whirling direction, i.e., a signal which is advanced 90° with respect to the displacement of the rotational axis, and is added to a signal indicating the displacement of the rotational axis. This means that a force which is advanced by 90° in the direction of displacement and is proportional to the rotational speed of the rotary body is added to increase the damping force with respect to the whirling. Because the instability increases or decreases as the rotational frequency of the rotary body increases or decreases, an adjuster is provided to adequately augment or reduce the signal indicating the whirling direction in response to any increase or decrease in the rotational frequency of the rotary body, whereby optimum control can be achieved in correspondence with any variation in the rotational frequency. Thus, in accordance with the present invention an active type magnetic bearing system as set forth in the preamble of claim 1 is characterized by the features of the characterizing portion of said claim. Preferred embodiments of the invention are disclosed in the dependent claims. The above and other related objects, features and advantages of the invention will become more apparent on reading the following detailed description with reference to the accompanying drawings. Fig. 1 shows an embodiment of an unstable vibration prevention apparatus for use with an active type magnetic bearing according to the present invention; Fig. 2 is an explanatory view depicting the whirling caused by unstable vibration of a rotary body suspended by such an active type magnetic bearing; and Fig. 3 shows a block diagram illustrating one example of the constitution of a control circuit of the type depicted in Fig. 1. In these figures, like elements and components are marked with the same reference numerals or symbols. An outline description of the construction of a magnetic bearing system according to the present invention will hereinafter be given with reference to Figs. 1 and 2. A rotary body generally indicated at 1 is suspended by radial magnetic bearings 112 and 113 at two positions in the direction of the axis Z so that the rotary body is rotatable about the axis Z by means of a motor (depicted by a dotted line). The respective magnetic bearings 112 and 113 include a combination of permanent magnets and electromagnets. The permanent magnets operate to flow a bias flux through control magnetic pole pieces to make the relationship between the suspending force and the flux density linear. The position control of the rotary body 1 is effected by making addition or subtraction between the bias flux by the permanent magnets and the flux generated by the electromagnets. As illustrated in Fig. 2, planes perpendicular to the rotational axis Z at both ends of the rotary body 1 are assumed; and two pairs of axes X₁ and Y₁, and X₂ and Y₂ which are perpendicular to each other are set on these planes. These four axes are considered as control axes with respect to the rotary body 1. More specifically, displacement sensors 2, 3, 13, and 14 are disposed on the control axes X₁, X₂, Y₁ and Y₂, respectively; and outputs from the displacement sensors are supplied to a control circuit 118. The construction and operation of the control circuit 118 will be explained later in detail. Electromagnets 10, 12, 21, and 23 are provided on the control axes X₁, X₂, Y₁ and Y₂ so as to control the position of the rotary body 1. The control circuit 118 serves to process the signals transmitted from the displacement sensors 2, 3, 13, and 14, and, as a result of this processing, signals are fed back to the electromagnets 10, 12, 21 and 23, thereby obtaining the static and dynamic stabilities of the rotary body 1. These displacement sensors are the same in construction. Fig. 1 shows one example of the construction of the displacement sensor 2. The displacement sensor 2 includes a pair of non-contact type series-connected coils 114 and 114′ for detecting radial positions. One end of one of the coils 114′ is connected to a oscillator 116. The difference between the outputs from the junction of the coils 114 and 114′ shows the amount of displacement of the rotary body and is amplified by a differential amplifier 117, which, in turn, compares such a difference with a signal indicating the reference axial position. The difference therebetween is supplied to the control circuit 118. It should be noted that the numerals 115 and 115′ designate coils for detecting the radial positions within the displacement sensor 3 disposed on the control axis X₂. As described above, the magnetic bearings 112, 113 have the electromagnets 10, 12, 21 and 23 each combined with permanent magnets to obtain bias magnetic fluxes from these permanent magnets. Since the magnetic bearings 112 and 113 are capable of acquiring the axial restoring force of the rotary body 1 by utilizing the bias magnetic fluxes, no control in the direction of the rotational axis Z is applied to the rotary body 1. It should be noted that magnetic bearings having electromagnets but not permanent magnets may also be used in this embodiment. In such a case the magnetic force generated by the electromagnets is adjusted to control the suspending force. When the rotary body 1 is rotated about the axis Z by the motor, whirling caused by the unstable vibration of the rotary body 1 is produced in the rotary body 1. The whirling is effected either in the same direction as that of the rotation of the rotary body 1, i.e., in the forward rotation, or in the direction opposite to the direction of rotation of the rotary body 1, viz., in the backward rotation. Fig. 2 schematically shows the form this whirling takes. When rotating the rotary body 1 at an angular velocity ω in the direction indicated by an arrow A, the rotary body 1 whirls forward (in the direction indicated by the arrow A) at a slant angle ϑ with respect to the axis Z at an angular velocity ω nf , or whirls backwards (in the backward direction relative to the arrow A) at an angular velocity ω nb . Referring now to Fig. 3 which is a block diagram showing the constitution of the control circuit shown in Fig. 1, the above-described displacement sensors 2 and 3 constitute a part of a first slant angle detecting circuit A₁, while the displacement sensors 13 and 14 constitute in combination a part of a second slant angle detecting circuit A₂. Outputs from these displacement sensors 2, 3, 13 and 14 are supplied to corresponding phase compensators 4, 5, 15 and 16, respectively. These phase compensators are provided to improve the stability of the magnetic bearing control system and serve to secure the phase margin at frequency cross-over points. On the other hand, a difference signal a₁ representing the difference between the outputs from the displacement sensors 2 and 3 is obtained from a subtraction circuit and is then supplied to a first decision circuit B₁. A difference signal a₂ representing the difference between the outputs from the displacement sensors 13 and 14 is also obtained from another subtraction circuit and is supplied to a second decision circuit B₂. The difference signals a₁ and a₂ each indicate a slant angle ϑ (Fig. 2) with respect to the axis Z of the rotary body 1. The first and second decision circuits B₁ and B₂ are designed to decide whether the rotary body 1 is whirling forward or backwards. These two circuits have the same construction and each include a band-pass filter 6 or 17, a low-pass filter 7 or 18 and a phase inverter 28 or 29 connected to the low-pass filter. If the difference signal a₁ or a₂ includes a frequency component indicating forward whirling, a signal is output from the band-pass filter 6 or 17. If the difference signal a₁ or a₂ includes a frequency component indicating backward whirling, a signal is output from the low-pass filter 7 or 18. The output of the first and second decision circuits B₁ and B₂ is applied to corresponding gain adjusters 8 and 19. The magnitude of these output signal is increased or decreased in accordance with the rotational speed of the rotary body 1. To be specific, the rotational speed of the rotary body is detected by a rotation meter 24. An F/V converter 25 serves to generate a voltage signal corresponding to the thus detected rotational speed. Upon receipt of this voltage signal, a rotational frequency detector 26 outputs a signal C corresponding to the rotational frequency of the rotary body and then applies it to each of the gain adjusters 8 and 19, so that the gain adjusters 8 and 19 increase or decrease the outputs of the decision circuits B₁ and B₂ as the rotational frequency of the rotary body increases or decreases. An output signal d₁ from the gain adjuster 8 which represents the output signal b₁ from the decision circuit B₁ after it has been adjusted in accordance with the rotational speed of the rotary body is supplied to adders 27 and 30. An output signal d₂ of the gain adjuster 19 which represents the output signal b₂ of the decision circuit B₂ after it has been adjusted in accordance with the rotational speed of the rotary body is supplied to adders 31 and 32. The adder 27 adds the signal d₁ to the output of the phase compensator 15 disposed on the axis Y₁, while the adder 30 adds the signal d₁ to the output of the phase compensator 16 disposed on the axis Y₂. To perform control in the Y direction, the outputs of the adders 27 and 30 are applied through power amplifiers 20 and 22 to electromagnets 21 and 23 on the axes Y₁ and Y₂, respectively . The adder 31 serves to add the signal d₂ to the output of the phase compensator 4 disposed on the axis X₁, and the adder 32 adds the signai d₂ to the output of the phase compensator 5 disposed on the axis X₂. With a view to effecting control in the X direction, the outputs of the adders 31 and 32 are applied through power amplifiers 9 and 11 to electromagnets 10 and 12 provided on the axes X₁ and X₂, respectively. Next description will be focused on the operation of the control circuit 118. The description starts with the operation conducted in the case of controlling the rotary body in the direction of the Y-axis. When controlling the rotary body on the axis Y, the first step is to generate the different signal a₁ which represents the difference between the outputs from the displacement sensors 2 and 3 on the axis X in the first slant angle detecting circuit A₁ depicted in Fig. 3, i.e., the signal indicating a slant angle ϑ (see Fig. 2) of the rotational axis of the rotary body. If the whirling mode is the parallel mode, the slant angle ϑ = 0°, and the signal a₁ is also zero. If the signal a₁ is not zero, the whirling proves to be in the conical mode. Subsequently, the decision circuit B₁ decides the frequency component of the output a₁ of the slant angle detecting circuit A₁ in cooperation with the band-pass filter 6 and the low-pass filter 7. Judgement is made as to whether the whirling is effected in the same direction as that of the rotation of the rotary body, i.e., the forward rotation, or in the direction opposite to that of the rotation of the rotary body, i.e., the backward rotation, depending on whether the signal a₁ is in a lower frequency band region (a band region including the frequency of the signal indicating the backward whirling) or in a higher frequency band region (a band region including the frequency of the signal indicating the forward whirling). The output signal b₁ of the decision circuit B₁ is processed on the basis of the rotational speed of the rotary body in the gain adjuster 8. The output signal d₁ transmitted from the gain adjuster 8, viz., a signal which has been adjusted according to the rotational speed, is added to the signals from the phase compensators 15 and 16 placed on axes Y₁ and Y₂, respectively, by means of the adders 27 and 30. The output signals of the adders 27 and 30 are amplified by the power amplifiers 20 and 22 on the axes Y₁ and Y₂, and then these signals are supplied respectively to the electromagnets 21 and 23 on the axes Y₁ and Y₂ in order to perform the Y-axis directional control. As will be clear from the descriptions given above, the signals applied to the electromagnets 21 and 23 are those obtained by adding the signals indicating the Y-axis directional displacement and the signals which are advanced by 90° with respect to this displacement. Using these signals, it is possible to suppress the whirling in the direction of the axes Y₁ and Y₂. Concerning the backward whirling, the output of the low-pass filter 7 is subjected to 180-degree phase inversion by the phase inverter 28 to obtain a signal which is advanced by 90°. The thus converted signal is added to the signals transmitted from the phase compensators 15 and 16. The reason why the phase inverter 28 is needed is that the direction of phase advance of an output from the slant angle detecting circuit A₁ is reversed in case of the backward rotation, and thus, that the phase of this output should be inverted. In the second place, when controlling the rotary body with respect to the axis X, the slant angle ϑ of the rotational axis of the rotary body is detected by the second slant angle detecting circuit A₂ from the different signal a₂ representing the difference between the outputs from the displacement sensors 13 and 14 disposed on the axes Y₁ and Y₂, respectively. The output signal a₂ of the detecting circuit A₂ is sent to the band-pass filter 17 and the low-pass filter 18 of the decision circuit B₂, which in turn decides whether a frequency component contained in the signal a₂ falls within a higher frequency band region which includes the component indicating forward whirling or within a lower frequency band region that includes the component indicating backward whirling. The signal relating to the backward whirling undergoes phase inversion by the phase inverter 29. The output signal b₂ of the decision circuit B₂, i.e., the signal indicating the direction of rotation, is gain-adjusted corresponding to the rotational speed of the rotary body by the gain adjuster 19. The output signal d₂ from the gain adjuster 19 is added to the signals transmitted from the phase compensators 4 and 5 on the axes X₁ and X₂ by the adders 31 and 32. The outputs of the adders 31 and 32 are amplified by the power amplifiers 9 and 11 and are then applied to the electromagnet 10 on the axis X₁ as well as to the eleotromagnet 12 on the axis X₂. Thus, the damping force required for performing the X-axis directional control is produced. As discussed above, the active type magnetic bearing according to the present invention is capable of effectively suppressing or preventing unstable vibrations by detecting the whirling mode without causing either structural resonance or any increase in vibration during the rotation which is produced when increasing the capacity of a control circuit or augmenting stiffness.

Claims: An active type magnetic bearing system including: a rotary body (1) magnetic bearings (112, 113) adapted to suspend at least two portions of said rotary body in a non contact state to permit said rotary body to rotate about the rotational axis (Z) thereof; and a control circuit (118) for supplying signals for controlling the radial position of said rotary body, said control circuit comprising: a first displacement sensor (2) disposed at one end of said rotary body within a first plane that includes the rotational axis for sensing any displacement of the radial position of one end of said rotary body to generate a first displacement signal corresponding to such displacement; a second displacement sensor (3) disposed at the other end of said rotary body within said first plane for sensing any displacement of the radial position of the other end of said rotary body to generate a second displacement signal corresponding to such displacement; a third displacement sensor (13) disposed at one end of said rotary body within a second plane that includes the rotational axis vertical to the first plane for sensing any displacement of the radial position of one end of said rotary body to generate a third displacement signal corresponding to such displacement; a fourth displacement sensor (14) disposed at the other end of said rotary body within the second plane for sensing any displacement of the radial position of the other end of said rotary body to generate a fourth displacement signal corresponding to such displacement; a first slant angle detecting circuit (A₁) for generating a first angular signal indicating a slant angle of said rotary body within said first plane by obtaining the difference between said first and second displacement signals so as to detect a slant angle on the basis of such difference; a second slant angle detecting circuit (A₂) for generating a second angular signal which indicates the slant angle of said rotary body within the second plane by obtaining the difference between said third and fourth displacement signals to detect a slant angle on the basis of such difference, said system characterized in that said control circuit further comprises: a first decision circuit (B₁) for deciding whether or not the whirling direction of said rotary body is the same as that of the rotation of said rotary body on the basis of the first angular signal to generate a first direction signal which indicates such whirling direction; a second decision circuit (B₂) for deciding whether or not the whirling direction of said rotary body is the same as that of the rotation of said rotary body on the basis of the second angular signal to generate a second direction signal which indicates such whirling direction; a first adding circuit (31)(32) for adding together the second direction signal and the first and second displacement signals; and a second adding circuit (27)(30) for adding together the first direction signal and the third and fourth displacement signals, thereby controlling the whirling of said rotary body within the first plane by utilizing an output of said first adding circuit, and controlling the whirling of said rotary body within the second plane by utilizing an output of said second adding circuit. A control circuit as set forth in Claim 1, wherein said first and second decision circuits (B₁, B₂) each include a band-pass filter (6, 17) for outputting a signal which indicates that the whirling direction of said rotary body is the same as that of the rotation of said rotary body, and a low-pass filter (7, 18) for outputting a signal which indicates that the whirling direction of said rotary body is opposite to that of the rotation of said rotary body. A control circuit as set forth in Claim 2, wherein said magnetic bearing (112, 113) includes a first bearing member for suspending said rotary body on one side and a second bearing member for suspending said rotary body on the other side, said first and second bearing members each have electromagnets for controlling the radial position of said rotary body within said first and second planes, an output of said first adding circuit (31)(32) is supplied to said electromagnets provided within said first plane, and an output of said second adding circuit (27)(30) is supplied to said electromagnets provided within said second plane. A control circuit as set forth in any one of Claims 1 to 3, further comprising gain adjusting means (8, 19) for adjusting the magnitude of an input signal in accordance with the rotational speed of said rotary body, said gain adjuster adjusting the magnitudes of the outputs of said first and second decision circuits (B₁, B₂).

Inventors: HIYAMA, HIROKUNI, MURAKAMI, CHIKARA, NAKAJIMA, ATSUSHI, WATANABE, KAZUHIDE
Applicants: EBARA CORPORATION, EBARA RESEARCH CO., LTD., NATIONAL AEROSPACE LABORATORY
Application year: 1988
Main classification: F16C39
IPC codes: F16C39, F16C32, H02K7