Method and apparatus for controlling a magnetic bearing

Abstract: A controller of a magnetic bearing controls the rotor speed with a reduced drive current while suppressing rotor vibrations. In addition to the basic Proportional, Integral and Derivative (PID) control (4X,4Y), the controller implements further control methods to ensure the stable operation of the rotor, even at rotor resonance frequencies. An automatic balancing system ("ABS", gainβ) is applied in conjunction with at least one of: N-cross control system (cross coupling, synchronous with rotor speed, gainα); N-straight control (no cross-coupling, but synchronous, gainδ); Feed Forward (FF, gainγ) control where a control current is applied to generate vibrations equal and opposite to those created by the rotor. A computer (20) in the controller determines the activation of each control method in accordance with a predetermined control plan in response to the detected rotor speed by varying the control parameters α, β, γ, δ.

Description: This invention relates to a method and apparatus for controlling a magnetic bearing with the intention of ensuring the stable operation of a rotor, suppressing the vibration caused by the imbalance of the rotor, and reducing the drive current. The EP-A-0 201 894 discloses a control apparatus for a rotor supported by an electromagnetic bearing. According to the EP-A-0 201 894 N-cross control is used. From the US-A-4,912,387 a separate adaptive noise canceller for magnetic bearing auto-balancing is known. This separate adaptive noise canceller receives a reference input of a rotor speed synchronisation pulse and is connected in the X and Y axis positional control loops of a magnetic bearing utilized to position a rotor suspended thereby. Fig.1 through Fig.4 show various conventional control schemes for handling the vibration of a rotor supported by a magnetic bearing. The control scheme shown in Fig.5 is not yet publicized at the time of filing of the application of the present invention. Shown in Fig.1 is the well-known servo feedback control system. Shown in Fig.2 is the automatic balancing system (ABS) described in Japanese patent publication JP-A-52-93852, and it may be called "N-cut system" as it derives from the operational property of eliminating the rotation-synchronous vibration component. Shown in Fig.3 is the critical damping control system described in Japanese patent publication JP-A-52-93853. Shown in Fig.4 is the N-cross system which is an improved version of the critical damping control system shown in Fig.3, and it was proposed by the inventors of the present invention and described in Japanese patent publication JP-A-61-262225. Before entering into the explanation of these prior art systems, the concept of magnetic bearing control against the rotor vibration will first be explained. A rotor supported by a magnetic bearing is subjected to the floating control in the x and y directions orthogonal to each other. Vibrations of the rotor in the x and y directions are detected as vibration signals x and y, respectively. From the vibration signal x, an input signal Px to a pair of power amplifiers 5X which drive electromagnets 2X for the x direction is produced. The magnetic bearing produces a force, which acts on the rotor in the x direction, in proportion to the Px signal. Similarly, an input signal Py to power amplifiers 5Y for the y direction is produced from the vibration signal y. Control circuits (PID controllers) 4X and 4Y, produce Px from x and Py from y respectively, which provide individually for the x and y directions. A tracking filter 7 receives a rotation signal which is a pulse signal generated at each revolution of the rotor and the vibration signals x and y, and extracts rotation-synchronous components x N and y N (will be termed simply "synchronous components", or generically called "N component") from the vibration signals x and y. A two-phase oscillator 10 is a device for producing, from the rotation signal, a cos(Ωt)signal and sin(Ωt) signal which are synchronous with the rotation. A prime concern of the present invention is how to treat the rotation-synchronous components x N and y N and the cos(Ωt)and sin(Ωt)signals. Next, the prior art arrangements of Fig.1 through Fig.4 will be explained individually. Prior art system of Fig.1: A revolving body (rotor) 1 supported by the magnetic bearing can move off the central position (will be termed "neutral position") of the magnetic bearing due to the imbalance of the rotor or the influence of external forces, and therefore it is necessary to bring the rotor back to the neutral position through the control of the magnetic bearing. In the figure, two sets of electromagnets 2X and 2Y are disposed at the right, left upper and lower peripheries of the rotor 1, and the exciting currents supplied to these electromagnets are controlled thereby to control the rotor position in the magnetic bearing. Control circuits 4X and 4Y and power amplifiers 5X and 5Y are used to energize the electromagnets. The control circuits 4X and 4Y are PID controllers. The control circuit 4X receives the value x of displacement detected by a displacement sensor 3X for the x-axis direction, and the control circuit 4Y receives the value y of displacement detected by a displacement sensor 3Y for the y-axis direction. The displacement values x and y represent the displacements of the rotor position from the neutral position. The control circuits 4X and 4Y produce control voltages P X and P Y from the displacement values, and supply them to the sets of power amplifiers 5X and 5Y. A pair of power amplifiers 5X supply exciting currents i x to the right and left electromagnets 2X of the magnetic bearing and another pair of power amplifiers 5Y supply exciting currents i y to upper and lower electromagnets 2Y so that the horizontal and vertical displacements of the rotor from the neutral position is corrected. The control circuits 5X and 5Y base the operation on the processing of proportion (P), integration (I) and differentiation (D), and behave with various control characteristics depending on the manner of combination of the PID components. The servo feedback control system of Fig.1 is a fundamental control scheme for maintaining the rotor at the neutral position. However, it is not possible for the system to maintain the rotor at the neutral position over the entire range of rotor speed due to the presence of resonance points of bend mode in the high speed regions. When the rotor speed is raised gradually, the resonance phenomena of the first and second orders of rigid mode appear. When the rotor speed is further raised gradually, the rotor becomes to exhibit the transition from a rigid body to a resilient moving body attributable to the imbalance of the rotor. The property of resilient body yields the resonance phenomena of the first and second orders of bend mode. The rotor speeds at which these resonance phenomena appear will be called "resonant frequencies" or "resonant points". The resonant points of the first and second orders of rigid mode and the resonant points of the first and second orders of bend mode may be called in series the resonant points of the first, second, third and fourth orders, respectively. The resonance curve has peak amplitudes Nc 1 , Nc 2 , Nc 3 and Nc 4 corresponding to these resonant points as shown in Fig.6. The servo feedback control system of Fig.1 has its PID control parameters tuned so that the rotor is maintained at the neutral position in the whole range of speed including these resonant points. Actually, however, the control system of Fig.1 can at most maintain the rotor at the neutral position against the vibration at the resonant points of the first and second orders of rigid mode. It is difficult for this servo feedback control system alone to cover the speed regions of the resonant points of the first and second orders of bend mode in maintaining the rotor at the neutral position. Prior art system of Fig.2: This system derived from the one shown in Fig.1 is provided with a tracking filter 7, and the detected rotation signals x N and y N are fed back (negative feedback) to the inputs of the control circuits 4X and 4Y. The tracking filter 7 receives the displacement signals x and y and rotation pulses, and extracts the rotation-synchronous components x N and y N . Further provided are subtracting nodes 9X and 9Y, on which the rotation-synchronous components x N and y N multiplied by a proportion factor β (0 or 1), i.e., βx N and βy N , are subtracted from the displacement values x and y, and the results are delivered to the respective control circuits 4X and 4Y. The circuit arrangement of Fig.2, in which βx N and βy N are subtracted from x and y, causes the magnetic bearing to be unresponsive to the vibration attributable to the unbalanced rotation (insulation of vibration). Accordingly, the spring constant K N and damping constant C N are both zero for the rotation-synchronous components. The factor β is set to 1 when the optional ABS control is to be applied, or otherwise it is set to 0. The ABS control has an advantage of eliminating the need of the exciting currents for the suppression of the imbalance-caused vibration. The ABS control is turned off (β=0) during the passage of the resonant points, and it is turned on (β=1) after the rotor speed has passed the resonant points. Prior art system of Fig.3: This system derived from the one shown in Fig.1 is provided with a set of differentiation circuits 6X and 6Y and a tracking filter 7 with the intention of reducing the resonant amplitude of bend mode. The detected displacement values x and y delivered to the PID control circuits 4X and 4Y are also received by the differentiation circuits 6X and 6Y. The circuit 6X evaluates Kx+C(dx/dt) indicative of a value proportional to the change with time of the displacement x, and the rotation-synchronous tracking filter 7 detects a component, which is dependent on the rotor speed N, expressed as follows. [Expression 1] x 0 =Kx N +C(dx N /dt) where K is the spring constant, and C is the damping constant. Similarly, the differentiation circuit 6Y and rotation-synchronous tracking filter 7 detect a component, which is dependent on the rotor speed N, expressed as follows. [Expression 2] y 0 =Ky N +C(dy N /dt) Based on the detection of the displacements and extraction of rotation-synchronous components x 0 and y 0 , the magnetic bearing is controlled only against the vibration of rotation-synchronous imbalance. This system enables the adjustment of rigidity of the bearing (by varying the spring constant K) depending on the magnitude of displacement, and enables the adjustment of damping of the bearing displacement (by varying the damping constant C) depending on the rate of change with time. Consequently, the resonant amplitude of bend mode can be reduced. Generally, a differentiation circuit configured by electronic component parts creates highfrequency noises, and therefore its performance is limited to the approximation of mathematical operation at present. Prior art system of Fig.4: This system detects the rotor speed based on another scheme instead of using the differentiation circuits in the prior art system of Fig.3. The system is intended for the case of the imbalance-caused confronting vibration, and it utilizes the fact that the differentiated value of vibration in the x-axis direction is proportional to the -y displacement value, and that in the y-axis direction is proportional to the x displacement value. Accordingly, the displacement values -y N and x N are used in place of the outputs of the differentiation circuits 6X and 6Y shown in Fig.3. The rotation-synchronous components of the rate of change of both displacements can be extracted by the tracking filter 7. The N-cross control can be accomplished perfectly by means of differentiation circuits, and it has an equivalent function of providing the damping constant C N against the imbalance-caused vibration. Accordingly, this is an optional system arrangement employed during the passage of the resonant point. Fig.5 discloses a relatively new system arrangement for achieving the suppression of vibration which was filed on May 20, 1992 as U.S. Serial 07/885,980 (US 5 408 875) by MATSUSHITA et al., titling "Balancing method for flexible rotor and a balancer applicable to the balancing method". In contrast to the foregoing prior art systems in Figs. 1 to 4 which can fairly be categorized as feedback control systems because of the exciting current control by means of tracking filters in response to the displacement signals, the new system of Fig.5 does not include a tracking filter which deals with the displacement signals x and y, and is categorized to be a feed forward (FF) control system, in which a counter vibrating force is generated in the opposite phase relation with the imbalance-caused vibration of each direction thereby to offset the vibration. A two-phase oscillator 10 included in the system of Fig.5 produces a sine wave signal Asin(Ωt+) and a cosine wave signal Acos(Ωt+)for the rotor speed Ω in synchronism with rotation pulses. The cosine wave signal Acos(Ωt+)multiplied by a proportional factor γ is added to the output of the control circuit 4X and the sine wave signal Asin(Ωt+)multiplied by a proportional factor γ is added to the output of the control circuit 4Y, and the resulting signals are used for the exciting current control of the magnetic bearing. By choosing the amplitude A and phase  properly, counter vibration forces are generated to offset the imbalance-caused vibration forces at the resonant point, and consequently the resonant amplitude can be suppressed. This operation will be called "FF counter vibration". The two-phase outputs may be applied at the inputs of the control circuits 4X and 4Y as shown by the dashed lines in Fig.5. The FF control is another optional control function. Fig.7 is a table which summarizes the applied speed range and the property of control current (exciting current) carried out by the prior art systems in Figs. 1 to 4 and above F.F. counter vibration. The critical damping system described in Japanese patent publication JP-A-52-93853 produces the displacement rate signals dx/dt and dy/dt from the displacement signals x and y, and therefore it necessitates differentiation circuits, resulting in a complex circuit arrangement. This system involves another problem of a large vibration amplitude caused by the imbalance at the passage of the critical speed, which requires an increased amount of exciting current. The N-cross control described in Japanese patent publication JP-A-61-262225 involves the same problem of a large exciting current due to the introduction of the resonant vibration signals to the PID control circuit. These increased exciting currents demand a large dynamic range of power amplifiers, imposing the difficulty of practice. The ABS operation filters out the rotation-synchronous vibration components caused by the imbalance, and the exciting current can be reduced significantly. However, the activation of ABS control during the passage of the resonant creates an increased vibration, and therefore this control must be used only after the passage of the critical speed. Accordingly, the PID control without the accompaniment of ABS control during the passage of the critical speed expends a large amount of exciting current. The application of the ABS control further involves a problem of system instability such as drifting besides the insuffient reduction of imbalance-caused vibration. Fig.8 are examples of vibration characteristic curves showing the effectiveness of the ABS control. When attention is paid to the speed region near the third-order resonant point, the rotor is stable when the ABS control is absent or it is applied weakly. However, if the ABS control is strengthened, the vibration curve oscillates violently following the passage of the third-order resonant point. Accordingly, strong ABS control must be avoided in the speed regions near the resonant points, and it works effectively in a speed region distant from the resonant point, i.e., the speed region between the third-order and fourth-order resonant points in this example. On this account, the ABS control based on the technical level at present is limited to the application in speed ranges which exclude the resonant regions. It is an object of the invention to provide a method and an apparatus for controlling a magnetic bearing which ensures an improved stable operation of a rotor, suppressing the vibration caused by the imbalance of the rotor, and reducing the drive current. According of the invention this object is achieved by an apparatus for controlling a magnetic bearing, comprising displacement detection means which detects displacement values x and y from reference positions on two orthogonal axes perpendicular to the rotational axis of a rotor supported by said bearing; filter means for extracting rotation-synchronous components x N and y N from at least the detected displacement values x and y; first control means which receives a differential value between the x and a value proportional to the x N (by proportion factor β) and produces a first control signal; second control means which receives a differential value between the y and a value proportional to the y N (by proportion factor β) and produces a second control signal; first power amplifying means which amplifies a summed value of said first control signal and a value proportional to the x N (by proportion factor δ) and supplies a resulting exciting current to an electromagnet of said magnetic bearing; second power amplifying means which amplifies a summed value of said second control signal and a value proportional to the y N (by proportion factor δ) and supplies a resulting exciting current to another electromagnet of said magnetic bearing; and means of setting the proportion factors β and δ variably in accordance with the rotor speed. According to this invention, the damping force against the confronting vibration at the passage of the resonant point is improved and it ensures the passage at a small resonant amplitude of the imbalance-caused vibration even if the rotor balance is not much accurate. In consequence, the rotor balancing work is simplified. The rotor can operate across the resonant points at a small vibration even if the imbalance of rotor is worsened due to the sticking of extraneous objects on the rotor during the rotation, and consequently the exciting current of the electromagnets can be reduced and the durability of the power amplifiers is improved. Optimal control (small exciting current and small vibration) for each rotor speed range can be achieved by adjusting the values of the proportion factors α, β, γ and δ depending on the rotor speed. Operating the magnetic bearing by keeping the exciting current small results in an extended life time of electronic component parts used in the power amplifiers. The magnetic bearing has its applied force reduced, and the life time of mechanical component parts is also extended. The resonant point can be shifted upward by applying the N-straight control with the δ parameter, and consequently the rotor speed range can be extended upward. In a preferred embodiment the bearing control apparatus comprises a two-phase synchronous oscillator which produces a sine wave signal and cosine wave signal in synchronism with rotation pulses; means of adding a value proportional to said cosine wave signal (by proportion factor γ) to the input or output of said first control means; and means of adding a value proportional to said sine wave signal (by proportion factor γ) to the input or output of said second control means. Preferably said factor setting means is setting the proportion factor γ, besides the β and δ, variably in accordance with the rotor speed. According to the invention, the object is further achieved by a method of controlling a magnetic bearing, comprising the steps of: detection of displacement values x and y from reference positions on two orthogonal axes perpendicular to the rotational axis of a rotor supported by said bearing, using displacement detection means, extracting rotation-synchronous components x N and y N from the detected displacement values x and y using filter means, receiving a differential value between the x and a value proportional to the x N (by proportion factor β) by first control means, production of a first control signal by the first control means, receiving a differential value between the y and a value proportional to the y N (by proportion factor β) by second control means, production of a second control signal by the second control means, amplifying a summed value of said first control signal and a value proportional to the x N (by proportion factor δ) and supplying a resulting exciting current to an electromagnet of said magnetic bearing using first power amplifying means, amplifying a summed value of said second control signal and a value proportional to the y N (by proportion factor δ) and supplying a resulting exciting current to another electromagnet of said magnetic bearing using second power amplifying means, and setting the proportion factors β and δ variably in accordance with the rotor speed using setting means. Preferred embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows the conventional PID control; Fig. 2 shows the conventional ABS control; Fig. 3 shows the conventional critical control; Fig. 4 shows the conventional N-cross control; Fig. 5 shows the FF control; Fig. 6 is a diagram explaining the first-order to fourth-order resonant points; Fig. 7 is a diagram showing the relation between the applicable rotor speed range and the control current based on the conventional system and those of the preceding patent applications; Fig. 8 is a vibration response curve based on the ABS control; Fig. 9 shows the N-straiqht control; Fig. 10 shows an embodiment of this invention for carrying out the ABS control and N-straight control in coaction; Fig. 11 shows an embodiment of this invention for carrying out the ABS control, N-cross control and N-straight control in coaction; Fig. 12 shows an embodiment of this invention for carrying out the N-cross control, N-straigh control and FF control in coaction; Fig. 13 shows an embodiment of this invention for carrying out the ABS control, N-straight control and FF control in coaction; Fig. 14 shows an embodiment of this invention for carrying out the ABS control of feedback type, N-cross control, N-straight control and FF control in combination; Fig. 15 shows an embodiment of this invention for carrying out the ABS control of feedback type and N-straight control in coaction; and Fig. 16 shows an embodiment of this invention for carrying out the ABS control, N-cross control and N-straight control in coaction. Fig. 9 shows a system configuration which is not an embodiment of the invention. In this configuration, the rotation-synchronous components x N and y N multiplied by a proportion factor δ are added to the outputs of the control circuits 4X and 4Y. The configuration shown in Fig. 9 is intended to strengthen the spring constant K N , and it shifts the resonant point toward the higher speed region thereby to reduce the vibration. This optional control is named "N-straight control" in contrast to the N-cross control, and is used in the speed range below the resonant point. The proportion factor δ is set greater than or equal to zero, preferably. By setting δ=0, values to be added, i.e., δx N and δy N , are both zero, implementing the case without the N-straight control. With a positive large proportion factor δ being set, the x N and y N values multiplied by δ are added, implementing the typical N-straight control. With a negative proportion factor δ being selected, the proportion of the effect of N-straight control varies. The last case of a negative proportion factor δ is used with the intention of shifting the resonant point toward the lower speed region for the reduction of vibration by preventing the occurrence of resonation. (1) ABS plus N-straight control (Fig. 10) Fig. 10 shows the system configuration of this embodiment. This embodiment is the N-straight control shown in Fig.9 coacting with the ABS control. An advantage of this embodiment is the ability of shifting the resonant point toward the higher speed and thus providing a wide safe region between the resonant points, while leaving the ABS control active. This system configuration shown in Fig.10 is useful for the reduction of vibration amplitude for the rotor speed below the resonant point. The proportion factor δ can take any positive or negative value inclusive of zero. In the case of δ having a negative value, the control system shifts the resonant point toward the lower speed, thereby providing a wide safe speed region following the passage of the resonant point, and the vibration can be reduced. (2) ABS plus N-cross plus N-straight control (Fig.11) Fig.11 shows the system configuration of this embodiment. The embodiment applies the N-cross control and N-straight control in coaction, with the ABS control being active, and it can reduce the current consumption and the vibration amplitude. (3) ABS plus N-straight plus FF control (Fig.13) Fig.13 shows the system configuration of this embodiment. The embodiment applies the N-straight control and FF control in coaction, with the ABS control being active. (4) N-cross plus N-straight plus FF control (or any pair of these controls) (Fig.12) Fig.12 shows the system configuration of this embodiment. The embodiment applies the N-cross control, N-straight control and FF control in coaction, but excluding the ABS control. Other possible combinations are the N-cross control plus N-straight control, the N-straight control plus FF control, and the N-cross control plus FF control. (5) Feedback of x N and y N under ABS control (Figs.14-16) This embodiment is a modified version of the ABS control. The output signals x N and y N of the rotation-synchronous tracking filter 7 is fed back (negative feedback) to the crude displacement signals x and y. A proportion factor β' which represents the amount of feedback relates with the β as defined with respect to Fig.2 as follows. [Expression 3] β=1/(1+β'). Accordingly, in order for the ABS control to have a strong effect to extract an increased amount of the rotation-synchronous components x N and y N from the crude signal x and y, the β' is set to a large value. The condition of complete extraction is met when β' is infinity.

Claims: An apparatus for controlling a magnetic bearing, comprising displacement detection means (3X, 3Y) which detects displacement values x and y from reference positions on two orthogonal axes perpendicular to the rotational axis of a rotor (1) supported by said bearing; characterized by filter means (7) for extracting rotation-synchronous components x N and y N from at least the detected displacement values x and y; first control means (4X) which receives a differential value between the x and a value proportional to the x N (by proportion factor β) and produces a first control signal; second control means (4Y) which receives a differential value between the y and a value proportional to the y N (by proportion factor β) and produces a second control signal; first power amplifying means (5X) which amplifies a summed value of said first control signal and a value proportional to the x N (by proportion factor δ) and supplies a resulting exciting current to an electromagnet (2X) of said magnetic bearing; second power amplifying means (5Y) which amplifies a summed value of said second control signal and a value proportional to the y N (by proportion factor b) and supplies a resulting exciting current to another electromagnet (2Y) of said magnetic bearing; and means (20) of setting the proportion factors β and δ variably in accordance with the rotor speed. A magnetic bearing control apparatus according to claim 1, characterized by a two-phase synchronous oscillator (10) which produces a sine wave signal and cosine wave signal in synchronism with rotation pulses; means of adding a value proportional to said cosine wave signal (by proportion factor γ) to the input or output of said first control means; and means of adding a value proportional to said sine wave signal (by proportion factor γ) to the input or output of said second control means. A magnetic bearing control apparatus according to claim 2, characterized in that said factor setting means is setting the proportion factor γ, besides the β and δ, variably in accordance with the rotor speed. A method of controlling a magnetic bearing, comprising the steps of: detection of displacement values x and y from reference positions on two orthogonal axes perpendicular to the rotational axis of a rotor (1) supported by said bearing, using displacement detection means (3X, 3Y), extracting rotation-synchronous components x N and y N from the detected displacement values x and y using filter means (7), receiving a differential value between the x and a value proportional to the x N (by proportion factor β) by first control means (4X), production of a first control signal by the first control means (4X), receiving a differential value between the y and a value proportional to the y N (by proportion factor β) by second control means (4Y), production of a second control signal by the second control means (4Y), amplifying a summed value of said first control signal and a value proportional to the x N (by proportion factor δ) and supplying a resulting exciting current to an electromagnet (2X) of said magnetic bearing using first power amplifying means (5X), amplifying a summed value of said second control signal and a value proportional to the y N (by proportion factor δ) and supplying a resulting exciting current to another electromagnet (2Y) of said magnetic bearing using second power amplifying means (5Y), and setting the proportion factors β and δ variably in accordance with the rotor speed using setting means (20).

Inventors: ABE YOSHIAKI, FUKUSHIMA YASUO, HIROSHIMA MINORU, KANEKI TADASHI, MATSUSHITA OSAMI, SAKANASHI NAOFUMI, TAKAHASHI NAOHIKO, YONEYAMA MITSUHO, ABE, YOSHIAKI, FUKUSHIMA, YASUO, HIROSHIMA, MINORU, KANEKI, TADASHI, MATSUSHITA, OSAMI, SAKANASHI, NAOFUMI, TAKAHASHI, NAOHIKO, YONEYAMA, MITSUHO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: F16C39
IPC codes: F16C39