Abstract:
A magnetic bearing arrangement for a rotary device includes circuitry for generating a multiphase excitation signal for energizing the phase windings of a magnetic bearing element. According to an embodiment of the present invention, circuitry for detecting the radial position of a rotor of the rotary device generates a position signal indicative of the radial position of the rotor of the rotary device relative to a desired rotor position. The position signal is used to modify the excitation signal to produce a modified excitation signal. The modified excitation signal is used to energize the phase windings of the magnetic bearing, thus providing a low-cost and efficient means for dynamically suspending the rotor of the rotary device.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 12/044,835, filed Mar. 7, 2008 and fully incorporated herein by reference for all purposes. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed to magnetic bearings and to magnetic bearings in general, and to the use of magnetic bearings as configured in bearingless drives. 
     Broadly, a magnetic bearing supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact; for example, they can levitate a rotating shaft of the rotor of a motor and permit relative motion without friction or wear. Magnetic bearings are used in lieu of a rolling element or fluid film journal bearings in some high performance turbo-machinery applications. Since there is no mechanical contact in magnetic bearings, mechanical friction losses are eliminated. In addition, reliability can be increased since there is no mechanical wear. Following are principles of operation, and examples, of conventional magnetic bearings. 
       FIG. 11  schematically represents a conventional magnetic bearing arrangement for a rotary device. A motor  1102  connected to a shaft  1104  is suspended by magnetic bearings  1106   a ,  1106   b . Each bearing  1106   a ,  1106   b  generates radial forces in the x- and y-directions. A thrust bearing  1108  is optionally provided for positioning in the z-direction. 
     The motor  1102  is a conventional brushless motor comprising a stator element  1112  and a permanent magnet rotor element  1114 . The stator  1112  comprises a set of phase coils which are energized in a specific manner to produce rotational torque about the z-axis. The construction of stators and rotors are well understood and need no further elaboration. An example of motor  1102  is a three-phase motor, where the stator  1112  comprises three phase coils. A suitable three-phase voltage source and a controller serve to energize the phase coils of the stator  1112  which magnetically interact with the rotor  1114  to produce rotational torque. 
     The magnetic bearings  1106   a ,  1106   b  each typically comprises four coils (see  FIG. 12 ) arranged in respective bearing stators  1122   a ,  1122   b . The bearing rotors  1124   a ,  1124   b  comprise a magnetic material. For each bearing stator  1122   a ,  1122   b , two coils are arranged on the x-axis on opposing sides of the shaft  1104 , and two coils are arranged on the y-axis on opposing sides of the shaft. Differing amounts of current are made to flow in each coil in order to affect the radial position of the shaft  1104  in the x-y directions. For example, when current flows in the coils, opposing magnetic forces are generated which act on the rotor  1124   a . A radial force in the x-direction can therefore be generated by creating a difference in the magnetic forces generated by the x-axis coils; likewise for the y-direction. 
     Coil currents in the bearing stators  1122   a ,  1122   b  are regulated by suitable power and control circuitry. For example, a single phase voltage source can be provided for each coil (a total of eight for both magnetic bearings  1106   a ,  1106   b ). 
     Further detail of one of the bearing stators  1122   a ,  1122   b  is illustrated in  FIG. 12  along with additional electronics detail. Referring to  FIG. 12  then, a bearing stator  1222  is shown in cross-section and reveals an example of the arrangement of coils which comprise the bearing stator. Coils  1226   x   1 ,  1226   x   2  constitute the x-axis coils and coils  1226   y   1 ,  1226   y   2  constitute the y-axis coils. In the example shown, each coil (e.g., coil  1226   x   1 ) comprises a pair of coils arranged in a horseshoe configuration. 
     Conventionally, the radial position of the rotor  1224  is sensed by a series of gap sensors  1202 . Output from the sensors  1202  feed into suitable gap sensor electronics  1204  to produce a usable signal for a controller  1206 . The controller  1206  drives power amplifiers  1208  (two are illustrated) to supply sufficient current to the coils  1226   x   1 ,  1226   x   2 ,  1226   y   1 ,  1226   y   2  to energize the coils in a manner that positions the rotor to a desired position in the x-y plane. 
     Typically, the controller  1206  and/or the gap sensor electronics  1204  process signals in the digital domain. In other words, the position signals output from the gap sensors  1202  (usually analog) are processed as digital signals by the controller  1206  to determine the amount of correction in the x-direction and in the y-direction that is needed to place the rotor in a desired position. The x- and y-direction correction data are then converted back to analog signals so that the power amplifiers  1208  can produce suitable drive currents to energize the coils  1226   x   1 ,  1226   x   2 ,  1226   y   1 ,  1226   y   2  appropriately. 
     In the magnetic bearing arrangement shown in  FIG. 11 , the magnetic bearings  1106   a ,  1106   b  are elements separate from the motor  1102 . However, the magnetic bearings can be incorporated in the construction of the motor in an arrangement referred to as a “bearingless drive”. 
       FIG. 13  shows an example of a bearingless drive. The bearingless drive shown in the figure comprises two bearingless motor units  1302   a ,  1302   b . Each bearingless motor unit  1302   a ,  1302   b  comprises, respectively, a stator element  1322   a ,  1322   b  and a rotor element  1324   a ,  1324   b . Although not explicitly shown in the figure, each stator element  1322   a ,  1322   b  comprises two set of windings; there is one set of windings (phase coils), called the motor windings, for torque production and there is a separate set of windings, called the suspension windings, for rotor suspension. The motor windings are energized by a source of drive currents to produce rotational torque. The suspension windings are energized by a separate source of drive currents to produce radial forces for positioning in the x-y direction (i.e., the radial direction). 
     The motor windings in each stator element  1322   a ,  1322   b  are connected in parallel. A generator produces drive currents to energize both sets of motor windings for rotary operation. Each of the suspension windings in each stator element  1322   a ,  1322   b , on the other hand, is energized by its own generator in order to provide independent suspension control by each suspension winding. 
     BRIEF SUMMARY OF THE INVENTION 
     A magnetic bearing system for a rotary device according to the present invention includes receiving a drive signal and adjusting the drive signal in accordance with deviations of the rotor of the rotary device from a neutral (desired) rotor position. Deviation in the rotor is detected by a position sensor. An output of the position sensor is used to adjust the drive signal to produce an adjusted drive signal. The adjusted drive signal is applied to phase coils of the stator of the rotary device. When energized by the adjusted drive signal, the phase coils generate both the rotational torque to produce rotary motion to operate the rotary device and the radial forces to affect the radial position of the rotor during operation of the rotary device thereby magnetically suspending the rotor an in particular to return the rotor to its neutral (desired) radial position. Typical rotary devices that the present invention can be used with include brushless AC motors and brushless DC motors. 
     In an alternative embodiment of the present invention, a magnetic bearing for a rotary device includes receiving a multiphase drive signal and adjusting the multiphase drive signal in accordance with deviations of the rotor of the rotary device from a neutral rotor position. More specifically, a position sensor produces an output signal that indicates the position of the rotor of the rotary device relative to a neutral rotor position. The output signal is used to adjust the multiphase drive signal to produce a modified signal which is then applied to a set of phase coils comprising the magnetic bearing. More specifically, the neutral or zero point of the multiphase drive signal is shifted. The resulting modified signal can then be applied to phase coils comprising the magnetic bearing. The phase coils comprising the magnetic bearing are energized by the positioning signal and generate radial forces to suspend the rotor during operation of the rotary device. This particular embodiment of the present invention is suitable for motors, generators, and the like. In the case of motors, the phase coils comprising the magnetic bearing are provided to operate in conjunction with the torque generating component of the rotary device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a high-level schematic diagram of the circuitry of a magnetic bearing in accordance with the present invention. 
         FIG. 2A  explains the principles of operation of a radial position sensor used in the present invention. 
         FIGS. 2B and 2C  illustrate an example of an inductive radial position sensor. 
         FIG. 2D  shows an example of a capacitive radial position sensor. 
         FIG. 3  is a more detailed diagram of an example of a neutral point shifting circuit in accordance with the present invention. 
         FIG. 4  is a schematic representation of a three-phase Y-Y transformer. 
         FIG. 5  shows additional detail of the amplifier  106  shown in  FIG. 1 . 
         FIG. 6  is a more detailed schematic diagram of the circuitry for a magnetic bearing shown in  FIG. 1 . 
         FIGS. 7A and 7B  illustrate the principles of operation of the present invention. 
         FIGS. 8A-8C  show variations of the present invention for Y-connected and Δ-connected components. 
         FIG. 9  illustrates an example of a magnetic bearing of the present invention in a bearingless motor configuration. 
         FIG. 10  illustrates an example of a magnetic bearing unit of the present invention configured with a rotary device having a motor component and a separate magnetic bearing component. 
         FIG. 11  illustrates a conventional magnetic bearing arrangement for suspending a rotor. 
         FIG. 12  shows additional detail of the convention magnetic bearing shown in  FIG. 11 , along with conventional circuitry. 
         FIG. 13  shows an example of a conventional bearingless drive. 
         FIGS. 14 ,  15 A,  16 A, and  16 B illustrate some principles of the present invention. 
         FIG. 15B  illustrates a digital circuit embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Following is a description of various aspects and embodiments of the present invention presented in conjunction with the figures identified above. First, a description of some principles underlying the present invention will be given. The discussion will then proceed with a description of specific embodiments of the present invention. 
     The present invention is directed to the operation of magnetic bearings and bearingless drives for use in rotary devices. An important aspect of the present invention is the departure from the use of a conventional position sensing system based on Cartesian (X-Y) coordinates to the use of a polar (ρ, φ) coordinate system. Where the conventional position sensing system produces an X-position signal and a Y-position signal that represents the displacement of the rotor of the rotary device in an X-Y coordinate system, a polar coordinate position sensing system in accordance with the present invention produces a time-varying signal whose magnitude is proportional to the absolute value of displacement of the rotor from a neutral (desired) position and a phase shift that represents the direction of the displacement, where the phase shift is relative to the time varying signal when the rotor is at its neutral position. Thus,
 
 u=U  sin (ω t +φ),
 
where U represents magnitude (ρ) and φ represents phase shift (“phase”).
 
     Using a system of polar coordinates permits the use of the time-varying output signal u of the position sensor and is a most straightforward and natural way to represent the radial position of the rotor. The presentation of information in the form of a single harmonic signal whose parameters are directly correlated with the parameters of displacement, namely magnitude and phase, greatly simplifies the processing circuitry used to determine position, resulting in simple and efficient bearingless systems. 
     Refer now to  FIGS. 14-16B , keeping in mind that an aspect of the present invention is to combine position sensors, bearings, and torque production elements in a single construction. Consider the relationship between polyphase signals U A , U B , U C  for driving the rotor of a polyphase motor and a position signal u of the position sensor indicating the radial position of the rotor relative to a neutral position of the rotor.  FIG. 14  illustrates a vector representation of the polyphase drive signals U A , U B , U C , having a neutral point at O 1 , for driving the polyphase motor. The position signal, represented in the figure as a vector u, is the output signal of a position sensor which represents a displacement of the rotor of the motor from a neutral radial position of the rotor. More specifically, the position signal u is a time-varying signal whose amplitude represents the distance of the displacement from the neutral position and whose phase represents the direction of the displacement. An example of such a position sensor is disclosed in commonly-owned, co-pending U.S. application Ser. No. 12/044,835, which is incorporated herein in its entirety for all purposes. 
       FIG. 15A  is a vector-based explanation of how the position signal (represented as vector u in  FIG. 14 ) of the position sensor can be used to produce a drive signal in accordance with a novel aspect of the present invention, namely producing a drive signal to drive the phase coils of a polyphase motor so as produce (1) rotational torque forces and (2) rotor suspension forces.  FIG. 15A  shows that the position signal u can be decomposed into components that project onto the vector representation of the polyphase drive signals U A , U B , U C . Thus, the position signal u comprises vector components U* A , U* B , U* C  respectively projected onto vectors U A , U B , U C . The vector components U* A , U* B , U* C  can then be combined with the respective drive signals U A , U B , U c  to modify them. The modified drive signals, represented in the figure as vectors U′ A , U′ B , U′ C , are then applied to drive the phase coils of the polyphase motor. When driven by the modified drive signals, the phase coils will produce a torque force in order to cause rotation of the rotor and a suspension force in order to cause repositioning of the rotor to its neutral radial position. A particularly salient aspect of the present invention is to be noted here, namely that, in accordance with the present invention, only one set of phase coils are driven by the modified signal. Whereas prior art configurations provide one set of phase coils for torque production and a separate set of phase coils for rotor suspension (see  FIGS. 11 and 13 ), the present invention uses only one set of phase coils for both torque production and rotor suspension. 
       FIG. 15B  shows a circuit implementation of the process shown in  FIG. 15A . The circuit implementation comprises analog-to-digital (A/D) converters  1502   a - 1502   f , each of which converts one of the signal components to produce a suitable digital signal for subsequent processing. For example, the polyphase drive signals U A , U B , U C  are A/D converted respectively by A/D converters  1502   a ,  1502   c ,  1502   f  to produce digital signals which respectively feed into summation circuits  1504   a - 1504   c . Likewise, the component signals U* A , U* B , U* C  of the position signal u are A/D converted respectively by A/D converters  1502   b ,  1502   d ,  1502   e . The resulting digital signals are fed into the summation circuits  1504   a - 1504   c . Digital-to-analog conversion circuitry  1506   a - 1506   c  convert the digital output of the summation circuits  1504   a - 1504   c  to produce the resulting modified drive signals U′ A , U′ B , U′c which are then fed to the phase coils of the motor. Thus, for example, the drive signal U A  is converted to its digital form and summed via circuit  1504   a  with the digital version of component signal U* A  to produce a modified digital signal. The output of digital output summation circuit  1504   a  is converted to an analog signal U′ A  via converter  1506   a . Alternate implementations using microprocessor circuits are well within the knowledge of those of ordinary skill in the relevant microprocessor and digital processing arts and thus need not be discussed herein. 
     The modified polyphase signals U′ A , U′ B , U′ c  are applied to respective phase coils of the motor. For example, the modified polyphase signals U′ A , U′ B , U′ c  constitute a three-phase signal for driving a three-phase motor, and in particular the three phase coils comprising the three-phase motor. More specifically, the signal U′ A  drives one of the three phase coils, the signal U′ B  drives a second one of the three phase coils, and the signal U′ C  drives the third one of the three phase coils. When the phase coils of the motor are driven by the drive signals U′ A , U′ B , U′ C , the resulting magnetic fields produced by the phase coils are able to produce both a torque force to provide rotation of the rotor and suspension force to suspend the rotor in a manner so as to maintain the radial position of the rotor at its neutral position. This aspect of the present invention is discussed in more detail below. 
       FIG. 16A  illustrates an alternative approach, in accordance with the present invention, for producing the modified drive signals U′ A , U′ B , U′ c . In the approach illustrated in  FIG. 16A , the output signal of the position sensor (represented in  FIG. 14  as vector u) is used to shift the neutral point O 1  of the polyphase drive signals U A , U B , U c  from O 1  to O 2 . By so doing, the drive signals U A , U B , U c  are altered to produce drive signals U′ A , U′ B , U′ c . The resulting drive signals U′ A , U′ B , U′ C  also provide both torque generation and rotor suspension when applied to energize the phase coils of the motor as explained above. In this configuration of the present invention, the only operation that is required is suitable amplification of the output signal of the position sensor, an operation that can be achieved by any of a number of known amplification techniques.  FIG. 16B  shows the decomposition of the vectors representing the modified drive signals U′ A , U′ B , U′ c , illustrating that the neutral point O 1  can be effectively shifted using digital circuitry similar to that shown in  FIG. 15B . 
     Returning to  FIG. 16A , a one step solution in accordance with the present invention that is an improvement over the embodiment of the present invention illustrated in  FIGS. 15A ,  15 B, and  16 B will now be described. The apparatus of  FIG. 15B  requires considerable circuitry/processing, namely signal conversion between analog and digital formats and digital processing, in order to produce the desired drive signals U′ A , U′ B , U′ c . Following is a description of an embodiment of the present invention which does not require the A/D and D/A circuitry  1504   a - 1504   f , or the summation circuitry  1506   a - 1506   c.    
     Referring to  FIG. 1 , a schematic diagram of a magnetic bearing drive system  100  that embodies the principles discussed above in  FIG. 16A  is presented. The magnetic bearing drive system incorporates a magnetic bearing in accordance with the present invention is shown for a specific illustrative embodiment, namely a three-phase AC brushless motor. It will be appreciated from the discussion that follows that the magnetic bearing of the present invention can be readily adapted for use with other similar rotary devices. The arrangement shown in the figure is referred to as a bearingless drive. The discussion will now turn to a description of specific illustrative embodiments of the present invention, incorporating the principles set forth above. 
     In  FIG. 1 , a signal generator component  102  generates an N-phase sinusoidal drive signal. In the specific embodiment shown in the figure, the signal generator  102  is a three-phase generator which comprises three signal generating elements  102   a ,  102   b ,  102   c . Each signal generating element produces a sine wave signal that is 120° out of phase relative to the sine wave signals output from the other two signal generating elements. The figure represents these signals as U A , U B , U. The configuration of the signal generating elements  102   a ,  102   b ,  102   c  is referred to as a Wye-configured (“Y-configured”), or a “star-configured” connection, having a neutral (or central or zero) point O 1 . The neutral point O 1  is at some voltage potential which serves as a reference voltage potential for each signal generating element  102   a ,  102   b ,  102   c ; e.g., the neutral point O 1  can be ground potential. 
     The signal generator  102  is connected to one side of a circuit referred to herein as the neutral point shifting circuit  122 . The neutral point shifting circuit comprises transformers T 1  and T 2 , an amplifier  106 , and a position sensor  108 . Operation of the neutral point shifting circuit  122  will be explained below. A three-phase brushless motor  104  is connected to the other side of the neutral point shifting circuit  122 . The motor  104  comprises three phase coils  104   a ,  104   b ,  104   c.    
     The signal generator  102  is connected to a transformer T 1  of the neutral point shifting circuit  122 . The terminals of the transformer T 1  are identified with respect to the primary and secondary windings which comprise the transformer. On the primary side are terminals p 1 , p 2 , p 3 , p 0 . On the secondary side are terminals s 1 , s 2 , s 3 , s 0 . Notable is the connection of the neutral point O 1  to the terminal p 0  of the transformer T 1 . The significance of this connection will be discussed below. 
     The transformer T 1  is connected to an amplifier element  106  and to another transformer T 2 . Additional detail of the amplifier element  106  will be presented below. As for the transformer T 2 , it comprises primary windings and secondary windings, along with corresponding terminals. Thus, the primary side of transformer T 2  has terminals p 1 , p 2 , p 3 , p 0 , while the secondary side of transformer T 2  has terminals s 1 , s 2 , s 3 , s 0 . 
     The secondary side of the transformer T 2  of the neutral point shifting circuit  122  outputs modified signals U′ A , U′ B , U′ C  which are coupled to the corresponding phase coils (also referred to as phase windings)  104   a ,  104   b ,  104   c  which comprise the stator element of the three-phase motor  104 . The generalized illustration of the motor  104  shown in  FIG. 1  represents only the phase coils  104   a ,  104   b ,  104   c  of the motor  104 . The phase coils  104   a ,  104   b ,  104   c  are arranged in a Y-configuration, having a neutral (or central, or zero) point denoted by O 2 . Notable is the connection of the neutral point O 2  to the terminal s 0  of the transformer T 2 . The significance of this connection will be discussed below. The construction of multiphase motors, such as motor  104 , are very well known and do not require additional discussion. 
     The amplifier  106  includes terminals a, b, c, d. The terminal a of the amplifier  106  is connected to the terminal s 0  on the secondary coil of transformer T 1 . The terminal b of the amplifier  106  is connected to the terminal p 0  on the primary coil of transformer T 2 . The terminals c and d of the amplifier  106  are connected to the output of a radial position sensor  108 . The position sensor  108  provides a signal that indicates the displacement of the radial position of the rotor of the motor  104  from a neutral radial position of the rotor. 
     Reference is now made to  FIGS. 2A-2C  for a brief explanation of the operation of the radial position sensor  108 , additional detail being provided in U.S. application Ser. No. 12/044,835.  FIG. 2A  illustrates the principles of operation of the radial position sensor  108 . A head-on view of a rotor shaft  200  of a motor is shown; the shaft is shown rotating in a clockwise direction. Typically, it is desirable that the shaft  200  does not deviate from its neutral position during operation of the motor (the neutral position can be referred to as an “initial position” or a “desired position”). However, in practice the shaft  200  is likely to deviate from its neutral position, referred to as a radial displacement, during operation of the motor.  FIG. 2A  illustrates, in an exaggerated manner, radial displacement of the shaft  200  to a displaced position at  200 ′. 
       FIG. 2A  includes a simplified diagram showing the neutral position A of the axis of rotation of shaft  200  and the new position A′ of the axis of rotation of the shaft when it is radially displaced. The displacement of the axis of rotation from A to A′ can be represented by polar coordinates, as shown in the figure. The distance of the displacement from position A to position A′ is represented by D. The angle Θ represents the angle subtended between an axis X the line A-A′. 
       FIG. 2B  shows an example of a radial position sensor  108 , and in particular an inductive radial position sensor suitable for generating an output signal representative of the radial displacement of the rotor of a motor (in terms of polar coordinates) vis-à-vis displacement of the rotor shaft  200 .  FIG. 2C  is a circuit schematic representation of  FIG. 2B . Additional detail of this sensor is provided in U.S. application Ser. No. 12/044,835. Briefly, the position sensor  108  comprises a set of plates  202 ,  204 ,  206 , and a polyphase signal generator. As can be seen in  FIG. 2C , the output signal U OUT    222  of the position sensor  108  is shown connected to terminals c and d of the amplifier  106  ( FIG. 1 ). 
     Emitters  202   a ,  202   b ,  202   c  are fixedly disposed about a rotating plate  204  to which the shaft  200  ( FIG. 2A ) is fixed. These emitters  202   a ,  202   b ,  202   c  are electromagnets, each emitting a magnetic field when energized. A rotating plate  204  is connected to the shaft  200  and rotates in proximity to the emitters  202   a ,  202   b ,  202   c .  FIG. 2C  shows a three-phase signal source connected to the emitters  202   a ,  202   b ,  202   c  where each phase U A , U B , U C  of the signal source is connected to one of the emitters. The magnetic fields emanating from the emitters  202   a ,  202   b ,  202   c  couple to the rotating plate  204  and to the shaft  200 . An output signal U OUT  can be obtained by wrapping a coil of wire about the shaft  200  (assuming that the shaft is of a suitable magnetic material) and measuring the current flow induced in the coil. 
     The amount of coupling of each phase U A , U B , U C  to the rotating plate  204  and shaft  200  will depend on the distances d 1 , d 2 , d 3  between the emitters and the rotating plate. The distances d 1 , d 2 , d 3  will vary as the shaft  200  moves about from its neutral position during operation of the motor, and consequently so will the amount of coupling of the phase U A , U B , U C  to the rotating plate  204  and shaft  200 . The closer an emitter (e.g.,  202   a ) is to the rotating plate  204 , the greater the coupling, and vice versa. Thus, the output signal U OUT  appearing will vary depending on the radial position of the rotating plate  204  relative to the emitters  202   a ,  202   b ,  202   c . Suppose the input signals (phases U A , U B , U C ) are represented by the following:
 
 U   A   =U  sin ω t  
 
 U   B   =U  sin (ω t+ 120°)
 
 U   A   =U  sin (ω t+ 240°),
 
where U is the magnitude of the sinusoidal signal. The output signal U OUT  of the position sensor  108  is generally represented by:
 
 U   OUT   =U   m (sin ω t+φ ),
 
where U m  is the magnitude of the signal U OUT  and φ is a phase shift of the signal U OUT . The position sensor  106  defines a “neutral position” for the rotor so that when the radial position of the rotor is at this neutral position the output signal U OUT  is substantially zero; i.e., U m  substantially zero (i.e., signal magnitude is substantially zero), and of course φ is zero since there is no phase shift when there is no signal.
 
     Suppose now that the rotor is displaced from its neutral position to a displace position. The output signal U ouT  of the position sensor  108  will be:
 
 U   OUT   =U   mx (sin ω t+φ ),
 
where U mx  is the magnitude of the output signal U OUT , and φ is the phase of the output signal relative to the output signal when the rotor was in its neutral position. The output signal U OUT  at this displaced position of the rotor represents the radial displacement of the rotor in polar coordinate terms, where U mx  is proportional to the displacement distance D ( FIG. 2A ) and the phase shift φ is equal to the subtended angle Θ ( FIG. 2A ).
 
       FIG. 2D  shows another example of a radial position sensor  108 , and in particular a capacitive radial position sensor suitable for generating an output signal representative of the radial displacement of the rotor of a motor (in terms of polar coordinates) vis-à-vis displacement of the rotor shaft  200  ( FIG. 2A ). Additional detail of this sensor is provided in U.S. application Ser. No. 12/044,835. The position sensor  108  comprises a set of plates  202 ′,  204 ′,  206 ′, and a polyphase signal generator. The output signal U OUT  of the position sensor  108  is shown connected to terminals c and d of the amplifier  106  ( FIG. 1 ). 
     Stationary plates  202 ′ and  206 ′ are fixedly disposed about the shaft  200  ( FIG. 2A ). These plates  202 ′,  206 ′ are stationary relative to the shaft. A rotating plate  204 ′ is connected to the shaft  200 . The stationary plate  202 ′ comprises three electrically isolated conductive wedges A, B, C. The stationary plate  206 ′ is an electrically conductive plate. The rotating plate  204 ′ is of a suitable dielectric material. The plates  202 ′,  204 ′,  206 ′ are arranged in proximity to each to allow for capacitive coupling between the stationary plates  202 ′,  206 ′. A three-phase signal source  222 ′ is connected to the stationary plate  202  such that each phase U A , U B , U C  of the signal source is connected to one of the wedges A, B, C of the stationary plate. The stationary plate  206 ′ is capacitively coupled to the stationary plate  202 ′ and has a resulting output signal U OUT  that represents a superposition of the phases U A , U B , U C  capacitively coupled from plate  202 ′ via plate  204 ′. 
     The discussion will now turn to  FIG. 3  and an explanation of the neutral point shifting circuit  122  shown in  FIG. 1 .  FIG. 3  illustrates additional details of the neutral point shifting circuit  122 . In the particular embodiment shown in the figure, the transformer T 1  is a three-phase transformer. More specifically, the transformer T 1  is a three-phase transformer comprising a Y-configured primary winding and a Y-configured secondary winding, a so-called “Y-Y transformer.” The primary-side terminals p 1 , p 2 , p 3  of transformer T 1  are connected to the phases of the signal generator  102 , while the primary-side terminal p 0  of transformer T 1  is connected to the neutral O 1  (e.g., ground potential) of the signal generator  102 . The secondary-side terminals s 1 , s 2 , s 3 , s 0  of transformer T 1  correspond respectively to terminals p 1 , p 2 , p 3 , p 0  of the transformer. 
     The transformer T 2  in the embodiment shown in  FIG. 3  is also a Y-Y transformer. The secondary-side terminals s 1 , s 2 , s 3  of transformer T 2  are connected to the phase coils  104   a ,  104   b ,  104   c  of the motor  104 , while the secondary-side terminal s 0  of the transformer is connected to the neutral O 2  of the phase coils. The primary-side terminals p 1 , p 2 , p 3 , p 0  of transformer T 2  correspond respectively to terminals s 1 , s 2 , s 3 , s 0  of the transformer. 
       FIG. 4  shows a wiring diagram for a three-phase Y-Y transformer. The transformer comprises three transformer elements T a , T b , T c . The primary windings of the transformer elements T a , T b , T c  are connected in a Y-configuration, and likewise the secondary windings of the transformer elements T a , T b , T c  are connected in a Y-configuration. The terminals p 1 , p 2 , p 3 , p 0 , s 1 , s 2 , s 3 , s 0  of the transformer elements T a , T b , T c  are connected so as to correspond with the terminal arrangement shown in  FIGS. 1 and 3 . 
       FIG. 5  shows an example of the amplifier  106  in accordance with the present invention. Bipolar transistors Q 1  to Q 5  are shown, but the design can be realized using FETs (field effect transistors) as well. Specific values for the resistor and capacitor elements can be readily determined by one of ordinary skill. The “Input” signal is obtained from the output of the position sensor  108 . More particularly, the output of the position sensor  108  is coupled to the terminals c and d of amplifier  106  (see  FIG. 1 ), which respectively are tied to the amplifier input and ground. A portion of the transformer T 1  is shown in this figure. The terminal s 0  of transformer T 1  is connected to terminal a of amplifier  106 , which is also tied to ground. Likewise, a portion of the transformer T 2  is shown; the terminal p 0  of the transformer is connected to terminal b of amplifier  106 , which is tied to the amplifier “Output”. 
       FIG. 6  is a circuit schematic diagram illustrating the connection of the circuitry shown in FIGS.  1  and  3 - 5 . The amplifier illustrated in  FIG. 5  is a conventional power amplifier, and it will be appreciated that other conventionally known amplifier designs can be used. For example, magnetic amplifiers are a well established technology that are well suited for use with the present invention. 
     Referring now to  FIG. 7A , operation of the present invention will be explained. The signal generator  102  produces input drive signals U A , U B , U C  relative to a neutral point O 1 . As discussed above, the neutral point O 1  is typically ground potential. The input drive signals U A , U B , U C  are represented in vector notation in the figure. When the rotor of the motor  104  moves from its neutral position, the radial position sensor  108  senses the displacement and outputs a signal that represents the displacement, where the output signal provides information indicative of the position of the displaced rotor relative to the neutral position. The output signal of the position sensor  108  is represented in  FIG. 7A  by the designation U. 
     In accordance with the present invention, the output signal U is used to shift the neutral point O 1  of the input signals U A , U B , U C  to produce modified drive signals U′ A , U′ B , U′ C  having a shifted neutral point O 2 . In other words, the reference potential of the modified drive signals U′ A , U′ B , U′ C  is different from the reference potential of the input signals U A , U B , U C . The modified drive signals U′ A , U′ B , U′ C  are applied to the phase coils  104   a ,  104   b ,  104   c  comprising the stator of the motor  104 . 
     The modified drive signals U′ A , U′ B , U′ C  are modified in that their neutral point O 2  is shifted with respect to the neutral point O 1  of the input drive signals U A , U B , U C . The modified drive signals U′ A , U′ B , U′ C  can be viewed as having offset components that are superimposed on the sinusoidal components of the input drive signals U A , U B , U C . The offsets in the modified drive signals U′ A , U′ B , U′ C  energize the phase coils  104   a ,  104   b ,  104   c  to produce radially directed magnetic forces, in addition to producing rotational torque forces. Thus, the phase coils  104   a ,  104   b ,  104   c , when energized by signals U′ A , U′ B , U′ C , will generate torque forces and radial forces. By comparison, conventionally produced drive signals simply result in torque production only. 
     The amount of offset superimposed on each of the modified signals U′ A , U′ B , U′ C  will vary depending on where the neutral point O 1  is shifted. The radial force generated by each phase coil  104   a ,  104   b ,  104   c  will therefore differ in strength. Consequently, the rotor will be biased in a direction depending on the relative strengths of the radial forces produced by the phase coils  104   a ,  104   b ,  104   c  and exerted on the rotor. 
       FIG. 7B  shows a generalized circuit that embodies the principles set forth in  FIG. 7A . Signal generator  702  produces an N-phase drive signal; the figure shows an example for a three-phase signal generator. The phase signals U A , U B , U C  are applied to the phase coils  704   a ,  704   b ,  704   c  of the stator of motor  704 . Conventionally, the phase signals U A , U B , U c  generate only a rotational torque because the neutral point of the generator and the neutral point of the phase coils are at the same potential, typically ground potential. 
     However, as the figure shows, the neutral point O 1  of the phase signals U A , U B , U C  is shifted based on the output signal U of the position sensor  708 . The resulting modified signals U′ A , U′ B , U′ C  have a shifted neutral point O 2 . Due to the shifted neutral point O 2  of the modified signals U′ A , U′ B , U′ C , the phase coils generate radially directed magnetic forces in addition to torques, thus affecting the radial position of the rotor in order to suspend the rotor. The modified signals U′ A , U′ B , U′ C  therefore obviate the need for a separate system of bearings to suspend the rotor. Referring to  FIG. 1 , the inclusion of the transformers T 1  and T 2  serve to isolate the neutral points O 1  and O 2  in order to avoid short circuiting the signal generator  102  and the phase coils  104   a ,  104   b ,  104   c.    
     The foregoing embodiment of the present invention is adapted for a motor  104  ( FIG. 1 ) having Y-connected phase coils  104   a ,  104   b ,  104   c . The present invention can be readily adapted for a motor having Δ-connected phase coils.  FIG. 8A  shows the schematic diagram for the phase coil connections  804   a ,  804   b ,  804   c  of a Δ-connected motor  804 . 
       FIG. 8A  illustrates an embodiment of the present invention suitable for a Δ-connected motor  804 , where the phase coils  804   a ,  804   b ,  804   c  are connected in a Δ-configuration. Referring to  FIG. 1  or  FIG. 6 , the Y-connected motor  104  is replaced with the Δ-connected motor  804  of  FIG. 8A . To accommodated a Δ-connected motor, the Y-Y transformer T 2  in the neutral point shifting circuit  122  is replaced by the Y-Δ transformer T′ 2 .  FIG. 8B  shows an example of the wiring for a Y-Δ transformer. 
     In the embodiment shown in  FIGS. 8A and 8B , the neutral point O 1  of the input drive signals U A , U B , U C  is shifted by the amplifier  106 , in the manner discussed above, and coupled to the motor  804  by way of the Y-Δ transformer T′ 2 . The resulting modified drive signals U′ A , U′ B , U′ C  include the sinusoidal torque-generating signals which energize the phase coils  804   a ,  804   b ,  804   c  to generate rotational torque. The modified drive signals U′ A , U′ B , U′ C  also include offset components which energize the phase coils  804   a ,  804   b ,  804   c  to also generate radially directed magnetic forces to adjust the radial position of the rotor element of the motor  804 . 
       FIG. 8C  shows the transformer configuration for other combinations of Y-connected and Δ-connected signal generator types  802 ′ and motor types  804 ′. Four configuration combinations  822 ,  824 ,  826 ,  828  are shown using schematic representations that indicate the connection type, namely Y- or Δ-connected. Combination  822  represents the configuration shown in  FIG. 1 , where both the signal generator  802 ′ and the motor  804 ′ are Y-connected devices. The transformers T 1 , T 2  serve to isolate the potential difference that exist between the neutral points O 1 , O 2 . Combination  824  represents the configuration explained in  FIGS. 8A and 8B , where the signal generator  802 ′ is a Y-connected device and the motor  804 ′ is a Δ-connected device. Combination  826  represents a configuration where the signal generator  802 ′ is a Δ-connected device and the motor  804 ′ is a Y-connected device. Here, the transformer T 1  is connected in a Δ configuration on its primary side and in a Y configuration on its secondary side. The transformer T 2  is connected in a Y configuration on its primary side and on its secondary side. Combination  828  shows a configuration where both the signal generator  802 ′ and the motor  804 ′ are Δ-connected devices. Here, the transformer T 1  is connected in a Δ configuration on its primary side and in a Y configuration on its secondary side, while the transformer T 2  is connected in a Y configuration on its primary side and in a Δ configuration on its secondary side. 
       FIG. 9  is an illustration of a particular motor configuration according to the embodiment of the present invention depicted in  FIG. 1 . The example shown in  FIG. 9  depicts a motor  900  having two motor units  904   a ,  904   b  operating together as a single motor. Each motor unit  904   a ,  904   b  is represented in  FIG. 1  by the motor  104 . The output of the neutral point shifting circuit  122  is connected to the motor unit  904   a  in the manner shown in  FIG. 1 . The motor unit  904   a ,  904   b  are connected such that respective windings of each motor unit are connected in series. 
     The foregoing embodiments of the present invention are adapted for a magnetic bearing system in which the motor&#39;s phase coils are energized by a drive signal that generates rotational torque and to generate radial force; i.e., a bearingless motor. The result is that only a single set of phase coil windings are required to operate the motor (i.e., torque production) and to provide control over the radial position of the rotor (production of suspension forces), thus suspending an operating rotor without the need for physical bearings or lubrication. The dual functionality that the phase coils made possible by the present invention represents a significant advantage over conventional magnetic bearing systems which require a separate set of windings to provide the functionality of a magnetic bearing. However, it will be appreciated that the present invention can be readily adapted for use where only the suspension functionality is needed. 
     In the previously described embodiments, the element  104  is a motor and the phase coils  104   a ,  104   b ,  104   c  are components of the motor. The phase coils thus operate to produce rotational torque in conjunction with a rotor element comprising an arrangement of permanent magnets. However, the present invention can be used in a suspension-only mode where the element  104  shown in  FIG. 1  operates simply as a magnetic bearing and the phase coils  104   a ,  104   b ,  104   c  do not contribute to or are otherwise involved in producing rotational torque, but rather provide radial force to adjust the radial position of the rotor of a motor to control suspension of the rotor. This is readily achieved where the phase coils  104   a ,  104   b ,  104   c  are arranged to magnetically interact with some portion of the motor (e.g., a portion of the rotor shaft) other than the rotor component of the motor. Thus, in the suspension-only embodiment, the modified drive signals U′ A , U′ B , U′ C  might be more aptly referred to as suspension signals, repositioning signals, or the like. 
       FIG. 10  shows an example of a suspension-only embodiment of the present invention. Here, a motor  1000  comprises a motor unit  1002  (comprising a motor stator and a motor rotor) and a suspension unit  1004 , corresponding to element  104  in  FIG. 1 . The suspension unit  1004  comprises a stator component  1012  which in turn comprises windings configured in the same manner as coils  104   a ,  104   b ,  104   c  shown in  FIG. 1 . The rotary component  1014  is a single element of magnetic material, rather than a plurality of individual permanent magnets as in the case of a rotor of a motor. The suspension unit  1004  is driven by a polyphase source (not shown) comprising signals U A , U B , U C . 
     The suspension-only embodiment shown in  FIG. 10  still represents a significant improvement over conventional magnetic bearing systems. Whereas conventional magnetic bearing systems require significant amounts of digital processing to produce suitable polar coordinate correction signals, the present invention as embodied in  FIG. 10  produces a position sensing signal (via a position sensor such as position sensor  108  in  FIG. 1 ) which is then used to shift the neutral point O 1  of the polyphase source signals U A , U B , U C . The resulting neutral point shifted signals U′ A , U′ B , U′ C  are then applied to the phase coils  104   a ,  104   b ,  104   c  of suspension unit  1004 .