Patent Publication Number: US-11378090-B2

Title: Compact centrifugal pump with magnetically suspended impeller

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of, and claims the benefit to, U.S. patent application Ser. No. 16/032,316, filed Jul. 11, 2018, the contents of which are incorporated herein by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to pumps for handling fluids such as blood that are sensitive to mechanical stress. More particularly, the present invention relates to centrifugal pumps in which an impeller is suspended and rotated using magnetic fields without mechanical contact between the impeller and the pump housing. 
     BACKGROUND 
     Various types of rotary blood pumps have been developed for clinical use as either implantable or extracorporeal devices. Implantable blood pumps, also known as ventricular assist devices, are used for saving lives of heart failure patients. Some extracorporeal blood pumps are used for temporary ventricular assist, and others are an integral part of the heart-lung system during open-heart surgery, or part of the extracorporeal membrane oxygenator (ECMO) that provides life support for patients with heart and lung dysfunctions. One particular challenge in the design of these pumps pertains to the fact that blood cells and proteins in blood are prone to damage due to non-physiological flow in the pump, leading to hemocompatibility issues including hemolysis (broken red blood cells) and thrombosis (clotting of blood). In addition, implantable blood pumps may be miniaturized to lessen invasiveness of surgical implantation. These pumps need to be highly reliable since they are life-saving devices, and need high power efficiency to prolong the time interval between changes of the carry-on batteries. 
     How the pump impeller is suspended may have a significant impact on the pump&#39;s performance in handling blood or other stress-sensitive fluids. Three types of impeller suspension are known, including mechanical, hydrodynamic and magnetic suspension. Mechanical suspension relies on physical contact between the rotor and stationary part in the pump housing. A typical design of a mechanical suspension impeller can be found in U.S. Pat. No. 8,088,059 which incorporates a pair of mechanical bearings immersed in blood. Another design can be found in U.S. Pat. No. 6,155,969 where the entire suspension consists of a mechanical bearing (pivot bearing) and a magnetic bearing with permanent magnets. Although simple in construction, mechanical suspension is associated with blood damage due to excessive shear stress in the flow field near the bearing and heat generation on the bearing surfaces. Mechanically suspended impellers also suffer from durability issues due to mechanical wear of the bearing surfaces. 
     Apart from mechanical suspension, hydrodynamic suspension relies on localized pressures in a thin layer of fluid film, blood film in the case of a blood pump, that keeps the bearing couple separated. The bearing couple surfaces are specially designed so that when the rotor moves to a speed beyond a threshold, localized high pressure is established in the fluid filling in between the bearing couple. A typical blood pump with hydrodynamic suspension is described in U.S. Pat. No. 7,976,271 in which the hydrodynamic suspension is accompanied by a set of permanent magnetic suspensions to achieve full stability in all degrees-of-freedom. Although hydrodynamic suspension avoids direct physical contact, the suspension gap must be extremely small to maintain high enough localized pressure. This induces excessive shear stress in the flow field within the gap, which may cause damage to the blood or other stress-sensitive fluid in the gap to a comparable extent as that of a mechanical bearing. 
     Magnetic suspension differs from mechanical or hydrodynamic suspension by employing a magnetic force, which is inherently non-contact, eliminating the need for a fluid as a medium to suspend the pump impeller. It has been demonstrated that a rotor can be fully suspended with desired stiffness in all degrees-of-freedom by using actively controlled electromagnets alone or in combination with permanent magnets. Unlike hydrodynamic suspension, magnetic suspension allows a significantly greater suspension gap so that blood in the gap is subjected to less shear stress, which helps to improve blood compatibility. Another advantage of magnetic suspension is the lack of physical contact between the components, eliminating any mechanical wear on the parts of the suspension. 
     Pumps capable of handling stress sensitive fluids without mechanical wear may be implemented in other applications aside from pumping blood. For example, chemical-mechanical planarization (CMP) using a slurry of precise particles is a common process for polishing a wafer surface in the integrated circuit industry. It has been observed that excessive stress in slurry mixtures during transportation causes aggregation of the suspended particles, and the oversized particles lead to defect scratches on the wafer surface. This issue can be addressed by replacing the diaphragm pump in the conventional process with a fully magnetically suspended pump that can avoid excessive stress in the slurry. Another area of application pertains to transportation of ultra-pure fluids, e.g. ultra-pure water for manufacturing of microelectronic components. Using full magnetic suspension the mechanical wear inside the pump can be reduced and thusly avoid contamination of wear-off debris into the pure fluid. 
     The rotor in a magnetically suspended pump can be classified into shaft-like and disc-like types. A shaft-like rotor has greater axial dimension than radial dimension and is usually suspended with two sets of radial/journal bearings that are distinctly separated along the rotor&#39;s rotational axis. A disc-like rotor has greater radial dimension than axial dimension or may have substantially similar axial and radial dimensions and is usually suspended with a single set of radial bearing. Inclination of a shaft-like rotor is usually stabilized with a torque resulting from the difference in the radial bearing forces that are apart from each other along the shaft axis. Conversely, inclination of a disc-like rotor is usually stabilized with the overall effect of the distributed forces on the rotor which results in a net torque about the inclination axes. The distributed forces may be provided by a special tilt bearing unit arranged around the rotor, or by a single unit of magnetic bearing that serve the dual functions of radial and tilt suspension. 
     SUMMARY 
     Embodiments of the present invention include a pump with a fully magnetically suspended rotor to improve blood compatibility when pumping blood, or other fluid with similar fluid dynamic characteristics. In particular, it is desirable to have such a pump that stabilizes radial displacements of a disc-like rotor with active control through separate electric motor and magnetic bearings to improve the pump&#39;s critical performances including device packaging size, system simplicity and reliability, stiffness and other dynamic performances of suspension, power efficiency, and others. 
     One embodiment of the invention includes a pump apparatus with a housing having inlet and outlet for respectively receiving and discharging fluid and a central axis. A rotor may be positioned within the interior of the housing to be rotatable about the central axis. The rotor may have an impeller for pumping fluid between the inlet and the outlet, and may be magnetically suspended to maintain a flow channel between the rotor and the housing. An electric motor may be adapted to drive the rotor about a rotational axis substantially coincident with the central axis. The electric motor may include a motor rotor assembly disposed within the rotor and a motor stator assembly disposed within the housing. The pump apparatus may further include a magnetic suspension device including an annular rotor primary pole piece mounted within the rotor coaxially with the rotational axis. The annular rotor primary pole piece may comprise a ferromagnetic material for channeling magnetic flux and have a first end surface, a second end surface, and a cylindrical side surface configured to serve as a rotor pole face. A plurality of electromagnet units mounted within the housing and circumferentially distributed at regular intervals about the central axis. Each electromagnet unit may include a pole shoe having a first end surface, a second end surface, and a side cylindrical surface configured to serve as a casing pole face. An iron core may extend from the pole shoe and a back yoke may connect two or more of the iron cores of different electromagnet units together. A coil may be wound around the iron core for conducting electric current. The pole shoe, iron core, and back yoke may comprise ferromagnetic material for channeling magnetic flux and the first end surface of the rotor primary pole piece and the first end surfaces of all the pole shoes are on a same side along an axial direction. The rotor pole face and each casing pole face may oppose to each other and define a primary suspension gap thereinbetween. The primary suspension gaps may be axially aligned with each other and circumferentially separated from each other. 
     At least one permanent magnet may generate a plurality of bias magnetic fluxes. Each bias magnetic flux may radially pass through one of the primary suspension gaps, and pass through the interior of the rotor primary pole piece and of the pole shoe of electromagnet unit. The at least one permanent magnet may be magnetized in such a direction that all the bias magnetic fluxes pass through the primary suspension gaps in a same polar direction. A plurality of position sensors may be disposed circumferentially around the rotor and mounted within the housing for detecting a radial position of the rotor pole face. 
     The pump apparatus may further include a feedback control system for generating and delivering electric current into the coils of the plurality of electromagnet units according to a real-time output of the position sensors. The feedback control system may include a control strategy adapted to achieve stability of radial positioning of the rotor. The plurality of the electromagnet units may be electrically and magnetically connected to jointly generate a modulating magnetic flux for active control of the position of the rotor along any one of two orthogonal radial axes. A first radial axis may have a first side and a second side divided by a second radial axis. The modulating magnetic flux may radially pass through a plurality of the primary suspension gaps and superimpose the bias magnetic fluxes to enhance the bias magnetic flux in the primary suspension gap located on the first side of the radial axis and to weaken the bias magnetic flux in the primary suspension gap located on the second side of the radial axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of facilitating an understanding of the subject matter sought to be protected, there are illustrated in the accompanying drawings embodiments thereof, from an inspection of which, when considered in connection with the following description, the subject matter sought to be protected, its construction and operation, and many of its advantages should be readily understood and appreciated. 
         FIG. 1  is a top front perspective of a pump in accordance with an embodiment the present invention. 
         FIG. 2  is an exploded view of the pump of  FIG. 1 , showing the pump&#39;s interior construction for fluid flow through the pump in accordance with an embodiment the present invention. 
         FIG. 3  is a front cross-sectional view of the pump of  FIG. 1  in accordance with an embodiment the present invention. 
         FIG. 4  is an exploded isometric view of the assemblies of the magnetic suspension and the electric motor in the pump of  FIG. 1  in accordance with an embodiment the present invention. 
         FIG. 5  is a top cross-sectional view of the pump of  FIG. 1 , in accordance with an embodiment the present invention. 
         FIGS. 6( a ), 6( b ), 6( c ), and 6( d )  depict an elementary passive suspension unit consisting of magnetically coupled annular members in the rotor and the casing respectively, in accordance with an embodiment the present invention. 
         FIG. 7  depicts elementary passive suspension units in which a magnetic flux does not link the rotor and casing members. 
         FIG. 8  is a front cross-sectional view of an exemplary passive suspension unit in accordance with an embodiment the present invention. 
         FIGS. 9( a ), 9( b )  are cross-sectional views of the magnetic suspension assembly in the pump of  FIGS. 3 through 5 . 
         FIG. 10  is a schematic drawing of the feedback control loop for active suspension control in accordance with an embodiment of the present invention. 
         FIGS. 11( a ), 11( b ), 11( c )  are cross-sectional views of various elementary magnetic suspension units that can be employed in a pump in accordance with an embodiment of the present invention, 
         FIG. 12  is a cross-sectional view of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIGS. 13( a ), 13( b )  are cross-sectional views of an embodiment of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention, 
         FIG. 14  is a cross-sectional view of an embodiment of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIG. 15  is a cross-sectional view of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIG. 16  is a cross-sectional view of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIGS. 17( a ), 17( b ), 17( c )  are cross-sectional views of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIGS. 18( a ), 18( b )  are cross-sectional views of the magnetic suspension assembly of a pump in accordance with an embodiment of the present invention. 
         FIG. 19  is the magnetic circuit for the electromagnet units of  FIG. 18  in accordance with an embodiment the present invention. 
         FIG. 20  is a top front perspective of another pump in accordance with an embodiment of the present invention. 
         FIG. 21  is an exploded view of the pump of  FIG. 20 , showing the pump&#39;s interior construction for fluid flow through the pump. 
         FIG. 22  is a front cross-sectional view of the pump of  FIG. 20 , showing the constructions of the rotor and the housing with emphasis on the magnetic suspension and electric motor. 
         FIG. 23  is an exploded isometric view of the assemblies of the magnetic suspension and the electric motor in the pump of  FIG. 20 , shown in partial cross-sectional views. 
         FIG. 24  depicts the magnetic suspension assembly in the pump of  FIGS. 20 through 23 . 
     
    
    
     DETAILED DESCRIPTION 
     While this disclosure is susceptible of embodiments in many different forms, there is shown in the drawings, and will herein be described in detail, certain embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the disclosure and is not intended to limit the broad aspect of the disclosure to embodiments illustrated. 
     Referring to  FIGS. 1-3 , a pump apparatus  10 , according to an embodiment of the present disclosure, includes a housing  12  with an inlet  11  to receive working fluid and an outlet  13  to discharge the working fluid. A housing  12  consists of a continuous inner wall that borders an interior chamber  20 , within which a rotor  30  with an integrated impeller  32  is mounted. The housing  12  also consists of an outer wall which, with the inner housing wall, forms a space of substantial volume for containing structural components of the magnetic suspension and electric motor. An outlet  13  extends into the housing chamber  20  and communicates with the pump volute  22  that is advantageously constructed for obtaining pressure rise from the kinetic energy of a fluid. 
     The rotor  30  is disposed for rotation about the central axis z of housing  12 , as depicted in  FIG. 2 . The impeller  32  is composed of a plurality of blades  33  that transfers energy to the working fluid when the impeller  32  rotates. The rotor  30  contains components of the magnetic suspension and electric motor that interact with the corresponding components within the housing  12  to provide the force and torque necessary to suspend and revolve the rotor  30 . 
     According to an embodiment of the present disclosure, the rotor  30  may take an annular shape, and the housing interior chamber  20  may have a corresponding annular channel  24  that accommodates the annular rotor  30 . The inner wall of annular channel  24  forms a central post  15  that projects from the bottom surface of annular channel  24 . The outer wall of the annular channel  14  attaches to the exterior casing  16  that is a portion of the space between the inner and outer walls of the housing  12 . Either the central post  15  or the exterior casing  16 , or both, may contain components of the magnetic suspension and/or electric motor. 
     An xyz coordinate system is represented on the housing  12  as shown in  FIGS. 2 through 5 . The z axis overlaps with the central axis of the cylindrical surface of central post  15 . The xy plane passes through the middle height of the electromagnet pole shoes  83   a - d  ( FIGS. 3, 4, 9 ( a )). 
     When the pump  10  is assembled with the rotor  30  placed in the annular channel  24 , and the magnetic suspension takes effect properly, the rotor is fully suspended by magnetic forces such that in normal operation no part of the rotor  30  is in physical contact with the housing surface. In this way, the surface of the rotor  30  and the corresponding surface of the annular channel  24  define a U-shaped suspension gap  25  ( FIG. 3 ) therebetween. Also, as the rotor  30  is properly suspended, the passageway of the impeller  32  (with blades  33 ) becomes aligned with the passageway of the volute  22 . Therefore, when the rotor  30  rotates, working fluid entering into the pump  10  through inlet  11  is pushed by the impeller blades  33  to flow radially outwards through the impeller passageway and enters into the volute  22 . The fluid is collected by the volute  22  and discharged out of the pump  10  through outlet  13 . 
     Due to pressure differences, a fractional amount of fluid flows through the U-shaped suspension gap  25  and forms a secondary flow around the rotor  30 . Since the pressure at the outer opening of the U-shaped suspension gap  25  is greater than the pressure at the inner opening of the “U”, the secondary flow is created by fluid entering the outer opening of the gap, flowing downwards on the outer side of the rotor  30 , inwards at the bottom of the rotor  30 , and upwards on the inner side of the rotor  30 , and exiting the U-shaped suspension gap  25  from the inner opening. It can be appreciated that such a secondary flow path, according to an embodiment of the present invention, is straightforward and free from obstructive object or structure, such as zigzag structure, that would otherwise cause flow stagnation or significantly hinder the flow. Consequently, the secondary flow produces unimpeded wash out on the entire rotor surface, which helps to prevent blood clotting, among other benefits in handling stress-sensitive fluids. 
     According to an embodiment of the present invention, a thin-walled jacket of any suitable material that is compatible with the fluid the pump handles, such as a titanium alloy, or suitable coating may be applied on the rotor surface and the housing inner surface to keep the parts within the rotor  30  and the housing  12  from direct contact with the working fluid. However, for the sake of clarity, such a jacket or coating is not shown in the drawings herein. Further, when addressing principles of operation of the motor and magnetic suspension, the term “air gap,” as used herein, designates the gap between magnetic parts although in actual practice such a gap may be filled with fluid and/or any other nonmagnetic materials, or even a vacuum, rather than air. 
     Now turning to  FIGS. 3-5 , one example embodiment of an electric motor  40  and magnetic suspension  60  is illustrated. The motor  40  is preferably of a brushless DC or brushless AC type, although various other types, such as induction motor, can be employed by one skilled in the art based on the general principles disclosed herein. As is illustrated in  FIGS. 3 and 4 , a brushless motor  40  consists of a stator assembly  41  mounted within the housing  12  and a rotor assembly  42  mounted within the rotor  30 . In this embodiment, the stator assembly  41  is located within the central post  15 , but it may alternatively be disposed in another portion of the housing  12 , such as exterior casing  16 , for example. 
     The motor stator assembly  41  is disposed closely adjacent to an air gap  43  to favor power efficiency. The motor stator assembly  41  includes a plurality of coils  46  that are grouped into windings of multiple phases, for example 3 phases, as commonly known by those having skill in the field. According to an embodiment of the present disclosure, motor coils  46  are wound around teeth of a motor stator core  47 , which is made of a ferromagnetic material, such as soft iron or silicone steel, with a laminated or non-laminated structure. However, the motor stator core  47  may be made in part or entirely of any non-magnetic material in order to reduce or eliminate unbalanced magnetic pull on the rotor  30 . The unbalanced magnetic pull is the magnetic force induced between the rotor magnets and the stator magnetic material when the rotor and stator are not in perfect alignment geometrically and magnetically in a radial direction. Such force is generally unwanted, especially in a design with magnetic suspension, since it causes negative stiffness that has to be counterbalanced by magnetic suspension. Therefore, although a motor stator having a ferromagnetic core may contribute to increased power efficiency, such a core structure may not be necessary for optimizing the overall performance of the apparatus, i.e. a coreless motor structure may be used. 
     The motor rotor assembly  42  includes a plurality of permanent magnet segments  48  installed around the inner periphery of the rotor  30 , adjacent to the air gap  43 . These permanent magnet segments  48  are mounted piece by piece circumferentially and are configured with alternating polarization to form the magnetic poles of the motor rotor, which generates a circumferentially alternating magnetic field in the air gap  43 , as commonly known to one skilled in the art. Preferably, a Halbach array can be employed to form concentrated magnetic field towards air gap  43 . Also, a magnetic yoke may be used on the back side of the permanent magnets  48  to advantageously facilitate assembly and enhance magnetic performance. However, it may not be necessary for other considerations, such as reducing the rotor size. According to an embodiment of the present disclosure, the annular pole member or piece  73 , a constructional part of the magnetic suspension assembly  60 , also serves for the back yoke of permanent motor magnets  48 . 
     Although  FIGS. 3-5  show motor  40  located in the inner portion of the pump  10 , the motor  40  can be disposed within the outer portion of the pump  10  by inversing its structures inside-out. In that way, the motor stator assembly  41  will be mounted within the exterior casing  16  of the pump housing  12 , with the stator core  47 , if any, and the winding coils  46  inverted such that the motor coils  46  reside adjacent to the air gap  63 . Accordingly, the motor rotor assembly  42  will be moved to the outer periphery of the rotor  30 , with permanent magnets  48  and back yoke, if any, inverted inside-out so that magnetic poles are formed in the air gap  63 . 
     In another alternative embodiment, the motor  40  may be disposed within the base portion of the housing  12 , beneath the air gap  62  that corresponds to the bottom of the U-shaped gap  25  ( FIG. 3 ). In such a configuration, an axial flux motor similar to that described herein in  FIGS. 22 and 23  will be constructed by one skilled in the art in accordance with the principle of the present disclosure. 
     Turning now to the principle and construction of the magnetic suspension in pump  10 , coordinate system xyz, as stated above, is used for referring the five degrees of freedom (DOFs) of the rotor  30  to be stabilized. These five DOFs include one axial displacement along the z axis, two radial displacements along the x and y axis respectively, and two tilting displacements (angular displacement) about the x and y axis respectively. The radial displacements are stabilized through feedback control of an electric current feeding into electromagnetic coils in a hybrid structure of an electromagnet and a permanent magnet. The other DOFs are stabilized by passive suspension, or utilization of permanent magnets. 
     In accordance with an embodiment of the present disclosure, the passive suspension is comprised of one or several elementary units each including co-axial annular members, respectively, installed in the rotor  30  and stationary casing (within the pump housing  12 ). The rotor  30  and casing members are separated by a radial air gap, or in other words, they oppose to each other across a radial air gap. Without loss of generality, the concept detailing the outer member on the casing is explained below. 
     One embodiment of the elementary passive suspension unit is shown in  FIG. 6 . In this example, both rotor and casing members are advantageously formed into substantially equivalent thickness, although this is not necessary for successful practice of the present invention. In addition, both members  101 ,  102  are preferably permanent magnets, but any one of them may be replaced with a soft iron part without deviating from the general principle disclosed herein. However, stronger magnetic flux can be produced by using permanent magnets, so that increased suspension stiffness can be obtained in the same amount of space. Using permanent magnets can also reduce negative stiffness in the radial direction, and thus facilitate the radial suspension design for better performance. 
     As used herein, the term “permanent magnet” or “magnet” refers to a part made of a ferromagnetic material that has a large remanence and a large coercivity, and is magnetized to serve as a source of a magnetic field, such as NeFeB, as commonly known to one skilled in the art. A “soft iron”, as used herein refers to a part made of laminated or non-laminated ferromagnetic material that has a small remanence and a small coercivity, and is used for channeling magnetic flux, such as pure iron, silicone steel, or Hiperco alloy, as commonly known to one skilled in the art. 
     A coordinate system xyz is represented on the stationary casing of  FIG. 6 . As shown in  FIGS. 6( a ) through 6( c ) , annular members  101 ,  102 , separated by an air-gap  104  are both magnetized along the z axis, but in opposite directions. Letters “N” and “S” denote the north pole and south pole, respectively. Accordingly, these magnets produce a series of loops of working magnetic flux  103  that lie in meridian plan and pass through the interior of both annular members  101 ,  102 , or link these members. Note that the term “working magnetic flux”, as used herein stands for the magnetic flux that contributes to the primary forces for suspension, in contrast to the leakage flux. 
     The passive stability can be appreciated with the principle that a magnetic flux loop tends to minimize its total reluctance. Therefore, annular members  101 ,  102  tend to align with each other about the center of thickness (along the z axis) as shown in  FIG. 6( a ) . If annular member  102  experiences an upward displacement as shown in  FIG. 6( b ) , then the net attracting force on the left and right cross-sectional areas of annular member  102  from casing member  101 , f 1  and f 2  respectively, become inclined with respect to the x-y plane. The sum of these forces forms a net force F that pulls annular member  102  downwards, restoring alignment with member  101 . This mechanism stabilizes the rotor in axial direction. 
     As shown in  FIG. 6( c ) , if annular member  102  gets an angular displacement (tilting) about the x axis, then distributed attracting forces are induced on annular member  102  from annular member  101 . The net force on the right (positive y) cross-sectional area, f 2 , inclines from the x-y plane towards the negative z direction, while the net force on the left (negative y) cross-sectional area, f 1 , inclines from the x-y plane towards the positive z direction. If the thicknesses of annular members  101 ,  102  are sufficiently small relative to the diameter of the air gap  104  and the tilting angle is sufficiently small, then the acting point of force f 2  locates above the acting point of force f 1 . Therefore, a net torque T on annular member  102  occurs thereby tending to realign the rotor member with the casing member. This mechanism provides tilting stability of the rotor with passive suspension. 
     The suspension described above is inherently unstable in the radial direction. If the annular member  102  becomes misaligned with the casing member  103  in the radial direction of  FIG. 6( a ) , a net attracting force may be induced on annular member  102  to push it further away from the center, increasing the misalignment until the annular member  102  touches the inner surface of the casing member  103 . In fact, the passive suspension of embodiments of the present invention is characterized by distributed attracting forces in a radial direction between the rotor and casing members, rather than repulsive forces in the radial direction or attracting forces in an axial direction. If otherwise two concentric annular members  111 ,  112  are magnetized in the same direction as illustrated in  FIG. 7 , then a distributed repulsive force is brought about between annular members  111 ,  112  in radial direction. As such, the working magnetic flux of any of the annular members  111 ,  112  of  FIG. 7  completes a loop (e.g. flex loop  114 ) that merely passes through the interior of that magnet member itself (annular member  112 ), but not through the other member (annular member  111 ). A similar effect may occur with other respective annular members (e.g. flux loop  113 ). In other words, the magnetic flux does not link the rotor and casing members that oppose to each other across a radial air gap  115 . Such a configuration does not serve for passive suspension of this invention. 
     Therefore, in accordance with an embodiment of the present invention, passive suspension is achieved with working magnetic flux loop that links rotor and casing members that oppose to each other across a radial air gap. As long as the overall thickness of the suspension unit is sufficiently small in comparison with the diameter of the air gap, passive stability in axial displacement and tilting displacement can be obtained. This principle is referred to as the principle of flux loop linkage and is the sufficient criteria for achieving passive suspension in this disclosure. For example, a valid suspension remains if one of the members  101 ,  102  of  FIGS. 6( a )-( c )  is replaced with a soft iron, since the flux loop still passes through the interior of the both members. 
     Although the magnets of  FIG. 6( a )  are axially polarized, various other arrangements may be employed by one skilled in the art to create the same effect of passive suspension based on the principles disclosed herein. For example, as shown in  FIG. 6( d ) , the annular member magnets  105 ,  106  are polarized in a radial direction, which creates a working flux loop  107  that links the magnets  105 ,  106 . This construction can serve substantially the same function of passive suspension for axial displacement and tilting. Other combinations of polarization of magnets, e.g. one axially polarized and the other radially polarized, may also be used. Such operable examples also include magnets polarized in an inclined direction with respect to the z axis. 
     The elementary suspension unit described above can be enhanced by adding annular plates of soft iron onto one or both ends of any axially magnetized permanent magnet of  FIGS. 6( a )-( c ) . Such a plate, namely end pole piece, serves to concentrate magnetic flux into the soft iron and brings about intensified magnetic flux density in the air gap. A magnetic force applied on a surface of highly permeable magnetic material depends not only on the total flux over the surface, but also on the flux density on the surface. For the same total flux going into or out of a surface, the higher the flux density on the surface is, the greater the magnetic force the surface experiences. Therefore, by adding end pole pieces on the ends of the permanent magnets in an elementary suspension unit of  FIGS. 6( a )-( c ) , increased suspension stiffness can be obtained with same volume of permanent magnet. 
     The annular members of permanent magnet or soft iron in the rotor of a magnetic suspension assembly of the embodiments of the present invention are preferably complete rings substantially uniform in geometry and magnetic characteristics around the circumference. An otherwise discontinuous structure that produces a significantly varying magnetic field around the circumference of rotor can bring about undesirable effects when the rotor rotates. For example, the variation of the magnetic field may lead to an unsteady suspension force and stiffness as the rotor rotates, which can cause vibration and other undesirable dynamic effects. It also induces an eddy current in electrically conducting materials in the casing, which can cause energy loss and heating. 
     Adversely, some or all of the annular members in the casing of a magnetic suspension assembly of the embodiments of the present invention may be formed of geometrically or magnetically non-uniform or interrupted structures. This is because such an alternative structure by itself does not cause a variation of suspension force or an eddy current as the rotor rotates. For example, a set of arcuated segments of permanent magnets or soft iron evenly distributed along a circle, especially if the segments together cover the majority of the circle, can suitably serve for the magnetic suspension of the embodiments of the present invention. 
     The elementary passive suspension unit, discussed above, may be used as an independent structure, or by forming a stack of multiple units in an arrangement of alternating magnetic polarizations between neighboring units.  FIG. 8  illustrates an embodiment of such a stacked structure in accordance with this principle. As discussed above, tilt stability of an elementary passive suspension unit, such as that of  FIGS. 6( a )-( d ) , requires sufficiently small thickness of annular magnetic members in comparison with the diameter of the air gap. According to the same principle, in order to achieve tilt stability of a stacked structure, the overall thickness of the stack is made sufficiently small relative to the diameter of the air gap. 
     As shown in  FIG. 8 , the passive suspension assembly  160  consists of symmetrical upper and lower portions. The upper portion includes an annular permanent magnet  184  disposed within the casing and an annular permanent magnet  174  within the rotor. These magnets are preferably of substantially equivalent thickness and face to each other across a radial air gap  163 . An annular end pole piece  185  of soft iron is attached to the top end of the magnet  184 . This end pole piece  185  may or may not project from the inner cylindrical surface of the magnet  184  towards air gap  163 , depending on an analysis of design optimization. Correspondingly, an annular end pole piece  175  of soft iron is attached to the top end of the magnet  174 , and it may or may not project from the outer cylindrical surface of the magnet  174  towards the air gap  163 . The end pole pieces  175 ,  185  are preferably of substantially equivalent thickness and oppose to each other across air gap  163 . In addition, a pole member  183  of annular soft iron is attached to the bottom end of magnet  184 . Pole member  183  may advantageously have an annular groove cut on the inner cylindrical surface to form a tooth  188  and a tooth  187  on the upper and lower ends of the pole member  183  respectively, both projecting towards the air gap  163 . Correspondingly, another pole member or piece  173  of annular soft iron is attached to the bottom end of the magnet  174 , and it may have an annular groove cut on the outer cylindrical surface to form teeth  178 ,  177  that project towards the air gap  163 . The pole members  183 ,  173  are preferably of substantially equivalent thickness, as well is the thickness of each couple of teeth  188 ,  178 ,  187 ,  177 , consistent with the same feature of the coupled end pole pieces  185 ,  175 . 
     The annular permanent magnets  184 ,  174  are both magnetized across thickness (along axis z) but in opposite directions. The soft iron members sandwiching these magnets serve to channel the magnetic flux through the magnetic materials and air gap. Therefore, annular permanent magnet  184 ,  174  generate a group of magnetic flux loops  153 , which passes through the annular permanent magnet  174 , the end pole piece  175 , the air gap  163 , the end pole piece  185 , the annular permanent magnet  184 , the pole member  183 , the air gap  163 , and the pole member  173 . A group of rotor members  173 ,  174 ,  175 , and a group of casing members  185 ,  184 ,  183  are thus linked by the working magnetic flux loop  153 . 
     The structure in the lower portion of assembly  160  can be formed by mirroring the upper structure about the x-y plane that extends through the middle of thickness of the pole members  183 ,  173 . Accordingly, coupled members of magnets  182 ,  172 , end pole pieces  181 ,  171 , teeth  187 ,  177  are formed. A flux loop  154  links the magnetic members in the rotor with the magnetic members in the casing. The magnets sandwiching the pole members  183 ,  173  are magnetized in opposite directions such that the working magnetic flux loops  153  and  154  circle in opposite directions. 
     Therefore, the rotor members and casing members of the stacked structure  160  is linked by a group of magnetic flux loops  153 ,  154 . In addition, the overall thickness of the assembly  160  is made sufficiently small relative to the diameter of the air gap  163 . Therefore, according to the above stated principle of flux loop linkage, the assembly  160  characterized by the magnetic flux loops  153 ,  154  can serve for passive suspension for axial and tilting stability. 
     The pole members  183 ,  173  play the same role of focusing magnetic flux into a confined air gap area as do the end pole pieces  185 ,  175 . The teeth  188 ,  178 ,  187 ,  177  in these pole members may contribute to further focusing the magnetic flux into an even narrower air gap in between the opposing teeth compared to the air gap in between the entire pole members. However, part or all of the tooth structures are not necessary in some applications depending on design optimization, which means any or both of the grooves on the pole members  183 ,  173  may not be needed. 
     It should be noted that the components of  FIG. 8  may or may not be a continuous annular piece along circumference. For example, any member such as the casing pole piece  183  can be replaced with a plurality of arcuated segments disposed in the original space of the annular piece  183 . This alteration does not deviate from the principle of magnetic suspension disclosed herein, although certain suspension performances may be affected. Specifically, if a rotor member is made with an interrupted structure, an unsteady suspension force and an eddy current may be induced when the rotor spins, which may impair power efficiency, dynamic performance and possibly other performances. 
     The pole members  183 ,  173  can be made the same as the end pole pieces  185 ,  175  if the assembly  160  is employed merely for passive suspension. However, the construction with thicker pole members  183 ,  173  can be adapted to form a hybrid magnetic suspension of  FIGS. 3-5  that serves an additional function of active suspension for radial stability. Returning to  FIGS. 3-5 , the suspension assembly  60  may have nearly the same construction as the assembly  160  of  FIG. 8 . In fact, the reference numerals of each component of  FIG. 8  corresponds to those of  FIGS. 3-5 , albeit with a trailing 0 (i.e. changing  160  to  60 ). Each reference numeral of  FIG. 8  (with a trailing 0) can find a similar numeral in  FIG. 3  with the associated structural members matching with each other, except for the pole member  183 . The pole member  183  of  FIG. 8  is replaced by a plurality of electromagnet pole shoes  83   a - d  ( FIG. 5 ) distributed circumferentially around the air gap  63  in order to serve for the electromagnet functions to be discussed below. This group of pole shoes can be viewed as being made by cutting off some sections along the circumference of the continuous annular pole member  183 . Such replacement of a continuous ring with interrupted annular segments does not change the principle of passive suspension, and will not cause significant change in suspension performance since a majority of circumferential space is still occupied by soft iron. Therefore, the passive suspension assembly in the pump  10 , in accordance with an embodiment of the present invention, is constructed with the above examples. 
     Turning now to the principle and construction of the active suspension in pump  10  of  FIG. 1 . the active suspension is based on a principle of magnetic flux modulation on bias flux. The bias magnetic flux is established by permanent magnets, and the modulating magnetic flux is generated by electromagnets. 
     Referring to  FIGS. 3-5 , a magnetic suspension assembly  60  includes a rotor assembly  62  and a casing assembly  61  separated by an air gap  63 . The rotor assembly  62  includes, among others, a primary pole piece  73  sandwiched by permanent magnets  72 ,  74  possessing opposite polarizations. The casing assembly  61  includes, among others, a group of pole pieces  83   a - 83   d  that are sandwiched by permanent magnets  82 ,  84  possessing opposite polarizations. In addition, the casing assembly  61  consists of a group of electromagnet units  90   a - d  evenly distributed around the periphery of the casing assembly  61 . Each electromagnet unit has substantially the same construction. Therefore, for simplicity, they are described with a representative unit subtracting the alphabetic suffix from the numeral. For example, unit  90  is a representative of any of the four units  90   a - d . This convention is used throughout this document. 
     Thus, an electromagnet unit  90  is comprised primarily of a coil  91 , an iron core  92 , a pole shoe  83 , and a back yoke  95  that is shared by a set of electromagnet units. The iron core  92  is a cubic piece made of soft iron with a cross sectional shape such as circular, rectangular with rounded corners, or others that are known to one skilled in the art to be suitable for construction of electromagnet core. The iron core  92  is advantageously mounted into the assembly  61  by aligning its longitudinal axis in a radial direction, like a spoke of a wheel. A coil  91  for conducting electric current is wound around the iron core  92 . A pole shoe  83  is attached to one end of the iron core  92  on the side towards the air gap  63 . A back yoke  95  is attached to the other end of iron core  92 . 
     The pole shoes  83   a - 83   d  are evenly distributed circumferentially around the air gap  63 . Each pole shoe serves for coupling with the rotor primary pole piece  73  to form concentrated magnetic flux through the air gap. Accordingly, teeth  88 ,  87  are constructed on the inner surface of the pole shoe  83  to oppose the teeth  78 ,  77  of the primary pole piece  73  respectively, if the latter teeth are present. For optimal design of active suspension, the circumferential gap between the neighboring pole shoes is determined to minimize flux leakage in between the pole shoes while maximizing the inner surface of each pole shoe for best conducting working flux through the air gap. An annular end pole piece  85  of soft iron is attached to the top end of the magnet  84 . This end pole piece  85  may or may not project from the inner cylindrical surface of the magnet  84  towards air gap  63 , depending on an analysis of design optimization. Correspondingly, an annular end pole piece  75  of soft iron is attached to the top end of the magnet  74 , and it may or may not project from the outer cylindrical surface of the magnet  74  towards the air gap  63 . The end pole pieces  75 ,  85  are preferably of substantially equivalent thickness and oppose to each other across air gap  63 . 
     Referring to  FIGS. 3-5 , in accordance with an embodiment of the present disclosure, a back yoke  95  is configured to connect the electromagnet units that jointly serve for control of one DOF. Particularly, the electromagnet units  90   a ,  90   b  are connected by back yoke  95  to jointly control the rotor&#39;s radial position along the y axis, and electromagnetic units  90   c ,  90   d  are connected to control the rotor along the x axis. In  FIGS. 3-5 , one back yoke  95  connects all electromagnet units, which is beneficial for simplicity and compactness, among other advantages. However, in some applications, coupling between magnetic flux from different sets of electromagnet units is to be strictly limited to suppress interference between the x axis control and y axis control. In that case, separate back yokes may be configured so that each back yoke only connects those electromagnetic units that merely work for controlling one particular radial displacement (along the x or y axis). Such an alternative construction can be readily conceived by one skilled in the art in light of the principle disclosed herein. 
     Active control of the rotor&#39;s radial position is achieved through real time adjustment of magnetic force on the rotor from the casing, mainly the magnetic force on the primary pole piece  73  from the electromagnet pole shoes  83   a - 83   d . In the embodiment of  FIGS. 3-5 , radial displacement in the x or y direction is independently controlled, with two electromagnets  90   a ,  90   b  responsible for the y axis control, and two electromagnets  90   c ,  90   d  for the x axis control. Since the basic principle of control on each of the axes is the same, only the y axis control is to be discussed in detail below. The active suspension in the embodiments of the present invention is based on a mechanism called push-pull modulation of the bias magnetic flux in air gap. As illustrated in the upper portion of the symmetrical structure of  FIG. 9( a ) , permanent magnets  84 ,  74  generate a group of magnetic flux loops  53   a - 53   d  that pass through the air gap  51   a - 51   d  between the rotor primary pole piece  73  and the electromagnet pole shoes  83   a - 83   d  respectively. Such working magnetic flux in the air gap for suspension is referred to as bias flux. A length of flux loops  53   a ,  53   b  can be seen in  FIG. 9( b )  which is a cross sectional view of  FIG. 9( a )  with cutting plane A-A passing through the air gap  51   a ,  51   b . A dot inside a circle indicates flux going out of the page, and an “x” inside a circle indicates flux going into the page. The teeth on the pole members  73 ,  83  have an effect of focusing the bias flux in the confined areas in the air gap  51 . In a same manner, another set of bias magnetic flux loops  54   a ,  54   b  is established in the lower portion of the symmetrical structure of  FIG. 9( a ) . Since both sets of bias magnetic flux are substantially symmetrical and produce active control forces with the same mechanism, only active control with flux loops  53   a ,  53   b  is to be further discussed below. Note that the total active control force on the rotor is a sum of forces from these two sources. 
     A magnetic force on a tooth  78  of the rotor from the tooth  88   a  of the casing pulls the rotor in a negative y direction, and a magnetic force from the tooth  88   b  of the casing pulls the rotor in a positive y direction. Since the magnetic suspension assembly  60  of  FIGS. 9( a ) and ( b )  has a symmetrical construction about the x-z plan, when the rotor is set concentrically with the casing, bias flux in the air gap  51   a ,  51   b  are substantially identical in magnitude. Therefore, magnetic forces due to bias flux in the air gaps  51   a ,  51   b  substantially counterbalance each other, resulting in a practically zero net force. 
     Suppose electric current, i, is fed into the coils  91   a ,  91   b  in directions as shown in  FIGS. 9( a ) and ( b ) , where a dot inside a circle symbolizes current flowing out of the page, and an “x” inside a circle symbolizes current flowing into the page. The coils  91   a ,  91   b  are connected in series so that they work jointly with same current to produce substantially the same amount of magnetic flux in the iron cores  92   a ,  92   b  respectively. Such working magnetic flux for suspension generated by electromagnets is referred to herein as modulating flux. As shown in  FIG. 9 , since the overall suspension assembly  60  is symmetrical about the y-z plan and x-y plan, the modulating flux produced by electromagnets  90   a ,  90   b  makes either the modulating flux loop  55  in the upper portion of the assembly  60 , or the modulating flux loop  56  in the lower portion of the assembly  60 . The flux loops  55 ,  56  are substantially identical for the same reason as with the above bias flux loops  53 ,  54 , and so only the modulating flux loop  55  is analyzed below. The modulating flux loop  55  passes through the electromagnet iron core  92   a , teeth  88   a  of the pole shoe  83   a , the air gap  51   a , and enters the teeth  78  of the rotor primary pole piece  73  on the negative y side. It then passes along the periphery of the rotor primary pole piece  73  to the positive y side, exiting the teeth  78 , passing through the air gap  51   b , the tooth  88   b  of the pole shoe  83   b , the iron core  92   b , and entering the back yoke  95 , and finally passes along the periphery of the back yoke  95  to the negative y side to complete the loop. Since the magnetic flux passing through the iron cores  92   a ,  92   b  are substantially identical in magnitude, flux going into the other iron core through the air gap  51   c ,  51   d  in x direction, i.e. the flux leakage, is negligible. 
     The modulating flux  55  superimposes the bias flux  53   a ,  53   b  in the air gap  51   a ,  51   b . With the particular directions of the magnetic flux loops indicated in  FIGS. 9( a ) and ( b ) , but without loss of generality, the modulating flux  55  goes in the same direction with the bias flux  53   a  in air gap  51   a , but in opposite direction to the bias flux  53   b  in the air gap  51   b . Therefore, the magnetic flux in the air gap  51   a  is enhanced above the bias flux, and thus the magnetic force between the pole shoe tooth  88   a  and the rotor pole piece tooth  78  is increased. Meanwhile, the magnetic flux in the air gap  51   b  is reduced from the bias flux, and thus the magnetic force between the pole shoe tooth  88   b  and the rotor pole piece tooth  78  is decreased. These effects combine in a push-and-pull manner so that a net magnetic force on the rotor towards the negative y direction results. If the electric current increases, then the resultant force on the rotor increases in magnitude. Also, if the electric current reverses, then the resultant magnetic force changes to the opposite direction. The mechanism of imposing paired, opposite modulating flux on bias flux in the air gap to create controllable net magnetic force, the so-called push-pull modulation, is thus demonstrated. 
     Suppose the air gap flux density of the bias flux and the modulating flux is B and AB, respectively. The flux density in the air gap  51   a ,  51   b  becomes B+ΔB and B−ΔB respectively. According to magnetics theory, the magnetic force on a surface of highly permeable magnetic material is in approximate proportion to the product of the square of flux density on the surface and the surface area. Therefore, the above analysis yields the following net magnetic force
 
 F=k·S ·[−( B+ΔB ) 2 +( B−ΔB ) 2 ]=−4 k·S·B·ΔB   (1)
 
where S is the surface area of the inner surface of tooth  88  ( 88   a ,  88   b ) of the electromagnet pole shoe, and k is a constant.
 
     Further, the air gap flux density generated by the electromagnet unit, ΔB, is in proportion to electric current in the electromagnet, i, as long as the corresponding magnetic circuit is not saturated. Therefore, Equation (1) can be rewritten as
 
 F=c·B·i   (2)
 
where c is a constant.
 
     The air gap flux density, B, generated by the permanent magnet does not vary with electric current i. Therefore, Equation (2) shows that the net magnetic force is in direct proportion to the electromagnet current. That is, there is a linear relationship between the active control force and the control current. This attribute of the push-pull modulation is advantageous, since, among other advantages, it allows application of linear control strategy for achieving preferred active control performances. 
     It can be appreciated that according to embodiments of the present invention, the bias flux loop and the modulating flux loop take different pathways in a three dimensional configuration so that they only overlap in the vicinity of the air gap for active suspension control. In a non-limiting example, as shown in  FIGS. 9( a ) and ( b ) , the bias flux loops  53   a ,  53   b ,  54   a ,  54   b  lie in meridian plans and modulating flux loop  55 ,  56  lie in planes parallel to the equator plan. They overlap merely in the air gap  51  and neighboring pole pieces including the rotor primary pole piece  73  and the electromagnet pole shoe  83 . In general, in accordance with an embodiment of the present disclosure, the bias flux loops  53   a ,  53   b ,  54   a ,  54   b  do not pass through the iron core of electromagnet, and the modulating flux loop  55 ,  56  does not pass through the permanent magnet. This aspect of the present invention advantageously differs from the conventional designs such as those described in U.S. Pat. Nos. 8,288,906 and 8,596,999. The permanent magnet has extremely low magnetic permeability to external magnetic flux (close to vacuum) and thus exhibits high reluctance in a magnetic circuit energized by an electromagnet. Therefore, any configuration with the working magnet flux loop of electromagnet passing through permanent magnet will hamper power efficiency or cause significant increase of the coil size. On the other hand, if the working magnetic flux generated by the permanent magnet is configured to pass through the iron core of the electromagnet, then the cross-sectional area of the iron core must be enlarged to avoid saturation. In comparison with a modulating flux, the bias flux must be greater, often significantly, in magnitude in order to cover the entire variation range of modulating flux during operation. Therefore, the increase in the size of electromagnet due to involving its iron core in a permanent magnet circuit can be significant, and thus should to be avoided. 
     Active suspension for the radial displacements along the x and y axes is achieved with a feedback control system based on the principle of bias flux modulation disclosed herein. In one embodiment of the present invention, the displacement along x or y axis is independently controlled, so two substantially identical control systems can be employed. As schematically shown in  FIG. 10 , such a control system  200  includes a position sensor  201  to detect the real time displacement of the rotor along x or y axis. A controller  202  processes the displacement signal coming from the sensor  201  with an appropriate control strategy, and yields commands of control. Various control strategies, such as the proportional differentiation (PD) control, commonly known to those having skills in the magnetic suspension field can be adopted. The control commands are fed into a current amplifier  203  to produce a time-varying electric current with sufficient power capability for actuating the electromagnets. This current flows into the coils of the electromagnet  204  to create the modulating magnetic flux and thus fulfills the goal of active suspension control. The rotor position sensor  201  can be any suitable type for noncontact measurement of the rotor&#39;s position, such as an eddy current displacement sensor or Hall effect sensor that is commonly known to one skilled in the field of magnetic suspension. For example,  FIG. 5  shows a number of eddy current sensor probes  98 , constructed with coils for working with high frequency excitation current, distributed in the gap between electromagnet pole shoes  83   a - 83   d  right-adjacent to air gap  63 . Correspondingly, an annular piece  97  made of an electric conductor such as copper is installed in the outer surface groove of the rotor, in  FIGS. 3 and 5 , right adjacent to the air gap  63  and directly facing the eddy current probe  98 , to serve as the target of the eddy current sensor probe  98 . The rotor&#39;s radial displacements along the axes pointing to the sensor probes  98  are transformed to yield displacements along the x and y axes. Two or more sensor probes  98  are used to obtain the necessary displacement signals. 
     It can be appreciated that according to embodiments of the present invention, the bias flux not only constitutes the basis of active suspension, but also by itself can serve for passive suspension. This is because a bias flux loop links members in the rotor and the casing that oppose each other across a radial air gap. According to the principle of magnetic flux linkage discussed above for  FIG. 6 , such a flux loop can serve the function of passive suspension for axial and tilting stability. Therefore, the hybrid magnetic suspension construction of  FIGS. 9( a ) and ( b )  can be advantageously simplified by including fewer members that serve for passive suspension. In general, an elementary hybrid suspension unit according to an aspect of this disclosure may merely include generation of bias magnetic flux and modulating magnetic flux in an air gap that is defined by an annular rotor primary pole piece and a plurality of circumferentially distributed pole shoes of electromagnet units. Various alternative embodiments can thusly be conceived. A few such examples are shown in  FIG. 11 . 
       FIG. 11( a )  shows an exemplary hybrid magnetic suspension assembly  310  that is simplified from  FIGS. 9( a ) and ( b )  and still holds the fundamental function of full magnetic suspension in accordance with embodiments of the present invention. The rotor assembly is extensively simplified into a single piece of annular soft iron  314 , which serves the same function of the rotor primary pole piece  73  of  FIGS. 9( a ) and ( b ) . The casing assembly is constructed according to the same fundamental concept of  FIGS. 9( a ) and ( b )  with end pole pieces on the ends of permanent magnets being omitted for constructional simplicity. A number of electromagnets are distributed around the air gap  326 , each including a pole shoe  313   a ,  313   b , an iron core  315   a ,  315   b , a coil  317   a ,  317   b , and a back yoke  316 . The cross-sectional view of  FIG. 11( a )  depicts two electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. The annular permanent magnets  311  and  312 , which are preferably continuous rings, sandwich the pole shoes  313   a ,  313   b  with opposing magnetic polarizations. Two substantially symmetric bias flux loops  324 ,  325  are thus generated on both ends of the pole shoes  313   a ,  313   b . These flux loops link the rotor member  314  with a group of casing members  311 ,  313   a ,  313   b , and  312 . 
       FIG. 11( b )  shows an elementary hybrid suspension unit that is further simplified from  FIG. 11( a )  by including one permanent magnet  331  in the casing. This magnet generates a bias flux  341  that links the rotor primary pole piece  334  with casing members including the permanent magnet  331  and the electromagnet pole shoe  333   a ,  333   b . A number of electromagnets are distributed around the air gap  343 , each including a pole shoe  337   a ,  337   b , an iron core  335   a ,  335   b , a coil  337   a ,  337   b , and a back yoke  336 . The cross-sectional view of  FIG. 11( b )  depicts two electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. The configuration of  FIG. 11( b )  fulfills the fundamental function of magnetic suspension in this invention, although many additional suspension performances, such as compactness, dynamics, and power efficiency, may be different. Since the casing of  FIG. 11( b )  is not symmetrical about the x-y plan, with the passive suspension in the axial direction, the rotor primary pole piece  334  will find an equilibrium position by offsetting a distance from aligning with the pole shoe  333  towards the permanent magnet  331 . Accordingly, the modulating flux line  343  in the air gap  345  is tilted with respect to the x-y plane, as well as the active control force that points along flux line  343 . The active control force thus gets an axial component that pulls the rotor primary pole piece  334  axially towards pole shoe  333   a ,  333   b . This axial force can be counterbalanced by the passive suspension if a proper design is adapted. However, during operation of the pump, the active control force is adjusted in real time to maintain suspension stability. So, the active control will induce a time-varying axial force on the rotor, which is an internal disturbance on the passive suspension. This disturbance may stimulate axial vibration or even resonance of rotor, among other undesirable dynamic issues, since the passive suspension does not possess an active mechanism to adequately damp the vibration. Therefore, the asymmetric construction of  FIG. 11( b )  may be less preferable than a symmetric one such as that of  FIG. 11( a )  in terms of disturbance of active control on passive suspension. 
       FIG. 11( c )  shows another elementary hybrid magnetic suspension unit that is constructed by moving the permanent magnet of  FIG. 11( b )  from the casing to the rotor. A number of electromagnets are distributed around the air gap  365 , each including a pole shoe  353   a ,  353   b , an iron core  355   a ,  355   b , a coil  357   a ,  357   b , and a back yoke  356 . The cross-sectional view of  FIG. 11( c )  depicts two electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. A bias flux  361  generated by magnet  358  links casing member  353   a ,  353   b  with the rotor members including magnet  358  and primary pole piece  354 . Similar to  FIG. 11( b ) , the axial equilibrium position of the rotor is shifted from aligning with the pole shoe  353  towards the reverse side of the rotor magnet  358 . The same effect of disturbance of active suspension control on passive stability as in  FIG. 11( b )  is expected to occur, which may be regarded as less preferable than a symmetric configuration like that of  FIG. 11( a ) . 
     The configurations of  FIGS. 11( b ) and 11( c )  have advantages in simplicity and cost effectiveness, among others. In order to remedy disturbance of active control force on passive stability, one can combine a pair of those elementary suspension units to form a symmetric configuration that generates active control force in practically pure radial directions. An exemplary embodiment according to this principle is shown in  FIG. 12 . A magnetic suspension assembly  410  comprises a pair of substantially identical elementary suspension units disposed along the axial direction. The upper and lower unit respectively comprises an annular primary pole piece  422 ,  423  in the rotor, and a group of electromagnet units  403   a ,  403   b ,  404   a    404   b  in the casing. The electromagnets are distributed around the air gap  426 , each including an iron core  414   a ,  414   b ,  416   a ,  416   b , a coil  415   a ,  415   b ,  417   a ,  417   b , and a back yoke  418 ,  419 . The cross-sectional view of  FIG. 12  depicts two electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. Each primary pole piece  422 ,  423  may, or may not, have multiple teeth (not shown) formed on its outer surface by cutting out one or more annular grooves on that surface. Each electromagnet unit includes a pole shoe, an iron core, a coil, and a back yoke, the same as in  FIGS. 9( a ) and ( b )  and  11 . Each pole shoe preferably has substantially same thickness as the rotor primary pole piece, and has same tooth structure on the inner surface as the tooth structure, if any, on the outer surface of the corresponding rotor primary pole piece  422 ,  423 . 
     The upper and lower elementary hybrid magnetic suspension units are connected together by annular permanent magnets  411 ,  421 . The permanent magnet  411  is sandwiched in between the upper pole shoes  412   a ,  412   b  and lower pole shoes  413   a ,  413   b . These pole shoes may advantageously have inner surfaces projected from the inner surface of magnet  411  towards air gap  426 . The other permanent magnet  421  is sandwiched in between the rotor primary pole pieces  422 ,  423 . It may advantageously have outer cylindrical surface indented from the outer surfaces of these primary pole pieces. The magnets  411  and  421  have substantially same thickness so that the upper and lower pole shoes  412   a ,  412   b  and  413   a ,  413   b  are in alignment with the rotor primary pole pieces  422  and  423  respectively. Such configuration serves for focusing magnetic flux into the projected structures adjacent to the air gap  426  and thus obtaining intensified magnetic forces, as discussed above on  FIG. 6 . 
     Permanent magnets  411 ,  421  are magnetized in axial directions opposing to each other. Therefore, they jointly generate bias flux loops  427  that lie in meridian plans of the assembly. The magnetic flux loop  427  serves as the bias flux of both the upper and lower elementary hybrid magnetic suspension units. Moreover, the overall thickness of the rotor assembly, measured from the upper end surface of the primary pole piece  422  to the lower end surface of the primary pole piece  423 , is made sufficiently small in comparison with the diameter of the air gap  426 . Therefore, according to the above principle of flux loop linkage, passive suspension in axial displacement and tilting is achieved. 
     The four electromagnet units  403   a ,  403   b ,  404   a ,  404   b  shown in  FIG. 12  are connected in series to work jointly to provide active control of the radial displacement in the y direction. Identical electric current is fed into the coils  415   a ,  415   b ,  417   a ,  417   b  so that the modulating magnetic flux loops  428 ,  429  are generated. The symbols with a dot inside a circle and an “x” inside a circle on the cross sections of the rotor primary pole piece  422 ,  423  and the back yoke  418 ,  419  indicate modulating flux going out of or into the cross sectional area, respectively. These fluxes passing from one cross sectional area extend their paths along the periphery of the primary pole piece or the back yoke to reach the other cross sectional area on the opposite side about the z axis. As can be seen from  FIG. 12 , the bias flux  427  and the modulating flux  428  in the air gap  426  on the positive y side go in opposite directions, while these fluxes in the air gap  426  on the negative y side go in the same direction. This leads to a net magnetic force on the rotor primary pole piece  422  in the negative y direction. A similar analysis on the effects of the bias flux  427  and the modulating flux  429  yields a net magnetic force on the rotor primary pole piece  423 , also in the negative y direction. Therefore, active control of the radial magnetic force with the mechanism of push-pull modulation of bias flux in the air gap is achieved with the configuration of  FIG. 12 . 
     An alternative embodiment that has a stacked structure of the elementary hybrid magnetic suspension units can be made by replacing the rotor members  421 ,  422 ,  423  of  FIG. 12  with a single annular member of soft iron  441  of  FIG. 13 . Rotor member  441  allows magnetic flux to pass through along the axial direction, in a similar way as does the magnetic flux of  FIG. 12  that passes through rotor magnet  421 . Also, the rotor member  441  has two distinct pole edges  442 ,  443  arranged on the upper and lower end portions of the outer surface, respectively. These pole edges  442 ,  443  may be formed by simply cutting out an annular groove on the central portion of the outer surface of rotor member  441 . The pole edges  442 ,  443  serve fundamentally the same function of channeling magnetic flux as does the primary pole pieces  422 ,  423  of  FIG. 12 , respectively. 
     The casing assembly of  FIG. 13  has a similar construction as that of  FIG. 12 . A number of electromagnets are distributed around the air gap  446 , each including an iron core  434   a ,  434   b ,  436   a ,  436   b , a coil  435   a ,  435   b ,  437   a ,  437   b , and a back yoke  438 ,  439 . The cross-sectional view of  FIG. 13  depicts four electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. The four electromagnet units  405   a ,  405   b ,  406   a ,  406   b  shown in  FIG. 13( a )  are connected in series to work jointly to provide active control of the radial displacement in the y direction. Identical electric current is fed into the coils  435   a ,  435   b ,  437   a ,  437   b  so that the modulating magnetic flux loops  448 ,  449  are generated. The symbols with a dot inside a circle and an “x” inside a circle on the cross sections of the rotor  441  and the back yoke  438 ,  439  indicate modulating flux going out of or into the cross sectional area, respectively. Therefore, the permanent magnet  431  generates a bias magnetic flux  447  that forms substantially the same loop as does the bias magnetic flux  427  ( FIG. 12 ) that is jointly generated by the permanent magnets  411 ,  421  ( FIG. 12 ). On the other hand, electromagnets  405   a ,  405   b ,  406   a ,  406   b  generate modulating magnetic fluxes  448 ,  449  that form substantially the same loops as do the modulating magnetic flux  428 ,  429  ( FIG. 12 ), respectively, provided that the central portion of rotor member  441  is so designed such that the bias magnetic flux  447  causes sufficient saturation therein. If the central portion of the rotor member  441  is not saturated to such extent, then it allows crossover of the modulating magnetic fluxes  448 ,  449  through the central portion of the rotor member  441 . As a result, the modulating magnetic fluxes  448 ,  449  of  FIG. 13( a )  may be replaced by magnetic flux  445  of  FIG. 13( b ) . The modulating flux  445  passes from one pole edge  442 ,  443  to the other pole edge of rotor member  441  along the axial direction in a same meridian plane instead of extending along circumferential direction to the other side of the same pole edge. However, no matter whether the modulating magnetic flux extends through the paths of  FIG. 13( a )  or the path of  FIG. 13( b ) , the hybrid magnetic suspension device  430  can provide active control in radial directions according to the same mechanism of push-pull modulation of bias magnetic flux as disclosed herein. 
     The configuration of  FIG. 13  employing a single rotor member of soft iron is advantageous with respect to cost effectiveness, among others, in comparison with  FIG. 12 . However, with the rotor magnet being omitted, less-strong bias magnetic flux is generated, and thus the configuration of  FIG. 13  may be associated with less suspension stiffness of passive suspension, less power efficiency due to less magnetic force per unit current of active suspension, among other issues. 
     The hybrid magnetic suspension assembly including, but not limited to, those depicted in  FIG. 11, 12 or 13  can be reinforced by adding one or more elementary passive suspension units as described herein to obtain increased suspension stiffness and other required performances. Such an embodiment is shown in  FIG. 14 , as an example. A hybrid magnetic suspension unit adopted from  FIG. 12 , is depicted. In addition, a pair of annular permanent magnets  463 ,  453  is attached respectively to the end surfaces of the rotor primary pole piece and the pole shoes of electromagnet units on the upper end of the hybrid magnetic suspension unit. Also, a pair of annular end pole pieces of soft iron  464 ,  454  is attached to the other end surface of the magnets  463 ,  453  respectively. Advantageously, these magnets and end pole pieces may be configured in a way similar to the corresponding members in the upper portion of  FIG. 8 , which is a typical elementary passive suspension unit of the present invention. In addition, another elementary passive suspension unit comprising magnets  462 ,  452  and end pole pieces  461 ,  451  is installed on the lower end of the hybrid magnetic suspension unit of  FIG. 14 , which may advantageously be configured in a similar way as the lower portion of  FIG. 8 . 
     In comparison with  FIG. 12 ,  FIG. 14  involves two additional bias flux loops  457 ,  458  symmetrically located on the both ends of the assembly  450 . These flux loops can provide additional stiffness of passive suspension. Moreover, they enhance the magnetic fields in the air gap where modulating flux also passes through, and thus can advantageously contribute to an increase in magnetic force per unit current of active suspension. However, the construction becomes more complicated. In addition, the overall thickness of the rotor assembly  450  of  FIG. 14  relative to the air gap diameter has to be sufficiently small in order to ensure passive stability of tilting. 
     Whereas in  FIG. 14  the additional passive suspension units are mounted to the outward ends of the elementary hybrid suspension units, a passive suspension unit can also advantageously be integrated into the middle of the stacked structure of elementary hybrid suspension units. According to this principle, various other embodiments of the present invention can be made, and one such example is illustrated in  FIG. 15 . The upper and lower elementary hybrid suspension units of  FIG. 12  are adopted for the construction of  FIG. 15 , but the connection between these units are modified to allow installation of an elementary passive suspension. As shown in  FIG. 15 , magnetic suspension device  470  comprises a pair of elementary hybrid suspension unit  488 ,  489  disposed on the upper and lower portion of the device respectively. The upper unit  488  comprises an annular rotor primary pole piece  482  and a plurality of pole shoes  472   a ,  472   b  of the electromagnet units for active control along the y axis. The lower unit  489  comprises an annular rotor primary pole piece  483  and a plurality of pole shoes  473   a ,  473   b  of the electromagnet units for active control along the y axis. The cross-sectional view of  FIG. 15  depicts four electromagnets, however it will be appreciated that in the embodiment described, additional electromagnets may be contemplated, however, due to the cross sectional view are not shown. In addition, a passive suspension unit being disposed in the middle of the device  470  comprises an annular first pole member  481  within the rotor, and an annular second pole member  471  within the casing. Both members  471  and  481  are made from soft iron, and preferably have substantially equivalent thickness. The outer cylindrical surface of the first pole  481  and the inner cylindrical surface of the second pole  471  oppose to each other and define an annular air gap  478  for the secondary passive suspension. Three layers of magnetic poles for primary hybrid suspension and additional passive suspension are thus constructed. 
     An annular permanent magnet  484  is sandwiched in between the rotor primary pole piece  482  and the first pole member  481 . Preferably, the outer cylindrical surface of magnet  484  is indented from the outer surfaces of the pole members  482 ,  481  in order to form a concentration of magnetic field around the poles. Another annular permanent magnet  474  is sandwiched in between the pole shoes  472  and the second pole member  471 . Preferably, the inner cylindrical surface of the annular permanent magnet  474  is indented from the inner surfaces of the pole members  472 ,  471 , for the same purpose of magnetic field concentration. Annular permanent magnets  484  and  474  are magnetized along axial directions in opposite to each other. Therefore, they jointly generate a magnetic flux  476  that serves for the bias magnetic flux of the hybrid suspension unit  488 . The same flux  476  also serves for the working magnetic flux of the additional passive suspension through the secondary suspension gap  478 . The lower portion of device  470  is constructed in symmetry with the upper portion about the x-y plan that passes through the middle of pole members  471 ,  481 . Therefore, another magnetic flux loop  477  is generated by the annular permanent magnets  485 ,  475 , and serves for both the bias flux of the hybrid suspension unit  489  and the working flux of the additional passive suspension unit. 
     Modulating magnetic fluxes  486 ,  487  are generated by the electromagnet units in the upper and lower hybrid suspension units respectively. It can be appreciated that whereas the modulating fluxes  428 ,  429  of the configuration shown in  FIG. 12  flow in opposite directions in the meridian plane, the modulating fluxes  486 ,  487  flow in the same direction, in the configuration shown in  FIG. 15 . 
     The pole members  471 ,  481  may preferably be made sufficiently thin in thickness to generate highly concentrated magnetic field in the secondary passive suspension gap  478 . In this manner, the secondary passive suspension can play the major role of passive suspension for axial and tilting stability, as compared to the functions of passive suspension of the hybrid suspension units. The hybrid suspension units, on the other hand, can be optimized for the role of active control for radial stability with less constraint of passive suspension performances. Therefore, the configuration shown in  FIG. 15  may be preferable for achieving particular goals of design optimization of a compact magnetically suspended centrifugal pump. 
     Analogous to the variation of configurations shown in  FIGS. 12-13 , an alternative embodiment of the present invention can be made by replacing all of the rotor members of  FIG. 15  with a single rotor member  495  of  FIG. 16 . The rotor member  495  may be made from soft iron and has three pole edges  491 ,  492 ,  493  formed on the outer cylindrical surface. The pole edges  492 ,  493  play same the role as the rotor primary pole pieces  482 ,  483  of the configuration shown in  FIG. 15  coupling with the corresponding pole shoes of the electromagnet units for the hybrid suspension. In addition, the pole edge  491  couples with pole member  498  to define the additional passive suspension gap. Three layers of magnetic poles for the primary hybrid suspension and additional passive suspension are thus constructed in the same manner as that of  FIG. 15 . The magnetic flux loops  496 ,  497 , although generated solely by permanent magnets in the casing, fulfill the same functions of bias magnetic flux of hybrid suspension and working flux of additional passive suspension. 
     According to the embodiment shown in  FIG. 16 , the modulating magnetic fluxes from upper and lower hybrid suspension units would not substantially crossover since they pass through the rotor member  495  in parallel. This is in contrast to the configuration shown in  FIG. 13  where modulation fluxes from different layers of the device may crossover as indicated in  FIG. 13( b ) . In this sense, the configuration shown in  FIG. 16  is preferable, especially when the additional passive suspension is designed as the major contributor to the passive suspension performances, since the magnetic flux in the secondary suspension gap is to a great extent not interfered by the modulating magnetic flux, meaning that the passive suspension is not interfered by the active suspension. 
     The embodiments of the present invention shown in  FIGS. 12-16  have a stacked structure of two layers of electromagnet units so that active control forces at different levels are generated. These forces sum up to result in a net radial force on the rotor, but if the two forces are different in magnitude, then a torque is also induced, which tends to cause the rotor to tilt. Due to the imperfection of materials, dimensional tolerance, operational environment, and other factors involved in practical applications, difference in these forces cannot be entirely avoided. Therefore, such a stacked structure, although preferable for certain applications, may be associated with the issue that active control of radial displacements causes disturbance on passive suspension for tilting stability. This issue may be resolved through proper design considerations such as separating the layers by proper distance, or alternative designs such as that of  FIG. 17 . 
     An alternative embodiment of the hybrid magnetic suspension assembly in accordance with this invention,  500 , is shown in  FIG. 17 . It includes a stacked structure with two layers similar to the configuration shown in  FIG. 12 , but in contrast to that configuration, each layer of the hybrid magnetic suspension comprises only two electromagnet units and the electromagnet units of different layers are circumferentially shifted by 90 degrees to independently control different axis of radial displacements. 
       FIGS. 17( b ) and 17( c )  are respectively cross-sectional views of the upper layer and lower layer of the stacked structure  FIG. 17( a ) . Two electromagnet units  510   a ,  510   b  are situated in the upper layer of the stacked structure, each unit consisting of an iron core  511   a ,  511   b , a pole shoe  512   a ,  512   b , a coil  513   a ,  513   b , and a back yoke  514 . These units are symmetrically arranged along the y axis and oppose each other. The back yoke  514  of annular continuous soft iron connects the iron cores  511   a ,  511   b  of these electromagnets. The pole shoes  512   a ,  512   b  face a rotor primary pole piece  515  of annular continuous soft iron. The other group of two electromagnet units  510   c ,  510   d  are situated in the lower layer of the stacked structure of  FIG. 17 , and disposed along the x axis, each unit consisting of an iron core  511   c ,  511   d , a pole shoe  512   c ,  512   d , a coil  513   d ,  513   d , and a back yoke  517 . The back yoke  517  of annular soft iron connects the iron cores  511   c ,  511   d . The pole shoes  512   c ,  512   d  face a rotor primary pole piece  518  of annular soft iron. 
     Bias flux loop  522  is generated by annular permanent magnets with the aid of annular rotor primary pole pieces in a same way as that of the configuration shown in  FIG. 12 . When an electric current is delivered into the coils  513   a ,  513   b , shown in  FIG. 17( b ) , a modulating flux loop  524  is generated. That flux loop  524  passes through the first electromagnet unit  510   a , the air gap  527   a , a rotor primary pole piece  515 , the air gap  527   b , the second electromagnet  510   b , and closes the loop by passing through the back yoke  514 . The combination of modulating flux and bias flux in the two air gaps  527   a ,  527   b  along the y axis constitutes push-pull modulation of the bias magnetic flux. Therefore, the group of electromagnet units in the upper layer fulfills the function of active control of the rotor&#39;s radial displacement in y axis. A similar analysis applies to the lower layer of the assembly  500 , as shown in  FIG. 17( c ) , and readily leads to the group of electromagnets in the lower layer fulfilling the function of active control of the rotor&#39;s radial displacement in x axis. When an electric current is delivered into the coils  513   c ,  513   d , shown in  FIG. 17( c ) , a modulating flux loop  525  is generated. That flux loop  525  passes through the first electromagnet unit  510   c , the air gap  528   c , a rotor primary pole piece  518 , the air gap  528   d , the second electromagnet  510   d , and closes the loop by passing through the back yoke  517 . The combination of modulating flux and bias flux in the two air gaps  528   c ,  528   d  along the x axis constitutes push-pull modulation of the bias magnetic flux. Therefore, the group of electromagnet units in the lower layer fulfills the function of active control of the rotor&#39;s radial displacement in x axis. 
     Whereas in the embodiment shown in  FIGS. 9( a ) and ( b )  the modulating flux for active control along x or y axis passes through the same back yoke and same rotor primary pole piece, the embodiment shown in  FIG. 17  works with separate modulating fluxes that passes through different back yoke and different rotor primary pole piece for different axis of control. It can be appreciated that when the rotor of  FIGS. 9( a ) and ( b )  deviates from the ideal equilibrium center in an arbitrary radial direction, modulating flux generated by electromagnets for the control of one (say, y) axis may, to more or less extent, enters into the electromagnets for control of the other (say, x) axis. This may cause undesirable coupling between the controls of different axes. In addition, since the pole shoes of the neighboring electromagnet units are situated in the same plane and thus relatively close to each other, magnetic flux leakage between these pole shoes may also cause unacceptable interference between the controls of different axes. On the other hand, in the configuration of  FIG. 17 , the modulating flux and the corresponding pole shoes for control of one axis and those for control of the other axis are situated in different planes which are separated by a substantial distance. Therefore, the configuration of  FIG. 17  can effectively avoid coupling and interference between controls of different axis, and better achieve independent control of radial displacements along each axis. 
     Moreover, unlike the stacked structures such as those shown in  FIGS. 12-16  in which active control of one radial axis relies on two radial forces respectively located in the upper and lower layers of the stacked structure, which may induce a tilting torque, the configuration of  FIG. 17  uses a single radial force for control of one radial axis. For example, the radial displacement in the y axis is controlled by merely one force in the upper layer of the stacked structure. Therefore, the configuration of  FIG. 17  can also advantageously resolve the issue of disturbance of active control on tilting stability. 
     According to an embodiment of the present disclosure, the hybrid magnetic suspension assembly can use three electromagnet units to achieve active suspension of radial displacements. An example of such a configuration can be made by replacing the four electromagnet units of  FIGS. 9( a ) and ( b )  (also  FIGS. 3 through 5 ) with three units, as shown in  FIG. 18 . One skilled in the art will appreciated that other embodiments such as those shown in  FIGS. 11-17  and  FIGS. 22-24  can be modified in the same way to yield alternative configurations which, for purposes of brevity are not discussed herein. 
     As shown in  FIG. 18 , three electromagnet units  551 A,  551 B,  551 C are evenly distributed around the annular air gap. Each electromagnet unit  551  consists of an iron core  561 , a pole shoe  562 , and a coil  563 ; and an annular back yoke of soft iron,  564 , connects the three iron cores. Three axes α, β, γ as indicated in  FIG. 18( b ) , extend from the origin of the coordinate system x-y-z outwards through the iron cores of the electromagnets  551 A,  551 B,  551 C respectively. Each of these axes passes through the peripheral center of the pole shoe surface and coincides with the central axis of the iron core of the corresponding electromagnet. Therefore, magnetic force between any electromagnet and the rotor lies in the direction of the corresponding axis of α, β, or γ. In the particular configuration of  FIG. 18 , these axes are evenly apart from each other by 120 degrees, although various alternative configurations can be made in accordance with the principle of this disclosure. An annular primary pole piece  565  made of soft iron is disposed in the rotor and opposes the pole shoes of the electromagnets  551 A,  551 B,  551 C across the air gap  556 A,  556 B,  556 C respectively. The rest of the construction of  FIG. 18  is fundamentally the same as that of  FIGS. 9( a ) and ( b ) . Two series of bias flux loops  552 ,  553  are generated by permanent magnets and are situated symmetrically in the upper and lower portion of  FIG. 18( a ) . The upper or lower portion of the assembly contributes a substantially identical magnetic force because of constructional symmetry, and a sum of the forces yields the overall force on the rotor. Only the force at the upper portion will be discussed below. 
     The three coils of the electromagnets are connected in such a way that electric currents flowing into these coils are balanced. For example, the Y connection or Delta connection that is commonly used in three-phase electric machinery may be employed. Correspondingly, the sign of electric current in a coil is defined such that a positive current produces magnetic flux passing through the core of the coil outwards from the origin of the coordinate system. Now, suppose electric currents I A , I B , I C  are supplied into coils  563 A,  563 B,  563 C respectively, and these currents satisfy
 
 I   A   +I   B   +I   C =0  (3)
 
     The current I A  in coil  563 A produces two symmetric groups of magnetic flux  554 AB and  554 AC as indicated in  FIG. 18 . The magnetic flux  554 AB goes from the rotor primary pole piece  565  which possess a reference magnetic potential, and passes through air gap  556 A in an outward direction (corresponding to positive current I A ). The magnetic flux  554 AB continues to flow outward through the pole shoe  562 A and the iron core  561 A of the electromagnet  551 A, and enters the back yoke  564  which possesses a substantially uniform magnetic potential over the circumference. The magnetic flux then goes along the periphery of the back yoke  564  and reaches one end of the electromagnet  551 B. It then passes through the iron core  561 B and the pole shoe  562 B of electromagnet  551 B in an inward direction, and then through the air gap  556 B. It finally enters into the rotor pole piece  565  and completes the loop. The other magnetic flux produced by the current I A  in coil  563 A,  554 AC, also passes through electromagnet  551 A in outward direction, but it then goes through electromagnet  551 C before completing the loop. In a same way, each of currents I B , I C  in the other two electromagnet units produces a couple of magnetic flux loops, each linking a pair of electromagnets. A total of 6 groups of such flux loops are produced. These flux loops are designated by the numeral  554  followed by two suffix alphabets, the first alphabet representing the electromagnet that energizes the flux loop, and the second alphabet representing the electromagnet that the flux loop also passes through, i.e. links with. For example, flux loop  554 AB is energized by the electromagnet  551 A and it also links the electromagnet  551 B. For another example, the flux loop  554 BA is energized by the electromagnet  551 B and it also links the electromagnet  551 A. The flux loop  554 AB and  554 BA overlap but flow in opposite directions. They sum up as signed numbers and results in the net magnetic flux along that common path. 
     The above flux loops and the associated magnetic components of  FIG. 18  can be modeled with the magnetic circuit of  FIG. 19 . The magnetic reluctance of the soft irons is assumed negligible, so the rotor primary pole piece  565  and the back yoke  564  can be modeled as single points. The R A , R B , R C  represent the reluctance of air gaps  556 A,  556 B,  556 C respectively. The NI A , NI B , NI C  represent the magnetomotive forces of the electromagnets  551 A,  551 B,  551 C respectively, where N is the number of turns of the coil and I is the electric current in the coil. The Φ A , Φ B , Φ C  are magnetic flux through the corresponding air gaps in the circuit, generated by the electromagnets. This magnetic circuit leads to the following equation
 
Φ A +Φ B +Φ C =0  (4)
 
     Also, analysis of magnetomotive forces for each branch of the magnetic circuit yields that magnetomotive force rise in the electromagnet equals to magnetomotive force drop over the corresponding air gap, for example, NI A =Φ A R A . Also note that the three air gaps have the same dimensions, and thus R A =R B =R C . Therefore, Equation (4) is in consistent with Equation (3). 
     The bias flux loops  552  include three groups  552 A,  552 B,  552 C that respectively pass through the air gaps  556 A,  556 B,  556 C. They produce the same flux density in these air gaps because of the constructional symmetry of the configuration shown in  FIG. 18 . Therefore, the bias magnetic flux density in any air gap is denoted as B. 
     Suppose the magnetic flux density in the air gaps  556 A,  556 B,  556 C due to the modulating fluxes Φ A , Φ B , Φ C  are ΔB A , ΔB B , ΔB C  respectively. Since magnetic flux density is in proportion to magnetic flux, the following relationship can be obtained from Equation (4)
 
Δ B   A   +ΔB   B   +ΔB   C =0  (5)
 
     According to magnetics theory, the magnetic force on a surface of highly permeable magnetic material is in proportion to the product of the square of flux density on the surface and the surface area. Therefore, the magnetic forces on the rotor, F A , F B , F C , in air gaps  556 A,  556 B,  556 C from electromagnet units  551 A,  551 B,  551 C, respectively, are
 
 F   A   =k·S ·( B+ΔB   A ) 2   (6)
 
 F   B   =k·S ·( B+ΔB   B ) 2   (7)
 
 F   C   =k·S ·( B+ΔB   C ) 2   (8)
 
where S is the surface area of the inner surface of the electromagnet pole shoe, and k is a constant.
 
     These forces are directed along the α, β, γ axes shown in  FIG. 18( b ) , respectively, with positive force pointing outwards from the center of the assembly. 
     The net signed force in the α axis is
 
 F   α   =F   A   −F   B  cos(60°)− F   C  cos(60°)  (9)
 
     From Equations (5) through (9), the force can be represented as: 
     
       
         
           
             
               
                 
                   
                     F 
                     α 
                   
                   = 
                   
                     
                       3 
                       ⁢ 
                       k 
                       ⁢ 
                       S 
                       ⁢ 
                       B 
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                         B 
                         A 
                       
                     
                     + 
                     
                       
                         1 
                         2 
                       
                       ⁢ 
                       k 
                       ⁢ 
                       
                         S 
                         ⁡ 
                         
                           ( 
                           
                             
                               Δ 
                               ⁢ 
                               
                                 B 
                                 A 
                                 2 
                               
                             
                             + 
                             
                               2 
                               ⁢ 
                               Δ 
                               ⁢ 
                               
                                 B 
                                 B 
                               
                               ⁢ 
                               Δ 
                               ⁢ 
                               
                                 B 
                                 C 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The bias flux is generated by permanent magnets and the modulating flux is produced by electromagnets. The permanent magnet creates much higher magnetomotive potential rise than electromagnet does. Therefore, bias flux density B is usually much greater than any of the modulating magnetic flux densities ΔB A , ΔB B , or ΔB C . Equation (10) can be approximated with
 
 F   α =3 k·S·B·ΔB   A   (11)
 
     The air gap flux density ΔB A  generated by electromagnet unit  551 A is in proportion to electric current in that electromagnet, i A , as long as the corresponding magnetic circuit is not saturated. Therefore, (11) can be rewritten as
 
 F   α   =c·B·i   A   (12)
 
where c is a constant.
 
     This shows a linear relationship between the magnetic force along the α axis and the electric current flowing into the electromagnet unit that resides on the α axis, similar to the linear relationship of Equation (2). Further, the linear relationship is resulted from summation of force F A  towards the positive α axis, and projections of forces F B , F C  towards the negative α axis. A push-pull mechanism similar to that involved in Equation (1) can be observed. 
     By symmetry, similar expressions are obtained for the magnetic forces along the β and γ axes, as follows
 
 F   β   =c·B·i   B   (13)
 
 F   γ   =c·B·i   C   (14)
 
     Magnetic forces F x  and F y  along the x and y axes, respectively, of  FIG. 18  can be obtained through a linear transformation from Equations (12) through (14). Therefore, in a configuration of  FIG. 18  where bias flux density is much greater than modulating flux density, magnetic force for active control of any of the two radial displacements is in linear relationship with the electric currents in the electromagnet units. This attribute of active control forces is highly desirable, the same as in the case of the embodiments such as  FIGS. 9( a ) and ( b )  where four electromagnet units are employed, since it facilitates application of linear control algorithms, among many other potential advantages. Also, the linear relationship is a result of the push-pull modulation of the bias magnetic flux in the air gap, which is fundamentally the same mechanism involved in the other embodiments of the hybrid magnetic suspension disclosed in this invention. 
     The above discussion illustrated the general principle and construction of the hybrid magnetic suspension assembly equipped with four or three electromagnet units. According to the same fundamental principle, other numbers of electromagnet units, evenly or unevenly disposed around the air gap, can be employed to yield various alternative designs by one skilled in the art. The above discussion also illustrates the method for deriving expressions of active control force in terms of electric current and bias magnetic flux density. This method and resultant expressions can be used to understand the mechanism of magnetic flux modulation in accordance with this invention, so that various design configurations especially those with an advantageous linear relationship for active control, can be readily conceived by those having skills in the field. 
     The pump  10  of  FIGS. 1-3  is exemplified with a magnetic suspension assembly  60  consisting of a casing assembly  61  mounted within an exterior casing  16  of a housing  12  as illustrated in  FIGS. 3-5 . An alternative embodiment may configure the magnetic suspension assembly  60  in the inner portion of the pump so that the casing assembly  61  is mounted within central post  15 , and the rotor assembly  62  is flipped over to the inner side of the rotor  30  so that the casing assembly  61  and the rotor assembly  62  oppose each other across the air gap  43  of  FIG. 3 . Such a configuration can be made to achieve the same fundamental functions of magnetic suspension in this invention by simply reversing the original configuration about the air gap. Specifically, in the alternative configuration, the teeth  87 ,  88  on the pole shoes  83  of the electromagnets is disposed adjacent to the air gap  43 , and the iron core  92  extends inward from the pole shoes and connects the back yoke  95  that constitutes the innermost member of the magnetic suspension assembly. The other components in the casing assembly  61 , as well as the rotor assembly  62 , are flipped inside-out in the same manner. Besides, this method of constructing a magnetic suspension unit in the inner portion of the pump can be equally applied to any of the other embodiments such as those of  FIGS. 11-18  to yield additional alternative embodiments of the present disclosure. 
     The structural and hydraulic features of the pump, as disclosed herein, are not to be taken in a limited sense, and they are made merely for the purpose of illustrating the general principle and construction involved in the present invention, especially with respect to the magnetic suspension. For example, the chamber of the exterior casing  16  of the configurations shown in  FIGS. 1-3  may not be necessary if the magnetic suspension unit is disposed in the inner portion of the pump, as discussed above. On the other hand, the central post  15  may not be necessary for successful practice of the present invention, as long as the components of magnetic suspension and motor can be disposed within the other portion of the housing. In addition, the rotor  30  may take other shapes for its inner and/or outer surfaces such as a conical shape rather than the right cylindrical surfaces as illustrated in  FIG. 2 . 
       FIGS. 20-22  illustrate an alternative pump  610  in accordance with an embodiment of the present disclosure. It includes a housing  612  with an inlet  611  to receive working fluid into the pump and an outlet  613  to discharge the pressurized fluid out of the pump. The side towards the inlet  611  is referred to herein as the front side of the pump  610  and the opposite side as the rear side. The housing  612  has a continuous inner wall that borders an interior chamber  620 , which communicates with the inlet  611  and outlet  613 . The chamber  620  is enclosed by a cylindrical side surface, a substantially flat end surface on the front side of the pump  610 , and a curved end surface on the rear side of the pump  610 , corresponding to a nose cone structure  615  that projects from the pump rear end towards the pump inlet  611 . The housing  612  also has an outer wall, which together with the inner housing wall form a space therebetween for mounting stationary components of an electric motor and magnetic suspension. Particularly, an exterior casing  616  is formed in between the cylindrical surface of the chamber  620  and the outer cylindrical wall of the housing  612 , and an end casing  617  is formed in between the end surface of chamber  620  and the front-end outer wall of the housing  612 . A volute  622  is constructed on the periphery of the interior chamber  620  for collecting fluid discharged from pump impeller  632 , and communicating with the pump outlet  613 . 
     A rotor  630  is disposed within the pump interior chamber  620  and is fully magnetically suspended without any physical contact with the surface of the chamber  620 . An impeller  632  consisting of a plurality of blades is mounted on the rotor  630  to transfer energy to the working fluid. Unlike the configuration of  FIGS. 2 and 3  where the impeller is attached to the front end of the rotor, the impeller  632  is attached to the rear end of the rotor  630 , which may be conventionally named a reverse impeller. The nose cone  615 , together with the rear surface of pump interior chamber  620 , is constructed to form a streamlined flow path for the working fluid to pass through the impeller blades  632  radially outward. The rotor is magnetically suspended so that an “L” shaped flow gap  625  is formed in between the front end surfaces of the rotor  630  and the pump interior chamber  620  for one arm of the “L”, and in between the outer cylindrical surface of the rotor  630  and the inner cylindrical surface of the pump interior chamber  620  for the other arm of the “L”. The mainstream of fluid flow passes from the inlet tubing  611  through the impeller  632  into the volute  622 . In the meanwhile, a secondary flow is generated due to pressure gradients through the “L” shaped gap  625 . The fluid of the secondary flow passes into the outer cylindrical gap towards the front end, and then flows inward in the annular end gap, and finally merges into the main flow inside the inlet tubing. The secondary flow takes a fractional amount of the main flow but plays an important role in washing the blood-contacting surfaces in the suspension gap  625  to prevent blood clotting, among other advantages in handing stress-sensitive fluids. It can be appreciated that this secondary flow path, like the secondary flow path of  FIG. 3 , is straightforward and free from a zigzag structure or any other obstructive feature in the flow channel so that unimpeded wash out on the entire rotor surface can be achieved. 
     According to an embodiment of the present invention, an electric motor  640  including a stator assembly  641  and a rotor assembly  642 , is disposed within the front end portion of the pump, as shown in  FIGS. 22 and 23 . Unlike the motor  40  in the pump  10  of  FIG. 3  which works with magnetic flux in a radial direction, the motor  640  is an axial flux motor that works with substantially axial magnetic flux. The rotor assembly  642  consists of a plurality of permanent magnet pieces  648  evenly distributed in an annular space of the front end of rotor  630 . Each magnet piece is preferably made into a shape like a fan so that these pieces can be assembled side by side circumferentially to form a solid ring centered about the rotational axis of rotor  630 . These magnet pieces are magnetized with regularly varying polarities to form magnetic poles in axial directions, which create the working magnetic flux passing through the air gap in axial direction. The variation of polarities may follow any pattern that is known to ones having skill in this field, for example, a Halbach array configuration that can advantageously create enhanced magnetic field on the air gap side. An annular piece of soft iron  649  may preferably be disposed on the back end of the magnet ring to serve as a back iron for fixing the magnet pieces in place and also completing the magnetic flux loop of the magnetic poles. However, it can be replaced with a nonmagnetic material or may not be needed without deviating from the general principle of this invention. 
     Motor stator  641  is mounted within the end casing  617  of the housing  612  closely adjacent to the air gap  643 . It includes a plurality of motor coils  646 , evenly distributed circumferentially in the annular space opposing to the rotor magnet ring. The coil axis is orientated substantially parallel with the rotor&#39;s rotational axis so that the rotor magnetic flux passes through the end surface area enclosed by the coil turns, or the flux links the coils. The coils  646  are connected into groups of windings of multiple phases, for example 3 phases, in a way commonly known to one skilled in the art. The coils  646  may be wound on cores  645  of soft iron to improve power efficiency of the motor. However, they may alternatively be wound on a core of nonmagnetic material, or without a core, in order to avoid or alleviate magnetic attracting force between the stator iron and rotor magnets. This is especially an advantage for a magnetically suspended rotor since the attracting force creates negative stiffness in axial direction that has to be compensated by positive stiffness provided by the magnetic suspension, which requires additional volume and weight of the magnetic suspension assembly, among other potential issues. Although the coils  646  shown in  FIGS. 22 and 23  are distinctly wound around cores  645 , alternatively, they can be constructed without a core and arranged in an overlapped manner with one side of a coil residing in the core area of another coil. This adds more flexibility in making use of space for a compact device. An annular plate of soft iron, the stator back yoke  647 , may be disposed on the back side of the stator coils  646  to increase magnetic flux linking the coil turns. In addition, a structure of partial iron cores that fills merely a portion of thickness of the coil core space may be made on the end surface of the back yoke  647 , to further increase the magnetic flux linkage and bring about increased efficiency. However, these structural features may not be needed, especially if they cause unacceptable magnetic attracting force between the motor stator and rotor. 
     It should be noted that the general principle and constructional features of the radial flux motor and axial flux motor disclosed herein can lead to other preferred configurations of electric motor of this invention. For example, the motor may possess an air gap of a straight conical or curved conical shape, so that the working magnetic flux passing through the air gap forms an angle with respect to the rotational axis of the rotor. In this way, the electric motor as discussed above can be readily adapted to various configurations of fluid pathway and magnetic suspension for handling stress sensitive fluids, by those having skill in the field. 
     As shown in  FIGS. 22 through 24 , the magnetic suspension assembly  660 , in accordance with an aspect of the present disclosure, includes a rotor assembly  662  and a casing assembly  661 . The casing assembly  661  is mounted within the exterior casing  616  of the pump housing  612 . The rotor assembly  662  is mounted on the outer side of the pump rotor  630 . The magnetic suspension assembly  660  is a hybrid structure of permanent magnet and electromagnet for passive and active suspensions according to the principle described in this disclosure, for example, in  FIGS. 9( a ) and ( b ) . 
     The rotor assembly  662  consists of a primary pole piece  673 , which is an annular plate of soft iron having an outer cylindrical surface opposing the air gap  663 . An annular permanent magnet  674  magnetized across its thickness is mounted on the rear end surface of the primary pole piece  673 . In an alternative configuration (not shown), another annular magnet is mounted on the front end surface of the primary pole piece to form a symmetric structure similar to the magnetic suspension construction of  FIGS. 9( a ) and ( b ) . This may enhance bias magnetic flux at the cost of increased volume of the pump. An end pole piece  675 , an annular plate of soft iron, is attached to the other end surface of permanent magnet  674  to serve for focusing magnetic flux into a concentrated area in air gap. The end pole piece  675  may not be needed, however, as long as sufficient suspension stiffness can be obtained, for example. 
     The casing assembly  661  consists of four substantially identical electromagnet units  690   a - d , evenly distributed around the periphery of the assembly. Each electromagnet  690   a - d  includes a pole shoe  683   a - d  which is primarily a circumferential segment of an annular soft iron. The four pole shoes  683   a - d  are disposed around the annular space, separated by gaps in between the neighboring pole shoes. The pole shoe  683   a - d  has an inner cylindrical surface opposing the rotor primary pole piece  673 , preferably with substantially equivalent thickness of the latter. An annular permanent magnet  684  magnetized across its thickness is installed on the rear end surfaces of the pole shoes  683   a - d . An end pole piece  685  of annular soft iron is mounted on the rear end of permanent magnet  684 . The magnet  684  and end pole piece  685  are preferably of substantially equivalent thickness as those of the opposing members  674 ,  675 , respectively. A symmetric configuration (not shown) that also includes a permanent magnet and/or end pole pieces mounted on the front end surfaces of the pole shoes  683   a - d , corresponding to the above mentioned alternative rotor configuration, may be employed as alternative embodiments of this invention. 
     Each electromagnet unit  690   a - d  also includes an iron core  692   a - d , a coil  691   a - d  that is wound around the iron core  692   a - d , and a back yoke  695  that is shared by all electromagnet units. Iron core  692   a - d  is primarily a cubic piece made of soft iron, with cross sectional shape of circular, rectangular with rounded corners, or any other suitable shape commonly known to one skilled in the art. One end of the iron core  692   a - d  is attached to an end surface of pole shoe  683   a - d , and the other end of the iron core  692   a - d  is attached to an end surface of back yoke  695 , which is an annular plate of soft iron and serves as the base circle to structurally connect all electromagnet units together. Unlike the iron cores  92   a - d  of  FIGS. 9( a ) and ( b )  that extend radially like spokes of a wheel, the iron cores  692   a - d  extend axially like legs that connect the base circle and top members (pole shoes). Magnetically, the back yoke  695  connects one electromagnet to the opposing electromagnet residing on the same radial axis (e.g.  690   a  and  690   b  on y axis) so that a pair of electromagnets works jointly for control of displacement in that axis. It should be appreciated that although the construction of electromagnet  690  of  FIG. 24  for pump  610  ( FIG. 22 ) and the construction of the electromagnet unit  90  of  FIGS. 9( a ) and ( b )  for pump  10  ( FIG. 3 ) appear different from each other, the general topology and magnetic circuit remains substantially similar Both electromagnets are constructed with the same fundamental building blocks including the pole shoe, iron core and back yoke, in a fundamentally same way of connecting these building blocks to form a magnetic circuit. One construction can be viewed as a result of bending and stretching the other construction without changing the structure of the magnetic circuit. However, a different aspect ratio of the magnetic suspension assembly is obtained by such a different configuration so that the assembly  60  best fits into the pump  10  of  FIG. 3  and the assembly  660  best fits into the pump  610  of  FIG. 22 . In that way, each individual pump can be optimized for the smallest overall pump size. Based on this discussion, various other embodiments of the magnetic suspension can be conceived according to the general principle of this invention by one skilled in the art, to best utilize the available space within a pump housing to create the most compact pump. 
     The magnetic suspension assembly  610  fulfills the function of passive suspension according to the same principle of magnetic flux linkage as the other embodiments of the present invention. Referring to  FIG. 24 , permanent magnets  674  and  684  together generate a group of magnetic flux loops  653   a - b . Each flux loop  653   a - b  links the rotor members including permanent magnet  674 , end pole piece  675 , and primary pole piece  673 , with the casing members including the permanent magnet  684 , the end pole piece  685 , and the electromagnet pole shoe  683   a - b . These rotor members and casing members oppose to each other across a radial air gap  663 . In addition, the overall thickness of the rotor assembly including members  673 ,  674 , and  675  is sufficiently small in comparison with the diameter of the air gap  663 . Therefore, the flux loop  653  has the attribute of flux loop linkage as defined herein and thus can provide axial and tilting stability. Note that while the  FIG. 24  shows magnetic flux loops  653   a - b  and pole shoes  683   a - b  due to the cross-sectional view, one skilled in the art will appreciate two additional flux loops and pole shoes are present in the embodiment, but are not shown in the cross-section. 
     The active suspension is achieved with the same mechanism of push-pull modulation of bias flux in air gap as the other embodiments of this invention. As shown in  FIG. 24 , a group of bias flux loops  653   a - b  are generated by permanent magnets  674 ,  684 . Two electromagnet units  690   a ,  690   b  work together for active control in y axis; and two other electromagnet units  690   c ,  690   d  work together for active control in x axis. Without loss of generality, only control in y axis is discussed below. Coils  691   a ,  691   b  are connected in series so that when electric current is supplied, they jointly generate a magnetic flux loop  655 , i.e. the modulating flux. The flux loop  655  is completed by passing through the perimeters of the rotor primary pole piece  673  and of the back yoke  695 , in addition to the iron core  692   a  and the pole shoe  683   a  of the electromagnet unit  690   a , the iron core  692   b  and the pole shoe  683   b  of the electromagnet unit  690   b , and air gap  663  on both positive and negative sides of the y axis. Therefore, the bias flux and modulating flux superimpose in the air gaps in between the rotor primary pole piece  673  and the casing pole shoes  683   a ,  683   b . These fluxes add up in the air gap in between the primary pole piece  673  and the pole shoe  683   b , on the positive side of the y axis. The bias flux is deducted by modulating flux in the air gap in between the primary pole piece  673  and the pole shoe  683   a , on the negative side of the y axis. Unbalanced magnetic force on the rotor primary pole piece  673  is thus resulted, pointing to the positive direction in y axis. The magnetic force can be controlled by adjusting the electric current in coil pairs  691   a ,  691   b . This shows the mechanism of push-pull modulation of bias flux, with which the rotor can be actively controlled by real time adjustment of electric current in electromagnets. In addition, the push and pull effects leads to linearity of control force with respect to control current, as explained above with the other preferred embodiments of this invention. 
     The bias flux loop  653  and the modulating flux loop  655  take different pathways in three-dimensional space so that they merely overlap in the air gap  663  and its surrounding pole members. The modulating flux does not pass through any permanent magnet, and the bias flux does not pass through any iron core of the electromagnets. 
     The various aspects of the present invention as discussed above can be used independently or jointly to best address design optimization of a fully magnetically suspended pump for handing stress-sensitive fluids such as blood. Particularly, they are presented to allow the electric motor and magnetic suspension to be adapted to the flow path that is configured to mitigate mechanical stress in fluid, to avoid flow stagnation, and to promote wash out of the fluid-contacting surfaces. Moreover, the electric motor and magnetic suspension are configured in various ways to allow optimization of the pump performances including pump compactness, power efficiency, reliability, suspension stiffness and other dynamic performances of the suspension, among others. In this context, optimization is addressed at the system level rather than components level. Therefore, the present invention should be regarded as a novel pump with the flow path, electric motor, and magnetic suspension configured and integrated in a unified way so that better performance of the entire pump is achieved. 
     Therefore, according to an aspect of this invention the electric motor and magnetic suspension unit are configured as separate components of the pump, in contrast to some of the conventional magnetically suspended pumps where bearingless motor or combined motor and bearing are employed. The so-called bearingless motor or combined motor and bearing may take various forms but fundamentally features a single rotor assembly serving for both electric motor and magnetic suspension. This is achieved by interaction of one magnetic field of rotor with two groups of coils in stator to respectively create rotational torque of the motor and translational forces of the magnetic suspension. Such a rotor magnetic field may be generated with a plurality of permanent magnets in the rotor and may possess multiple poles. Or, it may be a unipolar magnetic field built on a reluctance rotor that has varying, around the circular periphery, magnetic reluctance of the magnetic circuit energized by permanent magnets or electromagnets in the stator. In general, the rotor of a bearingless motor carries a magnetic field that spatially varies in a regular pattern in circumferential direction. When the rotor rotates, the rotor magnetic field at any point in the air gap varies with time. Although such a variation of rotor magnetic field constitutes a unique characteristic of a bearingless motor, it is to be avoided. In a magnetic suspension construction formed according to one embodiment of the present invention, variation of rotor magnetic field in the air gap when the rotor rotates can cause variations of magnetic force and torque of the magnetic suspension, which acts as an internal disturbance on suspension and thus compromises dynamic performance, e.g. causing vibration. Moreover, since the active suspension of the embodiments of the present invention is based on the mechanism of bias flux modulation, variations of the bias flux with rotational angle of the rotor can lead to unsteady relationship between the control force and current, which can significantly deteriorate control performances including stability robustness, response to external disturbance, suspension stiffness, damping, and so forth. In addition, such a rotor magnetic field can induce eddy current in stator members of electrically conductive material when the rotor rotates, and hampers power efficiency of the entire pump. 
     Therefore, in contrast to the bearingless motor, the rotor of a magnetic suspension assembly according to an embodiment of the invention is not intended to create regularly varying magnetic field in circumferential direction. Instead, substantially uniform rotor magnetic field for magnetic suspension is preferable. In addition, by separating the electric motor and magnetic suspension, this invention can make better use of available space within pump housing around the specific flow path of this invention, so that the overall pump dimensions can be minimized without compromising the other system performances such as power efficiency and suspension dynamics. 
     While embodiments of the invention presented herein may describe permanent magnets as annular or ring-shaped, one of skill in the art will recognize that other shapes and configurations of permanent magnets may be implemented to accomplish the desired effect. For example, the permanent magnets may be in the form of annulate segmental magnets. 
     The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. While particular embodiments have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the broader aspects of applicants&#39; contribution. The actual scope of the protection sought is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.