Abstract:
There is provided a magnetically levitated apparatus wherein an impeller has one side supported by an electromagnet and the other side supported and magnetically levitated by an attractive force created between a permanent magnet and a permanent magnet of a motor rotor rotated by a motor stator to rotate the impeller and a magnetic bearing sensor provides an output which is in turn rectified and thus shifted to have a gain adjusted and subsequently a notch filter removes a carrier wave frequency component used in the magnetic bearing sensor, to prevent a PID compensator from causing voltage saturation attributed to high frequency noise.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to magnetically levitated (maglev) apparatus and more specifically to those magnetically levitating an impeller to deliver liquid such as blood.  
           [0003]    2. Description of the Background Art  
           [0004]    [0004]FIG. 7 is a vertical cross section of a maglev liquid pump apparatus as one example of a conventional maglev apparatus and a block diagram of a controller thereof. In FIG. 7 a magnetically levitated liquid pump apparatus  100  is configured with an electromagnet unit  120 , a pump unit  130  and a motor unit  140  housed in a cylindrical housing  101 . Electromagnet unit  120  has an electromagnet  121  and a magnetic bearing sensor  122  incorporated therein. Housing  101  has on one side a side wall having a center provided with an inlet  102  introducing a liquid. At least three electromagnets  121  and at least three magnetic bearing sensors  122  surround inlet  102 . Electromagnets  121  and magnetic bearing sensors  122  are attached to a partition  103  separating electromagnet unit  120  and pump unit  130  from each other.  
           [0005]    In pump unit  130  an impeller  131  is rotatably housed and it has a portion closer to electromagnet unit  120 , or closer to one side, that is supported by electromagnet  121  contactless through partition  103 , and magnetic bearing sensor  122  senses the distance as measured from one side of impeller  131 . Impeller  131  has the other side with a permanent magnet  132  buried therein. Motor unit  140  houses a motor  141  and a rotor  142 . Rotor  142  has a surface facing pump unit  130  and having a permanent magnet  143  buried therein, opposite to permanent magnet  132  buried in impeller  131 , with partition  104  disposed therebetween.  
           [0006]    In the liquid pump apparatus thus configured, magnetic bearing sensor  122  provides an output which is in turn input to a sensor circuit  201  included in a controller  200  and sensor circuit  201  detects the distance between one side of impeller  131  and magnetic bearing sensor  122 . Sensor circuit  201  provides an output which is in turn input to a PID compensator  202  to provide PID compensation and PID compensator  202  provides an output which is in turn amplified by a power amplifier  203  and thus applied to electromagnet  121  to control attractive force provided toward the opposite side of impeller  131 .  
           [0007]    Furthermore impeller  131  has a portion closer to motor unit  140  that is affected by the attractive force introduced by permanent magnets  132  and  143  and impeller  131  is magnetically levitated by a non-controlled bearing provided by permanent magnets  132  and  143  and a controlled bearing provided by electromagnet  121 . A motor control circuit  205  provides a control signal which is in turn applied to a power amplifier  204 . Power amplifier  204  drives motor  141  and the motor&#39;s driving force rotates impeller  131  and blood or any other similar liquid introduced through inlet  102  is output through an outlet (not shown) formed at pump unit  130 .  
           [0008]    The FIG. 7 magnetic bearing sensor  122  is a reluctance sensor using a carrier wave. This reluctance sensor is provided opposite to impeller  131  with the FIG. 7 partition  103  disposed therebetween. Note that partition  103  is formed of conductive material, particularly titanium for blood pumps owing to its compatibility with blood. The reluctance sensor generates a magnetic field, which introduces an eddy current in the conductive titanium and would thus impair the sensor&#39;s sensitivity.  
           [0009]    In order to avoid this the carrier wave is adapted to have a low frequency range. This, however, may significantly affect controlling the magnetic bearing. More specifically, a reluctance sensor using a carrier wave provides a detection with a phase delay for down to a frequency approximately two decades lower than that of the carrier wave, e.g., 100 Hz for a carrier wave frequency of 10 kHz. To compensate for this to reliably control the magnetic bearing, PID compensator  202  needs to be constructed to lead a phase to a high frequency range. Consequently PID compensator  202  has a gain increased and a component of the carrier frequency contained in the sensor output causes voltage saturation in a circuit portion internal to PID compensator  202  and thus prevents reliable control.  
         SUMMARY OF THE INVENTION  
         [0010]    Therefore the present invention mainly contemplates a magnetic bearing apparatus capable of removing a carrier wave frequency component used in a magnetic bearing sensor, to provide reliably control.  
           [0011]    Generally the present invention provides a magnetically levitated apparatus including: a drive unit driving and thus levitating a body to be levitated; a magnetic position detection circuit using a carrier wave signal to detect a position of the body as the body levitates; a controlled magnetic bearing unit operative in response to an output of the magnetic position detection circuit to support the body without contacting the body; a control circuit operative in response to a signal output from the magnetic position detection circuit to control the controlled magnetic bearing unit, wherein between the magnetic position detection circuit and the body there exists a partition formed of a conductive material; and a filter connected between the magnetic position detection circuit and the control circuit to remove a carrier wave signal output from the magnetic position detection circuit.  
           [0012]    Thus in the present invention a filter can remove a carrier wave frequency component from a sensor output before amplification. Thus voltage saturation in the control circuit can be prevented to provide reliable control.  
           [0013]    More preferably the magnetic position detection circuit includes: a reluctance sensor provided adjacent to the body and having an inductance varying as the distance between the reluctance sensor and the body varies; and a sensor circuit operative in response to an output of the reluctance sensor to output a signal varying as the inductance varies.  
           [0014]    Furthermore the present apparatus further includes a carrier wave generation circuit feeding the magnetic position detection circuit with a carrier wave signal, wherein: the sensor circuit outputs the carrier wave signal with the amplitude varying as the spacing between the magnetic position detection circuit and the body varies; and the filter removes a center frequency of the carrier wave signal.  
           [0015]    Furthermore the filter is a band eliminating filter arranged immediately subsequent to the sensor circuit.  
           [0016]    Furthermore, the drive unit includes a non-controlled magnetic bearing unit magnetically coupled with the body at one side and a drive unit operative to rotate the body via the magnetic bearing unit, and the controlled magnetic bearing unit is magnetically coupled with the body at the other side.  
           [0017]    Furthermore the body is an impeller rotated to output a liquid and the magnetically levitated apparatus configures a magnetically levitated pump.  
           [0018]    The magnetically levitated apparatus further includes a drive unit rotatably driving the impeller through magnetic-coupling.  
           [0019]    Furthermore the impeller is rotated to output blood and the magnetically levitated apparatus configures a blood pump apparatus.  
           [0020]    Furthermore the impeller is rotated to output blood and the magnetically levitated apparatus configures a blood pump apparatus.  
           [0021]    More preferably the body is rotated to rotate a vane and the magnetically levitated apparatus configures a turbo molecular pump.  
           [0022]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    In the drawings:  
         [0024]    [0024]FIGS. 1A and 1B show a first embodiment of the present invention;  
         [0025]    [0025]FIG. 2 is a cross section taken along line II-II of FIG. 1A;  
         [0026]    [0026]FIG. 3 is a cross section taken along line III-III of FIG. 1A;  
         [0027]    [0027]FIG. 4 is a schematic block diagram showing a controller controlling a magnetically levitating apparatus of the present invention;  
         [0028]    [0028]FIG. 5 represents a frequency cut-off characteristic of a notch filter shown in FIG. 4;  
         [0029]    [0029]FIG. 6 is a cross section of a spindle for a turbo molecular pump in a second embodiment of the present invention; and  
         [0030]    [0030]FIG. 7 is a vertical cross section of a conventional magnetically levitated liquid pump apparatus and a block diagram of a controller. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    [0031]FIGS. 1A and 1B show a magnetically levitated liquid pump apparatus in one embodiment of the present invention. More specifically, FIG. 1A is a vertical cross section thereof and FIG. 1B is a cross section thereof taken along line IB-IB of FIG. 1A. FIG. 2 is a cross section taken along line II-II of FIG. 1A and FIG. 3 is a cross section taken along line III-III of FIG. 1A. FIG. 2 shows a sensor unit in a simple manner and FIG. 3 omits the sensor unit.  
         [0032]    With reference to FIGS. 1A and 1B, the liquid pump apparatus includes a cylindrical casing  1  axially separated by partitions  11 ,  12 ,  13  and  14  to have sections housing a magnetic bearing unit  20 , a pump unit  30  and a motor unit  40 , respectively. Casing  1  is formed for example of plastic, ceramic, metal or the like, although of casing  1 , partition  12  provided between magnetic bearing unit  20  and pump unit  30  and partition  13  provided between pump unit  30  and motor unit  40  are not allowed to be formed of magnetic material and they are accordingly formed of non-magnetic material.  
         [0033]    At pump unit  30  casing  1  is internally provided with a pump chamber  33  in which an impeller  31  rotates to output through an outlet  16  shown in FIG. 1B a fluid introduced through an inlet  15 . Impeller  31  has a plurality of vanes  34  spirally provided, as shown in FIG. 1B. Impeller  31  includes a non-magnetic member  35  having a permanent magnet  32  configuring a non-controlled magnetic bearing and a soft magnetic member  36  corresponding to a rotor of a controlled magnetic bearing. Permanent magnet  32  is divided in a circumferential direction of impeller  31  and adjacent magnets are magnetized to have opposite magnetic poles.  
         [0034]    Note that by coating the entire interior of pump chamber  33  with heparin serving as an anticoagulant, formation of thrombus can be prevented therein and the liquid pump apparatus can thus be used as a blood delivering pump. In this example, the heparin coating can effectively limit activation of coagulation system, protect platelets, limit activation, activation of inflammation system, activation of fibrinolysis system, contagion, and the like.  
         [0035]    In FIG. 1A, the dotted portion of impeller  31  is formed of soft magnetic material  36  and the remainder thereof is shown formed of nonmagnetic material. If the pump is used to deliver a corrosive fluid such as blood, the soft magnetic material is preferably a highly corrosive-resistant, ferritic stainless steel (SUS447J, SUS444 or the like) and the non-magnetic material is preferably a highly corrosive-resistant, austenitic stainless steel (SUS316L or the like) or titanium alloy, pure titanium or the like.  
         [0036]    Opposite to a side of impeller  31  having permanent magnet  32 , a cylindrical member  48  is provided in motor unit  40 , extending from a center of partition  13  toward partition  14 . Cylindrical member  48  has an external peripheral surface provided with a motor bearing  49  provided in the form of a ball or roller bearing which supports a motor rotor  46  rotatably. Cylindrical member  48  has an end with a motor stator  47  attached thereto. Motor rotor  46  is driven by motor stator  47  to rotate. Motor rotator  46  is provided with the same number of permanent magnets  45  as permanent magnets  32  of impeller  31  opposite thereto to provide attractive force. Adjacent permanent magnets  45  are magnetized to have opposite magnetic poles.  
         [0037]    Note that while the motor is a synchronous motor including a DC brushless motor, a non-synchronous motor including an induction motor, or the like, it may be any kind of motor.  
         [0038]    Provided in electromagnet unit  20  are an electromagnet  23  and a magnetic bearing sensor  24 , attached on a wall of partition  12  provided between electromagnet unit  20  and pump unit  30 , opposite to that side of impeller  31  having soft magnetic member  36 . Electromagnet  23  and magnetic bearing sensor  24  allow impeller  31  to be held at the center of pump chamber  33 , matching the attractive force produced between permanent magnets  32  and  45 .  
         [0039]    Thus the heat generated at electromagnet  23  can be transferred to partition  12  and thus cooled by a liquid existing in pump unit  30 . Similarly, the heat generated at motor stator  47  can also be transferred through cylindrical member  48  to partition  13  and thus cooled by the liquid existing in motor unit  30 . This can reduce heat transfer to outside casing  1  and also reduce heat transfer to magnetic bearing sensor  24  to provide a reliable sensing operation. Furthermore, if partitions  12  and  13  are increased in thickness to have a level of strength allowing electromagnet  23 , magnetic bearing sensor  24  and motor stator  47  to be attached thereto, housing  1  can advantageously have an outer-diameter portion reduced in thickness.  
         [0040]    Electromagnet  23  and magnetic bearing sensor  24  are arranged, as shown in FIGS. 2 and 3. More specifically, paired electromagnets  23  have magnetic poles  51  and  52  with a sensor  241  arranged therebetween, magnetic poles  53  and  54  with a sensor  242  arranged therebetween, and magnetic poles  55  and  56  with a sensor  243  arranged therebetween. Sensors  241  to  243  are typically a magnetic sensor, such as a reluctance sensor.  
         [0041]    Furthermore, as shown in FIG. 3, electromagnets  23  have their respective yokes  71 - 76  in the form of a column with electromagnet coils  8186  wound therearound, respectively.  
         [0042]    Circumferentially arranging magnetic poles  51 - 56  can increase the space housing electromagnet coils  81 - 86  that can be housed in electromagnet unit  20 . This ensures a large space for winding the coils without increasing the size of the pump. Increasing a space for housing a coil in turn allows an electromagnet coil to have an increased turn count and an increased wire diameter and can thus decrease the power consumption of the electromagnet.  
         [0043]    Furthermore, electromagnet yokes  71 - 76  in the form of a column can facilitate winding electromagnet coils  81 - 86  around electromagnet yokes  71 - 76 , respectively. Electromagnet yokes  71 - 76  having a simple geometry also ensure insulation from electromagnet coils  81 - 86 . While electromagnet yokes  71 - 76  are cylindrical, they may be in the form of a prism, which can facilitate winding coils and thus ensuring a withstand voltage between the coils and the yokes.  
         [0044]    Furthermore while in FIGS. 2 and 3 electromagnet yokes  71 - 76  and electromagnet coils  81 - 86  are all arranged in a single circle, they may not thus be arranged if required to effectively ensure a space for housing the same.  
         [0045]    With the magnetic bearing having each electromagnet with its magnetic pole and yoke arranged circumferentially, the magnetic bearing unit is not required to have a large space and the electromagnet yoke can be provided in a cylinder or a prism.  
         [0046]    [0046]FIG. 4 is a block diagram showing an example of a controller controlling a liquid pump as a magnetically levitated apparatus in the first embodiment of the present invention.  
         [0047]    In FIG. 4, the FIG. 1A magnetic bearing sensor  24  has an inductance L and receives a carrier wave signal for example of 100 kHz from a carrier wave generation circuit  60  via a capacitor C 2 . Magnetic bearing sensor  24  outputs via a capacitor C 3  to a rectifying circuit  61  a detection signal with the carrier wave having an amplitude varying as the spacing between sensor  24  and the FIG. 1A impeller  31  varies. Rectifying circuit  61  rectifies the detection signal to provide a dc signal which is in turn output via a shift circuit  62  to a gain adjustment circuit  63 .  
         [0048]    Shift circuit  62  and gain adjustment circuit  63  adjust the sensor output&#39;s zero point and gain and the resultant sensor signal is fed to a notch filter  64  to have the carrier wave&#39;s center frequency component removed.  
         [0049]    [0049]FIG. 5 represents a frequency cut-off characteristic of the FIG. 4 notch filter  64 . As shown in FIG. 5, notch filter  64  is a band elimination filter abruptly attenuating a center frequency fo of a carrier wave and it can thus remove a component of a carrier wave frequency from a component of a sensor output. Consequently if a PID compensator  65 , which is connected at a subsequent stage, is increased in gain a circuit portion internal thereto would not have voltage saturation and reliable control can thus be provided. PID compensator  65  provides an output which is in turn input to a power amplifier  66  to drive electromagnet  23 .  
         [0050]    Note that while notch filter  64  is connected at a stage immediately preceding PID compensator  65  it may alternatively be connected closer to the output of magnetic bearing sensor  24 .  
         [0051]    [0051]FIG. 6 is a cross section of a spindle for a turbo molecular pump as a magnetically levitated apparatus in a second embodiment of the present invention. In FIG. 6 a rotation shaft  151  has a radial direction supported by radial magnetic bearings  152 ,  153  configured of vertically arranged electromagnets and a thrust direction supported by a thrust magnetic bearing  154 . Thrust bearing  154  is configured of a disk  161  fixed on a lower side of rotation shaft  151  and permanent magnets  163 ,  264  vertically sandwiching the disk. Radial magnetic bearings  152 ,  153  are vertically sandwiched by radial position sensors  155 ,  156  provided in the form of a magnetic sensor corresponding to a reluctance sensor and sensing a gap as measured from rotation shaft  151 . Rotation shaft  151  has an upper portion with a vane  165  attached thereto to rotate to serve as a vacuum pump.  
         [0052]    Between radial magnetic bearings  152  and  153  there is provided a motor  157  to rotatably drive rotation shaft  151 . Below radial magnetic bearing  152  and radial position sensor  156  are arranged protecting ball or roller bearings  158 ,  159  supporting rotation shaft  151  for example when power supply is cut and rotation shaft  151  is thus not supported by radial magnetic bearings  152 ,  153 . Rotation shaft  151  has a lower portion provided with a thrust position sensor  160  sensing a thrust position of rotation shaft  151 . Thrust position sensor  160  is also provided in the form of a magnetic sensor corresponding to a reluctance sensor.  
         [0053]    Such a turbo molecular pomp spindle thus configured has a disadvantage similar to that as has been described with reference to the conventional example as radial position sensors  155 ,  156  and thrust position sensor  160  are all reluctance sensors.  
         [0054]    Accordingly if the FIG. 4 controller is arranged for each sensor and radial position sensors  155  and  156  and thrust position sensor  160  each have an output connected to a notch filter provided at a stage preceding a PID compensator a carrier wave frequency component can be removed and if the notch filter&#39;s output is used to control the corresponding radial magnetic bearings  152 ,  153  and thrust magnetic bearing  153  reliable control can be provided.  
         [0055]    Thus in an embodiment of the present invention a filter removing a carrier wave can be connected between magnetic position detection means and a control unit controlling a magnetic bearing unit. This can prevent a circuit connected at a stage subsequent to the filter from causing a voltage saturation attributed to a carrier wave component used by a sensor circuit. Thus reliable control can be provided.  
         [0056]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.