Abstract:
A centrifuge, especially a flow-through centrifuge free of rotating seals, for centrifuging biological fluids has a stand on which a frame ( 1 ) is rotatably mounted. A centrifuge chamber ( 3 ) is rotatably mounted on the rotating frame ( 1 ) to rotate about the axis of the latter. The centrifuge chamber ( 3 ) is driven in the same direction of rotation as the frame ( 1 ) but at twice the rotational speed. Coupling elements which are engaged by magnetic forces and are designed in the manner of a clutch disk or a gearwheel are used to transmit the torque to the centrifuge chamber or the rotating frame. Force is transmitted in a non-contact and wear-free manner. No lubrication is necessary, which also reduces the buildup of dust and dirt. In addition, there is little generation of noise.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a centrifuge, in particular a flow-through centrifuge for centrifuging biological fluids, such as blood, and capable of being free of rotating seals. 
     RELATED TECHNOLOGY 
     With such a centrifuge, the biological fluid is centrifuged in a flow-through process, with the fluid flowing through a line into and out of the rotating centrifuge chamber. The line routing proves to be problematical because of the relative movement between the centrifuge chamber and the stationary connection point of the line. Traditional flow-through centrifuges use rotating joints to prevent the line from twisting. German Patent Application No. 3,242,541 A discloses a blood centrifuge free of rotating seals, with the line being guided in a loop around the centrifuge chamber at half the rotational speed of the centrifuge chamber. To do so, the line is connected to a rotating frame which rotates at half speed in comparison with the centrifuge chamber. To drive the centrifuge chamber and the rotating frame, it is proposed that the rotating frame be connected to a hollow shaft, and that the centrifuge chamber be driven with a drive shaft extending through the hollow shaft. A belt drive is used to transmit the torque from the drive shaft to the centrifuge chamber. A blood centrifuge with a belt drive is also disclosed in U.S. Pat. No. 4,425,112. 
     International Patent Application No. WO 96/40322 discloses a blood centrifuge characterized by a very compact design. The centrifuge chamber and the line pusher are driven at half the rotational speed in the same direction of rotation as the chamber by a toothed gear. One disadvantage is the relatively loud running noises of the gearwheels, which noises are perceived as unpleasant by both the donor and the personnel. Furthermore, using gearwheels requires a high-precision manufacturing process for the centrifuge, which is therefore very expensive. Furthermore, the gearwheels must be lubricated, which not only increases the maintenance cost of the centrifuge but also leads to a buildup of dust and dirt. Therefore, the gear should be completely closed. However, arranging the gear in a closed casing in turn leads to problems in dissipating the resulting heat loss. In addition, the gearwheels are subjected to constant wear. 
     International Patent Application No. WO 96/04996 discloses a centrifuge in which the centrifuge chamber is designed as the rotor of an electric motor. However, this known centrifuge is not a flow-through centrifuge where there is the problem of twisting of the line. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to create a centrifuge capable of being free of rotating seals that will have low running noise and a drive that is largely maintenance-free while permitting operation at high rpms and relatively low driving power. 
     The present invention provides a centrifuge having: 
     a stand ( 21 ) on which a frame ( 1 ) is rotatably mounted, 
     a separation unit ( 3 ) rotatably mounted on the rotating frame ( 1 ), 
     a first drive train for transmitting the torque to the separation unit ( 3 ), 
     a line ( 7 ) for supplying and/or removing at least one fluid, leading from a stationary connection point ( 6 ) around the separation unit ( 3 ) and connected to the separation unit ( 3 ) on a side of the separation unit ( 3 ) facing away from the stationary connection point, and 
     a second drive train for transmitting a second torque to the rotating frame ( 1 ), with the separation unit ( 3 ) and the rotating frame ( 1 ) being driven so that the separation unit ( 3 ) rotates in the same direction as the rotating frame ( 1 ) but at twice the speed. 
     The centrifuge of the present invention is characterized in that the first and/or second drive train has coupling elements ( 17  to  20 ) arranged with a distance between them and designed so that the torques can be transmitted using magnetic forces. 
     The present invention also provides a centrifuge characterized in that the first and/or second drive train has at least one stator ( 143 ) with a first and/or second coil arrangement ( 142 ) and a mechanism ( 145 / 136 ) for magnetic coupling, designed so that the torques can be transmitted by magnetic forces. 
     The centrifuge according to the present invention has two drive trains, with the first drive train serving to transmit the torque to the centrifuge chamber and the second drive train serving to transmit the torque to the rotating frame. The centrifuge chamber and the rotating frame can be driven by a common motor or by separate motors. 
     The first and/or second drive train has coupling elements arranged with a spacing between them for transmitting the torque; said coupling elements are designed so that the torque can be transmitted by magnetic forces. Force is transmitted in a non-contact and wear-free manner. Lubrication is not necessary, which therefore also reduces the accumulation of dust and dirt. In addition, little noise is generated. It is also advantageous that the line for supplying and/or removing the fluid can be passed through the gap between coupling elements, thereby simplifying the spatial arrangement of drive elements. The coupling elements may be in one piece with the centrifuge chamber or the rotating frame. However, they may also be spatially separated from the centrifuge chamber and the rotating frame, with the torque being transmitted from the respective coupling element to the centrifuge chamber or the rotating frame by way of additional coupling elements which are magnetically engaged, or additional gear elements of a wide variety of designs. The magnets may be attached to the top or bottom side of the clutch disks. However, they may also be integrated into the clutch disks or they may be of one piece with the centrifuge chamber. For example, the gaps between the magnets may be filled with a casting compound to produce smooth surfaces. 
     It has been found in experiments that a relatively great torque can be transmitted in particular when the coupling elements have magnets arranged on a circumference such that the magnetic poles of adjacent magnets of a coupling element are arranged in opposition to one another. The magnets are preferably permanent magnets. In principle, however, electromagnets can also be used for transmission of torque. 
     The coupling elements may be designed in the manner of a clutch disk. In a preferred embodiment of the centrifuge, two coupling elements designed in the manner of a clutch disk mounted to rotate about a common axis may be provided in the first and/or second drive train(s), with the magnets being arranged in opposition to one another along the circumference on the top or bottom side of the coupling elements. Such an arrangement serves to transmit torques with the shafts of the drive train aligned. 
     It has surprisingly been found that a relatively high torque can be transmitted even when the clutch disks are mounted rotatably about two parallel axes, with the magnets being arranged in opposition to one another only along part of the circumference on the top or bottom side of the coupling elements. Such an arrangement serves to transmit torques when the shafts are not aligned. 
     However, the coupling elements may also be designed in the manner of a gearwheel. In a preferred embodiment, two coupling elements mounted rotatably about perpendicular axes are provided in the first and/or second drive train of the centrifuge. The coupling elements may be designed as bodies in the form of truncated cones with the magnets arranged on the conical faces, or the coupling elements may be designed as disk-shaped bodies, with the magnets arranged on the top or bottom side thereof. 
     In an especially preferred embodiment, the magnets have a rectangular cross section with one narrow side and one long side. The magnets preferably are arranged on the circular disk-shaped coupling elements in such a way that their longitudinal axes run radially. This arrangement allows an especially large torque to be transmitted at a low angle offset. The lateral spacing between the magnets of a coupling element is optimal when it corresponds essentially to the spacing between the magnets of one coupling element and the opposing magnet of the other coupling element which is magnetically engaged with the first coupling element. 
     In an alternative embodiment of the blood centrifuge, which is based on the same principle, however, namely the principle of magnetic coupling, the centrifuge includes a stator with a coil arrangement for generating a first and a second magnetic field and a mechanism for magnetic coupling of the rotating frame such that the centrifuge chamber can be driven by the first magnetic field of the stator and the mechanism for magnetic coupling of the centrifuge chamber such that the centrifuge chamber can be driven by the second magnetic field of the stator. The magnetic fields of the stator are designed so that the centrifuge chamber is driven in the same direction of rotation as the rotating frame but at twice the rpm. 
     An important advantage of the alternative embodiment is the especially compact design. It is also characterized by low running noise and extensive freedom from maintenance and can be operated at high speeds and at a relatively low drive power. 
     The rotating frame of the centrifuge preferably has a top and a bottom carrying plate, with the centrifuge chamber being rotatably mounted on the top carrying plate and the stator being arranged between the centrifuge chamber and the bottom carrying plate. The line connected to the centrifuge chamber can be guided to the stationary connection through the air gap between the centrifuge chamber and the stator. 
     The mechanism for magnetic coupling of the rotating frame and the centrifuge chamber are preferably permanent magnets arranged on the rotating frame or the centrifuge chamber, distributed around the periphery on the bottom side of the centrifuge chamber or the bottom carrying plate of the rotating frame, with the magnetic poles of adjacent magnets being aligned in opposition to one another. The centrifuge chamber and the rotating frame are driven by magnetic rotating fields generated by the coil elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Several embodiments of the present invention are described in greater detail below with reference to the drawings, in which: 
     FIG. 1 shows a schematic diagram of a first embodiment of a centrifuge of the present invention, with the coupling elements designed in the manner of a gearwheel; 
     FIG. 2 shows a schematic diagram of a second embodiment of the centrifuge, with the coupling elements designed in the manner of a gearwheel; 
     FIG. 3 shows a schematic diagram of a third embodiment of the centrifuge, with the coupling elements designed in the manner of a clutch disk; 
     FIG. 4 a  shows a cross section through two clutch disks which are magnetically engaged, and 
     FIG. 4 b  shows a top view of one of the clutch disks; 
     FIG. 5 a  shows a schematic diagram of a fourth embodiment of a centrifuge, with the coupling elements designed in the manner of a clutch disk; 
     FIG. 5 b  shows a block diagram to illustrate the arrangement of the magnets on the clutch disks of the centrifuge from FIG. 5 a;    
     FIG. 6 a  shows a schematic diagram of a fifth embodiment of the centrifuge, with the coupling elements designed in the manner of a clutch disk; 
     FIG. 6 b  shows a block diagram to illustrate the arrangement of the magnets on the clutch disks of the centrifuge from FIG. 5 a;    
     FIG. 7 shows an alternative embodiment of the blood centrifuge, where the centrifuge chamber and the rotating frame are driven by magnetic rotating fields. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic diagram of a first embodiment of a centrifuge free of rotating seals for centrifuging biological fluids, especially blood. The centrifuge has a rotating frame  1  with top and bottom carrying plates  1   b ,  1   a  and two side parts  1   c ,  1   d . Rotating frame  1  is driven by an electric motor  2  whose drive shaft  3  is connected to bottom carrying plate  1   a  of the rotating frame. A centrifuge chamber  3  arranged in the rotating frame can rotate about the vertical axis of rotation of frame  1 . On its top side, centrifuge chamber  3  has a shaft  4  mounted in a bearing  5  on the top carrying plate  1   b  of rotating frame  1 . However, it may also be arranged above the top carrying plate. 
     A flexible line  7  which may combine one or more tubes for supplying blood or blood constituents to centrifuge chamber  3  and removing them from the centrifuge chamber leads from a stationary connection  6  around the centrifuge chamber and is connected to the bottom side of the chamber. Line  7  extends through a central recess  8  of rotating frame  1 . A line connection  9  ending in an eye  10  where line  7  is secured is mounted on a side part  1   d  of rotating frame  1 . However, the line may also be loosely guided without being connected to the rotating frame. 
     Two carrier disks  13 ,  14  in the form of a shallow truncated cone are mounted in bearings  11 ,  12  on side parts  1   c ,  1   d  of rotating frame  1  to rotate about a common horizontal axis; permanent magnets  15 ,  16  are mounted at a uniform spacing on their conical faces  13   a ,  14   a . Carrier disks  13 ,  14  themselves are preferably made of a ferromagnetic material. Magnets  15 ,  16  are arranged on the peripheral faces  13   a ,  14   a  of carrier disks  13 ,  14  so that the magnetic poles of adjacent magnets of one carrier disk are aligned in opposition to one another. Carrier disks  13 ,  14  with magnets  15 ,  16  form intermediate coupling elements  17 ,  18  designed like a gearwheel in magnetic engagement with other coupling elements, forming a gear, and disposed in an intermediate position between third coupling element  19  and fourth coupling element  20 . 
     Intermediate coupling elements  17 ,  18  mounted rotatably on side parts  1   c ,  1   d  of rotating frame  1  are magnetically engaged with a third coupling element  19  mounted on the bottom side of centrifuge chamber  3 , and with a fourth coupling element  20  which is connected to a stationary stand  21 . Although carrier disks  13 ,  14  of the vertical, or intermediate coupling elements  17 ,  18  each have an even number of magnets, the same number of magnets  23  and  24 , also an even number, is distributed at equal intervals on conical faces  21   a  and  22   a  of ferromagnetic carrier disks  21  and  22  of the horizontal coupling elements  19  and  20 . 
     The drive of centrifuge chamber  3  and rotating frame  1  operates as follows. Electric motor  2  drives rotating frame  1  at speed n. The vertical coupling elements  17 ,  18  which are magnetically engaged with rotationally fixed horizontal coupling element  20  are driven in the opposite direction of rotation by the rotation of rotating frame  1 . The vertical coupling elements  17 ,  18  in turn drive horizontal coupling element  19  which is connected to centrifuge chamber  3  in the same direction of rotation, but at twice the speed. Since rotating frame  1  moves line  7  around the chamber at half the speed of centrifuge chamber  3 , twisting of the line is prevented. 
     FIG. 2 shows a schematic diagram of a second embodiment of the centrifuge. This embodiment differs from the embodiment described with reference to FIG. 1 in that the carrier disks of the coupling elements are cylindrical and only one vertical coupling element is mounted on the side part of the rotating frame, with the rotating frame being open on the side opposite the vertical coupling element. The rotating frame which is open at the side facilitates monitoring of the phase limit in the centrifuge chamber. 
     In this embodiment, rotating frame  25  comprises a bottom frame half  26  and a top frame half  27 , with the top frame half  27  being mounted on the bottom frame half  26  to pivot about an axis, so that the rotating frame can be opened up. 
     Shaft  29  of centrifuge chamber  30  is mounted rotatably in bearing  28  of top frame half  27  and is connected to horizontal carrier disk  31  on whose peripheral face magnets  32  are distributed at uniform intervals around the circumference. Carrier disk  33  of the vertical coupling element  34  mounted rotatably in bearing  35  on bottom frame half  26  carries magnets  36  on its top side at uniform intervals along the circumference. Vertical coupling element  34  is in turn magnetically engaged with coupling element  37  mounted on centrifuge chamber  30  and with coupling element  39 , which is mounted on stationary stand  38  and is designed like coupling element  37  of centrifuge chamber  30 . 
     Centrifuge chamber  30  and rotating frame  25  are driven with the same electric motor  40  whose drive shaft  41  is mounted on the bottom frame half  26  of rotating frame  25 . If electric motor  40  drives rotating frame  25  at speed n, centrifuge chamber  30  will rotate at double speed 2n in the same direction of rotation. A line is provided for supplying fluids to and removing them from the centrifuge chamber. 
     FIG. 3 shows a schematic diagram of a centrifuge drive, where the coupling elements are designed as clutch disks. Rotating frame  42  is mounted to rotate about a vertical axis on a stand. A hollow shaft  43  mounted on the bottom carrying plate  42   a  of rotating frame  42  interacts with a pulley  44  with a belt  45  leading to a pulley  48  mounted on drive shaft  46  of an electric motor  47 . 
     Centrifuge chamber  49  is mounted rotatably on the top carrying plate  42   b  of rotating frame  42  with a bearing  50  as in the embodiments described with reference to FIGS. 1 and 2. A circular disk-shaped carrier plate  50  mounted on the bottom side of centrifuge chamber  49  has magnets  51  distributed around its circumference at equal intervals, so that the magnetic poles of adjacent magnets are aligned in opposition to one another. The first clutch disk  53  is magnetically engaged using magnets  54   a  with a second clutch disk  54  of the same design. The latter is mounted on a drive shaft  55  which extends through hollow shaft  43  of rotating frame  42  and is mounted in a bearing  56 , which is inserted into hollow shaft  43 , and rotates about the axis of rotating frame  42 . Drive shaft  55  carries a pulley  56 , with a belt  57  leading to a pulley  58  mounted on drive shaft  46  of electric motor  47 . Pulley  44  of hollow shaft  43  has a diameter twice as large as that of pulley  56  of drive shaft  55 , while pulleys  48 ,  58  mounted on drive shaft  46  of electric motor  47  have the same diameter, so that rotating frame  42  is driven the same direction of rotation as drive shaft  55  of clutch disk  54 , but at half its rotational speed. Clutch disk  54 , mounted on drive shaft  55  and magnetically engaged with clutch disk  53  mounted on centrifuge chamber  49 , then also drives the centrifuge chamber in the same direction of rotation as the rotating frame but at twice the rotational speed. Line  59  for supplying blood and/or blood constituents to the centrifuge chamber and removing them from the centrifuge chamber is guided to the stationary connection  59   a  through the air gap between clutch disks  53 ,  54 . 
     FIG. 4 a  shows a cross section through two clutch disks  60 ,  61  which are magnetically engaged. FIG. 4 b  shows a top view of one of the clutch disks  61 . Magnets  62   a ,  62   b  are distributed with a uniform spacing around the circumference of the bottom side of carrier plate  62  of the top clutch disk  60 , with the magnetic poles of adjacent magnets being aligned in opposition to one another. Carrier disk  63  of the bottom coupling element  61  carries magnets  63   a ,  63   b  on its top side. The magnetic poles are labeled as north N and south S in FIG.  1 . When clutch disks  60 ,  61  are magnetically engaged, the magnets of the two clutch disks are aligned so that magnets of opposite polarities are opposite one another. An especially great torque can be transmitted with a small angle offset when the magnets have a rectangular cross section with long side  63   a  and narrow side  63   d . The magnets are arranged along the circumference of the carrier disks in such a way that their longitudinal axes  63   e  intersect at midpoint  61   a  of carrier disks  63 . Transmission of torque is optimal when the distance a between magnets on opposing clutch disks  60 ,  61  is essentially equal to distance b between the adjacent magnets on a clutch disk. 
     FIGS. 5 a  and  5   b  show a schematic diagram of another embodiment of the blood centrifuge. The blood centrifuge has a stationary stand  65  and a frame  66  comprising a cylindrical bottom frame half  66   a  and a cylindrical top frame half  66   b  having a smaller diameter than the bottom frame half. Bottom frame half  66   a  is mounted with a ball bearing  72  on stationary stand  65  to rotate about a vertical axis. Top frame half  66   b  accommodates centrifuge chamber  68  whose shaft  68   a  is mounted with a ball bearing  70  on the top plate of rotating frame  66  to rotate about the axis of the frame. A coupling element  69  mounted on the bottom side of centrifuge chamber  68  is designed in the manner of a clutch disk. Clutch disk  69  has a cylindrical ferromagnetic carrier plate  64  on whose bottom side are distributed at a uniform spacing an even number of magnets  64   a , e.g., circular magnets, arranged around the circumference with alternating polarities N, S (FIG. 5 b ) line  71  for supplying and removing blood and/or blood constituents passes through a side opening in the frame to the stationary connection. A drive shaft  73  mounted in a bearing  67  on stationary stand  65  is driven by an electric motor of a drive. Drive shaft  73 , arranged with an offset to the side of the axis of rotation of frame  66 , is connected to a second coupling element  74  which is designed in the manner of a clutch disk and is arranged in bottom frame half  66   a . The second clutch disk  74  has a ferromagnetic carrier disk  75  with a larger diameter than carrier disk  64  of the first clutch disk  69 . On the top side there are magnets  76  in an even number which is 50% larger than the number of magnets  64 . The second clutch disk  74  is magnetically engaged with the first clutch disk  69  over part of its circumference. On the other hand, the second clutch disk  74  is magnetically engaged over part of its circumference with rotating frame  66  around whose circumference are mounted magnets  77  (twice as many as magnets  64   a ) in a uniform spacing so that magnetic poles N, S of adjacent magnets are opposite one another. 
     The drive of the centrifuge operates as follows. The second clutch disk  74 , which is driven at speed 1.5n, in turn drives the first clutch disk  69 , which is connected to centrifuge chamber  68 , at speed 2n, and also drives rotating frame  66  at speed n in the same direction of rotation as centrifuge chamber  68 . 
     FIGS. 6 a  and  6   b  show another embodiment of the blood centrifuge. The blood centrifuge has a stationary stand  80  and a frame  81 . The bottom frame half  81   a  is mounted with a ball bearing  82  on stationary stand  80  to rotate about a vertical axis. The top frame half  81   b , which is mounted to pivot on the bottom frame half  81   a  or is designed in one piece with the bottom frame half, accommodates centrifuge chamber  83  whose shaft  84   a  is mounted on the top plate of frame  81  to rotate about its axis of rotation. A first coupling element  84  mounted on the bottom side of the centrifuge chamber is designed as a clutch disk with an even number of magnets  85 . Line  86  for supplying and removing blood and blood constituents leads to the stationary connection through a side opening in the rotating frame. A second clutch disk  87  of the same diameter as the first clutch disk  84  is mounted on a partition  81   c  of frame  81  at a distance from the first clutch disk  84  so that it can rotate about the axis of rotation of the frame. The second clutch disk  87  is connected to a third coupling element  89 , likewise designed as a clutch disk, by two coupling rods  88   a ,  88   b . Coupling rods  88   a ,  88   b  have an articulated connection to the second clutch disk  87  at two opposite points of a circle, while on the other hand having an articulated connection to the third clutch disk  89  at two opposite points of a circle with the same diameter (FIG. 6 b ). On its bottom side, the third clutch disk  89  has an even number of magnets  90  and is magnetically engaged over part of its circumference with rotating frame  81  on whose bottom plate are provided magnets  91  distributed at a uniform spacing over the circumference but the number of these magnets is twice as high as that of magnets  90 . The third clutch disk  89  is connected to a drive shaft  92  which is driven by an electric motor of a drive. 
     The drive of the blood centrifuge operates as follows. Drive shaft  92 , which is connected to the third clutch disk  89 , is driven at speed 2n. The third clutch disk  89  in turn drives frame  81  at speed n and also drives the second clutch disk  87  at speed 2n by way of coupling rods  88   a ,  88   b , and the second clutch disk in turn drives the first clutch disk  84 , which is connected to centrifuge chamber  83 , in the same direction of rotation as frame  81  but at twice the speed 2n. 
     FIG. 7 shows a schematic diagram of an alternative embodiment of the blood centrifuge. The blood centrifuge has a stand  130  on which a rotating frame  131  is rotatably mounted. Rotating frame  131  has a bottom frame half  131   a  with a bottom carrying plate  131   b  and two side walls  131   c ,  131   d  and a top frame half  131   e  with a top carrying plate  131   f  and two side walls  131   g ,  131   h . The rotating frame is open on two opposite sides. The bottom carrying plate  131   b  of rotating frame  131  is rotatably mounted with a roller bearing  133  on a vertical axis  132  extending from stand  130  into frame  131 . A centrifuge chamber  135  is mounted on top carrying plate  131   f  of rotating frame  131  with a roller bearing  134  so that it rotates about the axis of the rotating frame. Permanent magnets  136  are attached to the bottom side of centrifuge chamber  135 , distributed at a uniform spacing around the circumference, with the magnetic poles of adjacent magnets being aligned opposite one another. To this extent, the design of the centrifuge chamber corresponds to that of the centrifuge chambers described above. 
     A flexible line  138 , which may combine one or more hoses for supplying blood and/or blood constituents to centrifuge chamber  135  and removing them from the centrifuge chamber, leads from a stationary connection  137  around the centrifuge chamber and is connected to the bottom side of the chamber. Line  138  extends through rotating frame  131 , which is open at the side. A line connection  139  mounted on a side part  131   h  of rotating frame  131  ends in a loop  140  in which the line is secured. However, line  138  may also be carried loosely without being connected to rotating frame  131 . 
     A plate  141  which carries a coil arrangement  142  is mounted on vertical shaft  132  of stand  130 . A first coil  142   a  is mounted on the top side of plate  141 , while a second coil  142   b  is mounted on the bottom side of the plate. The two coils  142   a ,  142   b  are connected by electric connecting lines  143  to a control unit  144  in stand  130  of the blood centrifuge. 
     Additional permanent magnets  145  are distributed at an even spacing on a circumference on the bottom carrying plate  131   b  of rotating frame  131  at a distance from the second coil  142   b , with the magnetic poles of adjacent magnets being aligned in opposition to one another. 
     The blood centrifuge operates as follows. The first coil  142   a  generates a first magnetic rotational field, so that centrifuge chamber  135  is driven, while the second coil  142   b  generates a second magnetic rotational field which drives rotating frame  131 . The first and second coils  142   a ,  142   b  are driven by control unit  144  so that centrifuge chamber  135  is driven in the same direction of rotation as rotating frame  131  but at twice the speed (rpms) 2n. Twisting of line  138  coming out of the side of the rotating frame is prevented because the line rotates about the centrifuge chamber at half the speed of the latter.