Abstract:
A developer system and method for an electrographic printer is provided that provides a variable rate of developer flow to the photoconductor drum of the printer. The system includes a magnetic brush having a magnetic core surrounded by a toning shell that rotatably conveys a layer of developer to the photoconductor element; a sump containing a reservoir of developer, and a variable speed conveyor roller.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to commonly assigned, copending U.S. application Ser. No. 12/484,397, filed Jun. 15, 2009, entitled: “SYSTEM AND METHOD FOR PROVIDING A STABLE AND HIGH FLOW RATE OF DEVELOPER IN AN ELECTROGRAPHIC PRINTER” and U.S. application Ser. No. 12/484,409, filed Jun. 15, 2009, entitled: “DEVELOPER SYSTEM AND METHOD FOR PROVIDING A STABLE FLOW RATE OF DEVELOPER IN AN ELECTROGRAPHIC PRINTER.” 
     FIELD OF THE INVENTION 
     This invention generally relates to electrographic printers, and is specifically concerned with a system and method that provides stable image quality by controlling the flow rate of developer to the magnetic brush of a developer station through the use of a conveyor roller operating at a preferred variable speed range. 
     BACKGROUND OF THE INVENTION 
     Electrographic printers that use a rotating magnetic brush to apply a dry, particulate developer to a photoconductor member are known in the art. In such electrographic printers, the magnetic brush includes a rotatable magnetic core surrounded by a rotatable, cylindrical toning shell. The toning shell may be eccentrically mounted with respect to the axis of rotation of the magnetic core. The eccentric mounting of the toning shell defines an area of relatively strong magnetic flux where the shell corners closest to the magnetic core, and an area of relatively weak magnetic flux where the shell is farthest away from the core. The magnetic brush is mounted over a developer sump that holds a reservoir of dry, two-component developer including a mixture of ferromagnetic transport particles and toner particles capable of holding an electrostatic charge. A rotatable conveyor roller is disposed between the reservoir of developer in the sump and the toning shell. In operation, the rotatable conveyor roller attracts and transports developer from the sump to a region of the toning shell where the magnetic flux is relatively weak. The rotating toning shell in turn transports the developer toward the photoconductor member. At the line of closest approach between the toning shell and the photoconductor member, the particulate toner component of the developer is transferred to the photoconductor member as a result of electrostatic attraction between the toner particles and the electrostatic field on the photoconductor member and consequently develops a latent electrostatic image on the photoconductor member. The developed image is ultimately transferred to a substrate, such as a sheet of paper, where the toner image is fused into a permanent image via a fuser station. The combination of the magnetic brush, developer sump and conveyor roller is referred to as a developer station in this application. 
     In order to print images as quickly as possible, it is necessary for the conveyor roller to rapidly deliver developer to the toning shell. However, it is also necessary for the toning shell to deliver a constant, uniform flow of developer material across its width to the photoconductor member by minimizing variations in developer delivery from the sump. Usually, a “metered flow” is obtained by utilizing a metering skive spaced uniformly from the toning shell parallel to the axis of rotation of the toning shell. Such a metered flow of developer on the toning shell insures that a uniform layer or “nap” of developer material of the proper thickness is delivered to the nip with the photoconductor member. If the nap is non-uniform, the resulting image may have streaks or other undesirable artifacts that degrade the quality of the image. If the developer nap is too thick, developer material can jam the nip and be expelled from the developer roller resulting in contamination of other areas of the electrophotographic reproduction apparatus. If the nap is uniform but too thin, there might not be enough toner present to enable a high quality image. 
     Producing acceptable image quality is made more complicated by the phenomena of developer break-in. Periodically, the developer needs to be changed in a high volume apparatus because of a deterioration of the characteristics of the carrier. A known problem associated with this change in developer is that there is a break-in period for the new developer after which the setpoints in the apparatus must be readjusted for best results. If the ultimate setpoints are used to start, the image shows substantial mottle. If the setpoints are chosen to work well with fresh developer, the images gradually lose density as the developer is “broken in.” 
     As previously indicated, past attempts at providing a metered flow of developer material have included the use of a metering skive across the toning shell downstream from the line of developer delivery between the conveyor roller and the toning shell. The skive gap and its relationship to the toning shell must be tightly controlled to achieve both a uniform thickness of the developer nap along the axis of rotation of the developer roller, and a desired thickness that is neither too thick or too thin. Even very small errors in the metering skive gap can result in an unacceptably large error in either the nap uniformity or nap thickness of the developer. Accordingly, when a metering skive is used, it is positioned at the point of the lowest magnetic field strength from the developer roller&#39;s magnetic core. It has been found that such positioning significantly decreases the sensitivity of developer nap height to the metering skive gap by a factor of two to four times. This makes the metering skive gap easier to setup in manufacturing and the resulting developer nap thickness less sensitive to differences in that skive gap along the length of the developer roller. 
     However, the applicants have observed multiple disadvantages associated with such metering skives. First, such metering skives limit the printing speed that can be achieved by the photoconductor member, as they necessarily impose a limit on the amount of developer that the toning shell can deliver to the photoconductor member. Second, the positioning of such skives at the preferred point of the lowest magnetic field strength from the developer roller&#39;s magnetic core requires the developer to be applied a relatively large angular distance of nearly 180° away from the line of closest approach between the toning shell and the photoconductor element. Such a large angular distance results in a relatively long residence time for the developer on the toning shell. The applicants have observed that such a relatively long residence time in combination with the rapid rotation of the magnetic core of the brush to achieve high printing speeds can disadvantageously age the developer, rendering it less effective in developing the electrostatic latent image on the photoconductor member. Third, the metered flow obtained with the metering skive is sensitive to the uniformity of the material delivered by the conveyor roller. Fourth, the metering skive spacing is typically set at a fixed spacing that does not take into account developer break in or developer deterioration. 
     SUMMARY OF THE INVENTION 
     To solve these and other problems, the developer system of the invention includes the combination of (1) a self-metering conveyor roller having a magnetic core and an outer shell for conveying developer from the reservoir of said sump to said toning shell of said magnetic brush, wherein a maximum magnetic field strength of said outer shell is less than 1000 gauss and preferably less than 300 gauss, and a minimum magnetic field strength between magnetic poles of said outer shell is no less than about 30% of said maximum field strength, and (2) a driving assembly that rotates the conveyor roller from a minimum speed for feeding developer to a speed that saturates the capacity of the toning shell such that that a constant, high flow rate of developer is provided to the toning shell despite variations in the attraction and transport of developer from the sump by said conveyor roller. The driver assembly and the conveyor roller speed are controlled by a controller to maintain image quality, and in particular, to maintain image density within acceptable limits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which: 
         FIG. 1A  is a schematic side view of a typical electrographic development station suitable for use with the development station improvements of the invention; 
         FIG. 1B  is a cross sectional side view of the conveyor roller of the invention, schematically illustrating the distribution of the magnetic poles in the roller magnetic core and schematically illustrating the lines of flux therebetween; 
         FIG. 2  is a schematic diagram of developer flow as a function of development station set points; 
         FIG. 3  is a graph of measured developer flow as a function of developer station set points for a first conveyor roller; 
         FIG. 4  is a graph of measured developer flow as a function of developer station set points second conveyor roller; 
         FIG. 5  is a graph of measured developer flow vs. developer mass area density as a function of developer station set points for a first conveyor roller; 
         FIG. 6A  is a plot of the magnetic field of a first conveyor roller, and 
         FIG. 6B  is a plot of the magnetic field of a second conveyor roller. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  illustrates a developer station  10  that the invention is applicable to. Such a developer station  10  includes a housing  12 . A developer roller or magnetic brush  14 , mounted within the development station housing  12 , includes a rotating (counterclockwise in  FIG. 1A ) fourteen pole magnetic core  16  inside a rotating (clockwise in  FIG. 1A ) toning shell  18 . Of course, the core magnet  16  and the shell can have any other suitable relative rotation. The rotating toning shell  18  applies developer to the outer surface of a drum-shaped photoconductor element  20  shown in phantom in order to develop latent electrostatic images written on the drum by an optical writing station (not shown). A magnetic conveyor roller  21  having a stationary core  22  surrounded by a rotatable shell  24  delivers developer  25  from a developer reservoir or sump  26  located at the bottom of the housing  12  to the back side of the toning shell  18 . In the preferred embodiment, shell  24  is approximately 1.20 inches in diameter. A toner replenishing tube  27  delivers toner to the developer  25  in the reservoir  26  to maintain a proper ratio of toner and transport particles in the developer  25 . Feed augers  28  having suitable mixing paddles (rotating counterclockwise) mix toner from the tube  28  with the developer in the reservoir  26 , while return augers  30  mix toner-depleted developer removed from the toning shell  18  by stripping skive  36  back into the developer reservoir. A conveyor roller controller and driving assembly  42  is operably connected to the shell  24  of the conveyor roller and controls the speed of rotation of the shell  24 . While not shown in detail, the conveyor roller controller and driving assembly may include an electric motor, a densitometer for measuring image density and image quality, a digital controller that controls the speed of the motor by controlling the amount of current conducted thereto, and a gear train. 
     With reference now to  FIG. 1B , the stationary core  22  of the conveyor roller  21  preferably includes at least two magnets and preferably at least four magnets  23  arranged around the circumference of the core  22  between the six o&#39;clock position and about the ten or eleven o&#39;clock position as shown. Although individual magnets are shown, the stationary core can also include a single piece of magnetic material that is magnetized to produce a similar magnetic field. The poles of the magnets  23  are further arranged so that they alternate N-S-N-S from the six and ten-eleven o&#39;clock position, although they could just as easily alternate in an S-N-S-N pattern. The radial vector  40  of each of the fields of the magnetic poles is radially aligned with respect to a central axis C of the cylindrical core  22 . Tangential field lines  41  interconnect the alternating poles of the magnets  23  as shown. As is described in more detail hereinafter, the magnets  23  provide a maximum combined radial and tangential magnetic field strength at the outer surface of the rotating shell  24  of preferably between about 100 and 300 gauss, and more preferably between about 150 and 250 gauss, and most preferably of about 200 and 250 gauss. Additionally, the minimum tangential magnetic field strength between magnetic poles  23  around the outer surface of the rotating shell  24  is preferably no less than about 30% of the maximum radial field strength, and more preferably no less than about 35-50% of the maximum radial field strength. In the preferred embodiment, magnets  23  are flexible strip-type magnets formed from a mixture of magnetic particles mixed with a plastic material. Such magnets having the required field strengths are commercially available, and are easy to cut to the sizes required for the manufacture of the roller  21 . 
     In operation, the shell  24  of the conveyor roller  21  is rotated clockwise by the driving assembly  42  at speeds ranging from a minimum speed necessary to feed developer to at least a speed that saturates the capacity of the toning shell to receive and convey developer to the photoconductor element  20 . Core  22  remains stationary. The magnet  23  disposed in approximately the six o&#39;clock position is adjacent the developer reservoir  26  and draws the magnetic developer  25  onto the outer surface of the shell  24  as shell  24  rotates, forming a layer  32  of developer on the shell  24 . The field applied by the magnets  23  in combination with the rotational speed of the shell  24  provides a uniform thickness to the layer  32  as it rotates upwardly. The outer surface of the shell  24  includes a sprocket-like pattern of ridges and grooves as shown to enhance the grip that the shell  24  applies to the layer  32  of developer. While the average diameter of the shell  24  is 1.20 inches, the diameter varies from 1.27 to 1.15 inches between the ridges and grooves. 
     The developer layer  32  is delivered to a back portion of the toning shell  18  of the magnetic brush  14 . A metering skive  34  (illustrated in phantom) can be provided immediately downstream of the line where the conveyor roller  21  delivers developer to the toning shell  18 . The gap between the metering skive  34  and the toning shell  18  is carefully calibrated to ensure that the layer  32  of developer delivered to the photoconductor element  20  has a uniform nap of the proper thickness to produce acceptable image quality for a typical developer. However it is advantageous to adjust the flow for a new developer or for an old developer by changing the conveyor roller speed to maintain acceptable image quality and to maintain image density within acceptable limits. This can be done either within a metering skive present or with a self-metering conveyor roller that does not require a metering skive to produce a uniform layer of developer on the toning shell. 
     For a developer station  10  employing conventional conveyor roller and a metering skive  34 , specific examples of magnetic brush core and developing shell speeds, conveyor roller speeds, auger rotational speeds, metering skive and takeoff skive gaps with respect to the toning shell are given below for printer speeds of 70 ppm, 83.3 ppm and 100 ppm, respectively: 
     
       
         
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Parameter 
                 Low 
                 Nominal 
                 High 
                 Units 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 70 PPM 
               
             
          
           
               
                 Core Speed 
                   
                 800 
                   
                 RPM 
               
               
                 Shell 
                   
                 82 
                   
                 RPM 
               
               
                 Conveyor 
                   
                 60 
                   
                 RPM 
               
               
                 Mixer 
                   
                 436 
                   
                 RPM 
               
               
                 Metering Skive Spacing 
                 0.033 
                 0.035 
                 0.037 
                 inches 
               
               
                 Take-off Skive Spacing 
                 0.007 
                 0.012 
                 0.017 
                 inches 
               
               
                 Process 
                   
                 11.8 
                   
                 ips 
               
               
                 speed 
               
             
          
           
               
                 83.3 PPM 
               
             
          
           
               
                 Core Speed 
                   
                 952 
                   
                 RPM 
               
               
                 Shell 
                   
                 97.58 
                   
                 RPM 
               
               
                 Conveyor 
                   
                 71.4 
                   
                 RPM 
               
               
                 Mixer 
                   
                 519 
                   
                 RPM 
               
               
                 Metering Skive Spacing 
                 0.033 
                 0.035 
                 0.037 
                 inches 
               
               
                 Take-off Skive Spacing 
                 0.007 
                 0.012 
                 0.017 
                 inches 
               
               
                 Process 
                   
                 14.0 
                   
                 ips 
               
               
                 speed 
               
             
          
           
               
                 100 PPM 
               
             
          
           
               
                 Core Speed 
                   
                 1142.9 
                   
                 RPM 
               
               
                 Shell 
                   
                 117.1 
                   
                 RPM 
               
               
                 Conveyor 
                   
                 85.7 
                   
                 RPM 
               
               
                 Mixer 
                   
                 623 
                   
                 RPM 
               
               
                 Metering Skive Spacing 
                 0.033 
                 0.035 
                 0.037 
                 inches 
               
               
                 Take-off Skive Spacing 
                 0.007 
                 0.012 
                 0.017 
                 inches 
               
               
                 Process 
                   
                 16.9 
                   
                 ips 
               
               
                 speed 
               
               
                   
               
             
          
         
       
     
     Surface speed of the rollers can be calculated from the roller speed in rpm using the diameter of the roller. For example, at 70 ppm printing speed, a toning shell with a 2 inch diameter rotating at 82 rpm has a surface speed in inches per second (ips) of 8.6=2π×82/60. The conveyor roller rotating at 60 rpm with a 1.27 inch maximum diameter has a maximum surface speed of 4 ips=1.27π×60/60, and so forth for surface speeds of toning station components at other printing speeds. Toning shell speed is approximately 10.2 ips at 83.3 ppm and approximately 12.3 ips at 100 ppm. 
       FIG. 2  is a schematic drawing showing developer flow in a developer station  10  as a function of the rotational speed of a conveyor roller. The dashed-line lower curve A in  FIG. 2  shows the flow measured on the toning shell  18  with a metering skive  34  in place, and with a conveyor roller. At rotational speeds lower than P 1 , the conveyor roller delivers a raw flow of developer to the toning shell  18  that varies with the rotational speed of the roller. At rotational speeds greater than P 1 , the conveyor roller delivers a metered, constant flow of developer indicated by the flattening of the dashed line curve. This metered flow of developer remains substantially constant despite variations in the amount of developer transported from the sump from causes including variation of the rotational speed of the conveyor roller after P 1 . The solid-line upper curve B in  FIG. 2  shows the flow measured on the toning shell  18  without a metering skive  34 . At rotational speeds lower than P 2 , the conveyor roller  21  delivers a flow of developer to the toning shell  18  that varies with the rotational speed of the roller. At rotational speeds greater than P 2 , the conveyor roller  21  delivers a stable, constant flow of developer indicated by the flattening of the solid-line curve that is substantially greater than the metered flow achieved with a metering skive  34 . This stable flow of developer remains substantially constant despite variations in the amount of developer transported from the sump from causes including variation of the rotational speed of the conveyor roller after P 2 .  FIG. 2  can be divided into three regions. In Region I, both non-metered flow (solid line) and metered flow utilizing a skive (dashed line) increase proportionally with conveyor roller speed. In Region II, raw and metered flow approach saturation. In Region III, both raw flow and metered flow are saturated, and independent of conveyor roller speed. The present invention consists of setpoints for which the raw flow increases with increasing conveyor roller speed, as shown in Regions I and II of  FIG. 2 . A metering skive can be used to control flow to acceptable levels below a limit at which developer jams the nip and is expelled from the developer roller 
       FIG. 3  is a graph illustrating the performance of a first conveyor roller that is capable of operating in Region I and Region II of  FIG. 2 , but not Region III. The vertical axis indicates the developer flow rate to the photoconductor element  20  in grams per inch-second or g/(in.-sec.) in a direction perpendicular to the axis of rotation of the toning shell  18 , while the horizontal axis indicates the rotational speed of a first conveyor roller in revolutions per minute (rpm). The lower three curves (indicated by diamonds, squares and triangles) plot developer flow rate as a function of conveyor roller speed when a metering skive  34  is present in the developer station  10  for printing speeds of 70 pages per minute (ppm), 83.3 ppm and 100 ppm respectively. Stable, metered flow occurs with the combination of a prior art conveyor roller and a metering skive  34  at all three printing speeds at about 100 rpm to 150 rpm. The upper two curves (indicated by short and long dash marks) plot developer flow rate as a function of conveyor roller speed when a metering skive is not present in the developer station  10  for printing speeds of 83.3 ppm and 100 ppm respectively. Note that the flow of developer can be controlled by changes in conveyor roller speed for this range of roller speeds with such a roller when the metering skive is not present. Flow rate is proportional to conveyor roller speed for rotational speeds up to 150 rpm with no indication in either of the top curves of a stable plateau being reached. 
       FIG. 4  shows the performance of a second conveyor roller  21  compared to that of a first roller shown in  FIG. 3 . The second conveyor roller is capable of operating in Region III, where stable metered flow and stable non-metered flow are obtained for a range of conveyor roller speeds. Specifically, the upper two curves indicated by short and long dashes plot developer flow rate as a function of conveyor roller speed when a metering skive is not present and a second conveyor roller  21  is used in the developer station  10  for printing speeds of 83.3 ppm and 100 ppm, respectively. These two curves show that stable flow of developer that is robust to changes in conveyor roller speed or other fluctuations in the instantaneous rate of developer transport is achieved with the second roller  21  when the metering skive is not present. The non-metered flow varies by less than plus or minus 10% for conveyor roller speeds of 50 to 150 rpm, a speed range of 6.6 ips conveyor roller surface speed for toning shell speeds of 10.2 to 12.3 ips. The conveyor roller speed range that provides stable non-metered flow is more than 50% of the toning shell speed. The lower three curves indicated by diamonds, squares, and triangles plot developer flow rate as a function of conveyor roller speed when a metering skive  34  is present in the developer station  10  for printing speeds of 70 pages per minute (ppm), 83.3 ppm and 100 ppm respectively. The flatness of these three curves indicates that the combination of the second conveyor roller and metering skive also provides a stable, metered flow of developer despite variations in the rotational speed of the roller or transport of the developer from the sump. 
     Indication of the controllability of flow by conveyor roller speed is provided by the data of  FIG. 4  and analysis of  FIG. 5 , which shows the performance obtained with the first conveyor roller. In  FIG. 5 , the developer flow normalized by printer speed in pages per minute (flow/ppm) and developer mass area density on the toning shell (DMAD) in units of g/(sq in) are plotted for the first conveyor roller. All normalized flow curves for metered and non-metered flow at 70, 83.3, and 100 ppm fall on nearly the same diagonal line, indicating that, at all printing speeds tested, flow is a linear function of developer mass area density. For flow with a metering skive spacing of 0.035 in., flow/ppm increases proportionally with DMAD as conveyor roller speed is increased until an upper limit is reached for DMAD. This limit in DMAD and flow is caused by the metering skive spacing.  FIG. 5  also shows indirectly that flow is directly proportional to toning roller magnetic core speed and shell speed, as these speeds were increased proportionally to obtain greater printing speed, and flow normalized by printing speed (flow/ppm) is approximately the same for all toning station settings at the same DMAD. 
       FIG. 5  also shows, for the curves corresponding to metered flows and non-metered flows obtained without a metering skive, that a range of flows and DMAD can be obtained by increasing or decreasing conveyor roller speed at all printing speeds tested. With the first conveyor roller, normalized metered flows of approximately 0.04 to 0.05 g/(in sec) per ppm are used at 70, 83.3 and 100 ppm printing speed. These flows are obtained at the standard metering skive spacing of 0.035 inches. If the first conveyor roller is used without a metering skive, normalized flow of approximately 0.65 to 0.80 g/(in.-sec.) per ppm can be obtained by increasing or decreasing conveyor roller speed. The maximum conveyor roller speed tested was150 RPM. 
     For the second conveyor roller  21 , both metered and non-metered flow is constant as a function of conveyor roller speed, as shown in  FIG. 4 . This shows the maximum flow rates obtainable by saturating the developer layer on the toning shell for print speeds to 100 ppm. The flow data for the second conveyor roller shown in  FIG. 4  can be normalized by dividing by printing speed in ppm. Normalized non-metered flows (flow/ppm) average 0.07 g/(in sec) per ppm for the full range of conveyor roller speeds from 50 to 150 rpm for printing speeds of 83.3 and 100 ppm. This is almost 50% more than the normalized flow rate used for the first roller for printing speeds up to 100 ppm. Normalized metered flow for the second conveyor roller is approximately the same as the prior art roller. 0.04 to 0.05 g/(in.-sec.) per ppm. 
       FIGS. 6A and 6B  compare the magnetic field strengths at the surface of the rotatable shell  24  of a conveyor roller  21  between a first four-magnet conveyor roller and the second conveyor roller respectively. In both graphs, the vertical axis is calibrated in gauss while the horizontal axis is calibrated in angular degrees. Both the first conveyor roller and the second conveyor roller utilize four magnetic poles uniformly spaced around the magnetic core  22  of the roller between the six and ten-eleven o&#39;clock positions. The poles of the magnets are radially aligned with the central axis C of the core  22  as illustrated in  FIG. 1B . The magnetic field on the surface of the rotating shell  24  is the resultant sum of the radial and tangential components schematically shown by force vectors  40  and  41  illustrated in  FIG. 1B . The strength of the radial and tangential field components is plotted at each angular point around the shell  24  by the thin solid line and dotted line in  FIGS. 6A and 6B , respectively. In both graphs, the radial field strength is characterized by peaks and valleys as a result of the alternating polarity of the magnets  23 . The tangential field strength also undulates, and maximizes or minimizes at the zero crossing points of the radial field strength. The resultant field strength is illustrated by the bold solid line of both graphs. The magnetic field of the first conveyor roller is less than the magnetic field of the second roller where a useful field of at least 100 gauss is present. 
     In generating and plotting the data illustrated in  FIGS. 6A and 6B , the applicants discovered two unexpected characteristics in the magnetic fields surrounding conveyor rollers that contributed to the development of the claimed invention. First, the graph of  FIG. 6A  indicates that a precipitous minimum field strength occurs between the two middle magnets  23  located at about the seven and nine o&#39;clock positions in the first conveyor roller. Specifically, this graph shows that a maximum field strengths of 190 and 200 gauss occur at about 94° and 200°, respectively, but that a minimum field strength of only about 30 gauss occurs at about 150°. Stated somewhat differently, the graph of  FIG. 6A  indicates that the minimum field strength in the array of the magnets  23  is only 15% of the maximum field strength. The applicants have further observed that such relatively low field strength in the middle of the array of magnets  23  creates what amounts to a “hole” in the field which substantially compromises the ability of the first conveyor roller  21  to transport developer. Second, the graph of  FIG. 6B  indicates that the percentage difference between the maximum and minimum field strengths can be largely remedied by only a relatively moderate increase in the magnetic field strength of the magnets  23 . Specifically, the applicants discovered that if the tangential field strength of the magnets  23  between the two center magnets is increased from 30 gauss to 100 gauss the minimum field strength increases to 100 gauss, which is 38% of the maximum field strength. Hence by increasing the tangential field strength only 70 gauss, the ratio of minimum to maximum field strength increases by a factor of 250% (i.e. from 15% to 38%). 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.