Abstract:
A printing machine for printing on a recording medium moving along a path through a print zone, includes a printhead, adapted to deposit ink on the recording medium in the print zone; and a radiant dryer, disposed adjacently to the path, for heating the recording medium. The radiant dryer includes a reflector and a heat source. The reflector includes a first portion defining a first heat region preheating the recording medium at a position in the path prior to the print zone, and a second portion defining a second heat region heating the recording medium in or subsequent to the print zone. In this design, the first portion generates heat energy having a first temperature and said second portion generates heat energy having a second temperature greater than said first temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates generally to a liquid ink printing apparatus, and more particularly to a liquid ink printing apparatus having a dual function radiant dryer. 
     2. Description of Related Art 
     Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal printer, have at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink is contained in at least one channel, or preferably in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels. 
     In a thermal ink-jet printer, the power pulse is usually produced by a heater transducer or a resistor, typically associated with one of the channels. Each resistor is individually addressable to heat and vaporize ink in one of the plurality of channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially bulges from the channel orifice, followed by collapse of the bubble. The ink within the channel then retracts and separates from the bulging ink, to form a droplet moving in a direction away from the channel orifice and towards the recording medium. When the ink droplet hits the recording medium, a drop or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. 
     The ink jet printhead may be incorporated into either a carriage type printer, a partial-width-array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge. The combined printhead and cartridge assembly is attached to a carriage, which is reciprocated to print one swath of information (having a width equal to the length of a column of nozzles) at a time on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion of the swath, so that the next printed swath is contiguous or overlapping with the previously printed swath. This procedure is repeated until the entire page is printed. In contrast, the page-width printer includes a stationary printhead having a length sufficient to print across the width or length of a sheet of recording medium at a time. The recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. A page width ink-jet printer is described, for instance, in U.S. Pat. No. 5,192,959. 
     Many liquid inks, and particularly those used in thermal ink jet printing, include a colorant or dye and a liquid, which is typically an aqueous liquid vehicle, such as water, and/or a low vapor pressure solvent. The ink is deposited on the substrate to form an image in the form of text and/or graphics. Once deposited, the liquid component is removed from the ink and the paper to fix the colorant to the substrate by either natural air drying or by active drying. In natural air drying, the liquid component of the ink deposited on the substrate is allowed to evaporate and to penetrate into the substrate naturally without mechanical assistance. In active drying, the recording medium is exposed to heat energy of various types, which can include infrared heating, conductive heating and heating by microwave energy. 
     Active drying of the image can occur either during the imaging process or after the image has been made on the recording medium. In addition, the recording medium can be preheated before an image has been made to precondition the recording medium in preparation for the deposition of ink. Preconditioning the recording medium typically prepares the recording medium for receiving ink by driving out excess moisture, which can be present in a recording medium such as paper. Not only does this preconditioning step reduce the amount of time necessary to dry the ink once it is deposited on the recording medium, but this preconditioning step also improves image quality by reducing paper cockle and curl, which can result from too much moisture remaining in the recording medium. 
     Various drying mechanisms for drying images deposited on recording mediums are illustrated and described in the following disclosures, which may be relevant to certain aspects of this invention. 
     U.S. Pat. No. 4,970,528 to Beaufort et al., describes a method for uniformly drying ink on paper from an ink jet printer. The printer includes a uniform heat flux dryer system including a 180° contoured paper transport path for transferring paper from an input supply tray to an output tray. During transport, the paper receives a uniform heat flux from an infrared bulb located at the axis of symmetry of the paper transport path. Reflectors are positioned on each side of the infrared bulb to maximize heat transmission from the bulb to the paper during the ink drying process. 
     U.S. Pat. No. 5,029,311 to Brandkamp et al., describes a fluorescent lamp utilized in a document scanning system that is environmentally and thermally stabilized by means of a bifurcated heater control assembly. A heater blanket is wrapped around the entire surface of the lamp, including the end areas surrounding the filaments but exclusive of the aperture through which light is emitted. 
     U.S. Pat. No. 5,274,400 to Johnson et al. describes an ink path geometry for high temperature operation of ink jet printheads. A heating means is positioned close to a print zone for drying of the print medium. The heating means includes a print heater and a reflector, which serve to concentrate the heat on the bottom of the print medium through a screen. 
     U.S. Pat. No. 5,287,123 to Medin et al. describes a color ink jet printer having a heating blower system for evaporating ink carriers from the print medium after ink-jet printing. A print heater halogen quartz bulb heats the underside of the medium via radiant and convective heat transfer through an opening pattern formed in a print zone heater screen. 
     U.S. Pat. No. 4,982,207, to Tunmore et al. describes a heater construction for an ink jet printer having a rotary print platen for holding and transporting a print sheet through a print path. The platen heater includes a hollow shell having vacuum holes for sheet attachment. A heating foil is detachably mounted in a heat transfer relation with the interior periphery of the shell. 
     U.S. Pat. No. 5,005,025, to Miyakawa et al. describes an ink jet recording apparatus for recording, which fixes ink through evaporation of an ink solvent. The apparatus includes a heating member extending both upstream and downstream with respect to a recording area and a conveying direction of the recording sheet. The heating member contacts the recording sheet to assist in fixing the ink. 
     U.S. Pat. No. 5,406,321, to Schwiebert et al. describes an ink jet printer and a paper preconditioning preheater for the ink jet printer. The paper preconditioning preheater has a curved surface and a multi-purpose paper path component to accomplish direction reversal for the paper. The paper contacts the preheater, which dries and shrinks the paper to condition it for a printing operation. The preheater is a thin flexible film carrying heater elements that is suspended in air to provide extremely low thermal mass and eliminate the need for long warm up times. 
     SUMMARY OF THE INVENTION 
     Despite these various designs, a need exists for an ink jet printer dryer system that efficiently works to dry printed substrates. This need is particularly evident in the field of color ink jet printing, where one color printed area is preferably dried, either partially or completely, prior to printing with a second color. Such a system is required in order to minimize inter-color bleed, a problem that results when a subsequently printed color bleeds into a previously printed color, or vice-versa, causing a print image defect. That is, when two colors are printed in sequence, either on top of or adjacent to one another, the colors will unintentionally mix with each other if there is insufficient drying of the first printed color prior to printing the second color. This creates a gross deficiency in the print quality, affecting the sharpness of the edges and the overall resolution of the image. The problem of intercolor bleed can be dealt with by first printing one color and allowing time for this ink to penetrate the paper before printing the second color, but this limits the speed of the printing device. High speed ink jet printing devices utilize drying to control the intercolor bleed problem. 
     Current state-of-the-art printing systems typically utilize a multiple dryer mechanism, where, for example, one dryer is located in a preheat station and a second dryer is located in a primary drying station that is either within or subsequent to the printing zone. Such printing systems typically use a contact heater in the preheat station, to precondition (or predry) the paper prior to printing. As described above, such predrying is intended to improve latitude with respect to paper moisture and environment. 
     The predrying station removes excess moisture from the print substrate prior to the print substrate entering the print zone. Once in the print zone, the typical dryer configuration is the use of a radiant infra-red energy heat source that heats the paper through the back side of the paper while ink is being applied to the front side. Energy density in the order of 10 W/in 2  are supplied to the paper through the low color (infrared) temperature source, which typically is in the range of 1000-1300° K. Such a light source is used because water molecules have a strong affinity for infrared energy with a wavelength of 2.6 microns, which is abundant at this color temperature range. 
     However, a problem with such typical printer dryer designs is that the heat source causes the paper to dry very quickly. This in turn can cause instability and buckling in the print zone. This problem is further exacerbated in high moisture content papers. In such cases, the predrying station described above has been proposed as a solution. Such predrying stations generally have a much lower energy output, in the range of 2-3 W/in 2  with a dwell time of several seconds. 
     However, addition of such predrying stations to an ink jet printer pose economic and design problems. For example, the addition of the predrying station can be costly, requiring both separate heating elements and control components. This adds both to the manufacturing cost of the printer as well as to its operating and maintenance costs. 
     Furthermore, a need exists for printer dryer designs that will efficiently and quickly dry the printed image without causing image or paper defects. Such printer dryer designs will permit still higher increases in paper throughput speed, permitting higher speed printing. 
     This invention provides an active drying system that efficiently provides both primary and secondary (preheating) functions using a single component with a single control system, rather than by two different components having separate control systems. 
     The active drying system of this invention thus includes a dual function radiant dryer that can be incorporated into a printing apparatus, such as an ink jet printer, to provide higher printing efficiency, higher print quality, and lower cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of this invention will be apparent from the following, especially when considered with the accompanying drawings, in which: 
     FIG. 1 illustrates an exemplary printer drying design according to the present invention. 
     FIG. 2 is a schematic diagram of a dual function radiant heater according to an embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a dual function radiant heater according to another embodiment of the present invention. 
     FIG. 4 is a graph showing relative irradiance versus relative position of a dual function radiant heater according to an embodiment of the present invention. 
     FIG. 5 is a schematic diagram of a dual function radiant heater providing the irradiance graph of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Although the active dryer system discussed herein may be used for drying any image that is created by a liquid ink printer, the description of the active dryer system of this invention will be described in the environment of a liquid ink printer such as that shown in FIG.  1 . 
     FIG. 1 illustrates a schematic representation of a liquid ink printer  10  in a side elevation view. A translating ink jet printhead  12  printing black and/or colored inks is supported by a carriage  14 , which moves back and forth across a recording medium  16 , for instance, a sheet of paper or a transparency, on a guide rail  18 . Multiple printheads (not shown) printing different colors, or a full-width printbar (not shown) printing one or more colors, could also be used in the liquid ink printer  10 . The recording medium  16  moves along a recording medium path through the liquid ink printer  10  in the direction noted by the arrow  20 . Single sheets of the recording medium  16  are fed from a supply tray  22  by a document feed roll  24 . The document tray  22  is spring biased by a biasing mechanism  26 , which forces the top sheet of the stack of recording sheets held by the tray  22  into contact with the feed roll  24 . The topmost recording medium  16 , in contact with the drive roll  24 , is transported by the drive roll  24  into a chute  28 , which is defined by an outer guide member  30  spaced from an inner guide member  32 , each of which are curved to reverse the direction of the recording sheets  16  for printing by the printhead  12 . Once the recording medium exits the chute  28 , the recording medium  16  is driven by a drive roll  36  to advance the recording sheet  16  into a print zone  38 . 
     The print zone  38  is the area directly beneath the printhead  12  where droplets of ink  40  are deposited by an array of ink nozzles printing a swath of information and arranged on a front face of the printhead  12 . The front face of the printhead  12  is substantially parallel to the recording medium  16 . The carriage  14 , traveling orthogonally to the recording medium  16 , deposits the ink droplets  40  upon the recording medium  16  in an imagewise fashion. The printhead  12  receives ink from either an attached ink tank or from an ink supply tube (not shown). The image deposited upon the recording medium  16  can include text and/or graphic images, the creation of which is controlled by a controller, known to those skilled in the art, in response to electrical signals traveling through a ribbon cable  42  coupled to the printhead  12 . Before the recording medium  16  has completely left control of the drive roll  36 , an exit drive roll/pinch roll combination (not shown), or other known means, captures the leading edge of the recording medium  16  for transport to an output tray  44 , which holds printed recording medium. 
     To fix the liquid ink to the recording medium  16 , the moisture must be driven from the ink and the recording medium  16 . While it is possible to dry the ink by natural air drying, natural air drying can create certain problems such as cockle or curl and can also reduce the printing throughput of the liquid ink printer  10 . Consequently, active drying by applying heat energy to the printed recording medium  16  is preferred. To improve printing quality, the active drying system of this invention includes a dual function radiant dryer  50 , which is located along the inside of the inner guide member  32  from a position just after the supply tray  22  to a position just after the print zone  38 . The dual function radiant dryer  50  generates heat energy using a heat source  56  located within the dryer  50 . A portion of the heat generated by the heat source  56  is absorbed by the walls of the dryer  50 , and is transferred to and through the inner guide member  32  to heat the recording medium  16 . The dryer  50  is located within the liquid ink printer  10  such that the side of the recording medium  16  opposite the side to be printed receives the heat energy. Heat energy is delivered primarily through conduction. The inner guide section  32  can include apertures, such as round holes, diagonally placed slots, or raised areas to shorten warm-up times. Once the leading edge of the recording medium  16  has passed into the print zone  38 , ink is deposited on the recording medium  16  in the print zone  38 . During traversal through the print zone  38 , the recording medium  16  is supported by a support platen  52  that defines a substantially planar surface. The support platen  52  can be in any of various combinations, as will be known to one skilled in the art, and can be, among other designs, an extension of the piece forming the inner guide member  32  or can be a material having apertures in the support platen  52  to provide more direct access to the dryer  50  located beneath the support platen  52 . For example, a suitable structure of the support platen  52  having apertures is disclosed in U.S. Pat. No. 5,754,208, the entire disclosure of which is incorporated herein by reference. Alternatively, one or both of the inner guide member  32  and the support platen  52  can be omitted, and instead can be formed directly by the outer wall of the dryer  50 . 
     Furthermore, although the above description is with reference to heating the recording medium from the back side in relation to the printing side, the present invention is not limited to such embodiments. Rather, the dual function radiant dryer of the present invention can, in embodiments, be readily modified to heat the recording medium from the printing side thereof, or secondary heaters on the printing side can be suitably combined with the dual function radiant heater. 
     The operation of the dryer  50  will now be described in greater detail with reference to FIG.  2 . In FIG. 2, the dual function radiant dryer  50  generally comprises a reflector with a elliptically-shaped cross section  54  surrounding a radiant heat source  56 . The heat source  56  is preferably a low color temperature radiant energy source, such as an infra-red bulb or other known heat sources available in the art. Preferably, the heat source  56  is a low color temperature of from about 1000-1300° K. 
     In one exemplary embodiment of the active dryer system, the heat source  56  is preferably an elongated infrared bulb, which has a filament that extends along the central axis of the bulb. Typically, the bulb will have a length slightly wider than the width of the recording medium being used. Thus, for typical office and home applications where paper widths of up to 8.5 inches are used, the bulb will have a length of about 9 inches. The filament of the bulb should preferably be designed to emit a uniform heat flux from one end of the filament to the other. 
     An exemplary infra-red bulb suitable for use in the present invention would contain the filament suspended within a bulb housing. Preferably, suitable filaments include iron-aluminum chromium or nickel chromium alloy filaments in a quartz tube housing which forms the bulb. These are relatively simple wire wound filament construction and can be open to the atmosphere. Alternatively, the filament can be suspended in a gas-filled tube, such as in a quartz halogen tube, or in the case of tungsten filament application supported in an evacuated tube, which forms the bulb housing. These and other bulb designs are generally known in the art and can readily be incorporated into the radiant dryer of the present invention. 
     Furthermore, although it is preferred that the heat source emit a uniform heat flux from the entire source, i.e., in the case of a bulb from all angles of the bulb around its central axis, a reflective coating can be applied to a portion of the bulb. That is, for example, in the case the quartz tube bulb described above, a reflective coating can be applied to a portion of the bulb to provide a non-uniform heat flux from the bulb. In this case, the reflectiveness of the dryer reflector can be altered accordingly, so as to provide the desired preheat and primary heating functions. Thus, the use of a reflective coating on a portion of the bulb provides additional design latitude in designing specific shapes of the dryer reflector. 
     The dryer  50  generally can be described as having three different zones, labeled A, B and C in FIG.  2 . Zone A generally corresponds to the print zone  38  shown in FIG. 1, or to a portion of the print path within or subsequent to the print zone  38 , and can either be coextensive with the print zone  38 , or can extend on either side of the print zone to provide for pre- or post-printing drying, as desired. In this zone A, the reflector wall  54  of the dryer  50  receives and absorbs a majority of the energy generated by the heat source  56 . In this manner, the primary drying function of the dryer is focused into the zone A to provide primary active drying of the recording medium  16  located in or subsequent to the print zone  38 . In this zone A, the reflector  54  is subjected to an appropriate surface treatment to provide a low reflectivity (high absorptivity) of the reflector  54 . Preferably, in this zone A, the reflectivity of the reflector  54  is from about 0.0 to about 0.25, preferably from about 0.0 to about 0.1, and more preferably from about 0.05 to about 0.1. 
     Alternatively, in other exemplary embodiments of the active dryer system, the reflector  54  can be formed as shown in FIG. 3, wherein the zone A is an opening in the reflector  54 . In this exemplary embodiment, the opening forming zone A is preferably wider than the print zone  38 , which is shown in FIG. 3 as zone A′. In this embodiment, the reflector is shaped such that a higher energy output is realized in the portion of zone A corresponding to the print zone  38  or zone A′, with a lower energy output in the portions of zone A located adjacent to the print zone  38 . Alternatively, the higher energy output can be focused on an area overlapping with or subsequent to the print zone  38 , if active drying outside of the print zone  38  is desired. 
     Zone B of the dryer  50  generally corresponds to the chute area  28  shown in FIG. 1, and can be coextensive with the chute  28 , can be shorter than the chute  28  on either or both sides of the chute  28 , or can extend on either side of the chute  28 . In this zone B, the reflector  54  is subjected to an appropriate surface treatment to maintain a high reflectivity of the reflector  54 . Preferably, in this zone B, the reflectivity of the reflector  54  is from about 0.8 to about 0.97, preferably from about 0.85 to about 0.97, and more preferably from about 0.90 to about 0.95. 
     Accordingly, although most of the energy impacting on the inner wall of the reflector  54  in zone B is reflected toward the zone A, a portion of the impacting energy is absorbed by the inner wall of the reflector  54  in zone B. This absorbed energy is transferred through the inner guide member  32  to preheat the recording medium  16 . Alternatively, the inner guide member  32  can be formed by the outer wall of the reflector  54  of the dryer  50 , rather than as a separate component. In this embodiment of the active dryer system, the heat absorbed by the inner wall of the reflector  54  can be directly transferred to the recording medium  16  to preheat the recording medium  16 . 
     Generally, in the exemplary embodiments of the active dryer system, the temperature in the preheat zone B is maintained at a temperature lower than the temperature in the primary heating zone A. Thus, for example, the temperatures and energy densities are suitably maintained at levels so as to provide paper temperatures in the preheat region of generally from about 100 to about 150° F. and in the primary drying region of generally from about 175 to about 250° F. As such, a lower energy density is transferred to the recording medium in the preheat zone B than is transferred to the recording medium in the primary heat zone A. Generally, energy densities absorbed by the reflector, and transferred to the recording medium, in the preheat zone B are from about 1 to about 5 W/in 2 , preferably from about 2 to about 4 W/in 2 , and more preferably from about 2 to about 3 W/in 2 . Generally, energy densities transferred to the recording medium in the primary heat zone A (i.e., in or subsequent to the print zone) are from about 4 to about 10 W/in 2 , preferably from about 5 to about 8 W/in 2 , and more preferably from about 5 to about 6 W/in 2 . 
     Finally, the zone C of the reflector  54  is generally a flat reflector located at 45° to the major axis of the elliptical cylinder. The purpose of the zone C is to direct substantially all of the impacting energy to the zone A for heating the recording medium  16 . Thus, in this zone C, the reflector  54  is subjected to an appropriate surface treatment to maintain a high reflectivity of the reflector  54 . Preferably, in this zone C, the reflectivity of the reflector  54  is from about 0.8 to about 1.0, preferably from about 0.85 to about 1.0, and more preferably above about 0.90 
     Other areas of the reflector, that is, other than the particular areas discussed above, are also preferably subjected to an appropriate surface treatment to maintain a high reflectivity of the reflector  54 . Preferably, in these areas, the reflectivity of the reflector  54  is from about 0.8 to about 1.0, preferably from about 0.85 to about 1.0, and more preferably above about 0.90. 
     In the exemplary embodiments of the active dryer system described above, the reflector is preferably made of a heat conductive material, which can selectively be made reflective or conductive based on varying the surface treatment. In particular, the reflector is made of aluminum, but other materials such as stainless steel can also be used. Preferably, the reflector housing is made of a material that is thin enough to permit efficient conduction of absorbed heat, but thick enough to provide a rigid surface where rigidity is required. 
     The reflector surface, and particularly the inside surface, can be selectively surface treated to provide a desired range of reflectivity. For example, where an aluminum reflector is used, reflectivity of the surface can be increased by applying a high polish to the surface. Alternatively, secondary materials can be applied to the surface to alter its reflectivity and prevent environmental oxidation that can change refelctivity over time. For example, a thin layer of a highly reflective material, such as gold, silver, or other thin films, can be applied to the reflector surface. 
     In the exemplary embodiments of the active dryer system described above, the energy densities supplied by the heat source to the respective preheat and primary heating areas can be selectively changed and adjusted by altering the surface reflectivity of portions of the reflector surface facing the heat source. Thus, for example, a portion of the reflector surface can be painted or otherwise treated to decrease its reflectivity (i.e., increase its absorptivity). Similarly, regions of the reflector surface can also be painted or treated to alter the reflectivity, to alter the energy density profile in respective preheat and primary heating areas to provide non-uniform profiles. Such changes will be apparent to one skilled in the art based on the instant disclosure. 
     In the above discussion, the reflector design has been discussed with reference to a particular reflector design as demonstrated in the attached Figures. That is, the reflector has been described with respect to a particular embodiment where the cross section is a partial ellipse closed by a line intersecting the ellipse at an angle of from about 30 to about 60° to the major axis of the ellipse. Preferably the partial ellipse is closed by a line intersecting the ellipse at an angle of from about 40 to about 50° to the major axis of the ellipse, and more preferably about 45° to the major axis of the ellipse. However, it will be apparent to those skilled in the art based on the present disclosure that other reflector shapes can be used in the present invention. For example, with minimal design changes to the printing machine, a dual function radiant dryer could be provided where the reflector takes the shape of a parabola, where the heat source is located at the focus of the parabola, the primary heat zone is located at the “mouth” or open end of the parabola, and the preheat zone is located along a selected side of the parabola. Other shapes could also be selected based on appropriate design considerations. 
     Furthermore, in such standard geometric shapes, it is preferred that the dryer reflector design be such that there is at least one focal point within the reflector. In this case, the heat source can be located at the focal point, so that the majority of the energy given off by the heat source can be focused at a single area, such as the print zone. However, other reflector designs, not having a particular focal point, can also be used so long as a sufficient amount of energy can be concentrated at the print zone, and a lesser amount of energy can be provided to a secondary preheat area. 
     Furthermore, a particular advantage of the active dryer system, and especially in the exemplary embodiments having a geometric focal point within the reflector, is that both heating functions can be accomplished with a single heat source. However, as will be readily apparent, multiple heat sources can be incorporated into the reflector if desired. 
     The irradiance profile of one exemplary radiant heater, as shown in FIG. 5, is analyzed using the analytical reflector design tool. The radiant dryer  90  forms an ellipse, having a major axis of 43.2 mm and a minor axis of 21.5 mm, giving an eccentricity of 0.867. The infra-red source is located at the right focus of the ellipse, and a flat reflector is tilted to 45° to the major axis and intersects the major axis at 25.5 mm in front of the left focus of the ellipse. The energy distribution is adjusted to give a uniform distribution over a width of 12 mm, which is slightly wider than an exemplary print swath of a printhead. 
     FIG. 4 shows the distribution of the irradiance profile versus the relative position in the opening of the radiant dryer  90  shown in FIG.  5 . FIG. 4 shows that for the radiant dryer  90 , an almost uniform distribution of irradiance is achieved over about the 12 mm width, with substantially no irradiance outside of the 12 mm width. 
     One skilled in the art will recognize that the various aspects of the dual function radiant dryer discussed above may be selected and adjusted as necessary to achieve specific results for a particular printer application. 
     The foregoing exemplary embodiments are intended to illustrate and not limit this invention. It will be apparent that various modifications can be made without departing from the spirit and scope of the invention.