Abstract:
An isolation system comprising: a substantially rectangular stationary frame; an imaging assembly including a curved platen for supporting image media and an exposure assembly for imagewise exposing a supported image media to produce exposed image media, wherein the imaging assembly has a substantial rectangular footprint and is sized to fit within the stationary frame; and a cable and spring assembly for suspending the imaging assembly from the frame to substantially isolate the imaging assembly from sources of external vibration.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to laser imaging systems incorporating an internal drum scanner assembly and more particularly to an isolation system for isolating said internal drum scanner assembly from external vibration sources during the imaging process. 
     BACKGROUND OF THE INVENTION 
     Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of medical image scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and control subsystems for coordinating the operation of the laser imager and the film processor. 
     The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the control subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film. 
     The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists). 
     Film exposure systems are used to provide exposure of the image on photosensitive film. Known film exposure systems include a linear translation system and a laser or optical scanning system. The laser scanning system includes a laser scanner with unique optical configurations (i.e., lenses and mirrors) for exposure of the image onto the film. The linear translation system provides for movement of the laser scanning system in a direction perpendicular to the scanning direction, such that a full image may be scanned on a piece of photosensitive film. 
     In an internal drum type laser scanner assembly, a piece of film is positioned onto a film platen, wherein the film platen has a partial cylindrical or partial drum shape. The photosensitive film is positioned against the film platen. The laser or optical scanning system is positioned at the center of curvature of the photosensitive film for scanning a scan line across the photosensitive film surface. A linear translation system moves the laser or optical scanning system lengthwise along a longitudinal axis as defined by the center of curvature of the film to expose an entire image onto the film. 
     The film may be fed onto the film platen utilizing a film transport system which often incorporates a plurality of feed rollers. Once the piece of photosensitive film is fed onto the film platen, the film must be held tight against the curved surface of the film platen, and centered and aligned into a scanning position in order for an image to be correctly exposed onto the photosensitive film. Any skew of the film must also be removed. Often such methods and mechanisms for aligning and centering a piece of film on the internal surface of the film platen require multiple complex mechanical and electrical components and control systems. 
     U.S. Pat. No. 5,956,071, issued Sep. 21, 1999, inventors Mattila et al., discloses an assembly for positioning a film into a scanning position on a curved film platen in an internal drum scanner assembly. The film platen is defined by a first curved edge, a second curved edge, a film feed edge, and a film stop edge. The assembly comprises a first slider block assembly and a second slider block assembly which is spaced from the first slider block assembly a distance less than the width of the leading edge of the photosensitive film. A feed mechanism is positioned proximate the film feed edge, for feeding a piece of photosensitive film having a leading edge along the curved film platen. The leading edge of the film is fed from a location proximate the film feed edge towards the film stop edge. 
     When the photosensitive film is in the scanning position, the leading edge of the photosensitive film contacts the first slider assembly and the second slider assembly. The photosensitive film is tensioned against the curved film platen in alignment between the first slider assembly and the second slider assembly and the feed mechanism, thus removing any skew. 
     The laser scanning system and linear translation system must be protected from external vibration sources during the imaging process in order to minimize image degradation in the scanned film. Variation of placement of scan lines must be controlled very tightly to avoid banding artifacts. Vibration sources can effectively produce these same artifacts by exciting natural frequencies of the systems within the imaging assembly. More abrupt or short term sources, such as shock, can cause more visible artifacts at a given location on the scanned film. Therefore, to effectively manage the performance of the imaging assembly, vibration sources must be controlled. Prior designs to achieve isolation for internal drum scanning equipment typically uses commercially available vibration and shock mount made from various rubber-like materials. These components do not have low enough natural frequencies and also show significantly different values of natural frequency for the in-plane motion versus the normal direction. Air systems for achieving lower natural frequencies are typically cost prohibitive and sometimes undesirable in certain environments as leakage will disable the system. 
     There is thus a need for a vibration isolation system for a laser imaging system which is cost effective, which can control natural frequencies of the system in multiple directions, which has high performance, and which is reliable and minimally complex in design. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a solution to the problems discussed above. 
     According to a feature of the present invention, there is provided an isolation system comprising: a substantially rectangular stationary frame; an imaging assembly including a curved platen for supporting image media and an exposure assembly for imagewise exposing a supported image media to produce exposed image media, wherein said imaging assembly has a substantially rectangular footprint and is sized to fit within said stationary frame; and a cable and spring assembly for suspending said imaging assembly from said frame to substantially isolate said imaging assembly from sources of external vibration. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention has the following advantages. 
     1. Can control the natural frequencies in multiple directions—cable lengths and spring compression values can be chosen to carefully control the natural frequency in one plane of motion with completely independent values in the normal direction. 
     2. Low cost/High performance—can obtain natural frequencies typical of complex air isolation systems with relatively inexpensive equipment like cables and springs. 
     3. Can tune damping parameters of systems—geometry and material of damping foam can be used to control the damping characteristics in the three axes of translation. 
     4. Better control of system position—typical commercial mounts require large loads for low natural frequencies which makes them more unstable. As the system must line up with other subsystems for the reliable transfer of film, controlling the position of the platen becomes important as opposed to something that is positioned on a soft, low durometer material. Rigid cables better define the system position allowing for more accurate alignment with other subsystems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are isometric views of a laser imaging system incorporating the present invention. 
         FIG. 3  is an isometric view showing certain of the components of the system of  FIGS. 1 and 2 . 
         FIG. 4  is an isometric view of the cable suspension system and platen of the system of  FIGS. 1 and 2 . 
         FIGS. 5 and 6  are exploded views of the cable connections to the platen and frame respectively. 
         FIG. 7  is a graphical view of transmissability curves versus normalize forcing frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2  there is shown a laser imaging system incorporating the present invention. As shown, laser imaging system  10  is a subsystem of a laser imager such as a laser imager for producing medical images on photothermographic film. In such a laser imager, a digital medical image is reproduced on heat developable photothermographic film fed onto a curved platen. 
     After exposure, the exposed film is brought into contact with a rotating heated drum which thermally develops the exposed film. The film is then cooled and output to a user for diagnostic applications. 
     Laser imaging system  10  includes a rectangular frame  12 , an internal drum laser scanner assembly including concave, curved platen  14 , translation assembly  16  and optics assembly  18 . Optics assembly  18  is mounted by translation assembly  16  which is mounted on platen  14 . Platen  14  and assemblies  16  and  18  are supported by cables  22  from frame  12 . In operation, unexposed film is fed onto platen  14 , and once properly positioned on platen  14 , the film is exposed in a raster pattern by a rotating laser beam produced by optics assembly  18  which scans the film in consecutive lines as the optics assembly  18  is translated along the length of the film by translation assembly  16 . Translation assembly  16  is moved in the direction of arrow  20 . 
       FIG. 2  shows the cable  22  connections to frame  12  as being presented in greater detail in  FIGS. 5 and 6 . 
       FIG. 3  is an isometric view of the frame  12  and platen  14  without assemblies  16  and  18  to more clearly show the isolation system of the present invention. As shown, frame  12  is in the form of an open rectangular box-like configuration including upper members  24 ,  26 ,  28 ,  30 , side members  32 ,  34 ,  36 ,  38  and bottom members  40 ,  42 , (and others not shown). Platen  14  is suspended from frame  12  by cables  22 . (See FIG.  4 ). 
     As shown more clearly in  FIG. 5 , each cable  22  is connected to platen  14  by a compression spring  50 , coiled about support  52  on the bottom end  54  of cable  22 . Platen tab  56 , grommet  58  and cable button  60  complete the connection system. When platen  14  is supported, spring  50  is compressed between tab  56  and button  60  as a function of the compressibility of the spring and the weight of the platen and other assemblies supported by the individual cable  22 . 
     As shown in  FIG. 6 , the upper end  62  of cable  22  is supported from a frame member, such as, member  28 , by button  64  secured to the end of cable  22 . Button  64  is threaded through keyhole  66  in member  28 . 
     The purpose of the isolation system according to the present invention is to protect the imaging assembly from external vibration sources during the imaging process. Variation in the placement of the scan lines must be controlled very tightly to avoid banding artifacts. Vibration sources can effectively produce these same artifacts by exciting natural frequencies of the systems within the imaging assembly. More abrupt or short term sources, such as shock, can cause more visible artifacts at a given location on the film. Therefore to effectively manage the performance of the imaging assembly, vibration sources must be controlled. There are three options to controlling shock and vibration which include reducing the magnitude of the source, isolating either the source or the equipment where the response is measured, or by reducing the magnitude of the response. Numerous methods can be used to achieve these goals according to the invention there is used a combination of isolation along with alteration of the response frequencies. Isolating involves building a system between the source and the sensitive equipment to protect the system while the magnitude of the response is altered by adjusting the natural frequency of the system or various components. In particular this last methodology was used in the design of the platen  14  to stiffen the platen structure and drive its natural frequency as high as possible given the material and geometry constraints. 
     As described, the isolation system design according to the invention is composed of suspension cables  22  and springs  50  to isolate the assembly in the X, Y, and Z axes. The goal is to lower the natural frequency of the isolation system as much as possible so that the system will effectively be protected from frequencies above that level. Since frequencies up to the natural frequency of the system effectively transmit directly into it, the lower the system natural frequency the better because there are less low frequency sources available. For example driving the natural frequency down to a level of 1 Hz (typical of air suspension type systems) means that the system is isolated from frequencies above approximately 3 Hz. Since there are very few sources from which signals of 3 Hz and below are generated, the probability for success is high. On the other hand if the natural frequency of the system is designed to be 10 Hz, any sources from approximately 30 Hz and below can cause problems, therefore the system is susceptible to a much broader range of sources. 
     From the absolute transmissibility curves  FIG. 7 , it is evident that significant attenuation of the input does not occur until approximately a factor of 3ω n , where ω n  is the natural frequency of the isolation system. At that point the response magnitude is about one-tenth the level of the input. Of course, depending on the sensitivity of the systems involved, further attenuation levels may be required. Depending on the magnitude of the input, perhaps an attenuation level of one one-hundredth may be required based on the sensitivity of the system. In that case the natural frequency and damping characteristics would have to be adjusted to account for these magnitudes. In the case of the imaging assembly where even low magnitudes of vibration can present an artifact, image testing must be performed to ensure that the system is sufficiently protected. An alternate method would be to fully characterize the threshold magnitude of the imaging assembly at each frequency and evaluate that against the known input frequency and magnitudes. Given this information, the level of isolation could be calculated. This is a bit idealistic given the numerous input levels that would have to be quantified, therefore the target is to design an isolation system with as low a natural frequency as possible and verify the performance through testing. Potential external sources of shock or vibration during the imaging process can include the following: structure borne signals (either in buildings or mobile vans), inputs from air conditioners, fans, elevators, patient access doors opening and closing, hydraulic lifts, general workflow traffic levels, or from operators impacting the imager. Internal vibration sources include fans, electronic noise, and any mechanical sources from equipment operating during imaging like the pick-up assembly, the processor, and the sorter. Shipping is also a significant source of shock and vibration but it is handled separately (such as through lock-downs) as this is not a source while the machine is imaging. 
     For the in plane isolation, the cable suspension system essentially acts as a pendulum for which the natural frequency is defined by 
         ω   n     =       g   l           
 
where g is the acceleration of gravity and l is the length of the pendulum or in this case, the cable length. The effective pivot length for this design is approximately 225 mm, therefore 
         ω   n     =           9.806   ⁢           ⁢     (     m     s   2       )         0.255   ⁢           ⁢     (   m   )           =     6.60   ⁢     rad   s             
     or     
         6.60   ⁢     rad   s     *     cycle     2   ⁢           ⁢   π   ⁢           ⁢     (   rad   )           =       1.0   ⁢     cycles   s       =     1.0   ⁢           ⁢   Hz           
 
     As there are different cable lengths used to balance the center of gravity of the imaging assembly, the shortest effective pivot length was used for this calculation to determine a conservative value for ω n . The balance of the imaging assembly is critical as the system must be positioned such that the feed rollers from the platen are square to the rollers leading to the vertical transport system. Therefore different cable lengths were selected such that this was achieved with the carriage at the front of the translation system as this is when the film exits the platen. 
     For the vertical direction, springs  50  are used to isolate the system. For a mass-spring system, the governing equation is 
         ω   n     =       k   m           
 
where k is the stiffness of the spring, and m is the mass it supports. Coupling the spring force equation, 
       k   =     F     Δ   ⁢           ⁢   x           
 
with the common mass-acceleration-force equation, F=ma, a relationship for the natural frequency of the system as a function of the spring displacement can be derived as follows: 
         ω   n     =         k   m       =             F   /   Δ     ⁢           ⁢   x     m       =             ma   /   Δ     ⁢           ⁢   x     m       =         α     Δ   ⁢           ⁢   x         =       g     Δ   ⁢           ⁢   x                     
 
     Thus the natural frequency of the system is primarily dependent on the amount of spring deflection. The deflection for the springs  50  used in the present invention is slightly different due to an uneven weight distribution within the imaging assembly. Since the left side is much heavier due to the location of stepper motors and all the transport equipment, the springs on the left-hand side are compressed more than those on the right-hand side. In order for the platen to sit level, the cables on the left-hand side are actually shorter than those on the right-hand side. 
     As an example, assume the weight of the imaging assembly is 82 lbs. with approximately a 5.75 lbs. difference between the left-hand side and the right hand side. The commercial springs used for this application have a free length of 2.25 inches, and a spring rate of 24.24 lbs./in. for the stainless steel material (P/N 7e8491). The nominal deflection of all four springs is: 
         Δ   spring     =         (       W   imaging     4     )       k   spring       =         (       82   ⁢           ⁢   lbs     4     )       24.24   ⁢           ⁢     lbs   in         =       0.846   ⁢           ⁢   in     =     21.5   ⁢           ⁢   mm               
 
     where 4 is the total number of springs supporting the weight of the imaging assembly. To balance off the 5.75 lbs. side-to-side difference, the following difference in cable lengths is used: 
         Δ   cable     =       offsetweight     k   spring       =         5.75   ⁢           ⁢   lbs       24.24   ⁢           ⁢     lbs   in         =       0.237   ⁢           ⁢   in     =     6   ⁢           ⁢   mm               
 
     Therefore the cables on the left-hand side are shortened from nominal by 3 mm while the cables on the right-hand side are lengthened by 3 mm. The actual spring deflections are then calculated as:
 
Δ springleft =Δ springnominal +Δ springoffset =21.5 mm+3 mm=24.5 mm
 
Δ springright =Δ springnominal −Δ springoffset =21.5 mm−3 mm=18.5 mm
 
     Note that implicit to these calculations of spring deflection is the fact that the imaging assembly will remain level when placed into the machine. The assembly must be robust enough to operate within ±1° however as this is the levelness specification called out in the PRS. In terms of limiting the travel, the over-travel grommets will also account for this variability. 
     Knowing the spring deflection is the smallest on the right-hand side one can calculate the natural frequency in the vertical direction to give us the most conservative estimate. The deflection at that location is approximately 18.5 mm so the natural frequency is: 
         ω   n     =         g     Δ   ⁢           ⁢   x         =           9.806   ⁢           ⁢     (     m     s   2       )         0.0185   ⁢           ⁢   m         =       23.02   ⁢     rad   s       =     3.7   ⁢           ⁢   Hz               
 
     Again this is the natural frequency of the isolation system in the vertical direction. If it should be proven that this level is not sufficient under simulated vibration conditions, the natural frequency must be driven lower by choosing springs that deflect further than 18.5 mm. Note that to drive the natural frequency down to a level of either 2 Hz or even 1 Hz, a spring deflection of approximately 2.5 inches and nearly 10 inches would be required, respectively. 
     As shown from the transmissibility curves in  FIG. 7 , the roll-off or decay of the curve after the natural frequency greatly depends on the amount of damping ξ in the system. With zero damping the absolute transmissibility reaches extreme magnitudes at the natural frequency, but the roll-off after that point is very steep. In contrast a system that is critically damped can be held under more control at resonance but has a very flat roll-off curve after that point. Therefore there is a trade off between how much motion can be withstood at resonance and what level of isolation is required beyond that point. 
     As shown in  FIG. 3 , a foam material  80  with certain damping properties is used to impart a low level of damping to the system. The goal is to minimize the impact of damping on the system (in order to maintain a steep roll-off curve) but still damp out oscillations of the imaging assembly within one or two cycles. Dampers  80  are placed in the front and back of the isolation system between frame  12  and platen  14  to achieve the desired effect. The axial direction of the assembly is the most important as that is the same direction as the translation system and can cause the carriage and optics module to rock. The same thing can be done in the vertical direction if required, but again this direction is not as critical. To vary the level of damping ξ, the geometry of the foam dampers  80  are varied and the system can in effect be tuned to a specific level. Optimal levels of damping must be determined under various levels of external vibration while making images to quantify the results. Preliminary testing was done to determine that four dampers (two in the front, two in the back) of approx. 1 in. 2  under 25% compression are required to effectively damp out external vibration sources. 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. 
     PARTS LIST 
     
         
           10  laser imaging system 
           12  rectangular frame 
           14  curved platen 
           16  optics assembly 
           18  translation assembly 
           20  direction arrow 
           22  cables 
           23 , 26 , 28 , 30  upper members 
           32 , 34 , 36 , 38  side members 
           40 , 42  bottom members 
           50  compression spring 
           52  support 
           54  bottom end 
           56  platen tab 
           58  grommet 
           60  button 
           62  upper end 
           64  button 
           66  keyhole