Abstract:
An EMI shield that utilizes perforated dimples or other features configured to reflect eletromagnetic radiation while simultaneously permitting air flow through the shielding material.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates generally to electromagnetic interference (EMI) shielding in electronic systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    The operation of electronic circuitry used in many electronic devices is often accompanied by unwanted stray electromagnetic radiation. Stray electromagnetic radiation or “noise” can interfere with the performance of surrounding devices. Consequently, it is important to shield electronic devices to reduce electronic noise emanating from those devices.  
           [0003]    Redundant arrays of inexpensive or independent storage devices (RAID) are being employed by the mass storage industry to provide variable capacity storage. RAID systems use interconnected disk drives to achieve the desired capacity of mass storage. With this approach, a disk drive of one capacity may be manufactured and packaged with the same or different capacity drives to provide the required storage capacity. RAID systems eliminate the need to manufacture disk drives individually designed to meet specific storage requirements. Each disk drive in a RAID system is usually housed in an individual module for handling and installation. The modules slide into and out of a larger enclosure that houses the array of disk drives and provides the sockets, plug-ins and other connections for the electrical interconnection of the drives. Controllers orchestrate the interconnection and control access to selected disk drives for data reading and writing operations.  
           [0004]    Each module includes a plastic housing and, in most cases, some type of metal EMI shielding. Metal shielding is often constructed as metal plates, panels, partial enclosures and the like positioned within or about the housing. The metal attenuates stray electronic signals emanating from the module as well as stray signals coming from surrounding modules. The degree of attenuation increases with the amount, placement and composition of metal shielding. A closed metal box, for example, would provide excellent shielding. The housing, however, must also permit sufficient air flow to cool the device during operation. Hence, there must be adequate openings in the housing and the shielding to provide the necessary degree of cooling air flow.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is directed to an EMI shield that utilizes perforated dimples or other features configured to reflect electromagnetic radiation while simultaneously permitting air flow through the shielding material. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a perspective view of an enclosure for a disk drive or other electronic circuitry in which the front and rear portions of the enclosure are constructed as an EMI shield.  
         [0007]    [0007]FIG. 2 is a perspective view of the front of a housing for a group of individual device modules, such as the disk drive modules used in RAID data storage systems. The electronic device in one or more of the modules may be housed in an enclosure like the one illustrated in FIG. 1.  
         [0008]    [0008]FIG. 3 is a perspective view of the rear of the housing of FIG. 2.  
         [0009]    [0009]FIG. 4 is a close-up perspective view of the rear corner of the device module of FIG. 1 showing one embodiment of an EMI shield in which perforated parabolic dimples are used to attenuate electromagnetic radiation while allowing air flow through the shield.  
         [0010]    [0010]FIG. 5 is a detailed perspective view of one of the dimples of FIG. 4.  
         [0011]    [0011]FIG. 6 is a side view of the dimple of FIG. 5 looking directly into the inner dimple between bands of the outer dimple.  
         [0012]    [0012]FIG. 7 is a plan view of the orientation of the dimple in FIG. 6 looking down on the dimple as indicated by the line  7 - 7  in FIG. 6.  
         [0013]    [0013]FIG. 8 is a side view of the dimple of FIG. 5 looking along one band of the outer dimple.  
         [0014]    [0014]FIG. 9 is a plan view of the orientation of the dimple in FIG. 8 looking into the dimple as indicated by line  9 - 9  in FIG. 8 showing an alternative embodiment in which the bands overlap the underlying segments.  
         [0015]    [0015]FIG. 10 is a section view taken along the line  10 - 10  in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 illustrates an enclosure  10  for a disk drive, controller or other electronic circuitry in which the front panel  12  and rear portions  13  are constructed as an EMI shield through which cooling air can flow. A multiplicity of perforated dimples  14  are formed along front panel  12  and rear portions  13  of enclosure  10 . Dimples  14  project out from enclosure  10 . As described in more detail below, perforated dimples  14  permit cooling air flow while attenuating electromagnetic radiation or “noise” generated by the electronic circuitry inside enclosure  10 . Some type of ejector latch  15  is typically used on front panel  12  to facilitate the installation and removal of enclosure  10  in a group housing unit such as the one described below for FIGS. 2 and 3.  
         [0017]    [0017]FIGS. 2 and 3 illustrate one example of a data storage system  18  with which the invention can be used. Referring to FIGS. 2 and 3, data storage system  18  includes a group of individual device modules  20  and  22 , such as the disk drive modules used in a RAID data storage systems, housed in a housing  24 . FIG. 2 shows the front of housing  24 . FIG. 3 shows the rear of housing  24 . The electronic device in each module  20  and  22  is housed in an enclosure like the one illustrated in FIG. 1. System  18  may also include, for example, power supplies  26  and  28 , battery back-up units  30  and  32 , cooling fan modules  34  and  36  and input/output modules  38  and  40 . Power supplies  26  and  28  provide the necessary electrical power for system  18 . Battery back-up units  30  and  32  provide an alternative power source in the event of a failure of one or more of the power supplies  26  and  28 . Fan modules  34  and  36  circulate air through housing  24  to cool the components. The input/output modules  38  and  40  enable the system components to communicate with external devices. The front panels and other parts of the enclosures for power supplies  26  and  28  and battery back-ups  30  and  32  might also be constructed as EMI shields using the perforated dimples of the present invention.  
         [0018]    The details of one embodiment of dimples  14  will now be described with reference to FIGS.  4 - 10 . FIG. 4 is a close-up perspective view of the rear corner of the device module enclosure  10  of FIG. 1. FIGS.  4 - 10  show one embodiment of an EMI shield in which perforated parabolic dimples  14  are used to attenuate electromagnetic radiation while allowing air flow through the shield. FIG. 5 is a detailed perspective view of one dimple  14 . FIG. 6 is a side view of the dimple of FIG. 5 looking directly into the inner dimple between bands of the outer dimple. FIG. 7 is a plan view of the orientation of the dimple in FIG. 6 looking down on the dimple as indicated by the line  7 - 7  in FIG. 6. FIG. 8 is a side view of the dimple of FIG. 5 looking along one band of the outer dimple. FIG. 9 is a plan view of the orientation of the dimple in FIG. 8 looking into the dimple as indicated by line  9 - 9  in FIG. 8. FIG. 10 is a section view taken along the line  10 - 10  in FIG. 7.  
         [0019]    Referring to FIGS.  4 - 10 , each dimple  14  includes a segmented inner dimple  42  and a segmented outer dimple  44 . In the embodiment shown in the figures, each segment or band  46 ,  48 ,  50  and  52  of outer dimple  44  is offset from segments  54 ,  56 ,  58  and  60  of inner dimple  42  such that the bands of outer dimple  44  cover the gaps between the segments of inner dimple  42 . In the embodiment shown in FIG. 7, bands  46 ,  48 ,  50  and  52  are co-extensive with the gaps between segments  54 ,  56 ,  58  and  60 . In the embodiment shown in FIG. 9, bands  46 ,  48 ,  50  and  52  overlap the underlying segments  54 ,  56 ,  58  and  60 . Preferably, both inner dimple  42  and outer dimple  44  are parabolic. For parabolic dimples, the focal point of each dimple  42 ,  44  lies within enclosure  10  where the unwanted noise originates. In this way, noise is reflected off the parabolic dimples  42  and  44  back into enclosure  10 , as indicated by arrows  62  in FIG. 10, while allowing air to pass through the dimples, as indicated by arrow  64  in FIG. 10.  
         [0020]    “Dimple” characterizes the reflective feature as viewed from inside enclosure  10 . These same features might also be characterized as “bumps” when viewed from the outside of enclosure  10 . Hence, “dimple” is a relative term that refers generally to the desired reflective feature on enclosure  10  or, more generally, on any shielding material of interest.  
         [0021]    Conventional EMI shielding relies on holes and/or waveguides in metal sheeting to permit air flow. The extent to which conventional EMI shielding attenuates stray electromagnetic radiation (i.e., “noise”) depends on the cut-off frequency of the holes or waveguides and the number of holes or waveguides. The degree of electromagnetic attenuation of the dimpled structure of the present invention, by contrast, depends on the reflective properties of the electromagnetic radiation. If the electromagnetic radiation is below the cut-off frequency of a conventional waveguide, then the waveguide rapidly attenuates the signals. If, however, the electromagnetic radiation is above the cut-off frequency, then the signals pass readily through the waveguide. The dimpled structure of the present invention can be made to reflect part or all frequencies of electromagnetic radiation back into the enclosure, with the greatest reflective effect when the dimples are in the far-field for the frequency of interest. A dimple is in the far-field if the source of the electromagnetic radiation is at least λ/2 from the dimple where λ is the wavelength. The wavelength of the radiation decreases as the frequency increases according to the relationship λ=c/f, where λ is the wavelength, c is the speed of light and f is the frequency. For example, the far-field for electromagnetic radiation at a frequency of 5 GHz is about 9.5 mm and the far-field is closer than 9.5 mm for frequencies higher than 5 GHz. Hence, the dimpled construction of the present invention offers an effective alternative to conventional waveguides particularly for higher frequency radiation.  
         [0022]    While it is expected that the parabolic dimples illustrated in FIGS.  4 - 10  will usually be formed in sheet metal using conventional stamping processes, they may be molded in conductive plastic sheets or plates or other fabrication materials and processes may be used. For sheet metal, a solid inner dimple is first pressed into the sheet and then the inner dimple and a bit of the surrounding sheet is lanced through and pressed out to form the cross shaped outer dimple  44  while leaving a segmented inner dimple  42 . The size and shape of dimples  14  are dependent on the EMI shielding and air flow requirements for the particular device, module and system in which the dimples are used. For a typical disk drive or controller module in a RAID system in which higher frequency noise targeted for this new dimpled shielding, generally 1 Ghz and above, it is expected that inner dimple  42  and outer dimple  44  will be from a few millimeters deep to a few tens of millimeters deep.  
         [0023]    The present invention has been shown and described with reference to the foregoing exemplary embodiment in which parabolic inner and outer dimples are used. Other embodiments are possible. For example, the EMI shielding may use only an outer dimple or only an inner dimple in cases where more air flow is necessary or desired and less noise attenuation may be tolerated. Although parabolic dimples are preferred for the reasons noted above, other shapes may be adequate in some applications. It should be understood, therefore, that the invention is to be construed broadly within the scope of the following claims.