Abstract:
A method and apparatus is provided for reducing or eliminating the amount of electromagnetic radiation being emitted from a system assembly comprising one or more subassemblies which comprise electrical circuits which generate electromagnetic radiation. The method and apparatus of the present invention also reduce or prevent electromagnetic radiation generated by sources outside of the system assembly from interfering with and adversely affecting the electrical circuits comprised in the subassemblies. To achieve these goals, the present invention provides an electromagnetic compatibility (EMC) solution which comprises a plurality of lossy elements, and/or elements comprised of a combination of lossy and conductive material, to reduce electromagnetic radiation. Thus, the conventional EMC solution, which utilizes conductive gaskets on system assemblies and subassemblies, and which requires adequate contact between parts in order to be effective, is replaced by the EMC solution of the present invention, which is more effective at higher frequencies and does not have stringent contact requirements.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to a method and apparatus for controlling electromagnetic radiation being emitted from electrical components. More particularly, the present invention relates to a method and apparatus for using lossy materials, or a combination of lossy and conductive materials, on the exterior surfaces of electrical modules, or subassemblies, comprised within a larger electrical system assembly, such as, for example, a computer system assembly or a data storage system assembly, to prevent or reduce electromagnetic radiation emissions. The use of lossy material or a combination of lossy and conductive materials in this manner also provides immunity to electromagnetic radiation interference by preventing electromagnetic radiation generated by sources outside of the electrical system assembly from adversely affecting the electrical circuitry of the system assembly and subassemblies. 
     BACKGROUND OF THE INVENTION 
     Electrical components emit electromagnetic radiation. The Federal Communications Commission (FCC) regulates the amount of electromagnetic radiation that can be emitted form various classes of electrical and electronic devices by promulgating regulations that dictate the maximum amount of electromagnetic radiation that may be emitted form different types of electrical and electronic devices. With respect to computer systems and data storage systems, manufacturers typically control the amount of electromagnetic radiation emissions by suing conductive enclosures and gaskets to prevent electromagnetic radiation from being emitted from the assemblies. Conductive gaskets and conductive enclosures reflect radiation impinging on them and thereby prevent the radiation from escaping from the assemblies into the outside environment. 
     Conductive gaskets and enclosures generally are superior at controlling emissions as long as contact between conductive parts of the enclosure is continuous and the interfaces between parts of the enclosure are of low impedance. If contact between parts is not continuous, or if the interfaces between conductive parts are of high impedance, the effectiveness of the conductive enclosure or gasket as a shield will be greatly diminished. Therefore, the efficiency of the shield may be adversely affected by surface conditions and/or by inadequate contact pressure between parts is not maintained at all times. 
     One problem associated with using these types of conductive shields is that it is difficult to ensure that adequate contact pressure is maintained between parts due to mechanical tolerance variations. Also, aging of materials used to maintain contact between parts, such as, for example, polymers which function as springs to force conductive parts into contact, often results in adequate contact being lost over time. Therefore, it is difficult, in many cases, to ensure that adequate contact will be maintained over time, and therefore difficult to ensure the effectiveness of the conductive shielding over time. 
     Also, ensuring adequate contact between conductive parts of enclosures of electrical components often significantly increases the overall size of the assembly comprising the subassemblies since additional structure or treatment processes (e.g., flanges, collars, surface treatment, etc.) is required in order to ensure contact between the enclosures. Furthermore, additional structure and/or surface treatment generally translates into additional costs. The requirement that surface conditions not prevent adequate contact between parts also affects the appearance of the system assembly and/or subassemblies. For example, in many cases, the surfaces must be left exposed and cannot be painted. Furthermore, the implementation of subassemblies provided by different vendors (e.g., power supplies provided by different vendors) is limited by the requirement that the surface conditions of the various subassemblies provide adequate contact. Slight variations in surface characteristics make it difficult to achieve effective shielding. 
     Another problem associated with conductive shielding is that, with high frequencies, it is extremely difficult to ensure that contact between parts is adequate to control emissions. Therefore, as frequencies increase, conductive shields become less effective and other methods must be implemented to control electromagnetic radiation emissions. 
     It is generally known that lossy materials can be used to control electromagnetic radiation emissions. Lossy materials are materials which absorb and attenuate, and only partially reflect, electromagnetic radiation energy. The ratio of absorption-to-reflecting is determined by the electromagnetic radiation properties of the material and by the electromagnetic properties of the radiation impinging on the lossy material. However, it is not known to use lossy materials on the outside of enclosures, or housings, and between subassembly housings and a chassis on which the subassembly housings are mounted, to provide an EMC solution. With respect to these types of system assemblies, conductive gaskets and materials have been used as an EMC solution for a variety of reasons. 
     Low RF frequencies can be effectively dealt with using conductive gaskets and materials because conductive contact between parts does not have to be as continuous in order to ensure effective shielding, i.e., larger gaps can exist between parts while still maintaining effective shielding. Also, with these types of system assemblies, conductive shielding has been used not only for electromagnetic compatibility, commonly referred to as EMC, but also to ground various subassembly housings in order to eliminate residual voltages which may exist between parts of the chassis of the system assembly. Thus, implementing conductive shielding has been viewed as being advantageous since it performs both of these functions. 
     Currently computer system assemblies and data storage system assemblies implement EMC solutions which utilize conductive enclosures and conductive gaskets. As computer systems and data storage systems are developed which operate at exceedingly higher frequencies, with significant emissions at frequencies larger than approximately 1 GHz, a need will exist for an EMC solution which is effective in controlling emissions at higher frequencies and which does not diminish ineffectiveness over time due to aging or with increasing frequency. At higher frequencies, conductive shielding is less effective because continuity of contact between conductive subassemblies must be maintained in order to ensure effective shielding, which, as discussed above, is very difficult in many cases, especially above 1 GHz. 
     In order to meet this need for an EMC solution which is effective for controlling electromagnetic radiation emissions in system assemblies at higher frequencies, the present invention provides and EMC solution which utilizes lossy materials, or a combination of lossy and conductive materials, within an electrical system assembly, such as a computer system assembly or a data storage system assembly, on the exterior of subassembly housing and/or on the chassis of the assembly. 
     The EMC solution of the present invention controls the emission of electromagnetic radiation from the system assembly enclosure into the outside environment as well as the emission of electromagnetic radiation within the system assembly enclosure from subassemblies, or modules, of the system which may interfere (i.e., EMI) with the operation of other subassemblies or modules of the system. Furthermore, the EMC solution of the present invention is relatively inexpensive and overcomes problems associated with surface conditions required for ensuring contact between subassembly enclosures and the overall system enclosure. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for reducing or eliminating electromagnetic radiation being emitted from a system assembly comprising one or more subassemblies which comprise electrical circuits which generate electromagnetic radiation. The present invention also reduces or eliminates electromagnetic radiation generated outside of the system assembly form adversely affecting the electrical circuits comprised in the subassemblies. To achieve these goals, the present invention provides an electromagnetic compatibility (EMC) solution which comprises a plurality of lossy elements, and/or elements comprised of a combination of lossy and conductive material, to reduce electromagnetic radiation. 
     The elements comprising the EMC solution of the present invention may be placed on the enclosures of the subassemblies and/or on the chassis of the system assembly and are placed in the electromagnetic energy path between the source of the electromagnetic energy and the outside world. The elements comprising the EMC solution may be comprised of materials including, but not limited to, conductive polyethylene, conductive rubber and magnetically-filled silicon rubber. The EMC solution of the present invention is more effective at higher frequencies than conductive gaskets are and problems associated with surface contact requirements and surface characteristics are avoided. 
     These and other features and advantages of the present invention will become apparent from the following description, drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective view of an electrical system assembly designed to hold a plurality of electrical component modules which may emit electromagnetic radiation. 
     FIG. 2 is a rear perspective view of the electrical system assembly shown in FIG.  1 . 
     FIG. 3 is a rear perspective view of a power supply module housed in the electrical system assembly shown in FIGS. 1-2. 
     FIG. 4 is a rear perspective view of an air mover pack module housed in the electrical system assembly shown in FIGS. 1-2. 
     FIG. 5 is a perspective view of a daughter board module housed in the electrical system assembly shown in FIGS. 1 and 2. 
     FIG. 6 illustrates one embodiment of the EMC solution of the present invention which utilizes a gasket comprised of lossy material. 
     FIGS. 7A and 7B illustrate exemplary forms of the EMC elements of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate an electrical system assembly  10  with which the present invention can be utilized in order to control electromagnetic radiation emissions. The electrical system assembly  10  may be, for example, a data storage system assembly. Those skilled in the art will understand that the present invention may be utilized with any type of electrical system assembly that generates electromagnetic radiation emissions that need to be controlled. Before a discussion is provided of the manner in which the method and apparatus of the present invention can be utilized to control electromagnetic radiation emissions, a discussion of the structure of the electrical system assembly  10  and of the modules housed therein will first be provided and then a discussion will be provided of the manner in which the EMC solution of the present invention is utilized in conjunction with the electrical system assembly  10  and the modules housed therein. 
     The electrical system assembly  10  comprises a substantially rectilinear enclosure, or chassis,  12  having front and rear ends  14  and  16 . Pivotally mounted at the front end  14  of the subassembly enclosure  12  is an enclosure door  18  which provides access to the electrical system assembly  10 . The electrical system assembly  10  is adapted to receive a plurality of modules  22  therein, which may be, for example, data storage modules  22 . The modules  22  are received in the enclosure  12  in a plurality of bay slots  24 . Each data storage module  22  may comprise a data storage device  28 , such as, for example, a disk drive. 
     Positioned inside of the subassembly enclosure  12  is a main circuit board, generally referred to as a backplane  36 . The backplane  36  includes a plurality of electrical connectors  38 , such as, for example, multiple pin connectors, and a plurality of airflow openings  40 . Typically, there is one electrical connector  38  and one airflow opening  40  aligned with each bay slot  24  of the subassembly enclosure  12  such that each module  22  aligns with its own electrical connector and airflow opening. 
     With reference to FIG. 2, the electrical system assembly  10  further comprises a transverse wall  42  which spans the width of the assembly  10 . The transverse wall  42  is oriented substantially parallel to the backplane  36 . Extending from the transverse wall  42  to the end of the subassembly enclosure  12  are longitudinal walls  48 . Together, the longitudinal walls  48 , the transverse wall  42 , and the walls of the subassembly enclosure  12  define two power supply module housings  52 . Extending between the longitudinal walls  48  in substantially parallel planes are a plurality of trays that, together with the longitudinal walls  48  define a first daughter board housing  56 , a first air mover pack housing  58 , a second air mover pack housing  60 , and a second daughter board housing  62 . 
     FIGS. 3,  4  and  5  depict a power supply module  64 , a modular air mover pack  66 , and a daughter board assembly  111 , respectively. The power supply module  64 , the modular air mover pack  66 , and the daughter board assembly  111  are adapted to be slidably inserted into the power supply module housings  52 , the air mover pack housing  58  and  60 , and the daughter board housings  56  and  62 , respectively. All of these subassemblies generate electromagnetic radiation emission which need to be controlled. 
     As shown in FIG. 3, the power supply module  64  comprises a substantially rectilinear enclosure  68  that includes front and rear ends  70  and  72 , respectively, and a top side  74 , a bottom side  76 , a front side  78 , a rear side  80 , and opposed lateral sides  82 . At the front end  70  of the module  64  are a plurality of perforations  84  which together form an air inlet. Similarly, the rear end  72  of the enclosure  68  is provided with a plurality of perforations  86  which together form an exhaust outlet. Attached to the rear side  80  of the module  64  is a handle  88  which facilitates insertion and removal of the module  64  into the subassembly enclosure  12 . 
     As illustrated in FIG. 4, the air mover pack  66  comprises a substantially rectilinear enclosure  92  that includes front and rear ends  94  and  96 , and at least top and rear sides  98  and  100 . The top side  98  is provided with at least one perforated area  102  which forms an air inlet. In addition, the rear side  100  of the enclosure  92  is provided with a plurality of perforations  104  which form an exhaust outlet. Mounted inside the pack enclosure  92  is a set  105  of at least one air mover  106 . Normally, two such air movers  106  are arranged directly adjacent each other in series, each being formed as a centrifugal fan having an inlet port  108  and an exhaust port  110 . The air movers  106  are positioned within the enclosure  92  such that the inlet ports  108  are aligned with one of the perforated areas  102  of the enclosure and the outlet port  110  face the perforations  104  of the rear side  100  of the enclosure. The air movers  106  draw in air from outside the enclosure  92  through the perforated areas  102  and expel it from the enclosure through the perforations  104 . 
     FIG. 5 is a perspective view of a daughter board module  111  while is housed in the first daughter board housing  56  shown in FIG.  2 . This daughter board module  111  is a source of EMI which needs to be contained or attenuated. An identical daughter board module may be housed in the second daughter board housing  62  shown in FIG.  2 . The daughter board module  111  may be, for example, an input/output (I/O) card. The manner in which the EMC solution of the present invention is applied to the electrical system assembly  10  and to the subassemblies housed therein will now be described. However, it should be noted that the following discussion of potential applications of the EMC solutions of the present invention is only an exemplary demonstration of the manner in which the EMC solution of the present invention may be implemented. Those skilled in the art will understand that the present invention is not limited with respect to the types of lossy and/or conductive materials utilized, with respect to the dimensions or mechanical characteristics of the EMC solution, or with respect to the locations of a system assembly at which the EMC solution of the present invention is applied. 
     With respect to FIG. 1, in accordance with one exemplary implementation of the present invention, the door  18  has the EMC solution applied thereto in the form of a strip of lossy material, or a combination of lossy and conductive material, attached to a molded indentation  19  about the periphery of the door  18 . Preferably, the EMC solution applied in this particular application is a strip comprising a combination of lossy and conductive material. The lossy material occupying the center of the strip and the conductive material occupying the periphery of the strip. 
     The lossy material, and/or the lossy/conductive material, may be secured to the indentation  19  by any suitable attachment mechanism, such as, for example, adhesive (not shown). The door may be comprised of, for example, a plastic material having a metalized surface (not shown). The door  18  and the chassis of the enclosure  12  preferably are designed so that when the door  18  is placed in the closed position (not shown), the conductive material of the EMC strip contacts the conductive enclosure  12  and the lossy material fills any gaps where continuous contact may not be provided between the conductive material of the EMC strip and the enclosure  12 . In this way, any emissions not shielded as a result of the contact between the conductive material of the EMC strip and the enclosure  12  will be absorbed by the lossy material of the EMC strip. An EMC element which is comprised of a lossy material or a combination of lossy and conductive materials can be designed and implemented in a variety manners. FIGS. 6-7B, discussed below, demonstrate some exemplary embodiments of the manner in which the EMC solution of the present invention may be designed and implemented. 
     The transverse wall  42  and the longitudinal walls  48  have air openings  44  formed therein to enable air to be circulated through the assembly  10  by the air mover packs  66 . Preferably, sheets (not shown) of lossy material having openings formed therein which match up with the air openings  44  are secured to the transverse walls  42  and  48  to allow air to be circulated within the enclosure  12  while also attenuating electromagnetic radiation impinging on the sheets of lossy material. Also, preferably, lossy material is placed in the cavity  53 , which normally has a significant amount of empty space. A sheet of lossy material having appropriate air openings and openings for connectors may also be placed between wall  42  and backplane  36 . However, as stated above, all of these locations are merely preferred, or candidate, locations for the lossy material or lossy and conductive material of the present invention. 
     With respect to FIG. 2, the EMC solution of the present invention preferably is applied at a plurality of locations. For example, a strip  17  of lossy material, or of lossy and conductive material, is applied to the side walls  19  and  48  and to the top walls  21  of the power supply housings  52 . A strip  23  of lossy material, or a strip of lossy and conductive material, is applied to the side wall  48  of the daughter board housings  56  and  62 . Although only one strip  23  is shown in FIG. 2 affixed to the side wall  48  of housing  56 , an identical strip (not shown) is affixed to the opposite side wall  48  designing housing  56  and to the side walls defining the housing  62  in an identical manner. 
     Preferably, the strip  17  is comprised of a comprised of a combination of lossy and conductive material, with the conductive material being disposed on a portion of the strip  17  closest to the rear end  16  of the chassis  12 . Similarly, preferably the strip  23  is comprised of a combination of lossy and conductive material, with the conductive material being disposed on a portion of the strip  23  closest to the rear end  16  of the chassis  12 . By using a combination of conductive and lossy material for the strips  17  and  23  in this manner, electromagnetic radiation propagating toward the rear  16  of the chassis  12  will be partially absorbed by the lossy material of the strips and then reflected toward the front end of the chassis  12  where it is absorbed by the lossy material of the strips. 
     The daughter board module  111  has a horseshoe-shaped strip of lossy material  115  secured thereto by fasteners  121 ,  124 ,  126  and  127 . These fasteners can be any type of fastener such as, for example, screws, rivets, etc. Also, a different type of securing mechanism such as, for example, an adhesive, may also be used for this purpose. A second strip of lossy material  118 , which is thicker than strip  115 , is attached to the daughter board  111  by fasteners  119  at the locations shown.. Alternatively, the strips  115  and  118  may be comprised of a combination of lossy and conductive materials. 
     In the exemplary embodiment of FIG. 5, the locations of the strips  115  and  118  was chosen for convenience and because it was determined that the strips would be effective at these locations for absorbing emissions. The locations at which the EMC elements of the present invention are disposed can be any suitable location, but typically will be disposed at locations where the field strength is the greatest and where they can be conveniently located. Also, the EMC elements can vary in thickness and can be comprised as a single, integrated unit or as a plurality of units. 
     FIG. 6 illustrates another form of the EMC solution of the present invention which utilizes a gasket comprised of lossy material. As shown in FIG. 6, a module  130  to be mounted to the chassis  12  of the assembly  10  has opposing flanges  132  and  133  which are separated from the chassis  12  by a lossy gasket  135 . Conventional techniques implement a metal, compressible gasket (not shown) in order to provide continuous contact between the metal flanges  132  and  133  and the chassis  112 . However, as stated above, the conventional technique may be very difficult to implement and its results are not very effective for shielding electromagnetic radiation emissions when relatively high frequencies are involved. By using a lossy gasket  135  in the manner shown in FIG. 6 about the periphery of a module  130 , electromagnetic radiation emissions are effectively absorbed and the difficulties associated with implementing a metal gasket are avoided. 
     It should be noted that it is unnecessary of the lossy material to completely fill the gap between the subassemblies, nor is it necessary for the lossy material to be in physical contact with the subassemblies. This eliminates the need for contact pressure and provides many options with respect to the surface characteristics, such as, for example, aesthetic design, mechanical tolerances of subassemblies, etc. 
     FIGS. 7A and 7B illustrate other forms of the EMC elements of the present invention. FIG. 7A illustrates an EMC element  150  which is composed of lossy material  151  in the center and of conductive material about the edges of the element  150 , which are designated by the shaded areas within the regions defined by numerals  153  and  154  on one end of the material  151 . Radiation propagating through the gap between subassemblies A and B in the direction indicated by arrow  155  will be absorbed by the lossy material  151  as it passes through the lossy material  151  and any radiation that is not absorbed will be reflected by the conductive material within the regions defined by the numerals  153  and  154 . Any reflected radiation will then be further absorbed by the lossy material  151 . 
     In FIG. 7B, the EMC element  160  is also comprised of a combination of lossy material and conductive material. The “X” designated by numeral  161  represents the direction of the electromagnetic radiation with respect to the EMC element  160  and subassemblies A and B (i.e., radiation going into the plane of the drawing page). The conductive material is indicated by the bands  165  which pass through a cross-section of the element  160  and the lossy material  162  is comprised elsewhere in the element  160 . The conductive material  165  reflects radiation and the lossy material  162  absorbs radiation. This configuration is suitable in cases where, for example, implementing a conductive gasket would require so much contact pressure that the subassemblies A and B would be deformed by the pressure. 
     A variety of materials are suitable for creating the EMC element of the present invention. Preferably, a conductive polyethylene is used for the lossy material. Desco Corporation manufacturers a dielectrically lossy polyethylene, known as conductive polyethylene material P, which is suitable for use as the EMC element of the present invention. Another material which is suitable for this purpose is a dielectrically lossy polyethylene sold by Com-Kyl, Inc., known as Crystal X. Cumming Microwave Corporation manufacturers several materials which are suitable as the EMC element of the present invention. Examples include the dielectrically lossy C-RAM ALU-25-neoprene and the magnetically-filled silicon rubber material C-RAM GDSS and C-RAM FDSS. However, it will be understood by those skilled in the art that the present invention is not limited to these materials which is known as 
     It should be noted that the present invention has been described with respect to particular embodiments but that the present invention is not limited to these embodiments. It will be understood by those skilled in the art that modifications may be made to the embodiments discussed herein which are within the scope of the present invention.