Abstract:
An improved electromagnetic seal is provided between body parts of an enclosure by providing mating surfaces of the body parts with projections to improve electrical contact between the body parts. The invention may be applied to various forms of mating surfaces, including complementary tongue and groove or stepped surfaces. Similar projections may also be introduced into slots for mounting of panels in order to improve electrical contact between the body parts and the panels.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of enclosures for electrical or electronic equipment in general and to the electromagnetic sealing of such enclosures in particular. 
     Enclosures, both conductive (e.g. metal) and non-conductive (e.g. plastic) are used to house electrical and electronic equipment. Where electromagnetic compatibility (EMC) is a problem, e.g. either in the form of electromagnetic interference (EMI) from outside the enclosure affecting sensitive apparatus inside, or vice versa, measures are taken to make the enclosure an effective electromagnetic (EM) screen. 
     Metal enclosures are obviously to be preferred from an EMC point of view, although cost, aesthetic and other factors mean that plastic enclosures are often used. In order to offer EM screening, plastic enclosures are coated with a conductive skin (e.g. metallic paint) or, alternatively, conductive, metal loaded plastic may be used. These enclosures are typically assembled from two matching halves mated together. Despite the use of conductive materials, EM leakage may occur at the join between the two halves. A known method to reduce EM leakage at the join is to insert a soft, metalised gasket along the join. A complementary measure is to include frequent fixing points (i.e. for fixing the two halves together) around the enclosure in the vicinity of the join to counteract the tendency of the join to open at sections away from the fixing points. 
     The introduction of extra fixing points leads to increased complexity of the mould or the machining and increased assembly time. The use of gaskets increases cost and assembly time. 
     Another means to improve EM screening is to create a tongue and groove joint between the two halves. A continuous groove is moulded or machined into the mating surface of one half of the enclosure and a complementary tongue (or wall) is moulded or machined on the corresponding mating surface of the other half. Disassembly is a particular problem where metallic coatings such as metallic paint are applied to the tongue and groove. In order to ensure ease of disassembly, the fit between the tongue and groove is designed to be generous. 
     However, this generous fit leads to poor electrical contact being obtained between the mating surfaces resulting in gaps in the EM seal which allow the passage of unwanted EM radiation. 
     SUMMARY OF THE INVENTION 
     Use of the present invention provides good EM sealing of enclosures with low cost and easy assembly. 
     The present invention provides an enclosure for electrical or electronic equipment comprising a first and a second electrically conductive body part, the first and second body parts each comprising a mating surface for mating with the mating surface of the other body part on assembly in which at least one of the mating surfaces comprises a plurality of electrically conductive projections for contacting the other mating surface when assembled; in which the mating surfaces are disposed at the periphery of the relevant body part, in which the plurality of projections are dispersed along the periphery, and in which each projection is substantially electro-magnetically shielded by at least one of the body parts when assembled; in which the electromagnetic shielding comprises portions of the first and second body parts which overlap each other when assembled; in which the electromagnetic shielding comprises a tongue on one mating surface and a groove on the other mating surface. 
     In a further preferred embodiment the invention provides an enclosure comprising a panel groove for mounting a panel, in which the panel groove comprises one or more electrically conductive projections on the groove surface for contacting the panel when assembled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of example with reference to the drawings in which 
     FIGS. 1 and 2 show first and second halves of an enclosure according to the present invention; 
     FIGS. 3 to  13  show parts of the enclosure of FIGS. 1 and 2 according to the present invention in more detail. 
    
    
     All drawings are to scale, although the present invention is not limited to the specific size ratios illustrated therein. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show two mating halves of an enclosure according to a first embodiment of the present invention. FIG. 1 shows one half  10  of the enclosure comprising base  12  and peripheral wall  14 . Peripheral wall  14  is provided with gap  16  for mounting a panel, eg for presenting switches and indicators to the user and indentations or “cut-outs”  17  which introduce gaps in the top surface  15  of wall  14 . Wall  14  also comprises a number of holes  18  for mounting connectors, switches, etc. In addition a plurality of slots  20  are provided for ventilation and protrusions  22  are provided for mounting the enclosure, eg on the wall of a room. The top surface  15  of wall  14  (the “mating surface”) is provided with a tongue  24  which may be continuous or divided (as shown in the figure) into a number of sections for ease of manufacture (eg at some corners) or due to gaps  16 ,  17  in the top surface  15  of wall  14 . Plurality of bosses  26  are incorporated in the base  12  for co-operating with corresponding bosses  46  of the second part (see FIG. 2) for fixing the two halves together. In addition, supplementary bosses  28  are provided for mounting equipment, eg printed circuit boards inside the enclosure. 
     FIG. 2 shows the second, mating half  40  of the enclosure of the first embodiment comprising base  42  and peripheral wall  44 . Peripheral wall  44  is provided with gap  46  for mounting a panel in co-operation with gap  16  in peripheral wall  14  of fist half  10  and indentations or “cut-outs”  47  which introduce gaps into the top surface  45  (the “mating surface”) of wall  44 . Wall  44  also comprises a plurality of slots  50  for ventilation. The top surface  45  of wall  44  is provided with a groove  54  which may be continuous or divided (as shown in the figure) into a number of sections for ease of manufacture or due to gaps  46 ,  47  in the top surface  45 . A plurality of bosses  56  are incorporated into base  42  for co-operating with bosses  26  of first half  10 . In addition, supplementary bosses  58  are provided for mounting equipment, eg printed circuit boards inside the enclosure. 
     Tongue  24  and groove  54  are arranged to co-operate on assembly of the two halves of the enclosure such that tongue  24  is at least partially accommodated in groove  54 . 
     For the purpose of clarity the term “height” in relation to a tongue or groove refers to that dimension substantially at right angles to the mating surface  15 ,  45  on which or in which the tongue or groove extends and the term “width” refers to a dimension through the tongue or across the groove substantially parallel to the relevant mating surface  15 ,  45 . The term “height” in relation to a projection refers to the dimension substantially at right angles to the surface (eg. of a tongue or a groove) from which it projects. A projection projecting from more than one surface may have a different height from each such surface. 
     FIG. 3 shows area A of the enclosure half  10  of FIG. 1 at larger scale with the tongue  24  shown in more detail. Other elements of the enclosure half  10  already described in relation to FIG. 1 are given the same reference numerals and will not be described further. As shown in FIG. 3, wall  24  is provided with a plurality of projections  30  on the top surface thereof. According to further embodiments, similar projections ( 60 , see FIG. 11) are provided on the lower surface of co-operating groove  54  either in addition to or instead of the projections  30 . The projections function, whether on the surface of the tongue or of the groove or of both, to improve mechanical contact between the two halves  10 ,  40  of the assembled enclosure. Where the tongue and groove are conductive or covered with a conductive skin the improved mechanical contact results in improved electrical contact between the two halves at the projections and helps to improve EM screening. Although shown having a rounded profile, various alternative shapes of the projections including rectangular, triangular or ridged/grooved may be used according to the present invention, depending on factors such as material used. 
     FIGS. 4 and 5 show part of FIG. 3 at successively larger scales. Features common to FIG. 3 are given the same reference numerals and are not described further. 
     FIG. 5 in particular shows an elevation of the enclosure half  10  of FIG. 3 with suitable dimensions of key features for an enclosure moulded from Acrylonitrile Butadiene Styrene (ABS). As shown in FIG. 5, by way of example, tongue  24  has a height of 1.5 mm above the mating surface  15  of wall  14 , and a width of 1.0 mm at the base (i.e. where it meets the mating surface  15 ) tapering at an angle of 10° plus/minus 0.5° away from the mating surface  15 . Projection  30  is shown as a curve projecting a further 0.5 mm above the top surface of tongue  24  (i.e. extending 2.0 mm plus/minus 0.1 mm in total from the mating surface  15 ), with the curve having a radius of 4.0 mm. 
     According to a further embodiment of the present invention, the height of the projections  30 ,  60  whether on the surface of the tongue  24  or groove  54 , are varied according to their position around the join between the two halves  10 ,  40  of the enclosure. The height of projections  30 ,  60  increases with increased distance from the nearest fixing point (i.e. boss  26  or  56 ) so that the tendency of the join to gape open midway between fixing points  26  and  56  is compensated for by the varying height of projections  30 ,  60  and so that good electrical contact is achieved at all projections  30 ,  60  irrespective of position around the join. Advantageously, this reduces the number of fixing points required. 
     FIG. 6 shows area B of the enclosure half  10  of FIG. 1 at larger scale with the tongue  24  shown in more detail. Other elements of the enclosure half  10  already described in relation to FIG. 1 are given the same reference numerals and will not be described further. As shown in FIG. 6, wall  24  is provided with a plurality of second projections  32 . Unlike projections  30 ,  60  described above, the projections  32  of FIG. 6 project from the top and side surfaces of wall  24 . According to further embodiments of the present invention, similar projections (not shown) are provided on the lower and side surfaces of co-operating groove  54  (i.e. forming a ‘U’ shaped projection inside groove  54 ) either in addition to or instead of the projections  32  on tongue  24 . As with projections  30 ,  60  projections  32  function, whether on the surfaces of the tongue or of the groove or of both, to improve mechanical contact and, hence, electrical contact between the two halves  10 ,  40  of the assembled enclosure. 
     According to a further embodiment the height of the projections  32  from the top surface of the tongue or the bottom surface of the groove are varied according to their position around the join between the two halves  10 ,  40  of the enclosure. Thus the height of these projections increase with increased distance from the nearest fixing point (i.e. boss  26  or  56 ) to compensate for the tendency of the join to gape open midway between fixing points  26  or  56 . 
     Advantageously, for moulded parts, adjustment of the heights of projections may be achieved by a “metal off” process whereby height of a projection may be increased as necessary by removing material (typically metal) from the mould. This allows fine adjustments to be made at a late stage in preparations for manufacture. 
     FIGS. 7 and 8 show part of FIG. 6 at successively larger scales to more clearly show projection  32  on tongue  24 . Features common to FIG. 6 are given the same reference numerals and are not described further. 
     FIGS. 9 and 10 show parts C and D respectively of the enclosure half  40  of FIG. 2 at larger scale. In particular, FIGS. 9 and 10 show groove  54  in mating surface  45  of wall  44  comprised of several parts separated at gaps (i.e. indentations  47  in mating surface  45  of wall  44 ) and at some comers. Other features common to earlier figures described above are given the same reference numerals as in the earlier figures and are not described further. 
     Although the groove  54  of FIGS. 9 and 10 is shown without projections, such projections may be present according to embodiments of the present invention, as described above. 
     FIG. 11 shows a cross-section of part of the enclosure half  40  of FIG. 10 with suitable dimensions of key features for an enclosure moulded from ABS. As shown in FIG. 11, by way of example, groove  54  has a depth of 1.0 mm plus/minus 0.1 mm and a width of 1.0 mm at the top of the groove (i.e. where it meets the mating surface  45  of wall  44 ), tapering at an angle of 10° plus/minus 0.5° away from the mating surface  45 . 
     FIGS. 12 and 13 show parts E and F of the enclosure halves  10  and  40  of FIGS. 1 and 2 respectively at larger scale and showing the corresponding gaps  16 ,  46  in walls  14 ,  44  respectively for mounting a panel (not shown). As shown in FIG. 12, gap  16  is bordered by panel grooves  34  (one shown) which are formed in each end of wall  14  (one shown), one at either end of gap  16 . Panel grooves  34  open into and are linked by further panel groove  36  which is formed in base  12  of enclosure half  10 . According to an alternative embodiment, gap  16  is formed by a section (not shown) of wall  14  of reduced height and panel groove  36  is formed in the top surface of the reduced height section of wall  14 . This embodiment advantageously allows for accommodating smaller panels. 
     As shown in FIG. 13, gap  46  is bordered by panel grooves  64  (one shown) which are formed in each end of wall  44  (one shown), one at either end of gap  46 . Panel grooves  64  open into and are linked by further panel groove  66  which is formed in base  42  of enclosure half  40 . According to an alternative embodiment, gap  46  is formed by a section (not shown) of wall  44  of reduced height and panel groove  66  is formed in the top surface of the reduced height section of wall  44 . This embodiment advantageously allows for accommodating smaller panels. 
     FIGS. 12 and 13 also show projections  38 ,  68  in one side wall of panel grooves  36 ,  66  respectively. According to further embodiments (not shown), similar projections may be provided on the bottom faces of the panel grooves  36 ,  66  or on the faces opposite to the ones shown in FIGS. 12 and 13 in combination with or in place of the projections  38 ,  68  shown. 
     According to further embodiments (not shown), similar projections may be provided on one or more faces of panel grooves  34 ,  64  either in addition to or in place of the projections described above with reference to panel grooves  36 ,  66 . 
     According to a further preferred embodiment, the projections are distributed around the join substantially equally spaced with the maximum spacing between two adjacent projections being determined by the minimum wavelength of radiation it is desired to screen. 
     Although described above in relation to an enclosure consisting of two body parts, the present invention is equally applicable to enclosures with a greater number of body parts. The tongue and groove are used due to the EM shielding they provide, i.e. in preventing a clear, straight path for radiation to cross the join.