Abstract:
A compact programmable logic controller provides a floating mounting of circuit cards carried by opposed terminal blocks of the industrial controller the latter only which are directly mounted to the housing. Positioning of the cards between opposed connector blocks prevents torsion on the cards resulting from unbalanced forces on the terminal blocks and provides an extremely rigid structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application hereby claims the benefit of provisional application 60/169,421, filed Dec. 7, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to industrial controllers used for the control of industrial processes and machines, and in particular to a rugged assembly method for compact industrial controllers. 
     Industrial controllers are special purpose computers used for controlling industrial processes and manufacturing equipment on a real-time basis. Under the direction of a stored program, a processor in the industrial controller examines a set of inputs reflecting the status of the controlled process and changes a set of outputs controlling the industrial process. The inputs and outputs may be binary, that is on or off, or analog, providing a value within a continuous range. Typically analog signals are converted to binary words for processing. 
     The inputs and outputs are processed by input/output (I/O) circuitry which performs any necessary data type conversion, level shifting, isolation and amplification of the signals to and from the processor so as to be compatible with the signals required by the industrial controller. Processor and I/O circuitry for industrial control are well known in the art. 
     Compact industrial controllers may be produced in which the processor and the necessary I/O circuits are in a single housing. It is desirable that the base of the housing, defining its footprint when attached to the back of a cabinet, be small to conserve cabinet space. This small footprint may be accommodated by dividing the internal circuitry of the controller into multiple circuit cards stacked one on top of another. 
     Interconnections between the circuit cards may be accommodated by conductive connectors typically having gold plated pins necessary to communicate the low voltage logic level signals used by the processor. The multiple cards must be rigidly mounted so that vibration to be expected in the industrial environment does not cause abrasion of the gold plating layer. 
     In addition to holding the processor and I/O circuits, such a compact industrial controller must support terminal blocks or the like providing a means to connect the signals of the controlled equipment to the I/O circuitry. Two sets of terminal blocks separated across the front of the housing may be provided, one for input signals and one for output signals. The terminal blocks may also provide connections for line power in, user power out, and other signals known in the art. 
     Normally the terminal blocks will be attached directly to the circuit card holding the I/O circuitry by mechanically rigid conductors being continuations of the stampings which form part of the connector block. Forces involved in making the connections to the terminal blocks using a screwdriver or the like, therefore can be transmitted through these terminal blocks to the I/O circuit card. Unless the terminal blocks are firmly mounted, movement of the terminal blocks under such forces can cause distortion of the attached circuit card and possible damage to its printed circuitry or components. 
     Precise, rigid and strong mountings of each of the circuit cards and the terminal blocks against the housing are extremely difficult to achieve. “Stack-up” of tolerances between the molded components of the housing and terminal blocks prevent designs requiring close fits between each of these parts and the circuit cards. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention recognizes a hierarchy of positional relationships between the various components of the industrial controller. At the root of the hierarchy are the terminal blocks which must be solidly affixed against the housing to resist the forces incident to attachment of wires to the terminal blocks. The two terminal blocks must in turn be fixed with respect to each other and the I/O circuit card so as to prevent stresses in the I/O circuit card caused by relative movement of any of these three elements. Further, the two circuit cards must be held rigidly with respect to each other to prevent wear on their electrical interconnections. Importantly, however, there need be no direct connection between the circuit cards and the housing. 
     The invention therefore contemplates a “floating” mounting of the circuit cards attached to the terminal blocks and not to the housing. Rigidity is provided by the circuit cards themselves which interconnect the terminal blocks in a box-like structure. The restraint on the terminal blocks provided by their contact with the housing, channels any forces transmitted to the box-like structure into tension and compression of the circuit cards along their planes, a mode in which they are extremely strong. 
     Specifically, the present invention provides an industrial controller that has a housing with walls extending upward from the base and a first and second circuit card positioned within the housing. First and second terminal blocks having upper faces exposed at the upper edges of the opposite walls of the housing reveal terminal blocks for the attachment of input and output lines and hold the first and second circuit cards spaced apart, therebetween by first attachments and without requiring substantial restraining contact between the circuit cards and the housing. 
     Thus it is one object of the invention to provide a floating support of the circuit cards with respect to the housing eliminating the need for precise housing tolerances or stresses caused by movement of the circuit cards with respect to the housing. 
     The first and second terminal blocks may provide for downwardly extending legs attached to opposing edges of the upper surface of a first of the circuit cards. The second circuit card may be positioned within the housing above the first circuit card and between the terminal blocks. The first and second terminal blocks may further include at outer edges of the first and second terminal blocks, attachments to the upper edges of opposing walls of the housing. At inner portions of the first and second terminal blocks, attachments may be made to opposing edges of the process circuit card. 
     Thus it is another object of the invention to provide a mounting of circuit cards on the terminal blocks such as forms a rigid structure resisting torsional motion between the terminal blocks and the I/O circuit card. Downward or upward forces on the terminal blocks of the first and second terminal blocks are converted into edgewise-tensile/compressive forces on the second circuit card, a direction in which the second circuit card is extremely strong. 
     The industrial controller may further include a housing cover attachable to the upper edges of the upwardly extending walls of the housing and restrained from downward movement by the upwardly extending walls. The housing cover may include attachments engaging inner portions of the first and second terminal blocks. 
     Thus it is another object of the invention to convert downward forces on the terminal blocks such as might produce torsional movement with respect to the I/O card into downward compressive forces on the upwardly extending housing walls through the housing cover. 
     At least one of the first and second terminal blocks may include an upwardly exposed conductive ledge positioned above the I/O circuit card and attached via the downwardly extending legs to a receiving ground conductor of the I/O circuit card whereby the lower surface of the second circuit card may be received above the upper surface of the conductive ledge to establish a ground connection between the second circuit card and the I/O circuit card. 
     Thus it is another object of the invention to provide a low impedance ground connection between the two circuit cards as may be realized by the relatively robust conductors of the terminal blocks. 
     The housing may include a plurality of ventilation slots in the upstanding walls and the legs may provide for the passage of air from the ventilation slots between the terminal blocks and the I/O circuit card. 
     Thus it is another object of the invention to promote cooling airflow by removing the terminal blocks from the surface of the I/O circuit card such as may be done without undue torsion between these two members through the bracing system of the present invention. 
     The housing may include attachments attaching edges of the I/O card to the housing. 
     Thus it is another object of the invention to provide for a supplemental restraining of the I/O circuit card as may be useful during assembly, but which is loose enough not to promote stresses with motion of the terminal blocks to which the I/O card is attached. 
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the assembled compact industrial controller of the present invention; 
     FIG. 2 is an exploded perspective view similar to that of FIG. 1 showing the housing, cover, and assembled processor and I/O cards held by terminal blocks; 
     FIG. 3 is a side elevational view of the terminal blocks of FIG. 2 removed from the circuit cards; 
     FIG. 4 is a simplified cross-section along lines  4 — 4  of FIG. 1 showing the floating mounting of the circuit cards on the terminal blocks with respect to housing and the operation of the terminal blocks and housing to convert downward forces on individual terminal blocks into compressive and tensional forces along the planes of the circuit cards; 
     FIG. 5 is a fragmentary perspective detail view of an auxiliary attachment of the processor circuit card to the terminal blocks via a grounding boss to provide ground connections between the circuit cards through the conductors of the terminal block; 
     FIG. 6 is a plan view of the I/O card and terminal block assembly of FIG. 2 with the processor card removed to show bosses of the terminal blocks used to hold the processor circuit card; 
     FIG. 7 is a side elevational view of the cover of FIG. 2 showing hooks for engaging the housing and separate hooks for supporting the inner edges of the terminal blocks; and 
     FIG. 8 is a side elevational view of the cover of FIG. 7 showing engagement of the hooks with respect to the terminal blocks for additional support of the terminal blocks. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1 and 2, an industrial controller  10  of the present invention provides an outer, box-shaped housing  12  having upstanding sidewalls  14  and  16  and upstanding end walls  18  and  20  attached at their lower edge about the periphery of a rectangular base  22 . Directions of up and down in the following description will be with reference to the base and the sidewalls. However, it will be understood that the invention is not limited by any particular orientation of the controller  10 . 
     Sidewalls  14  and  16  are perforated by a number of ventilation slots  24  that allow convective flow of air through the housing volume when the housing  12  is mounted against a vertical panel with sidewalls  14  and  16  vertically opposed. The housing  12  may be attached to the panel via attachment ears  15 . 
     A housing cover  26  fits against upper edges of the walls  14  through  20  and includes opposed cutouts  28  positioned, respectively, over the sidewalls  14  and  16 . The cutouts  28  expose terminal blocks  30  and  32  which present on their upper surfaces, screw terminals  42  to which input and output lines for the industrial controller  10  may be attached. 
     The housing cover  26  is retained against the upper edges of the wall  14  through  20  by downwardly extending flexible hooks  34  in each of the corners of the cover  26  engaging in corresponding slots  37  in the sidewalls  14  and  16 . The cover  26 , when so placed against the housing  12 , engages with a circuit card resting on cantilevered bosses  36  to be described below on the terminal blocks  30  and  32  to restrict access by the user only to functions and components relevant to the end user when the controller  10  is fully assembled. 
     Referring now to FIG. 3, the terminal blocks  30  and  32  include an upper insulating barrier portion  40  providing insulating walls between the screw terminals  42 . The barrier portion  40  may support two rows of screw terminals  42  in a stepped-tier arrangement well known in the art. Each screw terminal  42  communicates with a stilt conductor  46  extending downward in unsupported fashion beneath the barrier portion  40 . The stilt conductors  46  are staggered horizontally and are stiff and thus transmit forces between the terminal blocks  30  and  32  and the I/O circuit card  52  to which they are attached. While it is possible to isolate these forces through the use of highly flexible connectors, this non-standard design would increase the complexity of assembling the terminal blocks  30  and  32  to the I/O circuit card  52 , would have reduced current carrying capacity, and would require a more complex manufacturing process to produce the terminal blocks  30  and  32  which currently allows a relatively high gauge component of the screw terminals  42  to simply be extended to form the stilt conductor  46 . 
     The barrier portion  40  of the screw terminals  30  and  32  also provides, at their ends, downwardly extending insulating legs  48  of lengths substantially equal to the stilt conductor  46 . The insulating legs hold sidewardly extending cantilevered bosses  36  as have been mentioned and will be described in more detail below. Insulating legs  48  may also include outwardly extending tabs  72  engaging corresponding slots  74  in sidewalls  14  and  16 . 
     The terminal blocks  30  and  32  are held separated by a processor circuit card  50  and an I/O circuit card  52  oriented in a horizontal plane. The processor circuit card  50  and an I/O circuit card  52 , in a preferred embodiment are constructed of epoxy impregnated fiberglass according to techniques well known in the art. The I/O circuit card  52  extends horizontally to receive the lower most ends of the stilt conductors  46  by means of holes in the I/O circuit card  52  where the stilt conductors  46  are secured to printed circuitry by soldering techniques understood in the art. The insulating legs  48  have lowermost pegs (not shown) received by corresponding holes in the I/O circuit card  52  to stabilize them against lateral motion. 
     The processor circuit card  50  is positioned parallel to and above the I/O circuit card  52  to span the inner opposed edges of the barrier portions  40  of the terminal blocks  30  and  32 . Referring to FIG. 4 in particular, processor circuit card  50  may rest on inwardly extending ledges  53  formed integrally with the barrier portion  40  of the terminal blocks  32  and  30 . Upward motion of the processor circuit card  50  away from ledge  53  is prevented by barbs  54  extending over processor circuit card  50  but displaced outward by slight flexure of the I/O circuit card  52  and the stilt conductors and insulating legs  48  during assembly. The processor circuit card  50  may also be held by screws through corresponding holes in the processor circuit card  50  as received by the bosses  36 . 
     Referring again to FIG. 2, logic level signals may communicate between I/O circuit card  52  and processor circuit card  50  through multi-pin conductor  56  having mating halves, one attached to the upper surface of I/O circuit card  52  and one attached to the lower surface of processor circuit card  50 . I/O circuit card  52  may be restrained during assembly by means of tabs  58  extending from edges of the I/O circuit card  52  to be received in loose slots  60  in end walls  18  and  20 . Referring to FIG. 4, in a first embodiment, these loose slots  60  provide only a general location of the I/O circuit card  52  and, when the industrial controller  10  is fully assembled, do not provide significant restraining engagement between the housing  12  and I/O circuit card  52  as a result of their relatively large size. 
     Referring still to FIG. 4, the barrier portion  40  of the terminal blocks  32  and  30 , are positioned on the I/O circuit card  52  and flanking processor circuit card  50  so that their outer edges  62  engage upper edges of sidewalls  14  and  16 . A downward force  64  on either of terminal blocks  30  and  32  such as may occur when the terminal blocks are being tightened or loosened with a screwdriver will be met with an outer upward force  66  between the insulating barrier portion  40  and the upper edges of sidewalls  14  and  16 . Such a force would normally be expected to exert a torque on the terminal blocks  32  and  30  about their long axes such as would bend the I/O circuit card  52 . However, this torque is resisted by the presence of the processor circuit card  50  between the barrier portion  40  of terminal blocks  30  and  32  which experiences a compression  68 . At the same time, the I/O circuit card  52  attached to the terminal blocks  32  and  30  via the stilt conductors  46  (not shown in FIG. 4) and the insulating legs  48  experiences a tension  70 . Processor circuit card  50  and I/O circuit card  52  are relatively strong in tension and compression along their planes as opposed to a bending which may produce breaks in the fine wiring of their printed surfaces. . 
     Thus it will be understood that processor circuit card  50  and I/O circuit card  52  are attached directly only to the terminal blocks  32  and  30  and as a result of the large tolerances of loose slots  74  and  60  essentially float with respect to the housing  12 . Forces on the terminal blocks  30  and  32  causing them to move separately are resisted by the structure formed by the processor circuit card  50  and I/O circuit card  52 . For this reason, the processor circuit card  50  and I/O circuit card  52  do not experience bending strains as might be caused by relative movement of the terminal blocks  30  and  32  or forces between the processor circuit card  50  and I/O circuit card  52  the housing  12 . 
     The I/O circuit card  52  is supported above the base  22  to provide room for additional circuit cards, for example, a power supply circuit card  76  such as may be connected to the other circuit cards by a relatively low number of flexible connectors. 
     In an alternative embodiment, the rigid structure formed by the processor circuit card  50  and I/O circuit card  52  and the terminal blocks  32  and  30  may be attached to the housing  12  through the I/O circuit card  50 , for example, by stand-offs to the power supply circuit card  76  or the like, instead of or in addition to the attachment between the terminal blocks  32  and  30  and the housing  12 . In these cases the rigidity of the processor circuit card  50  and I/O circuit card  52  and the terminal blocks  32  and  30  is independent of the attachment but may be augmented by it. 
     Referring now to FIG. 6 as mentioned, barrier portion  40  provides for cantilevered bosses  36  extending laterally toward the end walls  18  and  20  and having an upper surface positioned to receive the lower surface of processor circuit card  50  (not shown in FIG.  6 ). Processor circuit card  50  has corresponding holes matching holes in the bosses  36  so that screws may be used to retain the processor circuit card  50  against the barrier portion  40  for additional security. Other types of fasteners or no fasteners may also be used. 
     Referring now to FIG. 5, one boss  36  may include an overlying conductive ledge or plate  80  extending downward along insulating leg  48  to be received like stilt conductors  46  in a hole of I/O circuit card  52  connected to a ground trace  81 . The ground trace  81  provides a solid ground connection through this substantial conductive plate  80  between the two circuit cards  50  and  52 . 
     Referring now to FIGS. 7 and 8, cover hooks  82  extending downward from the cover  26  to engage a lower surface of the barrier portion  40  of terminal blocks  32  and  30  thereby providing a retaining connection to a major structural component of the controller  10  (the terminal blocks) as opposed to latching hooking it just to the housing  12  which has relatively flexible walls. 
     A marked up version of the above paragraphs is attached hereto. 
     The above description has been that of a preferred embodiment of the present invention. It will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made.