Abstract:
A electrical unit consisting of a proximity switch and a cable terminal part is described, where the proximity switch has an outside housing and an insulation part, positioned on one end face of the outside housing and having a terminal element with terminals which lead to the outside. The terminals of the terminal element include terminal sockets. The cable terminal part includes a cable and a connecting part, and the cable is mounted in the connecting part and the ends of the leads of the cable are electrically connected to the terminal sockets of the terminal element. The electrical unitcable terminal part also has a cap which surrounds at least the connecting part and is attached to the proximity switch.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to an electrical unit comprising a proximity switch and a cable terminal part and a process for its manufacture. The proximity switch having an outside housing and an insulation part, with the insulation part being on one end face of the outside housing and having a terminal element with terminals which lead to the outside. The terminals of the terminal element which lead to the outside being made as terminal sockets. The cable terminal part having a cable and a connecting part, the cable being mounted in the connecting part and the ends of the leads of the cable being connected electrically to the terminal sockets of the terminal element.  
           [0003]    2. Description of the Related Art  
           [0004]    The prior art discloses different possibilities for connecting detachable and fixed proximity switches to a cable. Plug-and-socket connections, that is detachable cable joints which are made using plug connectors which can be connected to the terminals of the proximity switch, are known for example in German patents 195 28 678 and 44 19 023. Another detachable cable joint, specifically a screw-type terminal, is shown by German patent 195 29 181. A proximity switch with a fixed or non-detachable cable terminal is shown, by way of example, in the German patent 42 25 267 which describes a housing termination comprising hot-melt cement through which the cables, which are connected directly to the electronics in the proximity switch, are routed out of the proximity switch.  
           [0005]    Both the detachable joints of cables on proximity switches as well as the nondetachable cable terminals for proximity switches which are known from the prior art have a series of defects.  
           [0006]    Detachable joints generally require a relatively large amount of space. Since recently the trend has been toward smaller and smaller proximity switches, only little space is available for one such detachable joint. In designs with plug connectors, it is also disadvantageous that they are much more expensive to manufacture than proximity switches to which the cables are directly connected.  
           [0007]    In the last two decades, especially in the field of inductive proximity switches, it has been possible to further increase the operating distance at the same outside diameter of the coil which is used for the inductive proximity switch. This means that at a uniform operating distance the outside diameter of the proximity switch which is essentially dictated by the coil diameter has been reduced. This is advantageous to the user, but entails problems in forming connections, particularly with smaller devices, which are conventionally made in thread sizes M 6 to M 12. That is for cable terminal or plug terminal technology, the maximum plug diameter in cylindrical proximity switches generally will not be greater than the threaded or smooth sleeve diameter of the proximity switch in order to enable any installation in walls of different thickness.  
           [0008]    In proximity switches with a non-detachable cable terminal, either the cable is permanently connected to the electronics and then the entire electrical unit after adjustment is inserted into the housing of the proximity switch and potted, or the electronics in the housing of the proximity switch is connected via flexible wires to the cable or plug, and only after final testing are the cables and plugs mechanically connected non-detachably and permanently to the housing, for example, by potting. These types of direct connection enable the shortest overall lengths, due to the long wires however a large amount of space is necessary.  
           [0009]    Since the users of proximity switches, when selecting one type of proximity switch, ask for very many cable colors, cable materials and cable sizes which are different from one another, the diversity of types of proximity switches provided with a nondetachable cable is quite large; this leads to considerable warehousing costs for storage of all the conceivable types of proximity switches.  
         SUMMARY OF THE INVENTION  
         [0010]    The aforementioned problem is solved by the electrical unit set forth in related U.S. patent application Ser. No. 09/770,228, the entire disclosure of which is hereby incorporated by reference, in that the proximity switch is completely finished, checked using the electrical terminals present in the form of terminal sockets, and can then be warehoused. Connection to a cable requested by the user takes place using the cable terminal part which includes the connecting part which fits the proximity switch and the cable requested by the user attached in the cable terminal part. The proximity switch and cable terminal part can then be joined quickly, easily and reliably.  
           [0011]    The development of the subject matter of the present invention and the related U.S. patent application Ser. No. 09/770,228 is based on the object of making available an electrical unit which meets the highest demands for tightness and strength.  
           [0012]    This is achieved by the electrical unit essentially by a cable terminal part which has a cap which surrounds at least the cable terminal part and is attached, via the cap, to the proximity switch or to the outside housing of the proximity switch. Providing a cap can further increase the tightness of the electrical unit and its mechanical strength. In addition, through an appropriate choice of material for the cap, the electrical unit can be quickly and easily matched to different requirements. Thus, for example, due to the smooth form and corresponding selection of the material of the cap, an electrical unit can be achieved which meets the requirements of the food industry, and specifically ensures ease of cleaning.  
           [0013]    The cap can be connected in different ways to the outside housing depending on the choice of material of the cap and the material of the outside housing of the proximity switch. Preferably, the cap is attached via a catch connection and/or stamping to the outside housing of the proximity switch. But in addition other techniques such as screwing, welding or cementing are also possible.  
           [0014]    According to one preferred embodiment of the current invention, the cap is made such that the end of the cap facing away from the proximity switch is conically tapered. In this embodiment of the cap, it is possible for the cap to be used not only to connect the cable terminal part to the proximity switch, but also to at least partially to undertake sealing to the cable at the same time. To do this, the connecting part is radially compressed by the conically tapered end of the cap and thus pressed against the cable. Sealing via the cap and the connecting part is especially good when the length of the cap is dimensioned such that the connecting part is compressed when the cap is mounted on the proximity switch or the outside housing of the proximity switch. The connecting part thus has the function of a sealing element which is activated by attaching the cap to the outside housing of the proximity switch. Thus, the connecting part is compressed both axially and also radially in order that the connecting part seals both axially against the insulation part and also radially against the cable.  
           [0015]    According to one alternative embodiment of the initially described electrical unit, a high level of tightness and high mechanical strength, and inparticular a high strain relief of the cable, is achieved in that a ring is assembled with the connecting part and the ring can be shifted out of a first form, in which the inside diameter of the ring is greater than or equal to the outside dimensions of the connecting part or greater than or equal to the outside dimensions of the cable, into a second form in which the inside diameter of the ring is less than the outside dimensions of the connecting part or less than the outside dimensions of the cable. When the inside diameter of the ring is reduced, the cable is squeezed and fixed in the connecting part such that the tension forces acting on the cable are kept away from the connecting joint between the ends of the cable wires and the terminal sockets of the terminal element.  
           [0016]    It was stated above that the ring in the first form has an inside diameter which is greater than or equal to the outside dimensions of the connecting part or greater than or equal to the outside dimensions of the cable. If, according to one preferred embodiment of the invention, the ring is located in a groove which is made in the connecting part, the ring in its first form has an inside diameter which is greater than or equal to the outside dimensions of the connecting part. But, if the ring is located within the connecting part so that it is located between the cable and the connecting part, the ring in its first form has an inside diameter which is greater than or equal to the outside dimensions of the cable.  
           [0017]    Generally, cables with a circular cross section are used so that the connecting part also has a circular cross section and thus the outside dimensions of the cable correspond to the outside diameter of the cable.  
           [0018]    Strain relief of the cable, for different cable materials and cable diameters, can be achieved through a suitable choice both of the material and also the dimensions of the ring—diameter and width.  
           [0019]    In one preferred embodiment, the ring is made of metal, especially high quality steel, and the reduction of the inside diameter is roughly 25%, the ring being concentric in the compressed form, i.e., with reduced inside diameter, and thus surrounding the connecting part without a gap.  
           [0020]    Preferably, but not necessarily, the electrical unit has both the above described cap and also the above described ring, by which overall especially high tightness and mechanical strength are achieved.  
           [0021]    One especially simple connection between the cable terminal part a proximity switch is achieved when there is an alignment aid for alignment of the ends of the wires of the cable. By using such an alignment aid, the wires are arranged such that they correspond in terms of position to the terminal sockets in the insulation part.  
           [0022]    The alignment aid can be made as a separate part or can be located on the insulation part or the connecting part. The insertion of the ends of the wires of the cable is then especially simple when the alignment aid has holes with funnel-shaped or cup-shaped openings for receiving the ends of the wire.  
           [0023]    In order to preclude pin confusion or polarity reversal when the proximity switch is joined to the cable terminal part, according to another preferred development of the invention, the electrical unit is constructed such that the insulation part and the cable terminal part each have codes which correspond to one another, and when joining the proximity switch and cable terminal part indicate the correct alignment of the cable terminal part to the insulation part. As a result, alignment of the two components to one another with the desired contact-making of the wires to the terminal elements is achieved. Alternatively or additionally, it is preferable that the insulation part and the cable terminal part each have a shape such that they can only be joined in the correct alignment of the cable terminal part to the insulation part. In this way it is no longer possible for the two components to twist before final attachment of the cable terminal part to the proximity switch.  
           [0024]    Different techniques are possible for connecting or attaching the ends of the wires of the cable to the terminal sockets of the terminal element. For example, the terminal sockets can be slotted or can be made at least partially elastic so that the stripped ends of the wires of the cable, which can be tin-coated and/or bear at least partially conductive sleeves, can be inserted into the terminal sockets and as a result of the spring action of the terminal sockets they are easily and securely held in them and have good contact to them.  
           [0025]    According to another preferred embodiment of the invention, the electrical unit is designed such that the terminal sockets can hold a connecting agent and are composed of a ferritic material. Preferably solder or conductive cement, especially conductive silver cement, is used as the connecting agent. The stripped ends of the cable wires, which are preferably tin-coated and/or are provided at least partially with conductive sleeves, then project into the terminal sockets and are soldered or cemented in the terminal sockets. The advantage of ferritic material for the terminal sockets is that eddy current effects propagate especially well in these materials, such that inductive soldering can be done easily.  
           [0026]    To attach the insulation part in the outside housing, different techniques can be used. According to a preferred development of the invention, the insulation part is attached in the outside housing by stamping it. To attach the insulation part in the outside housing techniques, which are described for example in the German Utility Model 299 01 548, can be used. To ensure reliable sealing of the proximity switch, it is preferable that a seal, preferably an elastomer O-ring, is provided between the insulation part and the housing.  
           [0027]    The process for producing an electrical unit comprising a proximity switch, which has an outside housing and an insulation part, and a cable terminal part, which has a connecting part and a cable, is characterized by the following steps:  
           [0028]    providing the insulation part with a terminal element which runs through; its terminals which lead to the outside are made as terminal sockets,  
           [0029]    connecting the outside housing and the insulation part,  
           [0030]    attaching the cable in the connecting part,  
           [0031]    attaching the cable terminal part to the proximity switch and  
           [0032]    providing an electrically conductive connection of the ends of the wires of the cable to the terminal sockets of the terminal element,  
           [0033]    which is set forth in related U.S. patent application Ser. No. 09/770,228 and further is characterized in that an induction coil, through which current flows, is located in the outside area of the electrical unit in the vicinity of the terminal sockets, but at an axial distance to the end of the outside housing.  
           [0034]    By placing an induction coil in the outside area of the electrical unit, a simple and secure connection of the ends of the wires of the cable to the terminal sockets of the terminal element can be achieved by inductive soldering. If there are conductive materials located in the vicinity of the terminal sockets which are to be inductively heated, this leads to a limitation of the soldering capacity and to an undesirable heating of the conductive materials. This problem arises in the electrical unit mainly when the outside housing of the proximity switch is made of metal. To prevent unwanted heating of the outside sleeve of the proximity switch, the induction coil is located at an axial distance from the end of the outside housing. This axial distance of the induction coil to the outside housing of the proximity switch is preferably at least 5 mm. At this distance of the induction coil from the outside housing of the proximity switch, the soldering points, i.e., the terminal sockets, are heated adequately and it is necessary for the terminal sockets to project far out of the outside housing. In order to increase the desired heating of the soldering point, it is necessary that the magnetic field of the induction coil be kept away from the parts not involved in the inductive soldering, i.e. the outside housing of the proximity switch. In order to do this, according to one preferred embodiment of the process, the electrical unit is pushed through a conductive shielding sheet so that the shielding sheet is located between the end of the outside housing and the induction coil. The shielding sheet preferably has a distance of roughly 0.5 to 1.5 mm from the end of the outside housing and consists of aluminum which is easily handled and can be adapted very well to the respective circumstances.  
           [0035]    According to another alternative embodiment of the process set forth in related U.S. patent application Ser. No. 09/770,228, an especially high-degree of tightness and high mechanical strength of the electrical unit can be easily accomplished by reducing the diameter of the ring, which is part of the connecting part, by a multi-segment press after the connecting part has been pushed over the cable. The advantages which accrue with the use of this ring have already been addressed above. By using a multi-segment press, the diameter of the ring can be easily reduced, while ensuring that the concentricity of the ring is preserved when the diameter of the ring is reduced. In addition, the degree of pressing, i.e., the reduction in the diameter of the ring, can be adjusted to any amount using the multi-segment press according to the dimensions of the cable or connecting part. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]    [0036]FIG. 1 shows a proximity switch connected with a cable terminal part according to the first preferred embodiment of the invention in a section,  
         [0037]    FIGS.  2 A- 2 C illustrate the electrical unit as shown in FIG. 1 as an integral electrical unit and broken down into the proximity switch and the cable terminal part,  
         [0038]    FIGS.  3 A- 3 B show two different embodiments of the cap of a cable terminal part,  
         [0039]    FIGS.  4 A- 4 C show three different embodiments of the connecting part of the cable terminal part, each with one ring,  
         [0040]    FIGS.  5 A- 5 B showa preferred embodiment of part of the electrical unit, and in particular an alignment aid,  
         [0041]    FIGS.  6 A- 6 F show production of an electrical unit of the invention using six installation steps, and  
         [0042]    [0042]FIG. 7 shows an arrangement of an induction coil and a shielding sheet during inductive soldering of the ends of the wires of the cable with the terminal sockets of the terminal element.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    [0043]FIG. 1 shows an electrical unit comprising a proximity switch  1  and a cable terminal part  2  in the connected state. The proximity switch  1  has a sleeve-shaped outside housing  3  with one end face closed by a pot-shaped cover  4  and there is an insulation part  5  on the other end face. If the proximity switch  1  is an inductive proximity switch, there is a coil which is not shown here in the cover  4 . Alternatively, the one end face of the proximity switch  1  can also be closed by the outside housing  3  itself. The insulation part  5  is securely inserted sealed into the outside housing  3 . To do this, there is a seal  6 , preferably an elastomer O-ring, positioned between the insulation part  5  and the outside housing  3 .  
         [0044]    A terminal element  7  which is connected to electronics (not further shown) in the proximity switch  1  runs through the insulation part  5 . The externally routed terminals of the terminal element  7  are made as cup-shaped terminal sockets  8 . Joined to the proximity switch  1  is the cable terminal part  2  which has a cable  9  and a connecting part  10 . The cable  9  is of the type and form which the user of the proximity switch  1  desires and has ordered according to the particular requirements for the proximity switch  1 . The ends of the wires  11  of the cable  9  are stripped and tin-coated, inserted into the terminal sockets  8 , and are connected electrically to them by soldering.  
         [0045]    In the electrical unit of the invention, an especially high tightness and mechanical stability electrical unit is achieved by the cable terminal part  2  having a cap  12  through which the cable terminal part  2  is attached to the outside housing  3  of the proximity switch  1 . The cap  12  surrounds the connecting part  10  with the end of the cable  9  inserted therein. The connection between the cap  12  and the outside housing  3  of the proximity switch  1  is made via a catch connection  13 .  
         [0046]    Depending on the application of the proximity switch, and after choosing the material for the cap  12 , the attachment of the cap  12  to the outside housing  3  of the proximity switch  1  can also be established by screwing, welding, cementing or stamping. Preferably, the cap  12  is made of metal, especially high quality steel, and it is pressed onto the outside housing  3  which likewise is made of metal over the catch connection  13 .  
         [0047]    In the production of the electrical unit shown in FIG. 1 and FIG. 2A, according to one preferred embodiment of the invention, first the proximity switch  1  itself is completely finished, then checked, and then stored. FIG. 2B shows such a proximity switch  1  in the disassembled state. If the user orders a proximity switch  1  with a certain type of cable  9 , a corresponding cable terminal part  2  comprising the desired cable  9 , the connecting part  10  and the cap  12 , is shown in FIG. 2C, is joined to the proximity switch  1 . The cable terminal part  2  can either be pre-fabricated or fabricated only according to the user&#39;s order. If both the insulation part  5  and the connecting part  10  are made of plastic, a fixed connection of these two components to one another can be achieved by ultrasonic welding.  
         [0048]    [0048]FIGS. 3A and 3B show two embodiments of a cap  12 . In the preferred embodiment of the cap  12 , as shown in FIG. 3A, the end  14  of the cap  12  which faces away from the proximity switch  1  tapers conically. This results in that the connecting element  10 , which is preferably composed of an elastomer and has the function of the sealing element, is radially compressed by the conical part of the cap  12 , by which a good seal is ensured between the cable  9  and the connecting part  10 . Alternatively, the end  14  of the cap  12  facing away from the proximity switch  1  can also be made such that it has a constant, but smaller diameter than the end  15  of the cap  12  facing the proximity switch  1 . One such embodiment of the cap  12  is shown in FIG. 3B. Here, the diameter reduction of the end  14  of the cap  12  facing away from the proximity switch  1  is accomplished only after final installation of the electrical unit.  
         [0049]    The tightness of the electrical unit, which can be achieved by the cap  12  in conjunction with the connecting part  10 , is preferably further increased by the length of the cap  12  being dimensioned such that the connecting part  10  is axially compressed when the cap  12  is attached to the outside housing  3  of the proximity switch  1 . Axial compression of the connecting part  10  causes good sealing on the radial contact surfaces between the insulation part  5  and the connecting part  10  and between the connecting part  10  and the end of the cap  12 , as well as causes radial expansion of the connecting part  10 , by which good sealing on the axial contact surfaces is ensured between the insulation part  5  and the connecting part  10  and between the connecting part  10  and the cap  12 .  
         [0050]    In particular, FIGS. 3A and 3B moreover show that the cap  12  has several viewing holes  16  through which a display LED connected to the electronics of the proximity switch  1  is visible. Since the display LED  17  is surrounded at least in part by the insulation part  5 , the insulation part  5  is then made of transparent plastic.  
         [0051]    As is shown in FIG. 1, the connecting part  10  is not only pressed by the cap  12  onto the cable  9 , but the connecting part  10  has a ring  18  which is located in a groove  19  situated in the connecting part  10  and presses the connecting part  10  radially against the cable  9 . FIG. 4A illustrates the connecting part  10  and the ring  18  in the unassembled state in which the ring  18  has an inside diameter which is so great that the ring  18  can be pushed over the end  20  of the connecting part  10  into the groove  19 . In the mounted state of the ring  18 , which is shown in FIGS. 1 and 2C, the ring  18 , on the connecting part  10 , has an inside diameter which is smaller than the outside diameter of the connecting part  10  in the area of the groove  19 .  
         [0052]    After the connecting part  10 , the cable  9  and the ring  18  have been combined, the diameter of the ring  18  is reduced, preferably in a multi-segment press. By using a multisegment press, it is ensured that the concentricity of the ring  18  is preserved, and the diameter reduction of the ring  18 , which is preferably made of high quality steel, can be set very differently such that depending on the degree of pressing for different cable diameters the optimum strain relief for the respective cable  9  can be achieved. Preferably the reduction in the diameter of the ring  18  is roughly 25%.  
         [0053]    [0053]FIGS. 4B and 4C each show alternative possibilities for arrangement of the ring  18  relative to the connecting part  10 . In the embodiment shown in FIG. 4B, the ring is connected securely to the connecting part  10  such that the ring  18  is extrusion coated with plastic so that the connecting part  10  and the ring  18  are made in one part. In the embodiment shown in FIG. 4C, the ring  18  is located within the connecting part  10 , and thus, it is directly on the cable  9 .  
         [0054]    In order to facilitate joining of the cable terminal part  2  to a proximity switch  1 , particularly for insertion of the ends of the wires  11  into the terminal sockets  8  of the terminal element  7 , according to the preferred embodiment shown in FIG. 1, there is an alignment aid  21  for alignment of the ends of the wires  11  to the terminal sockets  8 . One preferred embodiment of such an alignment aid  21  is shown in FIGS. 5A and 5B. The alignment aid  21  is a separate component and has holes  22  with funnel-shaped openings  23 . One conical tip  24  provides for spreading of the individual wires  11  of the cable  9 . Alternatively to the construction of the alignment aid  21  as a separate component, the alignment aid  21  can also be integrated in the insulation part  5  or the connecting part  10 .  
         [0055]    In order to prevent problems as a result of misorientation of the cable terminal part  2  relative to the proximity switch  1 , both on the insulation part  5  and on the connecting part  10  there is provided coding which indicates the correct alignment of the two components relative to one another, i.e., that alignment in which the individual wires  11  of the cable  9  are inserted into the terminal sockets  8  which are intended for them. In addition, the insulation part  5  and the connecting part  10  each have a form which allows combining of these two components only in a single alignment. In this way, polarity reversal is reliably precluded when the proximity switch  1  is joined to the cable terminal part  2 .  
         [0056]    [0056]FIGS. 6A to  6 F show different steps of the installation sequence of the electrical unit of the invention. If there is an order from a user for a proximity switch  1  with a certain type of cable  9 , a corresponding length of the cable  9  is cut off the cable reel  25  and then on one end of the cable  9  first the cable insulation and then the insulation of the individual wires  11  are removed. After tin-plating the stripped ends of the wires  11 , the prepared cable  9  which is shown in FIG. 6B results. A corresponding cap  12  is then pushed onto this cable  9 , and the cable  9  is inserted into the connecting part  10 . At this point, the diameter of the ring  18 , which is located in the groove  19  of the connecting part  10 , is reduced in a multi-segment press, so that the cable terminal part  2  shown in FIG. 6C results, in which the cable  9  is securely connected via the ring  18  to the connecting part  10 .  
         [0057]    In the next step, then, the proximity switch  1 , which has been completely finished and checked, has the cup-shaped terminal sockets  8  of the terminal elements  7  filled with a solder paste. This is done, preferably, with a suitably dimensioned dispenser  26 . An electrically conductive connection between the stripped and tin-plated ends of the wires  11  of the cable  9  and the terminal sockets  8  is achieved in the proximity switch  1  with an induction coil  27  through which current flows and which is placed in the outside area of the electrical unit in the vicinity of the terminal sockets  8 at an axial distance from the end  28  of the outside housing  3 . In this state, current flows through the induction coil  27  which causes heating of the terminal sockets  8 , preferably composed of ferritic material, and thus, the solder and the tin-plated ends of the wires  11 , so that the wires  11  are soldered to the terminal sockets  8 .  
         [0058]    Finally, the cap  12  is pressed onto the outside housing  3  of the proximity switch  1  such that the catch connection  13  locks, and theLED  17  is located in the proximity switch  1  so that the LED can be recognized from the outside through at least one viewing hole  16  of the cap  12 .  
         [0059]    [0059]FIG. 7 shows another preferred arrangement of the electrical unit of the invention for inductive soldering of the tin-plated ends of the wires  11  of the cable  9  with the terminal sockets  8  of the terminal element  7 . In this embodiment, the electrical unit, which comprises the proximity switch  1  and the cable terminal part  2 , before pressing on the cap  12 , is pushed through a conductive shielding sheet  29  so that the end  28  of the metal-coated outside housing  3  is located on one side of the shielding sheet  29  and the terminal sockets  8  project through a hole  30  in the shielding sheet  29  on the other side of the shielding sheet  29 . The induction coil  27 , which is located in the vicinity of the terminal sockets  8 , is thus separated by the shielding sheet  29  from one end  28  of the metallic outside housing  3 . In this way, undesirable heating of the metallic outside housing  3  by the induction coil  27  is prevented. The shielding sheet  29  is preferably composed of aluminum, has a thickness of roughly 0.5 to 1.5 mm, preferably 0.75 mm, and is located at a distance of roughly 0.5 to 1.5 mm relative to the end  28  of the outside housing  3 .