Patent Publication Number: US-7221236-B2

Title: Waveguide communication system

Description:
The present invention relates in general to a system for transferring signals from a sender to a receiver, either the sender or the receiver, or both, being mobile. Specifically, the present invention relates to a communication system for use in an industrial apparatus for manufacturing products, of the type where a mobile actuator performs tasks at a range of locations, such as for instance picking up components in one location and placing the components in a different location. Such actuator needs to be given commands or control signals from a source in the fixed world. 
     In the following, the invention will be more specifically explained for a situation where a sender is fixed while a receiver is mobile. However, it is to be understood that the present invention is not restricted to such situation. In contrast, the present invention is likewise applicable in a situation where a sender is mobile while a receiver is fixed, and also in a situation where both the sender and the receiver are mobile. Further, it is possible to use the invention in a case of multiple mobile stations, each functioning as sender/receiver in multipoint communication system. 
     Conventionally, signals are transferred as electrical signal by electrical cables. However, the use of electrical cables has some disadvantages. 
     First, the electrical cable must be able to follow the movements of the receiver, so the cable must be mounted as a loose cable. 
     Second, because of the repeating movement of the receiver and thus of the repeated movement of the cable, the cable is vulnerable, and in fact it may eventually break. When this happens, the apparatus concerned must be shut down in order to repair the cable. Also, if signals do not reach the actuator because of a broken cable, it is possible that the actuator causes further damage to the apparatus. 
     Third, apart from the chance on failure, the moving actuator must exert mechanical forces on the cable in order to pull the cable along with the actuator, and such forces may affect the accuracy of positioning. 
     For these and other reasons, it is already known to use a wireless communication path from a control unit to an actuator. It is possible to use wireless communication in “open air”, but this involves the risk of interference by electromagnetic fields from other sources, and/or generating electromagnetic fields which may disturb other electronic components. In order to avoid this problem, a wireless communication path comprises a microwave RF signal guided by a waveguide. The waveguide is typically attached to the fixed world. A microwave signal is inputted into the waveguide at one end thereof. The movable actuator is provided with a coupler, movably associated with the waveguide, so that the coupler can pick up a signal from the waveguide within a range of positions. 
       FIG. 1  schematically illustrates a waveguide according to the state of the art. 
     A prior art waveguide  10  is a box-like structure with a rectangular cross-section, having a bottom  11  with a width W, sidewalls  12  and  13  with height H, and an upper wall  14 . The walls  11 ,  12 ,  13 ,  14  are electrically conductive; typically, they are made from iron or steel. A slot  15  runs in the longitudinal direction of the center of the upper wall  14 . The slot  15  is flanked by upright flanges  16 . The bottom  11 , and walls  12 ,  13 ,  14  enclose a waveguide chamber  17 , in which an RF wave can be generated by means not shown in  FIG. 1 . A pickup coupler, schematically shown as a square  19  in  FIG. 1 , is mounted on a support  18  which extends through the slot  15 , and which is movable in the longitudinal direction of the waveguide  10 , so that the coupler can travel the length of the waveguide  10 . The support  18  is associated with a movable actuator, and is capable of carrying signals from the coupler  19  to the actuator, which is not shown in  FIG. 1 . 
     This known waveguide  10 , invented by H. Dalichau and disclosed in, for instance, “Adapters and vehicles-couplers for slotted waveguide systems”, Frequenz 36 (1982), p.169–175, has some serious disadvantages. The most important disadvantage is that the state of the art waveguide  10  has a narrowband transfer characteristic and has especially to be designed for one predetermined carrier frequency. As such, in order to have a bandwidth less than an octave, the width W of the bottom  11  must be equal to λ, and the height H of the sidewalls  12  and  13  must be equal to λ/2, wherein λ is the wavelength of said predetermined carrier wave. 
     This limits the data transfer capacity of the wave guide. Further, since the carrier frequency is determined by the sender, different waveguides must be designed for different senders using different carrier frequencies. 
     Another problem relates to the size. At present, commercially available communication modules work at frequencies lower than 6 GHz. Then, the characterizing dimension W of the waveguide is larger than 5 cm. This means that the waveguide occupies a substantial amount of space within an apparatus. 
     An important objective of the present invention is to overcome the above-mentioned disadvantages. 
     Specifically, an objective of the present invention is to provide an improved waveguide which has smaller dimensions and has a broadband transfer characteristic. More particularly, the present invention aims to provide a waveguide capable of transferring waves with a frequency in the range of 1 GHz or lower to 6 GHz or higher. 
     According to an important aspect of the present invention, a waveguide comprises two parallel conductors, one being hollow and enclosing a waveguide chamber, the other being arranged inside this waveguide chamber. The hollow outer conductor confines the electromagnetic energy of the transferred signal substantially completely to the interior of said waveguide chamber. The hollow outer conductor has at least one slot, allowing a coupler to be introduced into said waveguide chamber, and to be displaced along the length of the waveguide, such as to pick up (or introduce) energy from (or to) the waveguide at any desired location along the length of the waveguide. 
     It is noted that so-called “leaky waveguides” exist, which are intentionally constructed such that a predetermined portion of the electromagnetic energy of the transferred signal leaks out towards the surroundings. Such leaky waveguide is typically implemented as a coaxial cable, having a hollow outer conductor and an inner conductor placed coaxially inside the outer conductor, the space between the inner conductor and the inner wall of the outer conductor being completely filled with a dielectric material. The outer conductor is provided with a plurality of small openings, in a regular pattern, through which electromagnetic field can leave the interior of the outer conductor. The openings have dimensions typically smaller than the wavelength. Such a leaky waveguide, too, allows pick up of signal at any desired location along its length, but in this case by using an antenna outside the waveguide. A typical example of an application of such leaky waveguide is in a tunnel, for providing radio signals to cars. The waveguide is, however, not suitable for the introduction of a travelling coupler into the interior of the waveguide. 
    
    
     
       These and other aspects, features and advantages of the present invention will be further explained by the following description of preferred embodiments of the waveguide according to the present invention with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
         FIG. 1  schematically shows a perspective view of a prior art waveguide; 
         FIG. 2  schematically illustrates some basic elements of a waveguide according to the present invention; 
         FIGS. 3A–3E  are cross-sections of inner conductors of a waveguide according to the present invention, illustrating several design possibilities; 
         FIGS. 4A–4E  are cross-sections of outer conductors of a waveguide according to the present invention, illustrating several design possibilities; 
         FIGS. 5A–5D  are cross-sections of outer conductors of a waveguide according to the present invention, illustrating several design possibilities; 
         FIGS. 6A and 6B  are a cross-section and a longitudinal partial section, respectively, of an embodiment of a waveguide according to the present invention; 
         FIG. 7A  is a longitudinal section of an end portion of a shield conductor, schematically illustrating a terminator; 
         FIG. 7B  is a perspective view of an end portion of a waveguide, schematically illustrating another terminator; 
         FIG. 7C  is a longitudinal section of an end portion of a waveguide, on an enlarged scale, schematically illustrating a feed through connector; 
         FIG. 8  is a perspective view schematically illustrating a waveguide of strip line type; 
         FIGS. 9A–9C  are perspective views schematically illustrating several embodiments of a coupler; 
         FIG. 10  is a perspective view schematically illustrating the use of a coupler with a waveguide; 
         FIGS. 11A and 11B  schematically illustrate an apparatus with a waveguide communication system in accordance with the present invention. 
     
    
    
     The present invention proposes a multiple conductor waveguide  100  comprising a first conductor  110  enclosed in a second conductor  120 , also indicated as shield conductor, that also provides a shielding of the electromagnetic field. 
       FIG. 2  schematically illustrates some basic elements of a first embodiment of a multiple-conductor waveguide  100  proposed by the present invention. In this first embodiment, the shield conductor  120  in general has the shape of a box extending around the first conductor  110 . The second conductor  120 , in the shape of a hollow box extending around the first conductor  110 , thus defines an inner space or waveguide chamber  121  in which the first conductor  110  is located.  FIG. 2  also illustrates that the second conductor  120  is provided with a longitudinal slot  122 , of which the function will be explained later. 
     In use, a signal will be applied to the first conductor, and travels the length of the conductors, causing an electromagnetic field in the inner space  121 . As will be clear to a person skilled in the art, the electromagnetic field will be confined within this interior  121 , i.e. no or very little electromagnetic field will be generated outside the second conductor  120 , so no or very little interference with other electronics will be caused. Conversely, outside electromagnetic fields will not penetrate into the interior  121 , so that no or very little interference from outside electromagnetic fields will result. 
       FIGS. 3A–D  illustrate some design details of the shape of the first conductor  110 . As illustrated in  FIG. 3A , the first conductor  110 A may have a circular cross-section. As illustrated in  FIGS. 3B–3E , the first conductor may also have at least one flat side surface  111 . In the embodiment  110 B illustrated in  FIG. 3B , the first conductor  110 B has a substantially D-shaped cross-section with only one flat side surface  111 . In a third embodiment illustrated in  FIG. 3C , the first conductor  110 C has a rectangular square cross-section, having four substantially flat side surfaces. In a fourth embodiment illustrated in  FIG. 3D , the first conductor  110 C has a square cross-section, having four substantially flat side surfaces. In a fifth embodiment illustrated in  FIG. 3E , the first conductor  110 D has a substantially triangular cross-section, having three substantially flat side surfaces. 
       FIG. 4  illustrates some design elements of the second conductor  120  (the slot  122  being omitted in  FIG. 4  for sake of simplicity). As illustrated in  FIG. 4A , the second conductor  120 A may have a substantially rectangular or even square cross-section, similar to the cross-section of the state of the art waveguide  10  illustrated in  FIG. 1 . However, the design of the second conductor  120  is no longer limited to a rectangular design. As illustrated in  FIG. 4B , the second conductor may have a substantially circular shape. As illustrated in  FIG. 4C , the second conductor may have a substantially square shape. As illustrated in  FIG. 4D , the second conductor  120 D may have a substantially D-shaped cross-section. As illustrated in  FIG. 4E , the second conductor  120 E may have a substantially triangular cross-section. 
     In fact, the second conductor  120  may have any suitable shape, wherein the main design criterion will be the fact that the second conductor should envelope the first conductor  110  such that the field lines are confined to the interior  121  of the second conductor  120 . Design choices relating to the shape of the second conductor  120  will now be made mainly with a view to manufacturing. 
     In this respect, it is pointed out that, in the state of the art waveguide  10  as illustrated in  FIG. 1 , there are no design options relating to the shape of the waveguide: as mentioned, it must have a rectangular cross-section having a width W twice as large as the height H and being equal to the wavelength λ of the design carrier frequency. In contrast, no such limitations apply to the second conductor  120  of the multiple-conductor waveguide  100  proposed by the present invention. Not only is it possible to use, basically, any shape of cross-section, but also the dimensions of the cross-section can be chosen much smaller. 
     As already mentioned with reference to  FIG. 2 , the second conductor  120  of the multiple-conductor waveguide  100  of the present invention comprises at least one longitudinal slot for allowing introduction of a coupler, examples of which will be described later. For sake of convenience, such slot has not been shown in  FIGS. 4A–4E . Such slot  122  is illustrated in  FIG. 5 , in which  FIGS. 5A–5D  illustrate several design possibilities. Details of the slot  122  will be explained in  FIG. 5  in conjunction with the rectangular embodiment  120 A illustrated in  FIG. 4A  and the triangular second conductor  120 E illustrated in  FIG. 4E , but it should be clear that the same principles apply to all other types of second conductors. 
     As illustrated in  FIG. 5A , the slot  122  may be located symmetrically, in the center of a side wall  123  of the second conductor  120 . However, according to the present invention, the second conductor  120  is not limited to this design, as is the prior art waveguide  10  illustrated in  FIG. 1 . As illustrated in  FIG. 5B , the slot  122  may also be located near a corner of the profile, i.e. the slot  122  may be arranged near the edge of a side wall  123 , adjacent a neighboring side wall  124 . In fact, the slot  122  may be located at any suitable position on a side wall. 
     The slot  122  may be very narrow, depending on the size of a coupler to be introduced in the slot  122 . If the slot  122  is sufficiently narrow, an electromagnetic field having a frequency in the range considered (about 1 GHz to about 6 GHz or even higher) hardly passes such a slot. A further improvement in this respect can be offered by arranging flanges  125 ,  126 , extending substantially parallel to each other on opposite sides of the slot  122 . Such flanges  125 ,  126 , may be arranged on opposite sides of a slot  122  in the center of a wall  123  as illustrated in  FIG. 5A ; in that case, such flanges will both be arranged substantially perpendicular to said sidewall  123 . This is not illustrated separately. In the embodiment illustrated in  FIG. 5C , where the slot  122  is located in a corner of the profile, the flanges may be arranged such that a first flange  125  extends in line with the adjacent side wall  124  while the second flange  126  extends in parallel to the first mentioned flange  125 . 
     As illustrated in  FIG. 5C , the flanges  125 ,  126  may extend outwards from the second conductor  120 . Preferably, however, the flanges  125 ,  126  may extend inwards, as illustrated in  FIG. 5D . As can be seen from  FIG. 5D , the slotted second conductor  120  now effectively comprises only one additional flange  126  extending from the edge of said side wall  123  into the interior  121 , in parallel to said adjacent side wall  124 . The portion of this adjacent side wall  124  which overlaps with said additional flange  126  now effectively performs the function of flange  125 . 
     An important advantage of the embodiments illustrated in  FIGS. 5C and 5D  when comparing with the embodiment of  FIG. 5A  when provided with flanges, is that the embodiments of  FIGS. 5C and 5D  are more easy to produce. Typically, production will involve folding a box-like structure from a flat sheet or plate of metal, the side walls and flanges being produced by folding this sheet or plate of metal. In the case of  FIG. 5C , the first flange  125  does not involve a folding operation, since it is formed as a simple extension of the adjacent side wall  124 . In the case of  FIG. 5D , the further advantage is achieved that the flanges do not project outwards from the shield conductor  120 , and providing the first flange  125  does not involve addition of material. 
     An important advantage of the embodiments illustrated in  FIGS. 5C and 5D  is that they provide a better confinement of the electromagnetic field within the interior  121 , because the electromagnetic field decays exponentially between the flanges, whose distance is less than half the wavelength. This advantage applies in the embodiment of  FIG. 5D  and in the embodiment of  FIG. 5C . 
     In a special embodiment, the second conductor  120  has a rectangular shape (such as illustrated in  FIG. 4A ), having two opposite long sidewalls and two opposite short sidewalls, wherein the slot  122  is arranged in one of the short sidewalls. Now the length of the short sidewall may be equal or only slightly larger than the width of the slot  122 , so that effectively the slot  122  occupies the entire length of the short sidewall. The second conductor  120  now might be considered as having only three sidewalls in a U-shaped configuration, wherein the two opposite long sidewalls effectively perform the function of flanges as mentioned above. 
     The first conductor  110  in the interior  121  of the second conductor  121  may be hanging free, suspended at its ends. Depending on the cross-sectional shape of the first conductor  110 , among else, the first conductor  110  may have sufficient stiffness and/or may be subjected to tension forces in order to be directed according to a straight line as much as possible, if the longitudinal shape of the waveguide is straight. However, in practice a more or less degree of sagging will then hardly be avoidable. In order to avoid such sagging, it may be desirable to arrange one or more supports in the interior  121 , to support the first conductor  110  with respect to the second conductor  120 . However, such supports will locally involve a change in impedance, which may cause reflections, which is undesirable. Preferably, the impedance of the waveguide is as constant as possible over its length. Therefore, in case a support for the first conductor  110  is desirable, such support preferably is a continuous support, i.e. extending over the entire length of the first conductor  110  with continuous properties. By way of example,  FIGS. 6A and 6B  illustrate an embodiment of a waveguide  100  comprising a second conductor  120  as illustrated in  FIG. 5D  and a first conductor  110 C as illustrated in  FIG. 3C , the first conductor  110 C being supported by a continuous support  130  of a non-conductive material, such as for instance plastics. Alternatively, a discontinuous support can be used as long as the dimensions of and distances between the support structures are significantly smaller than the wavelength. 
     It is not desirable to have the second conductor  120  open-ended.  FIGS. 7A–7C  illustrate several possibilities for an end construction of the second conductor  120 . 
     As illustrated in  FIG. 7A , the second conductor  120  may be ended by a conductive end wall  140 , electrically connected to the longitudinal walls of the second conductor  120 . Such end wall  140  may be implemented as a plate welded to the ends of the walls of the second conductor  120 , but the end wall  140  may also be implemented as a substantially cylindrical cap having a bottom  140  and a cylindrical side wall  141 , having a contour corresponding to the contour of the second conductor  120 , as schematically illustrated in  FIG. 7A . Such conductive end wall  140  will substantially reflect travelling electromagnetic fields, and will therefore also be referred to as reflector  140 . 
     In case it is desirable to avoid such reflections, the end construction may comprise a terminator  150  having an impedance matching the impedance of the waveguide  100 . Alternatively to a terminator, the signal can be extracted from the construction for example via a connector and used otherwise, for example to be inserted into another wave-guide. Multiple wave-guides can be connected in a chain-configuration and be used as the back bone of a network with multiple mobile couplers in different waveguides.  FIG. 7B  schematically illustrates an example of a waveguide  100  having a first conductor  110  with a substantially circular cross-section, as the first conductor  110 A illustrated in  FIG. 3A , and a second conductor  120  having a substantially circular profile, as illustrated in  FIG. 4B . The terminator  150  in this example comprises a plurality of resistors mounted in a star-like configuration, each resistor  151  being substantially radially directed between the first conductor  110  and the second conductor  120 , having one terminal connected to the main conductor  110  and having the other terminal connected to the second conductor  120 , wherein the resistors  151  are distributed evenly around the main conductor  110 . In effect, all resistors  151  are connected in parallel between the main conductor  110  and the second conductor  120 , and present an effective resistance, as will be clear to a person skilled in the art, which should match the impedance of the waveguide  100 . 
     Instead of a plurality of individual resistors  151 , the terminator  150  may also comprise an annular-shaped conductor arranged between the first conductor  110  and the second conductor  120 , this annular resistor presenting the matching resistance between first conductor  110  and second conductor  120 . Also microwave absorber materials can be used to terminate the waveguide. 
       FIG. 7C  illustrates, on an enlarged scale, a modification of the embodiment illustrated in  FIG. 7A . Again, the end construction comprises a conductive plate  140  extending substantially perpendicular to the longitudinal direction of the waveguide  100 . In the embodiments illustrated in  FIG. 7C , the end wall  140  is provided with a feed through connector  160  of the coaxial type. The feed through connector comprises a cylindrical outer conductor  161  with a circular profile, provided with screw thread  162  at one end and a mounting flange  163 . An inner conductor, also called pin,  164  extends coaxially within the outer conductor  161  and is connected to the first conductor  110 . An end portion  166  of the first conductor  110  is tapered such as to bring the cross size of the first conductor  110  down to the cross size of the pin  164  in order to reduce reflections and undesired effects, such as fringing electromagnetic fields. A dielectric insulator  165  is arranged between the pin conductor  164  and the outer conductor  161 . The end plate  140  is provided with a hole  146 , through which at least the pin conductor  164  of the connector  160  extends. The connector  160  is suitable for connecting a coax cable (not shown) carrying a signal to be transferred, wherein a connector of the coax cable will be screwed onto the connector  160 . In the interior  121  of the second conductor  120 , an end of the first conductor  110  will be connected to the pin conductor  164  of the connector  160 , as illustrated. 
     The waveguide  100  is preferably implemented as a rigid, self-supporting structure, directed according to a straight line. However, this is not essential, and alternatives may even be advantageous in some cases. For instance, it may be advantageous that the waveguide follows at least partially a curved path. Also, it may be advantageous if the waveguide is bendable, in order to be able to adapt its shape to the actual location of implementation. 
     Hereinafter, a second embodiment of the multiple-conductor waveguide will be explained with reference to  FIG. 8 . The second embodiment of the multiple-conductor waveguide  200  is of microstrip type. This microstrip waveguide  200  comprises a strip  201  of a dielectric material, having a first surface  202  (in this case: the bottom surface) carrying a strip  210  of a conductive material. This first or bottom surface  202  will hereinafter also be referred to as front surface. A second surface  203  opposite the front surface  202 , hereinafter referred to as back surface  203 , carries a second strip of conductive material  204 . This second strip  204 , which will also be referred to as back conductor  204 , has a width wider than the width of the strip conductor  210 , which will also be referred to as the first conductor, and preferably equal to the transversal dimensions of the back surface. In a preferred embodiment, the first conductor  210  and back conductor  204  are implemented as layers of a conductive material, preferably copper, arranged on the dielectric strip  201 . More preferably, the dielectric strip  201  with the opposite conductors  210 ,  204  is implemented as a strip of PCB. 
     In order to reduce leakage of electromagnetic field from the microstrip waveguide, a shield conductor  205  is located opposite the first conductor  210 , at a suitable distance. 
     Preferably, but not necessarily, the back conductor  204  is electrically connected to the shield conductor  205  by means of a side conductor  207 . 
     This side conductor  207  may be implemented as a strip of metal. The side conductor  207  may be soldered to the back conductor  204  and the shield conductor  205 . Then, the combination of back conductor  204 , side conductor  207 , and shield conductor  205  will form a combined conductor having a substantially U-shaped cross-section, with the first conductor  210  being located in an interior space  221  between the two legs  204 ,  205  of this U-shaped combination. The interior space  221  is accessible from the side opposite the side conductor  207  through a slot  222 . Further, the side conductor  207  may serve to keep the strip conductor  201  and the shield conductor  205  at a predetermined distance from each other, with a gap  209  between them. 
       FIGS. 9A–9C  illustrate several embodiments of a coupler according to the present invention, specifically suitably for introduction into a slot  122  of the outer waveguide conductor  120  and coupling with the inner waveguide conductor  110 . A coupler  300  illustrated in  FIG. 9A  has a general planar shape. The coupler  300  comprises a carrier plate  301  of a dielectric material, having a front surface  302  and a back surface  303 , and two opposite side edges  304 ,  305 , intended to be placed in the longitudinal direction of the waveguide. On the front surface  302 , a coupling conductor  320  is arranged. The coupling conductor  320  may advantageously be implemented as a conductive layer on the front surface  302 . On the back surface  303 , a back conductor  309  is arranged. The back conductor  309  covers a large area of the back surface  303 , and preferably covers the entire back surface  303 . Advantageously, the back conductor  309  is formed as a metallic layer on the back surface  303 . Advantageously, the carrier plate  301  with the coupling conductor  320  and the back conductor  309  may be implemented as a double-sided PCB. 
     Preferably, and as illustrated in  FIG. 9A , the coupler  300  comprises a connector  310  for connecting a coaxial cable (not shown), which advantageously is mounted at a side edge  304 . The coaxial connector  310  comprises an inner conductor electrically connected to the coupling conductor  320 , and a cylindrical outer conductor, which is electrically connected to the back conductor  309 . The connector  310  may be mounted, as illustrated, with its central axis  311  in the plane of the front surface  302 . 
     The embodiment illustrated in  FIGS. 9B–9C  also may have such connector  310 , but this connector is not shown in the  FIG. 9B–9C  for the sake of simplicity. 
     In the following, a coupler in general will be indicated with reference numeral  300 ; in order to specifically refer to specific embodiments illustrated in  FIGS. 9A–9C , these embodiments will be distinguished by adding the character A, B, C, respectively. 
     In the couplers  300 , the coupling conductor  320  is implemented as a strip line, i.e. a flat strip of conductive material, typically copper, having a predetermined width and a predetermined thickness. In the coupler  300 A illustrated in  FIG. 9A , the coupling conductor  320 A has a substantially L-shaped contour, comprising a leg portion  321  and a foot portion  322 . The longitudinal direction of the foot portion  322  is substantially parallel to the second side edge  305 , opposite the first side edge  304  at which the coaxial connector  310  is mounted. The leg portion  321  has its longitudinal direction substantially aligned with the inner conductor  311  of the coaxial connector  310 . The width and thickness of the leg portion  321  and foot portion  322  are chosen such that the characteristic impedance of the coupler  320  is equal to the characteristic impedance of the cable to be connected to the connector  310 , which typically will be 50 Ohm, although other standard impedances are also known. 
       FIG. 10  is a perspective view illustrating the use of a coupler  300  in conjunction with a waveguide  100  of the present invention. In use, the coupler  300  is inserted into the slot  122  of the second conductor  120 , such that the foot portion  322  of coupling conductor  320  faces the first conductor  110  of the waveguide  100 . The second side edge  305  may take reference to a guide member, in this case a side wall  127  of the second conductor  120 . The coupler  300  can be displaced in the slot  122  of the second conductor  120 , as indicated by arrow A, in which case the coupling foot portion  322  of the coupling conductor  320  is displaced along the first conductor  110  of the waveguide, the mutual distance between this coupling foot portion  322  and the first conductor  110  of the waveguide remaining constant. 
     In the case of the coupler picking up signal from the waveguide, the coupling foot portion  322  of the coupling conductor  320  will pick up part of the electromagnetic field generated by the first conductor  110  of the waveguide, and this will be transferred to the connector  310  for further processing. Similarly, in the case of the coupler introducing signal into the waveguide, the first conductor  110  of the waveguide will pick up part of the electromagnetic field generated by the coupling foot portion  322  of the coupling conductor  320 , and this will be transferred along the first conductor  110  of the waveguide for further processing. During and after displacement of the coupler  300  in the longitudinal direction of the waveguide, the coupling area of the coupling conductor  320  is determined by the length D of its foot portion  322  and no physical contact occurs between the first conductor  110  of the waveguide and the coupling conductor  320 . 
     In order to keep the mutual distance between the first conductor  110  of the waveguide and the coupling conductor  320  constant, external supports not shown in the Fig. may be provided. Such supports should preferably be arranged such as to assure that the coupling conductor  320  stays free from flange  126  of the second conductor  120 , while preferably also assuring that the back conductor  309  stays free from side wall  124  of the second conductor  120 . If desired, one or more guiding rails  128  may be arranged on an inner wall  127  of the second conductor  120 , in order to effectively guide the second side edge  305  of the carrier plate  301  in order to avoid any possible movement of the carrier plate  301  in a direction perpendicular to the front surface  302 . 
     The design should be such that electrical contact between the conductive parts of the coupler  300  on the one hand, and the conductive parts of the waveguide  100  on the other hand, is avoided. This applies specifically to the coupling conductor  320 , but preferably also to the back conductor  309 . In a possible embodiment, the width of the slot  122  of the second conductor  120  is slightly wider than the thickness of the coupler  300 , so that there is little play in a direction perpendicular to the surface  302  of the coupler  300 . However, it is also possible that the width of the slot  122  of the second conductor  120  corresponds to the thickness of the coupler  300 , so that the coupler  300  is supported and guided by the flanges of the outer waveguide conductor. 
     Electrical contact between the leg portion  321  of the coupling conductor  320  on the one hand, and the flange  126  of the outer waveguide conductor  120  on the other hand, can be prevented in various ways. In the embodiment shown in  FIG. 9A , said leg portion lies exposed on the front surface  302 . Alternatively, said leg portion  321  may lie in a recessed portion or groove (not shown for sake of simplicity). The same applies to the foot portion  322  of the coupling conductor  320 , in order to prevent contact with the flange  126  or with the first waveguide conductor  110 . 
     Also, an insulating layer (not shown for sake of simplicity) may be applied over the coupling conductor  320 , or over the entire front surface  302  of the coupler  300 . 
     Also, an insulating layer (not shown for sake of simplicity) may be applied over the surface of the flange  126  facing the coupler  300 , or over the entire surface of the coupler  300 . 
     Electrical contact between the foot portion  322  of the coupling conductor  320  on the one hand, and the inner waveguide conductor  110  on the other hand, can be prevented in various ways. In the embodiment shown in  FIG. 9A , said inner waveguide conductor  110  is arranged at a higher level than the flange  126  of the outer waveguide conductor  120 . Also, an insulating layer (not shown for sake of simplicity) may be applied over the surface of the inner waveguide conductor  110  facing the coupler  300 . It is even possible that the inner waveguide conductor  110  is completely embedded in the non-conductive support material  130 . 
     In those cases where electrical contact is prevented by insulating material or by a recessed arrangement, the coupler  300  may physically bear against the inner waveguide conductor  110  and/or the outer waveguide conductor  120  for guidance. 
     The coupler  300 A illustrated in  FIG. 9A  is sensitive mainly to an electromagnetic field travelling in one direction of the waveguide.  FIG. 9B  shows a modification  300 B of the coupler  300 A, which is sensitive to waves travelling in any direction in the waveguide. In the coupler  300 B, the coupling conductor  320 B has a substantially T-shaped contour, having a leg portion  321  and two opposite foot portions  322  and  323 . 
     The coupler  300 C illustrated in  FIG. 9C  has a substantially Δ-shaped contour. The coupling conductor  320  is symmetrical with respect to a center line  330 , substantially perpendicular to the second side edge  305 . Similar to the second embodiment  300 B illustrated in  FIG. 9B , this third embodiment  300 C is sensitive to waves travelling in any of the longitudinal directions of the waveguide. A common connection portion  331  divides into two branches  332 A,  332 B, each branch  332 A,  332 B comprising a foot portion  333 A,  333 B, respectively, having its longitudinal direction substantially parallel to the second side edge  305 , these foot portions  333 A,  333 B each having a length D and terminating at a distance d from each other. At their ends facing away from each other, the coupling portions  333  are connected to the common connection portion  331  by leg portions  334 , each leg portion  334  having a first leg portion  335  directly adjacent the connection portion  331 , the first leg portion  335  having a length equal to λ/4 and having a characteristic impedance equal to √2 times the characteristic impedance of the connection portion  331 , whereas the remaining portion of the leg portion  334  and the coupling foot portion  333  each have a characteristic impedance equal to the characteristic impedance of the connection portion  331 . 
     With respect to the three exemplary embodiments  300 A,  300 B,  300 C of the coupler according to the present invention, the coupler  300 A represents the easiest design and the smallest dimensions. 
     The couplers  300 B and  300 C are examples of bidirectional couplers, having symmetrical structures. 
     Further, it is noted that the waveguide and coupler as illustrated are suitable for use in a wide range of operating frequencies. This applies also to the Δ-shaped coupler  300 C illustrated in  FIG. 9C , although to a lesser extent since the length of first leg portions  335  should be determined in relation to the operating frequency. Further, since the coupling efficiency depends on the length of the coupling foot portion ( 322 A;  322 B+ 323 B;  333 A,  333 B) of the coupling conductor  320  in relation to the operating frequency, it is possible to optimize coupling by adapting said length to a design operating frequency (a good value is about λ/4), although in this respect the couplers perform well also in a wide band around the design operating frequency. In the third embodiment  300 C, the distance d between the two foot portions  333 A,  333 B should be made small in relation to the design operating frequency (preferably a fraction of λ). 
     Further, it is noted that the mutual distance between first conductor  110  and strip conductor  322  can be optimized for optimal coupling efficiency, although this distance is not critical. Generally, the smaller the distance the better the coupling. However, if the distance is made too small, the properties of the waveguide itself are disturbed. One can conclude that there is an optimal distance between the coupler and the waveguide for each application or a range of distances where the performance is sufficiently good. 
       FIG. 11A  schematically illustrates an apparatus  400 , such as an industrial manufacturing apparatus, comprising a command unit  402  and an actuator  403 , wherein in this example the actuator  403  is mobile, as indicated with arrow A. Signals from command unit  402  to actuator  403  are transferred through a waveguide communication system  401 , which comprises a waveguide  100 ;  200  as discussed above and at least one coupler  300  as discussed above, slideably fitting to said waveguide  100 ;  200 . 
       FIG. 11B  schematically illustrates an apparatus  410 , such as an industrial manufacturing apparatus, comprising a detector  412  and a receiver  411 , wherein in this example the detector  412  is mobile, as indicated with arrow A. Signals from detector  412  to receiver  411  are transferred through a waveguide communication system  401 , which comprises a waveguide  100 ;  200  as discussed above and at least one coupler  300  as discussed above, slideably fitting to said waveguide  100 ;  200 . 
     It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that various variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
     For instance, in the above examples, the second conductor of the multiple-conductor waveguide of the present invention is illustrated as having one longitudinal slot for allowing introduction of a coupler. However, it is also possible that the second conductor of the multiple-conductor waveguide is provided with two or even more longitudinal slots, each such slot allowing introduction of a coupler. Then, respective couplers introduced in respective slots can be moved over the entire length of the waveguide, irrespective of each others position, because couplers introduced in respective slots can now pass each other. 
     Further, in the above examples, the coupler is illustrated as being substantially plate-shaped. However, it is also possible to use couplers of a different design, for instance a wire-type design. 
     In the above, it has been explained how a waveguide communication system can be designed, comprising a multi-conductor waveguide and a coupler sliding along such waveguide, such that a predetermined coupling conductor ( 322 ) couples with the first conductor of the waveguide. Further, a new design for a waveguide has been described, especially suitable for use in such a waveguide communication system, and a new design for a coupler has been described, especially suitable for use in such a waveguide communication system. However, the basic idea of the present invention, i.e. the use of a coupler to slide along a multi-conductor waveguide, is considered new and inventive per se, even when practiced with a multi-conductor waveguide known per se, because up to date a multi-conductor waveguide has never been used in the inventive way as proposed by the present invention. This applies specifically to a multi-conductor waveguide of microstrip type. With reference to  FIG. 8 , a bare microstrip type multi-conductor waveguide essentially consists of the first conductor  210  and the back conductor  204 , i.e. without shield conductor  205  and without side conductor  207 . The basic idea of the present invention can very well be practiced with such a bare microstrip type multi-conductor waveguide.