Abstract:
A measuring-cable position sensor is provided in a pressure tight, in particular explosion protected manner, wherein all functional elements besides the cable drum and the measuring cable are located in a pressure tight housing, which is stable enough, so that also in case of an explosion in the interior of the pressure-tight housing section no ignition spark can get through the gaps of the housing to the outside.

Description:
CROSS-REFERENCE RELATED APPLICATION 
   This application claims priority to German Application No. 102005037862.5 filed Aug. 10, 2005. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
   Not Applicable 
   INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention regards position sensors operating according to the distance measuring cable principle. 
   Hereby, a measuring cable, mostly wound onto a cable drum only in one layer, pre loaded in wind up direction, is pulled off and the revolutions or partial revolutions during the pulling off are precisely measured with a rotation signal sensor, typically an angle sensor. 
   Subsequently, an angle sensor is always briefly mentioned instead of a rotation signal sensor without limiting the invention thereupon. 
   In order to make such a kind of position sensor resistant against external pressure and thereby enabling also a deployment e.g. under water, the main problem is that the cable intake into the housing inevitably is not tight. 
   In order to operate such a position sensor also in an explosion-sensitive environment, the measuring cable, the cable drum, and the cable intake can be made from a non-spark generating material, in particular stainless steel. However, the risk of spark generation still exists at the electrical components, that is, at the angle sensor and the processing electronics. 
   Mechanical spark avoidance could be performed at the flat spiral spring but this is very complicated. 
   Spark avoidance at the electrical units, such as, by limiting the choice of the electrical components to such with current and voltage requirements so low that spark generation is not possible any more, would lead to a poor signal quality. 
   Housing the potentially spark generating electrical end electronic components in a cavity and filling the remaining cavity with hardening filler material, a non-hardening gel, or bulk material would deny housing the flat spiral spring in this cavity. 
   BRIEF SUMMARY OF THE INVENTION 
   It is the objective according to the present invention to provide a position sensor according to the distance measuring cable principle, pressure tight, in particular explosion proof, without having to abandon the design principles for such sensors which have been proven for decades. 
   This objective is accomplished through providing all parts of the position sensor, besides the cable drum and the measuring cable, with encapsulation against external and/or internal pressure, in the latter case in particular explosion proof. 
   The principle of interior pressure-tight encapsulation for explosion proofing according to the European Standard 50018 provides for allowing an explosion of the ignitable mixture initially present in the interior of the pressure-tight encapsulation, whereby, however the housing is provided in a way that, in this case, no ignition capable spark can penetrate the housing towards the outside, whereby the gaps in the housing are safe against ignition passage, and the housing is provided stable enough so that even through the pressure of the explosion in the interior it does not change in a way that makes the existing gaps lose their safety against ignition passage. 
   This is accomplished herein through the walls being stable enough and the gaps being narrow and long enough relative to the deformability of the walls. 
   The measuring cable and the space where the measuring cable is wound onto the cable drum can only be made dust tight but hardly pressure tight, due to the pass-through of the measuring cable to the outside. 
   Safety against ignition passage from the inside to the outside can be achieved under certain conditions by providing an annular gap around the measuring cable opposite to a cable outlet long enough relative to its width to provide safety against ignition passage. This is why the cable inlet is provided preferably as a radially protruding cable inlet turret with extended length, and wherein additionally under certain conditions one or several brushes for cleaning the measuring cable can be located. These brushes, however, have to be provided in a manner that prohibits static electricity buildup relative to the cable sliding by to avoid generating sparks. 
   However, through locating all other moveable and non-moveable but spark generating components, that is, a flat spiral spring, an angle sensor and maybe the processing electronics, in a pressure-tight housing, which is in particular pressure tight enough so that it is explosion proof, only on the parts located outside the housing, like the measuring cable and the cable drum, spark avoidance by design has to be accomplished e.g. through the use of stainless steel and additional plastic coating and generally through the avoidance friction of metal on metal. 
   The explosion-proof pressure-tightness hereby is accomplished, in particular, through running a shaft that leads from the pressure-tight housing section into the non-pressure tight housing section through this housing in an explosion proof manner, as well as possibly the cable that leads out of the pressure-tight housing. 
   Also, a non-contact data transfer, instead of a cable pass through, is possible here. Hereby, the signal can be transmitted through the closed housing wall e.g. via electromagnetic waves, whereby no pressure, or other tightness problems, can occur at this location. The energy transfer into the sensor from the outside can also be performed through the closed housing wall, e.g. also via electromagnetic waves, thereby also the operation of the processing electronics inside the sensor can be performed without a current carrying cable through the housing wall. 
   Preferably the whole pressure-tight housing section, in particular the whole housing, will be made from stainless steel for the said reasons. 
   Proof against explosion is furthermore accomplished through over-sizing the pressure-tight housing in a manner that, in case of an explosion inside the housing, the housing does not get deformed enough so that the gaps leading out of the housing would not be proof against ignition passage anymore. In particular, the housing has to be able to stand a pressure of far more than 100 bar. 
   When subsequently pressure tightness is mentioned, it has to be differentiated depending on the application, this means regarding pressure tightness against external or internal pressure. Also regarding the direction of the pressure, a differentiation has to made and respective seal elements etc. for the respective pressure direction have to be provided, in case of pressure tightness in both directions, this means from the inside out and also the outside in, if necessary also two seal elements have to be provided at the same sealing location, whereby each seals in one direction. 
   A particularly simple and still pressure-tight design can be accomplished through putting a head cover over a tubular, or pot shaped, housing body, wherein at least the interior diameter is rotationally symmetrical, preferably also the exterior diameter, which overlaps the tubular housing part through reaching into it or through exterior overlap so that the axial length of an annular gap forming in between relative to the width of the gap is large enough to be safe against ignition passage. 
   In case a pot-shaped part is used as a housing body, only one such annular gap is created. When using a tubular housing body with a head cover sealing on both sides, two annular gaps exist, whereby any opportunity to reduce such gaps or make them smaller statically helps to reduce risk. 
   The pressure tightness is accomplished in a simple manner by providing an annular gap between a head cover and the housing wall as a threaded connection and by providing a seal, preferably an O-ring, at one end of the actual axial extension of the thread. Hereby, only an axial extension of the thread, not also the one of the O-ring area, is considered gap length. 
   Through this design, a head cover can be tightened with a high torque relative to the housing wall, which provides for a high-pressure resistance in combination with a sufficient wall thickness of the housing parts. In one of the head covers, preferably in the center, an opening is provided to insert a cable grommet. The cable grommet, which by itself has to be proof against ignition passage, must also be mounted in a manner that is safe against ignition passage as described, this means with a respective ratio of gap length versus gap width relative to the surrounding housing. 
   Also, the cable grommet is preferably screwed with an outside thread into an inside thread of a pass-through opening wherein a seal is located at the end of the thread. 
   Also, in the other head cover, preferably, a central pass-through is located through which a shaft connecting a spiral spring and an angle sensor to the cable drum in a rotation proof manner passes through the housing. Also, a layout, which is proof against ignition passage, has to be achieved through the ratio of length versus width of the annular gap formed around the shaft, wherein the shaft is held in the same head cover, preferably, by a roller bearing after the annular gap, whereby the actual length of the bearing cannot be counted towards the length of the annular gap. 
   The length of the required annular gaps can be achieved in a simple manner by making the head cover from material thick enough so that, at least in the area of the holes through this head cover and of the annular gaps, the required length of the annular gap is achieved without additional measures. 
   The non-pressure proof housing section is pot shaped and placed onto the outside of the pressure proof housing section, preferably, at its front face, directly or indirectly. 
   Indirect mounting is preferred, wherein standoff plates are initially placed onto external front faces of head covers, radially protruding beyond the head covers whose external diameter is approximately equivalent to the diameter of the housing body, serving to place the pressure-proof housing section with its longitudinal axis parallel to a base plate. 
   These standoff plates are bolted through pass-through holes into blind holes in the head covers or in the bottom of the pressure-proof housing section. 
   For this purpose, the pass-through holes in one of the standoff plates are provided as arched, long holes centrally located around the middle to be able to align the rotation position of the second standoff plate flush with the first standoff plate, in spite of the rotation position of the head cover relative to the body of the pressure-proof housing not being determined before the mounting. 
   A particularly simple assembly method is provided through providing the connecting shaft at the inside end, this means at the end inside the pressure-proof housing section as a hollow shaft, and inserting a shaft end of the angle sensor in a rotation proof manner. 
   Similar, for mounting the printed circuit board which carries the processing electronics with a head space from the angle sensor and from the head cover opposite to the shaft, in particular in a plane perpendicular to the longitudinal axis of the rotationally symmetrical pressure proof housing section, where it is bolted to the spring housing e.g. through a spacer in order to be electrically connected with the angle sensor after mounting the spring, the spring housing and the angle sensor. 
   In order to avoid spark generation on the moving components in the non-pressure proof only dust proof housing section, this housing section and also the measuring cable is made from stainless steel and is coated in particular with plastic, e.g. nylon. Also, the outlet opening for the measuring cable from the non-pressure proof housing section is not made from metal but preferably from plastic. 
   Through this design of the particular components, a simple and fast assembly process can be realized, which is performed upon the pressure-proof housing section and upon the components contained therein so, that
         a shaft is pushed in from the inside to the outside, first through a roller bearing and, thereafter, through a head cover or the bottom of a pot-shaped housing body until a shoulder of the shaft contacts,   the spring housing, with an inserted flat spiral spring, is pushed onto the shaft from its rear free end, connecting the flat spiral spring rotation proof with the shaft,   a rotation proof connection is established, in particular a bolted connection of the spring housing with the head cover or the bottom,   rotation proof placement of an angle sensor onto the rear end of the shaft and supported through a torque link relative to the spring housing,   electrical connection of the angle sensor, in case processing electronics for the signals of the angle sensor, is also included in the housing, and the signals are not transmitted to processing electronics outside the sensor housing the assembly process continues as follows:   in case, instead of a cable pulled through a cable grommet through the housing, e.g. through the head cover, a connector or a connector socket on the outside of the housing, e.g. of the head cover is used, it is inserted into the head cover instead of the cable grommet and is electrically connected with the angle sensor.       

   Thereby, to facilitate all assembly steps described so far, as a first step, the first head cover is initially bolted together with a first standoff plate that is mounted to a base plate to be able to assemble all components carried by the first head cover more easily.
         mounting processing electronics to a spring housing through a spacer,   pulling a cable connected to the processing electronics through the head cover and the cable grommet,   screwing and tightening the head cover into a frontal opening of a housing body,   inserting the cable grommet into the head cover.       

   The further assembly of the sensor beyond the pressure tight housing section is then performed so that
         on its face at least a second standoff plate is bolted on and thereby the standoff plates are brought into a position relative to each other that is flush in an axial direction and   they are bolted onto a base plate if necessary.       

   Now, or after inserting the shaft through the first head cover and bolting down the first stand off plate,
         the non-pressure tight housing, with the attached wound cable drum, can be placed onto the one standoff plate and   a flat spiral spring can be wound up through relative rotation of the housing parts and   eventually the non-pressure tight housing part be can be bolted to the pressure-tight housing part in this pre-loaded rotation position.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments according to the invention are subsequently described in more detail in an exemplary manner. 
       FIG. 1  is a perspective view of the completely assembled position sensor of the present invention. 
       FIGS. 2   a - c  are outside views of the position sensor. 
       FIG. 3  is a longitudinal cut view of the sensor. 
       FIG. 4  is a partially assembled position sensor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows the sensor  1  comprising a cylindrical housing section  6   a , held in parallel to a base plate  21  at a distance through frontal standoff plates  9   a,b , whereby a second housing part  6   b  is mounted on an outside of one of these standoff plates, from where a measuring cable  3  runs out, whose mounting adapter  26  rests on a cable intake turret  27  when the cable is wound up. 
   Within the  FIG. 2 ,  FIG. 2   a  shows a side view perpendicular to a longitudinal axis  10 ,  FIG. 2   b  shows the top view and  FIG. 2   c  shows the front view from the side opposite to housing part  6   b.    
   The following details can be seen best with reference to the longitudinal cut view in  FIG. 3 . The section proof against outside and inside pressure and explosion proof, in particular, is housing section  6   a , which is rotationally symmetrical and provided from a cylindrical, tubular housing body  13  with several shoulders and steps in its interior diameter, which is closed on its front faces through head covers  8   a, b , which rest on the front face of housing body  13  with one shoulder and which reach into an interior diameter of housing body  13  with the majority of their length, where they are screwed into an inside thread. 
   Between an abutting flange and a threaded connection  15  of head covers  8   a,b , a respective circumferential O-ring  16  is located in a respective groove. Head covers  8   a,b  are screwed into housing body  13  strong enough so that a desired tightness is achieved. 
   Onto exterior surfaces of head covers  8   a,b , standoff plates  9   a,b  are placed and bolted from outside into blind holes  22  of the head covers, whereby standoff plates  9   a,b  radially protrude beyond housing body  13  in one direction by the same distance and can, therefore, be used for bolting onto base plate  21 . 
   Furthermore, one standoff plate  9   b  is also large enough in a radial direction so it protrudes over the remaining circumference section of housing body  13  and, thereby, beyond face plate  8   b  so that housing  6   b  can be put onto standoff plate  9   b  from the side facing away from housing section  6   a , again including a tubular housing body  13 ′ and a head cover  8 ′, whereby no pressure tight, but only a dust proof housing section  6   b  is formed. 
   In housing section  6   b , a cable drum  2  is held in a bearing and is wound with measuring cable  3 , preferably in one layer, which exits from housing section  6   b  through cable exit turret  27 , on whose external face mounting adapter  26  on a free end of the cable rests when the cable is completely wound up. 
   Since cable drum  2  has to be connected rotation proof with several other functional elements which are all prone to generate sparks and, therefore, are located in the pressure tight and, in particular, explosion protected housing section  6   a , the assembly is performed as follows: 
   Initially head cover  8   b , later facing housing section  6   b , can be bolted together with standoff plate  9   b , which is already bolted together with base plate  21 , whereby lip seals  28  are inserted there beforehand. 
   From an interior of housing section  6   a , preferably after the described bolting assembly, a connecting shaft  7  is inserted through an appropriate central pass-through bore  18 ′ in head cover  8   b  and roller bearings  24  located on the inside of head cover  8   b  and lip seals  28  on the outside of head cover  8   b  and on the inside of standoff plate  9   b  so that the free end protrudes from standoff plate  9   b  into an axial area, where cable drum  2  is located later. 
   Thereafter, in the interior of housing section  6   a , a flat spiral drum  4  and a receiving, detachable pot-shaped spring housing  19  are preferably pushed together onto a part of shaft  7  protruding from head cover  8   b  towards the inside, whereby flat spiral spring  4  is connected with its one, preferably outside, end to a spring housing  19  in a rotation proof manner and with its other, preferably interior, end to shaft  7  in a rotation proof manner, e.g. through axial insertion. 
   Thereafter, spring housing  19  is mounted to head cover  8   b  rotation proof, in particular, through bolting. 
   Thereby, also in the bottom of spring housing  19 , a roller bearing  24  is located at the inner end of shaft  7  protruding from spring housing  19 , which is a hollow shaft and open at its face end, so that the shaft end of an angle sensor  5  can be inserted therein and fixated against rotation through a clamping nut  29 . 
   In an open axial space between angle sensor  5  and an end of housing body  13 , which is still free, one or several printed circuit boards with processing electronics  12  are mounted, e.g. with a spacer  20  relative to spring housing  19 . 
   In addition, shaft  7  is supported against rotation with a torque link  25  with reference to spacer  20 . 
   Processing electronics  12  are connected to electrical terminals of angle sensor  5 , on the one hand, and to the ends of a cable  11 , on the other hand, which conducts output signals of processing electronics  12  to the outside. 
   Therefore, cable  11  is run to the outside through an explosion-proof cable grommet  17 , which is screwed into central pass-through hole  18  of the other head cover  8   a  in a pressure tight and explosion proof manner. 
   Head cover  8   a , like the other head cover  8   b , is provided with a flange and an external thread and O-ring  16  and screwed into a respective internal thread of housing body  13 , which was previously screwed onto first head cover  8   b.    
   Pulling cable grommet  17  close, which has cable  11  already running through it, and screwing it in is preferably performed after screwing head cover  8   a  into housing body  13  and tightening it. 
   Second stand off plate  9   b  is then placed onto the completely mounted and closed housing  6   a  and bolted down. 
   Since hereby the rotation position of blind holes  22  in head covers  8   a, b  for bolting on the standoff plates  9   a,b  depends on the final position of head covers  8   a,b  when screwing them into housing body  13 , on the one hand, and, on the other hand, the rotation positions of standoff plates  9   a,b  have to be flush with each other to be able to bolt housing section  6   a  onto base plate  21 , in one standoff plate  9   b , facing housing section  6   b  normal pass-through holes are provided for bolting onto the head cover, on the other hand standoff plate  9   a  opposed from here has slotted holes  23  arched around the center, as can be seen best in  FIG. 2   c.    
   Through placing and fixating cable drum  2  and inserting measuring cable  3  through cable outlet turret  27  from the outside to the inside and through mounting to cable drum  2 , the functional elements are complete and housing section  6   b  can be closed through inserting its head cover  8 ′ and bolting it through tubular housing body  13 ′ onto standoff plate  9   b.    
   This can already be performed immediately after passing shaft  7  through and bolting standoff plate  9   b  on. 
     FIG. 4  furthermore shows a partially assembled position sensor, which results from choosing the described, particularly advantageous assembly process: 
   Thereby, initially, standoff plate  9   b , located between the two housing parts  6   a  and  6   b  and carrying them, is mounted onto base plate  21 . 
   Subsequently, the components contained in the two housing parts  6   a  and  6   b  are mounted one after the other, whereby one has to begin with bolting the abutting head cover  8   b  to standoff plate  9   b , whereby respective seals, like lip seals  28 , have to be inserted into these two parts previously. 
   Thereafter, shaft  7  can be pushed through the pass-through holes of these two plates. 
   Thereafter, the parts connected with shaft  7  in a rotation proof manner can be inserted and installed from both sides, this means cable drum  2  in the later housing section  6   b  and on the other side the flat spiral spring and spring housing  19  receiving it, on which further components are located later on, like e.g. angle sensor  5 , which can only be partially seen in  FIG. 4 . 
   On the side of housing part  6   b , the ring forming housing body  13 ′ is put onto standoff plate  9   b  and bolted to it. Thereafter, measuring cable  3  is inserted through the outlet opening and through the cable inlet turret, which is typically attached on the outside of housing body  13 ′ and mounted to cable drum  2  with its free end. Thereafter, cable  3  is wound onto cable drum  2  through rotating spring housing  19  relative to standoff plate  9   b , and the flat spiral spring contained in the spring housing can be wound up, this means pre-loaded, and then spring housing  19  can be mounted in a rotation proof manner relative to the head cover, which carries it in this pre-loaded position. 
   This provides the state of assembly shown in  FIG. 4 , which further required assembly steps have already been explained. 
   REFERENCE NUMERALS 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               1 
               Positions sensor 
             
             
                 
               2 
               Cable drum 
             
             
                 
               3 
               Measuring cable 
             
             
                 
               4 
               Flat spiral spring 
             
             
                 
               5 
               Angle sensor 
             
             
                 
               6 
               Housing 
             
             
                 
               6a, b 
               Housing section 
             
             
                 
               7 
               Shaft 
             
             
                 
               7a 
               Shoulder 
             
             
                 
               8a, b, 8′ 
               Head cover 
             
             
                 
               9a, b 
               Standoff plate 
             
             
                 
               10 
               Axial direction 
             
             
                 
               11 
               Cable 
             
             
                 
               12 
               Processing electronics 
             
             
                 
               13, 13′ 
               Housing body 
             
             
                 
               14 
               Annular gap 
             
             
                 
               15 
               Threaded connection 
             
             
                 
               16 
               O-ring 
             
             
                 
               17 
               Cable grommet 
             
             
                 
               18, 18′ 
               Pass through opening 
             
             
                 
               19 
               Spring housing 
             
             
                 
               20 
               Spacer 
             
             
                 
               21 
               Base plate 
             
             
                 
               22 
               Threaded blind hole 
             
             
                 
               23 
               Slotted hole 
             
             
                 
               24 
               Roller bearing 
             
             
                 
               25 
               Torque link 
             
             
                 
               26 
               Mounting Adapter 
             
             
                 
               27 
               Turret 
             
             
                 
               28 
               Lip seal 
             
             
                 
               29 
               Clamping nut 
             
             
                 
               30 
               Brush 
             
             
                 
               31 
               Connector